Alan L. F. Lee

Two forms of aftereffects induced by transparent motion reveal multilevel adaptation

Visual adaptation produces remarkable perceptual aftereffects. However, it remains unclear what basic neural mechanisms underlie visual adaptation and how these adaptation-induced neural changes are related to perceptual aftereffects. To address these questions, we examined transparent motion adaptation and traced the effects of adaptation through the motion processing hierarchy. We found that, after adapting to a bidirectional transparent motion display, observers perceived two radically different motion aftereffects (MAEs): segregated and integrated MAEs, depending on testing locations. The segregated MAE yielded an aftereffect opposite to one of the adapting directions in the transparent motion stimulus. Our results revealed that the segregated MAE relies on the integration of local adaptation effects. In contrast, the integrated MAE yielded an aftereffect opposite to the average of the adapting directions. We found that integrated MAE was dominant at non-adapted locations but was reduced when local adaptation effects were weakened. These results suggest that integrated MAE is elicited by a combination of two mechanisms: adaptation-induced changes at a high-level processing stage and integration of local adaptation effects. We conclude that distinct perceptual aftereffects can be observed due to adaptation-induced neural changes at different processing levels, supporting the general hypothesis of multilevel adaptation in the visual hierarchy.

Global-motion aftereffect does not depend on awareness of the adapting motion direction

It has been shown that humans cannot perceive more than three directions from a multidirectional motion stimulus. However, it remains unknown whether adapting to such imperceptible motion directions could generate motion aftereffects (MAEs). A series of psychophysical experiments were conducted to address this issue. Using a display consisting of randomly oriented Gabors, we replicated previous findings that observers were unable to perceive the global directions embedded in a five-direction motion pattern. However, adapting to this multidirectional pattern induced both static and dynamic MAEs, despite the fact that observers were unaware of any global motion directions during adaptation. Furthermore, by comparing the strengths of the dynamic MAEs induced at different levels of motion processing, we found that spatial integration of local illusory signals per se was sufficient to produce a significant global MAE. These psychophysical results show that the generation of a directional global MAE does not require conscious perception of the global motion during adaptation.

Motion Integration Using Competitive Priors

Psychophysical experiments show that humans are better at perceiving rotation and expansion than translation [5][9]. These findings are inconsistent with standard models of motion integration which predict best performance for translation. To explain this discrepancy, our theory formulates motion perception at two levels of inference: we first perform model selection between the competing models (e.g. translation, rotation, and expansion) and then estimate the velocity using the selected model. We define novel prior models for smooth rotation and expansion using techniques similar to those in the slow-and-smooth model [23] (e.g. Green functions of differential operators). The theory gives good agreement with the trends observed in four human experiments.