Orientation tuning of surround suppression in lateral geniculate nucleus and primary visual cortex of cat

We previously suggested that orientation-tuned surround suppression of responses of cells in the primary visual cortex (V1) is primarily caused by a decrease in geniculocortical input for the cell [Ozeki H, Sadakane O, Akasaki T, Naito T, Shimegi S, Sato H (2004) Relationship between excitation and inhibition underlying size tuning and contextual response modulation in the cat primary visual cortex. J Neurosci 24:1428-1438]. To further test this hypothesis, we compared the strength of orientation and spatial phase selectivity of surround suppression, and the spatial extent of the extraclassical receptive field (ECRF) between the lateral geniculate nucleus (LGN) and V1 neurons of anesthetized cats. Extraclassical surround suppression in the LGN was well tuned to orientation-contrast and relative spatial phase between the classical receptive field (CRF) and ECRF stimuli. Significant orientation-tuned surround suppression was observed in 72.6% of the LGN neurons and the 66.7% of the V1 neurons tested. The degree of orientation selectivity of ECRF in LGN was comparable to that in V1; however, the strength of the relative spatial phase selectivity of ECRF in LGN was higher than that previously reported for V1 [Akasaki T, Sato H, Yoshimura Y, Ozeki H, Shimegi S (2002) Suppressive effects of receptive field surround on neuronal activity in the cat primary visual cortex. Neurosci Res 43:207-220; DeAngelis GC, Freeman RD, Ohzawa I (1994) Length and width tuning of neurons in the cat's primary visual cortex. J Neurophysiol 71:347-374]. In 70% of the LGN neurons that exhibited significant orientation-tuned extraclassical surround suppression, the effective orientation of the suppression varied according to a change in the orientation of CRF stimulus, while the remaining 30% exhibited a fixed preferred orientation of the suppression regardless of the orientation of the CRF grating. These results suggest that the basic properties of surround suppression, such as orientation and spatial phase tuning, already exist in cat LGN and that a decrease of surround suppression in excitatory inputs from LGN by surround suppression is the primary cause of surround suppression in V1. Corticogeniculate feedback may further elaborate the properties of surround suppression in LGN.