P. Hammond

Differential responsiveness of simple and complex cells in cat striate cortex to visual texture

SummaryThe responsiveness of 254 simple and complex striate cortical cells to various forms of static and dynamic textured visual stimuli was studied in cats, lightly anaesthetised with N2O/O2 mixtures supplemented with pentobarbitone.Simple cells were unresponsive to all forms of visual noise presented alone, although about 70% showed a change in responsiveness to conventional bar stimuli when these were presented on moving, rather than stationary, static-noise backgrounds. Bar responses were depressed by background texture motion in a majority of cells (54%), but were actually enhanced in a few instances (16%).In contrast, all complex cells were to some extent responsive to bars of static visual noise moving over stationary backgrounds of similar texture, or to motion of a whole field of static noise. The optimal velocity for noise was generally lower than for bar stimuli.Since moving noise backgrounds were excitatory for complex cells, they tended to reduce specific responses to bar stimulation; in addition, directional bias could be modified by direction and velocity of background motion.Complex cells fell into two overlapping groups as regards their relative sensitivity to light or dark bars and visual noise. Extreme examples were insensitive to conventional bar or edge stimuli while responding briskly to moving noise.In many complex cells, the preferred directions for motion of noise and of an optimally oriented black/white bar were dissimilar.The ocular dominance and the degree of binocular facilitation of some complex cells differed for bar stimuli and visual texture.Preliminary evidence suggests that the deep-layer complex cells (those tolerant of misalignment of line elements; Hammond and MacKay, 1976) were most sensitive to visual noise. Superficial-layer complex cells (those preferring alignment) were less responsive to noise.Only ‘complex-type’ hypercomplex cells showed any response to visual noise.We conclude that, since simple cells are unresponsive to noise, they cannot provide the sole input to complex cells. The differences in the response of some complex cells to rectilinear and textured stimuli throw a new light on their rôle in cortical information-processing. In particular, it tells against the hypothesis that they act as a second stage in the abstraction of edge-orientation.

Orientation tuning of cells in areas 17 and 18 of the cat's visual cortex

SummarySharpness and symmetry of orientation tuning were quantitatively investigated and compared in ninety-seven cells from areas 17 and 18 of the lightly-anaesthetised feline visual cortex.Halfwidths of orientation tuning at half-height ranged between 5 ° and 73 ° for long stimuli, with an extreme exception at 111 ° (excluding untuned cells).There was a tendency for cells in area 18 to be more broadly tuned than those in area 17, due largely to the relatively sharp tuning of area 17 simple cells. Confirming previous work, simple cells were more sharply tuned than complex cells in area 17. In area 18, there was no clear distinction in sharpness of tuning between complex type 1 cells (equated with area 17 simple cells), complex type 2 cells (equated with area 17 complex cells), or hypercomplex cells.Approximately 60% of cells in both areas were asymmetrically tuned for orientation: ratios of half-widths to either side of the optimal orientation ranged from 1.0–3.0, exceptionally 5.8. Asymmetry of tuning was more marked in area 18 than in area 17, except that area 18 complex type 2 cells as a group were relatively symmetrically tuned for orientation.Occasional cells with different preferred orientations for opposite directions of motion, for each peak of a bimodal response to a single direction, or for each half of the receptive field were also observed. The latter are described in the following paper (Hammond and Andrews, 1978b).

Neural correlates of motion after-effects in cat striate cortical neurones: monocular adaptation

SummaryMotion after-effects were elicited from striate cortical cells in lightly-anaesthetized cats, by adapting with square-wave gratings or randomly textured fields drifting steadily and continuously in preferred or null directions. The time-course and recovery of responsiveness following adaptation were assessed with moving bars, gratings or textured fields. Results were compared with controls in which the adapting stimulus was replaced by a uniform field of identical mean luminance, and also assessed in relation to the strength and time course of adaptation. Within 30–60 s adaptation, firing declined to a steady-state. Induced after-effects were direction-specific, and manifest as a transitory depression in response to the direction of prior adaptation, recovering to control levels in 30–60 s. Maximal after effects were induced by gratings of optimal drift velocity and spatial frequency. With rare exceptions after-effects were restricted to driven activity; no consistent effects on resting discharge were observed. The onset of adaptation, and the recovery period, were more rapid in simple cells, although after effects of comparable strength were elicited from simple and from standard complex cells. Special complex cells, including many of the more profoundly texture-sensitive neurones in the cortex, were more resistant to adaptation. The results support the conclusion that psychophysically measured adaptation and induced motion after-effect phenomena reflect the known properties of cortical neurones.

Hemifield differences in perceived velocity.

Measurements of the perceived velocity of a drifting grating were obtained as a function of the position of the grating in the visual field. Identical drifting gratings were presented at the same eccentricity in the left, right, upper, and lower hemifields, and the perceived velocities were compared. A group of ten subjects considered together showed no significant hemifield differences in perceived velocity. However, some individual subjects showed marked and systematic hemifield differences, the directions of which varied among the subjects. There were no hemifield differences in susceptibility to adaptation to moving gratings.

On the sensitivity of complex cells in feline striate cortex to relative motion

SummaryResponses of superficial-layer, texturesensitive complex cells in cat striate cortex to relative motion between an oriented bar stimulus and its textured background were recorded. Some cells responded best to motion in one particular direction across the receptive field of the cell, irrespective of whether the bar and background moved simultaneously in the same (in-phase) or opposite (antiphase) directions. Others showed a clear preference for either in-phase or antiphase relative motion, irrespective of direction of motion across the receptive field.

Receptive field mechanisms of sustained and transient retinal ganglion cells in the cat

SummaryReceptive field organization of 135 sustained and 45 transient retinal ganglion cells was investigated in lightly pentobarbitone-anaesthetised cats. Stimuli were concentric annuli presented alone or simultaneously with a small spot centred on the receptive field, against photopic, mesopic or scotopic backgrounds.The addition of the test spot led to reduction in diameter of the centre-surround boundary of receptive fields of sustained retinal ganglion cells (assessed with annuli), and a decrease in diameter of the annulus which was most effective on the surround. In transient cells there was only marginal reduction in diameter of the centre-surround boundary, measured with annuli, and little or no decrease in diameter of the most effective annulus.Reducing background intensity from photopic to scotopic induced changes in response patterns and receptive field organization of sustained and transient retinal ganglion cells which were independent of stimulus intensity.Against photopic backgrounds, large annuli evoked surround-type responses from the majority of transient ganglion cells and from all sustained cells. In the scotopic range, surround-type responses could still be evoked from sustained cells, whereas predominantly centre-type responses were obtained throughout the receptive fields of transient cells.With transition from cone to rod vision, receptive field surrounds of sustained and transient cells became progressively less responsive than centres; in consequence the diameter of the centre-surround boundary increased. The initial, high frequency burst of impulses in discharges at annulus onset or offset became less evident and response latency increased substantially.The results are consistent with a model in which the centre and surround receptive field mechanisms are spatially co-extensive in transient retinal ganglion cells, albeit of different shape, but only partially overlapping in sustained retinal ganglion cells. It is suggested that the surround mechanism in sustained cells is spatially more extensive than the centre mechanism but does not extend entirely through the centre of the field.

Directional tuning interactions between moving oriented and textured stimuli in complex cells of feline striate cortex.

In sixty‐five complex cells recorded from striate cortex of lightly anaesthetized, paralysed cats we investigated directional selectivity for motion of oriented and textured stimuli, both alone and when moving simultaneously in the same direction and at the same velocity. Monocular comparisons were made over a range of velocities for the dominant eye in all cells, and for the other eye in fourteen instances. For oriented stimuli, response magnitude varied with velocity, but preferred directions(s) and sharpness of tuning remained constant. For background texture motion, directional selectivity was typically unimodal at low velocities, but became increasingly bimodal at high velocities: a trough of depressed response (in directions optimal for oriented stimuli) separated two progressively more widely disparate preferred directions. Preferred velocity and velocity bandpass were typically higher for texture than for bar motion. Directional tuning interactions revealed no important class‐ or layer‐specific differences and were similar for each monocular input. Results for bar and texture combinations moving in unison could not be predicted from selectivity for each stimulus alone. At all velocities they closely resembled those for bar motion alone. Tuning curves for the combination stimulus were only marginally broader than those for oriented stimuli: much sharper and totally different in profile from those for texture. It is concluded that an oriented stimulus in motion induces potent blockade of complex‐cell sensitivity to moving textured backgrounds. Complex cells insensitive to relative motion between objects and backgrounds (Hammond & Smith, 1982a, 1983b) may thus be excellent candidates for resolving motion of objects regardless of the context in which they are seen.

Influence of spatial frequency on tuning and bias for orientation and direction in the cat's striate cortex

Directionality, orientation and spatial frequency tuning were determined for 108 neurones recorded extracellularly from the striate cortex of anaesthetized cats. Significant sharpening of orientation selectivity with increasing spatial frequency was seen in all simple neurones and the overwhelming majority of complex neurones. Orientation selectivity sharpened in 90 and broadened in only 10 of 100 fully characterized neurones. At least four distinct classes of neurone could be characterized on the basis of their directionality at optimal spatial frequency, and the presence or absence of changes in directionality over a range of spatial frequencies: in two classes, directionality was spatial-frequency dependent; in the remaining two it was invariant. With two exceptions Type A neurones (23 cells) were direction-selective; they were narrowly tuned for orientation and spatial frequency, and their directionality was invariant with spatial-frequency. The majority of neurones (52 cells) were Type B, most of which were direction-biased; their bias for direction varied systematically with spatial frequency. Type C were direction-biased and spatial-frequency selective (5 cells), but showed a clear reversal of bias with change in spatial frequency. Type D, a subset of direction-biased cells, were bidirectional and spatial-frequency invariant (8 cells), with comparable response strengths to motion in two opposing directions at all spatial frequencies. These response types crossed traditional boundaries between categories of simple and complex neurones, assigned on the basis of spatial summation, presence or absence of end-inhibition, and receptive field size.

Motion after-effects in cat striate cortex elicited by moving texture

Responses of striate cortical neurons to randomly-textured test patterns (static visual noise), or to bars of optimal orientation and width, moving back-and-forth with fixed velocity, were recorded in the lightly anaesthetized cat. Effects of prior adaptation with textured patterns drifting continuously in each cells preferred or null directions, were assessed. Alterations of directional bias and responsiveness to the test stimulus were assessed in relation to the degree and time-course of texture adaptation. The effects of adaptation to moving texture were qualitatively similar to our previously published data on adaptation to drifting gratings in the same or similar cells. Responses to the test stimulus were transiently depressed in the direction of adaptation and enhanced following adaptation in the opposite direction, compared with responses following exposure to stationary texture. However, even in cells that were driven strongly by the adapting texture, i.e. particularly the special complex cells in cortical layers III and V, the after-effects were always weak in strength compared with those elicited by moving gratings. We conclude that, as a group, cortical cells strongly sensitive to texture motion are relatively unsusceptible to adaptation.

Neural motion after-effects in the cat's striate cortex: Orientation selectivity

Single striate cortical neurones were recorded from adult cats, lightly anaesthetized with N2O/O2/halothane. The receptive fields for the dominant eye were subjected to direction-specific adaptation by a square-wave grating of optimal spatial frequency and velocity, drifting continuously in each neurones preferred direction. Recovery of the neural motion after-effect induced by prior adaptation was assessed with the same grating pattern which now moved alternately in the preferred and opposite directions. In controls the same tests for recovery followed a period of exposure to a uniform field of identical luminance to the adapting grating. Three sets of measurements were made to establish whether the adaptation was orientation- as well as direction-specific. In the first, test grating orientation was maintained constant and optimal for each neurone whilst adapting orientation was systematically varied. In the second, test orientation was varied whilst maintaining adapting orientation constant. In the third set, adapting and test orientations were initially fixed at each neurones optimum; they were next set, non-optimally to one side of the optimum. Results from the latter configuration were compared with similar tests in which the test grating remained at that non-optimal orientation whilst the orientation of the adapting grating was now altered to a new point on the other flank of each neurones orientation tuning curve that was matched for strength of adaptation. Thus the degree of adaptation was identical in each case, but zero orientation difference between adapting and test gratings in one case was contrasted with a substantial orientation difference in the other. The results from all three sets of data were unequivocal: in simple neurones, and in standard and intermediate classes of complex neurones, but not in special complex neurones, the sequential effects of adapting gratings on the responses and sensitivity to subsequently presented test gratings were maximal when their orientations were matched and optimal for each neurone, less marked when orientations were matched but non-optimal. In conclusion, adaptation induced by pattern motion was orientation- as well as direction-specific only in standard (length summating) and intermediate complex neurones, and in simple cells; in special complex neurones it was not.