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Biomedical subjects

B T O'Sullivan

Publications and source records attributed to B T O'Sullivan.

18 recordsLinked to original sources
PubMed2001

Human cortical processing of colour and pattern.

The present study investigates human visual processing of simple two-colour patterns using a delayed match to sample paradigm with positron emission tomography (PET). This study is unique in that we specifically designed the visual stimuli to be the same for both pattern and colour recognition with all patterns being abstract shapes not easily verbally coded composed of two-colour combinations. We did this to explore those brain regions required for both colour and pattern processing and to separate those areas of activation required for one or the other. We found that both tasks activated similar occipital regions, the major difference being more extensive activation in pattern recognition. A right-sided network that involved the inferior parietal lobule, the head of the caudate nucleus, and the pulvinar nucleus of the thalamus was common to both paradigms. Pattern recognition also activated the left temporal pole and right lateral orbital gyrus, whereas colour recognition activated the left fusiform gyrus and several right frontal regions.

Adult↗
PubMed1999

An exploration of varieties of visual attention: ERP findings.

A set of five tasks was designed to examine dynamic aspects of visual attention: selective attention to color, selective attention to pattern, dividing and switching attention between color and pattern, and selective attention to pattern with changing target. These varieties of visual attention were examined using the same set of stimuli under different instruction sets; thus differences between tasks cannot be attributed to differences in the perceptual features of the stimuli. ERP data are presented for each of these tasks. A within-task analysis of different stimulus types varying in similarity to the attended target feature revealed that an early frontal selection positivity (FSP) was evident in selective attention tasks, regardless of whether color was the attended feature. The scalp distribution of a later posterior selection negativity (SN) was affected by whether the attended feature was color or pattern. The SN was largely unaffected by dividing attention across color and pattern. A large widespread positivity was evident in most conditions, consisting of at least three subcomponents which were differentially affected by the attention conditions. These findings are discussed in relation to prior research and the time course of visual attention processes in the brain.

Adolescent↗
PubMed1998

Further evidence for a deficit in switching attention in schizophrenia.

In this study, sustained, selective, divided, and switching attention, and reloading of working memory were investigated in schizophrenia by using a newly developed Visual Attention Battery (VAB). Twenty-four outpatients with schizophrenia and 24 control participants were studied using the VAB. Performance on VAB components was correlated with performance of standard tests. Patients with schizophrenia were significantly impaired on VAB tasks that required switching of attention and reloading of working memory but had normal performance on tasks involving sustained attention or attention to multiple stimulus features. Switching attention and reloading of working memory were highly correlated with Trails (B-A) score for patients. The decline in performance on the switching-attention task in patients with schizophrenia met criteria for a differential deficit in switching attention. Future research should examine the neurophysiological basis of the switching deficit and its sensitivity and specificity to schizophrenia.

Adult↗
PubMed1997

Pharmacokinetics of dexamethasone and its relationship to dexamethasone suppression test outcome in depressed patients and healthy control subjects.

The pharmacokinetics of dexamethasone (DEX) were studied in 9 drug-free melancholically depressed patients and 10 healthy control subjects matched by sex and age. Each subject received 1 mg of DEX administered orally and by the (i.v.) route at 11:00 PM and serial blood samples were collected over the next 17 hours until 4:00 PM. There were no significant differences between the diagnostic groups and DEX bioavailability, peak plasma level, time to maximum concentration, or in elimination half-life after oral administration. Bioavailability estimates indicated that DEX absorption was incomplete and variable mean = 61%, SD = 14) in controls as well as depressed patients. In both groups there was a wide interindividual variability in plasma DEX levels following both oral and i.v. routes of administration. This variability could not be reliably predicted by differences in age, sex, or weight between subjects. The factors that accounted for most the variability in 4:00 PM plasma DEX levels after oral administration were clearance, bioavailability, and time to reach maximum concentration. Plasma DEX levels were lower in 3 depressed nonsuppressors compared to 3 matched controls who suppressed. No single pharmacokinetic factor was shown to be responsible for the lower DEX levels in the depressed nonsuppressors. These results indicate that plasma DEX levels need to be measured in each individual during the DST procedure so that this information may be taken into consideration when interpreting DST results.

Administration, Oral↗
PubMed1995

Somatosensory activations of the parietal operculum of man. A PET study.

We tested the hypothesis that somatosensory discrimination of roughness (microgeometry) but not of shape (macrogeometry) would activate the parietal operculum (PO) in man. It was also investigated whether a simple square pulse indentation of the skin on the index finger would activate the PO. Regional cerebral blood flow was measured with [15O]butanol and positron emission tomography in a total of 20 normal volunteers. Ten subjects used their right hand to discriminate objects that differed in roughness and similar smooth objects that differed in length. Ten other subjects pressed a button when they felt a square pulse indentation of the skin on their right index finger in a somatosensory reaction time task. Discrimination of roughness activated one field in the PO contralaterally and two fields ipsilaterally to the stimulated hand. The discrimination of length activated one field in the PO located ipsilaterally to the stimulated hand. The somatosensory reaction time task also activated one contralateral and two ipsilateral fields in the PO, and these fields partially overlapped the activated fields in the roughness discrimination task. Based on the extension of these fields and their overlaps we conclude that there exist at least one part of the contralateral PO and at least two parts of the ipsilateral PO that can be activated by somatosensory stimulation of the right hand. We argue further that the contralateral activated part contains a region than can be activated by roughness.

Adult↗
PubMed1995

Positron-emission tomography studies of cross-modality inhibition in selective attentional tasks: closing the "mind's eye".

It is a familiar experience that we tend to close our eyes or divert our gaze when concentrating attention on cognitively demanding tasks. We report on the brain activity correlates of directing attention away from potentially competing visual processing and toward processing in another sensory modality. Results are reported from a series of positron-emission tomography studies of the human brain engaged in somatosensory tasks, in both "eyes open" and "eyes closed" conditions. During these tasks, there was a significant decrease in the regional cerebral blood flow in the visual cortex, which occurred irrespective of whether subjects had to close their eyes or were instructed to keep their eyes open. These task-related deactivations of the association areas belonging to the nonrelevant sensory modality were interpreted as being due to decreased metabolic activity. Previous research has clearly demonstrated selective activation of cortical regions involved in attention-demanding modality-specific tasks; however, the other side of this story appears to be one of selective deactivation of unattended areas.

Brain↗
PubMed1994

Activity in the human primary motor cortex related to ipsilateral hand movements.

In two studies with positron emission tomography (PET), we found that somatosensory discrimination of length activated the ipsilateral MI, but somatosensory discrimination of shape did not. This occurred even though both tasks required the exclusive use of distal finger and hand movements which were also very similar in both tasks. The activation of the ipsilateral MI was correlated with activations of the premotor cortex in the other hemisphere, the prefrontal cortex and the posterior cingulate cortex, indicating that these areas together with the ipsilateral MI constitute a task-related active network.

Discrimination Learning↗
PubMed1994

A PET study of somatosensory discrimination in man. microgeometry versus macrogeometry.

The regional cerebral blood flow (rCBF) was measured with 15O-butanol and positron emission tomography (PET) in 10 healthy subjects in order to compare cerebral activation involved in the somatosensory discrimination of microgeometric features with cerebral activation associated with the discrimination of macrogeometric features. Subjects performed two-alternative forced choice (2-AFC) discriminations of pairs of stimuli from a series of quantified standardized stimuli that differed in roughness (microgeometry), and a separate 2-AFC task of smooth tactile stimuli that differed in length (macrogeometry). Results are presented from three conditions: (1) a roughness discrimination task; (2) a length discrimination task; and (3) a control trial in which subjects were required to reproduce similar exploratory finger movements only, but without a specific stimulus to feel. Mean subtraction images were computed using the computerized adjustable brain atlas of Greitz et al. (1991, J. Comput. Assisted Tomogr., 15, 26-38) and areas of significant blood flow change were identified. Both the roughness and the length discrimination tasks activated overlapping cortical fields contralaterally in the anterior and posterior lip of the postcentral sulcus. However, in the length discrimination, activation of the posterior lip of the postcentral sulcus extended deeper into the sulcus and there was also a separate additional area of activation in the anterior part of the precentral gyrus. Furthermore, the length discrimination task activated fields in the overt part of the supramarginal gyrus bilaterally as well as fields in the angular gyrus bilaterally. Thus roughness discrimination uses only a subset of the cortical regions that are needed for the recovery of length information, which requires more extensive somatosensory processing. This finding may be partly explained in that length perception needs both edge detection of the stimuli used, as well as integrated information of surface length and velocity, which is not necessary for roughness perception. Specific differences in the acquisition of necessary tactile information between the two discrimination tasks was reflected in different sampling strategies.

Adult↗
PubMed1994

Fields in human motor areas involved in preparation for reaching, actual reaching, and visuomotor learning: a positron emission tomography study.

The purpose of this study was to examine the functional organization of motor cortices in the human brain involved in reaching and visuomotor learning. All subjects were asked to learn the positions of seven circular targets projected on a screen. Each time the targets were turned off, they were required to close their eyes and keep them closed, and, after a delay, to point to the center of the targets in a prescribed order using their right hand. The regional cerebral blood flow (rCBF) was measured with 15O-butanol and positron emission tomography in 20 subjects during a rest state, an initial learning stage, and a later learning stage. Ten subjects constituted the reaching group in which rCBF was measured during actual reaching; the 10 other subjects constituted the preparation group in which rCBF was measured in the delay period between target exposure and actual reaching. Individual subtraction images (each stage minus rest) were calculated and transformed into a standard size and shape brain image by the adjustable computerized brain atlas and averaged, after which significant changes of rCBF were identified. In all reaching and preparation for reaching phases, cortical fields were activated in the left primary motor area (M1) and the left premotor area (PMA). Within M1, fields active in the delay phases were adjacent to the fields active only during actual reaching movements. During the course of learning, additional fields of activity appeared in both M1 and PMA. The results indicate that three types of fields occur in M1 and PMA: (1) fields directly engaged in the efferent control of peripheral muscle contraction, (2) fields engaged in preparatory activity for reaching, and (3) fields appearing after learning of the task has taken place.

Adult↗
PubMed1991

Effect of high plasma dexamethasone levels on DST sensitivity: dose-response study in depressed patients and controls.

The aim of this study was to examine cortisol dynamics over a range of plasma dexamethasone (DEX) levels using a two-dose dexamethasone suppression test (DST). Two doses of DEX (0.5 and 1.5 mg) were administered in a randomized crossover design to 29 inpatients with major depression and 26 controls to identify the upper range of plasma DEX levels that would allow reliable interpretation of DST results. It was hypothesized that due to inappropriately high plasma DEX levels following 1.5 mg, several depressed patients would switch from suppressors after the 1.5 mg dose to nonsuppressors after 0.5 mg. In contrast, the nondepressed controls with high DEX levels following 1.5 mg would remain suppressors after the lower dose. Fourteen patients were identified as having high 4 p.m. DEX levels (greater than 4 nmol/l) after the 1.5 mg DST. Cortisol was suppressed in all of the subjects with high DEX levels. After 0.5 mg, five of the eight depressed patients with high DEX levels switched to nonsuppressors. In contrast, all six controls with high DEX levels remained suppressors. These results indicate that patients with high DEX levels after a 1 mg DST should be retested with a lower dose. This strategy enhances the sensitivity of the DST without loss of specificity.

Depressive Disorder↗
PubMed1989

The plasma dexamethasone window: evidence supporting its usefulness to validate dexamethasone suppression test results.

Two doses of dexamethasone (DEX) (0.5 and 1.0 mg) were administered in a randomized crossover design to 31 patients with major depression, 9 healthy controls, and 14 nondepressed psychiatric patients. Using this modified Dexamethasone Suppression Test (DST), minimum DEX levels of 6 nmol/liter at 8:00 AM and 2.0 nmol/liter at 4:00 PM were required to achieve reliable suppression of cortisol in healthy controls and nondepressed psychiatric patients. Failure to achieve these minimum plasma DEX levels was associated with similar rates of nonsuppression in both depressed and nondepressed patients, thereby reducing the specificity of the DST. Conversely, high DEX levels greater than 13 nmol/liter at 8:00 AM or 4.0 nmol/liter at 4:00 PM were associated with abnormal "suppressibility" in depressed patients, thereby reducing the sensitivity of the test. Controlling for plasma DEX concentrations by selecting a test result that fell within a plasma DEX window at 8:00 AM and 4:00 PM increased the sensitivity and specificity of the DST. Significant differences in plasma DEX between suppressors and nonsuppressors were no longer evident when comparing patients with adequate DEX levels, thus ensuring that cortisol escape reflected HPA axis changes associated with depression and not peripheral mechanisms responsible for the availability of DEX. These results suggest that the clinical utility of the DST would be significantly enhanced by extending the standard 1.0-mg DST and retesting those patients with levels outside the DEX window with a higher or lower dose. The data also indicate that the measurement of plasma DEX is essential to validly interpret DST status and highlight the need to standardize DEX assays to compare DST results between research centers.

Adult↗
PubMed1988

Anorexia nervosa in a woman totally blind since birth.

A 19-year-old woman, blind since birth, lost 26 kg over a 7-month period. This was achieved by restriction of food intake, excessive exercise, laxative abuse, and self-induced vomiting. Although the visual experience is often believed to be an integral component of body-shape perception and the overvaluation of thinness in contemporary society, it does not preclude the development of anorexia nervosa.

Adult↗
PubMed1986

Growth hormone and cortisol secretion after oral clonidine in healthy adults.

The purpose of this study was to evaluate oral clonidine for testing growth hormone (GH) responsiveness in healthy adults. Oral clonidine (0.15 mg) produced a satisfactory GH response (greater than 4 ng/ml from basal) in eight out of 10 subjects, which is similar to rates reported after an equivalent intravenous dose. Elevated GH levels at baseline occurred in four out of five female subjects; this did not affect the clonidine-induced GH release. There were no significant differences at any time point in plasma prolactin or cortisol levels following clonidine, compared to placebo controls. Adequate plasma clonidine levels (greater than 0.4 ng/ml) were achieved in all subjects, with corresponding reductions in mean arterial blood pressure, but with only minimal adverse effects. Results from this study indicate that oral clonidine is a reliable method for testing GH responsiveness in adult subjects.

Administration, Oral↗
PubMedDate not supplied

Functional anatomy of reaching and visuomotor learning: a positron emission tomography study.

The purpose of this study was to identify the functional cortical fields involved in reaching for targets in extrapersonal space, and to identify the specific fields representing visual target information in long-term memory. Ten healthy subjects were asked to learn the positions of seven circular targets that were repeatedly projected on a screen. The regional cerebral blood flow was measured with positron emission tomography during a rest state, at an early learning stage, at a later learning stage, and finally at 30 min after the course of learning had been completed. Mean rCBF change images for each task minus rest were calculated and fields of significant rCBF changes were identified. In all three task states, cortical fields were consistently activated in the left motor and premotor areas, the posterior part of the superior parietal lobule, and the right angular gyrus. When learning of the target positions had been achieved, additional fields appeared bilaterally in the posterior part of the superior parietal lobule, the right superior occipital gyrus, the left motor and premotor areas, the medial aspect of the superior frontal gyrus, the postcentral gyrus, the superior part of the cuneus, the inferior part of the angular gyrus, and the anterior part of the insula. The results indicate that there are at least two different types of functional fields in the posterior part of the superior parietal lobule; one is active during reaching for the targets when guided by internal representations of target positions; the other likely represents the storage sites of visual target information that is addressed in long-term memory.

Adolescent↗
PubMedDate not supplied

Bilateral activation of fronto-parietal networks by incrementing demand in a working memory task.

Working memory (WM) is known to activate the prefrontal cortex. In the present study we hypothesized that when additional contingencies are added to the instruction of a WM task, this would increase the WM load and result in the activation of additional prefrontal areas. With positron emission tomography we measured regional cerebral blood flow in nine subjects performing a control task and two delayed matching to sample tasks, in which the subjects were matching colours and patterns to a reference picture. The second of the two delayed matching tasks had a more complex instruction than the first, with additional contingencies of how to alternate between the matching of colours and patterns. This task thus required the subjects not only to remember a stimulus to match but also to perform this matching according to a specified plan. Both delayed matching tasks activated cortical fields in the middle frontal gyrus, the frontal operculum, upper cingulate gyrus, inferior parietal cortex and cortex lining the intraparietal sulcus, all in the left hemisphere. When alternated delayed matching was compared to simple delayed matching, increases were located in the right superior and middle frontal gyrus and the right anterior inferior parietal cortex. The increased demand during alternated matching thus resulted in bilateral activation of both dorsolateral prefrontal and inferior parietal cortex. The area in the inferior parietal cortex has previously been coactivated with the dorsolateral prefrontal cortex in several WM tasks, irrespective of the sensory modality of the stimuli, and during tasks involving planning.

Adult↗