Biomedical subjects

Gregory G Brown

Publications and source records attributed to Gregory G Brown.

43 records · Page 3Linked to original sources

PubMed2002

Neural correlates of refixation saccades and antisaccades in normal and schizophrenia subjects.

BACKGROUND: Schizophrenia subjects demonstrate difficulties on tasks requiring saccadic inhibition, despite normal refixation saccade performance. Saccadic inhibition is ostensibly mediated via prefrontal cortex and associated cortical/subcortical circuitry. The current study tests hypotheses about the neural substrates of normal and abnormal saccadic performance among subjects with schizophrenia. METHODS: Using functional magnetic resonance imaging, blood oxygenation level-dependent (BOLD) data were recorded while 13 normal and 14 schizophrenia subjects were engaged in refixation and antisaccade tasks. RESULTS: Schizophrenia subjects did not demonstrate the increased prefrontal cortex BOLD contrast during antisaccade performance that was apparent in the normal subjects. Schizophrenia subjects did, however, demonstrate normal BOLD contrast associated with refixation saccade performance in the frontal and supplementary eye fields, and posterior parietal cortex. CONCLUSIONS: Results from the current study support hypotheses of dysfunctional prefrontal cortex circuitry among schizophrenia subjects. Furthermore, this abnormality existed despite normal BOLD contrast observed during refixation saccade generation in the schizophrenia group.

Adult↗

PubMed2002

Increased amygdala activation to angry and contemptuous faces in generalized social phobia.

BACKGROUND: Generalized social phobia (GSP) is characterized by fear of social interactions and sensitivity to disapproval by others. Given the established role of the amygdala as part of a distributed neural system for the processing of emotional cues, we hypothesized that subjects with GSP would exhibit greater amygdala activation in response to harsh (angry, fearful, and contemptous) vs accepting (happy) facial emotional expressions compared with healthy control subjects (HCs). METHODS: Fifteen subjects with DSM-IV GSP and 15 age-, sex-, handedness-, and education-matched HCs, free of psychotropic medication for at least 12 weeks, viewed 60 color photographs from a standardized set of human facial stimuli, during which the task was to identify the sex of the person in the photograph. Data were collected across 3 functional (echo-planar) runs using a Siemens 1.5-T magnet, and analyzed using Analysis of Functional Neuroimaging software (Medical College of Wisconsin, Milwaukee). RESULTS: In the left allocortex (including the amygdala, uncus, and parahippocampal gyrus), subjects with GSP produced a significantly greater percent blood oxygen level-dependent signal change than did HCs for contemptous compared with happy faces (GSP: 0.72% vs HC: -0.01%; F(1,29) = 9.56, P =.004, Cohen d = 1.15) and for angry compared with happy faces (GSP: 0.45% vs HC: -0.09%; F(1,29) = 6.78, P =.02, Cohen d = 1.00). Subjects with GSP and HCs did not produce a statistically different percent signal change for fearful or nonexpressive faces compared with the happy faces in this region. CONCLUSIONS: These findings are consistent with a role for differential amygdala (and associated limbic) functioning in GSP. The pronounced response to contemptuous and angry facial expressions suggests that the amygdala in GSP may be particularly active in the processing of disorder-salient stimuli.

Adult↗

PubMed2002

Error rate and outcome predictability affect neural activation in prefrontal cortex and anterior cingulate during decision-making.

Decision-making in the presence of uncertainty is a complex process that involves both affective and cognitive factors. Both error rate and predictability have been implicated in the process of response selection during decision-making. This study examined the hypothesis that the rate of errors during decision-making differentially affects the activation in prefrontal and cingulate cortex. BOLD echo-planar signal intensity was obtained during a two-choice prediction task across 90-s blocks with 20, 50, or 80% error rates. This study yielded three main findings. First, at chance level error rates, activation of the right dorsolateral (BA 9, 46), inferior prefrontal (BA 44), and precuneus (BA 7) during the two-choice prediction task replicated the finding previously reported. Second, premotor (BA 6) and parahippocampal (BA 36) areas were relatively more active at high error rates, and dorsolateral (BA 9, 46) and inferior prefrontal cortex (BA 44) as well as parietal (BA 40) and cingulate cortex (BA 25, 32) were more active during low error rates. Third, the relationship between the frequency of the dominant strategy underlying decision-making (win-stay/lose-shift) and the activation in the dorsolateral prefrontal cortex and the anterior cingulate was dependent on error rate or outcome predictability. These results support the hypothesis that error rates and predictability affect the activation patterns in the neural systems underlying decision-making because these structures maintain a representation of the reinforcement history for the available response alternatives to select an "optimal strategy."

Adult↗

PubMed2002

Behavioral and functional neuroimaging evidence for prefrontal dysfunction in methamphetamine-dependent subjects.

Stimulant-dependent subjects show dysfunctions in decision-making similar to those seen in subjects with ventromedial prefrontal cortex lesions. Studies of drug craving, reward association, and decision-making have implicated dysfunctions of the dorsolateral and orbitofrontal cortex as a key neural substrate in subjects with stimulant dependence. Here, a functional magnetic resonance imaging (fMRI) study was carried out to determine the relationship between decision-making dysfunction and neural activation in different prefrontal areas. This investigation tested the behavioral hypothesis that methamphetamine-dependent subjects in early sustained remission show decision-making dysfunctions that are consistent with an increased reliance on stimulus-contingent response selection. It was hypothesized that these decision-making dysfunctions are due to differences in task-related activation in the dorsolateral and ventromedial prefrontal cortex. Ten methamphetamine-dependent subjects were compared with ten age- and education-matched controls performing a two-choice prediction task and a two-choice response task during a fMRI session. Response bias, latency, and mutual information measures assessing the underlying strategies of the decision-making sequences were obtained. First, methamphetamine-dependent subjects were more influenced by the immediately preceding outcome during the two-choice prediction task relative to normal comparison subjects. Second, methamphetamine-dependent subjects activated less dorsolateral prefrontal cortex (BA 9) and failed to activate ventromedial cortex (BA 10,11) during the two-choice prediction task compared with the two-choice response task. These results support the basic hypothesis that stimulant-dependent subjects exhibit fundamental cognitive deficits during decision-making that are consistent with both orbitofrontal and dorsolateral prefrontal dysfunction.

Adult↗

PubMed2002

New semantic and serial clustering indices for the California Verbal Learning Test-Second Edition: background, rationale, and formulae.

The original California Verbal Learning Test (CVLT) employed a semantic clustering index that used the words recalled during a given trial as the baseline for calculating expected values of chance clustering (recall-based expectancy). Although commonly used in cognitive psychology, clustering indices that use recall-based calculations of expectancy are implied by the assumption that organizational processes do not occur until after words are retrieved from memory. This assumption contradicts the generally held assumptions among neuropsychologists that (1) organization is an antecedent to recall, and (2) increases in the use of organizational strategies will result in better recall performance. After reviewing a brief history of clustering metrics, we used Monte Carlo simulations, informative examples, and patient data to examine clustering indices that use the word list as a baseline for calculating expectancy and propose these list-based expectancy measures as a refinement of the clustering indices used on the original CVLT. These indices are used on the recently published CVLT-II.

Cluster Analysis↗

PubMed2002

Effects of task structure on category priming in patients with Parkinson's disease and in healthy individuals.

Lexical decision tasks have been used to study both shifts of attention and semantic processing in Parkinson's Disease (PD). Whereas other laboratories have reported normal levels of semantic priming among PD patients, our laboratory has reported abnormally large levels. In this study, two experiments were performed to determine the influence of task structure on the extent of semantic priming during lexical decision-making and pronunciation tasks among PD patients and neurologically healthy controls. In Experiment 1, the effect of Prime Dominance (the ratio of category to neutral trials) on lexical decision-making was studied. Although equal numbers of word and nonword trials were presented, half of the PD patients and controls were studied under Category Prime Dominance (category : neutral prime ratio of 2:1) and half were studied under Neutral Prime Dominance (category : neutral prime ratio of 1:2). In Experiment 2, PD and control participants were studied on lexical decision-making and pronunciation tasks where twice as many words as nonword trials were presented, consistent with other studies from our laboratory. In Experiment 1, we found no group differences in the magnitude of priming and no effect of Prime Dominance. Moreover, the findings were similar in pattern and magnitude to results published by Neely (1977). In Experiment 2, we observed larger priming effects among PD patients than among controls, but only on the lexical decision (LD) task. These results support the hypothesis that abnormally large category-priming effects appear in LD studies of PD patients when the number of word trials exceeds the number of nonword trials. Furthermore, increased lexical priming in PD appears to be due to processes operating during the decision-making period that follows presentation of the lexical target.

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PubMedDate not supplied

Functional MRI and novel picture-learning among older patients with chronic schizophrenia: abnormal correlations between recognition memory and medial temporal brain response.

The relationship between disordered brain function and learning deficits in chronic schizophrenia is unclear. The authors compared correlations of brain response to picture encoding with subsequent recognition memory between samples of clinically stable patients over age 45 with schizophrenia and demographically similar healthy individuals. Subjects were studied with functional magnetic resonance imaging during novel picture encoding and a control condition. Comparison subjects showed an inverse relationship between subsequent recognition memory and brain response in medial temporal areas. Among schizophrenia patients, brain response in these regions was positively correlated with recognition memory. Brain-behavior relationships during learning were thus found to be qualitatively different between schizophrenic patients and healthy subjects.

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