Progressive transneuronal changes in the brainstem and thalamus after long-term dorsal rhizotomies in adult macaque monkeys.
This study deals with a potential brainstem and thalamic substrate for the extensive reorganization of somatosensory cortical maps that occurs after chronic, large-scale loss of peripheral input. Transneuronal atrophy occurred in neurons of the dorsal column (DCN) and ventral posterior lateral thalamic (VPL) nuclei in monkeys subjected to cervical and upper thoracic dorsal rhizotomies for 13-21 years and that had shown extensive representational plasticity in somatosensory cortex and thalamus in other experiments. Volumes of DCN and VPL, number and sizes of neurons, and neuronal packing density were measured by unbiased stereological techniques. When compared with the opposite, unaffected, side, the ipsilateral cuneate nucleus (CN), external cuneate nucleus (ECN), and contralateral VPL showed reductions in volume: 44-51% in CN, 37-48% in ECN, and 32-38% in VPL. In the affected nuclei, neurons were progressively shrunken with increasing survival time, and their packing density increased, but there was relatively little loss of neurons (10-16%). There was evidence for loss of axons of atrophic CN cells in the medial lemniscus and in the thalamus, with accompanying severe disorganization of the parts of the ventral posterior nuclei representing the normally innervated face and the deafferented upper limb. Secondary transneuronal atrophy in VPL, associated with retraction of axons of CN neurons undergoing primary transneuronal atrophy, is likely to be associated with similar withdrawal of axons from the cerebral cortex and should be a powerful influence on reorganization of somatotopic maps in the somatosensory cortex. (+info)
Effect of attentive fixation in macaque thalamus and cortex.
Attentional modulation of neuronal responsiveness is common in many areas of visual cortex. We examined whether attentional modulation in the visual thalamus was quantitatively similar to that in cortex. Identical procedures and apparatus were used to compare attentional modulation of single neurons in seven different areas of the visual system: the lateral geniculate, three visual subdivisions of the pulvinar [inferior, lateral, dorsomedial part of lateral pulvinar (Pdm)], and three areas of extrastriate cortex representing early, intermediate, and late stages of cortical processing (V2, V4/PM, area 7a). A simple fixation task controlled transitions among three attentive states. The animal waited for a fixation point to appear (ready state), fixated the point until it dimmed (fixation state), and then waited idly to begin the next trial (idle state). Attentional modulation was estimated by flashing an identical, irrelevant stimulus in a neuron's receptive field during each of the three states; the three responses defined a "response vector" whose deviation from the line of equal response in all three states (the main diagonal) indicated the character and magnitude of attentional modulation. Attentional modulation was present in all visual areas except the lateral geniculate, indicating that modulation was of central origin. Prevalence of modulation was modest (26%) in pulvinar, and increased from 21% in V2 to 43% in 7a. Modulation had a push-pull character (as many cells facilitated as suppressed) with respect to the fixation state in all areas except Pdm where all cells were suppressed during fixation. The absolute magnitude of attentional modulation, measured by the angle between response vector and main diagonal expressed as a percent of the maximum possible angle, differed among brain areas. Magnitude of modulation was modest in the pulvinar (19-26%), and increased from 22% in V2 to 41% in 7a. However, average trial-to-trial variability of response, measured by the coefficient of variation, also increased across brain areas so that its difference among areas accounted for more than 90% of the difference in modulation magnitude among areas. We also measured attentional modulation by the ratio of cell discharge due to attention divided by discharge variability. The resulting signal-to-noise ratio of attention was small and constant, 1.3 +/- 10%, across all areas of pulvinar and cortex. We conclude that the pulvinar, but not the lateral geniculate, is as strongly affected by attentional state as any area of visual cortex we studied and that attentional modulation amplitude is closely tied to intrinsic variability of response. (+info)
Differential extrageniculostriate and amygdala responses to presentation of emotional faces in a cortically blind field.
Patient G.Y. is able to discriminate emotional facial expressions presented in his blind (right) hemifield despite an extensive lesion of the corresponding (left) striate cortex. One proposal is that this residual ability (affective "blindsight") depends on a subcortical visual pathway comprising the superior colliculus, posterior (extrageniculate) thalamus and amygdala. Here we report differential amygdala responses in G.Y. to presentation of fearful and fear-conditioned faces in his blind (right) hemifield. These amygdala responses exhibited condition-dependent covariation with neural activity in the posterior thalamus and superior colliculus. Our results provide further evidence that an extrageniculostriate (colliculo-thalamo-amygdala) neural pathway can process fear-related stimuli independently of both the striate cortex and normal phenomenal visual awareness. (+info)
A graphical anatomical database of neural connectivity.
We describe a graphical anatomical database program, called XANAT (so named because it was developed under the X window system in UNIX), that allows the results of numerous studies on neuroanatomical connections to be stored, compared and analysed in a standardized format. Data are entered into the database by drawing injection and label sites from a particular tracer study directly onto canonical representations of the neuroanatomical structures of interest, along with providing descriptive text information. Searches may then be performed on the data by querying the database graphically, for example by specifying a region of interest within the brain for which connectivity information is desired, or via text information, such as keywords describing a particular brain region, or an author name or reference. Analyses may also be performed by accumulating data across multiple studies and displaying a colour-coded map that graphically represents the total evidence for connectivity between regions. Thus, data may be studied and compared free of areal boundaries (which often vary from one laboratory to the next), and instead with respect to standard landmarks, such as the position relative to well-known neuro-anatomical substrates or stereotaxic coordinates. If desired, areal boundaries may also be defined by the user to facilitate the interpretation of results. We demonstrate the application of the database to the analysis of pulvinar-cortical connections in the macaque monkey, for which the results of over 120 neuro-anatomical experiments were entered into the database. We show how these techniques can be used to elucidate connectivity trends and patterns that may otherwise go unnoticed. (+info)
The subcortical anatomy of human spatial neglect: putamen, caudate nucleus and pulvinar.
Various studies have documented that right hemispheric lesions restricted to the basal ganglia or to the thalamus may evoke spatial neglect. However, for methodological reasons, the exact anatomical correlate of spatial neglect within these two subcortical structures still remained uncertain. The present study identified these locations by comparing the anatomy of subcortical lesions to the basal ganglia or thalamus between neglect and control patients. Analysis revealed that the putamen, the pulvinar and, to a smaller degree, the caudate nucleus are the subcortical structures typically associated with spatial neglect in humans. All these structures have direct anatomical connections to the superior temporal gyrus (STG), which recently has been identified as the neural correlate of spatial neglect in the human cortex. Therefore, it is assumed that the right putamen, caudate nucleus, pulvinar and STG form a coherent corticosubcortical anatomical network in the genesis of spatial neglect in humans. (+info)
Gross total removal of gliomas in the pulvinar and correlative microsurgical anatomy.
Tumors in the pulvinar tend to present as circumscribed lesions with exophytic growth into the lateral and third ventricles. These lesions may be best explored via a parietal-transcortical-transventricular approach. If the tumor extends posteriorly or inferiorly, a posterior-interhemispheric-transtentorial approach may provide a good angle of access. Gross total removal of the tumors in the pulvinar of two patients was achieved by surgical sectioning of the unilateral crus of the fornix or the splenium via a transventricular or interhemispheric approach with acceptable risk. These patients are now doing well as students about 6 years following the first operations. During tumor removal, a posterior-interhemispheric-transtentorial approach combined with above-mentioned approaches was useful for orientation of the critical structures in the posterior incisural space. Knowledge of the anatomical relationships of the pulvinar to the crus of the fornix and the choroid plexus, and to the critical structures located in the posterior incisural space is extremely important for neurosurgeons. (+info)
MR spectroscopic pulvinar sign in a case of variant Creutzfeldt-Jakob disease.
We report MR spectroscopic findings in a patient hospitalized with biopsy-proven variant Creutzfeldt-Jakob (vCJD) disease. N-acetyl aspartate was markedly decreased in the postero-medial part of the thalami (pulvinar) but was not diminished in the parieto-occipital white matter and cortical grey matter. These observations, which are in accordance with the pathological findings in this disease, suggest that MR spectroscopy, a highly sensitive method for the detection of subtle brain metabolic dysfunction, could be of interest for the diagnosis, prognosis and therapeutic follow-up of vCJD. (+info)
The role of the thalamus in the flow of information to the cortex.
The lateral geniculate nucleus is the best understood thalamic relay and serves as a model for all thalamic relays. Only 5-10% of the input to geniculate relay cells derives from the retina, which is the driving input. The rest is modulatory and derives from local inhibitory inputs, descending inputs from layer 6 of the visual cortex, and ascending inputs from the brainstem. These modulatory inputs control many features of retinogeniculate transmission. One such feature is the response mode, burst or tonic, of relay cells, which relates to the attentional demands at the moment. This response mode depends on membrane potential, which is controlled effectively by the modulator inputs. The lateral geniculate nucleus is a first-order relay, because it relays subcortical (i.e. retinal) information to the cortex for the first time. By contrast, the other main thalamic relay of visual information, the pulvinar region, is largely a higher-order relay, since much of it relays information from layer 5 of one cortical area to another. All thalamic relays receive a layer-6 modulatory input from cortex, but higher-order relays in addition receive a layer-5 driver input. Corticocortical processing may involve these corticothalamocortical 're-entry' routes to a far greater extent than previously appreciated. If so, the thalamus sits at an indispensable position for the modulation of messages involved in corticocortical processing. (+info)