Biomedical subjects

D A Nathaniel-James

Publications and source records attributed to D A Nathaniel-James.

4 recordsLinked to original sources

PubMed2002

The role of the dorsolateral prefrontal cortex: evidence from the effects of contextual constraint in a sentence completion task.

Although the prefrontal cortices, in particular the dorsolateral prefrontal cortex (DLPFC), have been examined in numerous imaging and neuropsychological studies, it has proved difficult to assign a specific function to this brain region. The aim of this study was therefore to delineate the function of the DLPFC first by using positron emission tomography with a word generation task and second by comparing these findings with a series of different circumstances in which the DLPFC is activated in association with response selection. Six healthy volunteers were presented with a sentence completion task based on the Hayling test. In each of two conditions, (A) response initiation and (B) response suppression, volunteers saw a sentence with the final word omitted. In condition A they had to provide a word that fitted at the end of the sentence and in condition B they had to provide a word that did not fit. The corpus of sentences used varied systematically in their level of contextual constraint, ranging from low to high. With all levels of constraint combined, significant greater activation was observed in the left DLPFC (BA46/9) under the suppression condition and in the medial orbitofrontal cortex (BA11) under the initiation condition. Under the high-constraint condition with both tasks combined, significant right middle temporal activity (BA21) was observed, whereas under low constraint, the left DLPFC was significantly activated. An interaction of task and constraint revealed that the left DLPFC was significantly more active in the suppression task at all levels of constraint, but only under the low-constraint conditions in the initiation task. The reaction time analyses mirrored the pattern of activity observed, with slower reaction times under conditions of suppression and under conditions of initiation with low constraint. By comparing these findings with other studies of response selection, we argue that the most likely single cognitive function of the DLPFC is to specify a set of responses suitable for a given task and to bias these for selection (sculpting the response space). This function resembles the biasing of competition between stimuli in the model of Desimone and Duncan and is analogous to the component of the Supervisory Attentional System that modulates the contention scheduling system.

Adult↗

PubMed1997

The functional anatomy of verbal initiation and suppression using the Hayling Test.

This study utilised positron emission tomography (PET) to identify the cortical areas involved in verbal initiation and suppression in normal subjects whilst performing a sentence completion test (the Hayling Test). In the first condition (response initiation) subjects were required to complete a sentence from which the last word was omitted, whereas in the second condition (response suppression) subjects were asked to complete a sentence with a word which made no sense in the context of the sentence. Subjects were also required to perform a control task in which they had to read out the last word of given sentences. Compared to the control task, response initiation was associated with left-sided activation of the frontal operculum, inferior frontal gyrus, middle temporal gyrus and the right anterior cingulate gyrus, whereas response suppression was associated with left frontal operculum, inferior frontal gyrus and right anterior cingulate gyrus activation. The difference in activation between the two conditions of the Hayling Test lay in the increased activation of the left middle temporal gyrus and the left inferior frontal gyrus during response initiation.

Adult↗

PubMed1996

Memory impairment in schizophrenia: its' relationship to executive function.

The presence of memory impairment in schizophrenia has frequently been documented but much less attention has been given to the qualitative aspects of this impairment and its association to executive function. Using a cognitive-process approach, we examined memory and executive function in 25 patients who met DSM-III-R criteria for schizophrenia. Patients were matched with 25 healthy volunteers. The schizophrenic group was found to have a significant impairment in immediate memory, with relatively spared long-delay memory. Performance on verbal learning and recognition memory was similar to that of controls. Memory deficits were present irrespective of the encoding strategies used and were unrelated to chronicity. In addition, the schizophrenics performed worse than controls on tests of executive function, but the degree of impairment was greater on tests of response initiation and suppression. This pattern of performance resembled that found in patients with subcortical or frontal lesions which was supported by some significant correlations between aspects of memory and executive function. Our results suggest that in schizophrenia, specific executive functions may make a selective contribution to the pattern of memory performance in schizophrenia which is subserved by frontal and to a lesser extent hippocampal/diencephalic systems.

Adult↗

PubMed1996

Confabulation in schizophrenia: evidence of a new form?

This study is an attempt to demonstrate confabulation in schizophrenia. Twelve patients who met DSM-III-R criteria for schizophrenia were matched for age, sex and pre-morbid IQ with 12 volunteers, 9 of whom were normal healthy subjects, with the remainder being depressed patients. To elicit confabulation, subjects were asked to recall narratives. In addition, subjects were examined on a number of neuropsychological tests. Confabulation was defined as recall of information not present in the narrative. Variable amounts of confabulation were observed in all schizophrenics, while only one control subject confabulated. The content and structure of their productions differed from previously reported forms of confabulation in that schizophrenic patients spontaneously rearranged the original narratives to produce new ideas. The amount of confabulation was found to be related to difficulties in suppressing inappropriate responses (Hayling test) and formal thought disorder, but unrelated to understanding of the gist or moral of the narratives. Tentative mechanisms for the process of confabulation are proposed, based on specific difficulties with comprehension, response monitoring and response suppression.

Adult↗