Two rules for callosal connectivity in striate cortex of the rat

Abstract

In the rat, callosal cells occupy lateral as well as medial portions of striate cortex. In the region of the border between areas 17 and 18, which contains a representation of the vertical meridian of the visual field, cells projecting through the corpus callosum are concentrated throughout the depth of the cortex. In contrast, in medial portion of striate cortex, where peripheral portions of the visual field are represented, callosal cells are preferentially found in infragranular layers. These differences in topography and laminar distribution suggest that these callosal regions, referred to as medial and lateral callosal regions in the present study, subserve different functions. We explored this possibility by analyzing the patterns of callosal linkages in these two callosal regions. We charted the location of retrogradely labeled cells within striate cortex of one hemisphere after placing restricted injections of one or more fluorescent tracers into selected sites in the contralateral striate cortex. We found the medial and lateral callosal regions have distinctly different topographic organizations. Injections into medial striate cortex of one hemisphere produced labeled cells predominantly in mirror-symmetric loci in medial portions of contralateral striate cortex. The arrangement of these connections suggests that they mediate direct interactions between cortical regions representing visual fields located symmetrically on opposite sides of the vertical meridian of the visual field. In contrast, the mapping in the lateral callosal region is reversed: injections into the 17/18a border produced labeled fields located medial to the contralateral 17/18a border, while injections slightly medial to the 17/18a border produced labeled fields located at the contralateral 17/18a border.(ABSTRACT TRUNCATED AT 250 WORDS)

Transient Synaptic Silencing of Developing Striate Cortex Has Persistent Effects on Visual Function and Plasticity

“…In rats, the whole extent of primary visual cortex contains numerous callosal cells and terminals (Olavarria and Van Sluyters, 1985;Lewis and Olavarria, 1995). This organization is different from that observed in higher mammals such as cats and primates, in which callosal terminals are mainly concentrated at the border between area 17 and 18 (Olavarria, 1996;Houzel and Milleret, 1999).…”

“…The callosal pathway matures quite early in development and links retinotopically corresponding loci in the two hemispheres (Lewis and Olavarria, 1995;Olavarria, 1996). Interhemispheric linkages serve important functions, including binding together the separate representations of the two halves of the visual field (Berlucchi and Rizzolatti, 1968;Engel et al, 1991).…”

Cross-modal reorganization of callosal connectivity without altering thalamocortical projections

“…However, binocularity seems not to play an important organizing role in this pathway, since strabismus does not affect the pattern or number of callosal cells (49,50). Rather, the outcome of deafferentation is predicted by the effect of the manipulation on retinotopic correspondence between the two hemispheres (18,19,44,49,50). In deaf animals, there may be no information available on which to base elimination of connections, as in the retention of overlapped corticocortical (51) or thalamocortical (52) arbors after visual deprivation.…”

“…Callosal connections unite functionally and͞or topographically related cells in the two cortical hemispheres, and they are specific and different in each sensory cortical area (15)(16)(17)(18)(19). The projection is exuberant early in postnatal development (20,21), and becomes progressively restricted under the influence of sensory experience (16).…”

Interhemispheric connections between primary visual areas: beyond the midline rule

“…In addition to dense callosal connections packed along the 17/18a border, the entire mediolateral extent of rat striate cortex contains numerous callosal cells and terminals, forming a continuous band in infragranular layers (43). Further experiments based on targeted injections of retrograde tracers suggested a dual connectivity scheme (44): a) at the 17/ 18a border, non-mirror symmetric reciprocal connections would link cortical loci with roughly matching retinotopic projections along the vertical midline representation, as in cats; b) within A17, in contrast, callosal connections would link mirror-symmetric cortical regions, representing mirror-symmetric positions in the periphery of the visual field.…”