THE CONTRIBUTION OF ONE-DIMENSIONAL MOTION MECHANISMS TO THE PERCEIVED DIRECTION OF DRIFTING PLAIDS AND THEIR AFTEREFFECTS

When motion aftereffects (MAEs) are measured by adapting to a drifting plaid (simultaneous adaptation) or by adapting to the plaid's compone nt gratings in alternation (alternating adaptation), it has been shown that the velocity and duration of the MAE are smaller in the latter c ase [Wenderoth, P., Bray, R. and Johnstone, S. (1988) Perception, 17, 81-91; Burke, D. and Wenderoth, P. (1993) Vision Research, 33,351-359] . However, Burke and Wenderoth additionally reported that the directio ns of MAEs induced by simultaneous and alternating adaptation were ide ntical, an apparent inconsistency if the differences in duration and v elocity were due to the presence of ''blobs'' at the component grating intersects in the simultaneous case. Presumably, the direction of the ''blobs'' should also affect perceived plaid direction during adaptat ion and, hence, the MAE direction. In five experiments, we have measur ed both perceived adapting plaid and MAE direction, tested with both a lternating and simultaneous adaptation, measured interocular transfer of plaid-induced MAEs and obtained MAE and plaid direction judgments u nder monocular and binocular viewing conditions. All of the data indic ate that there is a blob tracking mechanism which is preferentially st imulated by plaids whose component gratings have high spatial frequenc y, low temporal frequency and high contrast. Differences between simul taneous and alternating adaptation emerge only when more optimal blobs are used, thus accounting for Burke and Wenderoth's failure to find a difference. The data also support Burke and Wenderoth's claim that th e blob tracking mechanism is monocular: alternating and simultaneous a daptation produce identical MAEs under interocular transfer conditions , even using plaids with more optimal blobs, We also report the unexpe cted finding that plaids with more- and less-optimal blobs appear to d rift in directions 20 degrees apart yet their aftereffects differ in d irection by only 3-5 degrees. That is, more optimal blob plaids-compar ed with less optimal blob plaids-change both perceived plaid direction during adaptation and subsequent perceived MAE direction but the latt er change is much more modest. Possible explanations of this dissociat ion are considered.