Abstract
Stroke damage to the primary visual cortex (V1) causes loss of vision known as cortically-induced blindness (CB). Our prior work showed that high-contrast visual training improves visual functions in both subacute (<6 months post-stroke) and chronic (>6 months post-stroke) CB patients. However, contrast sensitivity remained abnormal. Here, we asked if training patients to discriminate visual stimuli of progressively lower contrasts can further improve contrast sensitivity, whether this is equally effective in subacute and chronic participants, and whether temporal frequency and/or direction in the stimulus alters efficacy. Thirteen subacute (age:55±13yr, 9M/4F) and 11 chronic (age:56±13yr, 5M/6F) participants were trained at 2 non-overlapping blind-field locations on either orientation discrimination of static non-flickering Gabors (TF=0Hz, SF=1cpd), orientation discrimination of static flickering Gabors (TF=10Hz,SF=1cpd) or direction discrimination of drifting Gabors (TF=10Hz, SF=1cpd). Gabors were 4 or 5° in diameter, with blurred edges (sigma=1°). After daily home training for 3-6 months, we computed changes in contrast thresholds and contrast sensitivity functions at all trained locations and corresponding intact locations. Training with static Gabors (flickering and non-flickering) failed to improve contrast sensitivity in both subacute and chronic groups. Direction discrimination training also failed to improve contrast sensitivity in chronics. However, in subacutes, it did improve contrast thresholds (Wilcoxon signed-rank test, p=0.01) and contrast sensitivity functions (Wilcoxon signed-rank test, p<0.05). Our results suggest that training chronic CB participants with low-contrast Gabors does not restore basic discrimination performance on trained tasks, even when spatio-temporal frequency parameters optimal for blindsight are used. Only subacutes benefited from low-contrast training, and only when stimuli contained direction information. This may reflect relative preservation of motion-selective units in subacute participants, which could underlie a greater but time-limited potential to recover behavioral contrast sensitivity after V1 damage.