Eye position modulation of visual cortex and the sensory set hypothesis

What we see depends on where we look. This is obvious as a statement about the nonuniformity of our external visual environment. But it is also true, in a much less obvious sense, as a statement about the internal neurophysiology of the visual system. What we see depends on where we look in the neurophysiological sense that eye position signals have a dramatic effect on the responsiveness of visual cortical neurons. This thesis empirically studies the way in which point of regard (what point in space the eyes are fixating) influences neurons in visual cortical areas V1 and V4 and then presents a theoretical exploration of how these two different ways in which "What we see depends on where we look" might be functionally intertwined.;The empirical data presented here adds to the growing body of evidence that eye position signals are ubiquitous in visual cortex, an observation which reopens speculation about the functional role that these signals might play in different visual cortical areas. The presence of eye position signals in visual areas of the ventral visual processing stream raises the possibility that these signals might facilitate object identity. Eye position signals might be exploited by visual cortex as a conditioned stimulus, which can become functionally linked to the responses of visual cortical neurons (unconditional response) through repeated pairing with the unconditioned stimulus, the retinal stimulus, in a classical conditioning paradigm. In this way the visual system would be capable of learning systematic relationships between point of regard and statistical characteristics of the visual environment. The learned response to the conditioned stimulus could then be exploited as a preparatory signal, to speed or otherwise alter visual processing to suit the current context. In exploring this theoretical viewpoint, we discuss the circumstances under which context dependent coding provides advantages and how a code switching strategy might be implemented through physiological parcellation mediated by gain control. Eye position signals are here considered to be one among many different types of extra retinal signals present in visual cortical areas, whose presence might be similarly exploited. As such, the data and theory presented here should be considered as contributing to the broader literature on the influence of signals from outside the classical receptive field.