Objectives: 

We investigated the influence of feed-back from the primary visual cortex (V1) on the spatial organization of the receptive fields of cells in the lateral geniculate nucleus (LGN) in awake monkeys, using the novel approach of suppression via repetitive transcranial magnetic stimulation (rTMS).

Materials: 

We recorded neuronal activity in the LGN of two awake behaving monkeys. Animals were trained to maintain visual fixation within a 0.5° window. Control visual stimuli consisted of circular patches of different diameters (0.6° and 6°) centered over the receptive field of the cell under test. Patches were filled with a grating of optimal spatial frequency (drifting or static). The stimulus presentation was then repeated (i) during blockade of V1 induced by rTMS at low frequency (0.8Hz for 4 minutes) and (ii) after a recovery period that lasted up to 20 minutes.

Results: 

We recorded 28 cells before and after induction of the cortical blockade. The most common effect was a reduction of spontaneous and visual evoked neuronal activity seen immediately after TMS application (21 cells). This effect was more pronounced when the small stimulus was used, appearing to diminishing the center response, significantly altering the centre/surround balance. Interestingly, in 10 cells (35% of the sample) cortical blockade appeared to "displace" the RF centre position (up to 4 degrees) while simultaneously increasing the inhibitory component of the RF, which might reflect the uncovering of otherwise suppressed excitatory drives contributing to the RF, or a contribution from cortical areas not captured in the local blockade.

Conclusions: 

Cortical feedback to the thalamus dynamically regulates the responses to stimuli over the RF by modulating center surround interactions, including the spatial distribution of separate RF elements