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.
|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|
|Keywords:||Lateral geniculate nucleus, Primary visual cortex, Selectivity, Suppression, Vision|
Copyright in each Dissertation and Thesis is retained by the author. All Rights Reserved
The supplemental file or files you are about to download were provided to ProQuest by the author as part of a
dissertation or thesis. The supplemental files are provided "AS IS" without warranty. ProQuest is not responsible for the
content, format or impact on the supplemental file(s) on our system. in some cases, the file type may be unknown or
may be a .exe file. We recommend caution as you open such files.
Copyright of the original materials contained in the supplemental file is retained by the author and your access to the
supplemental files is subject to the ProQuest Terms and Conditions of use.
Depending on the size of the file(s) you are downloading, the system may take some time to download them. Please be