VISUAL ZONE OF THE CLAUSTRUM SHOWS LOCALIZATIONAL AND ORGANIZATIONAL DIFFERENCES AMONG RAT, GUINEA-PIG, RABBIT AND CAT

The retrograde axonal transport method was used to compare the topogra phy and organization of the visual zone of the claustrum in rat, guine a pig, rabbit and cat. First, massive Fluoro-Gold injections were plac ed into the primary visual cortex and the secondary areas. Experiments showed differences in the location of the visual zone among the anima ls under study. In rat, the visual zone occupied the posteroventral pa rt of the claustrum and spread to its anterior pole. In guinea pig, ne urons projecting to the visual cortex were located dorsally in the pos terior half of the claustrum. In rabbit, similarly to the rat, they we re localized in the posteroventral part; however, they did not reach t he anterior pole. In cat, neurons that project to the visual cortex we re concentrated dorsally in the posterior fourth of the claustrum. In double-injection experiments, Fast Blue and Diamidino Yellow were plac ed into the primary and secondary visual areas in various combinations . The experiments showed that in the rat and the rabbit claustral neur ons project to primary visual cortex (area 17) as well as to both seco ndary visual areas (areas 18a and b). Populations of neurons sending a xons to the primary and secondary areas showed full overlap. The prese nce of double-labeled neurons indicates that some claustral neurons pr oject both to the primary and secondary fields. In cat, neurons that p roject to the primary visual cortex appear to be clearly separated fro m those connected with the secondary visual area, as no double-labeled neurons were found. In all studied species, the double injections pla ced into the visual and primary somatosensory cortex did not result in any double-labeling neurons. Our results indicate that the location o f the visual zone in the posterior part of the claustrum is a phylogen etically stable feature, whereas its dorsoventral shift as well as the extent toward the anterior pole is related to the particular species. The overlap of neurons projecting to the primary and secondary visual areas in the rat and rabbit as well as the separation of both project ions in cat appear to reflect the higher degree of complexity of the v isual system in the latter.