Humans born without a corpus callosum (CC) are often comparatively slow and clumsy on tasks requiring bilateral motor coordination. In this study, we attempted to identify correlates of CC agenesis in mice by examining an ecologically valid motor behavior: running-wheel performance. Mice with varying degrees of congenital CC deficits were tested on a running wheel apparatus for 7 consecutive days. The mice became more proficient at running with experience (Le., rotations, time spent running, length of running bouts, and maximum and modal running speeds increased while variability of rotation times decreased). Multiple regression analysis suggested that CC deficiency may be related to 2 out of 20 measurements of running (decreased variability of rotation times and shorter maximum running bout duration), but at levels below the traditional criteria for significance. Although these variables predicted CC deficiency at a level significantly greater than chance (71% correct) in a discriminant analysis, examination of the meaning of their relationship with CC size suggested that any connection may be spurious.Axons of the corpus callosum (CC) connect the cerebral hemispheres in placental mammals. Agenesis ofthe CC occurs in the embryo when axons are unable to cross the cerebral midline but instead grow longitudinally in the novel Probst bundle (Ozaki, Murakami, Toyoshima, & Shimada, 1987;Ozaki & Shimada, 1988) and soon return to the ipsilateral cerebral cortex (Ozaki & Wahlsten, 1993). In humans, the incidence of callosal agenesis is estimated to be 0.0005%-0.7% (Wisniewski & Jeret, 1994), and its occurrence is often associated with other neurological deficits (Aicardi & Chevrie, 1994;Andermann & Andermann, 1994). It is paradoxical that humans born without a CC exhibit comparatively few behavioral or performance deficits, especially in comparison with patients who have had their CC surgically split (Sauerwein & Lassonde, 1994;Sperry, 1982). Unlike callosotomy patients, acallosals can transfer information from one hemisphere to another, suggesting that plasticity exists in the formation of forebrain cortical connections. The most consistent functional finding is that acallosals often exhibit deficits on motor tasks requiring bimanual coordination, especially under speed stress (Silver & Jeeves, 1994).Four strains of mice (BALB/c, I/LnJ, 129, and ddN) frequently exhibit congenital agenesis of the CC to vary-
