Biology subjects

Rozman, A.

Publications and source records attributed to Rozman, A..

3 recordsLinked to original sources

bioRxiv2025

Colour desaturation in the periphery is explained by general mechanisms of contrast sensitivity and constancy

Colors appear less saturated in the visual periphery than in the fovea. We revisit this well-known phenomenon by characterizing parafoveal perceived contrast as a function of size. Observers (n = 20) matched perceived contrast of a parafoveally presented comparison disc (2{degrees} -0.33{degrees}) to a standard 2{degrees} disc. For chromatic stimuli, desaturation increased with decreasing size. Unexpectedly, a similar amount of desaturation occurred for luminance-defined discs, once their perceived contrast was adjusted to match the standard chromatic discs. Desaturation was reduced as standard stimulus contrast increased, in line with contrast constancy theory, which predicts constant appearance for stimuli that are sufficiently distant from threshold. Since chromatic contrast sensitivity is reduced away from the fovea, contrast constancy is unachievable within the monitor gamut. In conclusion and somewhat counter-intuitively, the appearance of color and luminance in the periphery is affected similarly, governed by general laws of contrast sensitivity and constancy. Public Significance StatementIn the early stages of visual signal processing, color and luminance (achromatic) information are separated. If we overgeneralize on the basis of such early separation, we may conclude that the desaturated appearance of small and peripherally presented color stimuli is specific to color processing mechanisms. Here we show that this is not the case and that chromatic and achromatic stimuli are processed similarly when presented in the periphery, implying that such processing is governed by more general contrast-processing mechanisms. Our work provides the basis for a better understanding of how the visual system builds a unified experience of color appearance across chromatic and achromatic dimensions from the information on contrast present in the image.

neuroscience↗

bioRxiv2024

CHARACTERISING REPRESENTATIONS OF HUE AND SATURATION IN THE CORTEX USING INFORMATION DECODING

One of the enduring questions in the field of colour vision research revolves around how features of colour appearance, such as hue and saturation are represented in the brain. While considerable progress has been made in understanding the transformation of physical colour signals during early processing stages, the mechanisms by which these signals are recombined in later processing, ultimately giving rise to our perceptual experience of colour, remain elusive. A promising avenue for capturing these representations involves decoding from EEG signals. We captured EEG signals in response to 8 evenly spaced isoluminant colours at two saturation levels, taken from a perceptually uniform CIE Lab colour space. Surprisingly, our main finding challenges the expectation that representations of colours that are perceptually more similar elicit more similar signals. Instead, our results show that isoluminant hues perceived as maximally dissimilar tend to evoke the most similar EEG signals, which is in line with predictions derived from opponent theory. Additionally, decoding performs better for lower saturation than for higher saturation hues, contrary to expectations that more distinct hue representations would be associated with highly saturated hues but in line with prediction of magnified differences in latency and amplitude of EEG signals between low-level opponent mechanisms at lower contrast levels. Finally, our findings also highlight the non- uniformity of the cortical representation space for isoluminant colour, indicating that neighbouring hues exhibit varying degrees of similarity. Put together, the results show that low-level features, such as contrast and cone-opponency, drive the EEG response to colour.

neuroscience↗

bioRxiv2023

Tuning of cortical color mechanism revealed using steady-state visually evoked potentials

Color information is thought to be received by the primary visual cortex via two dominant retinogeniculate pathways, one signals color variation between teal and red, and the other signals color variation between violet and lime. This representation is thought to be transformed in the cortex so that there are a number of different cell populations representing a greater variety of hues. However, the properties of cortical color mechanisms are not well understood. In four experiments, we characterized the tuning functions of cortical color mechanisms by measuring the intermodulation of steady-state visually evoked potentials (SSVEPs). Stimuli were isoluminant chromatic checkerboards where odd and even checks flickered at different frequencies. As hue dissimilarity between the odd and even checks increased, the amplitude of an intermodulation component (I1) at the sum of the two stimulus frequencies decreased, revealing cortical color tuning functions. In Experiment 1 we found similar broad tuning functions for cardinal and intermediate color axes, implying that the cortex has intermediately tuned color mechanisms. In Experiment 2 we found similar broad tuning functions for checkerboards with no perceptible edges because the checks were formed from single pixels ([~]0.096{degrees}), implying that the underlying neural populations do not rely on spatial chromatic edges. In Experiment 3 we manipulated check size and found that color tuning functions were consistent across check sizes used. In Experiment 4 we measured full 360{degrees} tuning functions for a cardinal cortical color mechanism and found evidence for opponent color responses. The observed cortical color tuning functions were consistent with those measured using psychophysics and electrophysiology, implying that tracking intermodulation using SSVEPs provides a useful method for measuring them.

neuroscience↗