Responses to coloured patterns in the macaque lateral geniculate nucleus: pattern processing in single neurones

Visual pathways and psychophysical channels in the primate

The main cell systems of the retina that provide input to the striate cortex are now well described, although certain aspects of their anatomy and physiology remain contentious. Under simple stimulus conditions and in a threshold context psychophysical performance can often be assigned to one or other of these systems, and an identification of psychophysical channels with afferent pathways is justifiable. However, results from psychophysical studies using more complex stimulus conditions are more difficult to relate to 'front end' channels, and it is more difficult to separate the physiological contributions of afferent pathways from those of cortical mechanisms, in particular the separation of dorsal and ventral streams.

Neuronal responses to borders with and without luminance gradients in cat visual cortex and dorsal lateral geniculate nucleus

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.

Colour-spatial vision

A critical survey is made of neurophysiological and psychophysiological investigations of colour vision. A neuronal model of colour-spatial vision is suggested. The model allows a unified explanation of the whole range of psychophysiological phenomena: the mixing of colours of high-frequency image components, the McCollough type colour after-effects, the simultaneous and successive colour contrast, the hue constancy perception, the appearance of non-spectral colours by mixing of monochromatic lights. A suggestion is made as to the existence of two main mechanisms of colour vision. The first of these, by means of Fourier transforms, gives a set of coefficients which describes the spatial distribution of light (quantity of energy) and hue (quality of energy) in the visual field. The second mechanism establishes colour names in each chromatically homogenous area of the field described by the first mechanism. Both mechanisms cooperate on the basis of their common spatial organization.

Representation of spatial details in textured patterns by cells of the cat striate cortex

The ability of single cells to represent the spatial details of textured stimuli was investigated. Two complementary aspects of cell response were considered, the ability to discriminate fine stimulus details and the property of integration over wider areas of a structure to encode differences in mean luminance. Responses of simple and complex cells were distinct in some respects. Spatial discrimination: Simple cells would encode orientation of line arrays as long as individual line elements could be spatially resolved. By contrast, complex cells were able to distinguish the orientation of texture areas even when the individual lines of the stimulus were not resolved in their response. Threshold sensitivity for texture orientation was of the same order in both cell classes despite differences in receptive field size. Spatial integration: Complex cells responded to texture luminance differences of much coarser patterns than did simple cells. These responses, however, were not biased for contour orientation unless finer patterns were used. Only with very fine textures did responses become indistinguishable from those to uniform stimuli for both simple and complex cells. For complex cells, there was a smooth transition from resolution to fusion of spatial details with increasing structural density. Simple cells were insensitive to both detailed and global properties of a stimulus pattern over a wide range of texture density. Implications for alternative measures of visual acuity of single cells are discussed.

The neon color effect in the Ehrenstein pattern. Dependence on wavelength and illuminance

The neon color effect can be described as an illusory spread of color surrounding colored lines embedded in certain line gaps. The effect is seen in the Ehrenstein pattern if colored crosses are added to the central gaps so as to connect the inner tips of the pattern. Experiments were conducted to explore the dependence of this neon color effect on the wavelength and retinal illuminance of the inducing lines. The following results were obtained: neon color effects are strong when the wavelength of the crosses is in the short- (less than 480 nm) or long-wave part of the spectrum (greater than 620 nm) and the wavelength of the Ehrenstein pattern is in the middle-wave part (500-580 nm). Effects are weak or absent when the crosses and the pattern have similar wavelengths. The neon color effect is just detectable when the ratio between the retinal illuminances of the Ehrenstein pattern and the crosses ranges from 0.1 to 0.8. The neon color effect is maximal at illuminance ratios ranging from 0.8 to 8. The strength of the neon color effect is independent of the illuminance level of the crosses if the illuminance ratio to the Ehrenstein pattern is maintained.

Responses to coloured patterns in the macaque lateral geniculate nucleus: analysis of receptive field properties

Response patterns to complex visual stimuli were further analysed. Patterns were correlated with linear or non-linear components of the stimulus pattern at various wavelengths. Resulting correlograms revealed the spatial and spectral structure of receptive fields; they showed peaks or troughs according to whether that wavelength was associated with an increase or decrease in cell firing. Spectral response curves as derived from linear correlograms were similar to those reported for monochromatic stimuli. Variability in responsiveness and crossover wavelengths was high between parvocellular layer (PCL) cells even of the same class. Spatial differences between excitatory and suppressive receptive field components, i.e. a centre-surround organisation, are not apparent in linear correlograms from PCL cells. In this respect, spectral response curves do not qualitatively change with stimulus size. Correlation in time and the derivation of impulse functions showed that, even in magnocellular layer (MCL) cells, responses to luminance steps are of mainly temporal origin and due to a transient component in the response. A description of cell responses based on linear processing accounted well for the response patterns obtained in our experiments. Of various non-linear interactions investigated, only some kind of non-linear spectral differentiation provided an improvement in the description of cell responses. This improvement, however, was only minor and not present in all cells.