Whereas these

movements have traditionally been viewed as random, it was recently discovered that microsaccade directions can be significantly biased by covertly attended visual stimuli. The detailed mechanisms mediating such a bias are neither known nor immediately obvious, especially because the amplitudes of the movements influenced by attentional cueing could be up to two orders of magnitude smaller than the eccentricity of the attended location. Here, we tested whether activity in the peripheral superior colliculus (SC) is necessary for this correlation between attentional cueing and microsaccades. We reversibly and focally inactivated SC neurons representing peripheral regions of visual space while rhesus monkeys performed a demanding covert Paclitaxel concentration visual attention task. The normal bias of microsaccade directions observed in each monkey before SC inactivation was eliminated when a cue was placed in the visual region affected by the inactivation; microsaccades were, instead, biased away from the affected visual space. When the cue was

placed at another location unaffected by SC inactivation, Cisplatin the baseline cue-induced bias of microsaccade directions remained mostly intact, because the cue was in unaffected visual space, and any remaining changes were again explained by a repulsion of microsaccades away from the inactivated region. Our results indicate that peripheral SC activity is required for the link between microsaccades and the cueing of covert visual attention, and that it could do so by altering the probability of triggering microsaccades without necessarily affecting the motor generation of these movements. Microsaccades are tiny eye movements that occur during gaze fixation. Although microsaccades have long been thought to be random and spontaneous, recent evidence has shown that these movements, like larger saccades, are influenced by visual and cognitive factors. The first explicit demonstration

of this was the finding that putative covert visual attention shifts affect microsaccades (Hafed & Clark, 2002; Engbert & Kliegl, 2003). In these first studies on this phenomenon, cueing attention to the periphery Farnesyltransferase biased microsaccades towards the cued location. The detailed mechanisms mediating such a bias are not immediately obvious, especially because the amplitudes of the movements influenced by cueing could be up to two orders of magnitude smaller than the eccentricity of the attended location. Thus, unlike the classic coupling between saccades and attention, which involves shifts to the same spatial endpoint (Rizzolatti et al., 1994; Sheliga et al., 1994), the coupling between microsaccades and attention involves shifts that could be in the same direction but of very different amplitudes. The existence of similar behavioral correlations between attention and microsaccades in monkeys (Hafed et al.