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Simon P Kelly

Publications and source records attributed to Simon P Kelly.

16 recordsLinked to original sources
PubMed2006

Response variability in attention deficit hyperactivity disorder: evidence for neuropsychological heterogeneity.

Response time (RT) variability is a common finding in ADHD research. RT variability may reflect frontal cortex function and may be related to deficits in sustained attention. The existence of a sustained attention deficit in ADHD has been debated, largely because of inconsistent evidence of time-on-task effects. A fixed-sequence Sustained Attention to Response Task (SART) was given to 29 control, 39 unimpaired and 24 impaired-ADHD children (impairment defined by the number of commission errors). The response time data were analysed using the Fast Fourier Transform, to define the fast-frequency and slow-frequency contributions to overall response variability. The impaired-ADHD group progressively slowed in RT over the course of the 5.5min task, as reflected in this group's greater slow-frequency variability. The fast-frequency trial-to-trial variability was also significantly greater, but did not differentially worsen over the course of the task. The higher error rates of the impaired-ADHD group did not become differentially greater over the length of the task. The progressive slowing in mean RT over the course of the task may relate to a deficit in arousal in the impaired-ADHD group. The consistently poor performance in fast-frequency variability and error rates may be due to difficulties in sustained attention that fluctuate on a trial-to-trial basis.

Adolescent↗
PubMed2006

Increases in alpha oscillatory power reflect an active retinotopic mechanism for distracter suppression during sustained visuospatial attention.

Human electrophysiological (EEG) studies have demonstrated the involvement of alpha band (8- to 14-Hz) oscillations in the anticipatory biasing of attention. In the context of visual spatial attention within bilateral stimulus arrays, alpha has exhibited greater amplitude over parietooccipital cortex contralateral to the hemifield required to be ignored, relative to that measured when the same hemifield is to be attended. Whether this differential effect arises solely from alpha desynchronization (decreases) over the "attending" hemisphere, from synchronization (increases) over the "ignoring" hemisphere, or both, has not been fully resolved. This is because of the confounding effect of externally evoked desynchronization that occurs involuntarily in response to visual cues. Here, bilateral flickering stimuli were presented simultaneously and continuously over entire trial blocks, such that externally evoked alpha desynchronization is equated in precue baseline and postcue intervals. Equivalent random letter sequences were superimposed on the left and right flicker stimuli. Subjects were required to count the presentations of the target letter "X" at the cued hemifield over an 8-s period and ignore the sequence in the opposite hemifield. The data showed significant increases in alpha power over the ignoring hemisphere relative to the precue baseline, observable for both cue directions. A strong attentional bias necessitated by the subjective difficulty in gating the distracting letter sequence is reflected in a large effect size of 2.1 (eta2 = 0.82), measured from the attention x hemisphere interaction. This strongly suggests that alpha synchronization reflects an active attentional suppression mechanism, rather than a passive one reflecting "idling" circuits.

Adult↗
PubMed2006

Early visual sensory deficits as endophenotypes for schizophrenia: high-density electrical mapping in clinically unaffected first-degree relatives.

CONTEXT: The imperative to establish so-called endophenotypes-quantifiable measures of risk for neurological dysfunction-is a growing focus of research in schizophrenia. Electrophysiological markers of sensory processing, observable in human event-related potentials, hold great promise in this regard, lying closer to underlying physiology than descriptive clinical diagnostic tests. OBJECTIVE: Early visual processing deficits, as measured by clear amplitude reductions in the occipital P1 component of the visual event-related potential, have been repeatedly demonstrated in patients with schizophrenia. However, before P1 amplitude may be considered as an endophenotypic marker for schizophrenia, it is necessary to establish its sensitivity to genetic liability. DESIGN, SETTING, AND PARTICIPANTS: Event-related potential responses to simple visual isolated-check stimuli were examined in 25 clinically unaffected first-degree relatives of patients with schizophrenia and 15 DSM-IV-diagnosed schizophrenia probands and compared with responses from 26 healthy, age-matched control subjects. Using high-density electrical scalp recordings, between-groups analysis assessed the integrity of the visual P1 component across the 3 groups. The study was conducted at St Vincent's Psychiatric Hospital in Fairview, Dublin, Ireland. RESULTS: Substantially reduced P1 amplitude was demonstrated in both relatives and probands compared with controls with topographical mapping and inverse source analysis localizing this deficit largely to midline regions in early visual sensory cortices and regions of the dorsal visual stream. Additional later differences between these groups, where the relatives actually show larger amplitude responses, may point toward compensatory mechanisms at play in relatives. CONCLUSIONS: Our findings demonstrate a deficit in early visual processing in clinically unaffected first-degree relatives of patients with schizophrenia, providing evidence that this deficit may serve as a genetic marker for this disorder. The efficacy of using P1 amplitude as an endophenotype is underscored by the observation of a large effect size (d=0.9) over scalp sites where the deficit was maximal.

Adolescent↗
PubMed2005

Smoking and cataract: review of causal association.

Several risk factors for the development of cataract have been identified. This review evaluates epidemiologic literature that has examined tobacco smoking as a risk factor for cataract formation using established causality criteria. Twenty-seven studies were included in this review. Evidence suggests that smoking has a 3-fold increase on the risk for incident nuclear cataract development. There was also evidence of dose response, temporal relationship, and reversibility of effect. There was limited evidence of an association between smoking and posterior subcapsular cataract, but little or no association with cortical cataract. Thus, the literature review indicated a strong association between smoking and the development of cataract, particularly nuclear cataract. The association fulfills the established criteria for causality. The association between smoking and other types of cataract is less distinct and requires further evaluation.

Cataract↗
PubMed2005

Neurophysiological markers of alert responding during goal-directed behavior: a high-density electrical mapping study.

The ability to dynamically modulate the intensity of sustained attention (i.e., alertness) is an essential component of the human executive control system, allowing us to function purposefully in accordance with our goals. In this study we examine high-density ERP markers of alert responding during the fixed sequence sustained attention to response task (SART(fixed)). This paradigm has proven to be a sensitive clinical metric in patient populations with deficits in their ability to sustain attention (e.g., attention deficit hyperactivity disorder). In this task subjects withhold a button press to an infrequent no-go target ('3') embedded within a predictable sequence of numbers ('1' to '9'). Our data reveal a complex pattern of effects across the trial sequence of the SART, with clear contributions from frontal and parietal cortices to sustained attentional performance. Over occipito-parietal regions, early visual attention processes were increased during trial 2 (i.e., trial in which the digit '2' was presented) and trial 3, giving rise to the so-called selection negativity (SN). Two prominent late components were manifest during trial 2: LP1 (550-800 ms) and LP2 (850-1150 ms) over occipito-parietal and central sites. We interpret the LP1 component on trial 2 as reflecting retrieval of the task goal and the subsequent LP2 as reflecting competition between the currently relevant go response and the subsequent no-go response. On trial 3, an enhanced "no-go N2" (250-450 ms) was seen fronto-centrally in the absence of the "no-go P3" that typically follows. Fronto-polar activity was also seen across all trials and may be indicative of subgoal processes to integrate the association between stimulus and goal. Prior to a lapse of attention (i.e., failure to inhibit a response to "3") the LP1 was significantly attenuated on the preceding trial 2 indicating a failure of anticipatory goal-directed processing. The results are discussed in terms of models of sustained attention involving frontal and parietal cortices.

Adolescent↗
PubMed2005

Visual spatial attention control in an independent brain-computer interface.

This paper presents a novel brain computer interface (BCI) design employing visual evoked potential (VEP) modulations in a paradigm involving no dependency on peripheral muscles or nerves. The system utilizes electrophysiological correlates of visual spatial attention mechanisms, the self-regulation of which is naturally developed through continuous application in everyday life. An interface involving real-time biofeedback is described, demonstrating reduced training time in comparison to existing BCIs based on self-regulation paradigms. Subjects were cued to covertly attend to a sequence of letters superimposed on a flicker stimulus in one visual field while ignoring a similar stimulus of a different flicker frequency in the opposite visual field. Classification of left/right spatial attention is achieved by extracting steady-state visual evoked potentials (SSVEPs) elicited by the stimuli. Six out of eleven physically and neurologically healthy subjects demonstrate reliable control in binary decision-making, achieving at least 75% correct selections in at least one of only five sessions, each of approximately 12-min duration. The highest-performing subject achieved over 90% correct selections in each of four sessions. This independent BCI may provide a new method of real-time interaction for those with little or no peripheral control, with the added advantage of requiring only brief training.

Adult↗
PubMed2005

A parametric feature extraction and classification strategy for brain-computer interfacing.

Parametric modeling strategies are explored in conjunction with linear discriminant analysis for use in an electroencephalogram (EEG)-based brain-computer interface (BCI). A left/right self-paced typing exercise is analyzed by extending the usual autoregressive (AR) model for EEG feature extraction with an AR with exogenous input (ARX) model for combined filtering and feature extraction. The ensemble averaged Bereitschafts potential (an event related potential preceding the onset of movement) forms the exogenous signal input to the ARX model. Based on trials with six subjects, the ARX case of modeling both the signal and noise was found to be considerably more effective than modeling the noise alone (common in BCI systems) with the AR method yielding a classification accuracy of 52.8+/-4.8% and the ARX method an accuracy of 79.1+/-3.9 % across subjects. The results suggest a role for ARX-based feature extraction in BCIs based on evoked and event-related potentials.

Adult↗
PubMed2005

Visual spatial attention tracking using high-density SSVEP data for independent brain-computer communication.

The steady-state visual evoked potential (SSVEP) has been employed successfully in brain-computer interface (BCI) research, but its use in a design entirely independent of eye movement has until recently not been reported. This paper presents strong evidence suggesting that the SSVEP can be used as an electrophysiological correlate of visual spatial attention that may be harnessed on its own or in conjunction with other correlates to achieve control in an independent BCI. In this study, 64-channel electroencephalography data were recorded from subjects who covertly attended to one of two bilateral flicker stimuli with superimposed letter sequences. Offline classification of left/right spatial attention was attempted by extracting SSVEPs at optimal channels selected for each subject on the basis of the scalp distribution of SSVEP magnitudes. This yielded an average accuracy of approximately 71% across ten subjects (highest 86%) comparable across two separate cases in which flicker frequencies were set within and outside the alpha range respectively. Further, combining SSVEP features with attention-dependent parieto-occipital alpha band modulations resulted in an average accuracy of 79% (highest 87%).

Adult↗
PubMed2004

Behavioural and physiological impairments of sustained attention after traumatic brain injury.

Sustaining attention under conditions of low external demand taxes our ability to stay on task and to avoid more appealing trains of thought or environmental distractions. By contrast, a stimulating, novel environment engages attention far more freely without the subjective feeling of having to override monotony. Our ability to maintain a goal-directed focus without support from the environment requires the endogenous control of behaviour. This control can be modulated by fronto-parietal circuits and this ability is compromised following traumatic brain injury (TBI) leading to increased lapses of attention. In this paper, we further explore a laboratory paradigm that we argue is particularly sensitive to sustained attention as opposed to other aspects of attentional control involving the selection and management of goals in working memory. The paradigm (fixed sequence Sustained Attention to Response Task--SARTfixed) involves withholding a key press to an infrequent no-go target embedded within a predictable sequence of numbers. We demonstrate that TBI patients in this study make disproportionately more errors than controls on this task. An analysis of response times (RTs) and EEG alpha power across the task demonstrates group differences preceding the critical no-go trial. Controls demonstrate a lengthening of RTs accompanied by desynchronization of power within the alpha band (approximately 10 Hz) preceding the no-go trial. Conversely, the TBI group showed a shortening of RTs during this period with no evidence of alpha desynchronization. These findings suggest that TBI patients may have dysfunctional alpha generators as a consequence of their injury that impairs endogenous control during the task.

Adult↗