Search PubMed⌕ Search

Biomedical subjects

M Petrides

Publications and source records attributed to M Petrides.

At least 73 records · Page 4Linked to original sources
PubMed1996

Planning and spatial working memory: a positron emission tomography study in humans.

Previous work with both neurosurgical and neurodegenerative patient groups has suggested that high level planning is mediated by neural circuitry which includes both the prefrontal cortex and the striatum. In this study, the functional anatomy of cognitive planning was investigated further, using positron emission tomography (PET). Regional cerebral blood flow (rCBF) was measured in 12 normal volunteers while performing easy and difficult versions of (i) a modified Tower of London planning task; (ii) a mnemonic variant of this task that required short-term retention and reproduction of problem solutions; and (iii) a control condition that involved identical visual stimuli and motor responses. Significant increases in rCBF were observed in the left hemisphere, in both the mid-dorsolateral frontal cortex and in the head of the caudate nucleus, when the difficult planning task was compared with the control condition. Moreover, subtraction of a simple planning condition from the more difficult one revealed focal increases in rCBF in the caudate nucleus and the thalamus only. During both mnemonic variants of the planning task, changes were also observed in the mid-dorsolateral frontal cortex and in more ventral frontopolar regions, bilaterally. When compared directly, the planning and memory conditions differed in terms of these ventral activation foci, but not in the pattern of activation observed in mid-dorsolateral frontal cortex. These findings further implicate frontostriatal circuitry in high-level planning and provide evidence for functionally distinct contributions from ventral and dorsolateral frontal regions to spatial working memory.

Adult↗
PubMed1996

Functional anatomy of visuomotor skill learning in human subjects examined with positron emission tomography.

The present study was designed to examine patterns of regional cerebral blood flow (CBF) associated with the learning of a repeated visuomotor sequence both in the early and late phases of the acquisition process. In addition, changes in blood flow related to the implicit versus explicit aspects of learning such a skill were investigated. Fourteen normal control subjects were scanned while performing the task (i) in both early and advanced learning stages of the visuomotor sequence; (ii) after having acquired explicit knowledge of the sequences; and (iii) in two control conditions (perceptual and random sequence). Subtraction of the random condition from the highly learned condition revealed specific areas of activity in the right ventral striatum and dentate nucleus of the cerebellum. Blood flow changes in the right hemisphere were also seen in the medial posterior parietal and prestriate regions, as well as in the anterior cingulate cortex. Finally, once the subjects had acquired explicit knowledge of the embedded sequence that was presented in the highly learned condition, increased CBF activity was observed only in the mid-ventrolateral frontal area in the right hemisphere. These findings confirm that both the striatum and the cerebellum are involved in the implicit acquisition of a visuomotor skill, especially in the advanced stages of the learning process, and furthermore that the ventrolateral prefrontal cortex contributes preferentially to the declarative aspect of this task.

Adult↗
PubMed1996

Neural systems for tactual memories.

1. The aim of this study was to investigate the neural systems involved in the memory processing of experiences through touch. 2. Regional cerebral blood flow was measured with positron emission tomography by means of the water bolus H2(15)O methodology in human subjects as they performed tasks involving different levels of tactual memory. In one of the experimental tasks, the subjects had to palpate nonsense shapes to match each one to a previously learned set, thus requiring constant reference to long-term memory. The other experimental task involved judgements of the recent recurrence of shapes during the scanning period. A set of three control tasks was used to control for the type of exploratory movements and sensory processing inherent in the two experimental tasks. 3. Comparisons of the distribution of activity between the experimental and the control tasks were carried out by means of the subtraction method. In relation to the control conditions, the two experimental tasks requiring memory resulted in significant changes within the posteroventral insula and the central opercular region. In addition, the task requiring recall from long-term memory yielded changes in the perirhinal cortex. 4. The above findings demonstrated that a ventrally directed parietoinsular pathway, leading to the posteroventral insula and the perirhinal cortex, constitutes a system by which long-lasting representations of tactual experiences are formed. It is proposed that the posteroventral insula is involved in tactual feature analysis, by analogy with the similar role of the inferotemporal cortex in vision, whereas the perirhinal cortex is further involved in the integration of these features into long-lasting representations of somatosensory experiences.

Adult↗
PubMed1996

Evidence for a dorso-medial parietal system involved in mental transformations of the body.

1. The neural systems underlying body-space mental representation were studied by measuring changes in regional cerebral blood flow (CBF) with positron emission tomography in human subjects. 2. The experimental paradigm involved identification of the left or the right hand of the experimenter presented in different orientations or the palm of the subject's right hand. The subjects were required to decide whether it was the left or the right hand that was presented. To perform this task, the subjects had to move mentally the position of their own arm to adopt that of the experimenter's arm. The control condition involved the same type of tactual stimulation without the requirement of mental transformations of the subject's body position. The distribution of CBF was measured by means of the water bolus H2(15)O methodology during the performance of these tasks. 3. Comparison of the distribution of CBF between the experimental and control tasks was carried out to reveal changes specific to the mental transformations of the subject's body. Significant blood flow increases were observed in the caudal superior parietal cortex, including the intraparietal sulcus, and the adjacent medial parietal cortex. These findings demonstrated that there is a dorsomedially directed parietal system underlying mental transformations of the body in interactive relation with external space.

Adult↗
PubMed1995

Neural correlates of mental transformations of the body-in-space.

Regional cerebral blood flow was measured with positron emission tomography in human subjects during the performance of a task requiring mental rotation of their hand and a perceptually equivalent control task that did not require such a process. Comparison of the distribution of cerebral activity between these conditions demonstrated significant blood flow increases in the superior parietal cortex, the intraparietal sulcus, and the adjacent rostralmost part of the inferior parietal lobule. These findings demonstrated that, in the human brain, there is a specific system of parietal areas that are involved in mental transformations of the body-in-space.

Adult↗
PubMed1995

Functional activation of the human ventrolateral frontal cortex during mnemonic retrieval of verbal information.

Regional cerebral blood flow was measured with positron emission tomography during the performance of a verbal free recall task, a verbal paired associate task, and tasks that required the production of verbal responses either by speaking or writing. Examination of the differences in regional cerebral blood flow between these conditions demonstrated that the left ventrolateral frontal cortical area 45 is involved in the recall of verbal information from long-term memory, in addition to its contribution to speech. The act of writing activated a network of areas involving posterior parietal cortex and sensorimotor areas but not ventrolateral frontal cortex.

Adult↗
PubMed1995

Impairments on nonspatial self-ordered and externally ordered working memory tasks after lesions of the mid-dorsal part of the lateral frontal cortex in the monkey.

Monkeys with lesions of the mid-dorsal part of the lateral frontal cortex, which extends above the sulcus principalis as far as the midline (MDL lesions), were shown to exhibit severe and long-lasting impairments on certain nonspatial working memory tasks: the self-ordered and externally ordered tasks (experiments 1, 2, 3, 5, 6, and 8). These tasks, which were modeled on similar ones previously used with patients, measure the capacity to monitor, within working memory, self-generated choices and the occurrence of externally ordered stimuli. Monkeys with lesions of the adjacent posterior dorsolateral frontal cortex, which surrounds the arcuate sulcus (PA lesions), performed as well as the normal control animals on these tasks. Experiments 4 and 5 showed that the critical variable accounting for the impairment on the self-ordered and externally ordered working memory tasks by monkeys with MDL lesions is the size of the set of stimuli that must be monitored. Furthermore, the MDL lesions did not affect basic recognition memory (experiment 6), or primacy and recency mnemonic effects [i.e., the capacity to discriminate between the initial (or final) items and other items in a list of stimuli (experiments 4 and 7), or the capacity to select from a set of stimuli on the basis of a learned fixed sequence (experiment 9)]. Thus, lesions of the mid-dorsal part of the lateral frontal cortex give rise to an impairment in working memory that depends on the size of the set of the stimuli that have to be monitored.

Animals↗
PubMed1994

Frontal lobes and behaviour.

A number of studies have appeared during the past year that contribute significantly to our understanding of the role of the frontal cortex in learning, memory, and response control. In addition to the traditional approaches that involve the investigation of the behavioural effects of lesions and the recording of the activity of single neurons, important information is now provided by functional activation studies carried out using positron emission tomography on normal human subjects.

Animals↗
PubMed1993

Localization and lateralization of stereoscopic processing in the human brain.

Positron emission tomography was used to investigate the neural substrate of stereopsis. Changes in cerebral blood flow were measured in nine volunteers while they judged the orientation of a rectangular cyclopean shape in random dot stereograms. This experimental condition was compared with two control conditions, one having a two-dimensional shape but no cyclopean stimulus, the other having neither shape nor cyclopean stimulus. Blood flow increases were observed in areas 17 and 18 in the right hemisphere when comparing experimental with control conditions; blood flow decreases occurred in the right inferotemporal cortex. Results indicate that stereopsis begins in posterior visual areas in the right cerebral hemisphere. Under ambiguous stereoscopic conditions, participation of more anterior areas in the temporal lobe may be elicited.

Cerebrovascular Circulation↗
PubMed1993

Memory impairments following lesions to the mammillary region of the rat.

The contribution of the mammillary region to learning and memory was investigated. It was demonstrated that lesions of this region impair performance on tasks that require memory for spatial information but that the deficit depends both on the amount of damage within the region and the difficulty of the task. A dissociation in the effect of such lesions on performance of comparable spatial and non-spatial memory tasks was shown. In contrast to the deficits observed on spatial memory tasks, the acquisition and retention of a complex non-spatial memory task was not impaired after extensive damage to the mammillary region. Such lesions also did not impair performance in a conditioned taste aversion task. These experiments suggest that the mammillary region may be selectively involved in spatial learning and memory.

Animals↗
PubMed1993

Dissociation of human mid-dorsolateral from posterior dorsolateral frontal cortex in memory processing.

Work with non-human primates had previously demonstrated that the mid-dorsolateral frontal cortex, which comprises cytoarchitectonic areas 46 and 9, plays a critical role in the performance of non-spatial self-ordered working memory tasks, whereas the immediately adjacent posterior dorsolateral frontal cortex (area 8) is critical for the learning and performance of visual conditional associative tasks. The present study used positron emission tomography with magnetic resonance imaging to demonstrate the existence, within the human brain, of these two functionally distinct subdivisions of the lateral frontal cortex. These findings provide direct evidence that, just as the monkey brain, the human lateral frontal cortex is functionally heterogeneous and that comparable anatomical areas underlie similar functions in the two species.

Adolescent↗
PubMed1993

Functional activation of the human frontal cortex during the performance of verbal working memory tasks.

Regional cerebral blood flow was measured with positron emission tomography during the performance of verbal working memory tasks. The same type of verbal response (i.e., reciting numbers) was required in the control and the two experimental tasks. In the control task, the subjects were required to count aloud. In the two experimental tasks, the subjects were required to maintain within working memory the numbers they generated (self-ordered task) or the numbers generated by the experimenter (externally ordered task). Examination of the difference in activation between these conditions revealed strong bilateral activation within the mid-dorsolateral frontal cortex during both experimental tasks. There was, however, no evidence of additional activation within the mid-dorsolateral frontal cortex when monitoring self-generated responses as compared with the monitoring of externally generated responses. These results provide evidence regarding the role of the mid-dorsolateral frontal cortex in mnemonic processing that are in agreement with recent findings from work with non-human primates.

Adult↗
PubMed1993

Role of the human anterior cingulate cortex in the control of oculomotor, manual, and speech responses: a positron emission tomography study.

1. Two experiments were aimed at investigating the functional organization of the human anterior cingulate cortex (ACC) in relation to higher-order motor control. 2. The 15O-labeled H2O bolus method was used to measure relative changes of regional cerebral blood flow (rCBF) in 18 healthy human subjects as they performed oculomotor, manual, or speech tasks. 3. Task-specific rCBF changes were obtained in distinct subregions of the ACC, depending on the output system employed. The oculomotor and the manual task-related foci were found in the rostral and caudal regions of the ACC, respectively, whereas the speech foci were localized within two cingulate subregions, the intermediate dorsal and the rostral ACC. 4. In the manual tasks, two groups of activation foci could be distinguished, one just behind and the other just in front of the vertical plane traversing the anterior commissure. 5. The above pattern of rCBF changes was observed only if there was concomitant activation within the lateral prefrontal cortex (except for the posterior group of foci obtained in the manual tasks). 6. The localization of output-specific rCBF changes within the human ACC is consistent with the known somatotopic organization of the cingulate cortex in the monkey. 7. It is tentatively proposed that the ACC participates in motor control by facilitating the execution of the appropriate responses and/or suppressing the execution of the inappropriate ones. Such a modulatory effect would be of particular importance when behavior has to be modified in new and challenging situations.

Adult↗
PubMed1991

Monitoring of selections of visual stimuli and the primate frontal cortex.

This investigation shows that lesions confined to the middle sector of the dorsolateral frontal cortex, i.e. cytoarchitectonic areas 46 and 9, cause a striking impairment in the ability of non-human primates to recall which one from a set of stimuli they chose, without in any way affecting their ability to recognize that they had previously seen those stimuli. By contrast, lesions placed within the adjacent posterior dorsolateral frontal cortex affect neither recognition of visual stimuli nor recall of prior choices. These findings delineate the mid-dorsolateral frontal cortex as a critical component of a neural system mediating the monitoring of self-generated responses.

Animals↗
PubMed1991

Functional specialization within the dorsolateral frontal cortex for serial order memory.

Monkeys with lesions restricted to two anatomically distinct regions of the dorsolateral frontal cortex were tested on a novel task that was developed to assess memory for the order of occurrence of stimuli. Monkeys with bilateral lesions of the mid-dorsolateral frontal cortex (cytoarchitectonic areas 46 and 9) were severely impaired, whereas monkeys with lesions of the posterior region of the dorsolateral frontal cortex (area 8 and rostral area 6) performed as well as the normal control animals. These results show that the primate mid-dorsolateral frontal cortex is a critical component of a neural circuit underlying the monitoring of the serial order of stimuli.

Animals↗
PubMed1990

Nonspatial conditional learning impaired in patients with unilateral frontal but not unilateral temporal lobe excisions.

The present study examined the effect of unilateral frontal- or temporal-lobe excisions on the acquisition of a conditional task requiring that the subjects respond to each one of six different coloured stimuli by selecting, from a set of six abstract designs, the correct design for each stimulus. Patients with excisions from the left or right frontal cortex were impaired in learning this task, whereas patients with left or right temporal-lobe excisions, with or without radical involvement of the hippocampal region, were not impaired. These findings demonstrate that the major role played by the frontal cortex in the acquisition of conditional responses is a general one and not restricted to situations involving different movements.

Adolescent↗