PubMed Health⌕ Search

Biomedical subjects

R S Frackowiak

Publications and source records attributed to R S Frackowiak.

At least 55 records · Page 3Linked to original sources
PubMed1998

Neural correlates of memory retrieval during recognition memory and cued recall.

Regional brain activity, measured by H215O PET, was investigated during recognition memory and word-stem cued recall of words in order to compare the neural correlates of two components of memory retrieval-effort and success-as a function of task. For each task there was a baseline and two retrieval conditions. In one retrieval condition (zero density), none of the test items corresponded to words encoded in a preceding study phase. Differences in activity between this condition and the baseline were employed to characterize the neural correlates of retrieval effort in each task. In the other retrieval condition (high density), 80% of the test items had been studied previously. Differences in brain activity between this condition and the zero-density condition were taken to represent the neural correlates of successful retrieval. The principal findings concern the right anterior prefrontal cortex, a region demonstrated previously to be active during episodic retrieval. Relative to baseline, this region showed no evidence of activation in the zero-density condition of the recognition task, but did show enhanced activity in the equivalent condition of the cued-recall task. In contrast, relative to the zero-density condition, the high-density condition was associated with increased right prefrontal activity during recognition, but reduced activity during cued recall. It is proposed that the right prefrontal cortex supports cognitive processes that operate on information retrieved in response to a test item and that these processes contribute to the evaluation of whether the information represents an appropriate prior episode.

Adult↗
PubMed1998

The preparation, execution and suppression of copied movements in the human brain.

We used positron emission tomography (PET) to measure movement set-related changes in regional cerebral blood flow (rCBF) when human subjects were asked to copy hand movements. Movement set-related activity in the brain is thought to reflect the processes of movement selection, preparation and inhibition. Four conditions were used. In the first condition, prepare and execute (PE), the hand stimulus to be copied was shown to subjects 3 s before an auditory "go"-cue instructed subjects to execute the movement; a large part of the scanning time was therefore spent in preparing to move. In the immediate execution condition (E), the hand stimulus and the go cue were presented simultaneously. The prepare-only condition (P) was similar to PE, except subjects only prepared to make the movement and did not actually execute any movement when they heard the auditory go-cue. The same stimuli were presented in a baseline condition (B), but the subjects were instructed to neither prepare nor execute movements. There were 5 principle findings: (1) In contrast to a previous study of human set-related activity in which movements were instructed by an arbitrary pattern of LEDs, preparing to make a copied movement causes rCBF changes in area 44 in posterior Broca's area; (2) set-related activity can be recorded in the cerebellar hemispheres and midline; (3) we confirmed that the supramarginal gyrus has a general role in preparing movements - there was more rCBF in the P than the E condition; (4) the cerebellar nuclei and the basal ganglia may be particularly involved in the initiation and execution of a planned movement; these regions were more active in the PE condition than the P condition; (5) the ventrolateral prefrontal cortex and a left anterior cingulate area are part of a distributed system involved in the suppression of a motor response; these areas were significantly more active in the P than the PE condition.

Adult↗
PubMed1998

Monitoring for target objects: activation of right frontal and parietal cortices with increasing time on task.

The right prefrontal and parietal cortices have been implicated in attentional processing in both neuropsychological and functional neuroimaging literature. However, attention is a heterogeneous collection of processes, each of which may be underpinned by different neural networks. These attentional networks may interact, such that engaging one type of attentional process could influence the efficiency of another via overlapping neural substrates. We investigated the hypothesis that right frontal and parietal cortices provide the neuroanatomical location of the functional interaction between sustained attention and the process of selectively monitoring for target objects. Six healthy volunteers performed one of two tasks which required either selective or non-selective responding. The task lasted continuously for 18 min, during which time 3 Positron Emission Tomography (PET) scans were acquired for each task. This was repeated to obtain 12 PET measurements of regional cerebral blood flow (rCBF) for each subject. The right inferior frontal and parietal cortices were differentially activated by increasing time on task during the selective (S) vs non-selective (NS) task. Specifically, rCBF decreased with increasing time spent performing the NS task but not the S task. This result suggests that the normal deactivation in these areas as time on task increases is counteracted by the extra cognitive demands of selectively responding to target objects. Therefore, we have confirmed our hypothesis that right frontal and parietal cortices provide the neuroanatomical location for the modulation of object selection by sustained attention. We also identified the neuroanatomical correlates of each process separately, and confirmed earlier reports of prefrontal cortex and anterior cingulate activation associated with selective responding, and a fronto-parietal-thalamic network associated with sustained attention.

Adult↗
PubMed1998

Analysis of temporal structure in sound by the human brain.

For over a century, models of pitch perception have been based on the frequency composition of the sound. Pitch phenomena can also be explained, however, in terms of the time structure, or temporal regularity, of sounds. To locate the mechanism for the detection of temporal regularity in humans, we used functional imaging and a 'delay-and-add' noise, which activates all frequency regions uniformly, like noise, but which nevertheless produces strong pitch perceptions and tuneful melodies. This stimulus has temporal regularity that can be systematically altered. We found that the activity of primary auditory cortex increased with the regularity of the sound. Moreover, a melody composed of delay-and-add 'notes' produced a distinct pattern of activation in two areas of the temporal lobe distinct from primary auditory cortex. These results suggest a hierarchical analysis of time structure in the human brain.

Auditory Cortex↗
PubMed1998

Right parietal cortex is involved in the perception of sound movement in humans.

Changes in the delay (phase) and amplitude of sound at the ears are cues for the analysis of sound movement. The detection of these cues depends on the convergence of the inputs to each ear, a process that first occurs in the brainstem. The conscious perception of these cues is likely to involve higher centers. Using novel stimuli that produce different perceptions of movement in the presence of identical phase and amplitude modulation components, we have demonstrated human brain areas that are active specifically during the perception of sound movement. Both functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) demonstrated the involvement of the right parietal cortex in sound movement perception with these stimuli.

Acoustic Stimulation↗
PubMed1998

The neural systems sustaining face and proper-name processing.

This PET study has revealed the neural system involved in implicit face, proper-name and object name processing during an explicit visual 'same' versus 'different' matching task. Within the identified system, some areas were equally active irrespective of modality (faces or names) or type of stimuli (famous and non-famous) while other areas exhibited differential effects. Our findings support the hypothesis that faces and names involve differential pre-semantic processing prior to accessing a common neural system of stored knowledge of personal identity which overlaps with the one associated with object knowledge. The areas specialized for the perceptual analysis of faces (irrespective of whether they are famous or non-famous) are the right lingual and bilateral fusiform gyri, while the areas specialized for famous stimuli (irrespective of whether they are faces or names) spread from the left anterior temporal to the left temporoparietal regions. One specific area, the more lateral portion of the left anterior middle temporal gyrus, showed increased activation for famous faces relative to famous proper names and for famous proper names relative to common names. The differential responsiveness of this region when processing familiar people suggests functional segregation of either personal attributes or, more likely, the demands placed on processes that retrieve stored knowledge when stimuli have highly similar visual features but unique semantic associations.

Adult↗
PubMed1998

Different activation patterns in the visual cortex of late and congenitally blind subjects.

A key issue in developmental neuroscience is the role of activity-dependent mechanisms in the epigenetic induction of functional organization in visual cortex. Ocular blindness and ensuing visual deprivation is one of the rare models available for the investigation of experience-dependent cortical reorganization in man. In a PET study we demonstrate that congenitally blind subjects show task-specific activation of extrastriate visual areas and parietal association areas during Braille reading, compared with auditory word processing. In contrast, blind subjects who lost their sight after puberty show additional activation in the primary visual cortex with the same tasks. Studies in blind-raised monkeys show that crossmodal responses in extrastriate areas can be elicited by somatosensory stimulation. This is consistent with the crossmodal extrastriate activations elicited by tactile processing in our congenitally blind subjects. Since primary visual cortex does not show crossmodal responses in primate studies, the differential activation in late and congenitally blind subjects highlights the possibility of reciprocal activation by visual imagery in subjects with early visual experience.

Adult↗
PubMed1998

The functional roles of prefrontal cortex in episodic memory. II. Retrieval.

Functional neuroimaging studies of memory retrieval show consistent activation of the right prefrontal and superior parietal cortex. We examined the specific role of the prefrontal cortex during retrieval with the hypothesis that this region mediates monitoring processes necessary for optimal recall. During functional neuroimaging with PET, subjects retrieved verbal material under two conditions. In the first, an organizational structure had been provided, prior to scanning, and this formed the basis for a monitored memory search while the scan took place. A comparison condition did not require a monitored search because recall was externally cued. In both conditions, when compared with baseline tasks prefrontal cortex and medial parietal activation was observed. Within the right prefrontal cortex activation an anatomical dissociation was seen between the dorsal and ventral prefrontal cortex. The dorsal region showed greater activation when monitoring demands were emphasized, while the ventral region showed greater activation when external cueing was emphasized. An unpredicted dissociation within the superior parietal activation was also observed, a dorsal region showing activation during the monitored search task and a more ventral region showing activation under the externally cued condition. The results provide evidence for functional specialization of the right prefrontal cortex for discrete cognitive processes during episodic memory retrieval.

Adult↗
PubMed1997

How the brain learns to see objects and faces in an impoverished context.

A degraded image of an object or face, which appears meaningless when seen for the first time, is easily recognizable after viewing an undegraded version of the same image. The neural mechanisms by which this form of rapid perceptual learning facilitates perception are not well understood. Psychological theory suggests the involvement of systems for processing stimulus attributes, spatial attention and feature binding, as well as those involved in visual imagery. Here we investigate where and how this rapid perceptual learning is expressed in the human brain by using functional neuroimaging to measure brain activity during exposure to degraded images before and after exposure to the corresponding undegraded versions. Perceptual learning of faces or objects enhanced the activity of inferior temporal regions known to be involved in face and object recognition respectively. In addition, both face and object learning led to increased activity in medial and lateral parietal regions that have been implicated in attention and visual imagery. We observed a strong coupling between the temporal face area and the medial parietal cortex when, and only when, faces were perceived. This suggests that perceptual learning involves direct interactions between areas involved in face recognition and those involved in spatial attention, feature binding and memory recall.

Brain↗
PubMed1997

Recalling routes around london: activation of the right hippocampus in taxi drivers.

Functional imaging to date has examined the neural basis of knowledge of spatial layouts of large-scale environments typically in the context of episodic memory with specific spatiotemporal references. Much human behavior, however, takes place in very familiar environments in which knowledge of spatial layouts has entered the domain of general facts often referred to as semantic memory. In this study, positron emission tomography (PET) was used to examine the neural substrates of topographical memory retrieval in licensed London taxi drivers of many years experience while they recalled complex routes around the city. Compared with baseline and other nontopographical memory tasks, this resulted in activation of a network of brain regions, including the right hippocampus. Recall of famous landmarks for which subjects had no knowledge of their location within a spatial framework activated similar regions, except for the right hippocampus. This suggests that the hippocampus is involved in the processing of spatial layouts established over long time courses. The involvement of similar brain areas in routes and landmarks memory indicates that the topographical memory system may be primed to respond to any relevant topographical stimulation; however, the right hippocampus is recruited specifically for navigation in large-scale spatial environments. In contrast, nontopographical semantic memory retrieval involved the left inferior frontal gyrus, with no change in activity in medial temporal regions.

Adult↗
PubMed1997

Hemispheric specialization for global and local processing: the effect of stimulus category.

Neuropsychological evidence indicates that the global aspect of complex visual scenes is preferentially processed by the right hemisphere, and local aspects are preferentially processed by the left hemisphere. Using letter-based hierarchical stimuli (Navon figures), we recently demonstrated, in a directed-attention task, lateralized neural activity (assessed by positron emission tomography) in the left prestriate cortex during local processing, and in the right prestriate cortex during global processing. Furthermore, temporal-parietal cortex was critically activated bilaterally in a divided-attention task that involved varying the number of target switches between local and global levels of letter-based hierarchical stimuli. Little is known about whether such stimulus categories influence such hemispheric lateralization. We now present data on brain activity, derived from positron emission tomography, in normal subjects scanned during either local or global processing of object-based hierarchical stimuli. We again observe attentional modulation of neural activity in prestriate cortex. There is now greater right-sided activation for local processing and greater left-sided activation for global processing, which is the opposite of that seen with letter-based stimuli. The results suggest that the relative differential hemispheric activations in the prestriate areas during global and local processing are modified by stimulus category.

Adult↗
PubMed1997

Brain regions supporting intentional and incidental memory: a PET study.

Regional brain activity associated with intentional and incidental memory retrieval was studied with PET. Previously studied and new words were presented in either an intentional or an incidental memory task. Type of task was crossed with an encoding manipulation ('deep' vs 'shallow') which varied the probability that studied items would be remembered. In both tasks, deeply encoded items were associated with greater activation in the left hippocampus than were items that had received shallow encoding, suggesting that the involvement of the hippocampus in memory retrieval is independent of whether remembering is intentional or incidental. Right prefrontal and bilateral parietal cortex were more activated during the international task than during the incidental task, irrespective of encoding condition. Thus, these regions play a more extensive role in memory retrieval when remembering is intentional.

Brain↗
PubMed1997

Neural correlates of thinking in sign language.

Positron emission tomography (PET) was used to identify the brain regions activated during the 'inner signing' of sentences in subjects who were profoundly deaf and normally communicated using sign language. Although this appeared to involve the internal representation of hand and arm movements in space, it activated the left inferior frontal cortex rather than visuo-spatial areas. The activated region corresponds to that engaged during the silent articulation of sentences in hearing subjects. This suggests that 'inner signing' is mediated by similar regions to inner speech, and is consistent with neuropsychological data implicating the left hemisphere in the generation of sign language.

Adult↗
PubMed1997

The activity in human areas V1/V2, V3, and V5 during the perception of coherent and incoherent motion.

We have used the technique of positron emission tomography to study and compare the cortical activity produced when humans view a pattern of small squares moving incoherently with respect to one another and when the same pattern moves coherently and unidirectionally. A stationary version of the stimulus acted as a control. Our choice of paradigm was inspired by psychophysical models and physiological studies in the macaque monkey which show that directionally selective cells in V5 respond optimally to unidirectional coherent motion, whereas those of V1 respond to motion within their receptive fields, regardless of the motion in surrounding parts. Our results show that human V1/V2, V3, and V5 are all activated by both types of motion stimuli. Incoherent motion, however, proved to be more effective than coherent motion in activating V1/V2 and V5. Thus the higher perceptual salience of unidirectional coherent motion in comparison to incoherent motion is not reflected by any increased activation of human area V5.

Adult↗
PubMed1997

Characterizing the relationship between BOLD contrast and regional cerebral blood flow measurements by varying the stimulus presentation rate.

This paper investigates the relationship between the blood oxygenation level dependent (BOLD) contrast effect and regional cerebral blood flow using the techniques of functional MRI (fMRI) and positron emission tomography (PET). A passive listening paradigm with parametric variation in word presentation rate was used to investigate the rate dependency of both BOLD contrast fMRI and H215O PET in primary auditory cortex. We attempted to equate the stimulus presentation acoustic environments by using prerecorded echoplanar imaging sounds during the PET paradigm. We show that there is a linear relationship between word presentation rate and cerebral blood flow in primary auditory cortex, whereas the relationship between BOLD contrast and stimulus presentation rate is highly nonlinear, showing a saturable effect. Two possible explanations for our results are discussed: a nonlinearity in the relationship between BOLD contrast and deoxyhemoglobin concentration or a nonlinear rate dependency of the physiological mechanisms causing changes in deoxyhemoglobin concentration.

Adult↗
PubMed1997

The functional anatomy of a hysterical paralysis.

The concept of a conversion disorder (such as hysterical paralysis) has always been controversial (Ron, M.A. (1996). Somatization and conversion disorders. In: B.S. Fogel, R.B. Schiffer & S.M. Rao (Eds.), Neuropsychiatry. Williams and Wilkins, Baltimore, MD). Although the diagnosis is recognised by current psychiatric taxonomies, many physicians still regard such disorders either as feigned or as failure to find the responsible organic cause for the patient's symptoms. We report a woman with left sided paralysis (and without somatosensory loss) in whom no organic disease or structural lesion could be found. By contrast, psychological trauma was associated with the onset and recurrent exacerbation of her hemiparalysis. We recorded brain activity when the patient prepared to move and tried to move her paralysed (left) leg and when she prepared to move and did move her good (right) leg. Preparing to move or moving her good leg, and also preparing to move her paralysed leg, activated motor and/or premotor areas previously described with movement preparation and execution. The attempt to move the paralysed leg failed to activate right primary motor cortex. Instead, the right orbito-frontal and right anterior cingulate cortex were significantly activated. We suggest that these two areas inhibit prefrontal (willed) effects on the right primary motor cortex when the patient tries to move her left leg.

Cerebral Cortex↗
PubMed1997

Neural mechanisms involved in the processing of global and local aspects of hierarchically organized visual stimuli.

We investigated the functional anatomy involved in sustaining or switching visual attention between different perceptual levels, using functional imaging measures of neural activity. Two experiments were carried out using hierarchically organized letters (i.e. large letters made out of small letters). In a divided-attention task, subjects were required to switch attention between local and global levels. The number of successive stimuli for which subjects had to sustain attention to either the global or local level co-varied significantly with temporal-parietal activations bilaterally. Other activations were also observed in the right orbitofrontal cortex, the right dorsolateral prefrontal cortex, and the right middle temporal gyrus. The number of switches between levels co-varied significantly with activations in the left supplementary motor area and the left medial parietal cortex. In the directed-attention task, subjects were required to attend to either the global or local level of the stimuli throughout all trials; attention to the global aspect resulted in significant activation of the right lingual gyrus while attention to the local aspect significantly activated the left inferior occipital cortex. We suggest that left hemisphere activations with increasing numbers of switches between perceptual levels reflect increased demands on an executive attentional system, while sustained attention to either level activates a predominantly right hemispheric network involving temporal-parietal and dorsolateral prefrontal regions. Overall, the results provide evidence for relative hemispheric specialization for global and local processing in accordance with previous neuropsychological studies. In addition, the findings demonstrate that early visual processing mechanisms in the prestriate cortex are influenced by an attentional system in temporal-parietal areas.

Adult↗