Michael Petrides

Lateral prefrontal cortex: architectonic and functional organization

A comparison of the architecture of the human prefrontal cortex with that of the macaque monkey showed a very similar architectonic organization in these two primate species. There is no doubt that the prefrontal cortical areas of the human brain have undergone considerable development, but it is equally clear that the basic architectonic organization is the same in the two species. Thus, a comparative approach to the study of the functional organization of the primate prefrontal cortex is more likely to reveal the essential aspects of the various complex control processes that are the domain of frontal function. The lateral frontal cortex appears to be functionally organized along both a rostral–caudal axis and a dorsal–ventral axis. The most caudal frontal region, the motor region on the precentral gyrus, is involved in fine motor control and direct sensorimotor mappings, whereas the caudal lateral prefrontal region is involved in higher order control processes that regulate the selection among multiple competing responses and stimuli based on conditional operations. Further rostrally, the mid-lateral prefrontal region plays an even more abstract role in cognitive control. The mid-lateral prefrontal region is itself organized along a dorsal–ventral axis of organization, with the mid-dorsolateral prefrontal cortex being involved in the monitoring of information in working memory and the mid-ventrolateral prefrontal region being involved in active judgments on information held in posterior cortical association regions that are necessary for active retrieval and encoding of information.

Dorsolateral prefrontal cortex: comparative cytoarchitectonic analysis in the human and the macaque brain and corticocortical connection patterns

The cytoarchitecture of the human and the macaque monkey dorsolateral prefrontal cortex has been examined in a strictly comparative manner in order to resolve major discrepancies between the available segmentations of this cortical region in the human and the monkey brain. In addition, the connections of the dorsolateral prefrontal cortical areas were re‐examined in the monkey. The present analysis showed that only a restricted portion of what had previously been labelled as area 46 in the monkey has the same characteristics as area 46 of the human brain; the remaining part of this monkey region has the characteristics of a portion of the middle frontal gyrus in the human brain that had previously been included as part of area 9. We have labelled this cortical area as 9/46 in both species. These two areas (i.e. 46 and 9/46), which constitute the lower half of the mid‐dorsolateral frontal cortex, have a well‐developed granular layer IV, and can easily be distinguished from area 9, on the upper part of the mid‐dorsolateral region, which does not have a well‐developed granular layer IV. Area 9 has the same basic pattern of connections as areas 46 and 9/46, but, unlike the latter areas, it does not receive input from the lateral parietal cortex. Caudal to area 9, on the dorsomedial portion of the frontal cortex, there is a distinct strip of cortex (area 8B) which, unlike area 9, receives significant input from the prestriate cortex and the medial parietal cortex. The present results provide a basis for a closer integration of findings from functional neuroimaging studies in human subjects with experimental work in the monkey.

Precuneus shares intrinsic functional architecture in humans and monkeys

Evidence from macaque monkey tracing studies suggests connectivity-based subdivisions within the precuneus, offering predictions for similar subdivisions in the human. Here we present functional connectivity analyses of this region using resting-state functional MRI data collected from both humans and macaque monkeys. Three distinct patterns of functional connectivity were demonstrated within the precuneus of both species, with each subdivision suggesting a discrete functional role: (i) the anterior precuneus, functionally connected with the superior parietal cortex, paracentral lobule, and motor cortex, suggesting a sensorimotor region; (ii) the central precuneus, functionally connected to the dorsolateral prefrontal, dorsomedial prefrontal, and multimodal lateral inferior parietal cortex, suggesting a cognitive/associative region; and (iii) the posterior precuneus, displaying functional connectivity with adjacent visual cortical regions. These functional connectivity patterns were differentiated from the more ventral networks associated with the posterior cingulate, which connected with limbic structures such as the medial temporal cortex, dorsal and ventromedial prefrontal regions, posterior lateral inferior parietal regions, and the lateral temporal cortex. Our findings are consistent with predictions from anatomical tracer studies in the monkey, and provide support that resting-state functional connectivity (RSFC) may in part reflect underlying anatomy. These subdivisions within the precuneus suggest that neuroimaging studies will benefit from treating this region as anatomically (and thus functionally) heterogeneous. Furthermore, the consistency between functional connectivity networks in monkeys and humans provides support for RSFC as a viable tool for addressing cross-species comparisons of functional neuroanatomy.

Three-Dimensional MRI Atlas of the Human Cerebellum in Proportional Stereotaxic Space

We have prepared an atlas of the human cerebellum using high-resolution magnetic resonance-derived images warped into the proportional stereotaxic space of Talairach and Tournoux. Software that permits simultaneous visualization of the three cardinal planes facilitated the identification of the cerebellar fissures and lobules. A revised version of the Larsell nomenclature facilitated a simple description of the cerebellum. This atlas derived from a single individual was instrumental in addressing longstanding debates about the gross morphologic organization of the cerebellum. It may serve as the template for more precise identification of cerebellar topography in functional imaging studies in normals, for investigating clinical-pathologic correlations in patients, and for the development of future probabilistic maps of the human cerebellum.

Motor conditional associative-learning after selective prefrontal lesions in the monkey.

Abstract Rhesus monkeys with selective lesions of the frontal cortex were tested on a motor conditional associative-learning task. Monkeys with lesions of the periarcuate area were severely impaired in acquiring this task, whilst monkeys with lesions of the principalis region showed only a mild retardation in learning.

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.

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 the 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.

Deficits on conditional associative-learning tasks after frontal- and temporal-lobe lesions in man

Patients with unilateral frontal- or temporal-lobe excisions were tested on a spatial and a nonspatial conditional associative task. These tasks required the learning of arbitrary associations between a set of stimuli and a set of responses. Patients with excisions from the left or right frontal cortex were severely impaired in learning both tasks. Patients with left or right temporal-lobe excisions that did not involve extensive damage to the hippocampal region were not impaired, whilst those with more radical involvement of the hippocampal region exhibited deficits that were material-specific and varied with the side of the lesion.

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.

Human cortical gustatory areas : A review of functional neuroimaging data

In an effort to define human cortical gustatory areas we reviewed functional neuroimaging data for which coordinates standardized in Talairach proportional space were available. We observed a wide distribution of peaks within the insula and parietal and frontal opercula, suggesting multiple gustatory regions within this cortical area. Multiple peaks also emerged in the orbitofrontal cortex. However, only two peaks, both in the right hemisphere, were observed in the caudolateral orbitofrontal cortex, the region likely homologous to the secondary taste area described in monkeys. Overall significantly more peaks originated from the right hemisphere suggesting asymmetrical cortical representation of taste favoring the right hemisphere.