Biomedical subjects

M Petrides

Publications and source records attributed to M Petrides.

At least 55 records · Page 3Linked to original sources

PubMed1998

Functional organization of spatial and nonspatial working memory processing within the human lateral frontal cortex.

The present study used functional magnetic resonance imaging to demonstrate that performance of visual spatial and visual nonspatial working memory tasks involve the same regions of the lateral prefrontal cortex when all factors unrelated to the type of stimulus material are appropriately controlled. These results provide evidence that spatial and nonspatial working memory may not be mediated, respectively, by mid-dorsolateral and mid-ventrolateral regions of the frontal lobe, as widely assumed, and support the alternative notion that specific regions of the lateral prefrontal cortex make identical executive functional contributions to both spatial and nonspatial working memory.

Brain Mapping↗

PubMed1998

Conditional associative learning and the hippocampal system.

Rats with lesions of the fornix, the dorsal hippocampus, or a control operation were trained on a spatial-visual conditional associative learning task in which they had to learn to associate particular locations with specific visual stimuli. Animals with damage of the fornix were able to learn the task at a rate comparable to that of the control animals, but the performance of the hippocampal rats was significantly impaired in comparison with that of both the control and the fornix groups. In a second experiment, lesions to the fornix or the dorsal hippocampus significantly impaired performance on a spatial working memory task, the eight-arm radial maze. These findings suggest that the interaction between the hippocampus and subcortical structures via the fornix may be critical only for certain types of spatial learning and memory.

Animals↗

PubMed1998

Spatial memory deficits in patients with lesions to the right hippocampus and to the right parahippocampal cortex.

Spatial memory tasks, performance of which is known to be sensitive to hippocampal lesions in the rat, or to medial temporal lesions in the human, were administered in order to investigate the effects of selective damage to medial temporal lobe structures of the human brain. The patients had undergone thermo-coagulation with a single electrode along the amygdalo-hippocampal axis in an attempt to alleviate their epilepsy. With this surgical technique, lesions to single medial temporal lobe structures can be carried out. The locations of the lesions were assessed by means of digital high-resolution magnetic resonance imaging and software allowing a 3-D reconstruction of the brain. A break in the collateral sulcus, dividing it into the anterior collateral sulcus and the posterior collateral sulcus is reported. This division may correspond to the end of the entorhinal/perirhinal cortex and the start of the parahippocampal cortex. The results confirmed the role of the right hippocampus in visuo-spatial memory tasks (object location, Rey-Osterrieth Figure with and without delay) and the left for verbal memory tasks (Rey Auditory Verbal Learning Task with delay). However, patients with lesions either to the right or to the left hippocampus were unimpaired on several memory tasks, including a spatial one, with a 30 min delay, designed to be analogous to the Morris water maze. Patients with lesions to the right parahippocampal cortex were impaired on this task with a 30 min delay, suggesting that the parahippocampal cortex itself may play an important role in spatial memory.

Adolescent↗

PubMed1998

Directed attention after unilateral frontal excisions in humans.

We investigated the ability of patients with frontal-lobe lesions to benefit from advance information in a simple reaction-time task. The task involved pressing a button in response to the appearance of a peripheral target (visual angle of 11.5 degrees). A cue, presented in the centre of the screen, preceded the target onset by either a short (average 500 ms) or a long (average 3000 ms) interval. In half of the trials, the cue was an arrow indicating the location, in the left or right hemifield, of the upcoming target; in the other half, the cue was an uninformative plus sign. In addition to patients with unilateral excisions of frontal cortex, we tested patients with anterior temporal-lobe excisions and normal controls. The frontal-lobe group was mildly impaired with respect to the temporal-lobe group in using advance spatial cues to speed response to the visual target. While the size of the cueing effect changed across the range of cue-target intervals tested, there was no variation across intervals in the size of the impairment exhibited by the frontal-lobe group. The site and the volume of the lesions were determined on the basis of magnetic resonance image (MRI) scans in 10 of the 17 patients in the frontal group. There was no correlation between lesion volume and benefit score in these patients.

Adolescent↗

PubMed1998

Memory and the region of the mammillary bodies.

The contribution of the mammillary region to several classes of learning and memory has been reviewed. There is considerable evidence that lesions of this region of the brain impair performance on tasks that require memory for locations that an animal has visited, but that the deficit depends both on the amount of damage within the region and the difficulty of the task. Such lesions, however, do not appear to impair performance on a variety of spatial conditional associative learning tasks which require the animal to form an association between a place or a scene and a stimulus embedded within it. In addition, damage to the region of the mammillary bodies does not impair the ability to learn a variety of non-spatial memory tasks. These studies suggest that the mammillary region may play a selective role in certain types of spatial learning and memory.

Animals↗

PubMed1998

Signal-enhancing switched protocols to study higher-order cognitive tasks with PET.

UNLABELLED: We tested the effectiveness of a switched protocol when it is used to detect signals that result from the study of a higher-order cognitive task with PET. Using language tasks that have been studied extensively in our laboratories, we compared the signal-to-noise ratio (S/N) and statistical significance of the activation signals detected in PET images of regional cerebral blood flow (rCBF), obtained using a standard activation protocol, and of activity concentration, obtained using a switched protocol. METHODS: Four volunteers were studied with PET while they were performing synonym-generation and word-repetition tasks (activation and baseline tasks, respectively). Each volunteer had three activation/baseline and three baseline/ activation scans. Data for each scan were collected in two frames (60 and 120 sec long). During the first 60 sec, data were collected using a standard activation protocol. Subjects then switched tasks, and acquisition continued for 120 sec. Two images were obtained from each scan: an rCBF image using the first frame and an activity-concentration image using both frames. Images were transformed into Talairach space, subtracted and averaged within and across subjects. Parametric t-statistic images were generated for each protocol, and the magnitude and significance of the activation signals yielded by the two acquisition methods were compared. RESULTS: All the activation foci detected using measurements of rCBF were detected when the switched protocol was used; this protocol, in addition, yielded better S/N values. The cognitive component introduced by task-switching in switched protocols did not yield extra statistically significant foci. In single subjects, the average improvement in the signal significance from regions of activation, at a 95% confidence level, was between 6% and 25%. When scans were averaged across subjects, the switched protocol yielded improvements in signal statistical significance of up to 38%. CONCLUSION: We present evidence suggesting that switched protocols can be used to study higher-order cognitive tasks and that they yield activation foci with S/N values that are greater than those of equivalent foci detected using an rCBF protocol. Switched protocols appear to be easy to apply to the testing of higher-order cognitive functions. However, the extra cognitive requirement of switching tasks during data acquisition may be a limiting factor when switched protocols are used to study memory processes.

Adult↗

PubMed1998

Applications of random field theory to functional connectivity.

Functional connectivity between two voxels or regions of voxels can be measured by the correlation between voxel measurements from either PET CBF or BOLD fMRI images in 3D. We propose to look at the entire 6D matrix of correlations between all voxels and search for 6D local maxima. The main result is a new theoretical formula based on random field theory for the p-value of these local maxima, which distinguishes true correlations from background noise. This can be applied to crosscorrelations between two different sets of images--such as activations under two different tasks, as well as autocorrelations within the same set of images.

Brain Mapping↗

PubMed1997

Flavor processing: more than the sum of its parts.

We used positron emission tomography to evaluate differential processing of olfactory, gustatory and combined olfactory and gustatory (flavor) stimuli as indicated by comparison of evoked cerebral blood flow (CBF) changes during these conditions. We found significant CBF decreases in primary gustatory and secondary gustatory and olfactory cortices during simultaneous presentation compared with independent presentations of identical stimuli, suggesting that flavor processing is not represented by a simple convergence of its component senses. Additionally, CBF increases in the amygdala and basal forebrain were observed in a mismatched flavor condition versus a matched flavor condition, suggesting a role for these structures in processing novel or unpleasant stimuli.

Adult↗

PubMed1997

A role for the right anterior temporal lobe in taste quality recognition.

We conducted two experiments to examine central processing of the taste of citric acid. In the first experiment, elevated citric acid recognition thresholds, but normal detection thresholds, were observed in a group of patients who had undergone a right anterior temporal lobectomy for the treatment of epilepsy, compared with a control group and a group of patients who had undergone the same operation in the left hemisphere. In the second study, using positron emission tomography, we compared regional cerebral blood flow (rCBF) in a condition in which citric acid was presented with one in which water was presented (with similar somatosensory stimulation across both conditions). We observed increased rCBF bilaterally in the caudolateral orbitofrontal cortex, in the right anteromedial temporal lobe, and in the right caudomedial orbitofrontal cortex. The elevated recognition thresholds exhibited in patients with resection of the right anteromedial temporal lobe may be accounted for by damage in an area corresponding to that of the rCBF increase. These results suggest that although taste sensation may be computed in the primary taste cortex, recognition requires further processing by structures located in the anteromedial temporal lobe. Furthermore, they point to preferential processing of this higher-order gustatory function by the right cerebral hemisphere.

Adolescent↗

PubMed1997

Functional activation of the human brain during mental rotation.

Regional cerebral blood flow was measured with positron emission tomography during the performance of tasks that required cognitive spatial transformations of alphanumeric stimuli. In the mirror image task, the subjects were required to discriminate between the normal and the mirror images of alphanumeric stimuli presented in the upright orientation. In the mental rotation task, the same judgement was required, but now the stimuli were presented in various orientations other than the upright one. The subjects therefore had to rotate the stimuli, in mind, into the upright position before making their decision. In relation to the control task, which involved discrimination of these same stimuli but not any form of mental transformation, there were significant increases in the right postero-superior parietal cortex and the left inferior parietal cortex in both experimental tasks. For mental rotation, specific activity was seen only within the left inferior parietal region and the right head of the caudate nucleus. These results specified the parietal areas involved in a purely cognitive spatial process and demonstrated a close interaction between these areas and the anterior neostriatum and certain lateral frontal cortical areas in the discrimination of rotated forms of stimuli.

Adult↗

PubMed1997

Visuo-motor conditional associative learning after frontal and temporal lesions in the human brain.

It has been shown that damage to the human lateral frontal cortex results in a severe impairment on conditional associative tasks requiring learning of arbitrary associations between a set of stimuli and a set of responses (Petrides, M., Neuropsychologia, 1985, 23, 601-614; 1990, 28, 137-149). In these studies, which first demonstrated the impairment after frontal lesions, training was by a trial-and-error procedure, during which the subject performed the various responses when a given stimulus was presented and the experimenter provided feedback until the correct response was performed. In the present experiment, patients with unilateral frontal- or temporal-lobe excisions were tested on a visuo-motor conditional associative task with a modified procedure. The subjects had to learn arbitrary associations between a set of coloured stimuli and a set of hand postures. Training in the present experiment consisted of a series of demonstration trials followed by test trials. In the demonstration trials, the experimenter showed the subject the associations between the stimuli and the responses and, in the test trials that followed, the subject was tested on these associations. If an error was made on the test trials, the correct response was demonstrated by the experimenter. Despite these changes in the training procedure, namely the demonstration of the stimulus-response associations and the provision of the correct response immediately following an error, patients with left or right frontal-lobe excisions were severely impaired in learning this task. These findings, together with those of the earlier studies (Petrides, M., Neuropsychologia, 1985, 23, 601-614; 1990, 28, 137-149), demonstrate that the impairment in conditional learning after frontal lesions is not dependent on the type of the training procedure and therefore that it reflects a specific impairment in learning.

Adolescent↗

PubMed1997

A neuroanatomical method to assess the integrity of fibers of passage following ibotenate-induced damage to the central nervous system.

In some behavioral-lesion experiments involving animals, ibotenic acid (IBO) is used as a means of damaging brain structures. Occasionally one needs to assess the status of fibers coursing through the damaged area to determine whether the deficits observed are the result of destruction of neurons rather than fibers. In such cases, IBO, is considered to be the method of choice since it destroys cell bodies but leaves fibers of passage intact. However, if the IBO dose injected in a given area is too high, both cell bodies and fibers of passage could be damaged. The anterograde and retrograde tracer wheat germ agglutinin-horseradish peroxidase (WGA-HRP) is a useful technique to verify that fibers are intact, and is a more powerful tool in comparison with a tracer, such as HRP, which has been used in previous studies.

Amygdala↗

PubMed1997

When less is more: cost-containment in transfusion medicine.

The drive to contain the rising cost of medical care provides numerous challenges to blood bankers. Not the least of these is the opportunity to improve quality of care while reducing the cost thereof. This paper explores various cost-reduction strategies, focusing primarily on those applicable to small and medium-sized transfusion services, and demonstrates methods by which they can be implemented to yield improved patient care.

Blood Banks↗

PubMed1996

Specialized systems for the processing of mnemonic information within the primate frontal cortex.

The lateral frontal cortex is involved in various aspects of executive processing within short- and long-term memory. It is argued that the different parts of the lateral frontal cortex make distinct contributions to memory that differ in terms of the level of executive processing that is carried out in interaction with posterior cortical systems. According to this hypothesis, the mid-dorsolateral frontal cortex (areas 46 and 9) is a specialized system for the monitoring and manipulation of information within working memory, whereas the mid-ventrolateral frontal cortex (areas 47/12 and 45) is involved in the active retrieval of information from the posterior cortical association areas. Data are presented which support this two-level hypothesis that posits two distinct levels of interaction of the lateral frontal cortex with posterior cortical association areas. Functional activation studies with normal human subjects have demonstrated specific activity within the mid-dorsolateral region of the frontal cortex during the performance of tasks requiring monitoring of self-generated and externally generated sequences of responses. In the monkey, lesions restricted to this region of the frontal cortex yield a severe impairment in performance of the above tasks, this impairment appearing against a background of normal performance on several basic mnemonic tasks. By contrast, a more severe impairment follows damage to the mid-ventrolateral frontal region and functional activation studies have demonstrated specific changes in activity in this region in relation to the active retrieval of information from memory.

Animals↗

PubMed1996

Memory for object features versus memory for object location: a positron-emission tomography study of encoding and retrieval processes.

Regional cerebral blood flow was measured with positron-emission tomography during two encoding and two retrieval tasks that were designed to compare memory for object features with memory for object locations. Bilateral increases in regional cerebral blood flow were observed in both anterior and posterior regions of inferior temporal cortex and in ventral regions of prestriate cortex, when the condition that required retrieval of object locations was subtracted from the condition that required retrieval of object features. During encoding, these changes were less pronounced and were restricted to the left inferior temporal cortex and right ventral prestriate cortex. In contrast, both encoding and retrieval of object location were associated with bilateral changes in dorsal prestriate and posterior parietal cortex. Finally, the two encoding conditions activated left frontal lobe regions preferentially, whereas the two retrieval conditions activated right frontal lobe regions. These findings confirm that, in human subjects, memory for object features is mediated by a distributed system that includes ventral prestriate cortex and both anterior and posterior regions of the inferior temporal gyrus. In contrast, memory for the locations of objects appears to be mediated by an anatomically distinct system that includes more dorsal regions of prestriate cortex and posterior regions of the parietal lobe.

Adolescent↗

PubMed1996

Specific involvement of human parietal systems and the amygdala in the perception of biological motion.

To explore the extent to which functional systems within the human posterior parietal cortex and the superior temporal sulcus are involved in the perception of action, we measured cerebral metabolic activity in human subjects by positron emission tomography during the perception of simulations of biological motion with point-light displays. The experimental design involved comparisons of activity during the perception of goal-directed hand action, whole body motion, object motion, and random motion. The results demonstrated that the perception of scripts of goal-directed hand action implicates the cortex in the intraparietal sulcus and the caudal part of the superior temporal sulcus, both in the left hemisphere. By contrast, the rostrocaudal part of the right superior temporal sulcus and adjacent temporal cortex, and limbic structures such as the amygdala, are involved in the perception of signs conveyed by expressive body movements.

Adult↗

PubMed1996

The effects of lesions to the mammillary region and the hippocampus on conditional associative learning by rats.

Rats with extensive lesions to the mammillary body region, the hippocampus, or rats which had received a control operation were trained postoperatively on two visuo-spatial conditional associative learning tasks in which they had to learn to associate spatial cues with particular visual/auditory stimuli. The animals were subsequently trained on a spatial working memory task, the eight-arm radial maze. Rats with lesions to the mammillary body region were able to acquire the conditional associative learning tasks at a rate comparable to that of operated control animals, whereas those with hippocampal lesions were not. By contrast, rats with a lesion of the mammillary body region or the hippocampus were significantly impaired in comparison with the operated control animals in the radial maze. The findings suggest that lesions to the mammillary body region impair spatial working memory without affecting the capacity to associate particular exteroceptive cues with spatial locations.

Animals↗

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↗