Callosal connections correlate preferentially with ipsilateral cortical domains in cat areas 17 and 18, and with contralateral domains in the 17/18 transition zone.

Previous studies have shown that the distribution of callosal connections in the 17/18 callosal zone of the cat is patchy at a small scale, but the mechanisms that determine this periodic pattern remain unclear. The present study investigated this issue by correlating the distribution of retrogradely labeled callosal cells with the underlying patterns of ocular dominance columns (ODCs) revealed transneuronally after intraocular injections of wheat germ agglutinin-horseradish peroxidase. The density of labeled callosal cells was found to vary significantly between adjacent territories dominated by different eyes, indicating that the distribution of callosal cells is significantly biased toward domains that are eye specific. Moreover, callosal connections relate to the pattern of ODCs in a rather unique way: callosal cells correlate preferentially with contralateral ODCs within the 17/18 transition zone (TZ), and with ipsilateral ODCs in regions of areas 17 and 18 located outside the TZ. Similar results were obtained in cats raised with strabismus, indicating that the overlap between right and left ODCs present in normal cats does not influence the correlation between callosal neurons and ODCs. The results are consistent with the hypothesis that callosal linkages are stabilized during development by interhemispheric correlated activity driven by bilateral projections from temporal retina. It is proposed that developmental constraints imposed by both this retinally driven mechanism and the pattern of ODCs are likely to determine not only the association of callosal clusters with specific sets of ODCs, but also important aspects of the functional characteristics of the callosal pathway in cat striate cortex. Copyright 2001 Wiley-Liss, Inc.