Contextual influences on orientation discrimination: binding local and global cues

We sought to determine how local and global features within an image intera ct by examining whether orientation discrimination thresholds could be modi fied by contextual information. In particular, we Investigated how local or ientation signals within an image are pooled together, and whether this poo ling process is dependent on the global orientation content present in the image. We find that observers' orientation judgments depend on surround con textual information, with performance being optimal when the center and sur round stimuli are clearly distinct. In cases where the center and surround were not clearly segregated, we report two sets of results. If there was an ambiguity regarding the perception of a global structure (i.e. a small mis match between local cues), observers' performance was impaired. If there wa s no mismatch and local and global cues were consistent with the perception of a single surface, observers performed as well as in the distinct surfac es case. Although some of our results can be largely accounted for by inter actions between differently oriented filters, other aspects are more diffic ult to reconcile with this explanation. We suggest that low level filtering constrains observers' performance, and that influences arising from image segmentation modify how local orientation signals are pooled together. (C) 2001 Elsevier Science Ltd. All rights reserved.