Simon P Kelly

CUNY Graduate Center, New York City, New York, United States

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Publications (56)217.13 Total impact

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    ABSTRACT: Surround suppression is a well-known example of contextual interaction in visual cortical neurophysiology, whereby the neural response to a stimulus presented within a neuron's classical receptive field is suppressed by surrounding stimuli. Human psychophysical reports present an obvious analog to the effects seen at the single-neuron level: stimuli are perceived as lower-contrast when embedded in a surround. Here we report on a visual paradigm that provides relatively direct, straight-forward indices of surround suppression in human electrophysiology, enabling us to reproduce several well-known neurophysiological and psychophysical effects, and to conduct new analyses of temporal trends and retinal location effects. Steady-state visual evoked potentials (SSVEP) elicited by flickering "foreground" stimuli were measured in the context of various static surround patterns. Early visual cortex geometry and retinotopic organization were exploited to enhance SSVEP amplitude. The foreground response was strongly suppressed as a monotonic function of surround contrast. Further, suppression was stronger for surrounds of matching orientation than orthogonally-oriented ones, and stronger at peripheral than foveal locations. These patterns were reproduced in psychophysical reports of perceived contrast, and peripheral electrophysiological suppression effects correlated with psychophysical effects across subjects. Temporal analysis of SSVEP amplitude revealed short-term contrast adaptation effects that caused the foreground signal to either fall or grow over time depending on the relative contrast of the surround, consistent with stronger adaptation of the suppressive drive. This electrophysiology paradigm has clinical potential in indexing not just visual deficits but possibly gain control deficits expressed more widely in the disordered brain. Copyright © 2014, Journal of Neurophysiology.
    Journal of neurophysiology. 11/2014;
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    ABSTRACT: Children born with very low birth weight perform poorly on executive function and attention measures. Any difficulties with sustained attention may underpin impairments in performance on tasks measuring higher order cognitive control. Previous sustained attention research in very low birth weight cohorts has used tasks that involve arousing stimuli, potentially spoiling the measure of sustained attention. The aim of this study was to compare the performance of very low birth weight and normal birth weight children on a well-controlled task of sustained attention. The Fixed and Random versions of the Sustained Attention to Response Task were given to 17 very low birth weight and 18 normal birth weight children. The very low birth weight group performed the Fixed and Random Sustained Attention to Response Tasks in a similar manner as the normal birth weight group on all measures except for the omission error and Slow Frequency Area under the Spectra variables on the Fixed Sustained Attention to Response Task. These measures index lapses in sustained attention that may be underpinned by declining arousal. The very low birth weight group showed no response inhibition deficits. Omission errors and slow-timescale response-time variability on predictable tasks may thus present sensitive indices of difficulties with sustained attention and arousal associated with premature birth and low birth weight.
    Frontiers in Human Neuroscience 10/2014; · 2.91 Impact Factor
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    ABSTRACT: Healthy subjects typically exhibit a subtle bias of visuospatial attention favouring left space that is commonly termed 'pseudoneglect'. This bias is attenuated, or shifted rightwards, with decreasing alertness over time, consistent with theoretical models proposing that pseudoneglect is a result of the right hemisphere's dominance in regulating attention. Although this 'time-on-task effect' for spatial bias is observed when averaging across whole samples of healthy participants, Benwell, Thut, Learmonth & Harvey (2013b) recently presented evidence that the direction and magnitude of bias exhibited by the participant early in the task (left biased, no bias, or right biased) was a stable trait that predicted the direction of the subsequent time-on-task shift in spatial bias. That is, the spatial bias of participants who were initially left biased shifted in a rightward direction with time, whereas that of participants who were initially right biased shifted in a leftward direction. If valid, the data of Benwell et al. are potentially important and may demand a re-evaluation of current models of the neural networks governing spatial attention. Here we use two novel spatial attention tasks in an attempt to confirm the results of Benwell et al.. We show that rather than being indicative of true participant subtypes, these data patterns are likely driven, at least in part, by 'regression towards the mean' arising from the analysis method employed. Although evidence supports the contention that trait-like individual differences in spatial bias exist within the healthy population, no clear evidence is yet available for participant/observer subtypes in the direction of time-on-task shift in spatial biases.
    Neuropsychologia 09/2014; · 3.48 Impact Factor
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    Simon P. Kelly, Redmond G. O'Connell
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    ABSTRACT: In the last two decades, animal neurophysiology research has made great strides towards explaining how the brain can enable adaptive action in the face of noisy sensory information. In particular, this work has identified neural signals that perform the role of a 'decision variable' which integrates sensory information in favor of a particular outcome up to an action-triggering threshold, consistent with long-standing predictions from mathematical psychology. This has provoked an intensive search for similar neural processes at work in the human brain. In this paper we review the progress that has been made in tracing the dynamics of perceptual decision formation in humans using functional imaging and electrophysiology. We highlight some of the limitations that non-invasive recording techniques place on our ability to make definitive judgments regarding the role that specific signals play in decision making. Finally, we provide an overview of our own work in this area which has focussed on two perceptual tasks – intensity change detection and motion discrimination – performed under continuous-monitoring conditions, and highlight the insights gained thus far. We show that through simple paradigm considerations such as avoiding sudden intensity transients at evidence onset, a neural instantiation of the theoretical decision variable can be directly traced in the form of a centro-parietal positivity (CPP) in the standard event-related potential (ERP). We recapitulate evidence for the domain-general nature of the CPP process, being divorced from the sensory and motor requirements of the task, and re-plot data of both tasks highlighting this aspect as well as its relationship to decision outcome and reaction time. We discuss the implications of these findings for mechanistically principled research on normal and abnormal decision making in humans.
    Journal of Physiology-Paris 09/2014; · 0.82 Impact Factor
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    Simon P Kelly, Redmond G O'Connell
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    ABSTRACT: We frequently need to make timely decisions based on sensory evidence that is weak, ambiguous, or noisy resulting from conditions in the external environment (e.g., a cluttered visual scene) or within the brain itself (e.g., inattention, neural noise). Here we examine how externally and internally driven variations in the quality of sensory evidence affect the build-to-threshold dynamics of a supramodal "decision variable" signal and, hence, the timing and accuracy of decision reports in humans. Observers performed a continuous-monitoring version of the prototypical two-alternative dot-motion discrimination task, which is known to strongly benefit from sequential sampling and temporal accumulation of evidence. A centroparietal positive potential (CPP), which we previously established as a supramodal decision signal based on its invariance to motor or sensory parameters, exhibited two key identifying properties associated with the "decision variable" long described in sequential sampling models: (1) its buildup rate systematically scaled with sensory evidence strength across four levels of motion coherence, consistent with temporal integration; and (2) its amplitude reached a stereotyped level at the moment of perceptual report executions, consistent with a boundary-crossing stopping criterion. The buildup rate of the CPP also strongly predicted reaction time within coherence levels (i.e., independent of physical evidence strength), and this endogenous variation was linked with attentional fluctuations indexed by the level of parieto-occipital α-band activity preceding target onset. In tandem with the CPP, build-to-threshold dynamics were also observed in an effector-selective motor preparation signal; however, the buildup of this motor-specific process significantly lagged that of the supramodal process.
    Journal of Neuroscience 12/2013; 33(50):19434-19441. · 6.91 Impact Factor
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    ABSTRACT: Here we summarize the points raised in our dialogue with Ales and colleagues on the cortical generators of the early visual evoked potential (VEP), and offer observations on the results of additional simulations that were run in response to our original comment. For small stimuli placed at locations in the upper and lower visual field for which the human VEP has been well characterized, simulated scalp projections of each of the visual areas V1, V2 and V3 invert in polarity. However, the empirically measured, earliest VEP component, "C1," matches the simulated V1 generators in terms of polarity and topography, but not the simulated V2 and V3 generators. We thus conclude that, 1) consistent with the title of Ales et al (2010a), polarity inversion on its own is not a sufficient criterion for inferring neuroelectric sources in primary visual cortex; but 2) inconsistent with additional claims made in Ales et al (2010a), the simulated topographies provide additional evidence for - not against - the tenet that the C1 component is generated in V1.
    NeuroImage 06/2013; · 6.25 Impact Factor
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    ABSTRACT: Objective. The steady-state visual evoked potential (SSVEP) is an electroencephalographic response to flickering stimuli generated partly in primary visual area V1. The typical 'cruciform' geometry and retinotopic organization of V1 is such that certain neighboring visual regions project to neighboring cortical regions of opposite orientation. Here, we explored ways to exploit this organization in order to boost scalp SSVEP amplitude via oscillatory summation. Approach. We manipulated flicker-phase offsets among angular segments of a large annular stimulus in three ways, and compared the resultant SSVEP power to a conventional condition with no temporal phase offsets. (1) We divided the annulus into standard octants for all subjects, and flickered upper horizontal octants with opposite temporal phase to the lower horizontal ones, and left vertical octants opposite to the right vertical ones; (2) we individually adjusted the boundaries between the eight contiguous segments of the standard octants condition to coincide with cruciform-consistent, early-latency topographical shifts in pattern-pulse multifocal visual-evoked potentials (PPMVEP) derived for each of 32 equal-sized segments; (3) we assigned phase offsets to stimulus segments following an automatic algorithm based on the relative amplitudes of vertically- and horizontally-oriented PPMVEP components. Main results. The three flicker-phase manipulations resulted in a significant enhancement of normalized SSVEP power of (1) 202%, (2) 383%, and (3) 300%, respectively. Significance. We have thus demonstrated a means to obtain more reliable measures of visual evoked activity purely through consideration of cortical geometry. This principle stands to impact both basic and clinical research using SSVEPs.
    Journal of Neural Engineering 04/2013; 10(3):036003. · 3.28 Impact Factor
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    ABSTRACT: In theoretical accounts of perceptual decision-making, a decision variable integrates noisy sensory evidence and determines action through a boundary-crossing criterion. Signals bearing these very properties have been characterized in single neurons in monkeys, but have yet to be directly identified in humans. Using a gradual target detection task, we isolated a freely evolving decision variable signal in human subjects that exhibited every aspect of the dynamics observed in its single-neuron counterparts. This signal could be continuously tracked in parallel with fully dissociable sensory encoding and motor preparation signals, and could be systematically perturbed mid-flight during decision formation. Furthermore, we found that the signal was completely domain general: it exhibited the same decision-predictive dynamics regardless of sensory modality and stimulus features and tracked cumulative evidence even in the absence of overt action. These findings provide a uniquely clear view on the neural determinants of simple perceptual decisions in humans.
    Nature Neuroscience 10/2012; · 15.25 Impact Factor
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    ABSTRACT: Deep brain stimulation (DBS) is highly effective neurosurgery for idiopathic Parkinson's disease (IPD), essential tremor (ET) and primary dystonia. DBS involves stereotactic surgical implantation of a battery-operated stimulator into deep brain nuclei. Irish patients are referred abroad for DBS and have to travel repeatedly for pre and post-operative care resulting in stress, anxiety and hardship. Safe pre and post-operative care of these complex, ageing patients is compromised by the absence of a DBS service in Ireland. Moreover, both DBS surgery and the subsequent post-operative care abroad incurs substantial cost to the state. The Dublin Neurological Institute at the Mater Misericordiae University Hospital (DNI) is a non-profit institute for the care of patients with neurological diseases. The DNI developed, in collaboration with the Mater Private Hospital (MPH) and the Walton Centre, Liverpool, a DBS programme in 2008/2009. We performed DBS at the Mater Campus on three carefully selected patients from a cohort of movement disorder patients attending the DNI and continue to provide pre-operative assessment and post operative care for patients following DBS in Ireland and abroad.
    Irish medical journal 09/2012; 105(8):278-80. · 0.51 Impact Factor
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    E C Lalor, S P Kelly, J J Foxe
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    ABSTRACT: The VESPA (visual-evoked spread spectrum analysis) method derives an impulse response function of the visual system from scalp electroencephalographic (EEG) data using the controlled modulation of some feature of a visual stimulus. Recent research using VESPA responses to modulations of stimulus contrast has provided new insights into both early visual attention mechanisms and the specificity of visual-processing deficits in schizophrenia. To allow a fuller interpretation of these and future findings, it is necessary to further characterize the VESPA in terms of its underlying cortical generators. To that end, we here examine spatio-temporal variations in the components of the VESPA as a function of stimulus location. We found that the first two VESPA components (C1/P1) each have a posterior dorsal midline focus and reverse in polarity across the horizontal meridian, consistent with retinotopic projections to calcarine cortex (V1) for the stimulus locations tested. Furthermore, the focal scalp topography of the VESPA was strikingly constant across the entire C1-P1 timeframe (50-120 ms) for each stimulus location, with negligible global scalp activity visible at the zero-crossing dividing the two. This indicates a common focal source underpinning both components, which was further supported by a significant correlation between C1 and P1 amplitudes across subjects (r=0.54; p<0.05). These results, along with factors implicit in the method of derivation of the contrast-VESPA, lead us to conclude that these responses are dominated by activity from striate cortex. We discuss the implications of this finding for previous and future research using the VESPA.
    Neuroscience 06/2012; 218:226-34. · 3.12 Impact Factor
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    ABSTRACT: The VESPA (visual-evoked spread spectrum analysis) method estimates the impulse response of the visual system using a continuously varying stimulus. It has been used recently to address both basic cognitive and neurophysiologic questions as well as those surrounding clinical populations. Although the components of the average VESPA response are highly reminiscent of the early components of the visual-evoked potential (VEP) when measured over midline occipital locations, the two responses are acquired in different ways and, thus, they cannot be regarded as being equivalent. To further characterize the relationship between the VESPA and the VEP and the generative mechanisms underlying them, we recorded EEG from 31 subjects in response to checkerboard-based VEP and VESPA stimuli. We found that, across subjects, the amplitudes of the VEP C1 component and the VESPA C1 component were highly correlated, whereas the VEP P1 and the VESPA P1 bore no statistical relationship. Furthermore, we found that C1 and P1 amplitudes were significantly correlated in the VESPA but not in the VEP. We believe these findings point to the presence of common generators underlying the VESPA C1 and the VEP C1. We argue further that the VESPA P1, in light of its strong relationship to the VESPA C1, likely reflects further activation of the same cortical generators. Given the lack of correlation between the VEP P1 and each of these three other components, it is likely that the underlying generators of this particular component are more varied and widespread, as suggested previously. We discuss the implications of these relationships for basic and clinical research using the VESPA and for the assessment of additive-evoked versus phase-reset contributions to the VEP.
    Experimental Brain Research 05/2012; 220(2):191-9. · 2.22 Impact Factor
  • Neurology 04/2012; 78(Meeting Abstracts 1):P01.139-P01.139. · 8.30 Impact Factor
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    ABSTRACT: Recently, a forward-model simulation study demonstrated that the upper and lower visual field projections to extrastriate visual cortical areas V2 and V3 have polarity-inverted electrical scalp projections, a property famously associated with potentials generated in primary visual cortex (V1) (Ales et al., 2010a). The authors use this finding, along with other findings from fMRI-constrained source modeling, to argue that the initial component "C1" of the human visual evoked potential may not be generated in V1 as has been widely believed, but may instead come from V2/V3. Here, we examine the validity of this claim with respect to the full set of anatomical and electrophysiological factors comprising the unabridged "cruciform" model linking C1 to V1. We find that the simulations in their current form do not present a valid test of the model, nor are their results inconsistent with it. We also review non-human primate neurophysiology findings that support the C1-V1 principle, and that can and should be taken into account in assessing the validity of constrained source models of human EEG in general.
    NeuroImage 04/2012; · 6.25 Impact Factor
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    ABSTRACT: Caffeine and L-theanine, both naturally occurring in tea, affect the ability to make rapid phasic deployments of attention to locations in space as reflected in behavioural performance and alpha-band oscillatory brain activity (8-14 Hz). However, surprisingly little is known about how these compounds affect an aspect of attention that has been more popularly associated with tea, namely vigilant attention: the ability to maintain focus on monotonous tasks over protracted time-periods. Twenty-seven participants performed the Sustained Attention to Response Task (SART) over a two-hour session on each of four days, on which they were administered caffeine (50 mg), theanine (100 mg), the combination, or placebo in a double-blind, randomized, cross-over fashion. Concurrently, we recorded oscillatory brain activity through high-density electroencephalography (EEG). We asked whether either compound alone, or both in combination, would affect performance of the task in terms of reduced error rates over time, and whether changes in alpha-band activity would show a relationship to such changes in performance. When treated with placebo, participants showed a rise in error rates, a pattern that is commonly observed with increasing time-on-task, whereas after caffeine and theanine ingestion, error rates were significantly reduced. The combined treatment did not confer any additional benefits over either compound alone, suggesting that the individual compounds may confer maximal benefits at the dosages employed. Alpha-band oscillatory activity was significantly reduced on ingestion of caffeine, particularly in the first hour. This effect was not changed by addition of theanine in the combined treatment. Theanine alone did not affect alpha-band activity.
    Neuropharmacology 02/2012; 62(7):2320-7. · 4.11 Impact Factor
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    ABSTRACT: Oscillatory entrainment mechanisms are invoked during attentional processing of rhythmically occurring stimuli, whereby their phase alignment regulates the excitability state of neurons coding for anticipated inputs. These mechanisms have been examined in the delta band (1-3 Hz), where entrainment frequency matches the stimulation rate. Here, we investigated entrainment for subdelta rhythmic stimulation, recording from intracranial electrodes over human auditory cortex during an intersensory audiovisual task. Audiovisual stimuli were presented at 0.67 Hz while participants detected targets within one sensory stream and ignored the other. It was found that entrainment operated at twice the stimulation rate (1.33 Hz), and this was reflected by higher amplitude values in the FFT spectrum, cyclic modulation of alpha-amplitude, and phase-amplitude coupling between delta phase and alpha power. In addition, we found that alpha-amplitude was relatively increased in auditory cortex coincident with to-be-ignored auditory stimuli during attention to vision. Thus, the data suggest that entrainment mechanisms operate within a delimited passband such that for subdelta task rhythms, oscillatory harmonics are invoked. The phase of these delta-entrained oscillations modulates alpha-band power. This may in turn increase or decrease responsiveness to relevant and irrelevant stimuli, respectively.
    Journal of Neuroscience 12/2011; 31(50):18556-67. · 6.91 Impact Factor
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    ABSTRACT: The neural processing of biological motion (BM) is of profound experimental interest since it is often through the movement of another that we interpret their immediate intentions. Neuroimaging points to a specialized cortical network for processing biological motion. Here, high-density electrical mapping and source-analysis techniques were employed to interrogate the timing of information processing across this network. Participants viewed point-light-displays depicting standard body movements (e.g. jumping), while event-related potentials (ERPs) were recorded and compared to ERPs to scrambled motion control stimuli. In a pair of experiments, three major phases of BM-specific processing were identified: 1) The earliest phase of BM-sensitive modulation was characterized by a positive shift of the ERP between 100 and 200 ms after stimulus onset. This modulation was observed exclusively over the right hemisphere and source-analysis suggested a likely generator in close proximity to regions associated with general motion processing (KO/hMT). 2) The second phase of BM-sensitivity occurred from 200 to 350 ms, characterized by a robust negative-going ERP modulation over posterior middle temporal regions bilaterally. Source-analysis pointed to bilateral generators at or near the posterior superior temporal sulcus (STS). 3) A third phase of processing was evident only in our second experiment, where participants actively attended the BM aspect of the stimuli, and was manifest as a centro-parietal positive ERP deflection, likely related to later cognitive processes. These results point to very early sensory registration of biological motion, and highlight the interactive role of the posterior STS in analyzing the movements of other living organisms.
    NeuroImage 05/2011; 56(1):373-83. · 6.25 Impact Factor
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    ABSTRACT: To examine the correlation between the change in PASAT and the change in P3 event-related potentials (ERPs) over a 12-month period in multiple sclerosis (MS) patients, and to compare the 12-month change in the P3 ERP between MS patients and controls. Forty-four subjects (27 MS patients, 17 controls) completed visual and auditory two-stimulus oddball and three-stimulus oddball tasks at Month 0 and Month 12. Data were recorded from a 128-scalp channel electroencephalography array. Data from scalp channels were converted into continuous interpolated images (incorporating the entire scalp and time). Amplitude, topographical differences and correlations were then tested using statistical parametric mapping. The change in visual and auditory P3a correlated significantly with the change in PASAT score (r=0.56, p<0.001 and r=0.48, p=0.003, respectively). Visual P3b and P3a showed greater decrease in 12 months in MS patients relative to controls. Visual P3b, auditory P3b and auditory P3a amplitudes had significantly decreased in MS patients after 12-month period. Change in visual and auditory P3a ERP amplitudes correlate with change in PASAT scores in MS patients. Visual modality is more sensitive to changes in P3 ERP amplitudes over 12-month period. P3 ERPs may have utility in monitoring the change in cognitive functioning in MS.
    Journal of the neurological sciences 03/2011; 305(1-2):45-52. · 2.32 Impact Factor
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    ABSTRACT: The N1 component of the auditory evoked potential (AEP) is a robust and easily recorded metric of auditory sensory-perceptual processing. In patients with schizophrenia, a diminution in the amplitude of this component is a near-ubiquitous finding. A pair of recent studies has also shown this N1 deficit in first-degree relatives of schizophrenia probands, suggesting that the deficit may be linked to the underlying genetic risk of the disease rather than to the disease state itself. However, in both these studies, a significant proportion of the relatives had other psychiatric conditions. As such, although the N1 deficit represents an intriguing candidate endophenotype for schizophrenia, it remains to be shown whether it is present in a group of clinically unaffected first-degree relatives. In addition to testing first-degree relatives, we also sought to replicate the N1 deficit in a group of first-episode patients and in a group of chronic schizophrenia probands. Subject groups consisted of 35 patients with schizophrenia, 30 unaffected first-degree relatives, 13 first-episode patients, and 22 healthy controls. Subjects sat in a dimly lit room and listened to a series of simple 1,000-Hz tones, indicating with a button press whenever they heard a deviant tone (1,500 Hz; 17% probability), while the AEP was recorded from 72 scalp electrodes. Both chronic and first-episode patients showed clear N1 amplitude decrements relative to healthy control subjects. Crucially, unaffected first-degree relatives also showed a clear N1 deficit. This study provides further support for the proposal that the auditory N1 deficit in schizophrenia is linked to the underlying genetic risk of developing this disorder. In light of recent studies, these results point to the N1 deficit as an endophenotypic marker for schizophrenia. The potential future utility of this metric as one element of a multivariate endophenotype is discussed.
    European Archives of Psychiatry and Clinical Neuroscience 12/2010; 261(5):331-9. · 3.36 Impact Factor
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    ABSTRACT: Using atypical symptoms at presentation as red flags to indicate an alternative diagnosis to MS has not been formally assessed. The purpose of this study was to assess the utility of defining the typicality and atypicality of symptoms at presentation in the diagnosis of patients referred as being "query MS". A 3-year review of patients referred as "query MS" was performed. Patients were classified on the basis of the first clinical assessment as having typical symptoms of MS or atypical symptoms suggesting an alternative diagnosis. The initial clinical opinion was correlated with the eventual diagnosis. Raters blinded to the diagnosis also assigned typical or atypical status to the patients. 247 patients were referred with suspected MS. 56% had MS and 44% did not. 129 patients had atypical symptoms of whom, 81% did not have MS; 98 patients had typical symptoms; 10% did not have MS. Atypical symptoms had a sensitivity of 85% and specificity of 87% for the diagnosis of "not MS". Positive predictive and negative predictive values were 90% and 81% respectively. Atypical features as indicators of not MS have a high sensitivity and specificity.
    Journal of neurology, neurosurgery, and psychiatry 11/2010; 81(11):e55. · 4.87 Impact Factor

Publication Stats

2k Citations
217.13 Total Impact Points

Institutions

  • 2009–2014
    • CUNY Graduate Center
      New York City, New York, United States
  • 2007–2014
    • Nathan Kline Institute
      Orangeburg, New York, United States
  • 2012–2013
    • City College of New York
      • Department of Biomedical Engineering
      New York City, New York, United States
  • 2010–2012
    • City University of New York - York College
      New York City, New York, United States
    • Columbia University
      New York City, New York, United States
  • 2004–2012
    • Trinity College Dublin
      • School of Psychology
      Dublin, L, Ireland
  • 2011
    • St Vincent's University Hospital
      Dublin, Leinster, Ireland
  • 2006–2009
    • Saint Vincent Hospital
      Worcester, Massachusetts, United States
  • 2008
    • St. Vincent’s Hospital, Fairview
      Dublin, Leinster, Ireland
  • 2003–2005
    • University College Dublin
      • School of Electrical, Electronic and Mechanical Engineering
      Dublin, L, Ireland
  • 2002
    • University College Cork
      • Department of Electrical and Electronic Engineering
      Cork, M, Ireland