Perceptual priming does not transfer interhemispherically in the acallosal brain

Acallosal participants usually do not display any disconnection signs in tasks requiring an explicit or declarative type of response. They can accurately compare stimuli placed in each hand and they perform normally on lateralized recognition tasks. They, however, show impairment in tasks assessing interdependent motor control, bilateral coordination and they are also unable to intermanually transfer an implicit procedural motor skill. These deficits suggest that compensation might be limited when a motor component is involved. Alternately, it is also possible that interhemispheric transmission in callosal agenesis might be limited when implicit or unconscious processes are involved (Berlucchi et al. in Neuropsychologia 33:923-936, 1995; De Guise et al. in Brain 122:1049-1062, 1999). The aim of the present study was to assess interhemispheric transfer in two distinct nondeclarative tasks, namely visuoperceptual skill learning and perceptual priming, with a lateralized version of the fragmented picture task developed by Snodgrass et al. (Behav Res Methods Inst Comp 19:270-274, 1987) that did not involve any motor component. The performance of five acallosal and one early-callosotomized individuals was compared to that of control participants divided into four groups (n = 10) according to which hemisphere was trained (left or right) and which response mode was used (manual or verbal). Visuoperceptual skill learning was observed in all control groups except for the group submitted to training of the left hemisphere in the manual modality of response. The acallosal and early-callosotomized participants did not show any implicit learning of the visuoperceptual skill on any of the conditions. The evaluation of the perceptual priming effect in the second part of the testing revealed that the priming effect was restricted to the trained hemisphere in participants without corpus callosum, as opposed to all neurogically intact participants who presented interhemispheric transfer of the priming effect. These findings indicate that the compensatory pathways, most probably subcortical commissures, are insufficient to allow interhemispheric transfer of perceptual priming, confirming the limits of neural plasticity in the absence of the corpus callosum. They also support the dissociation between declarative and nondeclarative memory in the split-brain and acallosal participants suggested by Berlucchi et al. (1995) and observed by De Guise et al. (Brain 122:1049-1062, 1999). These results are further discussed within the context of neurobiological theories of memory systems.