Attention modulates contexual processing in vision
Contextual information influences the neuronal processing and perception of visual stimuli. The functional significance of this influence may be to increase the efficiency of visual processing by taking advantage of redundancies in natural scenes. Increased efficiency may come at a cost of introducing errors, especially when stimuli are incongruous with the context. For optimal performance the visual system may therefore balance efficiency with accuracy by dynamically controlling the influence of contextual information. Attention is an appropriate mechanism to set this balance since attention is high when errors are costly and therefore accuracy is preferable over efficiency, but attention is low when accuracy can be sacrificed for efficiency. States of attention are associated with increased acetylcholine (ACh) efflux into the cortex. The effect of ACh on cortical processing has been investigated in a number of in vitro studies. They show that ACh causes a selective inhibition of intracortical synapses while thalamocortical synapses are unaffected or even enhanced. Thus, ACh effectively switches cortical processing in favour of feedforward inputs. In the visual system this switching would be expected to reduce contextual influences, thought to be mediated by intracortical processing. These findings suggest the hypothesis that attention will reduce contextual influences by the action of ACh. To investigate this hypothesis I present work from four separate experiments. I found that attention caused a reduction in contextual influences at the level of human perception (Experiment 1) and at the level of neurons in primate V1 (Experiment 3). I also found the application of ACh to cells in VI of anaesthetised primates caused a reduction in non-classical receptive field modulation (Experiment 2), similar to the effect of attention. Finally I found that attentional modulation of neuronal responses in macaque V1 was partially blocked by the application of a cholinergic antagonist, scopolamine (Experiment 4). Taken together my findings demonstrate that attention causes a suppression of contextual influences at the level of perception and at the level of the primary visual cortex. These effects were at least partly mediated by cholinergic mechanisms.