Measurement of visual aftereffects and inferences about binocular mechanisms in human vision

The aftereffect to relative motion does not show interocular transfer

The motion aftereffect is strongest after viewing a moving field embedded in a patterned stationary surround, which suggests that relative motion is an important signal for its generation. The contribution of relative motion to binocular aspects of the motion aftereffect was assessed. Subjects viewed uniformly moving random dots surrounded by a stationary random-dot annulus. These displays could be presented in a variety of combinations to each eye separately or to both eyes, during adaptation and test. It was found that, although the presence of relative motion during adaptation significantly extended the duration of the monocular motion aftereffect, it did not augment interocular transfer. The presence of stationary surround contours in the nonadapting eye did not influence the aftereffect in the adapting eye. The enhancement provided by stationary surround contours is largely dependent on their presence during adaptation. The presence or absence of surround contours during the test phase did not influence the duration of the aftereffect. These findings are consistent with previous suggestions that the motion aftereffect is, in part, the result of adaptation to relative motion that occurs relatively early in the visual pathway-before binocular integration.

Interocular transfer of expansion, rotation, and translation motion aftereffects

The motion aftereffect demonstrates the existence of direction-selective mechanisms in the visual system. However, direction-selective cells exist within many visual areas, including V1 and MT/V5. Can motion aftereffects be generated within each of these areas? In visual cortical areas beyond V1 almost all cells are binocular, whereas a smaller percentage are binocular in V1. The degree of binocularity can be revealed psychophysically by assessing interocular transfer. Interocular transfer of motion aftereffects generated from expanding rotating, and translating dynamic random-dot patterns were therefore compared, since these stimuli should activate cells in higher visual areas selectively. Partial interocular transfer was found that was greater for expansion and rotation than for translation. The results support the involvement of higher visual areas in motion aftereffects to complex animation sequences.

Aftereffects of apparent motion: the existence of an AND-type binocular system in human vision

There has been evidence for the existence of a purely binocular system in human vision that acts as an AND gate on information from the two eyes. There also has been evidence for the nonexistence of such a purely binocular system, indicating only the existence of an OR-type binocular system that responds to input from one or both eyes. As a result there are a number of possible explanations for the differing experimental results: the binocular system is an OR-type system only, it is a facilitating OR system that has AND-type characteristics, or it consists of independent OR and AND subsystems. Monocular adaptation, alternating monocular adaptation, or binocular adaptation were used to demonstrate the existence of the different systems, but in none of the previous experiments was the AND-type binocular system activated directly, and the existence of this AND system was deduced mostly because of differences in aftereffect strengths between monocular and binocular test conditions. Experiments are reported in which stimuli that activate the AND-type binocular system explicitly have been used, and the results show that we need the existence of such an AND-type binocular system to account for the results.

On the contribution of a binocular 'AND' channel at contrast threshold

Three experiments are reported in which an attempt was made to isolate the contribution of an AND channel by measuring aftereffects following alternating monocular adaptation. The first two were designed to test Wolf and Held's proposal that the binocular AND channel does not respond at contrast threshold. In the first experiment the relative sizes of monocular and binocular contrast threshold elevation were compared with the pattern of aftereffects obtained in a study of the suprathreshold tilt aftereffect. Identical patterns of results were obtained under the two adaptation conditions. In the second experiment, the monocular and binocular contrast-reduction aftereffect reported by Blakemore et al was measured over a wide range of reference contrasts. As in the previous experiment, the monocular effect was greater than the binocular effect. This occurred at all reference contrasts. These data support the conclusion that the AND channel contributes to visual performance in the same manner, irrespective of stimulus contrast. In the final experiment an alternative explanation for existing evidence against the existence of an AND channel was assessed.

On interocular transfer of motion aftereffects

A brief history of quantitative assessments of interocular transfer (IOT) of the motion aftereffect (MAE) is presented. Recent research indicates that the MAE occurs as a consequence of adapting detectors for relative rather than retinal motion. When gratings above and below a stationary, fixated grating are moved in an otherwise dark field the central, retinally stationary grafting appears to move in the opposite direction; when tested with stationary gratings an MAE is almost entirely confined to the central grating. The IOT of such an MAE was measured in experiment 1: the display was presented to one eye with a black field in the other. The IOT was about 30% of the monocular MAE. Similar values were found in experiment 2, in which the contralateral eye received an equivalent central stationary grating during adaptation and test. The dichoptic interaction of the processes involved in the MAE was examined by presenting the central gratings to both eyes and a single flanking grating above in one eye and below in the other (experiment 3). The MAE was tested with either the same or the contralateral pairing. Oppositely directed MAEs were found for the central and flanking gratings, but they were confined mainly to the conditions in which the configurations presented during adaptation were present in the same eyes during test. In experiment 4, the surround MAEs were compared after adaptation with two moving gratings in one eye or with a similar dichoptic configuration, and they were of similar duration. In a final experiment the MAE was tested either monocularly or binocularly after alternating adaptation of the left and right eyes and was found to be of the same duration. It is concluded that the MAE is a consequence of adapting relational-motion detectors, which are either monocular or of the binocular OR class.