title: The reference frame for encoding and retention of motion depends on stimulus set size.
abstract: The goal of this study was to investigate the reference frames used in perceptual encoding and storage of visual motion information. In our experiments, observers viewed multiple moving objects and reported the direction of motion of a randomly selected item. Using a vector-decomposition technique, we computed performance during smooth pursuit with respect to a spatiotopic (nonretinotopic) and to a retinotopic component and compared them with performance during fixation, which served as the baseline. For the stimulus encoding stage, which precedes memory, 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 items increased (Set Sizes 3 to 7), the spatiotopic reference frame was able to account for the performance. Finally, when the number of items became larger than 7, the distinction between reference frames vanished. We interpret this finding as a switch to a more abstract nonmetric encoding of motion direction. We found that the retinotopic reference frame was not used in memory. Taken together with other studies, our results suggest that, whereas a retinotopic reference frame may be employed for controlling eye movements, perception and memory use primarily nonretinotopic reference frames. Furthermore, the use of nonretinotopic reference frames appears to be capacity limited. In the case of complex stimuli, the visual system may use perceptual grouping in order to simplify the complexity of stimuli or resort to a nonmetric abstract coding of motion information.
authors.affiliation: Department of Electrical and Computer Engineering, University of Houston, Houston, TX, USA.
authors.forename: Duong
authors.initials: D
authors.lastname: Huynh
authors.affiliation: School of Optometry and Vision Science, University of Bradford, Bradford, UK.
authors.forename: Srimant P
authors.initials: SP
authors.lastname: Tripathy
authors.affiliation: Center for Neuro-Engineering and Cognitive Science, University of Houston, Houston, TX, USA.|College of Optometry, University of Houston, Houston, TX, USA.
authors.forename: Harold E
authors.initials: HE
authors.lastname: Bedell
authors.affiliation: Department of Electrical and Computer Engineering, University of Houston, Houston, TX, USA. ogmen@du.edu.|Center for Neuro-Engineering and Cognitive Science, University of Houston, Houston, TX, USA. ogmen@du.edu.|Department of Electrical and Computer Engineering, University of Denver, Denver, CO, 80208, USA. ogmen@du.edu.
authors.forename: Haluk
authors.initials: H
authors.lastname: Öğmen
mesh_terms: D000328:Adult; D005260:Female; D006801:Humans; D008297:Male; D008570:Memory, Short-Term / Q000502:physiology*; D009039:Motion Perception / Q000502:physiology*; D011698:Pursuit, Smooth / Q000502:physiology*; D013028:Space Perception / Q000502:physiology*; D055815:Young Adult
keywords: Iconic memory; Motion perception; Reference frame; Sensory memory; Short-term memory