The retinal input to calbindin-D28k-defined subdivisions in macaque inferior pulvinar.

Several studies have provided evidence for direct retinal input to the pulvinar of macaques monkeys, but there is no general agreement regarding the extent of this projection. Moreover, it is not known how retinal input correlates with chemoarchitectonic subdivisions recently recognized within the large, classical divisions of the pulvinar. The potential implications of this correlation have become more evident after reports that chemoarchitectonic subdivisions of the inferior pulvinar (PI) have specific patterns of connections with cortical visual areas. We have therefore re-examined the retino-PI projection using intraocular injections of horseradish peroxides, and correlated it with pulvinar subdivisions revealed using an antibody for calbindin-D28k. Retinal projections were found preferentially within the medial subdivision of the PI, with some involvement of the posterior and central calbindin-D28k defined subdivisions.

Organization of callosal linkages in visual area V2 of macaque monkey.

In visual area V2 of the macaque monkey callosal cells accumulate in finger-like bands that extend 7-8 mm from the V1/V2 border, or approximately half the width of area V2. The present study investigated whether or not callosal connections in area V2 link loci that are located at the same distance from the V1/V2 border in both hemispheres. We analyzed the patterns of retrograde labeling in V2 resulting from restricted injections of fluorescent tracers placed at different distances from the V1/V2 border in contralateral area V2. The results show that varying the distance of V2 tracer injections from the V1/V2 border led to a corresponding variation in the location of labeled callosal cells in contralateral V2. Injections into V2 placed on or close to the V1 border produced labeled cells that accumulated on or close to the V1 border in contralateral V2, whereas injections into V2 placed away from the V1 border produced labeled cells that accumulated mainly away from the V1 border. These results provide evidence that callosal fibers in V2 preferentially link loci that are located at similar distances from the V1/V2 border in both hemispheres. Relating this connectivity pattern to the topographic map of V2 suggests that callosal fibers link topographically mirror-symmetrical regions of V2, i.e., callosal fibers near the V1/V2 border interconnect areas representing visual fields on, or close to, the vertical meridian, whereas callosal connections from regions away from the V1/V2 border interconnect visuotopically mismatched visual fields that extend onto opposite hemifields.

A morphological anomaly of the dorsal lateral geniculate nucleus in Macaca fascicularis.

In the present report we describe a morphological anomaly of the thalamus. In three macaque monkeys (Macaca fascicularis), we observed up to five finger-like protrusions that emanated from the posterior pole of the dorsal lateral geniculate nucleus (LGN) and extended posteriorly between the lateral pulvinar and reticular nucleus of the thalamus. These anomalous fingers measured up to 1.7 mm in length and contained dense accumulations of neurons and glia. The fingers received a direct retinal input from the contralateral eye indicating that they were part of the LGN rather than of other adjacent thalamic nuclei. In order to determine with which subcompartment(s) of the LGN the fingers were associated (parvocellular, magnocellular, or intercalated layers), we examined the immunochemical properties and size of neurons in the fingers and LGN subcompartments. We concluded that the fingers were not associated with the intercalated layers, since neurons in the fingers did not stain with an antibody to calbindin-D28k, whereas intercalated neurons stained intensely with this antibody. In addition, neurons located in the fingers were significantly smaller than those found in the magnocellular layers but were not significantly different in size from neurons in the parvocellular layers. We therefore consider that the fingers are an anomaly of the parvocellular subcompartment of the LGN. Interestingly, in two of the three cases with anomalous fingers, we also observed subsidiary parvocellular laminae, suggesting that these two anomalies were related. In five additional animals, however, we observed subsidiary parvocellular laminae without anomalous fingers. Thus, if there are common mechanisms underlying the development of both anomalous fingers and subsidiary layers, our data indicate that they do not always result in the concomitant expression of both anomalies.

Distribution of neurons projecting to the superior colliculus correlates with thick cytochrome oxidase stripes in macaque visual area V2.

In visual area V2 of macaque monkeys, cytochrome oxidase (CO) histochemistry reveals a pattern of alternating densely labeled thick and thin stripe compartments and lightly labeled interstripe compartments. This modular organization has been associated with functionally separate pathways in the visual system. We examined this idea further by comparing the pattern of CO stripes with the distribution of neurons in V2 that project to the superior colliculus. Visually evoked activity in the superior colliculus is known to be greatly reduced by blocking magnocellular but not parvocellular layers of the lateral geniculate nucleus (LGN). From previous evidence that V2 thick stripes are closely associated with the magnocellular LGN pathway, we predicted that a significant proportion of V2 neurons projecting to the superior colliculus would reside in the thick stripes. To test this prediction, the tangential distribution of retrogradely labeled corticotectal cells in V2 was compared with the pattern of CO stripes. We found that neurons projecting to the superior colliculus accumulated preferentially into band-like clusters that were in alignment with alternate CO dense stripes. These stripes were identified as thick stripes on the basis of their physical appearance and/or by their affinity to the monoclonal antibody Cat-301. A significantly smaller proportion of labeled cells was observed in thin and interstripe compartments. These data provide further evidence that the spatial distribution of subcortically projecting neurons can correlate with the internal modular organization of visual areas. Moreover, they support the notion that CO compartments in V2 are associated with functionally different pathways.

The distribution of callosal connections correlates with the pattern of cytochrome oxidase stripes in visual area V2 of macaque monkeys.

In visual area V2 of monkeys, cytochrome oxidase (CO) histochemistry reveals a system of stripe-like subregions where densely labeled thick and thin stripes and pale interstripes can be recognized. Several lines of evidence suggest that CO stripe-like subregions are associated with functional streams in the visual cortex. In the present study, the distribution of retrogradely labeled callosal cells in V2 and the pattern of CO staining were correlated using tangential sections through the flattened cortex. Spectral and coherency analyses of the callosal and CO patterns were performed to assess quantitatively the degree of spatial correlation between these two patterns. The results showed that labeled callosal cells accumulated along the V1/V2 border and in finger-like bands that protruded up to 7-8 mm into V2. These callosal bands were in register with thick and thin CO stripes, with relatively few labeled callosal cells found in interstripe regions. This finding supports the notion that the distribution of callosal connections in the visual cortex is dictated not only by the topography of visual areas, but also by the arrangement of cortical functional streams. Further, these results extend to interhemispheric pathways the notion of functional specificity currently associated mainly with some visual intrahemispheric pathways.

The callosal pattern in striate cortex is more patchy in monocularly enucleated albino than pigmented rats.

We investigated the effect of neonatal monocular enucleation on the pattern of interhemispheric connections through the corpus callosum in occipital cortex of pigmented and albino rats. Callosal connections were revealed in tangential sections through the flattened cortex following multiple injections of horseradish peroxidase into the opposite hemisphere. In pigmented rats, we found that monocular enucleation induces the development of an anomalous band-like accumulation of callosal connections in middle portions of striate cortex in the hemisphere ipsilateral to the remaining eye, as reported previously. In one-eyed albino rats, we also found callosal connections anomalously placed in middle portions of striate cortex, but they tended to form several patches of labeling rather than a single continuous band as in pigmented rats. Densitometric analysis of the callosal patterns revealed that this difference between rat strains was statistically significant. The increased patchiness in the callosal pattern of one-eyed albino rats may reflect differences in the ipsilateral retinal projections in albino versus pigmented rats.