September 2005
Volume 5, Issue 8
Free
Meeting Abstract  |   September 2005
Binocular summation, dichoptic masking and contrast gain control
Author Affiliations
  • Mark A. Georgeson
    Neurosciences Research Institute, Aston University, Birmingham B4 7ET, U.K.
  • Tim S. Meese
    Neurosciences Research Institute, Aston University, Birmingham B4 7ET, U.K.
  • Daniel H. Baker
    Neurosciences Research Institute, Aston University, Birmingham B4 7ET, U.K.
Journal of Vision September 2005, Vol.5, 797. doi:10.1167/5.8.797
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      Mark A. Georgeson, Tim S. Meese, Daniel H. Baker; Binocular summation, dichoptic masking and contrast gain control. Journal of Vision 2005;5(8):797. doi: 10.1167/5.8.797.

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Abstract

We consider a classical question - how signals from the two eyes are combined - in the context of contemporary models of contrast gain control. In 2AFC experiments, observers had to detect the presence of a test grating (1 c/deg, 200 ms) in one or both eyes, in the presence or absence of a similar masking (‘pedestal’) grating in one or both eyes. We found a high degree of binocular summation when pedestal contrast was low or zero, while at higher contrasts we confirmed Legge's (1984) paradoxical finding that there was no advantage for detecting binocular contrast increments over purely monocular ones. In a new variant, however, we found that, on a binocular pedestal, binocular increments were better detected than monocular ones. This implies that there is binocular summation of test signals even in the suprathreshold task. Importantly, there is also binocular summation of suppressive (gain control) signals: monocular increments were harder to detect on a binocular pedestal than on a monocular one. The pattern of results can be largely, but not completely, understood through a binocular version of the standard gain control equation: Resp(binoc) = (Lp +Rp )/(sq+Lq+Rq), expressing the output of a binocular channel to contrasts L,R in the left and right eyes, with p,q,s constant (p∼2.4, q∼2). With additive noise, this mechanism correctly predicts the high thresholds and unusually steep, step-like psychometric functions that we observed in dichoptic masking (test in one eye, pedestal in the other). But this mechanism under-estimates both facilitation and binocular summation at low contrasts, so we shall consider what modifications are needed. Viable options include more than one output channel, and more than one stage at which nonlinear transduction and gain control operate.

Georgeson, M. A. Meese, T. S. Baker, D. H. (2005). Binocular summation, dichoptic masking and contrast gain control [Abstract]. Journal of Vision, 5(8):797, 797a, http://journalofvision.org/5/8/797/, doi:10.1167/5.8.797. [CrossRef]
Footnotes
 EPSRC grant GR/S74515/01
© 2005 ARVO
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