The two-dimensional spectral structure of simple receptive fields in cat striate cortex

1. A quantitative, general purpose method was developed for measuring the responses of visual neurons to stimuli distributed with high resolution over the two-dimensional (2D) spatial frequency domain. The stimuli consisted of drifting sinusoidal gratings of nonsaturating contrasts whose spatial frequency and orientation were drawn in random order from a 16 X 16 array of coordinates covering each neuron's responsive area. This method was applied to a population of 36 simple cells in area 17 of cat. 2. The response of each simple cell to drifting sinusoidal gratings appeared as a rectified sinusoidal modulation of the spike frequency. The degree of rectification varied from cell to cell, but for each cell, the form of the response was constant irrespective of stimulus spatial frequency, orientation, or contrast. The amplitude of the average response at the stimulus temporal frequency was used as the response metric at all spectral coordinates. Variations in this amplitude over two spectral dimensions forms a surface that we call the 2D spectral response profile. 3. For each cell, the 2D spectral response profile was localized to a limited region of the complete 2D spatial frequency domain. In bidirectionally responsive cells, there were two lobes in the surface disposed with mirror symmetry about the origin. In all cells, each lobe exhibited a single maximum and the response decayed smoothly in every direction away from the maximum. Isoresponse amplitude contours were elliptical and often, but not always, elongated about an axis of symmetry passing through the origin. 4. We tested the hypothesis that orientation and spatial frequency tuning are independent by forming scaled radial and angular sections through 2D spectral response profiles. In virtually every case polar separability did not obtain, that is, orientation selectivity depended on spatial frequency and vice versa. 5. In contrast, more than half the cells had 2D spectral response profiles that were Cartesian separable. The 2D spectral response profiles of most of the remaining cells were neither polar nor Cartesian separable, because the response profiles were elongated about an axis of symmetry that did not pass through the origin. 6. These results are discussed in terms of the constraints they place on models of the contributions simple cells make toward the neural representation of images.     

Gabor analysis of auditory midbrain receptive fields: spectro-temporal and binaural composition

The spectro-temporal receptive field (STRF) is a model representation of the excitatory and inhibitory integration area of auditory neurons. Recently it has been used to study spectral and temporal aspects of monaural integration in auditory centers. Here we report the properties of monaural STRFs and the relationship between ipsi- and contralateral inputs to neurons of the central nucleus of cat inferior colliculus (ICC) of cats. First, we use an optimal singular-value decomposition method to approximate auditory STRFs as a sum of time-frequency separable Gabor functions. This procedure extracts nine physiologically meaningful parameters. The STRFs of approximately 60% of collicular neurons are well described by a time-frequency separable Gabor STRF model, whereas the remaining neurons exhibited obliquely oriented or multiple excitatory/inhibitory subfields that require a nonseparable Gabor fitting procedure. Parametric analysis reveals distinct spectro-temporal tradeoffs in receptive field size and modulation filtering resolution. Comparisons between an identical model used to study spatio-temporal integration areas of visual neurons further shows that auditory and visual STRFs share numerous structural properties. We then use the Gabor STRF model to compare quantitatively receptive field properties of contra- and ipsilateral inputs to the ICC. We show that most interaural STRF parameters are highly correlated bilaterally. However, the spectral and temporal phases of ipsi- and contralateral STRFs often differ significantly. This suggests that activity originating from each ear share various spectro-temporal response properties such as their temporal delay, bandwidth, and center frequency but have shifted or interleaved patterns of excitation and inhibition. These differences in converging monaural receptive fields expand binaural processing capacity beyond interaural time and intensity aspects and may enable colliculus neurons to detect disparities in the spectro-temporal composition of the binaural input.     

Spatial structure and symmetry of simple-cell receptive fields in macaque primary visual cortex

I present measurements of the spatial structure of simple-cell receptive fields in macaque primary visual cortex (area V1). Similar to previous findings in cat area 17, the spatial profile of simple-cell receptive fields in the macaque is well described by two-dimensional Gabor functions. A population analysis reveals that the distribution of spatial profiles in primary visual cortex lies approximately on a one-parameter family of filter shapes. Surprisingly, the receptive fields cluster into even- and odd-symmetry classes with a tendency for neurons that are well tuned in orientation and spatial frequency to have odd-symmetric receptive fields. The filter shapes predicted by two recent theories of simple-cell receptive field function, independent component analysis and sparse coding, are compared with the data. Both theories predict receptive fields with a larger number of subfields than observed in the experimental data. In addition, these theories do not generate receptive fields that are broadly tuned in orientation and low-pass in spatial frequency, which are commonly seen in monkey V1. The implications of these results for our understanding of image coding and representation in primary visual cortex are discussed.     

The two-dimensional spatial structure of simple receptive fields in cat striate cortex

1. A reverse correlation (6, 8, 25, 35) method is developed that allows quantitative determination of visual receptive-field structure in two spatial dimensions. This method is applied to simple cells in the cat striate cortex. 2. It is demonstrated that the reverse correlation method yields results with several desirable properties, including convergence and reproducibility independent of modest changes in stimulus parameters. 3. In contrast to results obtained with moving stimuli, we find that the bright and dark excitatory subregions in simple receptive fields do not overlap to any great extent. This difference in results may be attributed to confounding the independent variables space and time when using moving stimuli. 4. All simple receptive fields have subregions that vary smoothly in all directions in space. There are no sharp transitions either between excitatory subregions or between subregions and the area surrounding the receptive field. 5. Simple receptive fields vary both in the number of subregions observed, in the elongation of each subregion, and in the overall elongation of the field. In contrast with results obtained using moving stimuli, we find that subregions within a given receptive field need not be the same length. 6. The hypothesis that simple receptive fields can be modeled as either even symmetric or odd symmetric about a central axis is evaluated. This hypothesis is found to be false in general. Most simple receptive fields are neither even symmetric nor odd symmetric. 7. The hypothesis that simple receptive fields can be modeled as the product of a width response profile and an orthogonal length response profile (Cartesian separability) is evaluated. This hypothesis is found to be true for only approximately 50% of the cells in our sample.     

Internal spatial organization of receptive fields of complex cells in the early visual cortex

The receptive fields of complex cells in the early visual cortex are economically modeled by combining outputs of a quadrature pair of linear filters. For actual complex cells, such a minimal model may be insufficient because many more simple cells are thought to make up a complex cell receptive field. To examine the minimalist model physiologically, we analyzed spatial relationships between the internal structure (subunits) and the overall receptive fields of individual complex cells by a two-stimulus interaction technique. The receptive fields of complex cells are more circular and only slightly larger than their subunits in size. In addition, complex cell subunits occupy spatial extents similar to those of simple cell receptive fields. Therefore in these respects, the minimalist schema is a fair approximation to actual complex cells. However, there are violations against the minimal model. Simple cell receptive fields have significantly fewer subregions than complex cell subunits and, in general, simple cell receptive fields are elongated more horizontally than vertically. This bias is absent in complex cell subunits and receptive fields. Thus simple cells cannot be equated to individual complex cell subunits and spatial pooling of simple cells may occur anisotropically to constitute a complex cell subunit. Moreover, when linear filters for complex cell subunits are examined separately for bright and dark responses, there are significant imbalances and position displacements between them. This suggests that actual complex cell receptive fields are constructed by a richer combination of linear filters than proposed by the minimalist model.     

Spatial arrangements of responses by cells in the cat visual cortex to light and dark bars and edges

Summary Detailed examination is made of the responses of visual cortical cells (area 17, border 17–18 and adjacent area 18) in the anaesthetized cat to stationary flashing bars and to bars (lines) and edges moving at their optimal velocities. Particular attention is given to the receptive field organization of cells in the simple family. While there is good general agreement between the main receptive field subregions revealed by stationary and moving stimuli, the responses to moving light and dark bars, supplemented by the responses to moving light and dark edges, provide a much more rapid, accurate and complete guide to the spatial organization of the receptive fields than do the response profiles to a stationary flashing bar. Moving light and dark bars between them generally reveal more subregions in the receptive fields of simple cells than is evident from the response profiles to a stationary flashing bar, particularly when the receptive fields have many subregions. In addition the responses to moving edges provide a rapid guide to spatial summation across the width of a subregion and the possible antagonistic effects of the next subregion in sequence.Two subclasses of cells in the simple family have been recognized: ordinary simple and fast simple cells. Two cell classes (A-cells and silent periodic cells) having properties intermediate between simple and complex types are discriminated and their properties described.     

Periodic simple cells in cat area 17

Quantitative, high-resolution static receptive-field plots (response planes) in cat area 17 revealed simple cells whose receptive fields were composed of four to six excitatory regions alternating in space with up to seven inhibitory regions. The size, shape, and spacing of the excitatory regions within these receptive fields were highly regular, giving the receptive field a periodic appearance in space. We call these periodic simple cells. A periodic simple cell's response to moving stimuli could, in general, be anticipated from the detailed spatiotemporal map of excitatory and inhibitory regions provided by response planes. This observation suggests that periodic simple cells, like the more common simple cells composed of one to three excitatory regions, sum spatially distributed inputs in a roughly linear manner. Based on a quantitative assessment of the spatial distribution and time course of response of single excitatory regions within periodic receptive fields, as described in the previous paper, we characterized periodic simple cells as either X-like or Y-like. Furthermore, we found that periodic simple cells classified as X-like gave a more sustained response to standing contrast and had significantly smaller excitatory regions than those cells classified as Y-like. Periodic simple cells were found in layer III and at the border between layers III and IVab. It is suggested that these cells, which reside outside the primary zone of geniculate termination and include both X-like and Y-like types, may be constructed hierarchically from the convergence of lower order simple cells. In the spatial-frequency domain, periodic simple receptive fields were predicted to have bandwidths at half-maximum ranging from 0.80 to 1.4 octaves. By comparison, the predicted bandwidths of cells composed of two or three excitatory regions ranged from 1.6 to 4.3 octaves. Thus as additional excitatory regions are added to the receptive fields of simple cells, their bandwidth narrows in the spatial-frequency domain.     

Spatial organization of subregions in receptive fields of simple cells in cat striate cortex as revealed by stationary flashing bars and moving edges

Summary For each of 74 simple striate cells a quantitative analysis was made of the width dimensions and spatial arrangements of the subregions responding either at light on (ON subregion) or at light off (OFF subregion). It was concluded that every cell has at least two and no more than four subregions. Cells with two subregions (57%) were much more commonly encountered than those with three (32%) or four (11%). For most cells adjacent subregions were significantly overlapped, the region of overlap responding both at light on and at light off. In the case of cells with two subregions, the overlap averaged 32% of the overall width of the two subregions. Despite the degree of the overlap, there was, on this basis, still a large measure of discrimination between cells in the simple family and those in the B-cell and complex families. In general the receptive field profiles of cells with three and four subregions were only marginally wider than those with only two subregions. In any given receptive field, the subregions tend to be roughly equal in width so that, in cells with four subregions, the subregions are, on the average, distinctly narrower than they are in cells with only two. Hypercomplex I cells tend to have receptive fields with three and four subregions much more commonly than simple cells and these cells are encountered much more frequently in cortical cell laminae 2, 3 and 4 than in lamina 6. In lamina 6 most of the cells in the simple family have receptive fields with only two subregions. The width dimensions and spatial sequences of the response peaks to moving light and dark edges were quantitatively analyzed in response profiles prepared from 82 cells. In general, for any given receptive field, the response peaks to moving edges have a one-to-one correspondence with the subregions to a stationary flashing bar. When this is not the case, the tendency is for the number of response peaks to edges to be less than the number of subregions rather than more.     

Local sensitivity to stimulus orientation and spatial frequency within the receptive fields of neurons in visual area 2 of macaque monkeys

We used dynamic dense noise stimuli and local spectral reverse correlation methods to reveal the local sensitivities of neurons in visual area 2 (V2) of macaque monkeys to orientation and spatial frequency within their receptive fields. This minimized the potentially confounding assumptions that are inherent in stimulus selections. The majority of neurons exhibited a relatively high degree of homogeneity for the preferred orientations and spatial frequencies in the spatial matrix of facilitatory subfields. However, about 20% of all neurons showed maximum orientation differences between neighboring subfields that were greater than 25 deg. The neurons preferring horizontal or vertical orientations showed less inhomogeneity in space than the neurons preferring oblique orientations. Over 50% of all units also exhibited suppressive profiles, and those were more heterogeneous than facilitatory profiles. The preferred orientation and spatial frequency of suppressive profiles differed substantially from those of facilitatory profiles, and the neurons with suppressive subfields had greater orientation selectivity than those without suppressive subfields. The peak suppression occurred with longer delays than the peak facilitation. These results suggest that the receptive field profiles of the majority of V2 neurons reflect the orderly convergence of V1 inputs over space, but that a subset of V2 neurons exhibit more complex response profiles having both suppressive and facilitatory subfields. These V2 neurons with heterogeneous subfield profiles could play an important role in the initial processing of complex stimulus features.     

Spatiotemporal mechanisms in receptive fields of visual cortical simple cells: a model

1. Simple cells in the visual cortex have been subdivided into nondirection-selective (NDS), direction asymmetric (DA), and direction-selective (DS) cells. DA cells reverse their preferred direction with reversal of the stimulus contrast; DS2 cells respond with the same preferred direction for light and dark stimuli, whereas DS1 cells respond only to one (light or dark) contrast. Also, four velocity response groups have been distinguished: velocity broadband, low-pass, high-pass, and -tuned cells. This study describes an analytic model of feed-forward spatiotemporal interactions within a receptive field that reproduces these basic features of cortical simple cell behavior in the cat. 2. The spatial structure of the receptive fields is simulated with Gabor functions. Two neurobiologically plausible mechanisms, temporal low-pass filtering and intracortical spatial distribution of activity, are modeled. The central feature of the study is the implementation of both mechanisms in a spatially continuous way. The model is analytic, but an equivalent neural network diagram was drawn and is used to explain the features of the model. 3. First-order temporal low-pass filtering is performed both after convolving the stimulus light-intensity function with the Gabor type receptive field and also at the final output step of the model. In the circuit diagram this would correspond to low-pass filtering in lateral geniculate nucleus (LGN) and cortical cells. Filtering was adjusted to have a -3-dB drop-off frequency of 2-3 Hz, corresponding to the drop-off frequencies observed in response to temporal modulation of sine-wave gratings. 4. The mechanism that we call intracortical distribution of activity is implemented along the axis of stimulus motion. A response elicited from the part of the receptive field that is stimulated at a given time will spread out in the receptive field, influencing regions that have not been stimulated. It is equivalent to spreading of activity on the cortical surface. This mechanism extends the existing ideas of discrete interactions between subfields to a continuous scheme throughout the whole receptive field. It is based on findings that intracortical interactions exist even within single subfields. The impact of distributing the activity is assumed to decrease exponentially with the Euclidian distance between the stimulated region and the region under consideration. 5. Thresholds are implemented only at the level of the cortex. Both the activity distributing mechanism and the output of the cell being studied are thresholded.(ABSTRACT TRUNCATED AT 400 WORDS)     

Properties of concentrically organized X and Y ganglion cells of macaque retina.

1. Macaque retinal ganglion cells having concentrically organized receptive fields were classified as X- or Y-cells on the basis of the linearity or nonlinearity of their spatial summation to a "null" test of alternating contrast and drifting gratings. 2. When an alternating-phase, bipartite field positioned at the middle of the receptive field was used as a stimulus, X-cells had a null position, whereas Y-cells showed a doubling of the response frequency. When drifting sine-wave gratings of low contrast were used as a stimulus, X-cells showed a periodic modulation of their discharge having the same mean value for different spatial frequencies, whereas Y-cells showed a large increase in the mean value of their discharges. 3. X-cells had opponent-color responses that received cone-specific signals, i.e., center and surround responses were mediated by input from spectrally different types of cone, whereas Y-cells had broad-band spectral responses receiving mixed-cone signals, i.e., center and surround responses were totally or partly mediated by input from the same type(s) of cone. In most Y-cells, the spatially opponent responses from the center and the surround were mediated by the same types of cone and were thus spectrally nonopponent; other Y-cells showed spectral opponency, since one of the types of cone mediating responses of one region of the receptive field (e.g., center) was absent in the responses of the other region (e.g., surround). 4. X- and Y-cells projected to the lateral geniculate body. Opponent-color X- and Y-cells did not project to the superior colliculus, whereas a fraction of spectrally non-opponent Y-cells projected to this structure. 5. X-cells tended to have longer conduction latencies, less transient responses to small stimuli, and a more central retinal distribution than Y-cells; these differences, however, represented tendencies and not invariant properties. 6. The results show that the X/Y dichotomy of ganglion cells is present in the retina of macaques and indicate that the degree of the linearity of spatial summation of incoming cone signals to the cells is related to the degree of cone specificity of spectral inputs to the receptive-field mechanisms.     

Interaction effects of visual contours on the discharge frequency of simple striate neurones

1. The discharge frequency of simple neurones in the cat striate cortex responding to the two edges of a slit of light moving over their receptive fields was studied as a function of slit width. While one edge of the slit was discharging the cell, the other edge had a modifying influence on that discharge either by way of facilitation or of inhibition.2. The most common form of the curve relating discharge frequency and slit width had a maximal discharge at narrow slit widths (< 0.5 degrees ) and relative inhibition at medium widths (between 0.5 degrees and 2 degrees ). At greater slit widths there was usually a region of facilitation before the effects of the two edges became independent of one another. Three other response patterns to slits of different width are described.3. The curve relating slit width and response amplitude for a particular cell provides an important clue to the various activity profiles for that cell. An activity profile plots the excitability of a cell along a line through the receptive field in the direction of stimulus movement. Each type of edge, light and dark, has its own set of activity profiles which differ depending upon stimulus parameters such as the direction of the movement of the edge.4. Two other methods were used to provide further data concerning the activity profiles and as a check on the evidence provided by the responses to slits of different width. One of these two methods used the test stimulus against the background of an artificially produced maintained discharge and the other involved the interaction of the two receptive fields of binocularly activated cells.5. A model is put forward to explain the receptive field organization of simple striate neurones which takes into account not only the main features of what is known concerning the synaptology of the visual cortex but also the new data provided by the present paper and the one which precedes it.     

Cortical maps of separable tuning properties predict population responses to complex visual stimuli

In the earliest cortical stages of visual processing, a scene is represented in different functional domains selective for specific features. Maps of orientation and spatial frequency preference have been described in the primary visual cortex using simple sinusoidal grating stimuli. However, recent imaging experiments suggest that the maps of these two spatial parameters are not sufficient to describe patterns of activity in different orientation domains generated in response to complex, moving stimuli. A model of cortical organization is presented in which cortical temporal frequency tuning is superimposed on the maps of orientation and spatial frequency tuning. The maps of these three tuning properties are sufficient to describe the activity in orientation domains that have been measured in response to drifting complex images. The model also makes specific predictions about how moving images are represented in different spatial frequency domains. These results suggest that the tangential organization of primary visual cortex can be described by a set of maps of separable neuronal receptive field features including maps of orientation, spatial frequency, and temporal frequency tuning properties.     

Responses of V1 neurons to two-dimensional hermite functions

Neurons in primary visual cortex are widely considered to be oriented filters or energy detectors that perform one-dimensional feature analysis. The main deviations from this picture are generally thought to include gain controls and modulatory influences. Here we investigate receptive field (RF) properties of single neurons with localized two-dimensional stimuli, the two-dimensional Hermite functions (TDHs). TDHs can be grouped into distinct complete orthonormal bases that are matched in contrast energy, spatial extent, and spatial frequency content but differ in two-dimensional form, and thus can be used to probe spatially specific nonlinearities. Here we use two such bases: Cartesian TDHs, which resemble vignetted gratings and checkerboards, and polar TDHs, which resemble vignetted annuli and dartboards. Of 63 isolated units, 51 responded to TDH stimuli. In 37/51 units, we found significant differences in overall response size (21/51) or apparent RF shape (28/51) that depended on which basis set was used. Because of the properties of the TDH stimuli, these findings are inconsistent with simple feedforward nonlinearities and with many variants of energy models. Rather, they imply the presence of nonlinearities that are not local in either space or spatial frequency. Units showing these differences were present to a similar degree in cat and monkey, in simple and complex cells, and in supragranular, infragranular, and granular layers. We thus find a widely distributed neurophysiological substrate for two-dimensional spatial analysis at the earliest stages of cortical processing. Moreover, the population pattern of tuning to TDH functions suggests that V1 neurons sample not only orientations, but a larger space of two-dimensional form, in an even-handed manner.     

Spectral receptive field properties of visually active neurons in the caudate nucleus

Recent studies stress the importance of the caudate nucleus in visual information processing. Although the processing of moving visual signals depends upon the capability of a system to integrate spatial and temporal information, no study has investigated the spectral receptive field organization of the caudate nucleus neurons yet. Therefore, we tested caudate neurons of the feline brain by extracellular single-cell recording applying drifting sinewave gratings of various spatial and temporal frequencies, and reconstructed their spectral receptive fields by plotting their responsiveness as a function of different combinations of spatial and temporal frequencies. The majority of the caudate cells (74%) exhibited peak tuning, which means that their spatio-temporal frequency response profile had a characteristic region of increased activity with a single maximum in the spatio-temporal frequency domain. In one-quarter of the recorded caudate neurons ridge tuning was found, where the region of increased activity, forming an elongated ridge of maximal sensitivity parallel or angled to the spatial or the temporal frequency axis, indicating temporal (16%), spatial (5%) or speed (5%) tuning, respectively. The velocity preference of the ridge tuned caudate nucleus neurons is significantly lower than that of the peak tuned neurons. The peak tuned neuron could encode high velocities, while the ridge tuned neurons were responsible for the detection of moderate and lower velocities. Based upon our results, we suggest that the wide variety of spatio-temporal frequency response profiles might represent different functional neuronal groups within the caudate nucleus that subserve different behaviors to meet various environmental requirements.     

Linear analysis of the responses of simple cells in the cat visual cortex

Summary Spatial response profiles to stationary and moving stimuli and spatial frequency tuning curves to drifting sinusoidal gratings were recorded from a series of cells in the simple family. The spatial response profiles were recorded both to stationary flashing bars and sinusoidal gratings as well as to light and dark bars and edges and gratings moving at the optimal velocity. On the assumption that cells in the simple family operate linearly, spatial response profiles recorded experimentally were compared with those predicted by inverse Fourier transformation of the spatial frequency tuning curves. Conversely, the spatial frequency tuning curves recorded experimentally were compared with those predicted from the response profiles to moving and stationary stimuli. As a result of these comparisons, it is clear that moving stimuli provide a more accurate estimate of the spatial organization of the receptive field than do stationary stimuli. Cells with the higher optimal spatial frequencies tended to have narrower bandwidths. The simple cell with the narrowest bandwidth (0.94 octave) had five, and possibly six, subregions in the spatial response profile to moving light and dark bars, the largest number of subregions we encountered.     

Cutaneous masking. II. Geometry of excitatory andinhibitory receptive fields of single units in somatosensory cortex of the cat.

1. The responses of single neurons in the primary somatosensory cortex of the cat to brief air-pulse stimuli were quantitatively examined. These controlled natural stimuli activated almost exclusively rapidly adapting hair units which, on systematic movement of the stimulus through the receptive field, gave unit-response profiles that showed the classical unimodal tent-shaped distribution. 2. Conditioning stimulus-induced inhibition of a response evoked by a fixed test stimulus was measured by systematically moving the conditioning stimulus through the receptive field. The spatial distribution of in-field inhibitory activity was unimodal and highly covariant with that of the conditioning excitation, the peak inhibition corresponding to the functional center of the excitatory receptive field. 3. Nearly one-half of the units studied evidenced inhibition extending beyond the excitatory receptive field, forming a "surround" inhibitory region; but these were usually restricted areas with rather weak inhibitory effects. 4. Time-course measuring revealed, on the average, inhibition effects measureable from 10 ms before to some 70 ms following conditioning stimulation, with peak inhibition delayed some 10--15 ms from the conditioning stimulus onset. We showed the backward inhibition, occurring with the test stimulus delivered before the onset of the conditioning stimulus, to be a property of the test response duration. Inhibition measured in the surround areas had essentially the same time course as the inhibition calculated from measurements made within the receptive fields. 5. The spatial and temporal profiles of the excitatory and inhibitory cortical unitary activity are thus very similar to the parametric features of psychophysical enhancement and masking. These findings suggest that the excitatory and inhibitory activities related to individual stimuli interact in multipoint stimulus paradigms so that simple unimodal composite profiles are synthesized.     

The influence of visual and auditory receptive field organization on multisensory integration in the superior colliculus

The spatial register of the different receptive fields of multisensory neurons in the superior colliculus (SC) plays a significant role in determining the responses of these neurons to cross-modal stimulus combinations. Spatially coincident visual-auditory stimuli fall within these overlapping receptive fields and generally produce response enhancements that exceed the individual modality-specific responses and can exceed their sum. Yet, in this context, it has not been clear how "spatial coincidence" is operationally defined. Given the large size of SC receptive fields, visual and auditory stimuli could be within their respective receptive fields even when there are substantial spatial disparities between them. Indeed, previous observations have raised the possibility that there may be a second level of determinism in how SC neurons deal with the relative spatial locations of within-field cross-modal stimuli; specifically, that multisensory response enhancements become progressively weaker as the within-field visual and auditory stimuli become increasingly disparate. While the present experiments demonstrated that SC multisensory neurons have heterogeneous receptive fields, and that the greatest number of impulses evoked were by stimuli that fell within the area of cross-modal receptive field overlap, they also indicate that there is no systematic relationship between cross-modal stimulus disparity and the magnitude of multisensory response enhancement. Thus, two within-field cross-modal stimuli produced the same proportionate change (i.e., multisensory response enhancement) when they were widely disparate as they did when they overlapped one another in space. These observations indicate that cross-modal spatial coincidence can be defined operationally by the borders of an SC neuron's receptive fields regardless of the size of those receptive fields and/or the absolute spatial disparity between within-field cross-modal stimuli. Electronic Publication     

Receptive-field properties and laminar distribution of X-like and Y-like simple cells in cat area 17

We examined the spatiotemporal organization of excitatory regions in 197 simple receptive fields from cat area 17 using the peristimulus time response-plane technique of Stevens and Gerstein (53). With this method we observed a striking similarity between the spatiotemporal organization of excitatory regions in simple receptive fields and the excitatory centers in X or Y geniculate receptive fields. This observation suggested to us the possibility that individual simple receptive fields may be differentially innervated by either X or Y geniculate afferents. To test this hypothesis, we devised a quantitative measure that could characterize the excitatory regions in simple receptive fields as being X-like or Y-like. This measure was based on an understanding of the spatiotemporal organization of geniculate X and Y receptive fields. Further evidence supporting this division of simple cells was derived from additional physiological and anatomical comparisons. When compared to Y-like simple cells, X-like simple cells, as a group, gave a more sustained response to standing contrast, had smaller excitatory regions, and preferred slightly slower moving stimuli. A comparison of the properties of end-zone inhibition and directional selectivity showed no additional difference between X-like and Y-like simple cells. We found a correlation between the laminar position of X-like and Y-like simple cells and the known patterns of termination of X and Y geniculate afferents. Y-like simple cells were found in layers III, IVab, and VI, but not in layer IVc, whereas X-like simple cells were found in layer III, all parts of layer IV, and layer VI. Inhibitory regions appeared to play a major role in defining the spatiotemporal structure of simple receptive fields and they further acted to diminish differences between the spatial widths and velocity sensitivities of X-like and Y-like simple cells. These data are discussed in terms of a parallel model of geniculostriate convergence and support the hypothesis that the X and Y systems, which originate in the retina, are maintained in parallel at the level of simple cells in striate cortex.     

Chromatic sensitivity and spatial organization of cat visual cortical cells: cone-rod interaction

1. Colour sensitivity and spatial organization were determined for the dominant-eye receptive fields of thirty-eight simple or complex cells in cat primary visual vortex. Receptive fields were all from the cortical area associated with central vision. Each cell was investigated with threshold or suprathreshold monochromatic stimuli, under scotopic, low and high mesopic adaptation.2. The Purkinje shift, well defined for all units, was consistent with dual input from each of only two receptor mechanisms, viz. 556 nm cones and 500 nm rods. With change of adaptation level there was a systematic change in the peak sensitivity of spectral response curves to suprathreshold monochromatic stimuli, equated for quantum flux but of different wave-length. Equally with change of adaptation, the relative shift in threshold between wave-lengths selective for cone or rod activation was in close agreement with the change predicted from the Dartnall nomogram curves for visual pigments 556 and 507 respectively.3. For ganglion cells with concentric fields rod input derives from a spatially larger area than cone input. Rod field centre and rod field surround are substantially larger than the corresponding centre and surround for cones (Andrews & Hammond, 1970b). For cortical cells a conclusive comparable change could only be demonstrated for one simple unit. Its receptive field consisted of a horizontal excitatory stripe with asymmetric inhibitory flanks. When light-adapted the weaker, upper flank was functionally undetectable, indicative of purely rod input to this sideband, and the preference for upward movement was enhanced.4. No difference in receptive field configuration, or in spatial extent of input mediated by cones or by rods, was detected for any other unit. The discrepancy between retinal and cortical findings is discussed. It is inferred that cortical fields are compounded essentially by convergent input from geniculate cell field-centres.