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E. Cameron, Michael Levine, Jennifer Anderson; The effect of task and target size on the north effect . Journal of Vision 2016;16(12):227. doi: https://doi.org/10.1167/16.12.227.
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© ARVO (1962-2015); The Authors (2016-present)
Several studies have reported that orientation discrimination in the upper visual field is poorest directly above fixation (the north effect). Performance improves as stimuli appear further from the vertical meridian (VM), up to ~1.5 deg from the VM (Cameron, Levine & Anderson, VSS 2015). The neural mechanism underlying the north effect is not well understood. One behavioral approach that might lead to a better understanding is to measure performance on tasks that do not rely on orientation-tuned mechanisms. In this study, we explored the effect of visual field location on a simple detection task. In addition, we manipulated stimulus size to explore the extent to which mechanisms near the VM could counteract the north effect. We measured performance on a yes-no detection task with gray squares of two sizes (0.15 and 0.3 deg.) presented at one of 36 locations, concentrated within 30 deg. of the VM at 4.5 deg. eccentricity. We also measured performance on a 2AFC orientation discrimination task of 2cpd (1 and 2 deg.) and 8cpd (0.5 and 1 deg.) Gabors. Target contrast was chosen for each observer and task in pilot threshold tests such that overall performance was maintained at ~80% correct. Visual performance fields were fit with hemi-ellipses (Anderson, Cameron & Levine, 2014) and we computed difference ratios (predicted performance from the hemi-ellipse fit minus observed performance, at each location) and examined how they varied with angular degree. In all conditions, performance improved (ratios decreased) as stimuli appeared further from the VM. A north effect was observed under all conditions, including simple detection. Surprisingly, size did not mitigate the effect. In fact, in some cases (8cpd and detection task) the effect became more pronounced. These results suggest that the north effect is not solely dependent on orientation mechanisms and may rely on more primitive mechanisms.
Meeting abstract presented at VSS 2016
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