Dissociation of neural mechanisms underlying orientation processing in humans.

Orientation selectivity is a fundamental, emergent property of neurons in early visual cortex, and the discovery of that property has dramatically shaped how we conceptualize visual processing. However, much remains unknown about the neural substrates of this basic building block of perception, and what is known primarily stems from animal physiology studies. To probe the neural concomitants of orientation processing in humans, we employed repetitive transcranial magnetic stimulation (rTMS), which can significantly attenuate neuronal spiking activity, hemodynamic responses, and local field potentials within a focused cortical region. Using rTMS to suppress neural responses evoked by stimuli falling within a local region of the visual field, we were able to dissociate two distinct components of the neural circuitry underlying orientation processing: selectivity and contextual effects. Orientation selectivity gauged by masking was unchanged by rTMS, whereas an otherwise robust orientation repulsion illusion was weakened after rTMS. This dissociation implies that orientation processing in humans relies on distinct mechanisms, only one of which was impacted by rTMS. These results are consistent with models positing that orientation selectivity is governed by patterns of convergence of thalamic afferents onto cortical neurons, with intracortical activity then shaping population responses amongst those cortical neurons.