Showing papers by "Raymond J. Dolan published in 1998"

Conscious and unconscious emotional learning in the human amygdala

Abstract: If subjects are shown an angry face as a target visual stimulus for less than forty milliseconds and are then immediately shown an expressionless mask, these subjects report seeing the mask but not the target. However, an aversively conditioned masked target can elicit an emotional response from subjects without being consciously perceived. Here we study the mechanism of this unconsciously mediated emotional learning. We measured neural activity in volunteer subjects who were presented with two angry faces, one of which, through previous classical conditioning, was associated with a burst of white noise. In half of the trials, the subjects' awareness of the angry faces was prevented by backward masking with a neutral face. A significant neural response was elicited in the right, but not left, amygdala to masked presentations of the conditioned angry face. Unmasked presentations of the same face produced enhanced neural activity in the left, but not right, amygdala. Our results indicate that, first, the human amygdala can discriminate between stimuli solely on the basis of their acquired behavioural significance, and second, this response is lateralized according to the subjects' level of awareness of the stimuli.

A neuromodulatory role for the human amygdala in processing emotional facial expressions

Abstract: Localized amygdalar lesions in humans produce deficits in the recognition of fearful facial expressions. We used functional neuroimaging to test two hypotheses: (i) that the amygdala and some of its functionally connected structures mediate specific neural responses to fearful expressions; (ii) that the early visual processing of emotional faces can be influenced by amygdalar activity. Normal subjects were scanned using PET while they performed a gender discrimination task involving static grey-scale images of faces expressing varying degrees of fear or happiness. In support of the first hypothesis, enhanced activity in the left amygdala, left pulvinar, left anterior insula and bilateral anterior cingulate gyri was observed during the processing of fearful faces. Evidence consistent with the second hypothesis was obtained by a demonstration that amygdalar responses predict expression-specific neural activity in extrastriate cortex.

The functional roles of prefrontal cortex in episodic memory. I. Encoding.

Abstract: Functional neuroimaging studies of episodic memory consistently report an association between memory encoding operations and left prefrontal cortex (PFC) activation. Encoding-related activation has been described in dorsolateral, ventrolateral and anterior prefrontal regions. We tested the hypothesis that a specific component of this left PFC activation reflects organizational processes necessary for optimal memory encoding. Subjects underwent PET scans while learning auditorily presented word lists under dual task conditions. The degree to which they were required to organize word lists semantically was systematically varied across three experimental conditions. A task in which words were already organized produced the least degree of left PFC activity whereas a task requiring subjects to generate an organizational structure was associated with maximal activity in this region. This activation was localized to a region just above the inferior frontal sulcus. The functional specificity of this increased activity for organizational processes was tested using a concurrent distracting task known to disrupt these processes. Distraction resulted in a significant attenuation of this activation during the task emphasizing organizational processes but not other encoding tasks. In contrast, the distraction task resulted in reduced activity in a more ventral/anterior PFC region expressed equally for all memory tasks. The findings indicate that a key function of left dorsolateral PFC at encoding relates specifically to the use of executive processes necessary for the creation of an organizational structure. Activity in more ventral and anterior left PFC regions would appear to reflect a less specific component of episodic memory encoding.

The functional roles of prefrontal cortex in episodic memory

Abstract: Summary Functional neuroimaging studies of episodic memory consistently report an association between memory encoding operations and left prefrontal cortex (PFC) activation. Encoding-related activation has been described in dorsolateral, ventrolateral and anterior prefrontal regions. We tested the hypothesis that a specific component of this left PFC activation reflects organizational processes necessary for optimal memory encoding. Subjects underwent PET scans while learning auditorily presented word lists under dual task conditions. The degree to which they were required to organize word lists semantically was systematically varied across three experimental conditions. A task in which words were already organized produced the least degree of left PFC activity whereas a task requiring subjects to generate an organizational structure was associated with maximal activity in this

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

Abstract: 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.

Brain Activations in Schizophrenia During a Graded Memory Task Studied With Functional Neuroimaging

Abstract: Background Functional neuroimaging experiments have implicated prefrontal cortex (PFC) in memory processes. Several studies of schizophrenic patients have shown failure of activation in the dorsolateral region of PFC (DLPFC). We used a graded memory challenge to characterize functional neuroanatomical differences between schizophrenic and control subjects. The graded manipulation of task demands enabled us to assess group differences in the context of normal and abnormal psychological task performance. Methods Memory-related activity was assessed using positron emission tomography in schizophrenic patients and age-matched controls during performance of a graded memory task. Subjects underwent scanning while learning and recalling word lists of variable length. Results We used a model that assessed linear and nonlinear effects of memory load. Nonlinear group differences in DLPFC activation were observed. Controls showed a steepening slope of DLPFC increase as task demands increased. By contrast, schizophrenic subjects showed initial DLPFC increases that fell away with increasing memory load. The DLPFC response in schizophrenic subjects was closely related to measured task performance. In addition, schizophrenic subjects failed to show task-related decreases in activity in the left superior temporal and inferior parietal gyrus. Conclusions Patients with schizophrenia showed a failure in DLPFC activation only in the face of diminished performance measures, suggesting that a full characterization of task-related changes in DLPFC activation must consider performance levels. However, striking failures of deactivation in superior temporal and inferior parietal regions were independent of task performance, possibly reflecting a core abnormality of the condition.

Abnormal neural response to feedback on planning and guessing tasks in patients with unipolar depression

Abstract: Background. It has been suggested that patients with unipolar depression show abnormal responses to negative feedback in the performance of cognitive tasks. Positron emission tomography (PET) has previously identified blood flow abnormalities in depressed patients during cognitive performance. We have also used PET to identify regions where there is differential neural response to performance feedback in normal volunteers. In this study we aimed to test the hypothesis that blood flow in these regions, the medial caudate and ventromedial prefrontal cortex, would be abnormal in depressed patients.Methods. Six patients with unipolar depression and six matched controls were scanned using PET while performing cognitive tasks in the presence and absence of feedback.Results. Compared with controls, depressed patients failed to show significant activation in the medial caudate and ventromedial orbitofrontal cortex. Blood flow was lower and a differential response, observed in normals, under different task and feedback conditions was not seen in the patients.Discussion. The findings suggest that the behavioural response to feedback in depressed patients is associated with an abnormal neural response within the medial caudate and ventromedial orbitofrontal cortex, regions implicated in reward mechanisms. We argue that the observed abnormalities may depend on a combination of psychological factors, with both cognitive and emotive components.

Experience-dependent modulation of tonotopic neural responses in human auditory cortex.

Abstract: Experience-dependent plasticity of receptive fields in the auditory cortex has been demonstrated by electrophysiological experiments in animals. In the present study we used PET neuroimaging to measure regional brain activity in volunteer human subjects during discriminatory classical conditioning of high (8000 Hz) or low (200 Hz) frequency tones by an aversive 100 dB white noise burst. Conditioning-related, frequency-specific modulation of tonotopic neural responses in the auditory cortex was observed. The modulated regions of the auditory cortex positively covaried with activity in the amygdala, basal forebrain and orbitofrontal cortex, and showed context-specific functional interactions with the medial geniculate nucleus. These results accord with animal single-unit data and support neurobiological models of auditory conditioning and value-dependent neural selection.

Neural Response during Preference and Memory Judgments for Subliminally Presented Stimuli: A Functional Neuroimaging Study

Abstract: Preexposing subjects to visual stimuli is sufficient to establish a subsequent preference, even when previous exposure is subliminal, such that explicit recognition is at chance. This influence of previous exposure on preference judgments, known as the “mere exposure effect,” is a form of unconscious memory. The present functional neuroimaging study examines the mechanism of this effect. Nine volunteer subjects were studied using functional imaging while making forced choice judgments about abstract stimuli on the basis of either preference or memory. Each judgment type was made under two conditions: under one condition one or the other stimulus had previously been presented subliminally, whereas under the second condition both stimuli were novel. Memory judgments were associated with activation of left frontopolar cortex and parietal areas, whereas preference judgments were associated with activation of medial prefrontal cortex and regions of occipital cortex. The modulation of preference by objective familiarity (implicit memory) was associated with right lateral frontal activation. Significant activation of hippocampal gyrus was seen in response to objective stimulus novelty, regardless of judgment type required. Our data thus demonstrate activations of a memory system independent of recollective experience. Dissociable activations within this system implicate a frontopolar involvement in explicit retrieval attempt and right lateral prefrontal cortex involvement in implicit memory expressed in preference judgments. Furthermore, the results suggest that hippocampal response to stimulus novelty can be independent of conscious reportability of familiarity.

Neural correlates of depth of processing effects on recollection: evidence from brain potentials and positron emission tomography

Abstract: The probability that words would be recollected during tests of recognition memory was varied by manipulating depth of processing at study. Experiment 1 employed scalp-recorded event-related potentials (ERPs), and identified as a correlate of recollection a late (onset c. 500 ms), strongly left-lateralized positive-going modulation of the ERP waveform. The findings from experiment 2, which employed positron emission tomography (PET), indicated that recollection was associated with activation of the left hippocampal formation together with an extensive region of left temporal and frontal cortex. The findings support current ideas about the role of the hippocampal formation in episodic memory retrieval, and provide complementary information about the time course and localization of the cortical correlates of the recollection of recently experienced words.

A cognitive affective role for the cerebellum.

Abstract: Traditional neurological teaching suggests that, functionally, the cerebellum is devoted entirely to motor control. Accordingly, the cerebellum is proposed to be primarily concerned with co-ordination in skilled voluntary movement, as well as control of motor tone, posture and gait. In its motor regulatory role, the cerebellum is assumed to function as a comparator that adjusts motor outputs on the basis of afferent signals relating to planned motor action and reafference signals concerning excecuted motor acts. This motor view of cerebellar function has withstood numerous challenges over the past half century (Dow and Moruzzi, 1958). In the past decade, a growing body of empirical data, largely derived from functional neuroimaging studies, have implicated the cerebellum in diverse higher cognitive functions (Leineret al., 1995). What has been lacking is convincing documentation of cognitive deficits in patients with pathology restricted to the cerebellum. In this edition of Brain, Schmahmann and Sherman (1998) provide such neuropsychological and behavioural evidence. Their paper provides detailed clinical descriptions of prominent non-motor deficits in a carefully documented cohort of patients with cerebellar pathology. The authors conclude that the pattern of deficit in these patients is characteristic enough to suggest that it forms a recognizable syndrome, the Cerebellar Cognitive Affective Syndrome. Its defining features are disturbances in executive function, spatial cognition, language and emotional regulation of behaviour. Unlike previous clinical reports, where deficits were mainly those that detailed neuropsychological evaluation, the core deficits were evident enough to be elicited by standard bedside evaluation. Given the complex anatomy of the cerebellum, an obvious question is whether components of the syndrome are associated with pathology of distinct cerebellar regions. Unfortunately, the size of the series and the often diffuse nature of the associated pathology do not allow clear answers to this question. Nevertheless, the authors note a strong tendency for posterior lobe lesions to be associated with the core syndrome, while patients with vermis lesions had pronounced affective disturbance. A widely accepted principle of neural organization is the interdependence of structure and function. Within this framework, it is instructive to consider whether cognitive deficits in patients with cerebellar pathology can be accounted for in terms of its known anatomy. Two facts about cerebellar anatomy seem pertinent. First, in relation to its intrinsic structure, the cerebellum accounts for over half the brain’s population of neurons. More importantly, in terms of its likely cognitive functions, the cerebellum receives inputs

The Origins of Forgetting in a Case of Isolated Retrograde Amnesia Following a Haemorrhage: Evidence From Functional Imaging

Abstract: Following a left superior dorsolateral prefrontal haemorrhage, a man in his 40s presented with a dense isolated retrograde amnesia for 19 years preceding the stroke. This period of his lif...

How we feel

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