Eect of luminance on successiveness discrimination in the absence of the corpus callosum Bettina Forster a, *, Paul M. Corballis b , Michael C. Corballis c a University of Verona, Human Physiology Section, Department of Neurological and Visual Science, Strada La Grazie 8, 37134 Verona, Italy b Department of Cognitive Neuroscience, Dartmouth College, Hanover, NH, USA c Research Centre for Cognitive Neuroscience, Department of Psychology, University of Auckland, Auckland, New Zealand Received 8 June 1998; received in revised form 26 April 1999; accepted 25 June 1999 Abstract Three split-brained subjects, one with full forebrain commissurotomy and two with callosotomy, were impaired at judging whether pairs of lights in opposite visual ®elds were successive or simultaneous. This impairment did not vary with luminance when the lights were grey against a dark background, but was more pronounced when the lights were equiluminant with a yellow background. All three subjects were also better able to discriminate succession from simultaneity when the lights were both in the left visual ®eld than when they were both in the right. A fourth subject with callosal agenesis was only slightly impaired relative to normal subjects, who were virtually errorless. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Temporal discrimination; Callosal agenesis; Corpus callosum; Interhemispheric integration; Split brain 1. Introduction Ramachandran et al. [27] have reported evidence that perception of apparent motion, induced by pairs of lights presented in succession on either side of the vertical midline, is surprisingly little impaired in people who have undergone complete forebrain commissurot- omy. One of the subjects, L.B., identi®ed both the pre- sence and direction of succession across the midline with 100% accuracy, while another, A.A., also scored signi®cantly above chance. Moreover, the upper threshold of apparent motion across the midline, where the perception of motion gives way to the per- ception of simple succession, was approximately the same in L.B. and A.A. as in normal subjects. Another commissurotomised subject, N.G., was highly accurate at discriminating leftward from rightward succession, but judged all simultaneous pairs of lights to be mov- ing. Ramachandran et al. suggested that apparent motion across the midline in the split brain might be mediated by the so-called second visual system, which bypasses the lateral geniculate and relays through the superior colliculus and pulvinar nucleus; indeed, this system may play a more general role in unifying the visual ®eld in commissurotomised patients [31]. Nevertheless, there are some indications that the detection of succession across the midline may be de- ®cient in split-brained people, and may perhaps be based on inference rather than perception of apparent motion. Gazzaniga [13] found that a callosotomised subject, J.W., was unable to discriminate pairs of lights presented in succession on either side of the midline from a single light presented in one hemi®eld, suggesting that the perception of succession depends on the set created by the experimental conditions. Nai- kar and Corballis [25] found, however, that the com- missurotomised subject L.B., unlike J.W., was able to distinguish single lights from successive pairs with high accuracy, and they also con®rmed that he could accu- rately discriminate leftward from rightward succession, Neuropsychologia 38 (2000) 441±450 0028-3932/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0028-3932(99)00087-1 www.elsevier.com/locate/neuropsychologia * Corresponding author. Tel.: +39-45-8098104; fax: +39-45- 580881. E-mail address: bettinaf@borgorma.univr.it (B. Forster).