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1 School of Psychology, University of Nottingham, Nottingham, United Kingdom
* To whom correspondence should be addressed. E-mail: bsw{at}psychology.nottingham.ac.uk.
A suppressive surround modulates the responsiveness of cells in the lateral geniculate nucleus (LGN), but we know nothing of its spatial structure or the way in which it combines signals arising from different locations. It is generally assumed that suppressive signals are either uniformly distributed or balanced in opposing regions outside the receptive field. Here, we examine the spatial distribution and summation of suppressive signals outside the receptive-field in extracellular recordings from 46 LGN cells in anaesthetized marmosets. The receptive-field of each cell was stimulated with a drifting sinusoidal grating of the preferred size and spatial- and temporal-frequency; we probed different positions in the suppressive surround with either a large half-annular grating or a small circular grating patch of the preferred spatial-and temporal-frequency. In many of the cells with a strong suppressive surround (29/46), the spatial distribution of suppression showed clear deviation from circular symmetry. In the majority of these of cells, suppressive signals were spatially asymmetrical or balanced in opposing areas outside the receptive-field. A suppressive area was larger than the classical receptive-field itself and spatial summation within and between these areas was non-linear. There was no bias for suppression to arise from foveal or nasal retina where cone density is higher, and no other sign of a systematic spatial organization to the suppressive surround. We conclude that non-classical suppressive signals in LGN deviate from circular symmetry and are non-linearly combined.
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