Biomedical subjects

G Syrkin

Publications and source records attributed to G Syrkin.

4 recordsLinked to original sources

PubMed1997

Colour and luminance interact to improve pattern recognition.

The authors have previously hypothesised that colour vision has evolved not only to encode colour per se but also, perhaps principally, to enhance luminance-based visual processing so that for colour information to be fully effective, luminance as well as chromatic variations should be present in visual targets. Results of previous experiments, testing detection of spatial gratings and detection and perceived brightness of Mach bands support the hypothesis. Further experiments are reported in which the hypothesis was tested by using a higher-level task of pattern recognition. Subjects had to discriminate between luminance (isochromatic), isoluminant (chromatic), or combined colour/luminance ellipses and circles. It was found that the ability to discriminate between a circle and an ellipse was greatly enhanced when both colour and luminance variations were present as compared with the pure luminance or colour presentations. Summation-square analysis shows linear colour-luminance summation which can be modeled by a single-analyser model.

Adolescent↗

PubMed1996

Nonlinear responses of simple cells to Mach band stimuli: evidence from early monocularly deprived cats.

We have previously shown that, cat simple cells respond linearly to edges of variable blur widths: cells with receptive fields (RFs) of even symmetry respond better to a luminance ramp (where Mach bands are observed); cells with RFs of odd symmetry respond better to a luminance step (where no Mach bands are perceived). Our evidence has also indicated the existence of inhibitory interaction between cells with RFs of even and odd symmetry as predicted by the Tolhurst-Ratliff Mach band model. Since monocular deprivation is known to impair cortical inhibitory mechanisms, we studied the responses of simple cells of adult cats monocularly deprived at the age of 8-10 weeks to Mach band stimuli in order to delineate specific changes in inhibitory interactions caused by monocular deprivation. In pattern-deprived cats, particularly for cells driven by the deprived eye, there were many cells that responded contrary to linear models: odd-symmetric cells responded maximally to blurred edges while even-symmetric cells responded maximally to sharp edges. Cells that responded maximally as predicted, responded, similarly to normal cat cells, less than expected at suboptimal widths. All cells in normal and light-deprived cats responded in a linear fashion to sinusoidal stimuli. We conclude, therefore, that intracortical inhibition shapes simple cells' responses to edges. Monocular deprivation impairs this mechanism, thus causing simple cells in monocularly deprived cats to respond nonlinearly to edges. All simple cells responded linearly to gratings since it is not the linear spatiotemporal RF of these simple cells that was impaired under monocular deprivation.

Animals↗

PubMed1994

Simple cells may lie at the basis of mach bands: evidence from physiological studies in the cat's visual cortex.

Mach bands are a visual illusion evoked by a luminance ramp dividing two luminance plateaux (blurred edges), but not by sharp edges. Recently, two physiology-based models have tried to cope with the psychophysical data concerning this phenomenon. The basic components of both models are neurons with even- or odd-symmetric receptive fields (RFs). Both models predict that odd-symmetric cells respond better to sharp edges, while even-symmetric cells respond better to blurred ones. We have measured the responses of 34 primary visual cortex simple cells of the cat to blurred edges of various degrees. Twenty-one cells had RFs of even symmetry, responding best to blurred edges than to sharp ones. The rest were odd-symmetric cells, of which 12 responded best to sharp edges, and only one exceptional cell responded best to a 0.85 degrees-wide edge. Thus, the different cell types responded as predicted by the two different Mach band models. Simple cells may thus serve as the physiological basis of the psychophysical phenomenon of Mach bands. Furthermore, our evidence suggests the existence of inhibition between odd- and even-symmetric cells, as predicted by one of the models.

Animals↗

PubMed1993

Color enhances Mach bands detection threshold and perceived brightness.

We have previously argued that unless color and luminance are shown to be processed independently, using isoluminant stimuli may not reveal the full contribution of color to visual functioning. Here we study the interaction of color and luminance in a task, Mach bands detection and perceived brightness, where color by itself is not effective at all. Subjects viewed luminance or color/luminance ramps and had to determine in either case the luminance contrast necessary for detecting Mach bands and, in another experiment, to compare the brightness of the bands in the luminance and in the combined displays. Isoluminant color displays did not generate any Mach bands, but the addition of color to the luminance display lowered Mach bands detection thresholds and enhanced their perceived brightness. It is thus concluded that the failure to perceive Mach bands in an isoluminant display is not indicative of the lack of color contribution to spatial vision but rather indicates that the strong effect that color has on contrast enhancement mechanisms can be revealed if color and luminance are allowed to interact.

Color Perception↗