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Abstract:
Voluntary attention affects both behavioral performance and neuronal activity, but the exact neural mechanisms underlying the effects of attention are poorly understood. Feedback projections from higher cortical areas are assumed to be instrumental in mediating the effects of attention, and cholinergic neuromodulation is also often implicated. To investigate the cholinergic influence on attention we recorded from cells in V1 of an alert macaque engaged in a task that required attention to be directed either towards or away from a receptive field (RF) of the neuron under study. Spatial cues indicated the required focus of attention. We then presented two bars of preferred orientation and variable length, one inside the neuron’s RF, the other in the opposite hemifield. The animal had to detect a small light increment in the center of the cued bar, and ignore light increments at the un-cued bar location. Cholinergic transmission was manipulated by applying the muscarinic antagonist scopolamine via a newly developed recording electrode/iontophoresis pipette, which allows dura penetration without the use of guide tubes, thus permitting access to V1 with minimal tissue damage. We recorded neuronal activity while the animal attended to and away from the receptive field of the neuron under study, and in the absence and presence of iontophoretic scopolamine application. Attending to the neuron’s RF generally increased neuronal activity compared to attending away. Scopolamine application always resulted in decreased neuronal activity. In a subset of neurons it also resulted in a significant reduction of the attention mediated response enhancement. This reduction was stronger at a bar length roughly matching the neuron’s RF diameter, while it was small or absent at bar lengths substantially larger than the neuron’s RF. These data demonstrate that the cholinergic system plays an important role in mediating effects of attention in V1 of the macaque monkey.
