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E. Leslie Cameron; Perceptual inhomogeneities in the upper visual field. Journal of Vision 2005;5(8):176. doi: 10.1167/5.8.176.
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© ARVO (1962-2015); The Authors (2016-present)
Performance on visual tasks is poor for stimuli presented on the vertical rather than the horizontal meridian, and gets poorer as spatial frequency and eccentricity increase (Carrasco, Talgar & Cameron, 2001). Performance is particularly poor for stimuli presented directly above fixation - the north effect. In many of the experiments that have demonstrated such results, targets were presented at one of 8 locations across the visual field, at a fixed eccentricity. Thus, the nearest location to the vertical meridian tested was 45 deg away. The first goal of the present study was to examine the extent of the north effect by examining performance at many locations in the upper visual field. The question was: does performance increase gradually or abruptly as stimuli are presented further from the vertical meridian? Covert visual spatial attention (manipulated with peripheral precues) is known to improve performance equally across the visual field (typically measured at 8 locations). A secondary goal of this study was to assess the effect of attention at a wider range of locations. Three observers (2 naive) performed an orientation identification task with 8 cpd Gabor patches that were tilted either 15 deg clockwise or counter clockwise. Stimuli were presented briefly (mean 35 ms) at 4.5 deg eccentricity, at one of 17 locations across the upper visual field. Half of the trials contained a neutral precue (indicating when a target would appear) and half of the trials contained a peripheral precue (indicating when and where a target would appear). We found that, as previously reported, performance was best for stimuli presented on the horizontal meridian, and worst for stimuli presented on the vertical meridian. Performance gradually improved as stimuli were presented further from the vertical meridian, suggesting that the extent of the north effect may be larger than previously thought. The peripheral precue significantly improved performance across the upper visual field.
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