Insights into the different exploits of colour in the visual cortex.

A new method that allows controlled masking of luminance contrast has been developed to study the use of chromatic signals in human vision. The method also makes it possible to examine the different uses of chromatic signals (e.g. the generation of perceived colour, or the construction and representation of object structure and form). By using this technique, we studied the threshold detection of chromatic signals in normal trichromats. The results show that chromatic signals are virtually unaffected by ongoing, randomly varying, luminance contrast changes. These findings suggest that chromatic signals are either processed independently or can be separated completely from any confounding luminance contrast components in the stimulus. Thresholds for detection of colour changes only, and for extraction of stimulus structure from chromatic signals in normal trichromats, in subjects with single cone receptor deficiency (i.e. dichromats) and in three subjects with abnormal colour vision caused by bilateral damage to ventromedial, extra-striate visual cortex (i.e. subjects with cerebral achromatopsia) have also been measured. No significant difference in thresholds for the two conditions was observed either in normal trichromats or in dichromats. Subjects with cerebral achromatopsia, however, reveal markedly different thresholds. The results suggest that chromatic signals are processed independently to generate perceived object colour or to construct spatially structured objects, and that these functions involve different neural substrates. The results help to explain, at least in part, why cerebral achromatopsia is a heterogeneous disorder, and why there can be significant differences in the effective use of chromatic signals in subjects described as cerebral achromatopsics.

Spatial and temporal response properties of residual vision in a case of hemianopia.

Residual vision in subjects with damage of the primary visual cortex (striate cortex) has been demonstrated in many previous studies and is taken to reflect the properties of known subcortical and extrastriate visual pathways. In this report we describe psychophysical experiments carried out on a subject clinically blind in half of his visual field (i.e. homonymous hemianopia) caused by striate cortex damage. They reveal the existence of two distinct channels mediating such vision. One channel responds to spatial structure and the other to light flux changes. The spatially tuned channel has a peak response at about 1.2 cycles per degree and shows rapid loss of sensitivity at both high and low spatial frequencies. This channel does not respond to diffuse illumination. The light flux channel, however, responds only to sudden increments in light flux levels on the retina and shows extensive spatial summation. Both channels require transient inputs, with a peak sensitivity at about 10 cycles per second and show virtually complete attenuation at temporal frequencies below 2 cycles per second. The spatiotemporal characteristics of these two channels account for much of the reported limits of visual performance attributed to subcortical or extrastriate pathways in some patients, and especially for their relatively good sensitivity for the detection of abrupt, transient stimuli or fast-moving targets. A new method is also applied to the measurement of the amount of light scatter in the eye. The measurements show that light scatter into the sighted hemifield could not account for the results obtained with the stimuli used to characterized the residual vision of this subject.

New method based on random luminance masking for measuring isochromatic zones using high resolution colour displays.

A new method of measuring normal hue discrimination ellipses and dichromatic zones using a high resolution colour monitor is described. The test involves the detection of chromatic bars on a grey background (x = 0.305, y = 0.323) having a luminance of 34 cd m-2. Elements of the background matrix of square checks are varied randomly in luminance in space and time to provide random luminance masking (RLM) which compensates for differences in the relative luminous efficiency of different observers. The measurement technique provides a rapid and comprehensive colour vision test. Typical results are presented for normal trichromats, protanopes and deuteranopes without RLM and with the RLM set of 25%. The size of the discrimination ellipse in normal observers is the same in both viewing conditions, but the use of the RLM technique reveals the extent of the isochromatic zones in colour deficient observers.

Pupillary responses to stimulus structure, colour and movement.

Pupillary responses to stimuli which favour the preferential stimulation of neural mechanisms involved in the detection of visual attributes such as colour, spatial structure, movement and light flux changes on the retina have been measured and compared. Pupil responses to a decrement in stimulus luminance (i.e., a flash of darkness), suggest that at least three components are involved in this response, their relative contribution being determined largely by stimulus size, contrast and presentation time. A comparison of pupil responses to gratings of equal and lower space-averaged luminance shows that the amplitude of pupillary constriction at grating onset for the equal luminance condition is about twice that measured with similar gratings in the lower luminance condition. Pupillary responses to chromatic isoluminant gratings are in general of longer latency when compared to responses of similar amplitude elicited by achromatic gratings. Small pupillary constrictions elicited by the onset of coherent movement in dynamic, random dot patterns are also demonstrated under stimulus conditions which eliminate pupillary responses to sudden light flux changes on the retina. The results support an earlier hypothesis which suggests that the onset of sudden changes in neural activity in the visual cortex when a visual stimulus is presented to the eye causes an overall perturbation which weakens transiently the regulatory inhibitory input to the pupillomotor nucleus. This, in turn, results in a transient increase in the efferent parasympathetic innervation of the iris sphincter muscle and hence the observed constriction of the pupil. The characteristics of the pupillary response reflect the properties of the mechanisms and the number of neurones which participate in the detection of each stimulus attribute.