Chun Wang
Department of Anatomy and Histology, Bosch Institute, University of Sydney, NSW 2006, Australia

Purpose:Neurons in area 17 (A17, striate cortex, area V1) are tuned to stimulus size. For many cells, after reaching a peak the magnitude of the response decreases with further increases in stimulus size. Some neurones exhibit a response rebound or “disinhibition” at larger stimulus sizes. A similar size-tuning property has been reported for some relay cells in the dorsal lateral geniculate nucleus as well as some retinal ganglion cells. Here, we investigated the influence of feedback on classical receptive field centre (cRF) and surround of A17 neurons by examining their size-tuning properties during reversible inactivation of cytoarchitectonic area 20 (A20, presumed homologue of primate inferotemporal cortex). Methods: Adult cats were anaesthetized, paralysed and artificially ventilated. The electroencephalogram, heart rate and expired CO2 were monitored continuously throughout the experiments. Single neurons were recorded extracellularly from the part of A17 visuotopically corresponding to the part of visual field represented in A20. Cells were tested with sinusoidal gratings optimal for individual cells at a contrast producing 80% of maximal response and were classified as simple (S-), if the phase variant Fourier component (F1) of their responses is larger than the phase invariant (F0) component, I.e. F1/F0 >1 or complex (C-), if F1/F0 <1. Reversible inactivation was achieved by cooling A20 to 10oC. Results: For twenty-nine cells examined, fourteen (48%) of them showed a significant change (P<0.05) in their size-tuning properties during inactivation of A20. Most of them (9/14; 64%) exhibited a predominant reduction (mean: 56%) in the magnitude of responses and the remainder (5/14; 36%) showed a facilitation (mean: 46%). Furthermore, for seven cells exhibiting a significant disinhibition at larger stimulus sizes inactivation of A20 tended to reduce the amount of disinhibition. We have also found that during inactivation of A20 or when a grating extending beyond cRF and activating silent suppressive surrounds a substantial number of C-cells responded like S-cells, I.e. the magnitude of F1 component exceeded that of F0 component or F1/F0>1. Conclusions: 1) Feedback from A20 exerts a mostly excitatory influence on both the centre and surround of the receptive fields of A17 neurons; 2) In a subgroup of cells, it contributes specifically to the disinhibition in a distal region of the surround; 3) Influence from higher-order area(s) and/or from silent surround play an important role in determining simple/complex dichotomy in A17.

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