Rees, Geraint Ellis; (1999) An investigation of the neural correlates of selective attention in humans using functional imaging. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

To what extent does perception depend on attention? The work presented here uses functional imaging to explore this question by examining whether stimuli produce brain activity when they are not directly attended. The effects of unattended stimuli on brain activity are explicitly measured by systematically varying the rate of presentation, identity or presence of ignored stimuli. First, the rate of presentation of visual and auditory information was varied to give an index of stimulus processing. Activity evoked by attended and unattended stimuli differs profoundly in a way that suggests that attention operates through two distinct physiological mechanisms. In sensory cortex, even ignored stimuli evoke rate correlated brain activity, suggesting processing of these unattended stimuli. Second, the consequences of such processing were studied by systematically manipulating the identity of ignored distractors. When distractors produced negative priming, subcortical structures including the striatum were active, suggesting a link between negative priming and implicit learning. Finally the determinants of unattended processing were explored in a series of experiments where perceptual load of the primary task was varied. The degree to which ignored stimuli were processed, even when highly salient, depended strongly on the perceptual load of the task and the availability of attention. In the case of single words, eliminating the availability of attention led to inattentional blindness, and no brain activity was evoked that related to their meaning. These findings suggest that attention has a pervasive influence on sensory processing in multiple cortical areas and that the perceptual load of a task is an important determinant of selectivity. Moreover the results suggest an intimate link between the availability of attention and perceptual awareness.

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