Chromatic fading following complete adaptation to unique hues.

Profound vision loss occurs after prolonged exposure to an unchanging featureless visual environment. The effect is sometimes called visual fade. Here we investigate this phenomenon in the color domain using two different experiments. In the first experiment we determine the time needed for a colored background to appear achromatic. Four backgrounds were tested. Each represented the observers' four unique hues. This adaptation time was compared with time to recover after adaptation Hue shifts at the end of the adaptation period were also measured. There were wide individual differences in adaptation times and recovery times. Overall recovery was faster than adaptation (p < 0.02). There were minimal shifts in hue. In the second experiment the changes in saturation (Munsell chroma) and lightness (Munsell value) of the background were monitored at six time intervals during the adapting process. Again asymmetric matching with Munsell samples was used. There were two distinct components to both the adaptation and recovery phases; one fast with time constant <1s, the other slow with time constant between 40 and 160s. The experiments show that the special case of visual fade involving color represents the sensory basis for many color-related effects involving adaptation.

Neural Model of Coding Stimulus Orientation and Adaptation.

The coding of line orientation in the visual system has been investigated extensively. During the prolonged viewing of a stimulus, the perceived orientation continuously changes (normalization effect). Also, the orientation of the adapting stimulus and the background stimuli influence the perceived orientation of the subsequently displayed stimulus: tilt after-effect (TAE) or tilt illusion (TI). The neural mechanisms of these effects are not fully understood. The proposed model includes many local analyzers, each consisting of two sets of neurons. The first set has two independent cardinal detectors (CDs), whose responses depend on stimulus orientation. The second set has many orientation detectors (OD) tuned to different orientations of the stimulus. The ODs sum up the responses of the two CDs with respective weightings and output a preferred orientation depending on the ratio of CD responses. It is suggested that during prolonged viewing, the responses of the CDs decrease: the greater the excitation of the detector, the more rapid the decrease in its response. Thereby, the ratio of CD responses changes during the adaptation, causing the normalization effect and the TAE. The CDs of the different local analyzers laterally inhibit each other and cause the TI. We show that the properties of this model are consistent with both psychophysical and neurophysiological findings related to the properties of orientation perception, and we investigate how these mechanisms can affect the orientation's sensitivity.

Fast cyclic stimulus flashing modulates perception of bi-stable figure.

Many experiments have demonstrated that the rhythms in the brain influence the initial perceptual information processing. We investigated whether the alternation rate of the perception of a Necker cube depends on the frequency and duration of a flashing Necker cube. We hypothesize that synchronization between the external rhythm of a flashing stimulus and the internal rhythm of neuronal processing should change the alternation rate of a Necker cube. Knowing how a flickering stimulus with a given frequency and duration affects the alternation rate of bistable perception, we could estimate the frequency of the internal neuronal processing. Our results show that the perception time of the dominant stimulus depends on the frequency or duration of the flashing stimuli. The duration of the stimuli, at which the duration of the perceived image was maximal, was repeated periodically at 4 ms intervals. We suppose that such results could be explained by the existence of an internal rhythm of 125 cycles/s for bistable visual perception. We can also suppose that it is not the stimulus duration but the precise timing of the moments of switching on of external stimuli to match the internal stimuli which explains our experimental results. Similarity between the effects of flashing frequency on alternation rate of stimuli perception in present and previously performed experiment on binocular rivalry support the existence of a common mechanism for binocular rivalry and monocular perception of ambiguous figures.

Chips in the sunshine: color constancy with real versus simulated Munsell chips under illuminants adjacent to the daylight locus.

Accurate color judgments rely on a powerful cognitive component. Here we compare the performance of color constancy under real and simulated conditions. Shifts in the u'v' color plane induced by illuminant A (2750 K) and illuminant S (>20,000 K) were measured using asymmetric color matching. A general linear model was used to predict performance from the following dependent variables: chroma ("4" and "6"), illuminant ("A" and "S"), presentation mode ("Real" and "Monitor"), and hue zone ("blue," "green," "yellow," "red," and "purple"). There was a strong overall effect [F(7,264)=78.65, p<0.001]. Post hoc analysis showed that performance was substantially superior under real [chromatic constancy index (cCI)=0.76] compared with simulated cCI=0.55) conditions.

Objective assessment of chromatic and achromatic pattern adaptation reveals the temporal response properties of different visual pathways.

The aim was to investigate the temporal response properties of magnocellular, parvocellular, and koniocellular visual pathways using increment/decrement changes in contrast to elicit visual evoked potentials (VEPs). Static achromatic and isoluminant chromatic gratings were generated on a monitor. Chromatic gratings were modulated along red/green (R/G) or subject-specific tritanopic confusion axes, established using a minimum distinct border criterion. Isoluminance was determined using minimum flicker photometry. Achromatic and chromatic VEPs were recorded to contrast increments and decrements of 0.1 or 0.2 superimposed on the static gratings (masking contrast 0-0.6). Achromatic increment/decrement changes in contrast evoked a percept of apparent motion when the spatial frequency was low; VEPs to such stimuli were positive in polarity and largely unaffected by high levels of static contrast, consistent with transient response mechanisms. VEPs to finer achromatic gratings showed marked attenuation as static contrast was increased. Chromatic VEPs to R/G or tritan chromatic contrast increments were of negative polarity and showed progressive attenuation as static contrast was increased, in keeping with increasing desensitization of the sustained responses of the color-opponent visual pathways. Chromatic contrast decrement VEPs were of positive polarity and less sensitive to pattern adaptation. The relative contribution of sustained/transient mechanisms to achromatic processing is spatial frequency dependent. Chromatic contrast increment VEPs reflect the sustained temporal response properties of parvocellular and koniocellular pathways. Cortical VEPs can provide an objective measure of pattern adaptation and can be used to probe the temporal response characteristics of different visual pathways.

Phases of daylight and the stability of color perception in the near peripheral human retina.

Typical daylight extends from blue (morning sky) to orangey red (evening sky) and is represented mathematically as the Daylight Locus in color space. In this study, we investigate the impact of this daylight variation on human color vision. Thirty-eight color normal human observers performed an asymmetric color match in the near peripheral visual field. Unique hues were identified using a naming paradigm. The observers' performance for matching was almost perfectly coincident with the Daylight Locus but declined markedly in other regions. Interobserver variability reached a conspicuous minimum adjacent to the Daylight Locus and was maximal in the red and yellowish-green regions. In the naming task, unique blue and yellow were virtually coincident with the Daylight Locus. The results suggest that the mechanisms of color perception mediated by the phylogenetically older (blue-yellow) color pathway have been strongly influenced by the different phases of daylight.

The relationship between peripherally matched invariant hues and unique hues: a cone-contrast approach.

A characteristic shift in hue and saturation occurs when colored targets are viewed peripherally compared with centrally. Four hues, one in each of the red, blue, green, and yellow regions of color space, remain unchanged when presented in the peripheral field. Apart from green, these peripherally invariant hues correspond almost exactly in color space with the unique hues. We explore this puzzling observation using asymmetric color-matching and color-naming experiments and computing cone contrasts for peripheral and central stimuli. We find that the difference between cone contrasts for the peripheral and central stimuli reaches a maximum at the chromatic axis corresponding to peripherally invariant green. We speculate that the effect is linked to a weakened signal from M-cones and probably associated with a reduced number of M-cones in peripheral retina.

Systematic violations of von Kries rule reveal its limitations for explaining color and lightness constancy.

Cone contrast remains constant, when the same object/background is seen under different illuminations-the von Kries rule [Shevell, Vis. Res. 18, 1649 (1978)]. Here we explore this idea using asymmetric color matching. We find that von Kries adaptation holds, regardless of whether chromatic constancy index is low or high. When illumination changes the stimulus luminance (reflectance), lightness constancy is weak and matching is dictated by object/background luminance contrast. When this contrast is masked or disrupted, lightness constancy mechanisms are more prominent. Thus von Kries adaptation is incompatible with lightness constancy, suggesting that cortical mechanisms must underlie color constancy, as expected from neurophysiological studies [Zeki, Nature 284, 412 (1980); Wild, Nature 313, 133 (1985)].

Bar-like S-cone stimuli reveal the importance of an intermediate temporal filter.

The relative involvement of different temporal frequency-selective filters underlying detection of chromatic stimuli was studied. Diverse spectral stimuli were used, namely flashed blue and yellow light spots, wide bars, and narrow bars. The stimuli were temporally modulated in luminance having constant wavelength. Although the bar-like stimuli apparently reduced the sensitivity at short and long wavelengths, the cone-opponent mechanism still remained responsible for the actual stimulus detection at different temporal frequencies. The bar-like stimuli increased sensitivity for temporal frequencies around 3-6 Hz, revealing involvement of an intermediate temporal frequency-selective filter in detection, the so-called transient-1 filter. A probability summation model for the method of adjustment was developed that assumes that detection depends on the properties of the temporal filters underlying the temporal frequency-sensitivity curve. The model supports the notion that at least two temporal frequency-selective filters are necessary to account for the shape of the sensitivity curves obtained for blue bar-like stimuli.

Influences of prolonged viewing of tilted lines on perceived line orientation: the normalization and tilt after-effect.

Gibson [J. Exp. Psychol. 16, 1 (1993)] observed that during prolonged viewing, a line perceptually rotates toward the nearest vertical or horizontal meridian (the normalization effect), and moreover, the perceived orientation of a subsequently presented line depends on the orientation of the adapting one (the tilt after-effect). The mechanisms of both phenomena remain poorly understood. According to our experimental results, the adapting line perceptually rotates to the nearest of three orientations: vertical, horizontal, and diagonal. We propose a simple neuronal model of orientation detectors whose responses are determined by the cardinal detectors. It is shown that both normalization and tilt after-effect may be explained by adaptation of these cardinal detectors.

Perceptual stability--going with the flow.

Historically, inflow and outflow hypotheses have been formulated as the primary explanations for perceptual stability. Central to these hypotheses is the postulation that, following an intended eye movement, knowledge of eye position cancels the consequences of the retinal image motion. Here, we reconsider the evidence for the extra-retinal signal and discuss whether this cancellation approach is compatible with the available empirical evidence. In particular, we propose that visual-oculomotor processing is a distributed process and that population-coding models of sensorimotor transformations are critical elements that need to be incorporated in any comprehensive explanation of spatial constancy.

Colour matching of isoluminant samples and backgrounds: a dimming effect.

Sequential asymmetrical colour matching of forty Munsell samples simulated under illuminant C and one of eight test illuminants was carried out. The subjects matched the appearance of each sample under illuminant C with its appearance under the test illuminant. Samples and background (N7) were presented for 1 s under the test illuminant and were isoluminant with each other. Subjects adjusted hue, chroma, and value under illuminant C. The experiments distinguished two groups of subjects; some observers needed to reduce the luminance of the sample to make a match while others did not. This 'dimming' occurred when the matches were close to cardinal axes, especially the tritanopic confusion line. A model of luminance and cone-opponent mechanisms contributing to brightness can account for the dimming effect. Details of analysis in cone-opponent space (L - M, L + M - S, L + M) are presented in the companion paper (Stanikunas et al, 2005 Perception 34 this issue).

Colour matching of isoluminant samples and backgrounds: a model.

A cone-opponent-based vector model is used to derive the activity in the red-green, yellow-blue, and achromatic channels during a sequential asymmetric colour-matching experiment. Forty Munsell samples, simulated under illuminant C, were matched with their appearance under eight test illuminants. The test samples and backgrounds were photometrically isoluminant with each other. According to the model, the orthogonality of the channels is revealed when test illuminants lie along either red-green or yellow blue cardinal axes. The red green and yellow-blue outputs of the channels are described in terms of the hue of the sample. The fact that the three-channel model explains the data in a colour-matching experiment indicates that an early form of colour processing is mediated at a site where the three channels converge, probably the input layer of V1.

Investigation of color constancy with a neural network.

The perceptual stability of an object's color under different illuminants is called color constancy. We created a neural network to investigate this phenomenon. The net consisted of one input channel for the background and one for the test object. Each channel had a set of three (L, M, and S) receptors that were transmitting to three opponent neurons. The signals from the opponent neurons were passed to hidden neurons, which were connected to the output neurons. The output signal was generated from the three components of a color vector. The neural net was trained to identify the color of Munsell samples under various illuminants using the back-propagation algorithm. Our study investigated the properties of a successfully trained neural network. Based on the cross-neuron weight analysis, we report that the successfully trained neural net calculates color differences between the test object and the background. By comparing the human visual system to the neural net, we conclude that to satisfy the color constancy phenomenon, the human visual system has to contain two separate components: one to approximate the background color and the other to estimate the color difference between the object and the background.