UC San Diego

I ask whether rats have pattern vision. Is the rat's visual system influenced by the spatial context surrounding a feature? Specifically, is the processing of an oriented target feature influenced by the relative orientation and relative position of nearby visual features?. In chapter 1, I provide an introduction to the early visual system and the role of contextual processing. I review the influence of spatial context at the level of neural responses, anatomy, theory, and psychophysics. In chapter 2, I describe the experimental paradigm we developed in the lab. The methods were sufficient to train rats on a range of visual tasks, including training them to detect small faint oriented visual targets in the presence of nearby flanking stimuli (̀flankers'). In chapter 3, I ask whether the geometric relationship between the flankers and the target influences the rats' ability to detect an oriented target. Indeed, of all the spatial configurations I tested, one condition was harder than the rest: when all the stimuli where collinear. This finding shows that rats are influenced by spatial context when detecting a faint target. In chapter 4, I ask whether a difference in the contrast of target and flanker will reverse the impairment observed in collinear stimuli. Evidence from human behavior and neural responses suggests that performance might improve when targets are faint and the surrounding contrast is strong. In no case did the presence of flankers improve the rat's ability to detect a target. A model is fit to the behavioral data to explain the animal's impairment and bias. In chapter 5, I include a pilot study to validate the feasibility of physiological recordings that isolate surround processing in the lateral geniculate nucleus (LGN) of the visual thalamus of rats. In one experiment, changes in surround contrast and luminance did not reliably influence the neural response to the target, suggesting that contrast normalization in the LGN is not substantial for briefly flashed targets with flankers. I also characterize basic response properties of LGN neurons in the awake and anesthetized rat. The differences in the response properties validate the importance of recording from neurons in awake animals