Disentangling locus of perceptual learning in the visual hierarchy of motion processing

Abstract

Visual perceptual learning (VPL) can lead to long-lasting perceptual improvements. While the efficacy of VPL is well established, there is still a considerable debate about what mechanisms underlie the effects of VPL. Much of this debate concentrates on where along the visual processing hierarchy behaviorally relevant plasticity takes place. Here, we aimed to tackle this question in context of motion processing, a domain where links between behavior and processing hierarchy are well established. Specifically, we took advantage of an established transition from component-dependent representations at the earliest level to pattern-dependent representations at the middle-level of cortical motion processing. We trained two groups of participants on the same motion direction identification task using either grating or plaid stimuli. A set of pre- and post-training tests was used to determine the degree of learning specificity and generalizability. This approach allowed us to disentangle contributions from both low- and mid-level motion processing, as well as high-level cognitive changes. We observed a complete bi-directional transfer of learning between component and pattern stimuli as long as they shared the same apparent motion direction. This result indicates learning-induced plasticity at intermediate levels of motion processing. Moreover, we found that motion VPL is specific to the trained stimulus direction, speed, size, and contrast, highlighting the pivotal role of basic visual features in VPL, and diminishing the possibility of non-sensory decision-level enhancements. Taken together, our study psychophysically examined a variety of factors mediating motion VPL, and demonstrated that motion VPL most likely alters visual computation in the middle stage of motion processing.