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David A. Remus, Jess R. Kerlin, Frank Tong; Effects of attention and cognitive load on cortical responses to irrelevant stimuli. Journal of Vision 2004;4(8):621. doi: 10.1167/4.8.621.
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It has been shown that spatial attention directed to an object can enhance early visual responses, and that attention directed away from an object can lead to suppressed visual responses. However, it is less clear whether manipulation of attention in a non-spatial domain, such as varying cognitive load, has a similar effect on the processing of irrelevant stimuli and the responses they evoke in early visual cortex. Here, we investigated the comparative effects of cognitive load (using a working memory task) and spatial attention on fMRI responses in human retinotopic visual cortex. We measured neural responses to a flickering checkerboard annulus presented in the periphery while subjects fixated on the center of the display. Letters were presented at central fixation (1 item every 2.5 seconds) and subjects were cued to perform a 1, 2, or 3-back working memory task on the letter stream, or to attend to a flicker of the checkerboard stimulus while maintaining fixation. Regions of interest within visual cortex were established using flickering checkerboard stimuli in separate functional scans and were matched to retinotopic maps obtained in a separate session. High-level retinotopic areas such as V3A and V4 showed greater fMRI activity when spatial attention was directed to the checkerboard than when subjects attended to the letters (statistical comparison: attend checks > 1, 2, 3-back, p<.05). However, their response did not vary as a function of working memory load, suggesting that the level of cognitive load did not affect the degree of suppression found in retinotopic visual areas. In contrast, V1 showed no significant effect of spatial attention and no effect of cognitive load. Our results suggest that the effects of cognitive load in retinotopic visual cortex are relatively small, or perhaps negligible, as compared to the impact of the effects of spatial attention.
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