Is the Imitative Competence an Asymmetrically Distributed Function?

This study reconsiders behavioral and functional data from studies investigating the anatomical imitation (AI) and the related mental rotation (MR) competence, carried out by our group in healthy subjects, with intact interhemispheric connections, and in split-brain patients, completely or partially lacking callosal connections. The results strongly point to the conclusion that AI and MR competence requires interhemispheric communication, mainly occurring through the corpus callosum, which is the largest white matter structure in the human brain. The results are discussed in light of previous studies and of future implications.

Blockade of retinal or cortical activity does not prevent the development of callosal patches normally associated with ocular dominance columns in primary visual cortex

Callosal patches in primary visual cortex of Long Evans rats, normally associated with ocular dominance columns, emerge by postnatal day 10 (P10), but they do not form in rats monocularly enucleated a few days before P10. We investigated whether we could replicate the results of monocular enucleation by using tetrodotoxin (TTX) to block neural activity in one eye, or in primary visual cortex. Animals received daily intravitreal (P6–P9) or intracortical (P7–P9) injections of TTX, and our physiological evaluation of the efficacy of these injections indicated that the blockade induced by a single injection lasted at least 24 h. Four weeks later, the patterns of callosal connections in one hemisphere were revealed after multiple injections of horseradish peroxidase in the other hemisphere. We found that in rats receiving either intravitreal or cortical injections of TTX, the patterns of callosal patches analyzed in tangential sections from the flattened cortex were not significantly different from the pattern in normal rats. Our findings, therefore, suggest that the effects of monocular enucleation on the distribution of callosal connections are not due to the resulting imbalance of afferent ganglion cell activity, and that factors other than neural activity are likely involved.

Retino-thalamo-cortical and callosal connections visualized with manganese-enhanced magnetic resonance imaging and validated with tract tracing techniques

Ocular dominance columns correlate with patchy callosal connections in Long Evans rats (Laing et al., 2015). We explored in vivo manganese-enhanced magnetic resonance imaging (MEMRI) as a possible strategy for longitudinal studies of plastic changes in the retino-thalamo-cortical and callosal pathways. MnCl 2 was injected either intraocularly or intracortically to label these pathways, respectively. The transport of the paramagnetic ion Mn 2+ was evaluated by comparing images acquired both before and 36 or 12 hours after intraocular or cortical injections, respectively. Images were acquired on a 3T magnet (Philips Achieva, Philips Healthcare, Andover, MA), using a custom surface coil and a T1-weighted MPRAGE image sequence (TR/TE = 23/11 ms; Ti=1000 ms; FA= 10 deg acquired matrix 432x432 mm over 118 slices, voxel size 0.11x0.11x0.2 mm 3 ). To validate the transport of Mn 2+ , each animal also received either an intraocular injection of the transneuronal tracer WGA-HRP, or cortical injections of HRP. Following monocular injections of MnCl 2 , MRI images showed significant, bilateral accumulations of Mn 2+ in regions of the SC, LGN and visual cortex that corresponded with regions labeled with HRP. In adult rats monocularly enucleated at birth, we injected MnCl 2 in the hemisphere contralateral to the remaining eye in an attempt to detect anomalies reported previously in the callosal pattern ipsilateral to the remaining eye. After the scans, the hemisphere injected with MnCl 2 was injected with HRP. MRI images revealed Mn 2+ patterns that closely resembled the callosal patterns demonstrated with HRP in the same animal. Our results suggest that both transneuronal retino-thalamo-cortical, as well as cortico-cortical transport of Mn 2+ provide potentially useful strategies for longitudinal studies of plastic changes in these pathways.

Correlation of callosal connections and optical imaging maps in visual cortex of mice lacking retinal activity waves due to deficiency in the ß2 subunit of the nicotinic acetylcholine receptor

Studies of visual callosal connections proposed that bilateral projections from temporal retina promote the formation of callosal linkages between cortical loci that are retinotopically matched and non-mirror symmetric with respect to the brain midline. It is therefore possible for a spontaneously active retinal locus to simultaneously activate retinotopically corresponding loci in both cortices, leading to Hebbian-like stabilization of connections between them before the eyes open. Interhemispheric correlated activity could stem from single ganglion cells that send axon branches to both sides, or from closely located cells that project to one side or the other, but which fire in synchrony due to spontaneously generated retinal activity waves. We hypothesized that lack of retinal waves could induce callosal map anomalies similar to those produced by neonatal enucleation. We studied mice lacking retinal waves due to deficiency in the ß2 subunit of the nicotinic acetylcholine receptor. The organization of callosal projections revealed with small tracer injections was correlated with V1 maps made by imaging intrinsic optical responses to drifting stimuli. Consistent with studies showing that retinofugal and geniculocortical projections are less focused in ß2 -/- mice, we found that the overall callosal pattern in V1 is markedly broader in ß2 -/- mice than in wild type mice. However, the fine topography of the callosal map in ß2 -/- mice is similar to that in wild type and ß2 -/+ mice, indicating that lack of retinal waves is not sufficient for inducing the reversal in the callosal map caused by neonatal enucleation.

Visual Interhemispheric and Striate-Extrastriate Cortical Connections in the Rabbit: A Multiple Tracer Study

Previous studies in rabbits identified an array of extrastriate cortical areas anatomically connected with V1 but did not describe their internal topography. To address this issue, we injected multiple anatomical tracers into different regions in V1 of the same animal and analyzed the topography of resulting extrastriate labeled fields with reference to the patterns of callosal connections and myeloarchitecture revealed in tangential sections of the flattened cortex. Our results extend previous studies and provide further evidence that rabbit extrastriate areas resemble the visual areas in rats and mice not only in their general location with respect to V1 but also in their internal topography. Moreover, extrastriate areas in the rabbit maintain a constant relationship with myeloarchitectonic borders and features of the callosal pattern. These findings highlight the rabbit as an alternative model to rats and mice for advancing our understanding of cortical visual processing in mammals, especially for projects benefiting from a larger brain.