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Most neurons in the primary visual cortex (V1) of mammals show sharp orientation selectivity and band-pass spatial frequency tuning. Here, we examine whether sharpening of the broad tuning that exists subcortically, namely in the retina and the lateral geniculate nucleus (LGN), underlie the sharper tuning seen for both the above features in tree shrew V1. Since the transition from poor feature selectivity to sharp tuning occurs entirely within V1 in tree shrews, we examined the orientation selectivity and spatial frequency tuning of neurons within individual electrode penetrations. We found that most layer 4 and layer 2/3 neurons in the same cortical column preferred the same stimulus orientation. However, a subset of layer 3c neurons close to the layer 4 border preferred near orthogonal orientations, suggesting that layer 2/3 neurons may inherit the orientation preferences of their layer 4 input neurons and also receive cross-orientation inhibition from layer 3c neurons. We also found that layer 4 neurons showed sharper orientation selectivity at higher spatial frequencies, suggesting that attenuation of low spatial frequency responses by spatially broad inhibition acting on layer 4 inputs to layer 2/3 neurons can enhance both orientation and spatial frequency selectivities. However, in a proportion of layer 2/3 neurons, the sharper tuning of layer 2/3 neurons appeared to arise also or even mainly from inhibition specific to high spatial frequencies acting on the layer 4 inputs to layer 2/3. Overall, our results are consistent with the suggestion that in tree shrews, sharp feature selectivity in layer 2/3 can be established by intracortical mechanisms that sharpen biases observed in layer 4, which are in turn inherited presumably from thalamic afferents. [ABSTRACT FROM AUTHOR]

A group of 27 healthy young adults solved a task involving the working memory (WM) activation, consisting of the comparison between the spatial orientations of two sequentially presented square-wave luminance gratings. We investigated the effective (directed) connectivity patterns between the frontal and postcentral cortical regions related to the visual system. The connectivity was assessed using vector autoregression modeling of EEG. It was shown that the strength of the top-down right-hemispheric connectivity patterns directed from the frontal cortex to the visual areas in θ frequency was significantly lower at the stage of stimulus retention in the WM than at the stage of stimulus anticipation. On the contrary, in the α band the descending influences were slightly more intense. The results of the study showed the frequency-dependent dynamics of the descending influences of the frontal cortex on visual areas and confirm that the frontal cortex plays the role of a controlling and modulating center in the brain system underlying WM. [ABSTRACT FROM AUTHOR]

Line orientation was evaluated under different observation conditions. In the first experiment, the distortion in the estimation of line orientation was 1°–9° in the presence of adjacent lines with different orientation (the tilt illusion). The illusion was stronger and the orientation discrimination thresholds were higher when the line length decreased up to 6 arcmin. For 6 arcmin lines, the illusion was approximated by a model where orientation was determined by the tilt of the minimal receptive fields (RFs) whose neurons had the maximal responses. The model was validated in the second experiment, when the minimal length for vertical and horizontal lines discrimination was determined. The minimal length was consistent with the width of the model RFs areas. The RF interactions affecting the illusion were evaluated in the third experiment where the orientation discrimination of lines with a length of 60 arcmin was studied. Age-related orientation dependencies allow a suggestion that the interactions between RFs reduce the tilt illusions. [ABSTRACT FROM AUTHOR]

Bisphenol A (BPA), a common environmental xenoestrogen, has been implicated in physiological and behavioral impairment, but the neuronal basis remains elusive. Although various synaptic mechanisms have been shown to mediate BPA-induced brain deficits, there are almost no reports addressing its underlying physiological mechanisms at the individual neuron level, particularly in the primary visual system. In the present study, using multiple-channel recording technique, we recorded the responses of single neurons in the primary visual system of cats to various direction stimuli both before and after BPA exposure. The results showed that the orientation selectivity of neurons in the primary visual cortex (area 17, A17) was obviously decreased after 2 h of intravenous BPA administration (0.2 mg/kg). Moreover, there were worse performances of information transmission of A17 neurons, presenting markedly decreased signal-to-noise ratio (SNR). To some extent, these functional decreases were attributable to the altered information inputs from lateral geniculate nucleus (LGN), which showed an increased spontaneous activity. Additionally, local injection of BPA (3.3 μg/ml) in A17 resulted in an obvious increase in orientation selectivity and a decrease in neuronal activity, involving enhanced activity of fast-spiking inhibitory interneurons. In conclusion, our results first demonstrate that acute BPA exposure can restrict the visual perception of cats, mainly depending on the alteration of the LGN projection, not the intercortical interaction. Importantly, BPA-induced-brain deficits might not only be confined to the cortical level but also occur as early as at the subcortical level. [ABSTRACT FROM AUTHOR]

Sparsity-driven image recovery methods assume that images of interest can be sparsely approximated under some suitable system. As discontinuities of 2D images often show geometrical regularities along image edges with different orientations, an effective sparsifying system should have high orientation selectivity. There have been enduring efforts on constructing discrete frames and tight frames for improving the orientation selectivity of tensor product real-valued wavelet bases/frames. In this paper, we studied the general theory of discrete Gabor frames for finite signals, and constructed a class of discrete 2D Gabor frames with optimal orientation selectivity for sparse image approximation. Besides high orientation selectivity, the proposed multi-scale discrete 2D Gabor frames also allow us to simultaneously exploit sparsity prior of cartoon image regions in spatial domain and the sparsity prior of textural image regions in local frequency domain. Using a composite sparse image model, we showed the advantages of the proposed discrete Gabor frames over the existing wavelet frames in several image recovery experiments. [ABSTRACT FROM AUTHOR]

We continued our study of the mechanisms of visual acuity (VA) in ontogenesis. We measured the VA and sensitivity to lines orientation and determined the minimal length of lines for discrimination of the vertical and the horizontal ones in subjects aged 7-25 years. It was found that the thresholds of discrimination of vertical and horizontal lines in subjects with normal VA decreased with age up to 9-10 years and then remained constant, while the orientation selectivity was improving up to the age of 14-16 years. The average VA almost did not depend on age. Individual thresholds of line lengths and orientation discrimination correlated with the VA of subjects. [ABSTRACT FROM AUTHOR]

In this study, we investigated orientation selectivity in cat primary visual cortex (V1) and its relationship with various parameters. We found a strong correlation between circular variance (CV) and orthogonal-topreferred response ratio (O/P ratio), and a moderate correlation between tuning width and O/P ratio. Moreover, the suppression far from the peak that accounted for the lower CV in cat V1 cells also contributed to the narrowing of the tuning width of cells. We also studied the dependence of orientation selectivity on the modulation ratio for each cell, which is consistent with robust entrainment of the neuronal response to the phase of the drifting grating stimulus. In conclusion, the CV (global measure) and tuning width (local measure) are signifi cantly correlated with the modulation ratio. [ABSTRACT FROM AUTHOR]

Orientation maps are a prominent feature of the primary visual cortex of higher mammals. In macaques and cats, for example, preferred orientations of neurons are organized in a specific pattern, where cells with similar selectivity are clustered in iso-orientation domains. However, the map is not always continuous, and there are pinwheel-like singularities around which all orientations are arranged in an orderly fashion. Although subject of intense investigation for half a century now, it is still not entirely clear how these maps emerge and what function they might serve. Here, we suggest a new model of orientation selectivity that combines the geometry and statistics of clustered thalamocortical afferents to explain the emergence of orientation maps. We show that the model can generate spatial patterns of orientation selectivity closely resembling the maps found in cats or monkeys. Without any additional assumptions, we further show that the pattern of ocular dominance columns is inherently connected to the spatial pattern of orientation. [ABSTRACT FROM AUTHOR]

A layered continual population model of primary visual cortex has been constructed, which reproduces a set of experimental data, including postsynaptic responses of single neurons on extracellular electric stimulation and spatially distributed activity patterns in response to visual stimulation. In the model, synaptically interacting excitatory and inhibitory neuronal populations are described by a conductance-based refractory density approach. Populations of two-compartment excitatory and inhibitory neurons in cortical layers 2/3 and 4 are distributed in the 2-d cortical space and connected by AMPA, NMDA and GABA type synapses. The external connections are pinwheel-like, according to the orientation of a stimulus. Intracortical connections are isotropic local and patchy between neurons with similar orientations. The model proposes better temporal resolution and more detailed elaboration than conventional mean-field models. In comparison to large network simulations, it excludes a posteriori statistical data manipulation and provides better computational efficiency and minimal parametrization. [ABSTRACT FROM AUTHOR]

The implementation of Hubel-Wiesel hypothesis that orientation selectivity of a simple cell is based on ordered arrangement of its afferent cells has some difficulties. It requires the receptive fields (RFs) of those ganglion cells (GCs) and LGN cells to be similar in size and sub-structure and highly arranged in a perfect order. It also requires an adequate number of regularly distributed simple cells to match ubiquitous edges. However, the anatomical and electrophysiological evidence is not strong enough to support this geometry-based model. These strict regularities also make the model very uneconomical in both evolution and neural computation. We propose a new neural model based on an algebraic method to estimate orientations. This approach synthesizes the guesses made by multiple GCs or LGN cells and calculates local orientation information subject to a group of constraints. This algebraic model need not obey the constraints of Hubel-Wiesel hypothesis, and is easily implemented with a neural network. By using the idea of a satisfiability problem with constraints, we also prove that the precision and efficiency of this model are mathematically practicable. The proposed model makes clear several major questions which Hubel-Wiesel model does not account for. Image-rebuilding experiments are conducted to check whether this model misses any important boundary in the visual field because of the estimation strategy. This study is significant in terms of explaining the neural mechanism of orientation detection, and finding the circuit structure and computational route in neural networks. For engineering applications, our model can be used in orientation detection and as a simulation platform for cell-to-cell communications to develop bio-inspired eye chips. [ABSTRACT FROM AUTHOR]

One of the reasons the visual cortex has attracted the interest of computational neuroscience is that it has well-defined inputs. The lateral geniculate nucleus (LGN) of the thalamus is the source of visual signals to the primary visual cortex (V1). Most large-scale cortical network models approximate the spike trains of LGN neurons as simple Poisson point processes. However, many studies have shown that neurons in the early visual pathway are capable of spiking with high temporal precision and their discharges are not Poisson-like. To gain an understanding of how response variability in the LGN influences the behavior of V1, we study response properties of model V1 neurons that receive purely feedforward inputs from LGN cells modeled either as noisy leaky integrate-and-fire (NLIF) neurons or as inhomogeneous Poisson processes. We first demonstrate that the NLIF model is capable of reproducing many experimentally observed statistical properties of LGN neurons. Then we show that a V1 model in which the LGN input to a V1 neuron is modeled as a group of NLIF neurons produces higher orientation selectivity than the one with Poisson LGN input. The second result implies that statistical characteristics of LGN spike trains are important for V1's function. We conclude that physiologically motivated models of V1 need to include more realistic LGN spike trains that are less noisy than inhomogeneous Poisson processes. [ABSTRACT FROM AUTHOR]

Accumulating evidence suggests that cognitive declines in old (healthy) animals could arise from depression of intracortical inhibition, for which a decreased ability to produce GABA during senescence might be responsible. By simulating a neural network model of a primary visual cortical (V1) area, we investigated whether and how a lack of GABA affects cognitive performance of the network: detection of the orientation of a visual bar-stimulus. The network was composed of pyramidal (P) cells and GABAergic interneurons such as small (S) and large (L) basket cells. Intrasynaptic GABA-release from presynaptic S or L cells contributed to reducing ongoing-spontaneous (background) neuronal activity in a different manner. Namely, the former exerted feedback (S-to-P) inhibition and reduced the frequency (firing rate) of action potentials evoked in P cells. The latter reduced the number of saliently firing P cells through lateral (L-to-P) inhibition. Non-vesicular GABA-release, presumably from glia and/or neurons, into the extracellular space reduced the both, activating extrasynaptic GABAa receptors and providing P cells with tonic inhibitory currents. By this combinatorial, spatiotemporal inhibitory mechanism, the background activity as noise was significantly reduced, compared to the stimulus-evoked activity as signal, thereby improving signal-to-noise (S/N) ratio. Interestingly, GABA-spillover from the intrasynaptic cleft into the extracellular space was effective for improving orientation selectivity (orientation bias), especially when distractors interfered with detecting the bar-stimulus. These simulation results may provide some insight into how the depression of intracortical inhibition due to a reduction in GABA content in the brain leads to age-related cognitive decline. [ABSTRACT FROM AUTHOR]

The estimating problem of 3D holograms orientation selectivity on angular, orthogonal, and azimuthal sensitivity parameters is formulated and solved. Tenfold increase of density 3D, 2D/3D holograms in comparison with 2D holograms at given selectivity for ones is shown in theory and experimentally. [ABSTRACT FROM AUTHOR]

Simulations of orientation selectivity in visual cortex have shown that layer 4 complex cells lacking orientation tuning are ideal for providing global inhibition that scales with contrast in order to produce simple cells with contrast-invariant orientation tuning (Lauritzen and Miller in J Neurosci 23:10201-10213, ). Inhibitory cortical cells have been shown to be electrically coupled by gap junctions (Fukuda and Kosaka in J Neurosci 120:5-20, ). Such coupling promotes, among other effects, spike synchronization and coordination of postsynaptic IPSPs (Beierlein et al. in Nat Neurosci 3:904-910, ; Galarreta and Hestrin in Nat Rev Neurosci 2:425-433, ). Consequently, it was expected (Miller in Cereb Cortex 13:73-82, ) that electrical coupling would promote nonspecific functional responses consistent with the complex inhibitory cells seen in layer 4 which provide broad inhibition in response to stimuli of all orientations (Miller et al. in Curr Opin Neurobiol 11:488-497, ). This was tested using a mechanistic modeling approach. The orientation selectivity model of Lauritzen and Miller (J Neurosci 23:10201-10213, ) was reproduced with and without electrical coupling between complex inhibitory neurons. Although extensive coupling promotes uniform firing in complex cells, there were no detectable improvements in contrast-invariant orientation selectivity unless there were coincident changes in complex cell firing rates to offset the untuned excitatory component that grows with contrast. Thus, changes in firing rates alone (with or without coupling) could improve contrast-invariant orientation tuning of simple cells but not synchronization of complex inhibitory neurons alone. [ABSTRACT FROM AUTHOR]

We have studied the contribution of feedback signals originating from one of the “form-processing” extrastriate cortical areas, area 21a (A21a), to orientation selectivity of single neurons in the ipsilateral area 17 (A17). Consistent with previous findings, reversible inactivation (cooling to 5–10°C) of area 21a resulted in a substantial reduction in the magnitude of the maximum response ( R max) of A17 cells accompanied by some changes in the half-width at half-height of the R max (HWHH). By fitting model functions to the neurons’ response profiles we found that in the vast majority of orientation-tuned A17 cells tested (30/39, 77%), inactivation of A21a resulted in a “flattening” of their orientation-tuning curves. It is characterised by a substantial reduction in the R max associated with either a broadening of the orientation-tuning curves (17 cells) or a relatively small reduction (12 cells) or no change (1 cell) in the HWHH. When the “flattening” effect was quantified using a simple ratio index or R/ W, defined as R max/HWHH, we found that R/ W was significantly reduced during inactivation of A21a. The change in R/ W is strongly correlated with the change in the maximum slope of the orientation-tuning curves. Furthermore, analysis of response variability indicates that “signal-to-noise” ratio of the responses of A17 neurons decreases during inactivation of A21a. Our results suggest that the predominately excitatory feedback signals originating from A21a play a role in enhancing orientation selectivity of A17 neurons and hence are likely to improve overall orientation discriminability. [ABSTRACT FROM AUTHOR]

We present a reduction of a large-scale network model of visual cortex developed by McLaughlin, Shapley, Shelley, and Wielaard. The reduction is from many integrate-and-fire neurons to a spatially coarse-grained system for firing rates of neuronal subpopulations. It accounts explicitly for spatially varying architecture, ordered cortical maps (such as orientation preference) that vary regularly across the cortical layer, and disordered cortical maps (such as spatial phase preference or stochastic input conductances) that may vary widely from cortical neuron to cortical neuron. The result of the reduction is a set of nonlinear spatiotemporal integral equations for “phase-averaged” firing rates of neuronal subpopulations across the model cortex, derived asymptotically from the full model without the addition of any extra phenomological constants. This reduced system is used to study the response of the model to drifting grating stimuli—where it is shown to be useful for numerical investigations that reproduce, at far less computational cost, the salient features of the point-neuron network and for analytical investigations that unveil cortical mechanisms behind the responses observed in the simulations of the large-scale computational model. For example, the reduced equations clearly show (1) phase averaging as the source of the time-invariance of cortico-cortical conductances, (2) the mechanisms in the model for higher firing rates and better orientation selectivity of simple cells which are near pinwheel centers, (3) the effects of the length-scales of cortico-cortical coupling, and (4) the role of noise in improving the contrast invariance of orientation selectivity. [ABSTRACT FROM AUTHOR]

In order to probe into the self-organizing emergence of simple cell orientation selectivity, we tried to construct a neural network model that consists of LGN neurons and simple cells in visual cortex and obeys the Hebbian learning rule. We investigated the neural coding and representation of simple cells to a natural image by means of this model. The results show that the structures of their receptive fields are determined by the preferred orientation selectivity of simple cells. However, they are also decided by the emergence of self-organization in the unsupervision learning process. This kind of orientation selectivity results from dynamic self-organization based on the interactions between LGN and cortex. [ABSTRACT FROM AUTHOR]

Responses to illusory contours (ICs) were sampled from neurons in cortical areas 17 and 18 of the anesthetized cats. For ICs sensitive cells, the differences of receptive field properties were compared when ICs and real contour stimuli were applied. Two hundred orientation or direction selective cells were studied. We find that about 42 percent of these cells were the ICs sensitive cells. Although their orientation or direction tuning curves to ICs bar and real bars were similar, the response modes (especially latency and time course) were different. The cells’ responses to ICs were independent of the spatial phases of sinusoidal gratings, which composed the ICs. The cells’ optimal spatial frequency to composing gratings the ICs was much higher than the one to moving gratings. Therefore, these cells really responded to the ICs rather than the line ends of composing gratings. For some kinds of velocity-tuning cells, the optimal velocity to moving ICs bar was much lower than the optimal velocity to moving bars. The present results demonstrate that some cells in areas 17 and 18 of cats have the ability to respond to ICs and have different response properties of the receptive fields to ICs and luminance boundaries via different neural mechanisms. [ABSTRACT FROM AUTHOR]

The effects of stimuli falling outside the ’classical receptive field’ and their influence on the orientation selectivity of cells in the cat primary visual cortex are still matters of debate. Here we examine the variety of effects of such peripheral stimuli on responses to stimuli limited to the receptive field. We first determined the extent of the classical receptive field by increasing the diameter of a circular patch of drifting grating until the response saturated or reached a maximum, and by decreasing the diameter of a circular mask in the middle of an extended grating, centred on the receptive field, until the cell just began to respond. These two estimates always agreed closely. We then presented an optimum grating of medium-to-high contrast filling the classical receptive field while stimulating the surround with a drifting grating that had the same parameters as the central stimulus but was varied in orientation. For all but five neurons (of 37 tested), surround stimulation produced clear suppression over some range of orientations, while none showed explicit facilitation under these conditions. For 11 cells (34% of those showing suppression), the magnitude of suppression did not vary consistently with the orientation of the surround stimulus. In the majority of cells, suppression was weakest for a surround grating oriented orthogonal to the cell’s optimum. Nine of these cells (28%) exhibited maximum inhibition at the optimum orientation for the receptive field itself, but for 12 cells (38%) there was apparent ’release’ from inhibition for surround gratings at or near the cell’s optimum orientation and direction, leaving inhibition either maximal at angles flanking the optimum (9 cells) or broadly distributed over the rest of the orientation range (3 cells). This implies the existence of a subliminal facilitatory mechanism, tightly tuned at or near the cell’s optimum orientation, extending outside the classical receptive field. For just two cells of 13 tested the preferred orientation for a central grating was clearly shifted towards the orientation of a surrounding grating tilted away from the cell’s optimum. The contrast gain for central stimulation at the optimal orientation was measured with and without a surround pattern. For nine of 25 cells tested, surround stimulation at the cell’s optimum orientation facilitated the response to a central grating of low contrast (≤0.1) but inhibited that to a higher-contrast central stimulus: the contrast-response gain is reduced but the threshold contrast is actually decreased by surround stimulation. Hence the receptive field is effectively larger for low-contrast than for high-contrast stimuli. Inhibition from the periphery is usually greatest at or around the cell’s optimum, while suppression within the receptive field has been shown to be largely non-selective for orientation. Inhibition by orientations flanking the optimum could serve to sharpen orientation selectivity in the presence of contextual stimuli and to enhance orientational contrast; and it may play a part in orientation contrast illusions. [ABSTRACT FROM AUTHOR]

We have examined, using both qualitative and quantitative techniques, binocular interactions of extracellularly recorded single neurons in the extrastriate cortical area 21a of anaesthetized and paralysed cats. Consistent with previous reports we have found that: (a) all area 21a neurons were orientation-selective, with about 65% of them preferring orientations within 30° of the vertical; and (b) over 75% of area 21a cells could be activated through either eye. Furthermore, a significant minority (4 cells; about 10%) of a subpopulation of 39 neurons in which binocular interactions were examined quantitatively, were 'obligatory binocular neurons', that is, they responded very weakly, if at all, to the monocular stimuli presented through either eye but responded vigorously to simultaneous stimulation through both eyes. Almost 70% (27/39) of neurons tested quantitatively for binocular interaction have shown significant modulation (over 50%) of their peak responses in relation to binocular positional retinal disparities. The majority of neurons sensitive to binocular positional disparities resembled either 'tuned excitatory' (22 cells; 56.5% of the sample) or 'tuned inhibitory' (2 cells; 5% of our sample) cells. In particular, they gave, respectively, maximal or minimal responses to optimally oriented, moving photic stimuli when the receptive fields plotted through each eye completely or partially overlapped. Although neurons recorded in area 21a have relatively large receptive fields (mean width 3.3±1.1°; range 2.0-5.6°), the mean width of the disparity tuning curve (2.8±1.0°; range 1.3-4.8°) for our sample of area 21a neurons was similar to those of neurons with significantly smaller receptive fields, recorded in areas 17 and 18 of cat's primary visual cortex. We conclude that area 21a of the cat, like areas 17 and 18 of primary visual cortex, is likely to play an important role in binocular depth discrimination and might constitute a 'higher order' area for stereoscopic binocular vision. [ABSTRACT FROM AUTHOR]

The influence of GABA receptors on orientation selectivity of cat complex cells was tested by iontophoresis of the GABA receptor blockers bicuculline and N-methyl-bicuculline while stimulating with drifting sinusoidal gratings. Reduction of orientation tuning was markedly less than reported in previous studies that used drifting bars as visual stimuli. Only 3/31 cells lost orientation selectivity, with an average increase in bandwidth of 33%, as opposed to half the cells losing selectivity and a bandwidth increase for the remainder of 47% as reported previously. Infusion of GABA blockers revealed a prominent stimulus onset transient response, lasting about 120 ms, that showed a broadening of orientation selectivity comparable to that found using drifting bars under similar circumstances. We believe that drifting gratings emphasize a steady-state response component that retains, in the presence of GABA blockers, significant orientation selectivity. Because the onset transient is initially unselective for orientation, we suggest that the steady-state, orientation-selective response component develops from an alternate inhibitory mechanism, possibly mediated by GABA receptors. [ABSTRACT FROM AUTHOR]

Intracortical inhibition is believed to enhance the orientation tuning of striate cortical neurons, but the origin of this inhibition is unclear. To examine the possible influence of ascending inhibitory projections from the infragranular layers of striate cortex on the orientation selectivity of neurons in the supragranular layers, we measured the spatiotemporal response properties of 32 supragranular neurons in the cat before, during, and after neural activity in the infragranular layers beneath the recorded cells was inactivated by iontophoretic administration of GABA. During GABA iontophoresis, the orientation tuning bandwidth of 15 (46.9%) supragranular neurons broadened as a result of increases in response amplitude to stimuli oriented about ±20° away from the preferred stimulus angle. The mean (±SD) baseline orientation tuning bandwidth (half width at half height) of these neurons was 13.08±2.3°. Their mean tuning bandwidth during inactivation of the infragranular layers increased to 19.59±2.54°, an increase of 49.7%. The mean percentage increase in orientation tuning bandwidth of the individual neurons was 47.4%. Four neurons exhibited symmetrical changes in their orientation tuning functions, while 11 neurons displayed asymmetrical changes. The change in form of the orientation tuning functions appeared to depend on the relative vertical alignment of the recorded neuron and the infragranular region of inactivation. Neurons located in close vertical register with the inactivated infragranular tissue exhibited symmetric changes in their orientation tuning functions. The neurons exhibiting asymmetric changes in their orientation tuning functions were located just outside the vertical register. Eight of these 11 neurons also demonstrated a mean shift of 6.67±5.77° in their preferred stimulus orientation. The magnitude of change in the orientation tuning functions increased as the delivery of GABA was prolonged. Responses returned to normal approximately 30 min after the delivery of GABA was discontinued. We conclude that inhibitory projections from neurons within the infragranular layers of striate cortex in cats can enhance the orientation selectivity of supragranular striate cortical neurons. [ABSTRACT FROM AUTHOR]

Responsiveness to slits and pattern stimuli was quantified in a total of 68 cells sampled in the posterior extreme of the lateral suprasylvian (PS) cortex as response indices. The cells were studied in relationship to their locations in several subareas of the PS cortex, including areas 19 ( n=15) and 21a ( n=32) and the posteromedial lateral suprasylvian cortex (PMLS; n=21). These subareas were identified based on retrograde labelling from area 17 and also supplemented with photic responsiveness. This analysis revealed that each cortical area contains cells expressing different combinations of stimulus features. Area 19 contained two major groups of cells: (1) those with strong end-stop selectivity combined with moderate orientation or direction selectivity, and (2) those with weak end-stop selectivity combined with strong orientation selectivity. The groups of cells with strong or moderate orientation selectivity showed a strong preference for stripe over visual noise patterns and relatively large modulatory responses to motion of individual stripes. The PMLS contained one major group of cells with strong end-stop and direction selectivities and with poor orientation selectivity. They also showed stronger preference for visual noise than cells in the other cortical areas and rather weak modulatory responses. Area 21a contained only one group of cells with strong orientation selectivity and length summation property rather than end-stop selectivity, and they also lacked direction selectivity. These cells exhibited a strong preference for stripe patterns and moderate or weak modulatory responses. Altogether, these findings indicate that each cortical area is specialized in expressing different stimulus features. The two groups of cells in area 19 may encode the position and motion of discontinuous visual elements such as corners and line ends and continuous elements such as lines and edges. PMLS cells may encode the motion of single elements or associated motion of multiple discontinuous elements such as textures and backgrounds. Area 21a cells may specifically encode the orientation of long, continuous elements such as lines and edges. In support of this view, two types of statistical analyses demonstrated that the combinations of the response properties expressed in individual PS cells are highly correlated with their locations in cortical areas and that the anatomical locations of individual PS cells are reliably predicted from the sets of response indices expressed in these cells. [ABSTRACT FROM AUTHOR]

The representation of the two eyes in striate cortex (V1) of Cebus monkeys was studied by electrophysiological single-unit recordings in normal animals and by morphometric analysis of the pattern of ocular dominance (OD) stripes, as revealed by cytochrome oxidase histochemistry in V1 flat-mounts of enucleated animals. Single-unit recordings revealed that the large majority of V1 neurons respond to the stimulation of either eye but are more strongly activated by one of them. As in other species of monkey, neurons with preference for the stimulation of the same eye are grouped in columns 300-400 µm wide, spanning all cortical layers. Monocular neurons are clustered in layer IVc, specially in its deeper half (IVc-beta), and constitute less than 10% of the population of other layers. Neurons with equal responses to each eye are more commonly found in layer V than elsewhere in V1. In the supragranular layers and in granular layer IVc-alpha neurons strongly dominated by one of the eyes tend to be broadly tuned for orientation, while binocularly balanced neurons tend to be sharply tuned for this parameter. No such correlation was detected in the infragranular layers, and most neurons in layer IVc-beta responded regardless of stimulus orientation. Ocular dominance stripes are present throughout most of V1 as long, parallel or bifurcating bands alternately dominated by the ipsi- or the contralateral eye. They are absent from the cortical representations of the blind spot and the monocular crescent. The domains of each eye occupy nearly equal portions of the surface of binocular V1, except for the representation of the periphery, where the contralateral eye has a larger domain, and a narrow strip along the border of V1 with V2, where either eye may predominate. The orderliness of the pattern of stripes and the relationship between stripe arrangement and the representation of the visual meridians vary with eccentricity and polar angle but follow the same rules in different animals. These results demonstrate that the laminar, columnar and topographic distribution of neurons with different degrees of OD in V1 is qualitatively similar in New- and Old World monkeys of similar sizes and suggest that common ancestry, rather than parallel evolution, may account for the OD phenotypes of contemporaneous simians. [ABSTRACT FROM AUTHOR]

Contour orientation discrimination accuracy is determined by the orientation bandwidth, response variance and response strength of single units that code for orientation. We measured the latter three properties for V1 cells of monkeys which were performing an orientation discrimination of the grating stimulating the cell under study. We recorded from 285 cells, of which 76% responded to the grating. The orientation bandwidth, measured as full width at half height of the tuning curve, varied over a wide range amongst cells. The median bandwidth was 41 degrees. The response variance of the cells also varied considerably between cells; on average it was about two times the response strength. We also studied the temporal properties of the responses. Most of our cells had a latency between 40 and 100 ms. The response variance was found to be smaller in the initial phases of the response than at the later response stages. In some cells the orientation tuning varied in successive stages of the response, while in others the orientation bandwidth and preferred orientation remained stable throughout the response. However, all orientation sensitive cells were orientation tuned from the start of the response, a property which contribute to the fast and reliable coding of contour orientation. These results provide for the first time an estimation of the orientation tuning properties of V1 cells during visual orientation discrimination. They will be very useful to compare single cell properties of other areas to as well as in simulation studies of models of primate visual discriminations. [ABSTRACT FROM AUTHOR]

1. Previous experiments have demonstrated that eye movements, acting through the extraocular muscle (EOM) proprioceptive afferents, are necessary for the development of orientation selectivity in the cells of the kitten visual cortex. New experiments were carried out to study the effect of the plane of eye movements on the preferred orientation acquired by the visual cortical cells. 2. Darkreared (DR) kittens were operated on at 5-6 weeks of age. In the first series of experiments, 4 out of the 6 EOMs were removed bilaterally in such a way that both eyes could only move in a single plane, either vertical or horizontal. In the second series of experiments, the same operation was performed on one eye which was also sutured shut and, on the other side, the EOM were deafferented by intracranial section of the ophthalmic branch of Vth nerve and the eye left open. 3. 1-4 days after surgery the kittens were given 6 h of visual experience and 12 h later were prepared for visual cell recording in Area 17. 4. In kittens of the first series: orientation selectivity developed in the majority (60-65%) of visual cells, most of which encoded horizontal orientations when the eyes had moved in the vertical plane and vertical orientations when the eyes had moved in the horizontal plane. These results show that the plane of eye movements during early visual experience influences the distribution of preferred orientations with an orthogonal relation. Ocular dominance histograms were 'strabismic like' 5. In kittens of the second series: orientation selectivity developed in 40-50% of cells, about half of which were tuned for the orientation orthogonal to the direction of movement of the occluded eye, as in experiment I. The seeing, deafferented eye, presumably would have sent normal visual inputs centrally, corresponding to displacements on the retina in every direction since the ocular motility of that eye had not been disturbed. However, proprioceptive information about its movements was suppressed. As only some of the EOMs of the occluded eye were still present and connected, the conclusion is that the observed influence of the plane of eye movements acts through the proprioceptive afferents. The ocular dominance histograms showed: 1) a powerful change in favour of the seeing eye after only 6 h of monocular visual experience; 2) a larger effect of monocular visual experience in the hemisphere contralateral to the seeing eye; 3) a linkage between acquisition of orientation selectivity and shift in ocular dominance. 6. Our results suggest that normal development of orientation selectivity in visual cortical cells results from the close association of visual and EOM afferent inputs. It is suggested that these two signals must occur with a precise temporal relationship. [ABSTRACT FROM AUTHOR]

Single unit recording from visual cortex areas 17 and 18 and in the border region between them was performed on adult cats with unilateral optic tract section (OTX) either on the day of the operation (acute) or three to six months postoperatively (chronic). Visual activity from both hemispheres was analyzed with respect to the responsiveness level, the ocular dominance distribution and selectivity to orientation and direction. The results showed almost a complete absence of visual responsiveness in the deafferented hemisphere and a considerable reduction of responsiveness in the intact hemisphere. Following surgery an increase in visually responsive cells was found in the intact hemisphere as postoperative recovery continued. In addition, a reduction in the proportion of cells selective for orientation and direction was also found in the intact hemisphere of the OTX animals as compared to the control cats. Furthermore, cortical binocularity was not affected in the intact hemisphere of all OTX cats. We conclude that an almost total absence of interhemispheric callosal transfer of visual functions from the intact to the deafferented hemisphere is induced as a result of the unilateral OTX in adult cats. Moreover, the fact that the absence of cortical binocularity in the hemisphere receiving direct geniculate input was not disrupted, indicates the absence of callosal transfer from the deafferented to the intact hemisphere. [ABSTRACT FROM AUTHOR]

Contrast sensitivity and orientation selectivity were measured for neurons in lamina IV of macaque striate cortex. Contrast sensitivity was determined for a range of spatial frequencies, using a staircase method. The stimuli were at the optimal orientation, direction and speed of drift for each neuron. The assignment of each recording site to a subdivision of lamina IV was made by histological reconstruction of each electrode penetration from sections reacted for cytochrome oxidase and stained for Nissl substance. Neurons in the magnocellular recipient zone of IVc (IVcα) have high contrast sensitivities, while those in the parvocellular recipient zone (IVcβ) have low contrast sensitivities. Both of the upper subdivisions of lamina IV (IVa and IVb) contain a mixture of neurons with high and low contrast sensitivities. There were orientation selective neurons within all subdivisions of lamina IV, even in IVc, whereas non-oriented neurons were found only in those subdivisions that receive a direct parvocellular geniculate input (IVa and IVcβ). [ABSTRACT FROM AUTHOR]

The relationship between orientation selectivity and spatial receptive field organization was analyzed. Receptive field maps were made with a dual stimulus technique where an optimally oriented activation slit was presented in the most responsive region to produce activity against which the effect of a test spot in various positions was determined. Both simple and complex cells had receptive fields which were subdivided into adjacent elongated and antagonistic subrogions. When the two stimuli were presented in phase (both ON or OFF simultaneously) the fields had a central enhancement region with a strong suppression flank on one or both sides. Optimal slit orientation was related to the location of the suppression flank relative to the location of the central enhancement region, and the degree of orientation selectivity to the shape of the subregions and the distance between them. Estimated orientation tuning curves calculated from the receptive field maps gave satisfactory first approximations to experimental curves. The relative contribution of enhancement and suppression to orientation selectivity was studied by presenting a test slit in different orientations in phase with an optimally oriented activation slit. The orientation selectivity was produced almost exclusively by the flank suppression indicating that orientation selectivity is produced by inhibitory input. The flank suppression lacked any specific orientation selectivity, and it occurred only when both the central region and the flanks were activated in phase. Orientation selectivity in both simple and complex cells is explained by a receptive field organization where the cells have input from partially overlapping excitatory and inhibitory fields which have their centers slightly displaced from each other. [ABSTRACT FROM AUTHOR]

Kittens were reared wearing masks that contained strong cylindrical lenses, which allowed them to see only contours of one orientation. Selective exposure was alternated between the two eyes on successive days, while each time the other eye was covered by the mask. The total duration of exposure was different in the two eyes, amounting to 50 h for one eye and at least 100 h for the other. This resulted in asymmetric distributions of ocular dominance: neurones preferring precisely the experienced orientation favoured the eye with longer exposure, whereas neurones preferring oblique orientations adjacent to the experienced one surprisingly were dominated by the eye with shorter exposure. Thus neurones originally belonging to this group and dominated by the longer exposed eye must have tuned their orientation preference to the experienced orientation as a result of the longer exposure. Such instructive changes seem to be limited by the original response borders of the cortical neurones, as predicted by Hebb's rules for synaptic plasticity. [ABSTRACT FROM AUTHOR]

The developmental properties of 573 neurones have been investigated in the primary visual cortex of eight binocularly intact and twelve unilaterally enucleated kittens. It is shown that removal of one eye at birth alters the development of orientation selectivity observed in the presence or absence of visual experience. In 6-week-old deprived kittens, there remain significantly more orientation selective cells in enucleated than in binocularly deprived kittens. These deprivation-resistant cells respond preferentially to horizontal or vertical orientations and are recorded mainly in the cortex contralateral to the remaining eye. In six-week-old kittens with visual experience, the process of tuning maturation appears to be unaffected by unilateral enucleation at birth. However, a larger over-representation of horizontal and vertical orientation preferences is observed in uniocular kittens than in binocularly intact kittens, suggesting that the development of oblique orientation preference depends upon the presence of binocular afferents in the visual pathway. [ABSTRACT FROM AUTHOR]

In normal observers preadaptation to a parallel grating increases the contrast threshold for a line whereas a perpendicular grating has no effect. Such orientation selectivity was not found in the amblyopic eye of two out of five squinters. Only a weak after-effect produced with a grating parallel to the line was obtained in the good eye of four of the amblyopes while all of them show an abnormal threshold reduction following adaptation to a perpendicular grating. This suggests a relationship between abnormal binocular interaction during visual development and the organization of orientational mechanisms but does not explain the loss of visual acuity in amblyopia. [ABSTRACT FROM AUTHOR]

The responses of cells in the cat's visual cortex to a moving bar of light have been analysed quantitatively, using an integration of the post-stimulus time histogram, with particular reference to orientation selectivity. The method is assessed as to its reliability and usefulness; it is shown that much precise information about a cell can be derived from its orientational tuning curve. Complex cells were found on average to show more spontaneous activity, greater response amplitude, and slightly broader orientational tuning, than either simple or hypercomplex cell types. Correlations between spontaneous and evoked activity and orientational selectivity give information as to the possible mechanisms of excitation and inhibition of the cells. The question of meridional variations in perception has been especially examined: the unexpected finding that many simple cells detecting orientations close to horizontal or vertical are very narrowly tuned, which is not the case for complex or hypercomplex cells, is discussed in relation to human psychophysical variations in orientation discrimination and contrast sensitivity. [ABSTRACT FROM AUTHOR]

The sensitivity to stimulus orientation - orientation selectivity - of simple and complex neurons in cat striate cortex was studied quantitatively. Orientation selectivity was found to be related to receptive field size - neurons with larger receptive fields being less sensitive to stimulus orientation than those with smaller receptive fields. Simple cells were more orientationally selective than complex cells. The orientation selectivity of simple cells was only weakly related to their receptive field geometry (i.e., receptive field width/length ratio), but simple cells with narrow, elongated excitatory receptive fields are more orientationally selective than those with squarer, excitatory receptive fields. These results indicate that orientation selectivity cannot be accounted for solely by the geometry of the excitatory zones and suggest that inhibitory influences sharpen the tuning of simple striate neurons for stimulus orientation. [ABSTRACT FROM AUTHOR]

Recent studies have shown that local cortical feedback can havean important effect on the response of neurons in primary visualcortex to the orientation of visual stimuli. In this work, westudy the role of the cortical feedback in shaping thespatiotemporal patterns of activity in cortex. Two questionsare addressed: one, what are the limitations on the ability ofcortical neurons to lock their activity to rotatingoriented stimuli within a single receptive field? Two, can thelocal architecture of visual cortex lead to the generation ofspontaneous traveling pulses of activity? We study theseissues analytically by a population-dynamic model of ahypercolumn in visual cortex. The order parameter thatdescribes the macroscopic behavior of the network is thetime-dependent population vector of the network. We firststudy the network dynamics under the influence of a weakly tunedinput that slowly rotates within the receptive field. We showthat if the cortical interactions have strong spatialmodulation, the network generates a sharply tuned activityprofile that propagates across the hypercolumn in a path thatis completely locked to the stimulus rotation. The resultantrotating population vector maintains a constant angular lagrelative to the stimulus, the magnitude of which grows with thestimulus rotation frequency. Beyond a critical frequency thepopulation vector does not lock to the stimulus but executes aquasi-periodic motion with an average frequency that is smallerthan that of the stimulus. In the second part we consider thestable intrinsic state of the cortex under the influence of isotropic stimulation. We show that if the local inhibitoryfeedback is sufficiently strong, the network does not settleinto a stationary state but develops spontaneous travelingpulses of activity. Unlike recent models of wave propagation incortical networks, the connectivity pattern in our model isspatially symmetric, hence the direction of propagation ofthese waves is arbitrary. The interaction of these waves withan external-oriented stimulus is studied. It is shown that thesystem can lock to a weakly tuned rotating stimulus if thestimulus frequency is close to the frequency of the intrinsic wave. [ABSTRACT FROM AUTHOR]