Biomedical subjects

M Petrides

Publications and source records attributed to M Petrides.

At least 19 recordsLinked to original sources

PubMed2004

Functional abnormalities in symptomatic concussed athletes: an fMRI study.

Our aim was to quantify with functional magnetic resonance imaging (fMRI) changes in brain activity in concussed athletes and compare the results with those of normal control subjects. Regional brain activations associated with a working memory task were obtained from a group of concussed athletes (15 symptomatic, 1 asymptomatic) and eight matched control subjects, using blood oxygen level dependent (BOLD) fMRI. The average percent signal change from baseline to working memory condition in each region of interest was computed. Symptomatic concussed athletes demonstrated task-related activations in some but not all the regions of interest, even when they performed as well as the control subjects. Furthermore, several concussed athletes had additional increases in activity outside the regions of interest, not seen in the control group. Quantitative analysis of BOLD signals within regions of interest revealed that, in general, concussed athletes had different BOLD responses compared to the control subjects. The task-related activation pattern of the one symptom-free athlete was comparable to that of the control group. We also repeated the study in one athlete whose symptoms had resolved. On the first study, when he was still symptomatic, less task-related activations were observed. On follow-up, once his symptoms had disappeared, the task-related activations became comparable to those of the control group. These results demonstrate the potential of fMRI, in conjunction with the working memory task, to identify an underlying pathology in symptomatic concussed individuals with normal structural imaging results.

Adult↗

PubMed2004

The mind's nose: Effects of odor and visual imagery on odor detection.

We examined odor imagery by looking for its effects on detection of weak odors. Seventy-two healthy subjects performed a forced-choice odor detection task in one of three conditions: after being told to imagine an odor (odor imagery), after being told to imagine an object (visual imagery), or without having received imagery instructions (no-imagery control). For the two imagery conditions, the presented and imagined stimuli were either the same (matched) or different (mismatched). There was a significant difference between detection in the matched and mismatched conditions for odor imagery, but not for visual imagery. We conclude that our paradigm does measure odor imagery and that the effect of imagery on detection is both content- and modality-specific. Further, the difference between conditions was due to lower detection with mismatched odor imagery than without imagery, indicating that interference underlies the effect.

Adolescent↗

PubMed2002

Morphology and morphometry of the corpus callosum in Williams syndrome: a T1-weighted MRI study.

Williams syndrome (WS) is characterised by a defined genetic aetiology and a specific cognitive profile. It provides an opportunity to examine associations between neuroanatomy, behaviour, and genetics. High-resolution T1-weighted MRI of the brain of 12 patients with WS and 12 normal control subjects were used to estimate the shape and volume of the corpus callosum (CC), as well as the voxel intensity values as a measure its water content. The CC of patients with WS was more convex than that of normal control subjects and overall smaller in volume, particularly in the splenium and in the caudal part of the callosal body. In addition, there were higher ratio values of voxel intensity (i.e. less water content) in the mid-section of the body and the caudal part of the body of the CC. These combined features indicate an aberrant development of the CC in patients with WS and document some of the anatomical abnormalities that may underlie some of the cognitive impairments observed in subjects with WS.

Adolescent↗

PubMed2002

Effects of lesions to the hippocampus or the fornix on allocentric conditional associative learning in rats.

Rats with lesions of the fornix, the hippocampus, or normal control animals were trained on a visual-spatial conditional associative learning task in which they had to learn to go to a particular location based on the presence of a specific visual cue; the rats approached the cues from different directions. Animals with damage of the fornix were able to learn the task at a rate comparable to that of the control animals. The performance of the hippocampal rats was significantly impaired as compared with the control group. Both the fornix and the hippocampal animals were significantly impaired on a spatial working memory task, the eight-arm radial maze. These findings suggest that, under certain conditions, a functional dissociation exists between the effects of damage to the fornix or the hippocampus and that the fornix may be only selectively involved in spatial learning and memory.

Animals↗

PubMed2002

Comparative cytoarchitectonic analysis of the human and the macaque ventrolateral prefrontal cortex and corticocortical connection patterns in the monkey.

A comparison of the cytoarchitecture of the human and the macaque monkey ventrolateral prefrontal cortex demonstrated a region in the monkey that exhibits the architectonic characteristic of area 45 in the human brain. This region occupies the dorsal part of the ventrolateral prefrontal convexity just below area 9/46v. Rostroventral to area 45 in the human brain lies a large cortical region labelled as area 47 by Brodmann. The ventrolateral component of this region extending as far as the lateral orbital sulcus has architectonic characteristics similar to those of the ventrolateral prefrontal region labelled by Walker as area 12 in the macaque monkey. We designated this region in both the human and the monkey ventrolateral prefrontal cortex as area 47/12. Thus, area 47/12 designates the specific part of the zone previously labelled as area 47 in the human brain that has the same overall architectonic pattern as that of Walker's area 12 in the macaque monkey brain. The cortical connections of these two areas were examined in the monkey by injecting fluorescent retrograde tracers. Although both area 45 and area 47/12 as defined here had complex multimodal input, they could be differentiated in terms of some of their inputs. Retrograde tracers restricted to area 47/12 resulted in heavy labelling of neurons in the rostral inferotemporal visual association cortex and in temporal limbic areas (i.e. perirhinal and parahippocampal cortex). In contrast, injections of tracers into dorsally adjacent area 45 demonstrated strong labelling in the superior temporal gyrus (i.e. the auditory association cortex) and the multimodal cortex in the upper bank of the superior temporal sulcus.

Aged↗

PubMed2002

The effect of spatial and temporal information on saccades and neural activity in oculomotor structures.

It has been argued that saccade generation is supported by two systems, a'where' system that decides the direction and extent of an impending saccade, and a 'when' system that is involved in the timing of the release of fixation. We evaluated the contributions of these systems to saccade latencies, and used functional MRI to identify the neural substrates of these systems. We found that advance knowledge of the direction and the timing of an impending target movement had both overlapping and discrete effects on saccade latencies and on neural activation. Knowledge of either factor decreased regular saccade latencies. However, knowledge of target direction increased the number of predictive and express saccades while knowledge of target timing did not. The brain activation data showed that advance knowledge of the direction or the timing of the target movement activated primarily overlapping structures. The precentral gyrus, in the region of the frontal eye fields, was more active in conditions in which some aspect of the target movement was predictable than in saccade control and fixation conditions. In the basal ganglia, activation discriminated between advance knowledge of target timing and target direction. The lenticular nuclei were more active when only target timing was known in advance, while the caudate was more active when only target direction was known in advance. These data suggest that the neural structures supporting the 'where' and 'when' systems are highly overlapping, although there is some dissociation sub-cortically. Knowledge of target timing and target direction converge in precentral gyrus, a region where there is strong evidence of context-dependent modulation of neural activity.

Adult↗

PubMed2001

Wisconsin Card Sorting revisited: distinct neural circuits participating in different stages of the task identified by event-related functional magnetic resonance imaging.

The Wisconsin Card Sorting Task (WCST) has been used to assess dysfunction of the prefrontal cortex and basal ganglia. Previous brain imaging studies have focused on identifying activity related to the set-shifting requirement of the WCST. The present study used event-related functional magnetic resonance imaging (fMRI) to study the pattern of activation during four distinct stages in the performance of this task. Eleven subjects were scanned while performing the WCST and a control task involving matching two identical cards. The results demonstrated specific involvement of different prefrontal areas during different stages of task performance. The mid-dorsolateral prefrontal cortex (area 9/46) increased activity while subjects received either positive or negative feedback, that is at the point when the current information must be related to earlier events stored in working memory. This is consistent with the proposed role of the mid-dorsolateral prefrontal cortex in the monitoring of events in working memory. By contrast, a cortical basal ganglia loop involving the mid-ventrolateral prefrontal cortex (area 47/12), caudate nucleus, and mediodorsal thalamus increased activity specifically during the reception of negative feedback, which signals the need for a mental shift to a new response set. The posterior prefrontal cortex response was less specific; increases in activity occurred during both the reception of feedback and the response period, indicating a role in the association of specific actions to stimuli. The putamen exhibited increased activity while matching after negative feedback but not while matching after positive feedback, implying greater involvement during novel than routine actions.

Adolescent↗

PubMed2001

Three-dimensional probabilistic atlas of the human orbitofrontal sulci in standardized stereotaxic space.

A quantitative analysis of the variability of human orbitofrontal anatomical landmarks is required if one is to describe properly the location of functional or structural changes within this brain region. The present study investigated the orbitofrontal sulci in 100 normal adult human cerebral hemispheres using magnetic resonance images that were transformed into the standardized proportional stereotaxic space most commonly used, that of Talairach and Tournoux (1988). Four orbitofrontal sulci were identified and marked on each hemisphere: the olfactory, the medial, the lateral, and the transverse orbital sulci. Probability maps were then constructed for each sulcus. These maps provide a statistical description of the variability of the location of each orbitofrontal sulcus within the three-dimensional coordinate system of Talairach and Tournoux (1988). These probability maps may be directly compared with any image transformed into the same standardized space, providing a valuable tool for identifying and describing the location of functional or structural changes in the orbitofrontal region of the human brain.

Adult↗

PubMed2001

Time-related changes in task performance after lesions restricted to the frontal cortex.

Behavioural studies in patients and functional neuroimaging studies in normal subjects suggest that the right frontal cortex plays an important role in sustaining attention to a task. However, the contribution of different regions of the frontal lobe to performance changes has not been clearly established. First, the assessment of sustained attention has not been carried out in patients with lesions that are restricted to cortical regions of the frontal lobe. Second, although sustained attention is often measured by performance on a task requiring effortful attention, it may be more meaningful to measure changes in performance as a function of time-on-task. Two studies were carried out in five groups of subjects: patients with left or right frontal cortical resections, patients with left or right anterior temporal lobe resections, and normal controls. In the first experiment, a brief (4-min) selective attention task was performed. The right frontal group, unlike the other groups, did not show improved reaction times after the first minute on the task. The second experiment involved a long (30-min) shifting attention task. Both the left and right frontal groups were slower than control groups; however, reaction times in the right frontal group increased with time spent on the task. We conclude that the right frontal cortex plays an essential role in modulating arousal during performance of a task and may also underlie aspects of learning during the early stages of task performance.

Arousal↗

PubMed2001

Selective activation of the ventrolateral prefrontal cortex in the human brain during active retrieval processing.

The present study examined the role of the prefrontal cortex in retrieval processing using functional magnetic resonance imaging in human subjects. Ten healthy subjects were scanned while they performed a task that required retrieval of specific aspects of visual information. In order to examine brain activity specifically associated with retrieval, we designed a task that had retrieval and control conditions that were perfectly matched in terms of depth of encoding, decision making and postretrieval monitoring and differed only in terms of whether retrieval was required. In the retrieval condition, based on an instructional cue, the subjects had to retrieve either the particular stimulus that was previously presented or its location. In the control condition, the cue did not instruct retrieval but shared with the instructional cues the function of alerting the subjects of the impending test phase. The comparison of activity between the retrieval and control conditions demonstrated a significant and selective increase in activity related to retrieval processes within the ventrolateral prefrontal cortical region, more specifically within area 47/12. These activity increases were bilateral but stronger in the right hemisphere. The present study by strictly controlling the level of encoding, postretrieval monitoring, and decision making has demonstrated a specific increase in the ventrolateral prefrontal region that could be clearly related to active retrieval processing, i.e. the active selection of particular stored visual representations.

Adult↗

PubMed2001

Cortico-cortical connectivity of the human mid-dorsolateral frontal cortex and its modulation by repetitive transcranial magnetic stimulation.

Modulation of cortico-cortical connectivity in specific neural circuits might underlie some of the behavioural effects observed following repetitive transcranial magnetic stimulation (rTMS) of the human frontal cortex. This possibility was tested by applying rTMS to the left mid-dorsolateral frontal cortex (MDL-FC) and subsequently measuring functional connectivity of this region with positron emission tomography (PET) and TMS. The results showed a strong rTMS-related modulation of brain activity in the fronto-cingulate circuit. These results were confirmed in a parallel experiment in the rat using electrical stimulation and field-potential recordings. Future studies are needed to provide a direct link between the rTMS-induced modulation of cortical connectivity and its effects on specific behaviours.

Adolescent↗

PubMed2000

Dissociable roles of mid-dorsolateral prefrontal and anterior inferotemporal cortex in visual working memory.

Functional neuroimaging in human subjects and studies of monkeys with lesions limited to the mid-dorsolateral (MDL) prefrontal cortex have shown that this specific region of the prefrontal cortex is involved in visual working memory, although its precise role remains a matter of debate. The present study compared the effect on visual working memory of lesions restricted to the mid-dorsolateral prefrontal cortex of the monkey with that of lesions to the anterior inferotemporal cortex, a region of the temporal cortex specialized for visual memory. Increasing the delay during which information had to be maintained in visual working memory impaired performance after lesions of the anterior inferotemporal cortex, but not after mid-dorsolateral prefrontal lesions. By contrast, increasing the number of stimuli that had to be monitored impaired the performance of animals with mid-dorsolateral prefrontal lesions, but not that of animals with anterior inferotemporal lesions. This demonstration of a double dissociation between the effects of these two lesions provides strong evidence that the role of the mid-dorsolateral prefrontal cortex in visual working memory does not lie in the maintenance of information per se, but rather in the executive process of monitoring this information. In addition, the present study demonstrated that lesions limited to area 9, which constitutes the superior part of the mid-dorsolateral prefrontal region, give rise to a mild impairment in the monitoring of information, whereas lesions of the complete mid-dorsolateral prefrontal region yield a very severe impairment.

Analysis of Variance↗

PubMed2000

Orbitofrontal cortex: A key prefrontal region for encoding information.

Little is known about the specific functional contribution of the human orbitofrontal cortex with regard to memory processing, although there is strong evidence from lesion studies in monkeys that it may play an important role. The present investigation measured changes in regional cerebral blood flow with positron emission tomography in normal human subjects who were instructed to commit to memory abstract visual patterns. The results indicated that the rostral orbitofrontal region (area 11), which is primarily linked with the anterior medial temporal limbic region and lateral prefrontal cortical areas, is involved in the process of encoding of new information.

Adult↗

PubMed2000

Orbitofrontal sulci of the human and macaque monkey brain.

The present study investigated the orbitofrontal sulci in 100 normal adult human cerebral hemispheres by using magnetic resonance images that were transformed into the standardized proportional stereotaxic space most commonly used, that of Talairach and Tournoux (Talairach and Tournoux [1988]. Co-planar stereotaxic atlas of the human brain. New York: Thieme). The patterns formed by the individual sulci were then examined and compared with those of the less convoluted macaque monkey brain. Four sulci forming a similar sulcal pattern were identified in both species. The olfactory sulcus occupies the most medial position forming the lateral border of the gyrus rectus. Lateral to this, the medial, lateral, and transverse orbital sulci form a pattern often resembling an "H," "X," or "K." These sulci divide the orbitofrontal cortex into four major gyri: the medial, lateral, anterior, and posterior orbital gyri. Three major types of sulcal pattern were identified in both species based on the arrangement of these orbital sulci. Additional sulci were observed in the human brain, creating more complex patterns. Probability maps were constructed for the four main orbitofrontal sulci of the human brain. These maps provide a statistical description of the variability of the location of the orbitofrontal sulci within the three-dimensional coordinate system of Talairach and Tournoux (Talairach and Tournoux [1988]. Co-planar stereotaxic atlas of the human brain. New York: Thieme). Because these maps may be directly compared with any image transformed into the same standardized space, they provide a valuable tool for identifying and describing the location of functional or structural changes in the orbitofrontal region of the human brain.

Adult↗

PubMed2000

Architectonic analysis of the human retrosplenial cortex.

The architecture of the macaque retrosplenial cortex, including its posteroventral extension around and below the splenium of the corpus callosum, was recently characterized (Morris et al. [1999a] Eur. J. Neurosci. 11:2506-2518.). This analysis was made possible by sectioning the posterior cingulate gyrus radially, i.e., in planes that were orthogonal to its line of curvature and that, therefore, preserved the laminar organization of this region. The aim of the present study was to examine the architecture and the limits of the human retrosplenial cortex. Cross sections through the entire posterior cingulate gyrus were obtained by applying the sectioning technique developed in the monkey, so that an explicit comparison could be made between the architecture of the human and the monkey retrosplenial cortex. The present analysis revealed that, as is the case in the macaque brain, the human retrosplenial cortex is composed of granular areas 29a-c and d, and dysgranular/agranular area 30. The human retrosplenial cortex, like that of the macaque monkey, runs, as an arch, around the splenium of the corpus callosum. In the macaque brain, the retrosplenial cortex remains buried within the callosal sulcus throughout its entire course around the splenium. In the human brain, however, the posteroventral segment of the retrosplenial cortex extends on the medial wall of the cerebral hemisphere to encompass most of the cortical region commonly referred to as the "isthmus of the cingulate gyrus."

Adult↗

PubMed2000

Selectivity of the spatial learning deficit after lesions of the mammillary region in rats.

Rats with lesions of the mammillary region (MB-R) or a control group (C) were trained on a visual-spatial conditional associative learning task in which they had to learn to go to one of two locations depending on the particular visual cue presented on each trial; the rats approached the cues from different directions. The animals were subsequently tested on a spatial working memory task, the eight-arm radial maze. Rats with damage to the MB-R were able to acquire the conditional associative task at a rate comparable to that of the C animals. By contrast, lesions of the MB-R led to a severe impairment on the radial maze. The present finding of a dissociation between the effects of MB-R lesions on two different classes of behavior suggest that the spatial learning deficit following damage of this region may be specific to remembering one or more places over a given time but not the ability to form associations between visual and spatial stimuli.

Animals↗

PubMed2000

Activity in ventrolateral and mid-dorsolateral prefrontal cortex during nonspatial visual working memory processing: evidence from functional magnetic resonance imaging.

Whole-brain functional magnetic resonance imaging was used to study five healthy human subjects while they performed two nonspatial visual working memory tasks and one control task. In the first memory task, the subjects were required to view a sequence of three pattern stimuli, randomly selected from a familiar set of four stimuli, and then identify which one of three simultaneously presented stimuli was the one that had not been presented in the previous array. In the other task, the subjects were required to observe an identical sequence of three randomly selected pattern stimuli and then to respond by selecting those same stimuli in the order presented. In comparison to a baseline control task, increases in signal intensity were observed, bilaterally, in the mid-dorsolateral frontal cortex and in the right ventrolateral frontal cortex in both memory tasks. When the two tasks were compared directly, however, the first memory task, which had the higher monitoring requirement, yielded significantly greater signal intensity changes in area 9/46 of the right mid-dorsolateral frontal cortex. These results provide further evidence for the precise functional contribution made by the mid-dorsolateral frontal cortex in visual working memory tasks and concur closely with findings in nonhuman primates.

Adult↗

PubMed2000

The role of the mid-dorsolateral prefrontal cortex in working memory.

Recent studies with nonhuman primates have shown that lesions of the mid-dorsolateral prefrontal cortex, which extends from the lip of the dorsal bank of the sulcus principalis to the midline (i.e., dorsal area 46 and 9/46 and area 9), give rise to severe and long-lasting impairments on self-ordered and externally ordered tasks designed to tax executive processing within working memory, rather than short-term memory per se. Lesions limited to area 9 give rise to a mild impairment on these tasks. Thus, the mid-dorsolateral prefrontal region has been shown to be critical for the monitoring of multiple events in working memory. The mid-dorsolateral prefrontal region receives visuospatial input from the posterior dorsolateral region (areas 8 and 6) and from the cortex within the middle part (sulcal area 46) and the caudal part (area 8) of the sulcus principalis. Nonspatial visual input originates from the ventrolateral prefrontal cortex. Thus, lesions focused on the middle to caudal part of the sulcus principalis would affect visuospatial input, but would not affect the flow of nonspatial visual object information that reaches the mid-dorsolateral prefrontal region from the ventrolateral prefrontal cortex. Lesions of the sulcus principalis produce a spatially selective impairment, whereas lesions of the mid-dorsolateral prefrontal region produce a more general impairment of the monitoring and manipulation of information in working memory. The results of recent functional neuroimaging studies with human subjects are consistent with the above findings from work with the monkey.

Animals↗