Neurophysiology and neuropharmacology of visual attention

Abstract

There is ample evidence that attention to stimuli can facilitate perception under different experimental tasks. For example, human observers are faster and more accurate at detecting an object in a visual scene when they know in advance its location, motion or colour. Previous electrophysiological studies on attentional modulation have characterized the effects of attention on firing rates and oscillatory activity. They have also studied how attention can change basic neuronal integration properties, such as the size of the classical receptive field or its summation area. However, the cellular mechanisms underlying this response modulation are not clear. In this thesis, I investigate which neurotransmitters and receptors contribute to attentional modulation in the primate brain. I use pharmacological manipulations on neurons in the primary visual cortex (V1) while monkeys perform a visual spatial attention task. The contribution of two main neuromodullatory systems, the cholinergic and glutamatergic system, to visual attention is examined. Findings reveal that the amount of attentional modulation in V1 is augmented when the cholinergic system is pharmacologically enhanced. This effect is mediated by the activation of muscarinic, but not nicotinic, receptors. Glutamatergic NMDA receptor activation also leads to enhanced attentional modulation in V1, although the effects are largely restricted to improved response reliability and less to increased response gain. We note that both attention and acetylcholine can alter basic neuronal coding, namely integration properties of V1 neurons. Our findings show that acetylcholine affects contextual integration through muscarinic receptors, while nicotinic receptors affect the gain with no changes in the integration. Additional investigations of the cholinergic (and gabaergic) mechanisms in extrastriate area (area MT) are conducted, as ACh may result in improved direction selectivity computations similar to those reported in the attention literature. It appears that acetylcholine does not increase neuronal sensitivities as measured by a sharpening in the motion--‐ direction tuning curve. Instead, it improves response reliability to optimal stimulus features. Taken together, these findings may have implications to neuromodulatory accounts of visual attention.