Direction discrimination training recovers fine orientation perception in V1-damage fields

The primary visual cortex (V1) is critical for the perception of oriented features, yet patients with V1 damage can relearn to perceive orientation in their blind field following deliberate training (Chokron et al., 2008; Das et al., 2014). Given the intimate link between orientation and direction processing, we asked if such patients can regain the ability to discriminate small orientation differences after training to discriminate motion direction. Thirteen V1-stroke patients were tested before and after motion discrimination training using random dot stimuli, with performance compared to 5 visually-intact controls. Participants were asked to indicate if the orientation of a 100% contrast Gabor was left or right of vertical (5° diameter, 1 cpd, 1° sigma, 500 or 50ms duration, followed by a 500ms bandpass noise mask). Orientation difference thresholds (72.5% correct) were estimated from a Weibull function fit to a 3:1 staircase of orientation difference in log steps (53°-0.1°). Patients were unable to discriminate orientation differences when stimuli were presented for 500 ms in their untrained blind field, but had normal orientation thresholds in their intact field (0.9+/-0.7° [CB]; 0.6+/-0.4° [controls]; t-test: p=0.42). After training, 67% of patients attained reliable orientation difference thresholds (5.4+/-4.6°); the rest failed to improve. Shorter stimulus durations (50ms) decimated post-training, blind-field performance, and halved the number of patients with orientation difference thresholds in their intact fields. Without visual training, CB participants are unable to discriminate fine orientation differences of static, high-contrast Gabors in their blind-field. Despite V1 damage, direction discrimination training restores conscious, fine orientation discrimination in the majority of participants, but only for long stimulus durations. Despite a lack of corresponding injury, the “intact” field was severely impaired in a subset of participants for brief stimulus presentations. Ongoing experiments aim to understand the neural substrates of fine orientation perception in V1-damaged visual systems.