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1 Department of Neuroscience, School of Medicine, University of Pennsylvania, Philadelphia, Pa, USA
* To whom correspondence should be addressed. E-mail: nolt{at}mail.med.upenn.edu.
Based on extracellular recordings from 69 LGN cells in the anesthetized cat, we find spatial summation within their receptive fields to be dependent on the contrast of the stimuli presented. By fitting the summation curves to a difference of Gaussians model, we attribute this contrast dependent effect to an actual change in the size of the center mechanism. Analogous changes in spatial frequency tuning were also observed, specifically increased peaks and cutoff frequencies with contrast. These effects were seen across the populations of both X and Y cell types. In a few cases, LGN cells were recorded simultaneously with one of their retinal ganglion cell (RGC) inputs (S-potentials). In every case, the RGCs exhibited similar contrast dependent effects in the space and spatial frequency domains. We propose that this contrast dependency in the retinal ganglion cells results directly from a reduction in the size of the center mechanism due to an increase in contrast. We also propose that these properties first arise in the retina and are transmitted passively through the LGN to visual cortex.
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