Visual cortical representations of bottom-up salience and perceptual continuity [electronic resource]

The amount of information available to the retina far exceeds the processing capacity of the visual system. Visual attention provides a means by which we can make use of the information available with optimal efficiency, allowing us to selectively process objects that are most likely to help us achieve our behavioral goals while filtering out others. Decades of research have revealed that visual representations of objects aligned with the locus of attention are enhanced, resulting in perceptual benefits. The attentional modulation of visual cortical responses, however, has been almost exclusively characterized within the context of volitionally deployed, top-down attention. Although the behavioral correlates of involuntarily deployed, bottom-up attention are well known, the underlying neural mechanism and its relationship to top-down attention are not. In Chapters 2 and 3 of this thesis, I describe experiments that investigate the neural correlates and mechanisms of bottom-up attention. In order to examine the neural correlates of bottom-up attention, we measured responses of single neurons in area V4 to "popout" stimuli, which differ uniformly from surrounding items and draw attention in a bottom-up manner, and compared them to responses to "conjunction" stimuli, which are composed of a combination of surrounding features and require top-down selection. We found that V4 responses are modulated by popout stimuli. This selectivity was more robust when larger numbers of surrounding items and multiple features were included in the display, and it was absent when only a few items were presented immediately outside the receptive field (RF). We next examined the relationship between bottom-up and top-down attention by measuring responses to these same stimuli while top-down attention was directed to locations distant from the RF. We found that popout selectivity was eliminated, indicating that the salience of popout stimuli is not sufficient to drive selection by V4 neurons under these conditions. These results demonstrate that neurons in feature-selective cortex are influenced by bottom-up attention, but that this influence is limited by top-down attention. Whether the source of the signal initiating attentional deployment is top-down or bottom-up, our gaze is typically aligned with the locus of our attention, making use of the high density of photoreceptors at the fovea of the retina. As we move our eyes around a visual scene to briefly fixate objects for further processing, we do not perceive the explosive onset of motion between each fixation that is created by these eye movements, nor do we perceive a series of individual snapshots from each fixation. Instead, we have a seamless visual experience of our surroundings. This perception of continuity across eye movements is crucial to efficient behavior, and yet the neural mechanisms that support a continuous visual experience are not well understood. In Chapter 4 of this thesis, I describe an experiment that investigates the neural representations of perceptual continuity during saccadic eye movements. In order to examine whether V4 neurons are involved in creating a continuous perceptual experience, we measured the responses of V4 neurons to probe stimuli presented during the preparation of saccadic eye movements to targets distant from the RF. We observed that the responses of area V4 neurons to probe stimuli depended on the similarity between the neuronal preference and the target of a saccade. Specifically, the suppression of V4 responses normally present during the preparation of saccades to targets outside of a neuron's RF was absent when the features of the target were congruent with the preference of the recorded neuron. This target-dependent modulation resulted in a substantial effect of the target match/non-match manipulation on neuronal selectivity. The results are consistent with a computational model in which saccade preparation automatically results in a global target-dependent modulation of visual cortical signals. We suggest that this modulation may contribute to the continuity of perception across saccadic eye movements. The results of this thesis demonstrate that V4 neurons are selective for stimuli that draw bottom-up attention. Surprisingly, this selectivity depends on the availability of top-down attentional resources, suggesting that these two distinct varieties of visual attention rely, at least in part, on a common mechanism. Additionally, this thesis provides evidence that V4 neurons contribute to a seamless perceptual experience by maintaining important visual information transaccadically.

Advisor: Moore Tirin
Knudsen Eric I. advisor. Thesis advisor
Raymond Jennifer L. advisor. Thesis advisor
Wagner Anthony David advisor. Thesis advisor