Hemispheric specificity of perceptual learning effects under hard conditions

Previously, we found hemispheric differences when testing feature search with arrays wholly in one hemifield (Pavlovskaya et al., Spatial Vision, 2001), rather than testing with central arrays and lateral target elements. In parallel, Ahissar & Hochstein (Nature, 1997) found that perceptual learning transfer across position or orientation depends on the spatial conditions of the task: Learning effects transfer for easy tasks (large target-distractor difference; limited target-position uncertainty; long test-to-mask stimulus onset asynchrony) and are considerably specific with harder conditions. These differences are presumably related to cerebral modification site: hard tasks are seen as requiring low-level (specific) representations while easy tasks are performed using high cortical level mechanisms alone. We now ask if inter-hemispheric transfer also depends on task difficulty. Hard task learning might not transfer since low-level neurons have local receptive fields. Easy tasks, on the other hand, might transfer more within than across hemispheres. Subjects performed color and orientation feature search, each with arrays in one hemifield. Separate sessions had easy or hard conditions. Following training, we switched the sides of the orientation and color tasks. We find nearly complete transfer for easy conditions, and considerably less with difficult conditions. Our results support the notion that feature search with easy conditions is performed at high cortical levels where representation mechanisms include much of the visual field on both sides of the vertical meridian.