The colour specificity of spatial adaptation: red-blue interactions

The Effect of Stationary Patterns on Adaptation for Movement: Evidence for Inhibitory Interaction

Contingent movement aftereffects (CMAEs) can be demonstrated by adapting to a red pattern rotating clockwise (cw) alternating with a green pattern rotating counterclockwise (ccw). After 5 min subjects typically report stationary test patterns as apparently rotating clockwise when they are green and counterclockwise when they are red. Also, luminance thresholds for motion now depend on both the colour and direction of the moving pattern. The thresholds for red—cw and green—ccw motion will be relatively greater than for the opposite colour motion pairings. This is called contingent threshold elevation.

When stationary dots the same colour as the moving patterns are added to the adapting stimuli, subjects report weak CMAEs but no contingent threshold elevation can be demonstrated.

When stationary dots opposite in colour to the moving patterns are added to the adapting stimuli, neither CMAEs nor contingent threshold elevation can be demonstrated. And yet colour specific adaptation does occur, and can be demonstrated in the colour specificity of the simple movement aftereffect. When stationary dots are added to the adapting pattern, the simple movement aftereffect though reduced, is greatest on a test pattern of the same colour as the moving dots. These findings suggest that the CMAE, contingent threshold elevation, and the colour specificity of the movement aftereffect involves neural processes differentially sensitive to the presence of stationary patterns.

Evidence on the local character of spatial frequency channels in the human visual system

The question has been raised as to whether there exists any transfer of information between neighboring retinal areas following presentation of spatially limited optical stimuli. Our experiments were aimed at mechanisms by which spatial frequency information could be transferred from one part of the visual field to another. Within a 0.9 degree X 2.5 degree section of the 3.6 degree X 2.5 degree testfield the observers adapt to a number of moving sinusoidally-modulated brightness or hue gratings of different spatial frequencies. As known by many publications the sensitivity to spatial frequencies changes in the adapted area. We find that this influence on the contrast threshold decreases as the distance to the adaptation section increases. Hence our results suggest mechanisms within the visual system mathematically best described by a local spectrum analysis.

A study of interocular transfer of spatial adaptation

The threshold-elevation aftereffect was measured ipsiocularly and interocularly following grating adaptation of one eye. The functions relating aftereffect magnitude to adapting contrast and adaptation time were similar under the two testing conditions, with interocular transfer remaining fairly constant; decay times were similar for ipsiocular and interocular aftereffects of comparable magnitude, and their frequency selectivities were the same. It is concluded that the stimulus-response characteristics of monocular and binocular spatial channels are fundamentally similar.

Adapting to two orientations: disinhibition in a visual aftereffect

The tilt aftereffect of adapating to two different orientations simultaneously is weaker than the aftereffect of adapting to the more effective of the two orientations alone. This finding is consistent with explanations of orientational after-effects in terms of lateral inhibition between cortical orientation detectors, but not with explanations in terms of neural "fatigue" from excitation.

Dichoptic induction of McCollough-type effects

If left-oblique and right-oblique black/white gratings are presented alternately to one eye, and unpatterned red and green fields are presented alternately to the other, orientation-sensitive chromatic after-effects are induced. With the colour-stimulated eye the hue usually seen on a test grating is complementary to that originally paired with its orientation, with the pattern-stimulated eye the hue is the same as that originally paired.

The experiments reported here show that: (a) the time course of decay of these chromatic after-effects (for each eye) fits approximately the same power law as that found for the normal McCollough effect; (b) analogous chromatic after-effects, opposite in the two eyes, can be dichoptically induced using pairs of stimuli other than gratings, such as dot patterns differing in magnification.

These results suggest that strongly coloured light in one eye can induce a weakly complementary bias in the colour-signalling system of the other eye, at a level peripheral to the site of the McCollough-type adaptation.

Is spatial adaptation an after-effect of prolonged inhibition?

1. The elevation of the thresholds for sinusoidal gratings of 4 c/deg and 6.7 c/deg was examined after adapting to gratings of 4 c/deg. Threshold elevation was determined as a function of the adapting contrast.2. It was confirmed that, when the adapting and testing spatial-frequencies were the same, the threshold elevation curve extrapolated to zero elevation at an adapting contrast equal to the pre-adaptation threshold for the adapting grating.3. It was found that the threshold elevation curve for 6.7 c/deg gratings also extrapolated to zero elevation at the pre-adaptation threshold for the adapting grating, even though the testing and adapting frequencies were different.4. It is argued that the latter result shows that adaptation is not simply an after-effect of prolonged excitation of a channel. Threshold elevation curves may represent the bandwidths of inhibitory interactions between channels.