Linear motion aftereffect induced by pure relative motion

The effect of adaptation to pure relative motion was investigated for the motion aftereffect (MAE) of linear translation motion. In experiment 1, MAE induced by adaptation in the surrounding area was tested. The relative motion signal significantly increased the magnitude of MAE while local MAE in the surrounds was not affected. In experiment 2, MAE observed in the same adapted area was examined while local adaptation was cancelled out. Substantial MAE was found only when the test stimuli included the surroundings, which is considered to be favourable for relative motion mechanisms. These results clearly indicate that MAE is induced by adaptation to pure relative motion as well as by local motion. MAE should be regarded as a composite phenomenon reflecting multiple sites of adaptation including the local and the relative motion levels. The results also provide evidence for the existence of independent detecting mechanisms for relative motion processing.

Re-evaluation of local adaptation for motion aftereffect

A motion aftereffect (MAE) can be induced by stimuli moving in surrounding areas. This suggests the relevance of mechanisms for relative motion, rather than early-level motion detectors, which are considered to work locally. Experiments are reported in which the role of local adaptation in the MAE with a stimulus configuration comprising relative motion has been discussed. Sinusoidal gratings were presented in three rectangular windows: a centre window, and two windows one above and one below the central one. The surrounding top and bottom windows, which were divided into left and right halves, had gratings presented in only one of the two halves. The MAE duration was measured after adaptation to motion either in the central or in the surrounding windows, by controlling the regions with the gratings. From this, the regions of surrounding gratings were found not to have a significant effect with adaptation in the centre window. With adaptation in the surrounds, however, these regions did affect the MAE; the MAE duration was reduced when the adapted region had no gratings in the test phase. Thus, for an MAE it is necessary for the adapted area to be covered with stimuli in the test phase, which indicates the dominance of local adaptation for the MAE even when relative motion is relevant in producing the MAE.

Cooperative Interaction between Change in Disparity and Size for the Perception of Motion in Depth

While an object is moving in depth, its retinal disparity and size are cooperatively changing along the viewing distance. We examined the effect of a cooperative relation between the two cues for the perception of motion in depth—changes in disparity and size—with the Wheatstone stereoscopic display. In experiment 1, we used a stereoscopic stimulus whose disparity and size were independently modulated with sine-wave form, but at different frequencies (0.7 Hz vs 0.8 Hz, and vice versa). So, the cooperative and the uncooperative phases between the two cues repeatedly followed each other. The subjects continuously pushed a response key when the stimulus was clearly perceived to be moving in depth. In general, the impression of motion in depth was clear when the two cues were simultaneously modulated in similar phase, but not in different phase. In experiment 2, we measured the perceived distance of a stimulus that is moving in depth, when the two cues were moderated in the same phase and in counterphases. The perceived distance was increased when the two cues moved in the same phase. We found that not only the effect of each cue, but also the effect of the cooperative change of the two cues was affecting the perception of motion in depth. These results suggest that the cooperative interaction of two cues, that is their relative phase, is important for the perception of motion in depth.

Temporal-discontinuity detection with contrast-modulated gratings

Three experiments on temporal-discontinuity detection were carried out. In experiment 1, temporal-discontinuity thresholds were measured for sinusoidal gratings by the use of the double-staircase method. A sinusoidal grating was presented twice successively. The subject judged whether or not an interval was present. The temporal-discontinuity threshold increased as the spatial frequency of the grating increased, but decreased as the contrast of the grating increased. In experiment 2, contrast-modulated gratings were used instead of the sinusoidal grating. The temporal-discontinuity threshold increased as the carrier frequency increased, and the threshold for each contrast-modulated grating was similar to that for the no-modulation (sinusoidal) grating whose contrast was the same as the maximum local contrast of the contrast-modulated grating. In experiment 3, temporal-discontinuity thresholds were measured for low-contrast (3%) sinusoidal gratings. The thresholds were very low, even for such low-contrast gratings. These results suggest that the low-spatial-frequency channels are not involved in detecting the modulation frequency of the contrast-modulated grating. Rather, the local contrast seems to be the determinant of the detection of the contrast-modulated grating itself.