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The cortical representation of visual perception requires the integration of several-signal processing distributed across many cortical areas, but the neural substrates of such perception are largely unknown. The type of firing pattern exhibited by single neurons is an important indicator of dynamic circuitry within or across cortical areas. Neurons in area PEc are involved in the spatial mapping of the visual field; thus, we sought to analyze the firing pattern of activity of PEc optic flow neurons to shed some light on the cortical processing of visual signals. We quantified the firing activity of 152 optic flow neurons using a spline interpolation function, which allowed determining onset, end, and latency of each neuronal response. We found that many PEc neurons showed multiphasic activity, which is strictly related to the position of the eye and to the position of the focus of expansion (FOE) of the flow field. PEc neurons showed a multiphasic activity comprised of excitatory phases interspersed with inhibitory pauses. This phasic pattern seems to be a very efficient way to signal the spatial location of visual stimuli, given that the same neuron sends different firing patterns according to a specific combination of FOE/eye position.

The visual perception of self-motion is mainly provided by optic flow. Eyes usually scan the environment during locomotion, and the gaze is not always directed to the focus of expansion (FOE) of the flow field. Such eye movements change the retinal FOE position with respect to the fovea. Here, we assess if optic flow selective neurons in parietal area PEc are modulated by eye position. We recorded single neuron activity during radial optic flow stimulation in two monkeys, varying eye and retinal FOE positions. We found that the majority of PEc neurons are modulated by the FOE retinotopic position with different tuning for expansion and contraction. Although many neurons did not show any gaze field without visual stimulation, the eye position modulated optic flow responses in about half of the cells. These novel results suggest that PEc neurons integrate both visual and eye position signals, and allow us to hypothesize their role in guiding locomotion as a part of a cortical network involved in FOE representation during self-motion. Visual and eye position interaction in this area could be seen as a contribution to the building of the invariant space representation necessary to motor planning., (Copyright © 2010 IBRO. Published by Elsevier Ltd. All rights reserved.)

In macaque monkeys, corticocortical connections between distinct parietotemporal visual areas (areas MST-FST, DP, and 7a) and frontal periarcuate areas are studied using tritiated aminoacids and WGA-HRP. While labeling within the banks of the principal sulcus, the dorsal part of the arcuate concavity, and the banks of the upper arcuate limb were present in both 7a and MST-FST injected animals; in the latter cases, additional projections were found towards frontal regions including the dorsomedial frontal cortex and the posterior bank of the arcuate ventral limb. Our results point to widespread frontal connections of the MST-FST complex, involving both prefrontal and premotor cortical regions.

The spike activity of neurons was recorded from the dorsal bank of the superior temporal sulcus (area MSTd) of alert behaving macaque monkeys performing visual fixation or target tracking tasks, with the aim of studying the tuning features of these neurons with both the direction of slow eye movement and the position of gaze. One hundred thirty-two neurons were tested for several fixation points and tracking directions. Many of them (43%) tuned to the direction of pursuit, regardless of the angle of gaze. Some (18%) showed a tonic discharge modulated by the static position of the eyes without pursuit direction specificity. A substantial number of cells (22%) were characterized by a discharge rate tuned to pursuit direction but influenced also by angle of gaze. Tuning curves for eye movement direction presented an average bandwidth of 130 degrees and turned out to be continuously overlapping, suggesting a sort of vector coding of smooth pursuit direction. Gaze fields of eye position (EP) neurons were mostly ramp-like, with center of ramps shifted away from the straight ahead, implying a form of scalar coding of gaze eccentricity. The different categories of cells were intermingled and close to each other, suggesting possible reciprocal interactions within the same cortical area. These results show that EP and pursuit direction are signaled mainly by separate neuronal elements in area MSTd. Moreover, some cells can integrate both signals. Taking into account the visual responses of MSTd neurons to large, textured, moving fields, it is suggested that this area could be the site of interaction between visuo-oculomotor signals related to visual motion detection, slow eye movement direction, and EP. This signal interaction may be important for integrative functions such as analysis of external or self-induced visual motion, cortical control of pursuit eye movements, and eye/head coordination.

The activity of parietal cortex neurones primarily related to eye position (EP neurones) was studied in macaque monkeys with the aim of precisely defining the neurones' gaze fields (GF) and comparing them in two functionally different areas, MSTd and 7a. Discharge rates of single neurones in the inferior parietal lobule and in the underlying cortex of the superior temporal sulcus were recorded in two Java monkeys while the animals fixated a steady visual target positioned at several different points on a video screen. The GFs were then drawn as a regression surface fitting the mean discharge rates. Cells tonically influenced by the angle of gaze were found in both areas. The GFs most often took the form of a nearly planar surface best characterized as a ramp tilted towards a hemifield or quadrant of the visual field, shifted eccentrically with respect to the straight ahead (primary position), and with a midpoint centred between 0 deg and 20 deg of gaze eccentricity and saturation between 10 deg and 35 deg. In a minority of cases, the discharge rate was nearly maximal at the primary position and decreased to a minimum within 35 deg of eccentricity. In other instances, the GFs were peaked surfaces, limited to a restricted part of visual space. EP neurones, while showing similar gaze fields in areas MST and 7a, were found intermingled with functionally different types of cells. The results suggest that EP neurones similar to those already described in several areas of the monkey parietal cortex are present also in area MST. These cells, by signalling the degree of gaze eccentricity from the primary position, encode gaze position in an orbito-centered frame extending up to 30-35 deg from the straight-ahead. The role of EP neurones might be to supply contiguous elements with a gaze eccentricity signal required for visuo-motor processes such as the control of tracking movements.

The influences of the visual background on the spontaneous and evoked activity of neurons in the striate cortex (V1) of the awake and behaving macaque were investigated using uniform (dark and bright) and textured (dynamic random-dot) large fields (10 degrees) centered on the receptive field of the cell under study. Rhesus monkeys were trained to fixate a small target while visual patterns were presented on monitor displays and the impulse activity of single cortical neurons recorded extracellularly with metal microelectrodes. The discharge rates of the ongoing, spontaneous activity of the vast majority of V1 neurons, as well as their responses to optimally adjusted bar stimuli, were not significantly influenced by the luminance of a uniform background. On the other hand, the activity of more than 50% of V1 neurons was clearly affected by a textured background. Comparison of the effects of a uniformly dark background and a background of dynamic random dots showed that the neuron's spontaneous discharge rate was typically higher in the presence of the textured background, while the evoked response was often reduced in amplitude or even suppressed. The opposite effects were observed in only a few neurons. These findings indicate that neurons in area V1 are highly sensitive to a textured background of dynamic random dots which exert on them an activating effect, chiefly by stimulation of the neuron's receptive field, with consequent increase in the ongoing discharge and a reduction of the dynamic range of impulse activity, leading to a reduction in the amplitude of the response evoked by a contrast stimulus.

Cortical projections from the visual region and adjacent polysensory region of the superior temporal sulcus (STs) to the lateral geniculate body (LGb) were investigated in the macaque monkey using an autoradiographic tracing method. Solutions of tritiated aminoacids were injected into different parts of the caudal half of the STs of five animals. A survival time of 7 days was allowed. Labels were found in both subdivisions of the LGb: the dorsal lateral geniculate nucleus (DLGn) and the pregeniculate nucleus (PGn). In particular, part of the visual cortical region adjacent to the middle temporal area (MT) projects into the DLGn as well as the PGn, whereas the MT itself and the superior temporal polysensory region project into the PGn only. Afferents to the DLGn terminate in the magnocellular layers and in their adjoining interlaminar zones, completely sparing the parvocellular layers. Afferents to the PGn terminate in separate regions of this nucleus; the MT and adjacent visual cortices project into the internal layer of the PGn, whereas the polysensosy region of the STs projects into the external retinorecipient layer of the PGn. Possible functional implications of these projections are discussed.

Projections from different visual areas of the cerebral cortex to the ipsilateral claustrum have been investigated in the cat by intracortical injections of [3H]proline. As a consequence of axonal flow an accumulation of radioactive material within a dorso-caudal region of the ipsilateral claustrum was found. The proline-transporting terminal arborizations of projections from areas 17, 18, 19 and the Clare-Bishop area overlap in the same claustral region. No point-to-point relation was observed between any one visual area and its claustral projection. Differences in the terminal distribution pattern were detectable according to the visual areas of origin.

Extracellular recordings were carried out in the visual cortex of behaving monkeys trained on a fixation/detection task, during which a target light was displayed stationary or suddenly moving on a tangent translucent screen. The responses of visual cortical cells to fast moving stimuli during steady fixation and those obtained during rapid eye movements (saccades) which moved their receptive field across a stationary stimulus, were studied. Areas V1 and V2 were explored. When tested with rapidly moving stimuli (500 deg/sec) during steady fixation, neurons in each area behaved in almost the same way. About one fourth of them were activated, the remainder showing either no response (little more than a half of them) or a reduction of the spontaneous firing rate. In both areas, some of the neurons activated during steady fixation did not respond or responded very weakly during eye motion at saccadic velocity (500 +/- 50 deg/sec). Neurons of this type, which we refer to as 'real motion' cells, could somehow contribute to the maintenance of visual stability during the execution of large eye movements.

1. The projections from cat cortical visuals areas to the contralateral superior colliculus (SC) were studied by the autoradiographical tracing method. Microinjections of L- [5-3H] proline were carried out in cortical visual areas 17, 18, 19 and the lateral suprasylvian visual area (LS) in different cats. Only one cortical area was injected in each animal. Survival times of 30 hours or 8 days were allowed. 2. Areas 17, 18, 19 and LS send projections to laminae, I, II and III of the frontal pole of the contralateral SC. Areas 19 and LS project also the contralateral pretectal nuclei, mainly to the posterior pretectal nucleus. 3. Cortical fibres reaching the contralateral SC pass through the brachium of the ipsilateral SC. They run along the caudal part of this structure, cross the midline and then run along the caudal part of the contralateral SC; finally, they turn anterolaterally and reach the rostal part of SC. This pathway is the tectal semidecussation (5). Cortical fibres from LS (in particular from PLLS; 15) reach the contralateral SC also via the commissure of SC. 4. Our data support the suggestion (3) that the frontal poles of SC, in the cat, may subserve straight-ahead attention and orientation to visual stimuli.

In II adult cats, areas 17, 18, 19 as well as the lateral suprasylvian area were separately injected with L-[5-3H] proline and their efferent projections to the frontal cortex were autoradiographically searched. Only area 19 and lateral suprasylvian area showed such projections; terminal sites were localized in the ventral and dorsal banks of the cruciate sulcus and in the adjacent mesial surface of the brain. The possibility that these labeled regions may correspond to the monkey's frontal eye field is discussed.

Solutions of tritiated aminoacids were injected into the visual cortical region hidden in the depth of the superior temporal sulcus (STs) of the macaque monkey. Injection sites involved the middle temporal visual area, extending into the surrounding visual cortices. Projections were found homolaterally in both the striatum and claustrum. In the caudate nucleus labeled material affected mainly the body and spread both to the tail and the head of the nucleus. Label was also seen in the caudal third of the putamen and in the postero-ventral claustrum. Compared with the scarcity of afferents arising from occipital visual areas, the present data point to a heavy projection system from additional visual areas of the STs to the basal ganglia in the macaque monkey.

Associated and commisural connections of visual areas 17 and 18 were studied in the light of the latest knowledge about the anatomo-functional organization of the cat's visual cortices. Injections of L-[5-3H] proline were placed, in different animals, in area 17 or 18 of the right hemisphere. Serial histological sections of the whole brain were processed by autoradiographic technique, after long (8-16 days) or short (30 hours) survival times. Cortical areas labelled by axonally transported radioactive material were then correlated with electrophysiological and cytoarchitectonical maps. Areas 17 and 18 were found to send associational projections to area 19, to the anterior and posterior subdivisions of the lateral suprasylvian visual area and to two regions lying on the crown of the suprasylvian gyrus (areas 21A and 20). In addition, area 18 sends projections to ipsilateral areas 17, 7 and 5. Contralaterally, projections from areas 17 and 18 reach the homonymous areas near the margin of the other. Homotopical callosal connections of area 17 affect the band of the contralateral 17 in which part of the ipsilateral visual hemi-field is represented. Heterotopical commisural projections from the areas studied affect areas 19 and LS in regions adjoining the central vertical meridian representations. These results suggest that, on an anatomical basis, the interactions between the cortical targets of the classical geniculo-striate and the estrageniculo-striate systems are more complicated than previously believed. Furthermore, the geniculo-striate system supplies some visual input to non-visual areas through area 18.

Single unitresponses to photic stimuli of variable duration were studied in the cat visual areas 17 and 18. The closed-chamber technique was used to record extracellularly impulsive electrical activity in locally anaesthetized, Flaxedil-paralyzed and artificially ventilated cats. Stimulus duration ranges between 0.020 and 10 sec, the background luminance and the intensity of stimuli being constant. We found a first group of cells which fire independently of the stimulus duration; a second group which fire in close relation to the stimulus duration in a rather wide range of values, and a third group which fire monotonously as long as the stimulus goes on. The three groups of cells are supposed to be three functionally different types of cortical cells.

1. The projections from the superior temporal sulcus (STS) visual area to pontine nuclei were studied in the macaque monkey by the autoradiographic tracing method. Microinjections of a mixture of L-[5-3H] proline and L-[4,5-3H] leucine were carried out in the posterior band and the floor of the caudal half of the STS. Survival time was always 7 days. 2. The STS visual area projects to the dorsolateral part of ipsilateral pontine nuclei. Terminal projections are distributed in patches in the whole rostro-caudal extent of the pons. 3. These findings support the view that the STS visual area in the macaque monkey is homologous to the postero-medial lateral suprasylvian area in the cat.

Direct projections from visual areas 17, 18, 19, and lateral suprasylvian visual area (LS) to the striatum were searched for in 12 adult cats using the autoradiographic technique to detect neuronal pathways. Striatal labels were found only after injections in areas 19 and LS. Projections homolateral to the injection sites were observed from both areas to the head and body of the caudate nucleus and to the putamen. Contralateral projections were found from both areas 19 and LS: however, area 19 did not project to the contralateral putamen. The extent of contralateral projections was smaller and they were confined within the same regions as the homolateral ones. Silver grains were often arranged in cluster-like patches, which were more evident ipsilaterally, in the head of the caudate nucleus and after injections in area LS. The present data support the view of a not strictly topographical segregation of striatal projections from the cat visual cortex.

Extracellular recordings were carried out in the primary visual cortex of behaving monkeys. Neurons were activated by moving a visual stimulus on their receptive fields during periods of steady fixation and by moving their receptive fields (smooth pursuit eye movements) on a motionless visual stimulus. Regarding non-oriented cells, they turned out to be activated by the visual stimulation both during steady fixations and smooth pursuit eye movements. Therefore, the non-oriented cells we studied seem not to receive an extraretinal signal related to the slow eye movements.

A simple system for simultaneous recording of eye position and spike activity during on-line experimentation is described. The system is based on a relatively inexpensive and widely distributed personal computer. While the hardware is configured from commercially available products, the software has been developed in our laboratory. Data acquisition is controlled by a few routines whose logic is outline, together with the main limitations and the possible improvements of the system itself.

Extracellular recordings were carried out in the primary visual cortex of behaving macaque monkeys. Neurons were activated by moving a visual stimulus across their receptive fields during periods of steady fixation and by moving their receptive fields (by visual tracking) across a motionless visual stimulus, taking care that the velocities of stimulus and eye movements were the same. The total cell population (108 neurons) ws divided into 3 groups according to the cell sensitivity to visual stimulus orientation (non-oriented cell and oriented cells) and to the presence or absence of antagonistic areas in in the receptive fields (oriented cells with antagonistic areas). All the non-oriented cells (n = 14) showed almost the same response to visual stimulation both during steady fixation and during visual tracking. Out of a total number of 86 oriented cells, 77 turned out to be activated by the visual stimulation both during fixation and tracking. Eight oriented cells gave a very weak response or no response at all to visual stimulation during smooth pursuit eye movements and one neuron of the same group showed a greater response during visual tracking than during fixation. Six out of 8 oriented cells with antagonistic areas showed almost the same response to the two types of visual stimulation, while the remaining two neurons showed very weak responses during smooth pursuit eye movements. Our results show that a small percentage (about 10%) of striate neurons in macaque monkeys gave very different responses to the same physical stimulation at retinal level, according to the presence or absence of slow eye movements (smooth pursuit eye movements). The activity of these neurons seems to be related to the real movement of something in the visual world, in spite of the retinal image movement per se.

Associational and commissural connections of area 19 and of the lateral suprasylvian visual area (LS) of the cat were studied by means of an axonal anterograde transport technique. Multiple injections of L-[5-3H] proline were placed, in different animals, in area 19 and in two different subdivisions of LS (PMLS, PLLS). Serial histological sections of the whole brains were processed by autoradiographic technique after long (8-12 days) and short (30 hrs) survival times. Cortical areas labelled by accumulation of radioactive material were correlated with electrophysiological and cytoarchitectonic maps of cortical areas. Results showed a number of projections of areas 19 and LS not previously described. Area 19 sends ipsilateral projections to area 5 and to the medial bank of the anterior suprasylvian sulcus. The postero-medial (PMLS) and postero-lateral (PLLS) subdivisions of LS project to ipsilateral areas 5, 6, 7, 20, 21, 35, to the suprasylvian fringe (SF), to the cingulate gyrus and to the insular cortex. Contralaterally, new projections are found from area 19 to areas 20 and 5, and from LS to areas 19, 20, 21, 5, 7, SF to the cingulate gyrus and to the insular cortex. Results lead to conclude that the intra-modal and cross-modal interactions between cortical areas processing visual input are more complicated than previously assumed. The crucial role of the cat's LS in the cortico-cortical distribution of the visual message in emphasized.

Contralateral projections from visual areas 17, 18, 19 and the Clare-Bishop area of the cerebral cortex to the claustrum have been investigated in the cat using intracortical injections of [3H]proline. Radioactive material was found in a dorsocaudal region of the contralateral claustrum. This region was homotopic with respect to that found for the ipsilateral projection from visual cortex. The contralateral connection is assumed to be a monosynaptic pathway. The pattern by which the corticofugal fibres terminate in the claustrum is quite similar to the one described for the opposite hemisphere [6].

Stimulus-response curves of simple cells of the visual cortex were obtained by using 500-msec stationary stimuli. Background influence on single unit responses was studied. The contrast sensitivity of simple cells increases as a function of background luminance. The resolution power of these cortical cells for detecting differences in stimulus contrast decreases at background levels above 0.09 cd/m2.

Tritiated amino acids were injected into the striate area and in single visual areas of the superior temporal sulcus (STS) of 7 cynomolgus monkeys, in order to trace visual cortical projections to the nuclei of the accessory optic system (AOS). Injections in STS separately involved the areas MT and MST, and resulted in labels within the lateral terminal nucleus of the AOS. In no case were labels found within the AOS nuclei in the brains injected in the striate area, or within the contralateral AOS. It seems likely that the areas MT and MST contribute signals--selectively related to visual motion processing--to the AOS, which is probably involved in the neuronal pathway subserving the optokinetic reflex.

The anatomical pathways supplying the visual signal to the cat orbito-insular cortex (OIC) from primary visual areas were studied by an anterograde axonal transport technique. L-[5-3H]proline was injected, in different animals, in each of areas 17, 18, 19 and the lateral suprasylvian visual area (LS). Serial histological sections were processed by autoradiographic technique after long (8-16 days) or short (30 h) survival times. The axonal flow labelled direct pathways from LS to the ipsilateral orbital gyrus and the ventral bank of the anterior ectosylvian sulcus; this region seems to correspond to that from which many authors recorded photically evoked potentials. Long survival animals injected in LS showed labels also in the contralateral OIC. No axonal flow could be demonstrated from areas 17, 18 and 19 to OIC, either at short of long survival times. The results suggest that, apart from possible sub cortical afferences, a critical visual input may reach OIC from the extrageniculostriate visual system through LS. The functional relevance of extrastriate input to OIC is discussed.

A substantial proportion of both simple and complex neurons in the cortex subserving central vision are differentially sensitive to binocular disparity of isolated line patterns (local stereopsis), a sensitivity based on a positional disparity between the neuron's receptive fields in the two eyes. In addition, a subset of cortical neurons, nearly all complex neurons, responds to dynamic random-dot stereograms containing no depth cues other than disparity. These neurons are capable of signaling the correct binocular matches among a multitude of false matches in the stereograms (global stereopsis). The discovery of cyclopean neurons in striate cortex, at early stages of the processing neural network for stereoscopic vision provides a new insight of the basic neural mechanisms underlying binocular depth perception.