FIGURE 4 | Interhemispheric transfer in AgCC.

Illustration of interhemispheric transfer (IHT) limitations in individuals with agenesis of the corpus callosum (AgCC). Panels a and b show an absence of interhemispheric conduction of the early visual evoked potential components that index sensory activity in the extrastriate visual cortex (that is, P1 and N1 components), in both patients with commissurotomy and individuals with AgCC⁶⁸. a | Visual evoked potential (EP) recording paradigm. Right visual field (RVF) stimuli (top, solid lines) first result in evoked responses from locations within the left hemisphere (bottom left), and then following IHT (middle, solid arrow) evoke responses in right hemisphere locations (bottom right). The bottom panels show samples of typical evoked potentials from the left and right hemisphere recording locations within a healthy brain in response to the RVF (solid lines) and left (dashed lines) visual field (LVF). b | Comparison of left hemisphere evoked responses to stimuli in the right (solid lines) and left (dashed lines) visual fields. In the normal brain (top), the delay created by IHT is indicated by the later and smaller P1 and N1 components to the LVF stimuli (dashed lines) compared with RVF responses (solid lines). P1 and N1 components for LVF stimulation are absent in left hemisphere recordings of both the person with AgCC (middle) and the patient with commissurotomy (bottom), indicating that the corpus callosum is necessary for the IHT of visual information. c,d | Experimental conditions that reveal limitations in the ability to compare visual information from right and left visual fields⁶⁸. Each square is an example of a stimulus used in a letter- (c) and dot pattern- (d) matching task. While participants looked at a central fixation point (solid diamond), two stimuli to be matched were flashed tachistoscopically in various configurations (bilateral or unilateral) in each trial. Patients with AgCC could make bilateral letter matches as well as control participants (presumably by using extra-callosal pathways). However, patients with AgCC could not successfully match bilaterally presented dot patterns, which is a more complex task that cannot use semantic simplification, suggesting that there is a limit on information transfer via non-callosal pathways. Panel a modified, with permission, from Ref. 147 © (1993) Elsevier Science.

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