PhD ThesisContextual 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.Medical Research Council studentship
G78/7853, Wellcome grant GR070380, Biotechnology and Biological Sciences
Research Council grant BBSB/09325,
Royal Society small grant scheme
57400. G503/2936