Functional characterisation of sensory ERPs using probabilistic ICA: effect of stimulus modality and stimulus location.

OBJECTIVE: To decompose sensory event-related brain potentials (ERPs) into a set of independent components according to the modality and the spatial location of the eliciting sensory stimulus, and thus provide a quantitative analysis of their underlying components. METHODS: Auditory, somatosensory and visual ERPs were recorded from 124 electrodes in thirteen healthy participants. Probabilistic Independent Component Analysis (P-ICA) was used to decompose these sensory ERPs into a set of independent components according to the modality (auditory, somatosensory, visual or multimodal) and the spatial location (left or right side) of the eliciting stimulus. RESULTS: Middle-latency sensory ERPs were explained by a large contribution of multimodal neural activities, and a smaller contribution of unimodal neural activities. While a significant fraction of unimodal neural activities were dependent on the location of the eliciting stimulus, virtually all multimodal neural activities were not (i.e. their scalp distributions and time courses were not different when stimuli were presented on the left and right sides). CONCLUSION: These findings show that P-ICA can be used to dissect effectively sensory ERPs into physiologically meaningful components, and indicate a new approach for exploring the effect of various experimental modulations of sensory ERPs. SIGNIFICANCE: This approach offers a better understanding of the functional significance of sensory ERPs.

Delayed onset of Huntington's disease in mice in an enriched environment correlates with delayed loss of cannabinoid CB1 receptors.

Huntington's disease (HD) is a late onset progressive genetic disorder characterised by motor dysfunction, personality changes, dementia and premature death. The disease is caused by an unstable expanded trinucleotide (CAG) repeat encoding a polyglutamine stretch in the IT15 gene for huntingtin, a protein of unknown function. Transgenic mice expressing exon one of the human HD gene with an expanded polyglutamine region develop many features of human HD. Exposure of these mice to an "enriched" environment delays the onset of motor disorders and slows disease progression [Nature 404 (2000) 721]. We have compared the levels of receptor binding of a range of basal ganglia neurotransmitter receptors believed to be important in HD, in normal mice and R6/1 transgenic HD mice housed in either enriched or standard laboratory environments. HD mice housed in a normal environment show a loss of cannabinoid CB1 and dopamine D1 and D2 receptors in the striatum and the corresponding output nuclei of the basal ganglia. HD mice exposed to an enriched environment show equivalent loss of D1 and D2 receptors as their "non-enriched" counterparts; in contrast, the "enriched" mice show significantly less depletion of CB1 receptors. In the brains of humans diagnosed with HD cannabinoid CB1 receptors are selectively lost from the basal ganglia output nuclei prior to the development of other identifiable neuropathology [Neuroscience 97 (2000) 505]. Our results therefore show that an enhanced environment slows the rate of loss of one of the first identifiable neurochemical deficits of HD. This suggests that delaying the loss of CB1 receptors, either by environmental stimulation or pharmacologically, may be beneficial in delaying disease progression in HD patients.

Anterior cingulate cortical transplantation in transgenic Huntington's disease mice.

Huntington's disease (HD) is an autosomal dominant disorder involving progressive neurodegeneration of the corpus striatum and cerebral cortex. Transgenic mice, in which exon 1 of the human HD gene with an expanded trinucleotide repeat is expressed, develop a neurodegenerative syndrome that closely models human HD. Transplantation of wild-type donor cortex into the anterior cingulate cortex of neonatal HD mice (R6/1 line) was found to delay the onset of a specific motor deficit, rear-paw clasping. However, transplantation did not significantly enhance motor performance on a suspended horizontal rod, a behavioural measure of fine motor co-ordination. Control experiments in which the anterior cingulate cortex was resected, but no donor cortical tissue was transplanted, showed no behavioural benefit. In fact, wild-type littermate mice that also underwent this surgical resection, were found to develop motor deficits similar to those exhibited by non-resected HD mice. These results suggest that the anterior cingulate cortex is an important area of pathology in this HD model, and that therapeutic approaches to HD may need to target cortical, as well as striatal areas.

PLC-beta1, activated via mGluRs, mediates activity-dependent differentiation in cerebral cortex.

During development of the cerebral cortex, the invasion of thalamic axons and subsequent differentiation of cortical neurons are tightly coordinated. Here we provide evidence that glutamate neurotransmission triggers a critical signaling mechanism involving the activation of phospholipase C-beta1 (PLC-beta1) by metabotropic glutamate receptors (mGluRs). Homozygous null mutation of either PLC-beta1 or mGluR5 dramatically disrupts the cytoarchitectural differentiation of 'barrels' in the mouse somatosensory cortex, despite segregation in the pattern of thalamic innervation. Furthermore, group 1 mGluR-stimulated phosphoinositide hydrolysis is dramatically reduced in PLC-beta1-/- mice during barrel development. Our data indicate that PLC-beta1 activation via mGluR5 is critical for the coordinated development of the neocortex, and that presynaptic and postsynaptic components of cortical differentiation can be genetically dissociated.

N-Acetylaspartate and DARPP-32 levels decrease in the corpus striatum of Huntington's disease mice.

Huntington's disease (HD) is an autosomal dominant condition involving progressive neurodegeneration, primarily the corpus striatum and cerebral cortex. We have used in vivo magnetic resonance spectroscopy (MRS) to assess specific neuronal markers in transgenic mice (R6/1 line) expressing exon I of the human huntingtin gene with an expanded CAG repeat. Levels of N-acetylaspartate (NAA), an indicator of healthy neuronal function, were significantly reduced (26%) in the corpus striatum of HD mice relative to wild-type littermates at 5 months of age. However, levels of cholines and creatine-phosphocreatine were not altered in the HD mice. Expression of dopamine- and cAMP-regulated phosphoprotein, 32 kDa (DARPP-32), was assessed by immunohistochemistry in the striatum of HD mice and found to be downregulated by 5 months and, even more dramatically, at 11 months of age. In contrast, expression of calbindin was not significantly decreased in HD mice. Our results suggest that the observed decreases in DARPP-32 and NAA may contribute to aberrant receptor signalling and neuronal dysfunction in HD.

Functional imaging of brain areas involved in the processing of coherent and incoherent wide field-of-view visual motion.

The brain areas involved in processing wide field-of-view (FOV) coherent and incoherent visual stimuli were studied using positron emission tomography (PET). The brains of nine subjects were scanned as they viewed texture patterns moving in the roll plane. Five visual conditions were used: (1) coherent clockwise (CW) wide-FOV (>100 degrees) roll motion; (2) coherent counter-clockwise (CCW) wide-FOV roll motion; (3) wide-FOV incoherent motion; (4) CCW motion confined to the central visual field (approximately 55 degrees); and (5) a stationary control texture. The region most activated by the coherent-motion stimulus relative to the static one was the medial-occipital cortex, whereas both the medial- and lateral-occipital cortices were activated by incoherent motion relative to a static texture. Portions of the retroinsular parietal-temporal cortex, superior insula, putamen, and vestibulocerebellum responded specifically to the coherence of the stimulus, whereas a widespread lateralized activation was observed upon subtracting the CW scans from the CCW scans. The results indicate separate neural regions for processing wide-FOV motion versus stimulus coherence.

Morphology and growth patterns of developing thalamocortical axons.

It is increasingly evident that the actions of guidance factors depend critically on the cellular and molecular context in which they operate. For this reason we examined the growth cone morphology and behavior of thalamic fibers in the relatively natural environment of a slice preparation containing the entire pathway from thalamus to cortex. Axons were labeled with DiI crystals and imaged with a laser-scanning confocal microscope for up to 8 hr. Their behavior was analyzed in terms of morphology, extension rates, shape of trajectory, frequency of branching, and percentage of time spent in advance, pause, and retraction. Thalamic fibers had distinct and stereotyped growth patterns that related closely to their position; within the striatum growth cones were small and elongated, rarely extending filopodia or side branches. Axons grew quickly, in straight trajectories, with minimal pauses or retractions. When they reached the ventral intermediate zone, axons slowed down, often coming to a complete stop for up to several hours, and their growth cones became larger and more complex. During pauses there were continuous extensions and retractions of filopodia and/or side branches. When advance resumed, it was often to a different direction. These results demonstrate consistent regional variations in growth patterns that identify an unexpected decision region for thalamic axons. They provide the basis for examining the roles of guidance cues in an accessible yet intact preparation of the thalamocortical pathway and allow for an evaluation of previously suggested pathfinding mechanisms.

Is "ambient vision" distributed in the brain? Effects of wide-field-view visual yaw motion on PET activation.

Ambient vision comprises the visual functions that are associated with the maintenance of spatial orientation and that depend on peripheral, preconscious visual inputs. Although a limited number of brain areas appear to be activated by coherent wide-field-of-view (WFOV) motion in more than one axis, a diffuse pattern of lateralized brain activity occurs in response to clockwise or counterclockwise ambient visual roll motion. In the present study involving positron emission tomography (PET), a similar finding was shown for rightward versus leftward yaw stimulation. A total of 18 PET scans were obtained from six subjects in response to either leftward or rightward WFOV motion in a collimated display subtending > 100 degrees horizontally. Rightward stimulation elicited mainly activation throughout the right hemisphere, whereas leftward stimulation elicited mainly activation throughout the left hemisphere. These findings provide further evidence that the ambient vision signal is either processed or transmitted throughout the entire brain, as befits a visual function that is fundamental to all other perceptual systems.

Formation of cortical fields on a reduced cortical sheet.

Theories of both cortical field development and cortical evolution propose that thalamocortical projections play a critical role in the differentiation of cortical fields (; ). In the present study, we examined how changing the size of the immature neocortex before the establishment of thalamocortical connections affects the subsequent development and organization of the adult neocortex. This alteration in cortex is consistent with one of the most profound changes made to the mammalian neocortex throughout evolution: cortical size. Removing the caudal one-third to three-fourths of the cortical neuroepithelial sheet unilaterally at an early stage of development in marsupials resulted in normal spatial relationships between visual, somatosensory, and auditory cortical fields on the remaining cortical sheet. Injections of neuroanatomical tracers into the reduced cortex revealed in an altered distribution of thalamocortical axons; this alteration allowed the maintenance of their original anteroposterior distribution. These results demonstrate the capacity of the cortical neuroepithelium to accommodate different cortical fields at early stages of development, although the anteroposterior and mediolateral relationships between cortical fields appear to be invariant. The shifting of afferents and efferents with cortical reduction or expansion at very early stages of development may have occurred naturally in different lineages over time and may be sufficient to explain much of the phenotypic variation in cortical field number and organization in different mammals.

Probing the human stereoscopic system with reverse correlation.

Our two eyes obtain slightly different views of the world. The resulting differences in the two retinal images, called binocular disparities, provide us with a stereoscopic sense of depth. The primary visual cortex (V1) contains neurons that are selective for the disparity of individual elements in an image, but this information must be further analysed to complete the stereoscopic process. Here we apply the psychophysical technique of reverse correlation to investigate disparity processing in human vision. Observers viewed binocular random-dot patterns, with 'signal' dots in a specific depth plane plus 'noise' dots with randomly assigned disparities. By examining the correlation between the observers' ability to detect the plane and the particular sample of 'noise' disparities presented on each trial, we revealed detection 'filters', whose disparity selectivity was remarkably similar to that of individual neurons in monkey V1. Moreover, if the noise dots were of opposite contrast in the two eyes, the tuning inverted, just like the response patterns of V1 neurons. Reverse correlation appears to probe disparity processing at the earliest stages of binocular combination, prior to the generation of a full stereoscopic depth percept.

Development of signals influencing the growth and termination of thalamocortical axons in organotypic culture.

Explants of embryonic or postnatal rat cortex, organotypically cultured in serum-free medium, maintain their structural integrity and their upper layers continue to mature. Coculture of portions of embryonic thalamus with cortical slices taken at different ages reveals a temporal cascade of cortical signals. (1) Slices of occipital cortex taken at E19 or earlier stimulate axonal outgrowth from explants of embryonic lateral geniculate nucleus but do not allow the fibers to invade. (2) In cortical slices taken after E19 but before P2, thalamic axons enter the slice, from any direction, and extend radially across the entire depth of the cortical plate without branching or terminating. (3) In slices taken after P2, fibers slow down, arborize, and terminate in the maturing layer 4 of the cortex. If the thalamic explant is placed against the pial surface of the cortical slice, axons still enter and branch in the same layer. These findings imply that the developing cortex expresses a diffusible growth-promoting factor and then itself becomes growth permissive, and finally the maturing layer 4 expresses a "stop signal." In triple cocultures of one thalamic explant with a "choice" of two neighboring slices, thalamic axons will not invade slices of cerebellum but behave indistinguishably in response to slices from any region of the hemisphere. Thus the initial tangential distribution of the thalamic projection in vivo (which is achieved by about E16) is unlikely to be controlled by regional variation in signals produced by the cortex. When cortical slices were precultured alone for 7-14 days before the addition of an explant of embryonic thalamus for 4 further days of coculture, the pattern of innervation was more appropriate to the chronological age of the slice than the age at which it was first taken. Thus the timing of the cascade of cortical properties is at least partly intrinsically determined. This sequence of expression of these signals suggests that they play a part in vivo in controlling the outgrowth of thalamic fibers, their accumulation under the cortical plate, their invasion of the plate, and their arborization in layer 4.

Characterization of nodular neuronal heterotopia in children.

Neuronal heterotopia are seen in various pathologies and are associated with intractable epilepsy. We examined brain tissue from four children with subcortical or periventricular nodular heterotopia of different aetiologies: one with severe epilepsy following focal brain trauma at 17 weeks gestation, one with hemimegalencephaly and intractable epilepsy, one with focal cortical dysplasia and intractable epilepsy, and one dysmorphic term infant with associated hydrocephalus and polymicrogyria. The connectivity of nodules was investigated using histological and carbocyanine dye (DiI) tracing techniques. DiI crystal placement adjacent to heterotopic nodules revealed numerous DiI-labelled fibres within a 2-3 mm radius of the crystals. Although we observed labelled fibres closely surrounding nodules, the majority did not penetrate them. Placement of DiI crystals within nodules also identified a limited number of projections out of the nodules and in one case there was evidence for connectivity between adjacent nodules. The cellular and neurochemical composition of nodules was also examined using immunohistochemistry for calretinin and neuropeptide Y (NPY), which are normally expressed in GABAergic cortical interneurons. Within heterotopic nodules from all cases, numerous calretinin-positive neurons were identified, along with a few cell bodies and many processes positive for NPY. Calretinin-positive neurons within nodules were less morphologically complex than those in the cortex, which may reflect incomplete differentiation into an inhibitory neuronal phenotype. There were also abnormal clusters of calretinin-positive cells in the overlying cortical plate, indicating that the migratory defect which produces heterotopic nodules also affects development of the cortex itself. Thus, heterotopic nodules consisting of multiple neuronal cell types are associated with malformation in the overlying cortical plate, and have limited connectivity with other brain regions. This abnormal development of connectivity may affect neuronal maturation and consequently the balance of excitation and inhibition in neuronal circuits, leading to their epileptogenic potential.

Different mechanisms underlie three inhibitory phenomena in cat area 17.

Recently, it has been proposed that all suppressive phenomena observed in the primary visual cortex (V1) are mediated by a single mechanism, involving inhibition by pools of neurons, which, between them, represent a wide range of stimulus specificities. The strength of such inhibition would depend on the stimulus that produces it (particularly its contrast) rather than on the firing rate of the inhibited cell. We tested this hypothesis by measuring contrast-response functions (CRFs) of neurons in cat V1 for stimulation of the classical receptive field of the dominant eye with an optimal grating alone, and in the presence of inhibition caused by (1) a superimposed orthogonal grating (cross-orientation inhibition); (2) a surrounding iso-oriented grating (surround inhibition); and (3) an orthogonal grating in the other eye (interocular suppression). We fitted hyperbolic ratio functions and found that the effect of cross-orientation inhibition was best described as a rightward shift of the CRF ('contrast-gain control'), while surround inhibition and interocular suppression were primarily characterised as downward shifts of the CRF ('response-gain control'). However, the latter also showed a component of contrast-gain control. The two modes of suppression were differently distributed between the layers of cortex. Response-gain control prevailed in layer 4, whereas cells in layers 2/3, 5 and 6 mainly showed contrast-gain control. As in human observers, surround gratings caused suppression when the central grating was of high contrast, but in over a third of the cells tested, enhanced responses for low-contrast central stimuli, hence actually decreasing threshold contrast.

Phospholipase C-beta1 expression correlates with neuronal differentiation and synaptic plasticity in rat somatosensory cortex.

Receptor-mediated signal transduction is thought to play an important role in neuronal differentiation and the modification of synaptic connections during brain development. The intracellular signalling molecule phospholipase C-beta1 (PLC-beta1), which is activated via specific neurotransmitter receptors, has recently been implicated in activity-dependent plasticity in the cat visual cortex. PLC-beta1 has been shown to be concentrated in an intermediate compartment-like organelle, the botrysome, which is present in 5-week-old, but not adult, cat cortical neurons. We have characterized the spatial and temporal regulation of PLC-beta1 expression in the developing rat cerebral cortex. PLC-beta1-positive botrysome-like organelles are observed during early postnatal cortical development, but not at postnatal day 14 or later stages. In the postnatal somatosensory cortex, there is also striking spatial variation in diffuse neuropilar immunoreactivity of layer IV and above, in a pattern corresponding to the thalamocortical recipient zones known as barrels. This expression pattern is specific to the developing barrel field and is most distinct at postnatal days 4-7, when cellular components of barrels are capable of activity-dependent modification. During later stages of cortical maturation, stained botrysomes disappear, expression of PLC-beta1 is down-regulated and only diffuse immunoreactivity remains in dendritic processes. Our results are consistent with a role for PLC-beta1 in activity-dependent, receptor-mediated neuronal plasticity during development of the somatosensory cortex.

Development of thalamocortical projections in the South American gray short-tailed opossum (Monodelphis domestica).

We determined the time-course and general pattern of thalamocortical development of Monodelphis domestica by tracing projections with carbocyanine dye in fixed postnatal brains between postnatal day 2 (P2) and P30. By P2, the first neurons have migrated to form the preplate of the lateral cortex and have sent out axons into the intermediate zone. By P3, fibers from the preplate of more dorsal cortex have entered the intermediate zone, and, by P5, they reach the primitive internal capsule. Crystal placements in the dorsal thalamus at P2-P3 reveal thalamic axons extending down through the diencephalon and growing out through the internal capsule among groups of back-labelled cells that already project into the thalamus. Thalamic axons arrive at the cortex after the arrival of cells of the true cortical plate has split the preplate into marginal zone and subplate. Axons from the ventral part of the dorsal thalamus reach the lateral cortex by P5: Dorsal thalamic fibers arrive at the extreme dorsal cortex by P9. The deeper layers of the cortex appear to mature relatively earlier in Monodelphis than in eutherian mammals, and the subplate becomes less distinct. Thalamic fibers and their side branches proceed into the cortex without an obvious period of waiting in the subplate, but they do not penetrate the dense cortical plate itself. Monodelphis could provide an excellent model species, because the development of its thalamocortical connections is entirely an extrauterine process: The period P0-P15 corresponds to that of E12-P0 in the rat.

Mechanisms underlying the early establishment of thalamocortical connections in the rat.

We labeled axonal projections using carbocyanine dyes in the developing rat brain to study cellular interactions that might underlie the establishment of thalamocortical connectivity. By embryonic day 14 (E14), groups of neurons in the ventral diencephalon and the primitive internal capsule have established projections to the dorsal thalamus, and thalamic fibers pass in topographic order among them. Simultaneously, axons from the early-born cells in both subplate and marginal zone (i.e., the original cortical preplate) establish an ordered array that fills the intermediate zone. Thalamic axons and preplate fibers meet in the lateral part of the internal capsule (at E15 for occipital cortex and dorsolateral thalamus). Subsequently, selective labeling of corresponding thalamic and early corticofugal projections reveals thalamic fibers growing in association with early corticofugal axons, right up to the cortical subplate. A small carbocyanine crystal implanted at any point in the cortex shortly after the arrival of thalamic axons (E16 for the occipital cortex) labels a single, tight bundle containing both descending and ascending fibers, rather than two separate tracts, providing further evidence for intimate topographic association of the two axon systems. Crystals placed in a row, parasagittally or coronally along the hemisphere, reveal separate, topographically distributed, discrete fiber bundles throughout the pathway, leading to spatially ordered groups of back-labeled thalamic cells. These results indicate that the topography of thalamic axons is maintained throughout the pathway and that they reach the cortex by associating with the projections of a number of preexisting cells, including the preplate scaffold.