The Organization of Callosal Connections in Primates

A hierarchical, retinotopic proto-organization of the primate visual system at birth

The adult primate visual system comprises a series of hierarchically organized areas. Each cortical area contains a topographic map of visual space, with different areas extracting different kinds of information from the retinal input. Here we asked to what extent the newborn visual system resembles the adult organization. We find that hierarchical, topographic organization is present at birth and therefore constitutes a proto-organization for the entire primate visual system. Even within inferior temporal cortex, this proto-organization was already present, prior to the emergence of category selectivity (e.g., faces or scenes). We propose that this topographic organization provides the scaffolding for the subsequent development of visual cortex that commences at the onset of visual experience.

Functional ultrasound imaging of intrinsic connectivity in the living rat brain with high spatiotemporal resolution

Long-range coherences in spontaneous brain activity reflect functional connectivity. Here we propose a novel, highly resolved connectivity mapping approach, using ultrafast functional ultrasound (fUS), which enables imaging of cerebral microvascular haemodynamics deep in the anaesthetized rodent brain, through a large thinned-skull cranial window, with pixel dimensions of 100 μm × 100 μm in-plane. The millisecond-range temporal resolution allows unambiguous cancellation of low-frequency cardio-respiratory noise. Both seed-based and singular value decomposition analysis of spatial coherences in the low-frequency (<0.1 Hz) spontaneous fUS signal fluctuations reproducibly report, at different coronal planes, overlapping high-contrast, intrinsic functional connectivity patterns. These patterns are similar to major functional networks described in humans by resting-state fMRI, such as the lateral task-dependent network putatively anticorrelated with the midline default-mode network. These results introduce fUS as a powerful novel neuroimaging method, which could be extended to portable systems for three-dimensional functional connectivity imaging in awake and freely moving rodents.

Cortical and thalamic connections of the representations of the teeth and tongue in somatosensory cortex of new world monkeys

Connections of representations of the teeth and tongue in primary somatosensory cortex (area 3b) and adjoining cortex were revealed in owl, squirrel, and marmoset monkeys with injections of fluorescent tracers. Injection sites were identified by microelectrode recordings from neurons responsive to touch on the teeth or tongue. Patterns of cortical label were related to myeloarchitecture in sections cut parallel to the surface of flattened cortex, and to coronal sections of the thalamus processed for cytochrome oxidase (CO). Cortical sections revealed a caudorostral series of myelin dense ovals (O1-O4) in area 3b that represent the periodontal receptors of the contralateral teeth, the contralateral tongue, the ipsilateral teeth, and the ipsilateral tongue. The ventroposterior medial subnucleus, VPM, and the ventroposterior medial parvicellular nucleus for taste, VPMpc, were identified in the thalamic sections. Injections placed in the O1 oval representing teeth labeled neurons in VPM, while injections in O2 representing the tongue labeled neurons in both VPMpc and VPM. These injections also labeled adjacent part of areas 3a and 1, and locations in the lateral sulcus and frontal lobe. Callosally, connections of the ovals were most dense with corresponding ovals. Injections in the area 1 representation of the tongue labeled neurons in VPMpc and VPM, and ipsilateral area 3b ovals, area 3a, opercular cortex, and cortex in the lateral sulcus. Contralaterally, labeled neurons were mostly in area 1. The results implicate portions of areas 3b, 3a, and 1 in the processing of tactile information from the teeth and tongue, and possibly taste information from the tongue.

Is a cortical spike discharge "transferred" to the contralateral cortex via the corpus callosum?: An intraoperative observation of electrocorticogram and callosal compound action potentials

PURPOSE

By means of the intraoperative electrophysiologic observation, we reevaluated the "transfer" theory that a transcallosal volley invoked by a cortical spike discharge in one hemisphere directly causes its contralateral counterpart via the corpus callosum (CC).

METHODS

Twenty-six patients who underwent corpus callosotomy were the subjects of this study. Intraoperatively, electrocorticograms from both hemispheres were simultaneously monitored with callosal compound action potentials (CCAPs) from the CC. Analysis was conducted on (a) the interhemispheric delay of bilaterally synchronous spike-and-wave discharges (BSSWs), and (b) the chronological relation between BSSWs and CCAPs.

RESULTS

The side of prior spike discharges was never fixed but was occasionally reversed. Interhemispheric delays between the BSSWs were not constant, regardless of direction, and fluctuated in all patients. Most of the interhemispheric delays were distributed within 20 ms with a mode of 0 ms. The waveform of the CCAP was characterized by slow-rising negative potential change that attained its peak after a cortical spike discharge. These findings were identical in all the patients regardless of whether the BSSWs were changed or unchanged after callosotomy.

CONCLUSIONS

If the "transfer" role of the CC is true, interhemispheric delays between BSSWs must be longer than interhemispheric axonal conduction time (about 20 ms), and a preceding cortical spike discharge must produce a CCAP and then a contralateral one in order of time. However, this hypothesis was not confirmed in the present study. We propose the interhemispheric recruitment of the epileptogenic state as a different role of the CC on epileptogenesis.

Cortical connections of striate and extrastriate visual areas in tree shrews

The ipsilateral and contralateral cortical connections of visual cortex of tree shrews (Tupaia belangeri) were investigated by placing restricted injections of fluorochrome tracers, wheat germ agglutinin-horseradish peroxidase, or biotinylated dextran amine into area 17 (V1), area 18 (V2), or the adjoining temporal dorsal area (TD). As previously reported, V1 was characterized by a widespread, patchy pattern of intrinsic connections; ipsilateral connections with V2, TD, and to a lesser extent, other areas of the temporal cortex; and contralateral connections with V1, V2, and TD. A surface-view of the myelin pattern in V1 revealed a patchwork of light and dark module-like regions. The ipsilateral connections with V2 and TD were roughly topographic, whereas heterotopic locations in V1 were callosally connected. Injections in V2 labeled as much as one third of V2 in a patchy pattern, and portions of ipsilateral V1 and TD in roughly topographic patterns. In addition, connections with several other visual areas in the temporal lobe were revealed. Contralaterally, most of the label was in V2, with some in V1 and TD. Injections in TD demonstrated connections within the region, and with adjoining portions of the temporal cortex, V2, and V1. There were sparse connections with an oval of densely myelinated cortex, which we have termed the temporal inferior area (TI). Callosal connections were concentrated in TD, but also included V2. The results provide further evidence for modular organizations within V1 and V2, and reveal for the first time the complete patterns of cortical connections of V2 and TD. The results are consistent with the proposal that at least three visual areas, the temporal anterior area, TA, the temporal dorsal area, TD, and the temporal posterior area, TP, exist along the rostrolateral border of V2 in tree shrews; suggest visual involvement of at least three other areas, the temporal inferior area, TI, the temporal anterior lateral area, and the temporal posterior inferior area located more ventrally in the temporal cortex; and fortify the conclusion that TD is the likely homologue of the middle temporal visual area of primates. Because tree shrews are considered close relatives of primates, the evidence for several visual areas along the border of V2 is more compatible with theories that propose a series of visual areas along V2 in primates, rather than a single visual area, V3.