Brian H.-P. Tsou

Orthogonal combination of the three visual channels

Abstract The responses of three cone types are differenced and summed by receptive fields reported in electrophysiological studies to form two opponent, or chromatic, and one luminance channel. The responses of these three transformed channels are orthogonally combined at a detector. The spectral sensitivity of the transformed channels is a function of intensity and adaptation; transformation equations are given for threshold and suprathreshold. The channel responses, represented as distance along axes in a vector space, have different temporal and spatial properties, depending upon the characteristics of the receptive fields associated with each axis. Spectral sensitivities, Stiless π 5 , Guths data on threshold subadditivity, saturation discrimination and hue discrimination are modeled in the space.

Red - Green Opponent Spectral Sensitivity: Disparity Between Cancellation and Direct Matching Methods

The spectral sensitivity at the opponent stage of the visual system is traditionally measured by a hue-cancellation procedure. Comparison with a direct hue-matching method shows that cancellation overestimates short-wavelength sensitivity by as much as a factor of 30. The observation implies that different mechanisms control the perception of short-wavelength and long-wavelength redness.

The achromatic channel—I. The non-linearity of minimum-border and flicker matches

Abstract Flicker and minimum-border matches made at one intensity do not hold at other intensities. For flicker matches, more saturated lights must be increased relative to less saturated lights as intensity increases, in agreement with the hypothesis that a compression precedes the addition of the signals from the receptors. Minimum-border matches are more nearly linear. The two methods give the same radiances for an equal-luminance spectrum for a 100 troland standard; the methods depart for lower and higher intensities.

Neural interactions between flicker-induced self-organized visual hallucinations and physical stimuli

Spontaneous pattern formation in cortical activity may have consequences for perception, but little is known about interactions between sensory-driven and self-organized cortical activity. To address this deficit, we explored the relationship between ordinary stimulus-controlled pattern perception and the autonomous hallucinatory geometrical pattern formation that occurs for unstructured visual stimulation (e.g., empty-field flicker). We found that flicker-induced hallucinations are biased by the presentation of adjacent geometrical stimuli; geometrical forms that map to cortical area V1 as orthogonal gratings are perceptually opponent in biasing hallucinations. Rotating fan blades and pulsating circular patterns are the most salient biased hallucinations. Apparent motion and fractal (1/f) noise are also effective in driving hallucinatory pattern formation (the latter is consistent with predictions of spatiotemporal pattern formation driven by stochastic resonance). The behavior of these percepts suggests that self-organized hallucinatory pattern formation in human vision is governed by the same cortical properties of localized processing, lateral inhibition, simultaneous contrast, and nonlinear retinotopic mapping that govern ordinary vision.

Flicker photometry and achromatic-channel structure

It is widely assumed that the near-perfect additivity of heterochromatic flicker photometry implies the existence of an achromatic channel in the visual system, which accurately sums R- and G-cone signals. For flicker photometry to be additive, the channel that detects the flicker stimulus need not add cone signals.

Fechner-Benham subjective colors do not induce McCollough after-effects

Fechner-Benham subjective color is widely believed to be governed by local interactions in early (probably retinal) mechanisms. Here we report three lines of phenomenological evidence that suggest otherwise: subjective colors seen in spatially extended stimuli (a) are dependent on global aspects of the stimuli; (b) can become multistable in position; and (c) even after being stabilized do not support the creation of McColloughs colored after-effects--a cortically based phenomenon generally thought to be more central than Fechner-Benham color. These phenomena suggest a central locus that controls perception of subjective color, characterized by pattern dependent interactions among cortical mechanisms that draw their inputs from peripheral units.