Search PubMed⌕ Search

Biomedical subjects

C Frith

Publications and source records attributed to C Frith.

At least 19 recordsLinked to original sources
PubMed2000

Unconscious activation of visual cortex in the damaged right hemisphere of a parietal patient with extinction.

Visual extinction is a sign classically associated with right parietal damage. The patient can see a single stimulus presented in the ipsilesional or contralesional visual field, but is characteristically unaware of the same contralesional stimulus during simultaneous stimulation of both fields. The ipsilesional stimulus is said to 'extinguish' the contralesional stimulus from awareness during bilateral stimulation, perhaps due to a pathological bias in attention towards the ipsilesional side. Recent psychophysical evidence suggests that, although extinguished stimuli are not consciously seen, they may undergo residual processing and exert implicit effects on performance. However, the neural structures mediating such residual processing for extinguished stimuli remain unknown. Here we studied the neural activity evoked by an extinguished visual stimulus, using event-related functional MRI (fMRI), in a patient with circumscribed right inferior parietal damage and profound left-sided extinction. Monochrome objects (faces or houses) were presented in the left or right field, either unilaterally or bilaterally on each trial, with the patient indicating by button press whether he saw an object on the left, the right or on both sides. He usually saw only the right object on bilateral trials, yet the fMRI data showed activation of visual cortex contralateral to the extinguished left stimulus on these trials (compared with right-only stimulation), in both striate and early extrastriate areas of the right hemisphere. This activity had a similar location and time-course to that resulting from a single stimulus in the left versus right visual field. Cortical pathways involved in the normal processing of a single seen stimulus can thus still be activated by an unseen, extinguished stimulus after right parietal damage. Comparison of fMRI responses for faces versus houses revealed some category-specific activation for extinguished stimuli in right fusiform regions, but only at low statistical threshold. These results are discussed in terms of theoretical accounts for parietal extinction and, more generally, for the neural substrates of visual awareness.

Aged↗
PubMed2000

Selective spatial attention in vision and touch: unimodal and multimodal mechanisms revealed by PET.

Two positron-emission tomography (PET) experiments explored the neural basis of selective spatial attention in vision and touch, testing for modality-specific versus multimodal activations due to attended side. In the first study, either light flashes or finger vibrations were presented bilaterally. Twelve healthy volunteers were scanned while sustaining covert attention on the left or right hemifield within each modality. The main effect for attending right minus left, across both modalities, revealed bimodal spatial attention effects in the left intraparietal sulcus and left occipitotemporal junction. Modality-specific attentional effects (again, for attending right vs. left) were found in the left superior occipital gyrus for vision, and left superior postcentral gyrus for touch. No significant activations were seen for attending left minus right. The second study presented only tactile stimuli, manipulating whether the eyes were open or closed, and including passive stimulation and rest baselines. The unimodal activation for tactile spatial attention in the left superior postcentral gyrus was replicated. The bimodal activation of the left intraparietal sulcus observed in the first study was now found for touch, but only when the eyes were open (hands visible), apparently confirming its multimodal nature. These results reveal mechanisms of sustained spatial attention operating at both modality-specific and multimodal levels.

Adult↗
PubMed1999

The neuroanatomy of autism: a voxel-based whole brain analysis of structural scans.

Autism is a biological disorder which affects social cognition, and understanding brain abnormalities of the former will elucidate the brain basis of the latter. We report structural MRI data on 15 high-functioning individuals with autistic disorder. A voxel-based whole brain analysis identified grey matter differences in an amygdala centered system relative to 15 age- and IQ-matched controls. Decreases of grey matter were found in anterior parts of this system (right paracingulate sulcus, left inferior frontal gyrus). Increases were found in posterior parts (amygdala/peri-amygdaloid cortex, middle temporal gyrus, inferior temporal gyrus), and in regions of the cerebellum. These structures are implicated in social cognition by animal, imaging and histopathological studies. This study therefore provides converging evidence of the physiological basis of social cognition.

Adult↗
PubMed1999

The neural correlates of conscious experience: an experimental framework.

Demonstrating that neural activity 'represents' physical properties of the world such as the orientation of a line in the receptive field of a nerve cell is a standard procedure in neuroscience. However, not all such neural activity will be associated with the mental representations that form the contents of consciousness. In some cases, such as when patients with blindsight correctly 'guess' the location of a stimulus, neural activity is associated with physical stimulation and with appropriate behaviour, but not with awareness. To identify the neural correlates of conscious experience we need to identify patterns of neural activity that are specifically associated with awareness. Experiments aimed at making such identifications require that subjects report some aspect of their conscious experience either verbally or through some pre-arranged non-verbal report while neural activity is measured. If there is some characteristic neural signature of consciousness, then this should be distinguishable from the kinds of neural activity associated with stimulation and/or behaviour in the absence of awareness. It remains to be seen whether the neural signature of consciousness relates to the location of the neural activity, the temporal properties of the neural activity or the form of the interaction between activity in different brain regions.

Journal Article↗
PubMed1998

Images of psychopathology.

Brain imaging continues to provide important data about brain structure, neurotransmitter function and the physiological basis of cognitive processes, as these relate to schizophrenia and mood disorders. A unifying theoretical perspective, however, that can clarify the precise nature of the biological basis of these diverse psychiatric conditions is lacking. It is becoming increasingly evident that a lesion model is inappropriate and that a more relevant characterisation will be found in terms of disorders of functional interconnections between brain regions.

Depression↗
PubMed1997

Brain mechanisms associated with top-down processes in perception.

Perception arises through an interaction between sensory input and prior knowledge. We propose that at least two brain areas are required for such an interaction: the 'site' where analysis of afferent signals occurs and the 'source' which applies the relevant prior knowledge. In the human brain, functional imaging studies have demonstrated that selective attention modifies activity in early visual processing areas specific to the attended feature. Early processing areas are also modified when prior knowledge permits a percept to emerge from an otherwise meaningless stimulus. Sources of this modification have been identified in parietal cortex and in prefrontal cortex. Modification of early processing areas also occurs on the basis of prior knowledge about the predicted sensory effects of the subject's own actions. Activity associated with mental imagery resembles that associated with response preparation (for motor imagery) and selective attention (for sensory imagery) suggesting that mental imagery reflects the effects of prior knowledge on sensory processing areas in the absence of sensory input. Damage to sensory processing areas can lead to a form of sensory hallucination which seems to arise from the interaction of prior knowledge with random sensory activity. In contrast, hallucinations associated with schizophrenia may arise from a failure of prior knowledge about motor intentions to modify activity in relevant sensory areas. When functioning normally, this mechanism permits us to distinguish our own actions from those of independent agents in the outside world. Failure to make this distinction correctly may account for the strong association between hallucinations and paranoid delusions in schizophrenia; the patient not only hears voices, but attributes (usually hostile) intentions to these voices.

Cognition↗
PubMed1997

Two modulatory effects of attention that mediate object categorization in human cortex.

Attentional modulation of cortical activity was examined by varying the rate of visual stimuli in object categorization tasks according to single and conjoined features. Activation of dorsolateral frontal cortex was independent of the stimulus presentation rate and elicited by the participant's attention to conjoined compared with single features. Several cortical regions showed attentionally modulated activity. In inferior temporal cortex, modulation was due to an additional bias signal underlying normal rate-correlated activity. In two other regions (premotor cortex and cerebellum), attention modified the correlation of activity and the stimulus presentation rate. Attentional effects in the human cortex are expressed by at least two physiologically distinct mechanisms acting on spatially distributed areas.

Adult↗
PubMed1996

'Theory of mind' in the brain. Evidence from a PET scan study of Asperger syndrome.

The ability to attribute mental states to others ('theory of mind') pervades normal social interaction and is impaired in autistic individuals. In a previous positron emission tomography scan study of normal volunteers, performing a 'theory of mind' task was associated with activity in left medial prefrontal cortex. We used the same paradigm in five patients with Asperger syndrome, a mild variant of autism with normal intellectual functioning. No task-related activity was found in this region, but normal activity was observed in immediately adjacent areas. This result suggests that a highly circumscribed region of left medial prefrontal cortex is a crucial component of the brain system that underlies the normal understanding of other minds.

Adult↗
PubMed1996

The role of the prefrontal cortex in self-consciousness: the case of auditory hallucinations.

Many patients with schizophrenia report hallucinations in which they hear voices talking to them or about them. Behavioural and physiological studies show that this experience is associated with processes occurring in auditory language systems associated with both the production and the reception of speech. I propose that hallucinations are experienced because patients have difficulty in distinguishing sensations caused by their own actions from those that arise from external influences. This distinction can be made by predicting the sensations that will result from executive commands (forward modelling). If the predicted sensation matches the actual sensation then no outside influences have occurred and perception of change can be 'cancelled'. At the physiological level this mechanism depends upon interactions between the prefrontal areas where the executive commands originate and posterior brain regions concerned with the resultant sensations. Evidence from functional brain imaging confirms that interactions between prefrontal (executive) areas and auditory association areas are abnormal in schizophrenia. However, this account needs to be extended before we can understand why patients experience the voices as emanating, not just from an external source, but from agents who are trying to influence their behaviour. Recent imaging studies suggest that medial prefrontal cortex is engaged when we think about other people, but the precise nature of the interaction of this brain area with other regions remains to be established.

Consciousness↗
PubMed1996

The role of the prefrontal cortex in higher cognitive functions.

The higher cognitive functions, working memory, mental imagery and willed action, are all intimately associated with consciousness. The common process underlying all these functions is that information is "held in mind" for a period of time. This information, which may be about stimuli or responses, can be derived from the past or generated for the future. Brain imaging studies show that "holding something in mind" is associated with activity in an extended system which involves both prefrontal cortex and more posterior areas whose location is determined by the nature of the information being held in mind. Automatic actions and perceptions which do not involve consciousness are associated with activity in the relevant posterior areas, but not in the prefrontal cortex. These studies demonstrate that activity occurs in the same posterior area whether the associated information comes from the outside world or is internally generated. This raises the problem of how we know whether our experience derives from mental imagery or from something happening in the outside world. There is evidence that patients with schizophrenia have precisely this problem since they perceive their own thoughts and even sub-vocal speech as coming from outside (hallucinations). Recent brain imaging studies suggest that there is a disconnection between prefrontal and posterior areas in these patients which could explain their characteristic misperceptions.

Choice Behavior↗
PubMed1996

Neuropsychology of schizophrenia, what are the implications of intellectual and experiential abnormalities for the neurobiology of schizophrenia?

The diagnosis of schizophrenia is largely based on reports of bizarre experiences such as having alien thoughts inserted into one's mind. Many patients with this diagnosis show a marked intellectual decline and particular problems with tasks involving certain kinds of memory or requiring mental flexibility. Similar patterns of performance can be seen in patients with damage in the prefrontal cortex. However, patients with schizophrenia show a very varied pattern of impairments relating to their current mental state. Chronic patients with negative features, such as poverty of speech, are most likely to show poor test performance, while the presence of severe hallucinations and delusions need not be associated with any impairment. A cognitive approach suggests that hallucinations and delusions result from the patient attributing his own actions to an external agency. This error is due to an inability to distinguish between external events and perceptual changes caused by his own actions. The basis of this failure could be a functional disconnection between frontal brain areas concerned with action and posterior areas concerned with perception.

Brain↗
PubMed1995

A functional neuroanatomy of hallucinations in schizophrenia.

Hallucinations, perceptions in the absence of external stimuli, are prominent among the core symptoms of schizophrenia. The neural correlates of these brief, involuntary experiences are not well understood, and have not been imaged selectively. We have used new positron emission tomography (PET) methods to study the brain state associated with the occurrence of hallucinations in six schizophrenic patients. Here we present a group study of five patients with classic auditory verbal hallucinations despite medication, demonstrating activations in subcortical nuclei (thalamic, striatal), limbic structures (especially hippocampus), and paralimbic regions (parahippocampal and cingulate gyri, as well as orbitofrontal cortex). We also present a case study of a unique, drug-naive patient with visual as well as auditory verbal hallucinations, demonstrating activations in visual and auditory/linguistic association cortices as part of a distributed cortical-subcortical network. Activity in deep brain structures, identified with group analysis, may generate or modulate hallucinations, and the particular neocortical regions entrained in individual patients may affect their specific perceptual content. The interaction of these distributed neural systems provides a biological basis for the bizarre reports of schizophrenic patients.

Adult↗