John P. Harris

Illusory contours and stereo depth

A theory for illusory contours, which fill gaps in certain figures, is proposed and subjected to an experimental test. We suggest that nearer masking objects are perceptually postulated to “account” for gaps when these are unlikely. The experiment shows that when stereoscopic depth information incompatible with this “perceptual hypothesis” is presented, the illusory contours are reduced in intensity or disappear.

Movement Aftereffects Contingent on Binocular Disparity

Five subjects adapted for 30 min to a textured disc lying in front of the fixation point with 0·1 deg(1) crossed disparity, which rotated clockwise at 4 rev/min, alternating with a disc behind the fixation point, with 0·1 deg of arc uncrossed disparity, which rotated anticlockwise. A stationary test field then appeared to rotate anticlockwise when it lay in front of the fixation point, and clockwise when it lay behind. Conversely, a test field in the plane of fixation briefly appeared to lie a few millimetres behind the fixation plane when it rotated clockwise, and in front when it rotated anticlockwise. The movement aftereffect contingent on disparity reappeared each time the test disparity was reversed, but the total duration of each successive aftereffect in the series decreased exponentially with elapsed time. Movement aftereffects contingent on disparity were very much stronger than those contingent on colour and won out over them when disparity was pitted against colour.

Fusion and Rivalry of Illusory Contours

The general question is raised: “Are visual contours given directly from striate-cortex feature-detector activity?‘’ Phenomena of ‘subjective’ or ‘cognitive’ contours are examined to challenge this view, on the ground that contours can be extrapolations across low-probability gaps. The contours may be curved and may have poor ‘gestalt’ qualities—so ‘gestalt closure’ is not appropriate, but may be a sub-class of these phenomena. It is suggested that these illusory contours (and brightness differences) are generated by perceptually postulated masking objects—these being part of perceptual ‘scene analysis strategy’, since strong evidence for nearer objects is provided by improbable gaps. Experiments are reported, in which each eye is given a different ‘cognitive’ contour figure such that there are disparate but illusory contours. It is found that these are fused to give three-dimensional illusory. surfaces bowing in front of the display. Masking objects must be in front of gaps; what happens here with reversal of stereo depth? Switching the eyes often gives rivalry of the illusory contours when masking is incompatible with the stereo depth. Implications for normal stereo vision are discussed.

Illusion-Destruction by Appropriate Scaling

The inappropriate constancy scaling theory of visual distortion illusions is tested by optically projecting typical models giving these figures by perspective. Appropriate or inappropriate stereoscopic disparities are then added–with the prediction that when perspective and stereo are geometrically correct the distortion should vanish. This is confirmed with measurements for the Müller-Lyer illusion and by observation of several other classical examples. It is suggested that much previous work has investigated ‘end stop’ conditions, given by angles too extreme to be generated as perspective. Conditions for appropriate scaling, giving zero or small distortions, are found to be critical but readily attainable.

Contrast, spatial frequency and test duration effects on the tilt aftereffect: Implications for underlying mechanisms

The tilt aftereffect (TAE) was measured with a forced-choice technique for gratings of different spatial frequencies, contrasts and adapting and test durations. At short test durations, a 2 c/deg grating gave a larger TAE than a 10 c/deg, while at long test durations the opposite occurred. Low contrast gratings tended to give smaller TAEs at short test durations, and larger TAEs at long durations, than high contrast. A longer adapting duration tended to produce larger TAEs at low contrast for any test duration, but larger TAEs at high contrast only at long test durations. We suggest that the spatial frequency effect reflects differential excitation by the test stimuli of transient and sustained channels, and that the contrast effects reflect both a non-linearity in the relationship between excitation and inhibition, and the adapting effects of the test grating.

Adaptation to peripheral flicker: Relationship to contrast detection thresholds

The time to disappearance of flicker of a temporally modulated uniform 1 degree field, steadily viewed with the temporal retina at an eccentricity of 12 degrees, was measured as a function of temporal frequency and depth of modulation (contrast). As found by others, for a fixed contrast, adaptation time declined as temporal frequency increased. To check whether this effect was genuinely temporal frequency-dependent, or reflected the amount above threshold of the adapting contrast, measurements were also made at contrasts which were multiples of the contrast threshold or matched across temporal frequencies. The results suggest that both temporal frequency and amount of adapting contrast above threshold are important in determining the speed of adaptation.

Dichoptic induction of movement aftereffects contingent on color and on orientation

Some comparative experiments on the dichoptic induction of the movement aftereffect (MAE) contingent on color and the MAE contingent on orientation are reported. Colorcontingent movement aftereffects could be evoked only when the eye which had viewed color during adaptation also viewed color during test sessions. When the apparent color of the test field was changed by binocular color rivalry, contingent movement aftereffects (CMAEs) appropriate to the suppressed color were reported. After dichoptic induction of the orientation-contingent MAE, aftereffects could be obtained whether the eliciting gratings and stationary test fields were presented together to either eye alone or were dichoptically viewed.

Contingent aftereffects: Lateral interactions between color and motion

A randomly dotted yellow disk was rotated at a speed of 5 rpm, alternating in direction every 10 sec. Its change in direction of rotation was paired with a change in surround color, which was either red or green. After 15 min of exposure, observers reported vivid motion aftereffects contingent on the color of both the stationary disk and the surround, even though during adaptation only motion or color was associated with either alone. In further experiments, it was established that a change in color (or direction of motion) of the disk could be associated with a change in direction of motion (or color) of the surround. Such lateral effects were found even when a wide (50) annulus was introduced between the disk and the surround during adaptation and testing. Furthermore, the aftereffects generalized to the annulus, which was not associated with either color or motion during adaptation. However, when the disk alone was adapted to color and motion, no generalization to the surround was found (and vice versa), suggesting that the effects are not produced by adaptation of large receptive fields or by scatter of light within the eye. The results appear to conflict with the ideas that contingent aftereffects are confined to the adapted area of the retina and that they are built up by links between single-duty neurones, and with an extreme view of the segregation of color and motion early in human vision.