Right hemifield deficits in judging simultaneity: A perceptual learning study

Prior reports demonstrate that simultaneity is judged less precisely in the right visual field (RVF) than in the left visual field (LVF). The present psychophysical study was conducted to provide new information about why and when (i.e., the visual information stage at which) RVF deficits arise in simultaneity judgments. In Experiment 1, participants judged either the simultaneity or the relative spatial frequency of Gabor targets in the right or left hemifield while distractors were randomly absent or present. When attention was not needed to exclude distractors, signal detection theory analyses revealed an RVF simultaneity deficit with an error pattern that implicates low RVF temporal acuity, not excessive RVF neural noise. Adding attentionally demanding distractors introduced a separate, significant RVF simultaneity deficit with error patterns that implicate the inappropriate integration of temporal asynchronies from distractor locations. Neither the distractor-independent RVF acuity deficit nor the distractor-induced RVF excessive spatial integration occurred for spatial frequency discrimination at the same retinal locations. In Experiment 2, a perceptual learning procedure significantly improved RVF simultaneity judgments. The learning was task-specific but generalized to the untrained (left) visual field and to novel retinal locations. This observation implicates the simultaneity decision as the visual information stage that sets the limit on performance.