Dissertation/Thesis Abstract

Selectivity and dynamics of responses in the central visual pathway of the macaque
by Sokol, Sach H., Ph.D., New York University, 2009, 177; 3380273
Abstract (Summary)

The lateral geniculate nucleus (LGN) has long been considered a source of linear signals for models of cortical processing. But recent studies implicate LGN and retinal processing in nonlinear response properties of neurons later in the visual stream and there is no shortage of nonlinearities in the response properties of LGN neurons. We measured the effect of contrast on the spatial frequency tuning of LGN neurons and found that tuning changes with contrast. LGN receptive fields are well described by a difference of two Gaussians, representing the center and surround mechanisms. A version of this model incorporating changes of both the radius of the center and surround with contrast describes the data well. We conclude that contrast-dependent spatial frequency tuning reflects a change in size of both the center and surround mechanisms, presumably arising from a retinal mechanism.

The response of a primary visual cortex neuron to an optimal grating is reduced by superimposing an orthogonal grating, an effect called cross-orientation suppression. The source of this suppression is unclear. Recent work has shown it can be evoked by temporal frequencies which elicit little or no response in cortex, but which are potent stimuli for neurons in LGN, suggesting that the suppression originates subcortically. We explored the origin of suppression by characterizing the spatiotemporal tuning of crossorientation suppression. The tuning of suppression was broad for both spatial and temporal frequency and extended to lower spatial and higher temporal frequencies than excitation. Suppression was untuned for orientation. These results support a subcortical origin for cross-orientation suppression.

We also examined the effects of cross-orientation suppression on response latency. Changing the contrast of a single grating determines both the magnitude and latency of responses, while changing its orientation alters response magnitude but not latency. In this context, contrast rather than response magnitude is the principal determinant of response latency. When a grating is superimposed with an orthogonal mask, latency increases rather than decreases and is better associated with response magnitude than contrast. This suggests that the temporal coding of contrast is based on effective, not local contrast.

Indexing (document details)
Advisor: Movshon, J. Anthony, Lennie, Peter
Commitee: Alonso, Jose-Manuel, Graham, Norma, Hawken, Michael, Simoncelli, Eero
School: New York University
Department: Center for Neural Science
School Location: United States -- New York
Source: DAI-B 70/12, Dissertation Abstracts International
Subjects: Neurosciences
Keywords: Lateral geniculate nucleus, Primary visual cortex, Selectivity, Suppression, Vision
Publication Number: 3380273
ISBN: 9781109506822
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