The time-course of colour vision

Four experiments are presented, each investigating temporal properties of colour vision processing in human observers. The first experiment replicates and extends an experiment by Stromeyer et al. (1991). We look for a phase difference between combined temporal modulations in orthogonal directions in colour space, which might null the often-claimed latency of signals originating from the short-wavelength sensitive cones (S-cones). We provide another estimate of the magnitude of this latency, and give evidence to suggest that it originates early in the chromatic pathway, before signals from S-cones are combined with those that receive opposed L- and M-cone input. In the second experiment we adapt observers to two stimuli that are matched in the mean and amplitude of modulation they offer to the cone classes and to the cardinal opponent mechanisms, but that differ in chromatic appearance, and hence their modulation of later colour mechanisms. Chromatic discrimination thresholds after adaptation to these two stimuli differ along intermediate directions in colour space, and we argue that these differences reveal the adaptation response of central colour mechanisms. In the third experiment we demonstrate similar adaptation using the same stimuli, measured with reaction times rather than thresholds. In the final experiment, we measure the degree to which colour constancy is achieved as a function of time in a simulated stimulus environment in which the illuminant changes periodically. We find that perfect constancy is not achieved instantaneously after an illuminant chromaticity shift and that constancy of colour appearance judgements increases over several seconds

Steady-State Visual Evoked Potentials Elicited from Early Visual Cortex Reflect Both Perceptual Color Space and Cone-Opponent Mechanisms

Funding Japan Society for the Promotion of Science (JSPS KAKENHI grant number JP18K13365 to S.K., JP18H04995 to I.K.) Notes S.K. was also supported by the grant from Building of Consortia for the Development of Human Resources in Science and Technology program by Japan Science and Technology Agency. S.K.A. is very grateful to Satoshi Shioiri for inviting him to Tohoku University, which enabled this collaboration. Part of this study has appeared in the form of conference proceedings (Kaneko et al. 2018). Data and codes used in this study are available at https://osf.io/m47df/.Peer reviewedPublisher PD

The science of color and color vision

A survey of color science and color vision

Chromatic sensitivity: effects of light level and selective photoreceptor adaptation

When the light reflected from an object differs in spectral composition to the surrounding background these spectral differences are reflected in the excitation levels produced in each class of photoreceptor. The ability to see colours and to notice small colour differences is strongly affected by both the spectral composition and luminance level of the adapting light. Knowledge of the limits of colour detection is important in setting safety standards and guidelines in visually demanding workplaces, as varying conditions of illumination and chromatic adaptation are often encountered in different working environments. It is therefore of both fundamental and practical interest to be able to predict accurately how a human observer’s chromatic detection performance changes with both light level and chromatic adaptation. The Colour Assessment and Diagnosis (CAD) test was employed to measure colour detection threshold ellipses under different states of chromatic adaptation and background light levels. The advantage of this new technique is that it isolates the use of colour signals by embedding the isoluminant chromatic stimulus in dynamic luminance noise. These measured threshold variations were analysed in terms of changes in L-, Mand S-cone excitation levels required for threshold in different colour directions. Models based on the measured chromatic threshold data are proposed that are capable of reconstructing entire detection ellipses. These models were based on experiments where observers had colour thresholds measured around a series of different chromatic adaptation points, over a range of light levels (typically from 0.3 to 31 cd m-2), and additionally away from the adaptation point. The findings reveal the independent adaptation states of individual cone classes on measured thresholds, i.e., the threshold in a given cone class depends only on the signal produced by the background in that cone class and is independent of the adaptation state of the other cone classes and hence independent of chromaticity and light level. The effect of adapting different areas of the peripheral retina when thresholds are measured foveally was also investigated. No long range retinal interactions were observed. The results show that the adaptation state of the periphery has no effect on colour detection thresholds made in central vision. Variations in L-, M- and S-cone contrasts curves were simulated to assess the influence that detection ellipse size and ellipse orientation have on them. This revealed a correlation between the L-cone contrast curve gradient and the corresponding ellipse orientation. This was additionally shown to correlate with the central 2.8° mean value of macular pigment optical density, hence providing a new method of estimating macular pigment level from colour detection ellipses. Steady state pupil sizes were analysed with the rod and cone excitations that produced them. These data indicate that when chromatically adapted, the steady state pupil size correlates strongest with the S-cone signal, and is independent of the actual chromaticity and luminance levels involved. Cone signal-to-noise ratios were extracted from repeated threshold measurements for a series of colour directions. Analysis of these revealed the existence of a constant signal-to-noise ratios over the full range of colour directions tested relative to a whitish background. The results show that as the cone contrast level increases in a particular cone class so does the associated noise

Cortical summation and attentional modulation of combined chromatic and luminance signals

The work was supported by BBSRC new investigator grant BB/H019731/1 to JM. We would like to thank Justyna Mordal and Zarko Milojevic for their help with data collection.Peer reviewedPostprin

BOLD human responses to chromatic spatial features

Animal physiological and human psychophysical studies suggest that an early step in visual processing involves the detection and identification of features such as lines and edges, by neural mechanisms with even- and odd-symmetric receptive fields. Functional imaging studies also demonstrate mechanisms with even- and odd-receptive fields in early visual areas, in response to luminance-modulated stimuli. In this study we measured fMRI BOLD responses to 2-D stimuli composed of only even or only odd symmetric features, and to an amplitude-matched random noise control, modulated in red-green equiluminant colour contrast. All these stimuli had identical power but different phase spectra, either highly congruent (even or odd symmetry stimuli) or random (noise). At equiluminance, V1 BOLD activity showed no preference between congruent- and random-phase stimuli, as well as no preference between even and odd symmetric stimuli. Areas higher in the visual hierarchy, both along the dorsal pathway (caudal part of the intraparietal sulcus, dorsal LO and V3A) and the ventral pathway (V4), responded preferentially to odd symmetry over even symmetry stimuli, and to congruent over random phase stimuli. Interestingly, V1 showed an equal increase in BOLD activity at each alternation between stimuli of different symmetry, suggesting the existence of specialised mechanisms for the detection of edges and lines such as even- and odd-chromatic receptive fields. Overall the results indicate a high selectivity of colour-selective neurons to spatial phase along both the dorsal and the ventral pathways in humans

Sensitivity to modulation of color distribution in multicolored textures

AbstractWe evaluated the discriminability of color distributions in square-element textures. Each texture contained 225 colors, represented by a distribution of color vectors in color space defined by the L–M and S–(L+M) axes. Each color distribution was systematically manipulated by modulating the distribution of the vector lengths sinusoidally as a function of the direction in the color space. The results showed that it is difficult to resolve a color distribution modulated in more than three cycles per 360° in the chromatic direction. The difference in components along the cardinal axes is not a critical factor in the discrimination of the color distribution. An analysis using a line-element model suggested that the discrimination of the color distribution is mediated by multiple chromatic channels that are tuned to a variety of directions in the color space with a half-height-half-bandwidth of about 40° in the chromatic direction

Neural differences between chromatic- and luminance-driven attentional salience in visual search

Acknowledgements JM was supported through a DAAD Reinvitation program. AH was supported by an Elphinstone PhD Scholarship from the University of Aberdeen. Open-source data: All data used are publicly available to view on OSF at: https://osf.io/jzc3m/?view_only =c841a07dcac549c2bc2828ee753a4b19.Peer reviewedPublisher PD

The Constructive Nature of Color Vision and Its Neural Basis

Our visual world is made up of colored surfaces. The color of a surface is physically determined by its reflectance, i.e., how much energy it reflects as a function of wavelength. Reflected light, however, provides only ambiguous information about the color of a surface as it depends on the spectral properties of both the surface and the illumination. Despite the confounding effects of illumination on the reflected light, the visual system is remarkably good at inferring the reflectance of a surface, enabling observers to perceive surface colors as stable across illumination changes. This capacity of the visual system is called color constancy and it highlights that color vision is a constructive process. The research presented here investigates the neural basis of some of the most relevant aspects of the constructive nature of human color vision using machine learning algorithms and functional neuroimaging. The experiments demonstrate that color-related prior knowledge influences neural signals already in the earliest area of visual processing in the cortex, area V1, whereas in object imagery, perceived color shared neural representations with the color of the imagined objects in human V4. A direct test for illumination-invariant surface color representation showed that neural coding in V1 as well as a region anterior to human V4 was robust against illumination changes. In sum, the present research shows how different aspects of the constructive nature of color vision can be mapped to different regions in the ventral visual pathway