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The boundaries of neo- and allocortical areas of Galago demidovii are analyzed with an automatic quantitative procedure using an image analyzer. The results are summarized in a cortical map and compared with a cortical map of Tupaia. Galago shows a highly differentiated temporal lobe and no homogeneous peristriate area comparable to the classical concept of Brodmann's Area 19. Primary motor, somatosensory, auditory, and visual areas are delineated and shown to be surrounded by distinct secondary areas.

In reviewing the literature on the anatomy and physiology of the somatosensory cortical areas, it has become apparent that there are many gaps in our knowledge of even the basic pattern of organization. There are at least four short latency representations of the cutaneous receptor sheet but it is not possible yet to conclude that there are not others. Nor is it possible to specify clearly the manner in which each differs from the others. The evoked potential, a subject of study for decades, clearly arises from postsynaptic cortical phenomena; it is an electrical signal produced by membrane polarization changes in asymmetrically oriented cortical neurons. Under some specified conditions it can be modeled by activity in a particular neuronal subset; however, the manner in which this neuronal population is organized in the plane horizontal to the cortical surface is unknown. Considerable information is available concerning the primary somatosensory cortical map for numerous species and all maps appear to be variations on a general plan. The other somatosensory representations are less well described. Some maps are available for SII. The construction of one map for SIII was reported. No maps are available for any other areas. The cellular anatomy of the somatosensory cortex is well documented for cat and monkey. The precision of afferent and efferent connections, the patterns of dendritic and axonal arborizations and the anisotropy of the cortical matrix are data for which there are few or no physiological equivalents. The functional characteristics of neurons in the somatosensory cortices have been documented by workers in only a few laboratories. Conceptual advances such as the cortical column hypothesis and the conception of the somatosensory map as a topological surface, have provided testable theories that sharpen the questions concerning the function of this cortical region. The next level of analysis will require more information about the response properties of selected populations of cortical neurons in order to better define them. Some of the properties which have been studied are inhibition, modality convergence, directional selectivity, and the quantitative relationships between stimulus and response. A description of neuronal populations using such attributes is needed for understanding the functional connectivity of the somatosensory system and its role in the behavior of the organism.

This study establishes a cortical map of the somatosensory periphery of the mouse head, with emphasis on the whisker pad. Data in the literature on the projection of the common hair follicles are confusing, notably the question whether or not this projection is separated from or overlaps with that of the facial vibrissae, the barrelfield. Microelectrode recordings in the barrelfield and its immediate surroundings upon natural stimulation of the periphery were followed by microlesions and histological reconstruction. Results show that the barrelfield consists of two parts: an anterior part where vibrissal follicles and the skin bearing them are represented, and a posterior part receiving only vibrissa inputs. The skin between these latter vibrissae is represented outside the barrelfield. We conclude that the partially dissociated cortical representation of skin and vibrissae may allow large vibrissae to be used in tasks requiring greater acuity than the shorter ones provide. This hypothesis is supported by the observation that, owing to special muscles, large vibrissae are more mobile than short ones. We also propose that the segregation of inputs from vibrissae and common fur is related to the number of nerve fibers serving one follicle, and we indicate an experimental model to test this possibility.

1. Single neurones were recorded with glass-coated tungsten electrodes from area 17 of the cat's visual cortex. The cats were anaesthetized and artificially respirated with a mixture of halothane, nitrous oxide and oxygen. 2. For local cortical inactivation a multibarrel pipette was placed 0.5-2.5 mm posterior (or anterior) to the recording site, at a depth of 400-600 micron. Four separate barrels of the pipette were filled with gamma-aminobutyric acid (GABA); the fifth was filled with Pontamine Sky Blue for labelling of the centre of the inactivation site. 3. Direction-selective cells, of differing optimal orientations and preferred directions of motion, were classified as simple or complex and tested with computer-controlled stimuli presented on an oscilloscope. 4. During continuous recording GABA was microionophoretically applied for different durations and with different ejection currents. The effectiveness of GABA microionophoresis was evident from the direct GABAergic effects (strong overall inhibition of the recorded cells) observed with high ejection currents and prolonged application. 5. Two discrete effects could be observed during local inactivation distant from the cortical cell under study: an increase of the response in either the non-preferred or the preferred direction; or a decrease of the response in the preferred direction. All GABA-induced changes were reversible. 6. The depressant action of GABA was independent of the relative topography between recording and inactivation site and affected mainly the response to the preferred direction of stimulus motion. 7. Disinhibition was only observed when the stimulus-evoked response moved on the cortical map in a direction from the GABA pipette towards the recording electrode. It is concluded that GABA reversibly silences inhibitory interneurones that are situated in the vicinity of the micropipette tip and are involved in generation of direction selectivity. 8. No fundamental differences between cells from different cortical layers were observed. The disinhibitory effects of GABA inactivation were more pronounced and more frequently seen in simple cells (61%) than in complex cells (38%), while the opposite was true for reduced excitation during lateral GABA inactivation (observed in 62% of the complex vs. 39% of the simple cells). Accordingly, lateral inhibition statistically prevails in simple cells and lateral excitation in complex cells. 9. Among the inhibitory and excitatory mechanisms affected by lateral GABA inactivation, inhibition is organized with a higher topographic specificity.(ABSTRACT TRUNCATED AT 400 WORDS)

The organization of the whisker representation within the neocortex of the rat is dependent on an intact periphery during development. To further investigate how alterations in this cortical map arise we examined the organization of thalamocortical afferents to the whisker representation in adult animals in which the infraorbital branch of the trigeminal nerve was cut on the day of birth. The disrupted pattern of thalamocortical projections to the vibrissae representation was apparent in the abnormal pattern of the anterograde transport of horseradish peroxidase from the thalamus, as well as in the abnormal pattern of succinate dehydrogenase activity. To determine the morphology of individual thalamocortical axons associated with this disrupted pattern, terminal arbors were “bulk-labeled” by injections of horseradish peroxidase into the white matter beneath the somatosensory cortex. Terminal arbors were identified by their laminar distribution of boutons corresponding to the specific thalamocortical afferent. The medial to lateral extent of these terminal arbors varied dramatically, from 350 to 1500 microns. In addition, terminal arbors innervating the same local area of cortex appeared to have varying degrees of overlap. Thus, the disruption of the neocortical vibrissae representation appears to involve the abnormal arborization of individual thalamocortical afferents. This finding supports the hypothesis that the fine-grain organization of the somatotopic map is dependent on the morphology and organization of individual thalamocortical arbors, which, in turn, are dependent on the periphery during development.

A procedure is described for constructing two-dimensional, unfolded representations of the cerebral cortex. The technique is based on information contained in outlines of histological sections, and it allows an entire hemisphere to be represented on a single cortical map. Maps for different hemispheres from individuals of the same species are similar in size, shape, and organization, and their configuration is largely independent of the plane of sectioning used for reconstruction. Many types of information pertaining to the location and organization of different functional subdivisions can be displayed on cortical maps; representative applications of the technique to mapping cerebral cortex in the macaque and the cat are shown. Areal measurements on cortical maps correspond closely (generally within 20%) to actual surface areas in the intact hemisphere. Therefore, the maps can also be used to provide accurate determinations of the absolute and relative extent of various anatomical and functional subdivisions of the cortex.

The cortical representations of the hand in area 3b in adult owl monkeys were defined with use of microelectrode mapping techniques 2-8 months after surgical amputation of digit 3, or of both digits 2 and 3. Digital nerves were tied to prevent their regeneration within the amputation stump. Successive maps were derived in several monkeys to determine the nature of changes in map organization in the same individuals over time. In all monkeys studied, the representations of adjacent digits and palmar surfaces expanded topographically to occupy most or all of the cortical territories formerly representing the amputated digit(s). With the expansion of the representations of these surrounding skin surfaces (1) there were severalfold increases in their magnification and (2) roughly corresponding decreases in receptive field areas. Thus, with increases in magnification, surrounding skin surfaces were represented in correspondingly finer grain, implying that the rule relating receptive field overlap to separation in distance across the cortex (see Sur et al., '80) was dynamically maintained as receptive fields progressively decreased in size. These studies also revealed that: the discontinuities between the representations of the digits underwent significant translocations (usually by hundreds of microns) after amputation, and sharp new discontinuous boundaries formed where usually separated, expanded digital representations (e.g., of digits 1 and 4) approached each other in the reorganizing map, implying that these map discontinuities are normally dynamically maintained. Changes in receptive field sizes with expansion of representations of surrounding skin surfaces into the deprived cortical zone had a spatial distribution and time course similar to changes in sensory acuity on the stumps of human amputees. This suggests that experience-dependent map changes result in changes in sensory capabilities. The major topographic changes were limited to a cortical zone 500-700 micron on either side of the initial boundaries of the representation of the amputated digits. More distant regions did not appear to reorganize (i.e., were not occupied by inputs from surrounding skin surfaces) even many months after amputation. The representations of some skin surfaces moved in entirety to locations within the former territories of representation of amputated digits in every monkey studied. In man, no mislocation errors or perceptual distortions result from stimulation of surfaces surrounding a digital amputation.(ABSTRACT TRUNCATED AT 400 WORDS)

Striate cortex topography derives from a stretching of retinal space along the optic axis. At the retina, relative distances are preserved in a mapping of retinal space onto a spherical surface in the environment. At the cortex, relative distances along visual meridia in the cortical map are preserved in a mapping of striate cortex onto an environmental conic surface whose base is in the plane of the eye. This eco-cortical relationship can be considered a reference frame through which spatial relationships at the cortex might provide information about the environment. The present analysis provides an explanation of changes in cortical magnification with visual eccentricity in the primate and a detailed three-dimensional model of striate topography for the macaque monkey. In man, a conic environmental surface is shown to be uniformly resolvable along meridia in the visual field. Finally, the implications of this analysis of the structural properties of the retino-striate pathway and visual resolution are considered in relation to depth and distance perception.

In previous work, it was suggested that the sequence regularity property of cortical neurons could be accounted for if the local geometric structure of the cortex were a recapituation of the global complex logarithmic structure of the retinotopic mapping. This model is developed in detail: the excitatory and inhibitory structure of cortical receptive fields may be approximated by a complex logarithmic local geometry, coupled with an intra-cortical lateral inhibition operator which may flow unidirectionally yet still create "rotating" receptive field structure. The direction of intra-cortical lateral inhibition follows the borders of cortical ocular dominance columns, which are the approximate images under the global complex logarithmic mapping, of exponentially spaced, horizontal straight lines in the visual field. Two different topological structures are discussed for the local cortical manifold. The binocular trigger features of cortical neurons follow from the same geometric model, and the ratio of binocular to monocular cortical cells is related to the size and shape of cortical dendritic tree's by an application of integral geometry. Recent results in optical pattern recognition are cited to suggest that the rotation and size invariant properties of the cortical map are essential to any cross-correlational basis for stereopsis. Finally, a meromorphic function is presented which is both locally and globally complex logarithmic in its structure, and therefore represents the model presented in this and previous papers in a concise mathematical form. This function is closely related to the description of a Karman vortex pattern, in fluid mechanics, and leads to the suggestion that the boundary conditions of layer IV of the cortex (i.e. periodic ocular dominance columns) are causally related to the existence of sequence regularity in the cortex. The developmental implications of this statement are that the specification of neural connections in the cortex may follow directly, both locally and globally, from the detailed nature of the cortical boundary conditions (i.e. anatomy), coupled with general physico-mathematical considerations of continuity and differentiability in the neural fiber flow.

The main factors limiting the performance of the peripheral parts of the visual system can be specified, and doing this clarifies the nature of the interpretive tasks that must be performed by the central parts of the system. It is argued that the critical factor that hinders development of better resolving power is the difficulty of confining light within the waveguide-like outer segment, and that for sensitivity this critical factor is the thermal decomposition of photosensitive pigments. Knowledge of these limits makes many surprising details of the eye intelligible. Understanding the difficulties posed by the narrow dynamic range of nerve fibres may give similar insight into the coding of the retinal image for transmission to the brain. Our level of understanding changes when we come to the visual cortex, for although we do not lack good anatomical and neurophysiological data, these do not make the principles of operation self-evident in the way that the structure of the eye immediately suggests that it is an image-forming device. The cortex converts the representation of the visual field that it receives into reliable knowledge of the world around us, and the trouble may be that we lack good models of how this can be done. A system that can respond to single quanta and resolve almost to the diffraction limit is unlikely to employ grossly inefficient methods for those higher functions upon which its whole utility depends, and so it is worth seeking out the limiting factors. The quality of human performance at certain higher perceptual tasks is high compared with the limit of reliable statistical inference; hence much of the sample of information available in a visual image must be effectively utilized. But there are strong limitations on the connectivity in the cortex, so that one is forced to consider how the relevant information can be collected together. Three stages of dealing with the visual image are proposed: the improvement of the cortical map in primary visual cortex by processes analogous to spatial and temporal interpolation ; the detection of linking features in this map; and the concentration of this information by non-topographical mapping in adjacent visual areas.

The cortical map of Tupaia belangeri, evaluated with an automatic quantitative procedure using the Micro-Videomat (Zeiss), shows a well differentiated allocortex and periallocortex with 19 areas, and proisocortex and isocortex with 20 areas. The boundaries of the different areas are based on quantitative differences and the areal pattern of the cortex is, in most cases, compatible with physiological results found in the literature. Tupaia belangeri has a cortex which is largely dominated by primary and secondary sensory areas in its iscortical part and by the impressive visual system. The well differentiated allo- and isocortex show conditions more similar to prosimians than to insectivores, especially when considering the occurrence of primary and supplementary regions. Quantitative analyses of the separate areas with a higher resolution (Zilles et al., in prep.) are necessary in order to provide a basis for further comparative investigations of the Tupaia cortex.

We have measured the responses of cells in the cats lateral geniculate body and the visual cortex to edges which were blurred to various degrees (cosinusoidal blur). For the same cells also the responses were determined to sinusoidal gratings of various fundamental frequency and to slits of various blur and width. All stimuli were moved across the receptive fields at various speeds. The responses of most cells increased with increasing edge sharpness, but usually reached a maximum at a blur corresponding to a high frequency cutoff at 0.6–1.2 c/deg. The responses to the sharpest edges were usually smaller than those to a blurred edge (up to −50% in individual cells and −15% in the average). After normalization, the responses predicted from the spatial frequency tuning curves and the Fourier transform of the edge stimuli corresponded well to the measured blur functions up to the maximum of the edge response which varied considerably between cells, however. At edge sharpness beyond that maximum, the predicted curves rose up to edge sharpness with high frequency cutoff 1.6–1.8 times above that which produced the experimental neuronal response maximum. On the other hand, responses could increase with edge sharpening in spatial frequency regions, in which no or only small responses were seen with sinusoidal gratings (e.g. at lower spatial frequencies in “band pass neurons”). Geniculate X- and cortical simple cells as well as those geniculate Y-cells which showed phase locked grating responses behaved similarly in all respects. We concluded that edge sharpness is not represented by response amplitude of individual neurons but by the spatial distribution of excitatory peaks across the representation of the retinotopic cortical map. Our findings further indicate that spatial models of receptive fields assuming linear signal summation have only a limited value for predicting edge sharpness.

Primary evoked responses and single neuron activity were recorded from the cerebral cortex of the virginian opossum following stimulation of the appendages, particularly a contralateral forepaw and its digits. The electrical activity and discharge patterns of single neurons were comparable to those in higher mammals; both inhibitory and occlusive interactions were observed. Primary evoked responses also resembled those in higher mammals. The areas of the cerebral cortex which responded to stimulation of the palm and the five digits were coextensive, comprising about one-third of the dorsal surface of the cerebral hemisphere. Topographic maps for the contralateral palm and digits constructed on the bases of peak-to-peak voltage and latency of primary evoked response differed in both location and arrangement. Latency-based maps were more medially displaced than amplitude-based maps obtained from the same data although these were usually a mirror image of the amplitude maps. Two possible explanations of this differing topographic arrangement are tendered, and it is suggested that the cortical map based upon minimal latency of the primary evoked response is the primary representation of the periphery at the cerebral cortex.

There are two prevailing explanations for the foveal deficit in texture segmentation reported in previous works. One is based on the spatial and temporal properties of the stimuli, which means in terms of physiology a strong contribution of the Magnochannel. The other one is purely spatial and assigns filters of different bandwidths to each eccentricity in the visual field. We have challenged the first explanation experimentally by using isoluminant stimuli. The central performance drop persisted although the Magno-channel is known to respond weakly to stimuli with low luminance contrast. Therefore, we agreed with the spatial explanation. But instead of the abstract filter theories from previous works we propose a computational neural model assuming local lateral interactions in a cortical map model. The psychophysical performance measures could be directly related to geometric properties of the primary visual cortex concerning its mapping geometry and its intrinsic interaction width. Our model accounts quantitatively for our own psychophysical data as well as for others from literature. In general, we claim that the high foveal retino– cortical magnification maps texture elements too far away from each other for being compared by local processes. 2002 Elsevier Science Ltd. All rights reserved.

The capacity for functional restitution after brain damage is quite different in the sensory and motor systems. This series of presentations highlights the potential for adaptation, plasticity, and perceptual learning from an interdisciplinary perspective. The chances for restitution in the primary visual cortex are limited. Some patterns of visual field loss and recovery after stroke are common, whereas others are impossible, which can be explained by the arrangement and plasticity of the cortical map. On the other hand, compensatory mechanisms are effective, can occur spontaneously, and can be enhanced by training. In contrast to the human visual system, the motor system is highly flexible. This is based on special relationships between perception and action and between cognition and action. In addition, the healthy adult brain can learn new functions, e.g. increasing resolution above the retinal one. The significance of these studies for rehabilitation after brain damage will be discussed.

Maps of sensory receptor epithelia and of derived or computed features of the sensory environment are a common feature of auditory, visual, and somatic sensory representations from the periphery to the cerebral cortex. Maps enhance the understanding of normal neural organization and its modification by pathology and experience. They underlie the derivation of the computational principles that govern sensory processing and the generation of perception. Despite their intuitive explanatory power, the functio of and rules for organizing maps and their plasticity are not well understood. Some puzzles of auditory cortical map organization are that only a few complete receptor maps are available, and that even fewer computational maps have been identified beyond primary cortical areas. Neuroanatomical evidence suggests equally organized connectional patterns throughout the cortical hierarchy that might nerlie map stability. Here we consider the implications of auditory cortical map organization and its plasticity and evaluate the complementary role of maps in representation and computation from an auditory perspective.

SummaryThe topographic representation of visual space is preserved from retina to thalamus to cortex. We have previously shown that precise mapping of thalamocortical projections requires both molecular cues and structured retinal activity. To probe the interaction between these two mechanisms, we studied mice deficient in both ephrin-As and retinal waves. Functional and anatomical cortical maps in these mice were nearly abolished along the nasotemporal (azimuth) axis of the visual space. Both the structure of single-cell receptive fields and large-scale topography were severely distorted. These results demonstrate that ephrin-As and structured neuronal activity are two distinct pathways that mediate map formation in the visual cortex and together account almost completely for the formation of the azimuth map. Despite the dramatic disruption of azimuthal topography, the dorsoventral (elevation) map was relatively normal, indicating that the two axes of the cortical map are organized by separate mechanisms.

Johnston (1986) proposed that the global geometry of the striate cortical map in primates can be understood with reference to a conic surface in visual space oriented with its base in the plane of the eye and axis along the line of sight. Here, cortical magnification data from the macaque, squirrel and cebus monkeys and distance data from topographic maps of the macaque and owl monkey cortex are used as a further test of this proposal. Though there is a substantial difference in the emphasis placed on foveal vision in the owl monkey and the macaque topographic maps, both can be accommodated by the conic model with a change of parameter. The theory has also been extended in an attempt to model the differential changes in mean cortical receptive field size and inverse magnification with eccentricity found in the macaque. Though cortical receptive fields increase in size with eccentricity when measured on a spherical screen, mean receptive field size should be constant if measured on a tangent planes to a cone with an apical half angle of around 6.4 deg.

The cerebral cortex of the guinea pig has been examined by means of a quantitative cytoarchitectonic method (Schleicher et al. 1978; Zilles et al. 1978a). In this method, a computer-controlled automatic image analyzer determines the grey level index of microscopic fields measuring 50 X 50 mu m in Nissl-stained sections by a systematic scanning procedure. Computer plots of serially sectioned histological slides from three hemispheres were produced by printing selected ranges of grey level indices. The delineation of cortical areas was worked out in these plots based on quantitative criteria. Cortical maps of the areal pattern were reconstructed graphically. The resulting cortical map of the guinea pig differs from that of Rose (1912), but it corresponds to the results of Friede (1960) and is in agreement with neurophysiological studies. In general, the areal pattern of the guinea pig is similar to that of the rat (Zilles et al. 1980), but there are also some differences. These differences are discussed with respect to functional considerations.

Many of our present day concepts of the organization of somatosensory cortex stem from the landmark study of Woolsey, Marshall and Bard, first published in 1937 (7) and then more completely in 1942 (21). These investigators documented important features of the overall organization of the postcentral parietal cortex in Macaca mulatta by recording evoked potentials from the surface of the brain. The basic procedure was to determine the portion of the body surface (the receptive field), where movements of hairs or punctate stimulations of glabrous surfaces with a cat’s vibrissa were capable of evoking responses at a given recording site and to repeat the procedure for a grid of closely spaced recording sites. By removing parts of the brain, they were even able to explore cortical surfaces buried in the central sulcus and on the medial wall of the cerebral hemisphere. These studies led to several important conclusions: (a) the region of postcentral cortex activated by tactile somatic stimuli includes the separate architectonic fields 3 (3a and 3b), 1 and 2 of Brodmann; (b) the cortex is activated almost exclusively from the contralateral half of the body; (c) there is an orderly representation of body parts, with a medial to lateral sequence across cortex from tail to tongue along the body; (d) the cortical organization does not exactly reflect the body surface, so that there are displacements in the cortical map; one clear example given was the separation in cortex of the representation of the face from the caudal head; and (e) the skin surfaces with the greatest tactile acuity have the largest cortical projection areas.

A cortical map is a localized neural representation of the signals in the outer world. A rough map is formed under the guidance of genetic information at the initial stage of development, but it is modified and refined further by self-organization. The persent paper gives a mathematical theory of formation of a cortical map by self-organization. The theory treats both dynamic of excitation patterns and dynamics of self-organization in a neural field. This not only explains the resolution and amplification properties of a cortical map, but elucidates the dynamical stability of such a map. This explains the emergence of a microcolumnar or mosaic structure in the cerebrum.

In an earlier study (Neuroscience 8, 33-55, 1983), we found that the cortex representing the skin of the median nerve within parietal somatosensory fields 3b and 1 was completely occupied by 'new' inputs from the ulnar and radial nerves, 2-9 months after the median nerve was cut and tied in adult squirrel and owl monkeys. In this report, we describe the results of studies directed toward determining the time course and likely mechanisms underlying this remarkable plasticity. Highly detailed maps of the hand surface representation were derived in monkeys before, immediately after, and at subsequent short and intermediate time stages after median nerve section. In one monkey, maps were derived before nerve section, immediately after nerve section, and 11, 22 and 144 days later. Thus, direct comparisons in cortical map structure could be made over time in this individual monkey. In other experiments, single maps were derived at given post-section intervals. These studies revealed that: (1) large cortical sectors were 'silenced' by median nerve transection. (2) Significant inputs restricted to the dorsum of the radial hand and the dorsum of digits 1, 2 and 3 were immediately 'unmasked' by median nerve transection. (3) These immediately 'unmasked' regions were topographically crude, and represented only fragments of this dorsal skin. They were transformed, over time, into very large, highly topographic and complete representations of dorsal skin surfaces. (4) Representations of bordering glabrous skin surfaces progressively expanded to occupy larger and larger portions of the former median nerve cortical representational zone. (5) These 'expanded' representations of ulnar nerve-innervated skin surfaces sometimes moved, in entirety, into the former median nerve representational zone. (6) Almost all of the former median nerve zone was driven by new inputs in a map derived 22 days after nerve section. At shorter times (3, 6 and 11 days), 'reoccupation' was still incomplete. (7) Very significant changes in map dimensions within and outside of the former median skin cortical field were seen after the 'reoccupation' of the deprived cortex by 'new' inputs was initially completed. (8) Progressive changes were recorded within the original ulnar and radial nerve cortical representational zones, as skin surfaces originally overtly represented wholly within these regions expanded into the former median nerve zone. (9) Throughout the studied period, the cortical representational loci of many skin sites appeared to change continually and often markedly. (10) The locations of map discontinuities also shifted significantly over time. (11) Concomitant with changes in representational magnification over time, inverse changes in receptive field sizes were recorded.(ABSTRACT TRUNCATED AT 400 WORDS)

The spatial inhomogeneity of the retino-striate system is summarized by the vector cortical magnification factor. The logarithm of retinal eccentricity provides a good fit to the integrated cortical magnification factor. Under the assumption that the cortical map is analytic (conformal), this implies that a complex logarithmic function of retinal coordinates describes the two-dimensional structure of the cortical representation of a visual stimulus. This hypothesis is in good agreement with the measured global structure of rhesus, squirrel, and owl monkey retino-striate mappings, as well as that of the upper visual field of the cat. The geometric structure of the local hypercolumnar unit of striate cortex may also be characterized in terms of the complex logarithmic mapping: thus, the retino-cortical system may be thought of as a concatenated complex logarithmic mapping. A simple developmental mechanism is capable of constructing a map of this form, and the general mathematical properties of conformal mappings allow some insight into the nature of the minimal coding requirements which must be specified to encode a neural map. Complex logarithmic mapping yields a cortical “Gestalt” which is pseudo-invariant to size, rotation, and projection scaling: these symmetries, for a given fixation point, result in a linear shift of an invariant cortical pattern. The term computational anatomy refers to the possibility that the anatomical structure of the retinotopic mapping may simplify certain aspects of perceptual coding. Similar uses of the complex logarithmic mapping, in computer pattern recognition, are cited in support of this concept. Furthermore, it is shown that columnar structure, together with topographic mapping, may also provide a direct computational function. If two topographic mappings are appropriately interlaced, by columns, then the difference mapping of the two independent inputs is encoded within a spatial frequency channel determined by the period of the columns. A quantitative model of the human visual cortex is developed and used to portray the detailed structure of certain visual stimuli, as they would appear at the level of the striate cortex. The local and global geometric structure of the striate map suggests a simple explanation for several visual illusions. Thus, it is demonstrated that the geometric structure of visual stimuli, at the level of striate cortex, may be of significance to perception. Finally, the concept of computational anatomy is discussed in relation to other contemporary notions of perceptual coding. It is argued that both single cell feature extraction models and Fourier analysis models of visual coding are inconsistent with the known properties of the visual system, and moreover, have never been adequately formulated in computational terms. The approach of the present paper is to suggest that the basic data structure of perceptual coding consists of two-dimensional laminar mapping, and that successive stages of remapping, along with columnar architecture, may provide important computational functions.

Detailed microelectrode maps of the hand representation were derived in cortical areas 3b and 1 from a series of normal adult owl and squirrel monkeys. While overlap relationships were maintained, and all maps were internally topographic, many map features varied significantly when exam­ ined in detail. Variable features of the hand representations among different monkeys included a) the overall shapes and sizes of hand surface represen­ tations; b) the actual and proportional areas of representations of different skin surfaces and the cortical magnifications of representations of specific skin surfaces, which commonly varied severalfold in area 3b and manyfold in area 1; c) the topographic relationships among skin surface representa­ tions, with skin surfaces that were represented adjacently in some monkeys represented in locations many hundreds of microns apart in others; d) the internal orderliness of representations; e) the completeness of representa­ tions of the dorsal hand surfaces; and f) the skin surfaces represented along the borders of the hand representation. Owl monkey maps were, in general, internally more strictly topographic than squirrel monkey maps. In both species, area 3b was more strictly topographic and less variable than was area 1. The degree of individual variability revealed in these experiments is difficult to reconcile with the hypothesis that details of cortical maps are ontogenetically specified during a period in early life. Instead, we propose that differences in the details of cortical map structure are the consequence of individual differences in lifelong use of the hands. This conclusion is consistent with earlier studies of the consequences of peripheral nerve tran­ section and digital amputation, which revealed that cortical maps are dy­ namically maintained and are alterable as a function of use or nerve injury in these monkeys CMerzenich et al., '83a,b, '84a; Merzenich, '86; Jenkins et al., '84; Jenkins and Merzenich, '87).

Publisher Summary The term computational anatomy has been used to refer to the fact that certain geometric symmetry operations are simpler, in computational terms, when the cortical structure of a visual stimulus is examined. Specific examples are size and rotation symmetry: For a given fixation point, the cortical image is an invariant “Gestalt” that undergoes a linear shift under size (or rotation) of the retinal projection of the stimulus. This aspect of symmetry in the complex logarithmic plane has been demonstrated to be of utility in computer pattern recognition applications and it suggests that perhaps the anatomy of the cortical map may be contributing to the computational aspects of visual processing. Other examples of computational anatomy are also evident: The existence of periodic interlacing of two slightly different maps of a stimulus causes a spatial frequency coding of the difference mapping of the stimuli. Thus, the ocular dominance column pattern of striate cortex might allow a simple feature extraction for depth simply by the application of a high pass spatial filter to the cortical representation of a stimulus.

The boundaries of iso- and allocortical areas of Microcebus murinus were analyzed with an automatic quantitative procedure using an image analyzer. The results are summarized in a cortical map and compared with a previous study of Microcebus murinus (Clark, 1931) and Galago demidovii (Zilles et al., 1979). A surprising correspondence could be observed between Microcebus and Galago with respect to the cortical areal pattern and the cytoarchitectonic structure of the various areas, including their position and extension, although both species are grouped into different suborders.

Target parameters such as reflectivity, range, velocity, and angular position are represented by ordered maps of tuned cortical neurons in insectivorous bats. It is suggested that the response of each neuron in such a map is determined by a hypothesis test conditioned on a particular value of the mapped parameter. The excitation of each neuron is then interpreted as a sample value of a conditional log‐likelihood ratio or a log‐likelihood function. Interpolation between the samples, which is needed to find the parameter that maximizes the mapped function (e.g., the maximum likelihood parameter estimate), can be accomplished with overlapped tuning curves. An attempt to portray a sharp peak by a weighted sum of relatively broad neuronal tuning curves or interpolation functions results in excitatory center/inhibitory surround behavior. Facilitation or antifacilitation of neurons that are likely to be excited by succeeding observations can be used for sequential detection and tracking. Interpolation and pulse‐to‐pulse data storage capability are required to explain range jitter sensitivity and to allow for moving target indication in bat sonar. If a cortical map represents an ordered array of hypothesis tests, then many such tests are implemented in parallel when target parameters are unknown. Detection performance is then degraded relative to the idealized situation in which all parameters are specified. Performance in noise may thus appear to be much worse than that of an ideal detector, even if each hypothesis test is optimally implemented.

In Siamese cats, many ganglion cell fibers from the temporal retina misproject to the contralateral hemisphere; as a result, each lateral geniculate nucleus contains an abnormally large representation of the ipsilateral visual field. The manner in which the visual cortex processes this aberrant visual information has been examined in several previous studies. In some Siamese cats, the region of the 17/18 border was found to contain an extensive, systematic map of the ipsilateral field, while in other animals no such map was found, and the 17/18 border appeared to represent the zero meridian of azimuth (as in normal cats). These results have led to the suggestion that there are two distinct types of Siamese cat ("Boston" and "Midwestern") which can be distinguished on the basis of cortical topography and the anatomical organization oif the geniculocortical pathway. In the present study, we have recorded from four Siamese cats in order to examine the visual field map in the region of the 17/18 border; in each cat we recorded at anterior coronal levels corresponding to the representation of the lower visual field, and also at more posterior levels near the horizontal meridian representation. In all of the animals we found that the anterior penetrations (corresponding to mean receptive field elevations inferior to -7 degrees) yielded 15-20 degrees of ipsilateral field representation at the 17/18 border; however, the posterior, horizontal meridian penetrations (with mean elevations from +1 degrees to -4 degrees) showed excursions of only about 5 degrees into the ipsilateral field. This large difference in the representation of azimuth was not due to rotation of the eyes during our recording sessions. The finding of appreciable differences in the amount of ipsilateral field represented at different anterior-posterior levels of the same animal might lead to the suggestion that there are not two distinct populations (or types) of Siamese cat with regard to the cortical map of the ipsilateral field. Rather, we raise the possibility that Siamese cats form one population in which there is a continuous variation in the extent of ipsilateral field represented in the cortex.

The purpose of this essay is to examine some parallels in the evolutionary and functional significance of replicated maps in the genetic material and the visual cortex. In particular, I would like to explore two related ideas. The first is that the differentiation of replicated maps is an important factor in the development of new functional capacities in evolution. The second is that the properties of maps are influenced by their context. The contextual influences are mediated in genetic systems by various forms of gene regulation. In the visual cortex, contextual influences are expressed by the effects of stimuli located outside the classical receptive fields of individual neurons making up each cortical map. These contextual influences may determine how the organs of the body are assembled by the genes and how percepts and thoughts emerge from the activity of mapped arrays of neurons.

The development in the early part of the present century of new methods for fixation and staining of the brain opened up new possibilities for detailed examination of the cytoarchitecture of the cerebral cortex. Through the work of Ramon y Cajal and others, it soon became apparent that the cerebral cortex could be subdivided into areas of specific structure. The first cortical map based on cytoarchi- tectural differences was published in 1905 by Campbell, who divided the cortex into 20 areas. A few years later (1909) Brodmann published a map that has been widely reproduced which divided the cortex into about 50 separate zones (Fig. 11.1). Other maps have since been suggested, but Brodmann’s labelling is still the one most extensively employed and it will therefore be used in the following (Fig. 11.2).

Much of vertebrate midbrain and mammalian cortex is dedicated to two-dimensional "maps" in which two or more stimulus parameters are encoded by the position of neural activation in the map. Moreover, there are a large number of such maps which interact in an unknown fashion to yield a unified perception of the world. Our research program is based on studying the structure and function of brain maps. In the present paper, we review a recently constructed system of computer aided neuro-anatomy which allows high resolution texture mapped models of cortical surfaces in two and three dimensions to be displayed and manipulated. At the same time, this work demonstrates some of the basic geometric patterns of architecture of the primate brain, such as columnar and topographic mapping.

It is suggested that the long fibres, descending to the spinal cord, address columns of motoneurones, such as have been described by Romanes. Anatomical and physiological evidence is adduced in support of this suggestion. The consequences of such a projection rule are examined for several descending tracts. It is shown that the organization of the motor cortical map may simply reflect the arrangement of the motoneurone column within the spinal cord. That is, the mapping from motor cortex to spinal cord is continuous if one adopts the correct topology and ‘tolerance’ for the motoneurone pools. Finally, some developmental mechanisms that may account for this projection rule are briefly mentioned.

Recent studies conducted principally in the somatosensory nervous system of primates have revealed that cortical maps are alterable as a function of experience in adults. The specific nature of this evident dynamism of cortical representations bears a series of important implications for consideration of the significance of features of sensory system organizations. In this report, we shall a) briefly summarize the basic conclusions drawn from these studies on the dynamics of cortical map structure, b) outline by example the results of studies from which these conclusions have been drawn, and c) then specifically consider some implications of these studies for our understanding of the significance of special organizational features of the auditory system.

We analyse a model for the development of orientation-selective receptive fields of simple cells in a locally connected network of cortical neurons. The Hebbian learning rule that underlies the development is described by a linear differential equation. The structure of the emerging cortical map can be predicted by deriving the eigenfunctions corresponding to the leading eigenvalues of the associated matrix. We show that the receptive fields have the typical form of a wavelet. Mathematically, receptive fields are given by a Hermitian polynomial with Gaussian cut-off and a phase factor. Both the phase of the wavelet and the orientation are changing periodically along the surface of the cortical map as suggested by previous simulation studies and as also found in experiments. In order to get orientation-selective receptive fields, the spatial correlation function of the inputs that drive the development must have a zero crossing.

Background: On the one hand, stimulating the motor cortex at different spots may activate the same muscle and result in a muscle-specific cortical map. Maps of different muscles, which are functionally coupled, may present with a large overlap but may also show a relevant variability. On the other hand, stimulation of the motor cortex at one spot with different stimulation intensities results in a characteristic input-output (IO) curve for one specific muscle but may simultaneously also activate different, functionally coupled muscles. A comparison of the cortical map overlap of synergistic muscles and their input-out curves has not yet been carried out. Objective: The aim of this study was to probe functional synergies of forearm muscles with transcranial magnetic stimulation by harnessing the convergence and divergence of the corticospinal output. Methods: We acquired bi-hemispheric cortical maps and IO curves of the extensor carpi ulnaris, extensor carpi radialis and extensor digitorum communis muscles by subjecting eleven healthy subjects to both monophasic and biphasic pulse waveforms. Results: The degree of synergy between pairs of forearm muscles was captured by the overlap of the cortical motor maps and the respective IO curves which were influenced by the pulse waveform. Monophasic and biphasic stimulation were particularly suitable for disentangling synergistic muscles in the right and left hemisphere, respectively. Conclusion: Combining IO curves and different pulse waveforms may provide complementary information on neural circuit dynamics and corticospinal recruitment patterns of synergistic muscles and their neuroplastic modulation.

There have been few investigations on hemodynamic responses in the human cortex resulting from dental stimulation. Identification of cortical areas involved in stimulus perception may offer new targets for pain treatment. This initial study aimed at establishing a cortical map of dental representation, based on non-invasive fMRI measurements. Five right-handed subjects were studied. Eight maxillary and 8 mandibular teeth were stimulated after the vibratory perception threshold was determined for each tooth. Suprathreshold stimulation was repeated thrice per session, in a total of three sessions performed on three consecutive days. Statistical inference on cluster level identified increased blood-oxygen-level-dependent signal during vibratory dental stimulation, primarily in the insular cortex bilaterally and in the supplementary motor cortex. No significant brain activation was observed in the somatosensory cortex with this stimulation protocol. These results agree with previous findings obtained from invasive direct electrical cortical stimulation of the human insula.

We combined behavioral assessment of texture discrimination and electrophysiological mapping of concomitant reorganization in the forepaw representation within the SI cortex. Rats were housed in enriched (EE) or impoverished (IE) environments which have been shown to remodel the forepaw map and possibly alter discriminative abilities. In addition, animals were trained to discriminate homogeneous floorboards of invariant roughness from heterogeneous floorboards of gradually decreasing roughness contrasts during locomotion. As reported recently, differences in perceptual abilities were not related to housing conditions, but to a predilection for a floorboard type [Bourgeon S, Xerri C, Coq JO. Abilities in tactile discrimination of textures in adult rats exposed to enriched or impoverished environments. Behav Brain Res 2004;153:217-231]. Consistently, the present study shows that cortical map remodeling resulting from short-duration daily experience can prevail over changes induced by housing conditions. The relative area of glabrous skin representation was related to the discrimination performance and learning abilities in the rats (H) with a predilection for heterogeneous floorboards, i.e. in the animals performing discrimination in the most challenging perceptual context. By contrast, this cortical area was influenced by the duration of sensory experience in rats (h) with a predilection for homogeneous floorboards. Both EE condition and training to discrimination selectively decreased the sizes of the SI neurons' receptive fields (RFs) located on glabrous skin. Smaller RFs and larger cortical areas serving glabrous skin were correlated with better perceptual performances and learning abilities in the H rats only. The present study shows that representational reorganization related to tactile discrimination performances depends upon the perceptual context.

Whisker movement is somatotopically represented in rodent neocortex by electrical activity in clearly defined barrels, which can be visualized in living brain slices. The functional architecture of this part of the cortex can thus be mapped in vitro with respect to its physiological input and compared with its anatomical architecture. The spatial extent of excitation was measured at high temporal resolution by imaging optical signals from voltage-sensitive dye evoked by stimulation of individual barrels in layer 4. The optical signals correlated closely with subthreshold EPSPs recorded simultaneously from excitatory neurons in layer 4 and layer 2/3, respectively. Excitation was initially (3 msec) spread in a columnar manner into layer 2/3 and then subsided in both layers after approximately 50 msec. The lateral extent of the response was limited to the cortical column defined structurally by the barrel in layer 4. Two experimental interventions increased the spread of excitation. First, blocking GABA(A) receptor-mediated synaptic inhibition caused excitation to spread laterally throughout wide regions of layer 2/3 and layer 5 but not into neighboring barrels, suggesting that the local excitatory connections within layer 4 are restricted to single barrels and that inhibitory neurons control spread in supragranular and infragranular layers. Second, NMDA receptor-dependent increase of the spread of excitation was induced by pairing repetitive stimulation of a barrel column with coincident stimulation of layer 2/3 in a neighboring column. Such plasticity in the spatial extent of excitation in a barrel column could underlie changes in cortical map structure induced by alterations of sensory experience.

Animal models of hearing loss and tinnitus observe pathological neural activity in the tonotopic frequency maps of the primary auditory cortex. Here, we applied ultra high-field fMRI at 7 T to test whether human patients with unilateral hearing loss and tinnitus also show altered functional activity in the primary auditory cortex. The high spatial resolution afforded by 7 T imaging allowed tonotopic mapping of primary auditory cortex on an individual subject basis. Eleven patients with unilateral hearing loss and tinnitus were compared to normal-hearing controls. Patients showed an over-representation and hyperactivity in a region of the cortical map corresponding to low frequencies sounds, irrespective of the hearing loss and tinnitus range, which in most cases affected higher frequencies. This finding of hyperactivity in low frequency map regions, irrespective of hearing loss range, is consistent with some previous studies in animal models and corroborates a previous study of human tinnitus. Thus these findings contribute to accumulating evidence that gross cortical tonotopic map reorganization is not a causal factor of tinnitus.

In mammalian auditory cortex, sound source position is represented by a population of broadly tuned neurons whose firing is modulated by sounds located at all positions surrounding the animal. Peaks of their tuning curves are concentrated at lateral position, while their slopes are steepest at the interaural midline, allowing for the maximum localization accuracy in that area. These experimental observations contradict initial assumptions that the auditory space is represented as a topographic cortical map. It has been suggested that a “panoramic” code has evolved to match specific demands of the sound localization task. This work provides evidence suggesting that properties of spatial auditory neurons identified experimentally follow from a general design principle- learning a sparse, efficient representation of natural stimuli. Natural binaural sounds were recorded and served as input to a hierarchical sparse-coding model. In the first layer, left and right ear sounds were separately encoded by a population of complex-valued basis functions which separated phase and amplitude. Both parameters are known to carry information relevant for spatial hearing. Monaural input converged in the second layer, which learned a joint representation of amplitude and interaural phase difference. Spatial selectivity of each second-layer unit was measured by exposing the model to natural sound sources recorded at different positions. Obtained tuning curves match well tuning characteristics of neurons in the mammalian auditory cortex. This study connects neuronal coding of the auditory space with natural stimulus statistics and generates new experimental predictions. Moreover, results presented here suggest that cortical regions with seemingly different functions may implement the same computational strategy-efficient coding.

Animal research has shown that tonotopic representation in the auditory cortex is not statically fixed in the adult organism but can be altered after deafferentation. The present study examines the plasticity of the human auditory cortex in patients with high frequency cochlear hearing loss by means of magnetoencephalographic measurements. The data show that the cortical map can reorganize such that cortical neurons deprived of their usual most sensitive afferent input now respond to tone frequencies adjacent to the frequency range of the partial hearing loss. The results suggest that deafferentation due to cochlear damage in adults may lead to functional reorganization of auditory cortical structures.

The mechanical properties of the muscular actuators are important for dynamics (compensation of internal/external loads and disturbances) as well as for spatial cognition. Consider the so called /spl lambda/-model (Feldman and Levin, (1995)). The key point, from the point of view of space representation, is that that the controlled variable has the same dimensions of the control variable. The proprioceptive space can be defined as a lower-dimensional manifold in the set of all possible configurations and can be represented in a distributed way in terms of a self-organising cortical map. Its dimensionality and geometry is coded into the pattern of lateral connections that are excitatory and recurrent. These connections can be adapted by means of unsupervised Hebbian learning within a (probably innate) behavioural strategy of circular reaction. The same strategy and unsupervised learning paradigm allow the emergence of an internal distributed representation of the exteroceptive space and the nonlinear mapping between the two spaces, exploiting a side-effect of circular reaction.

Cortical afferents transferring information destined for the cerebellum terminate in the pontine nuclei (PN) in a divergent and patchy fashion. We investigated whether the form of dendritic fields of pontine projection neurons which are postsynaptic to the cortical afferents are related to this patchy pattern. To this end we used a triple combination of (1) retrograde labeling (injection of Fluorogold into the brachium pontis), (2) anterograde labeling [injection of Dil into cortical areas A17 and Sml(forelimb)], and (3) subsequent intracellular fills of identified projection neurons (Lucifer yellow) in slightly fixed slices of pontine brainstem. In 64 projection neurons whose somata were located within 160 microns of the border defined by cortical afferent fields, most of the dendritic trees were found to respect the border. Strikingly, proximal dendrites which were oriented toward the border often bent in order to avoid the boundary. This observation was supported by a quantitative analysis. It revealed that overlap areas of dendritic fields with the neighboring compartment were significantly smaller than those of hypothetical, radially organized dendritic fields of the same size, indicating that the dendritic fields are indeed confined to single compartments. In a second series of experiments, double injections of the anterograde tracers Dil and DiAsp into adjacent sites within one cortical area (A17 or Sml) were made in order to test if the topology of the cortical map is preserved within individual pontine compartments. This, however, does not seem to be the case, since the terminal fields displayed a complex pattern of overlap and nonoverlap rather than a consistent shift of terminal fields expected in the case of preserved topology. The results of the present study are consistent with the view that pontine modules independently process information from different parts of individual cortical areas. We suggest that this characteristic property of the corticopontine projection system might be the morphological basis of the well established fact that somatotopically continuous sensory maps in the cortex are transformed into maps at the level of the cerebellar cortex, showing a fractured somatotopy.