Topographical organization of human corpus callosum: an fMRI mapping study.

The concept of a topographical map of the corpus callosum (CC) has emerged from human lesion studies and from anatomical tracing investigations in other mammals. Over the last few years, a rising number of researchers have been reporting functional magnetic resonance imaging (fMRI) activation in white matter, particularly the CC. In this study, the scope for describing CC topography with fMRI was explored by evoking activation through simple sensory stimulation and motor tasks. We reviewed our published and unpublished fMRI data on the cortical representation of tactile, gustatory, and visual sensitivity and of motor activation, obtained in 36 volunteers. Activation foci were consistently detected in discrete CC regions: anterior (taste stimuli), central (motor tasks), central and posterior (tactile stimuli), and splenium (visual stimuli). These findings demonstrate that the functional topography of the CC can be explored with fMRI.

[Functional topography of the human corpus callosum]. / Organisation topographique du corps calleux chez l'homme. Etude cartographique en imagerie par résonance magnétique fonctionnelle (f-IRM).

The concept of a topographical map of the corpus callosum (CC) has emerged from lesion studies in humans and from anatomical tracing investigations in other mammals. We conducted the first in vivo study aimed at outlining the topographical organization of the normal human CC, using non-invasive functional magnetic resonance imaging (fMRI). We tested cortical and callosal activation by the BOLD effect during simple sensory stimulation (tactile, gustatory and visual) and simple motor tasks in 38 volunteers. The axonal organization of callosal white matter was also studied in 16/38 subjects, using diffusion tensor imaging (DTI) and diffusion tensor tractography (DTT). Activation foci evoked by taste stimuli were detected in most subjects in the anterior part of the CC, those elicited by motor tasks lay in the central portion of the body of the CC, and those elicited by tactile stimulation of different body regions lay in the posterior part of the body. Activation foci evoked by visual stimulation were seen in the splenium of the CC. Callosal fibers interconnecting the primary cortical areas activated by taste stimulation, motor tasks, and tactile and visual stimuli were shown by DTT. Anatomical correlates of the BOLD activation foci were demonstrated in the CC, with fibers crossing it at the level of the genu, anterior and posterior body, and splenium, respectively. This study demonstrates for the first time that the functional topographical organization of the human CC can be explored by fMRI in vivo. Our findings may have clinical implications, especially for neurosurgical planning.

Mirror-image discrimination and reversal in the disconnected hemispheres.

Two callosotomized patients and 24 neurologically normal subjects performed simple binary discriminations between upright letters flashed in one or other visual field. Where discrimination of the letters F and R by name either showed a left-hemisphere advantage or no hemispheric effect, discrimination of whether the same letters were normal or backward showed a right-hemisphere advantage. These results suggest that discrimination of mirror-image letters depends on matching to an exemplar, for which the right-hemisphere is dominant, while letter naming depends on abstract category recognition. One commissurotomized patient, DDV, showed systematic left-right reversal of the letters in the left visual field, classifying the normal letters as reversed and reversed ones as normal, and persisted with this reversal when the letters were shown in free vision. This suggests that reversed exemplars of the letters may be laid down the right cerebral hemisphere. There was no such reversal in the other patient (DDC).

Now you see it, now you don't: variable hemineglect in a commissurotomized man.

We describe the case of a callosotomized man, D.D.V., who shows unusual neglect of stimuli in the left visual field (LVF). This is manifest in simple reaction time (RT) to stimuli flashed in the LVF and in judging whether pairs of filled circles in the LVF are of the same or different color. It may reflect strong left-hemispheric control and consequent attention restricted to the right side of space. It is not evident in simple RT when there are continuous markers in the visual fields to indicate the locations of the stimuli. In this condition, his RTs are actually faster to LVF than to right visual field (RVF) stimuli, suggesting a switch to right-hemispheric control that eliminates the hemineglect. Neglect is also not evident when D.D.V. responds by pointing to or touching the locations of the stimuli, perhaps because these responses are controlled by the dorsal rather than the ventral visual system. Despite his atypical manifestations of hemineglect, D.D.V. showed evidence of functional disconnection typical of split-brained subjects, including prolonged crossed-uncrossed different in simple reaction time, inability to match colors between visual fields, and enhanced redundancy gain in simple RT to bilateral stimuli even when the stimulus in the LVF was neglected.

Contribution of posterior corpus callosum to the interhemispheric transfer of tactile information.

Three total and three partial callosotomy patients underwent neuropsychological testing to evaluate interhemispheric transfer of tactile information. Tactile transfer is required to name objects presented to the left hand, to compare objects held in either hand, and to transfer topological information between hands. Tactile Naming, Same-Different Recognition, and Tactile Finger Localization Tests (intra- and intermanual tasks) were administered as specific tools. Results were compared with previous fMRI data from the same subjects and with the performance of a control group (20 age-matched subjects). Total callosotomy patients performed modestly: mean correct responses were 93% and 30% (right and left hand, respectively) in Tactile Naming; 68% in Same-Different Recognition; 84% and 76% (right and left hand stimulation, respectively) in intermanual Tactile Finger Localization, and 100% in the intramanual task. Partial callosotomy patients achieved 93-100% accuracy: all have an intact splenium, and one, and possibly all, also an intact posterior callosal body. Controls scored 99% in Tactile Naming, both hands, and Same-Different Recognition; 100% in intramanual Tactile Finger Localization; and 96% and 95%, with right and left hand stimulation, respectively, in the intermanual task. Differences between the two callosotomy groups were significant, as were those between total callosotomy patients and controls. The partial callosotomy group scored like the control subjects. Neuropsychological data agree with previous functional findings, further demonstrating that interhemispheric tactile transfer requires posterior corpus callosum integrity.

Bilateral cortical representation of the trunk midline in human first somatic sensory area.

The cortical representation of the trunk zone in the human first somatosensory area was studied with functional magnetic resonance imaging (fMRI) to establish whether the cutaneous regions close to the midline are represented in this area of both hemispheres. Cortical activation foci evoked by unilateral tactile stimulation of ventral trunk regions were detected in the postcentral gyrus of the contralateral hemisphere slightly medial to or just behind the omega-shaped region of the central sulcus and in the anterior bank of the postcentral sulcus. These regions probably correspond to the trunk ventral midline representation zones of areas 3a-3b and 1-2, respectively. Stimulation of cutaneous regions adjacent to the midline evoked activation foci also in the ipsilateral postcentral gyrus in regions symmetrical to those activated in the contralateral hemisphere. These data demonstrate that in humans, as in nonhuman primates, the cutaneous regions adjacent to the trunk midline are represented bilaterally in the first somatic sensory cortex. Whether the ipsilateral activation depends on callosal or extracallosal inputs remains to be elucidated.

Direct synaptic connections between corticothalamic axon terminals and interneurons in the cat ventrobasal complex.

Lesion-induced degeneration was combined with immunocytochemistry to study, with electron microscopy, the synaptic connectivity between corticothalamic axon terminals from the first and second somatosensory areas and local circuit neurons of the ipsilateral ventrobasal complex (VB), selectively labelled with an antibody raised against gamma-aminobutyric acid (GABA). Four days from the cortical ablation many degenerating axon terminals, forming asymmetric synapses, were found on dendritic trees of both labelled and unlabelled neurons of VB and occasionally on presynaptic dendrites. The main finding of the present paper is that 64.01% of degenerating axon terminals synapsed with GABA-immunopositive dendrites, suggesting that the principal target of the cortical projection to VB are local circuit neurons.

Postnatal development of the glutamate vesicular transporter VGLUT1 in rat cerebral cortex.

The expression of the vesicular glutamate transporter VGLUT1 in the rat neocortex was studied during postnatal development using immunocytochemistry and Western blotting. At all ages, VGLUT immunoreactivity is localized to puncta that coexpress the presynaptic marker synaptophysin. VGLUT1 immunoreactivity is faint at birth, increases in the subplate during the first postnatal week, invades the supragranular layers in the second week and reaches the adult pattern at P20-P30. Its spatial and temporal maturation patterns suggest that VGLUT1 may be the vesicular transporter in developing corticocortical connections.