PSYCHOPHYSICAL INVARIANTS OF ACHROMATIC COLOUR VISION: IV. Depth adjacency and simultaneous contrast

L ie , I. Psychophysical invariants of achromatic colour vision. IV. Depth adjacency and simultaneous contrast. Scand. J. Psychol ., 1969, 10 , 282–286.— Achromatic colour contrast as a function of adjacent subareas was investigated by a stereoscopic depth technique. The degree of colour contrast was found to be independent of stereoscopic depth.     

Depth adjacency and the rod-and-frame illusion

The effects of psychological set on perception of first and second pain were determined for 20 subjects. Percutaneous electrical shock intensities (6–8 mA, 3 msec) sufficient to evoke double pain responses were used in all subjects. Psychological sets included PAST (“Place yourself in a previous experience that was free of any significant emotional tone”), PRESENT (“Feel your foot that will be shocked”), and FUTURE (“Think to yourself that you are about to be shocked”). Perception of second pain was never perceived in PAST and FUTURE sets but was always perceived in the PRESENT set. Furthermore, at minimal rates of stimulation ( > 1/3 sec), summation of second pain occurred in the PRESENT set but not in the FUTURE set. All subjects startled in the FUTURE set and did not startle in PAST or PRESENT sets. Each subject reported that the aversiveness of the shock related to painful sensations in PAST and PRESENT sets and to ones own body responses in the FUTURE set.     

Depth adjacency and induced motion.

Induced motion was investigated as a function of the stereoscopic separation of the test and inducing object and the instructions to attend to or to ignore the inducing object. It was found that stereoscopically displacing the test object from the inducing object with both kinds of instructions resulted in a decrease in the magnitude of induction particularly with crossed disparity. These results are consistent with the adjacency principle and with the ability of attention as well as adjacency to modify the magnitude of the induced motion.     

Color-selectivity in simultaneous motion contrast

Thirty-two Ss were required to estimate the apparent motion of stationary vertical lines viewed against a background of moving vertical lines when both patterns were seen by the same eye (monoptic conditions) or the center pattern was seen by one eye and the surrounds by the other eye (dichoptic conditions). The stationary lines appeared to be moving from right to left as the surrounds moved left to right. The simultaneous motion contrast found under monoptic conditions was maximal when the center pattern and the surrounds were the same color and was reduced when they differed in color. The surrounds had limited influence on the apparent motion of the center section under dichoptic condition, and the color relationship was no longer important. Related color selectivity has been reported for the motion aftereffect (successive motion contrast), and both sets of data can be attributed to inlaibitory interaction (simultaneous in one case and successive in the other) among neural detectors tuned to wavelength as well as the direction of image motion.     

Peripheral viewing and simultaneous contrast

An experiment was done to find the effect on simultaneous brightness contrast of viewing the test patches foveally or peripherally, at a distance of 3° or 6° from the fovea. It was found that contrast was greater in the periphery. It was also found that contrast increased with prolonged viewing of the display. A further experiment showed that blurring the test patches produced an increase in contrast.     

Invisible motion contributes to simultaneous motion contrast

The purpose of the present study was two-fold. First we examined whether visible motion appearance was altered by the spatial interaction between invisible and visible motion. We addressed this issue by means of simultaneous motion contrast, in which a horizontal test grating with a counterphase luminance modulation was seen to have the opposite motion direction to a peripheral inducer grating with unidirectional upward or downward motion. Using a mirror stereoscope, observers viewed the inducer and test gratings with one eye, and continuous flashes of colorful squares forming an annulus shape with the other eye. The continuous flashes rendered the inducer subjectively invisible. The observers’ task was to report whether the test grating moved upward or downward. Consequently, simultaneous motion contrast was observed even when the inducer was invisible (Experiment 1). Second, we examined whether the observers could correctly respond to the direction of invisible motion: It was impossible (Experiment 2).     

Size of test patch and simultaneous contrast

Two experiments were done to find the effect on simultaneous brightness contrast of placing test patches in, as against outside, the fovea. When the display was magnified so that the test patches were outside the fovea no statistically significant effect was found. Very short exposures, to eliminate eye movements, did not appear to reduce contrast.     

Evidence for the role of post-retinal processes in simultaneous contrast

Summary Evidence is presented that the degree of simultaneous contrast produced by the bisecting line in the Koffka ring is greatly lessened by varying the perceived depth interval between this line and the Koffka figure. The retinal disparity between the line and the figure was varied to produce this effect. The slight lateral displacements of the bisecting line in the monocular images themselves necessary to produce retinal disparity with binocular viewing had no effect on the magnitude of simultaneous contrast. Interpretations based on processes operating at the retinal level such as lateral inhibition level are not supported by this finding. The results are interpreted in terms of the adjacency principle.This research was supported by National Institutes of Health Research Grant No. 00268 to the first author. Thanks are due to Donald Mershon and Walter Gogel for their helpful comments and criticisms of an earlier version of this paper.     

Amount of surround and test-inducing separation in simultaneous brightness contrast

Simultaneous brightness contrast is investigated in the fovea as a function of (1) amount of surround of the inducing field (Experiment 1) and (2) separation between the test and inducing fields (Experiment 2). Circular test and match fields subtending 14 min (radius) are used throughout. The inducing field, held constant in area, is a circular annulus (615 sq min) varying from a quadrant on one side of the test circle to an annulus completely surrounding the test circle. Test-field apparent brightness is not significantly affected by amount of inducing-field surround when the separation between centers of the test and inducing fields is held constant (Experiment 1). Experiment 2, though, shows that apparent brightness increases significantly as the separation between the centers of the test and inducing fields is increased.     

An adjacency effect in free recall

An adjacency effect was demonstrated at a high level of significance in the free recall, by 123 subjects, of a list of 40 high-frequency nouns presented in varying order on successive trials. The phenomenon referred to as the adjacency effect consists of the fact that when a subject is given repeated trials of study and free recall of a list of words (always presented in a different order), the probability of recalling a given item is greater when the item is presented temporally adjacent to an item which is already learned (as evidenced by recall on the previous trial) than when the item stands temporally between other items which are not yet learned. The enhancement of recall is greater when the item is presented between two previously learned items. The implications of the adjacency effect for verbal learning theory, particularly for the serial-position effect in serial learning and the concepts of interference and neural consolidation, are discussed.     

A new effect of luminance gradient on achromatic simultaneous contrast

A new effect in the domain of achromatic simultaneous contrast has been observed. A middle gray region placed at the center of an area filled by a linear achromatic gradient from black (outer part) to white (inner part) is perceived as being much darker than an identical middle gray region surrounded by a reversed gradient. By using a matching task in two experiments, it has been shown that this phenomenon is much stronger than the classical achromatic simultaneous contrast effect. The new effect is interpreted in terms of thealbedo hypothesis.