252 research outputs found
The association fiber system linking the mid-dorsolateral frontal cortex with the retrosplenial cortex and the posterior hippocampal region in the rhesus monkey /
No full textThe study of patients has shown that certain higher cognitive processes, such as those involved in the monitoring and the manipulation of information within working memory, depend on the integrity of both the dorsolateral frontal cortex and the medial temporal lobe memory system, as well as on their functional interaction (Petrides, 1994). Small surgical removal of the anterior temporal region, including the entorhinal cortex, is not sufficient to interrupt that fronto-hippocampal relationship. More extensive removals, however, that include a sizeable portion of the hippocampus and the surrounding parahippocampal cortex do disrupt such a fronto-hippocampal functional interaction (Petrides and Milner, 1982). Based on these data, it was postulated that the fronto-hippocampal functional interaction is not entirely dependent upon the integrity of the entorhinal cortex. To test this hypothesis, injections of tritiated amino acids were placed within individual cytoarchitectonic units of the frontal cortex, and the resulting labeling in the hippocampal region was analyzed. It was shown that the mid-dorsolateral frontal cortex, together with its medial cortical extension, is the only frontal region that sends efferent fibers, running caudally as part of the cingulum bundle, to the presubiculum, the posterior parahippocampal gyrus, as well as to the retrosplenial cortex. A light contingent of these fibers, congregating in the outermost layer of both the retrosplenium and the presubiculum, course into the molecular layer of the hippocampus proper. In complete agreement with the work with patients, these findings have confirmed the hypothesis that the mid-dorsolateral frontal cortex is closely affiliated with the hippocampal system, and demonstrated that this hodological relationship bypasses the entorhinal cortex.Another major contribution of the present work has been to provide the first architectonic analysis of a gross morphological region, referred to as the caudomedial lobule, which receives inputs from the mid-dorsolateral frontal cortex and its medial extension. This architectonic analysis has revealed that the caudomedial lobule is nothing but the postero-ventral extension, below the splenium of the corpus callosum, of areas 29 and 30, which together form the retrosplenial cortex, and of area 23, which partly forms the posterior cingulate cortex. Among the cortical fields that comprise the postero-ventral part of the retrosplenial cortex, area 30 is the major recipient of the mid-dorsolateral frontal inputs.By virtue of the close anatomical relation of area 30 with the mid-dorsolateral frontal cortex and its medial extension, it is suggested that this part of the retrosplenial cortex may be a critical relay-station along the dorsomedially directed fronto-hippocampal pathway. In order to validate this hypothesis, the connections of area 30 were investigated by placing injections of anterograde and retrograde tracers within the limits of this retrosplenial area. This study has demonstrated that area 30 is bi-directionally connected with and only with that part of the lateral frontal cortex that lies above the sulcus principalis, namely the mid-dorsolateral frontal cortex, along with all the structures of the posterior hippocampal region that are the recipients of the inputs from the mid-dorsolateral frontal cortex. Since the fronto-hippocampal association fiber system described in the present thesis is most probably subserving certain aspects of working memory, area 30, by virtue of its bi-directional connections with both the mid-dorsolateral frontal cortex and the posterior parahippocampal cortex, is in a privileged position to exert a major influence in working memory processing
The precentral insular cortical network for speech articulation
No full textApraxia of speech is a motor disorder characterized by the impaired ability to coordinate the sequential articulatory movements necessary to produce speech. The critical cortical area(s) involved in speech apraxia remain controversial because many of the previously reported cases had additional aphasic impairments, preventing localization of the specific cortical circuit necessary for the somatomotor execution of speech. Four patients with "pure speech apraxia" (i.e., who had no aphasic and orofacial motor impairments) are reported here. The critical lesion in all four patients involved, in the left hemisphere, the precentral gyrus of the insula (gyrus brevis III) and, to a lesser extent, the nearby areas with which it is strongly connected: the adjacent subcentral opercular cortex (part of secondary somatosensory cortex) and the most inferior part of the central sulcus where the orofacial musculature is represented. There was no damage to rostrally adjacent Broca's area in the inferior frontal gyrus. The present study demonstrates the critical circuit for the coordination of complex articulatory movements prior to and during the execution of the motor speech plans. Importantly, this specific cortical circuit is different from those that relate to the cognitive aspects of language production (e.g., Broca's area on the inferior frontal gyrus)
Copy and recall of the Rey Complex figure before and after unilateral frontal- or temporal-lobe excision.
No full textCopy and recall drawings of the Rey Complex Figure obtained during the standard clinical testing of patients with well-localized epileptic foci before and after left frontal-, left temporal-, right temporal-lobe resection were re-scored blind as to lesion site using standard protocol (18 elements scored 0, 1/2, 1, or 2 based on whether they are drawn and placed correctly for a total out of 36). They were also scored for which, and how many, elements were missing, distorted, displaced, and/or repeated. Contrary to previous findings, no main effects of side or lobe or side-by-lobe interactions were found on copy and recall scores obtained either before or after surgery, and all patients' recall improved equally from pre-operative to follow-up testing. Furthermore, patients' lesion site could not be predicted on the basis of any single measure or across all measures of performance. While group differences had been found on the previously assigned scores, the between-group overlap was almost complete and the original scoring was not done blindly. These results suggest that, despite previous claims, the Rey Complex Figure, a widely-used measure of non-verbal memory, is not an effective tool for localizing neural disturbance in temporal- and frontal-lobe epilepsy patients
A comparison of the role of the frontal cortex and the anterior temporal lobe in source memory and in the accurate retrieval of episodic information /
No full textIt has been argued that patients with frontal lobe lesions are impaired in temporal context memory and, more generally, in retrieving the source of one's knowledge or ideas. Furthermore, it has been speculated that a failure to retrieve source information may result in an increased susceptibility to distortions of episodic memories in patients with frontal lobe lesions. The precise role of the frontal cortex, however, in source or episodic retrieval is not clear. Does this region of cortex play a primary role or a secondary, executive role in the processing of such memories? Studies of patients with temporal lobe lesions have also shown impairments in episodic memory, including difficulties in the retrieval of source information. An important issue, therefore, is whether these two brain regions make different contributions to the processing of source information and to the retrieval of episodic memories.In the present experiments, patients with unilateral excisions restricted to frontal cortex or to the anterior temporal lobe were compared on various tasks examining source memory performance and the accurate retrieval of episodic information. The results of these studies failed to support the general contention that patients with frontal cortex excisions have source (or temporal context) memory impairments. Instead, differences between these patients and normal control subjects appeared to be contingent on whether strategic organizational or control processes were necessary for efficient processing of episodic information. The memory of patients with left temporal lobe excisions, on the other hand, was significantly impaired for both content and source information in most tasks. Furthermore, these subjects showed high rates of inaccuracies and distortions of memory. The false memories of this patient group were attributed to a combination of their poor memory for the specific items of the task and their over-reliance on semantic "gist" or on inferential knowledge about the events. Patients with right temporal lobe excisions were generally less severely impaired on the verbal memory tasks compared with those with left-sided lesions, but were impaired in their memory for the contextual aspects of an event
The orbitofrontal cortex : sulcal anatomy and cytoarchitectonic correlations
No full textThe sulcal patterns of the human orbitofrontal cortex have not been adequately characterized. Classical authors, as well as more recent investigators, have attempted to identify and label the sulci of this part of the brain. Nevertheless, there is considerable confusion regarding the orbital sulcal patterns with inconsistencies in the naming of orbitofrontal sulci in many modern texts. Moreover, a correlation between specific landmarks and architectonic areas has not been demonstrated.A clarification of the patterns of the orbitofrontal sulci and their relationship to architectonic subregions is necessary if the results of functional neuroimaging and other physiological and anatomical findings are to be properly interpreted. Although studies have reported altered activity in the orbitofrontal cortex in relation to various sensory processes and pathological states it has been difficult to relate these changes to specific orbitofrontal regions because of a limited understanding of the anatomical landmarks. The absence of reliable reference markers forces the use of vague terminology (e.g., "orbital frontal activation") in describing the location of functional changes in the orbital frontal cortex.The aim of this doctoral thesis was to gain a better understanding of the sulcal pattern of the human orbitofrontal cortex and its relation to the underlying cytoarchitecture. The first study resolved the confusion associated with the orbitofrontal sulci by identifying, quantifying, and precisely localizing the various orbital sulci from fifty human magnetic resonance scans that were transformed into the standardized stereotaxic space of Talairach and Tournoux (1988). The second study compared the individual sulci and sulcal patterns of these fifty human brains with the brains of fifty adult rhesus monkeys. Having examined the orbitofrontal sulci in these two species, a nomenclature for the human orbitofrontal sulci was established which was based on comparable sulci in the less convoluted macaque monkey brain while trying to preserve many of the familiar labels associated with this region in the human brain. The final part of this thesis examined the orbitofrontal cytoarchitecture of 10 human adult cerebral hemispheres to determine if a correlation exists between the different orbital sulci and the borders of the architectonic subregions
Effects of lesions to the anterior thalamic nuclei on two spatial, working memory tasks in rats
No full textThe experiments reported in the present thesis investigated the effects of lesions to the anterior nuclei of the thalamus (ATN) on the acquisition of two spatial, working memory tasks performed on the eight-arm radial maze. In the task used in Experiment 1 and 2, the animals were required to discriminate and remember all the eight arms of the maze simultaneously. Lesions of the ATN produced impairments in the acquisition of this task, but the degree of impairment depended on the amount of damage within this region. In the task used in Experiment 3, the animals were required to discriminate and remember only two arms at once. Lesions of the ATN were shown to impair its acquisition even though performance was facilitated by the addition of visual intra-arms cues. These experiments suggest that the ATN may be involved in spatial learning and in the retention of non-specific information over time
The neural basis of human auditory rhythm perception and production /
No full textMusic depends on the perception and production of complex temporal patterns, or rhythms, as a vital part of its power to communicate. These experiments investigated the neuroanatomical substrate of rhythm perception and production in patients with focal cerebral lesions, and in neuroimaging studies with normals. The hypotheses focused on the role of the primary and secondary auditory cortex, as well as the cerebellum and other motor-related areas.Experiment I. To estimate the extent of removals in the region of auditory cortex in neurosurgical patients, a 3D probabilistic map of Heschl's gyrus (HG) was developed from magnetic resonance imaging (NW scans of normals. The map was coregistered with patent MRIs, revealing that most removals involved only anterior secondary auditory cortical regions, with relatively little encroachment onto primary auditory cortex. Additionally, this experiment compared the volumes of HG between hemispheres and found a consistent L > R asymmetry in the volume of cortical white matter, which could contribute directly to the preferential left-hemisphere processing of speech.Experiment II. Using a paradigm contrasting reproduction of auditory and visual rhythms, four groups of patients were tested: those with right or left anterior temporal-lobe removals (RT-a, LT-a) and those with similar removals which also included HG (RT-A, LT-A). RT-A patients were impaired on auditory but not visual rhythms, particularly when accurate reproduction of stimulus durations was required. In contrast, LT-a, RT-a and LT-a patients were not impaired. These results demonstrated a role for the right anterior secondary auditory cortical regions in the retention of auditory temporal information.Experiment III. The rhythm reproduction paradigm was adapted for a positron emission tomography (PET) activation study in normals. These data confirmed the participation of secondary auditory regions in the retention of auditory temporal information. They also demonstrated a supra-modal contribution of the lateral cerebellar cortices, vermis and basal ganglia to the task, pointing to a distributed system of neural structures involved in the production of a timed motor response from external stimuli.The results are discussed in relation to possible interhemispheric differences in auditory temporal processing as well as the role of the cerebellum in motor and perceptual timing
Hypothalamic involvement in multiple system atrophy: A structural MRI study
No full textObjective: To investigate hypothalamic atrophy and its clinical correlates in multiple system atrophy (MSA) invivo. Background: MSA is characterized by autonomic dysfunction and parkinsonian/cerebellar manifestations. The hypothalamus regulates autonomic and homeostatic functions and is also involved in memory and learning processes. Methods: 11 MSA, 18 Parkinson's Disease (PD) and 18 Healthy Controls (HC) were included in this study. A validated and automated hypothalamic segmentation tool was applied to 3D-T1-weighted images acquired on a 3T MRI scanner. MSA hypothalamic volumes were compared to those of PD and HC. Furthermore, the association between hypothalamic volumes and scores of autonomic, depressive, sleep and cognitive manifestations were investigated. Results: Posterior hypothalamus volume was reduced in MSA compared to controls (t = 2.105, p = 0.041) and PD (t = 2.055, p = 0.046). Total hypothalamus showed a trend towards a reduction in MSA vs controls (t = 1.676, p = 0.101). Reduced posterior hypothalamus volume correlated with worse MoCA scores in the parkinsonian (MSA + PD) group and in each group separately, but not with autonomic, sleep, or depression scores. Conclusions: In-vivo structural hypothalamic involvement may be present in MSA. Reduced posterior hypothalamus volume, which includes the mammillary bodies and lateral hypothalamus, is associated with worse cognitive functioning. Larger studies on hypothalamic involvement in MSA and its clinical correlates are needed
Locus coeruleus neuromelanin, cognitive dysfunction, and brain metabolism in multiple system atrophy
No full textSpecialized systems for the processing of mnemonic information within the primate frontal cortex
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