Abstract
INTRODUCTION: Several studies have reported that performance in the upper visual field is poorest at locations directly above fixation (the north effect). This phenomenon is thought to be due to the pooling of orientation mechanisms in the early stages of cortical visual processing. Performance in studies of the north effect typically involve orientation discrimination. However, we have previously confirmed that this effect exists even for simple detection tasks (VSS, 2016). In this study we directly compare performance on detection and orientation discrimination tasks. METHOD: We measured performance on a yes-no detection task and a 2AFC orientation discrimination task for 2 and 8cpd Gabors (2 deg., 10 deg tilt). Targets were presented at one of 8 possible locations, equally spaced at 4.5 deg. eccentricity. Target contrast was determined for each observer in pilot tests, such that overall performance was maintained at about 80% correct in the discrimination task. The identical stimulus parameters were then used to measure target detection. Visual performance fields were fit with hemi-ellipses (Anderson, Cameron & Levine, 2014) and we computed difference ratios (predicted performance based on the hemi-ellipse fit minus observed performance, at the north location). RESULTS: We found that the north effect exist for both detection and discrimination. However, the north effect was larger for orientation discrimination than for detection. CONCLUSION: We confirm that the north effect is not solely dependent on orientation mechanisms and may also include more primitive mechanisms. However, these results indicate that the cortical mechanisms are not necessarily limited by the targets’ detectability but apparently provide a second stage that is orientation selective and also shows a north effect.