August 2012
Volume 12, Issue 9
Free
Vision Sciences Society Annual Meeting Abstract  |   August 2012
Tonic and phasic influences on perceived size: Effects of visual field, stimulus eccentricity, and smooth pursuit eye movements
Author Affiliations
  • Katsumi Minakata
    Psychology Department, Math and Sciences, North Dakota State University
  • Yamaya Sosa
    Psychology Department, Math and Sciences, North Dakota State University
  • Mark McCourt
    Psychology Department, Math and Sciences, North Dakota State University
Journal of Vision August 2012, Vol.12, 551. doi:10.1167/12.9.551
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      Katsumi Minakata, Yamaya Sosa, Mark McCourt; Tonic and phasic influences on perceived size: Effects of visual field, stimulus eccentricity, and smooth pursuit eye movements. Journal of Vision 2012;12(9):551. doi: 10.1167/12.9.551.

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      © ARVO (1962-2015); The Authors (2016-present)

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Abstract

Background. A tonic leftward bias exists in the perceived midpoint of horizontal lines due to the differential magnification of left hemispace by a prepotent contralateral vector of visuospatial attention by the dominant right hemisphere. Many early studies reported that visual scanning phasically modulated this tonic bias, where leftward bisection error increased with rightward scanning and vice versa. More recent studies report the opposite effect: leftward bisection error increases after execution of leftward smooth pursuit eye movements (SPEMs) and vice versa. Companion experiments found that rectangular stimuli presented ahead of an SPEM appeared larger than identical stimuli located the same distance behind the SPEM trajectory. We further explore the effect of SPEMs on size perception. Method. Stimuli were second-order isotropic blobs of Gaussian-enveloped binary noise (100% contrast; 15 comparison blobs, σ = 0.17o -0.31o; standard blob σ = 0.24o). In scanning conditions subjects smoothly tracked (7.7o/s) a target dot which moved 18.9o leftward or rightward toward the center of the display, whereupon standard and comparison blobs were presented for 50 ms to the left and right of fixation, respectively. Subjects (N = 21) judged which blob was larger. A fixed-gaze (no scan) condition was also included, such that there were three levels of scanning (leftward/rightward/none) and two levels of blob eccentricity (1o and 4o). Results. A 3 (Scanning) x 2 (Eccentricity) within-subjects ANOVA revealed a main effect of Scanning (p <.001), where scanning increased the perceived size of blobs ahead of the scan trajectory; a trend effect of Eccentricity (p = .085) where at 1o left blobs appeared larger than right blobs (reversed at 4o); and a Scanning x Eccentricity interaction (p <.001) where the magnitude of the scanning effect is largest at 4o blob eccentricity. Conclusions. Tonic visual field dependent differences in perceived size are modulated by eccentricity and SPEMs.

Meeting abstract presented at VSS 2012

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