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Abstract
The ability to retain, and manipulate visual information in the mind is one of our most important human cognitive abilities. This ability varies widely across individuals yet why these large variations exist is poorly understood. In this thesis, I investigate the use of visual imagery as a cognitive strategy in visual working memory, and how this might produce these large individual differences. First, I carried out a correlational study where I found that participants with strong visual imagery (measured using binocular rivalry) tend to have larger visual working memory capacities. Next a new methodology using binocular rivalry was developed to measure the capacity limits of visual imagery. I found that the capacity limits of visual imagery are very similar to those found in the visual working memory literature. I then investigated the use of visual imagery as a mnemonic during visual working memory by employing a behavioural interference task (using background luminance) and neuro-stimulation (using transcranial direct current stimulation (tDCS)), which have both been found to modulate visual imagery strength. In this thesis and previous studies, I have shown that luminance attenuates visual imagery strength. Similarly, in this thesis I found that decreasing visual cortex excitability (using tDCS) resulted in stronger visual imagery, while increasing cortical excitability lead to small decreases in strength. If imagery is used as a mnemonic during visual working memory these two types of visual imagery modulators, luminance and neuro-stimulation, should likewise alter visual working memory performance. I found that background luminance perturbed performance on a visual working memory task, but only for good and high capacity imagers. When tDCS was used to alter visual cortex excitability during a visual working memory task good imagers showed the greatest changes to their visual working memory performance. This collection of studies suggests that visual imagery is a cognitive strategy used by some participants to complete visual working memory tasks, and may help to explain, in part, the large individual differences found in visual working memory. The implications of these findings are then discussed in terms of theories of visual working memory and visual imagery.