Biomedical subjects

Michael Petrides

Publications and source records attributed to Michael Petrides.

8 recordsLinked to original sources

PubMed2003

Cognitive strategies dependent on the hippocampus and caudate nucleus in human navigation: variability and change with practice.

The human brain activity related to strategies for navigating in space and how it changes with practice was investigated with functional magnetic resonance imaging. Subjects used two different strategies to solve a place-learning task in a computer-generated virtual environment. One-half of the subjects used spatial landmarks to navigate in the early phase of training, and these subjects showed increased activation of the right hippocampus. The other half used a nonspatial strategy and showed, with practice, sustained increased activity within the caudate nucleus during navigation. Activation common to both groups was observed in the posterior parietal and frontal cortex. These results provide the first evidence for spontaneous variability and shift in neural mechanisms during navigation in humans.

Adult↗

PubMed2003

The mid-ventrolateral prefrontal cortex: insights into its role in memory retrieval.

Although it is widely known that the prefrontal cortex plays a role in memory, the specific contribution of particular prefrontal regions in mnemonic functions remains controversial. The present investigation examined whether the mid-ventrolateral prefrontal cortex is selectively involved in active memory retrieval in situations in which mnemonic traces are embedded in ambiguous relations and automatic recollection cannot lead to successful retrieval. Thirteen subjects participated in this event-related functional magnetic resonance imaging experiment. Throughout the scanning session, trials belonging to an experimental and a control condition were administered in a pseudorandom fashion. During the encoding phase of any particular trial, subjects were presented with a stimulus-complex that was a combination of a face and a spatial location on the screen. In the experimental active retrieval condition, a question cue following the encoding phase instructed the subjects to retrieve selectively one of the two aspects of the encoded stimulus-complex, i.e. the face or the location. In the control condition, the question cue that followed the encoding phase instructed the subjects simply to recall the initially presented stimulus-complex, so as to be able to make a decision during the test phase based on simple stimulus familiarity. The comparison of the signal obtained during the retrieval phase of these two conditions yielded an increase in activity selective to the right mid-ventrolateral prefrontal region. These results therefore establish a specific link between the mid-ventrolateral prefrontal cortex and active retrieval mechanisms.

Adult↗

PubMed2003

Greater orbitofrontal activity predicts better memory for faces.

Having demonstrated recently that the orbitofrontal cortex is selectively involved when novel abstract visual information is encoded, we investigated whether the same orbitofrontal area would be activated during the encoding of novel human faces. The present positron emission tomography study demonstrated that area 11 in the right orbitofrontal region, which is directly linked to the medial temporal lobe, is a critical frontal area selectively activated when normal human subjects are engaged in encoding visually presented faces. Furthermore, better face recognition performance correlated with greater cerebral blood flow specifically in this orbitofrontal area.

Adult↗

PubMed2003

Source versus content memory in patients with a unilateral frontal cortex or a temporal lobe excision.

It has been suggested previously that patients with a frontal lobe lesion might have a specific impairment in the retrieval of the source of information despite adequate memory for facts. Patients with an anterior temporal excision are known to have impairments in memory for facts and the question arises as to whether they are also impaired in source memory. The present study compared memory for facts and their source in patients with a unilateral frontal cortical or an anterior temporal excision in a situation in which both types of information were encoded explicitly. Patients with a unilateral frontal cortex or a temporal lobe excision watched videos of a game show and were instructed to attend to both the trivia facts and their source (the identity of the speaker or the relative time of presentation). Patients with a frontal cortex excision were not impaired on either fact or source memory. This was true even when a subgroup of patients with an excision involving the dorsolateral frontal cortex was examined. In contrast, patients with a left temporal lobe excision were impaired in both fact and identity source memory and right temporal lobe patients were impaired in identity source memory. All patients performed similarly to normal controls in temporal source memory. The present results are consistent with the view that source information is part of an associative network of information about an episode and that the medial temporal region is critical for both source and content memory. Furthermore, if source information is encoded explicitly, the frontal cortex does not appear to be necessary for its retrieval. Instead, it is proposed that the frontal cortex plays a metacognitive role in memory retrieval.

Adolescent↗

PubMed2002

Orbitofrontal cortex and memory formation.

Which one of the many regions of the anatomically heterogeneous prefrontal cortex is part of the critical core of the neural circuit for encoding? This positron emission tomography (PET) experiment measured changes in cerebral blood flow (CBF) in normal human participants during the presentation of abstract visual information in four conditions that varied in their encoding demands. As encoding increased across the different conditions, there was an increase in activity in the right orbitofrontal cortex and the right parahippocampal region. No significant activation peaks were present in any other region of the frontal or temporal lobe. These findings indicate that the orbitofrontal cortex, which is massively connected to the medial temporal cortex, is a critical frontal region for memory formation.

Adult↗

PubMed2002

Differential activation of the human orbital, mid-ventrolateral, and mid-dorsolateral prefrontal cortex during the processing of visual stimuli.

There is considerable uncertainty about the precise functional contribution of the different parts of the prefrontal cortex to mnemonic processing. Changes in regional cerebral blood flow were measured with positron emission tomography in normal human subjects exposed to abstract visual designs under various conditions. It was demonstrated that the processing of stimuli that deviate from expectations involves selectively the orbitofrontal cortex, namely the part of the frontal cortex that is preferentially linked with the limbic system. By contrast, when the subject is making an explicit decision on the contents of memory (e.g., judgments of relative stimulus familiarity), the mid-ventrolateral prefrontal cortex is involved. The mid-dorsolateral prefrontal cortex is engaged when monitoring of information within working memory is required.

Adult↗

PubMed2002

The mid-ventrolateral prefrontal cortex and active mnemonic retrieval.

The role of the mid-ventrolateral prefrontal cortex in memory retrieval is examined and compared with the role of the mid-dorsolateral prefrontal cortex in the monitoring of information in memory. It has been argued that the mid-ventrolateral prefrontal cortex (areas 47/12 and 45) is involved in the active retrieval of information from posterior cortical association areas. Active retrieval is required when stimuli in memory do not bear stable relations to each other and therefore retrieval cannot be automatically driven by strong, stable, and unambiguous stimulus or context relations. Data from functional activation studies with normal human subjects are presented that have demonstrated specific changes in activity within the mid-ventrolateral region of the frontal cortex in relation to the active retrieval of information from memory. By contrast, increases in activity in the mid-dorsolateral region of the frontal cortex occur when the performance of the tasks requires monitoring of information in memory.

Humans↗

PubMed2002

Distractibility after unilateral resections from the frontal and anterior cingulate cortex in humans.

The prefrontal and anterior cingulate cortex figure prominently in current models of directed attention. The efficacy of selective attention was studied in a distractibility study carried out with patients who had unilateral surgical excisions from the frontal cortex with or without involvement of the cingulate gyrus, patients with unilateral anterior temporal resections, and normal control subjects. In Task A, participants responded to target stimuli presented at the foveal location in the presence or absence of distracting peripheral stimuli that appeared 200 ms before the target's onset. The presence of a distractor caused a comparable increase in reaction times to targets across all groups. In Task B, a distractor appeared during every trial but it appeared at a new location on 12% of the trials. A comparable transient increase in reaction times was observed in patients with frontal cortical resections that spared the anterior cingulate gyrus, patients with anterior temporal lobe resections, and the normal control group. Thus, lesions restricted to the prefrontal cortex need not impair the ability to attend and respond to a target event in the presence of distraction. Interestingly, no distracting effect was observed during Task B in patients with lesions that invaded the anterior cingulate gyrus, suggesting that this cortical region is involved in responding to or habituating to environmental stimuli.

Adult↗