1. The problem of the interhemispheric transfer of visual information in humans has been approached psychophysically, making use of a visual discrimination task that shows a clear left field advantage and is subject to the phenomenon of perceptual learning. 2. For this task (discrimination of complex gratings differing only by the relative spatial phase of their harmonic components) there is a left field advantage and a lack of interhemispheric transfer of learning effects at all spatial frequencies tested for stimuli removed at least 5 deg from either side of the vertical meridian. 3. For stimuli close to the vertical meridian, the left field advantage disappears and there is a complete transfer of learning effects, provided the fundamental spatial frequency is 2 cycles/deg or lower. 4. At higher spatial frequencies the left field advantage is maintained and the learning effects do not transfer from one visual hemifield to the other, even at +/- 0.5 deg from the vertical meridian, unless the contrast is very high. 5. The transfer of learning effects obtained for spatial frequencies of 2 cycles/deg or lower is peculiar to regions placed close to the vertical meridian and symmetrically located on either side of it. No transfer is obtained between non-overlapping regions on the same side of the vertical meridian. 6. These findings are consistent with an interhemispheric transfer of visual information, preferential for low spatial frequencies and high contrasts, in agreement with that found for callosal transfer in the cat (Berardi, Bisti & Maffei, 1987).
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