Subjects performed two simultaneously presented visual tasks, the primary foveal task was a letter comparison task; the peripheral task was an arrow identification task. Previous studies, in which foveal cognitive load was manipulated, showed that in higher foveal load conditions reaction times (RTs) to peripheral stimuli slowed down equally throughout the visual field, suggesting general interference rather than tunnel vision. Moreover, a substantial amount of peripheral processing was performed in parallel with foveal task performance. In order to explore whether perceptual stages of both tasks can be processed in parallel, the present experiment manipulated both foveal and peripheral perceptual load. RTs for intact peripheral stimuli showed that a higher foveal perceptual load resulted in an overadditive effect of 27ms (general interference). A considerable amount of parallel processing was found at all four experimental conditions; much faster peripheral RT without slowing down foveal RTs. Underadditivity of the eccentricity effect in dual task conditions for intact peripheral stimuli and more parallel processing at high than at low eccentricities, indicated parallel performance of the perceptual stimulus pre-processing stage. An underadditive effect of peripheral degradation in high foveal perceptual load condition suggested a parallel performed perceptual feature extraction stage. An additive effect of peripheral degradation for low foveal perceptual load conditions, however, indicated that the amount of parallel processing depends upon the amount of `spare time' during peripheral task execution, induced by foveal task complexity. (A)
Abstract