Callosum-dependent binocular interactions in the lateral suprasylvian area of Siamese cats which lack binocular neurons in areas 17 and 18

1996

J. Comp. Neurol.

In the cat, callosal connections in area 17 are largely confined to a 5-6-mm-wide strip at the 17/18 border. It is commonly thought that callosal fibers extending from between the 17/18 border regions interconnect loci that are mirror-symmetric with respect to the midline of the brain, but this idea has not been tested experimentally. The present study examined the organization of callosal linkages in the 17/18 border region of normal adult cats by analyzing the patterns of connections revealed in one hemisphere after small injections of different fluorescent tracers into the opposite 17/18 callosal region. The location of the injection sites within areas 17 and 18 was assessed by examining architectonic data and by inspecting the labeling pattern in the ipsilateral visual thalamus. Area 17 and 18 were separated by a 1-1.5-mm-wide zone of cytoarchitectonic transition rather than by a sharp border. The results show that, in general, callosal fibers interconnect loci that are not mirror-symmetric with respect to the midline. Thus, area 17 injections placed nearly 3 mm away from the 17/18 transition zone produced discrete labeled areas located preferentially within the contralateral 17/18 transition zone. However, when the injection site was within the 17/18 transition zone, labeled cells were found primarily medial and lateral to, but not within, the 17/18 transition zone in the contralateral hemisphere. Previous studies have indicated that the 17/18 transition zone contains a representation of a strip of the ipsilateral visual field. Comparison of the retinotopy of the 17/18 border region with the mirror-reversed pattern of callosal linkages found in the present study suggests that callosal fibers link points that are in retinotopic correspondence in both hemispheres.

Section: Contribution Of the Callosal Pathway To The Representation Osupporting

confidence: 85%

Non-mirror-symmetric patterns of callosal linkages in areas 17 and 18 in cat visual cortex

1996

J. Comp. Neurol.

“…For instance, in Figure 13, the segment 5-6 in both the right visual hemifield and right nasal retina is projected contralaterally to the left lateral callosal region, and from there it is-sent to the right lateral callosal region as an indirect ipsilateral visual field representation. Support for this idea comes from studies showing that callosal fibers are capable both of activating cortical neurons (Choudhury et al, 1965;Berlucchi and Rizolatti, 1968;Lepore and Guillemot, 1982;Antonini et al, 19851, and of conveying a representation of ipsilateral visual fields to striate (Payne, 1990) and suprasylvian cortex (Zeki and Fries, 1980;Marzi et al, 1980) in the cat.…”

Section: Role Of the Lateral Callosal Regionmentioning

confidence: 92%

Two rules for callosal connectivity in striate cortex of the rat

Lewis

J of Comparative Neurology

1995

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)

“…Previous studies have shown that callosal connections are capable of generating binocular cells in lateral striate cortex of the rat (Diao et al, 1983) and extrastriate cortex of Siamese cats (Zeki and Fries, 1980;Marzi et al, 1980). However, the issue of whether callosal connections contribute to binocularity in striate cortex of the cat remains unresolved.…”

Section: Functional Implications Of Uniocular Mechanisms Guiding Callmentioning

confidence: 96%

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

J of Comparative Neurology

2001

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.