The Reference Frame for Encoding and Retention of Motion-Direction Information Depends on Stimulus Set-size

Abstract

We investigated the reference frames used in perceptual encoding and storage of motion-direction information. Observers (N=4) viewed multiple moving objects while (1) maintaining gaze on a fixation point (Experiments 1a-1b); or (2) performing smooth-pursuit eye-movement (Experiments 2a-2b). Their task was to report the direction of motion of a randomly selected object by adjusting the orientation of a bar. To characterize the initial encoding stage, in Experiments 1a and 2a the target to be reported was cued immediately after objects stopped moving and disappeared. Experiments 1b and 2b included varying cue delays to tap into sensory memory and VSTM. Using a vector-decomposition technique, we computed performance during smooth pursuit with respect to a spatiotopic (non-retinotopic) and a retinotopic component and compared them with performance during fixation, which served as the baseline. For the stimulus encoding stage, we found that the reference frame depends on the stimulus set-size. For a single moving target, the spatiotopic reference frame had the most significant contribution with some additional contribution from the retinotopic reference frame. When the number of objects increased (set sizes 3 to 7), the spatiotopic reference frame alone was able to account for performance. Finally, when set size exceeded 7, the distinction between reference frames vanished. We interpret this finding as a switch to a more abstract, non-metrical encoding of motion direction. Results with non-zero cue delays indicated that the retinotopic reference frame was not used in memory. Our results suggest that, whereas a retinotopic reference frame may be employed for controlling eye-movements, reference frames used in perception and memory are primarily non-retinotopic. Furthermore, the use of non-retinotopic reference frames appears to be capacity limited. In the case of complex stimuli, the visual system may use perceptual grouping to reduce complexity or resort to an abstract, non-metrical coding of motion information.

Meeting abstract presented at VSS 2017