Library

Notes: Summary Directional tuning for visual noise, bar and single spot stimuli was compared over a wide range of velocities in cells from areas 17 and 18 of the visual cortex in lightly-anaesthetized cats. In each area, S-cells were predominantly insensitive to motion of a field of visual noise. C-cells were more sensitive to noise motion than B-cells, but showed heterogeneity in noise sensitivity, which was associated with other response properties: strongly noise-sensitive C-cells had relatively high spontaneous activity and broad directional tuning, and were predominantly direction-selective and binocularly-driven. Frequently, directional tuning for noise was unimodal at low velocity, but became progressively more bimodal as velocity was increased: a trough of depressed response corresponding to the peak in tuning for the bar separated two progressively more widely disparate preferred directions. In area 18, cells with velocity tuned (VT) functions for bar motion developed bimodal tuning for noise well below the optimum velocity for bar or for noise motion, while velocity high-pass (VHP) cells became progressively more bimodally tuned for noise over a wide range of velocities, in parallel with a steep increase in response to bar and noise motion. A high proportion of VT and VHP cells was bimodally tuned for noise at all velocities, one VHP cell showing two discrete lobes of tuning for noise below the threshold velocity for bar motion. Among cells which remained unimodally tuned for noise, VT and VHP cells in area 18 had radically dissimilar preferred directions for noise and bar motion at all velocities. With the exception of VHP cells, velocity bandpass was higher for noise than for bar motion. These results, together with other novel observations on the modality of tuning for noise in preferred and opposite directions of motion, demonstrate that bimodality of tuning for noise cannot simply be an effect of upper cut-off velocity for bar motion (Movshon et al. 1980; Orban 1984). It is argued that the trough between the lobes of tuning arises through laterally-directed inhibitory convergence from superficial- and deep-layer, large basket cells. In 40% of noise-sensitive cells, tuning for bar motion was broader on the flank closest to the preferred direction for noise and for a moving spot, while some 25% of cells showed variations in tuning for bar motion with velocity, which were associated with velocity-dependent changes in tuning for noise. Thus, the broad, asymmetrical tuning of these cell types for bar motion presumably reflects to some extent stimulation of the directional mechanism by the moving bar. Quantitative comparisons showed that C-cells in each area had similar tuning for bar motion, which was substantially broader than that of S- or B-cells. S-cells had the narrowest tuning, though those in area 18 were more broadly tuned than those in area 17.

Notes: Summary We investigated responses of neurones in cortical areas 17 and 18 and in the dorsal lateral geniculate nucleus (dLGN) of the cat to a phase shift in a moving line pattern forming a border without a luminance gradient (“subjective contour”). In both areas 17 and 18, S cells and B cells respond only slightly or not at all along the phase shift while C cells respond strongly. The response of C cells is strongest for line patterns with medium line separation and decreases with smaller and larger separation. In the dLGN the relative magnitude of neuronal responses along a phase shift is similar to that of C cells. However, C cells respond uniformly along the entire phase shift, whereas geniculate cells merely respond to individual line ends along the phase shift. In addition we compared responses along a phase shift and those to a luminance gradient formed by a dotted line whose dots were separated by the same distance as the line ends along the phase shift. S cells and B cells respond preferentially to dotted lines whereas C cells and geniculate cells respond equally well along both phase shifts and dotted lines. Possible explanations for these results in terms of receptive field structure and differences in inhibitory input to the cells are discussed. Differential neurone responses may account for the perceptual distinctness of the contours with and without luminance gradients.

Notes: Summary The modulatory influence of a synchronously moving visual noise background on responsiveness to an optimally-oriented moving bar stimulus was investigated in visual cortical area 18 of the lightly-anaesthetized cat. The bar and noise background were swept along the axis orthogonal to bar orientation, with the same phase, velocity and amplitude of motion. Cells which were insensitive to motion of visual noise per se or weakly responsive to individual ‘grains’ in the noise sample showed suppression of bar-evoked responses by simultaneous motion of the noise background. Percent suppression declined with increase in bar length, over a range which could exceed the maximum estimate of receptive field length. The decline in percent suppression was non-linear, becoming progressively flatter in slope as bar length was increased until an asymptotic value was reached; observations on end-stopped cells and on end-free cells with restricted length summation verified that percent suppression was related specifically to the length of the comparison bar and not to the strength of response it evoked. Percent suppression and the extent over which it declined with increase in bar length were comparable for preferred and opposite directions of bar motion even in cells with radically different length-response functions in the two directions, including end-stopped cells with direction-selective end-zones. In contrast to end-inhibition, which was maximal at or near the preferred velocity for a bar of optimal length, percent suppression by motion of the noise background was essentially velocity-invariant; in velocity tuned and velocity high-pass cells, background motion reduced the slope(s) of the velocity-response function, implying that the suppressive action of moving noise backgrounds is divisive rather than subtractive. It is argued that the suppression derives predominantly from an axo-somatic noise-sensitive inhibitory input from superficial- and deep-layer, large basket cells in orientation ‘columns’ at some distance from those of their target cells.

Notes: Summary We investigated the contributions of lateral intracortical connections to the orientation tuning of area 17 cells using micro-iontophoresis of the inhibitory transmitter gamma-aminobutyric acid (GABA) to inactivate small cortical sites in the vicinity of a recorded cell. GABA was ejected from an array of micropipettes each with an average horizontal distance of 500 μm from the recording site. Of 54 cells tested, 33 showed a reduction and 3 a loss of orientation selectivity due to an increase in responses to non-optimal orientations during GABA inactivation. The response to the optimal orientation remained constant in more than half of the cells and increased or decreased in others. Given that a complete cycle of orientations occupies a tangential distance of 1000 μm, the observed broadening of orientation tuning is presumably due to GABA-mediated inactivation of inhibitory interneurones with different preferred orientations from those of their target cell.

Notes: Summary The colour of an object is changed by surround colours so that the perceived colour is shifted in a direction complementary to the surround colour. To investigate the physiological mechanism underlying this phenomenon, we recorded from 260 neurons in the parvo-cellular lateral geniculate nucleus (P-LGN) of anaesthetized monkeys (Macaca fascicularis), and measured their responses to 1.0–2.0° diameter spots of equiluminant light of various spectral composition, centered over their receptive field (spectral response function, SRF). Five classes of colour opponent neurons and two groups of light inhibited cells were distinguished following the classification proposed by Creutzfeldt et al. (1979). In each cell we repeated the SRF measurement while an outer surround (inner diameter 5°, outer diameter 20°) was continuously illuminated with blue (452 nm) or red (664 nm) light of the same luminance as the center spots. The 1.0–1.5° gap between the center and the surround was illuminated with a dim white background light (0.5–1cd/m2). During blue surround illumination, neurons with an excitatory input from S-or M-cones (narrowand wide-band/short-wavelength sensitive cells, NSand WS-cells, respectively) showed a strong attenuation of responses to blue and green center spots, while their maintained discharge rate (MDR) increased. During red surround illumination the on-minus-off-responses of NS- and WS-cells showed a clear increment. L-cone excited WL-cells (wide-band/long-wavelength sensitive) showed a decrement of on-responses to red, yellow and green center spots during red surround illumination and, in the majority, also an increment of MDR. The response attenuation of narrow-band/long-wavelength sensitive (NL)-cellls was more variable, but their on-minus-off-responses were also clearly reduced in the average during red surrounds. Blue surround illumination affected WL-cell responses little and less consistently than those of NL-cells, but often broadened the SRF also in the WL-cells towards shorter wavelengths. The M-cone excited and S-cone suppressed WM-cells were strongly suppressed by blue but only little affected by red surround illumination. The changes of spectral responsiveness came out clearly in the group averages of the different cell classes, but snowed some variation between individual cells in each group. The zero-crossing wavelengths derived from on-minus-off-responses were also characteristically shifted towards wavelengths complementary to those of the surround. The direction of changes of spectral responsiveness of P-LGN-cells are thus consistent with psychophysical colour contrast and colour induction effects which imply that light of one spectral region in the surround reduces the contribution of light from that same spectral region in the (broad band or composite) object colour. Surrounds of any colour also decrease the brightness of a central coloured or achromatic light (darkness induction). We calculated the population response of P-LGN-units by summing the activity of all WS-, WM- and WL-cells and subtracting that of all NS- and NL-cells. The SRF of this population response closely resembled the spectral brightness function for equiluminous lights rather than the photopic luminosity function. With red or blue surrounds, this population SRF was lowered nearly parallel across the whole spectrum to about 0.7 of the amplitude of the control. In a psychophysical test on 4 observers we estimated the darkness induction of an equiluminous surround in a stimulus arrangement identical to the neurophysiological experiment, and found a brightness reduction for white, blue, green and red center stimuli to 0.5–0.7 of the brightness values without surround. This indicates that the neurophysiological results may be directly related to perception, and that P-LGN-cells not only signal for chroma but also for brightness, but in different combinations. The results indicate that both an additive (direct excitation or suppression of activity) and a multiplicative mechanism (change of gain control) must be involved in brightness and colour contrast perception. As mechanisms for the surround effects horizontal cell interactions appear not to be sufficient, and a direct adaptive effect on receptors feeding positive or negative (opponent) signals into the ganglion cells receptive fields by straylight from the surround must be seriously considered. This will be examined in the following companion paper. The results indicate that changes of spectral and brightness responses in a colour contrast situation sufficient to explain corresponding changes in perception are found already in geniculate neurons and their retinal afferents. This applies to mechanisms for colour constancy as well in as much as they are related to colour contrast.

Notes: Abstract Directional tuning for motion of a long bar and a spot was compared quantitatively over a wide range of velocities in 23 simple cells of cat striate cortex whose “on” and “off” receptive field subregions had been mapped with optimally oriented, stationary flash-presented bars. Tuning curves were derived using stimuli whose polarity of contrast was appropriate for the dominant receptive field subregion of each cell (i.e. light stimuli for on-subregions and dark stimuli for off-subregions); stimulus sweep was centred accurately on the centre of that subregion. Bar stimuli were of optimal width, and spot diameter was equal to the width of the bars. In all simple cells, preferred axis of motion for a long bar was invariant with velocity, being orthogonal to preferred orientation, as assessed with a stationary flash-presented bar. In 20 of 23 simple cells, preferred axis for spot motion was approximately orthogonal to that for bar motion (i.e., parallel to preferred orientation) at all velocities tested, including those just above threshold for spot stimuli. However, tuning for the spot became sharper as velocity was increased, due to an increase in response to the spot moving along the preferred axis and a decrease in response to spot motion along other axes, including the preferred axis for the bar. Both preferred and upper cut-off velocity were consistently higher for spot than for bar motion. The remaining 3 simple cells showed no response to spot motion at any velocity, and their preferred axis of motion for the shortest bar which evoked a consistent response was the same as that for a long bar. We conclude that simple cells respond to motion of a spot per se and not just to its oriented components, and that in most simple cells preferred axis for spot motion is genuinely approximately orthogonal to that for motion of a long bar. A spatio-temporal filter model incorporating intracortical feedforward facilitation along the long axis of the receptive field can account for the observed differences in axis preference and velocity sensitivity for spot and bar motion.