Gatsby Computational Neuroscience Unit Alexandra House, 17 Queen Square, LONDON, WC1N 3AR, UK, Tel: +44 (0) 20 7679 1176, Fax +44 (0) 20 7679 1173 abla@gatsby.ucl.ac.uk, www.gatsby.ucl.ac.uk   ABSTRACTS Venue B10 Seminar Room, Alexandra House, 17 Queen Square London, WC1N 3AR Supported by The Gatsby Foundation |Index|Objective|Attendees|Programme|Travel information|Accommodation| Acetylcholine, Spatio-Temporal Integration, and Attention M. Roberts, W. Zinke, A. Thiele University of Newcastle upon Tyne, UK We propose a new hypothesis regarding the role of attention. According to this hypothesis selective attention specifically modulates the balance between prior (feedback) and afferent (feed forward) information processing, decreasing the impact of prior information, making subjects more reliant on information coming in from the senses. In a psychophysical single/dual task paradigm we were able to demonstrate that under conditions of divided attention subjects’ judgments were substantially more biased by spatio-temporal priors compared to single task conditions, i.e. spatial attention seems to boost the effect of feed-forward/afferent processing at the expense of the effect of lateral and feedback processing. A pharmacological mechanism underlying these changes could rely on increased Acetylcholine under conditions of focused spatial attention. Recent evidence from slice studies suggests that Acetylcholine (ACh) modulates the balance between intracortical/feedback and afferent thalamocortical/feed-forward information processing. This re-balancing decreases the efficacy of lateral and feedback cortical processing, and might thereby increase the relative efficacy of thalamo-cortical/feed-forward processing.  We hypothesized that such re-balancing would affect centre-surround integration in primary visual cortex. We predicted that application of ACh should alter V1 neurons’ length tuning preferences inducing a shift towards shorter length preference. In line with our prediction we found that ACh induced a shift of preferred length towards shorter stimuli in the majority of neurons in an anaesthetized marmoset preparation.   Thus ACh modulates the flow of information in a dynamic fashion - decreasing effects of lateral/feed-backward connections while increasing the impact of information coming in from the eye. We are currently testing whether similar changes are present in V1 responses of awake macaque monkeys when spatial attention is directed towards the receptive field. We will compare preliminary data from the awake macaque recordings (attention in the RF vs. attention in the opposite hemifield) to the marmoset data  (high vs. low external Acetylcholine).