June 2004
Volume 4, Issue 8
Free
Vision Sciences Society Annual Meeting Abstract  |   August 2004
Interactions between contrast, coherence and directional tuning
Author Affiliations
  • Ione Fine
    Doheny Eye Institute, USC/Second Sight, USA
  • Christina M. Anderson
    Department of Psychology, UCSD, USA
  • Geoffrey M. Boynton
    The Salk Institute, USA
  • Karen R. Dobkins
    Department of Psychology, UCSD, USA
Journal of Vision August 2004, Vol.4, 209. doi:10.1167/4.8.209
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      Ione Fine, Christina M. Anderson, Geoffrey M. Boynton, Karen R. Dobkins; Interactions between contrast, coherence and directional tuning. Journal of Vision 2004;4(8):209. doi: 10.1167/4.8.209.

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Abstract

The responses of motion mechanisms have been shown, neurophysiologically and psychophysically, to depend not only on the direction of stimulus motion, but also on contrast, coherence and speed. We examined interactions between contrast, coherence and directional selectivity by measuring directional tuning psychophysically across a wide range of motion coherence and contrast levels. A 2-IFC procedure was used; the ‘noise’ interval contained randomly moving dots and the ‘signal’ interval contained noise plus two superimposed fields of coherently moving dots. Observers reported which interval contained global motion. We tested 7 angular differences between the directions of the two dot fields (d=0, 22.5, 45, 67.5, 90, 135, and 180 ). We obtained two types of thresholds for 3 subjects: (1) coherence thresholds: the proportion of dots moving coherently in the ‘signal’ interval was varied to obtain a coherence threshold for several fixed contrasts (rms, 3.2 – 30.2%) and (2) contrast thresholds: dot contrast was varied to obtain a contrast threshold for several fixed coherence levels (20 – 100% coherence). Thresholds for a given contrast-coherence pairing did not depend on whether coherence was fixed and contrast varied, or vice versa. We assumed the response to each dot field depended multiplicatively on both the contrast and the coherence that dot field, and that the ability to detect global motion in the pair of dot fields depended on the amount of summation between the two fields. Contrast and coherence gain parameters were fixed based on neurophysiological estimates. This very constrained model describes the data well, and estimates a directional tuning bandwidth of ∼83 for directionally selective mechanisms across a wide range of contrasts and coherences. The good fit of the model demonstrates that directional tuning is nearly invariant across a wide range of luminance contrasts and coherence levels, as predicted by models of contrast normalization.

Fine, I., Anderson, C. M., Boynton, G. M., Dobkins, K. R.(2004). Interactions between contrast, coherence and directional tuning [Abstract]. Journal of Vision, 4( 8): 209, 209a, http://journalofvision.org/4/8/209/, doi:10.1167/4.8.209. [CrossRef] [PubMed]
Footnotes
 CR: None. Supported by NIH Grant EY12153 (KRD).
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