Rare Target Research Articles

Digital PCR and the QuantStudio™ 3D Digital PCR System.

The great promise of digital PCR is the potential for unparalleled precision enabling accurate measurements for detection and quantification of genetic material. This chapter walks the reader through the fundamentals of digital PCR technology including digital PCR modeling using Poisson statistics. It describes a highly successful implementation of digital PCR technology using the chip-based nanofluidic Applied Biosystems™ QuantStudio™ 3D digital PCR system. It reviews the large number of applications where digital PCR is poised to make significant impacts. These include applications where detection of rare genetic targets is prioritized such as liquid biopsy, rare mutation detection, confirmation of NGS variant detection, detection of fusion transcripts, detection of chimerism and GMO detection and monitoring. These further include applications where accurate quantification of genetic targets is prioritized such as generation of references and standards, copy number variation, and NGS Library quantification.

Beta power encodes contextual estimates of temporal event probability in the human brain.

To prepare for an impending event of unknown temporal distribution, humans internally increase the perceived probability of event onset as time elapses. This effect is termed the hazard rate of events. We tested how the neural encoding of hazard rate changes by providing human participants with prior information on temporal event probability. We recorded behavioral and electroencephalographic (EEG) data while participants listened to continuously repeating five-tone sequences, composed of four standard tones followed by a non-target deviant tone, delivered at slow (1.6 Hz) or fast (4 Hz) rates. The task was to detect a rare target tone, which equiprobably appeared at either position two, three or four of the repeating sequence. In this design, potential target position acts as a proxy for elapsed time. For participants uninformed about the target’s distribution, elapsed time to uncertain target onset increased response speed, displaying a significant hazard rate effect at both slow and fast stimulus rates. However, only in fast sequences did prior information about the target’s temporal distribution interact with elapsed time, suppressing the hazard rate. Importantly, in the fast, uninformed condition pre-stimulus power synchronization in the beta band (Beta 1, 15–19 Hz) predicted the hazard rate of response times. Prior information suppressed pre-stimulus power synchronization in the same band, while still significantly predicting response times. We conclude that Beta 1 power does not simply encode the hazard rate, but—more generally—internal estimates of temporal event probability based upon contextual information.

Microwell Array-based Digital PCR for Influenza Virus Detection

Digital polymerase chain reaction (dPCR) enables the accurate determination of deoxyribonucleic acid (DNA) copy numbers. In contrast to real-time PCR (qPCR), dPCR can perform quantification without requiring previous standard curve experiments. Furthermore, dPCR is more sensitive for rare targets in genetic samples. Despite the significant advantages of dPCR over qPCR, dPCR is not widely applied yet due to the complicated requirements of instrumentations regarding creating sample subvolumes and the equipment cost associated with this. In this paper, we present a microwell array that can be used for dPCR. The microwell array can be operated with little micro-fluidic expertise, while it can be manufactured at a unit cost of less than 1$. The amplification results of the microwell array can be analyzed with a custom-written Python script. With this method, we have been able to amplify and quantify complementary deoxyribonucleic acid (cDNA) samples for the H7N9 influenza virus in a concentration range spanning three orders of magnitude (1,000-100,000 copies/μL).

Abstract 406: Profiling plasma cell-free RNA (cfRNA) with the NanoString low input nCounter assay

Abstract Background: Liquid biopsy offers advantages over tissue biopsy for early disease diagnosis, monitoring of disease progression, and evaluation of therapeutic interventions because sample collection is rapid, easy, minimally invasive, and repeatable. In particular, measuring cell-free RNA (cfRNA) in the plasma offers the opportunity for direct monitoring of a tumor cell byproduct without the need for direct access to the tumor. However, robust cfRNA detection has been difficult historically due to weak and highly variable signal detection. The nCounter platform and low input profiling protocol have been adapted to enable robust and reproducible detection of cfRNA from plasma, which is demonstrated here in proof of concept experiments using synthetic targets and patient samples to quantify transcription of standard gene transcripts and fusion products. Methods: The standard nCounter detection platform which leverages direct hybridization of optical barcodes to RNA transcripts of interest has been adapted to enable detection of rare targets by leveraging an initial targeted amplification step to increase transcript abundance with minimal bias of relative transcript abundance. Feasibility of this process to detect cfRNA was assessed with synthetic DNA oligos and commercially sourced patient-derived plasma samples. Plasma cfRNA was isolated and concentrated using commercial cfRNA kits. Relative expression of targets was benchmarked to universal reference RNA and compared between samples by both nCounter and qPCR. Results: Using synthetic ultramer DNA oligo targets for 7 genes, the limit of detection of the assay was as low as 24 molecules per target with 100% specificity. Using cfRNA from healthy donors, the nCounter assay was shown to profile relative abundance of 40 targets. 9 genes were selected to benchmark nCounter against qPCR and showed high correlation (R2 = 0.84) with linearity (slope = 0.97). cfRNA from healthy donors or patients with advanced CRC or NSCLC was profiled by the 770-gene PanCancer IO360 panel with a pre-amplification by the adapted nanoString low RNA input protocol. Unsupervised hierarchical clustering revealed that CRC derived cfRNA had different transcriptional profiles from healthy donors, and NSCLC cfRNA had an intermediate profile. Furthermore, a number of therapeutically relevant targets, including EGF, EGFR, PD-1, and PD-L1 could be detected in cfRNA from cancer patients but not in cfRNA from healthy donors. Conclusion: The nCounter low input protocol can be used for transcriptional profiling of cfRNA from healthy and diseased patients with high sensitivity and specificity. The flexible primer design strategy enables profiling of a variety of transcripts, including fusion gene products, with the possibility of profiling up to 800 transcripts from a single sample. In summary, the nCounter platform (RUO) provides a robust flexible molecular profiling tool for liquid biopsy research. Citation Format: Chung-Ying (Alan) Huang, Sarah Warren, Jason Reeves, Megan Wurden, Tim Riordan, Rich Boykin, Mingdong Liu, David Henderson, Dan Hanson, Jay Gerlach, Joseph M. Beechem. Profiling plasma cell-free RNA (cfRNA) with the NanoString low input nCounter assay [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 406.

Spotting rare items makes the brain "blink" harder: Evidence from pupillometry.

In many visual search tasks (e.g., cancer screening, airport baggage inspections), the most serious search targets occur infrequently. As an ironic side effect, when observers finally encounter important objects (e.g., a weapon in baggage), they often fail to notice them, a phenomenon known as the low-prevalence effect (LPE). Although many studies have investigated LPE search errors, we investigated the attentional consequences of successful rare target detection. Using an attentional blink paradigm, we manipulated how often observers encountered the first serial target (T1), then measured its effects on their ability to detect a following target (T2). Across two experiments, we show that the LPE is more than just an inflated miss rate: When observers successfully detected rare targets, they were less likely to spot subsequent targets. Using pupillometry to index locus-coeruleus (LC) mediated attentional engagement, Experiment 2 confirmed that an LC refractory period mediates the attentional blink (`Nieuwenhuis, Gilzenrat, Holmes, & Cohen, 2005, Journal of Experimental Psychology: General, 134[3], 291-307), and that these effects emerge relatively quickly following T1 onset. Moreover, in both behavioral and pupil analyses, we found that detecting low-prevalence targets exacerbates the LC refractory period. Consequences for theories of the LPE are discussed.

Prediction errors in surface segmentation are reflected in the visual mismatch negativity, independently of task and surface features.

The visual system quickly registers perceptual regularities in the environment and responds to violations in these patterns. Errors of perceptual prediction are associated with electrocortical modulation, including the visual mismatch negativity (vMMN) and P2 event-related potential. One relatively unexplored question is whether these prediction error signals can encode higher-level properties such as surface segmentation, or whether they are limited to lower-level perceptual features. Using a roving standard paradigm, a triangle surface appeared either behind (featuring amodal contours) or in front of (featuring real contours) a second surface with hole-like windows. A surface layout appeared for two to five repetitions before switching to the other "deviant" layout; lighting and orientation of stimuli varied across presentations while remaining isoluminant. Observers responded when they detected a rare "pinched" triangle, which occasionally appeared. Cortical activity-reflected in mismatch responses affecting the P2-N2 and P300 amplitudes-was sensitive to a change in stimulus layout, when surfaces shifted position in depth, following several repetitions. Specifically, layout deviants led to a more negative P2-N2 complex at posterior electrodes, and greater P300 positivity at central sites. Independently of these signals of a deviant surface layout, further modulations of the P2 encoded differences between layouts and detection of the rare target stimulus. Comparison of the effect of preceding layout repetitions on this prediction error signal suggests that it is all or none and not graded with respect to the number of previous repetitions. We show that within the visual domain, unnoticed and task-irrelevant changes in visual surface segmentation leads to observable electrophysiological signals of prediction error that are dissociable from stimulus-specific encoding and lower-level perceptual processing.

Development of Hydrogel Microparticle based RT-qPCR for Advanced Detection of BCR-ABL1 Transcripts

Reverse transcription – quantitative polymerase chain reaction (RT-qPCR) is conventionally used method to analyze oncogenes, infected DNAs, and mutated tumor associated genes. However it is hard to detect rare genetic targets since the disturbance of undesired amplification often overrides the reaction of very few targets in a myriad of interfering genes. To solve the limitation, we developed primerimmobilized network (PIN) probe RT-qPCR which can detect target RNA with high selectivity and sensitivity. To conduct PIN probe RT-qPCR with high efficiency, design of probe, concentration of immobilized probe and qPCR condition were optimized. The LOD of PIN probe RT-qPCR was 40pg per particle with 89.2% efficiency. When extremely low concentration of RNA was used, result of PIN probe RTqPCR was showed “on/off” signal. Also, target was confirmed only in the “on” particle. The interference effects by non-target PCR products were minimized by target-capturing and washing process in the RT. Using PIN probe RT-qPCR, we successfully detected target RNA in a sample which has a ratio of 1:100,000 (positive RNA : negative RNA).

Mild cognitive impairment in mild brain injury (MBI) patients: An event related potential (ERP) and neuropsychology study.

Objectives: To evaluate auditory cognitive function in mild brain injury (MBI) patients, which is important to determine for rehabilitation and improve quality of their life.
 Methods: Participants (n=19/group) were divided into group 1 (G1-control), group 2 (G2/1st test-MBI/within 7 days of road traffic accident-RTA) and group 3 (G3/2nd test-MBI/2-6 months after RTA). Event related potentials (ERPs) were conducted using a 128-sensor net; participants counted silently rare target tone stimuli and ignored standard tones. Several neuropsychology tests like Verbal fluency test (PAS), Wisconsin Card Sorting Test (WCST), Rey Auditory Verbal and Learning Test (RAVLTIM, RAVLTDR and RAVLTTS) and Beck Depression Inventory (BDI) were subsequently administered.
 Results: Sensory (P50, N100) and cognitive (P300) ERP components were analysed from ERP waveforms. There were no significant group differences in amplitudes or latencies for all components across sites except P300 component amplitudes at T6 location. P50, N100 and P300 ERP components exhibited non-significantly increased amplitudes in G2 and G3 compared with G1 at all sites; non-significantly shorter latencies were identified at various sites. At several locations, G3 evoked non-significantly increased amplitudes and longer latencies with shorter latencies to other sites compared with G2 in all components. The MBI (G3) group exhibited significantly increased WCST, RAVLTIM and RAVLTDR scores compared with G1.
 Conclusion: These findings indicate MBI patients may have mild auditory, cognitive and executive dysfunctions with good auditory memory. MBI was associated with mild depression.
 Bangladesh Journal of Medical Science Vol.18(3) 2019 p.557-566

Characterizing the movement strategy of CD8+ T cells in finding malaria parasites in the liver

Abstract Movement strategies by which antigen-specific CD8+ T cells search for sites of infection in peripheral tissues are not fully understood. Measurable quantities, such as the distance traveled per unit of time, that follow Lévy statistics are common in nature, for example, in bacterial and higher animal foraging; in mussels, marine predators, monkeys. A recent study has shown that moving according to Lévy flights enables CD8+ T cells in the brain to find “rare targets” (Toxoplasma gondii-infected cells) in an order of magnitude more efficiently than using Brownian and other walking strategies. It has been, thus, suggested that such efficient strategy of T cell searching for infection may have been evolutionarily selected. However, T cells in the brain (or other tissues) are often constrained by the structure of the physical environment in which cells move. Whether the emerged statistical pattern of T cell movement is due to a cell-intrinsic program or a consequence of environmental constrains is, thus, unknown. Here, we address this question by using intravital imaging of movement of Plasmodium-specific CD8+ T cells in murine livers and mathematical modeling and analyses of the generated data. We investigate whether the physical structure of the movement paths in the liver plays a role in shaping the walking strategy of T cells in finding malaria parasites. Here, we map out liver sinusoids in 3D using intravital microscopy and use simulations to parameterize the alternative walking models by fitting them to empirical data having no references to the physical structure. Our preliminary data suggest that constraints imposed by the structure of liver sinusoids significantly impact the moving pattern of the T cells.

Hearing-impaired listeners show increased audiovisual benefit when listening to speech in noise

Recent studies provide evidence for changes in audiovisual perception as well as for adaptive cross-modal auditory cortex plasticity in older individuals with high-frequency hearing impairments (presbycusis). We here investigated whether these changes facilitate the use of visual information, leading to an increased audiovisual benefit of hearing-impaired individuals when listening to speech in noise. We used a naturalistic design in which older participants with a varying degree of high-frequency hearing loss attended to running auditory or audiovisual speech in noise and detected rare target words. Passages containing only visual speech served as a control condition. Simultaneously acquired scalp electroencephalography (EEG) data were used to study cortical speech tracking. Target word detection accuracy was significantly increased in the audiovisual as compared to the auditory listening condition. The degree of this audiovisual enhancement was positively related to individual high-frequency hearing loss and subjectively reported listening effort in challenging daily life situations, which served as a subjective marker of hearing problems. On the neural level, the early cortical tracking of the speech envelope was enhanced in the audiovisual condition. Similar to the behavioral findings, individual differences in the magnitude of the enhancement were positively associated with listening effort ratings. Our results therefore suggest that hearing-impaired older individuals make increased use of congruent visual information to compensate for the degraded auditory input.

The impact of the COMT genotype and cognitive demands on facets of intra-subject variability

Intra-Subject Variability (ISV), a potential index of catecholaminergic regulation, is elevated in several disorders linked with altered dopamine function. ISV has typically been defined as reaction time standard deviation. However, the ex-Gaussian and spectral measures capture different aspects and may delineate different underlying sources of ISV; thus reflecting different facets of the construct. We examined the impact of factors associated with dopamine metabolism, namely, Catechol-O-Methyltransferase Val158Met (COMT) genotype and Working Memory (WM) and response-switching on ISV facets in young healthy adults. The Met allele was associated with overall increased variability. The rather exclusive sensitivity of ex-Gaussian tau to frequencies below 0.025 Hz and the quasi-periodic structure of particularly slow responses support the interpretation of tau as low frequency fluctuations of neuronal networks. Sigma, by contrast, may reflect neural noise. Regarding cognitive demands, a WM load-related increase in variability was present for all genotypes and all ISV facets. Contrastingly, ISV facets reacted differently to variations in response-switching as, across genotypes, sigma was elevated for rare target trials whereas tau was elevated for frequent standard trials, particularly for Met homozygotes. Our findings support the significant role of COMT in regulating behavioural ISV with its facetted structure and presumed underlying neural processes.

Red Eye in Rural Environment: An Unusual Reason

Eye is a rare target organ of parasitic infestation due to winged fly larvae. The eye can be affected in three different form in ophthalmomyiasis: Orbital, internal, and external. External ophthalmomyiasis commonly seen in the rural environment and close contact with animals especially in spring and summer months. External ophthalmomyiasis can be diagnosed by biomicroscopy with presence of moving larvae and the diagnosis can be confirmed by light microscopy. The basis of treatment is the mechanical removal of the larvae. Topical antibiotic, steroid, and antihistaminic treatment protects from secondary infections and complications. External ophthalmomyiasis is a rare cause of red eye that should be taken into consideration especially in the presence of risk factors that may be associated with the history. Keywords: Myiasis; Ophthalmomyiasis; Red eye

Digital Single Virus Immunoassay for Ultrasensitive Multiplex Avian Influenza Virus Detection Based on Fluorescent Magnetic Multifunctional Nanospheres.

The fluorescence method has made great progress in the construction of sensitive sensors but the background fluorescence of the matrix and photobleaching limit its broad application in clinical diagnosis. Here, we propose a digital single virus immunoassay for multiplex virus detection by using fluorescent magnetic multifunctional nanospheres as both capture carriers and signal labels. The superparamagnetism and strong magnetic response ability of nanospheres can realize efficient capture and separation of targets without sample pretreatment. Due to their distinguishable fluorescence imaging and photostability, the nanospheres enable single-particle counting for ultrasensitive multiplexed detection. Furthermore, the integration of digital analysis provided a reliable quantitative strategy for the detection of rare targets. Based on multifunctional nanospheres and digital analysis, a digital single virus immunoassay was proposed for simultaneous detection of H9N2, H1N1, and H7N9 avian influenza virus without complex signal amplification, whose detection limits were 0.02 pg/mL. Owing to its good specificity and anti-interference ability, the method showed great potential in single biomolecules, multiplexed detection, and early diagnosis of diseases.

Rapid liquid biopsy for Mohs surgery: rare target cell separation from surgical margin lavage fluid with a high recovery rate and selectivity.

In melanoma surgery, it is difficult to identify residual scattered tumor cells at the surgical margin because of invasive growth. Mohs surgery, widely applied to increase the cure rate and decrease the recurrence rate of melanoma, involves examination of the tissue for tumor cells after tissue removal. Here, we established a liquid biopsy platform for rapid (<5 h), sensitive examination of residual tumor cells at the margin after Mohs surgery using clinical samples from patients with pigment nevus for a demonstration. The design involved highly sensitive, selective rare target cell separation from surgical margin lavage fluid (SMLF) through micropore-arrayed filtration. High recovery rates (86.7% ± 16.3% and 72.7% ± 46.7%, respectively) for separation of spiked 5 A375s (cultured human melanoma cells) and 1 A375 from 1 mL PBS were achieved for this platform. Detection of SMLF samples from patients with pigment nevus was performed, and many (66-7420) Melan-A-positive target cells were successfully recovered and identified, demonstrating the application performance of this rapid liquid biopsy for Mohs surgery in clinical practice. Moreover, a high-selectivity separation of larger target A375 cells from smaller background Jurkat cells was achieved with a high enrichment factor (4.2 ± 1.1). In clinical practice, high selectivity contributes to effective depletion of red blood cells (RBCs), thus ensuring verification of target cells from samples with severe RBC contamination. Furthermore, target cells were obtained with high purity (2.7-35.2%). The capability of this method for rare-cell separation with a high recovery rate and good selectivity may facilitate improvement of performance of Mohs surgery for real clinical practice, including shortening examination time and increasing detection sensitivity.

Space-time fractional diffusion in cell movement models with delay

The movement of organisms and cells can be governed by occasional long distance runs, according to an approximate Lévy walk. For T cells migrating through chronically-infected brain tissue, runs are further interrupted by long pauses and the aim here is to clarify the form of continuous model equations that describe such movements. Starting from a microscopic velocity-jump model based on experimental observations, we include power-law distributions of run and waiting times and investigate the relevant parabolic limit from a kinetic equation for resting and moving individuals. In biologically relevant regimes we derive nonlocal diffusion equations, including fractional Laplacians in space and fractional time derivatives. Its analysis and numerical experiments shed light on how the searching strategy, and the impact from chemokinesis responses to chemokines, shorten the average time taken to find rare targets in the absence of direct guidance information such as chemotaxis.

Effect of GAD1 genotype status on auditory attention and acute nicotine administration in healthy volunteers.

The effects of GABA modulating drugs and nicotine, the prototypical nicotinic cholinergic agonist, on attention have been investigated using subcomponents of the P300 event-related potentials (ERP), which index involuntary (P3a) and voluntary attention (P3b). However, investigations into how such pharmacologic effects interact with genetic features in the GABA system remain unclear. This study examined the moderating effects of a single nucleotide polymorphism (rs7557793) in the glutamic acid decarboxylase 67 (GAD1) gene, which is implicated in the conversion of glutamate to GABA, on P300-indices of auditory attentional processing; the influence of nicotine administration was also assessed. The effects of GAD1 genotype (TT/CC/CT) were examined on the P3a/b in response to an auditory selective attention task in healthy, nonsmoking male volunteers (N=126; 18-40years). Participants responded to rare target stimuli (P3b-eliciting) and ignored frequent nontarget stimuli as well as rare distractor stimuli (P3a-eliciting). In a subsample (N=59), P3a/b profiles to acute nicotine (vs. placebo) administration were examined as a function of GAD1 genotype. As a secondary aim, earlier sensory processes were assessed with N200 ERP subcomponents elicited by novel (N2a) and target (N2b) auditory stimuli. GAD1 allelic variation moderated early sensory processes, enhancing N2a amplitudes in CT versus TT carriers. Further, TT homozygotes exhibited larger P3b amplitudes than CC homozygotes in the placebo versus nicotine condition. Regardless of genotype, nicotine versus placebo moderated the N200 ERP. These findings expand our knowledge regarding the attentional effects of GAD1 genetic variants in relation to nicotine.

Cholinergic Mechanisms of Target Oddball Stimuli Detection: The Late "P300-Like" Event-Related Potential in Rats.

Event-related potentials (ERPs) and oscillations (EROs) provide powerful tools for studying the brain's synaptic function underlying information processing. The P300 component of ERPs indexing attention and working memory shows abnormal amplitude and latency in neurological and psychiatric diseases that are sensitive to pharmacological agents. In the active auditory oddball discriminant paradigm, behavior and auditory-evoked potentials (AEPs) were simultaneously recorded in awake rats to investigate whether P300-like potentials generated in rats responding to rare target oddball tones are sensitive to subcutaneous modulation of the cholinergic tone by donepezil (1 mg/kg) and scopolamine (0.64 mg/kg). After operant training, rats consistently discriminate rare target auditory stimuli from frequent irrelevant nontarget auditory stimuli by a higher level of correct lever presses (i.e., accuracy) in target trials associated with a food reward. Donepezil attenuated the disruptive effect of scopolamine on the level of accuracy and premature responses in target trials. Larger P300-like peaks with early and late components were revealed in correct rare target stimuli trials as compared to frequent tones. Donepezil enhanced the peak amplitude of the P300-like component to target stimuli and evoked slow theta and gamma oscillations, whereas scopolamine altered the amplitude of the P300-like component and EROs to target stimuli. Pretreatment with donepezil attenuated effects of scopolamine on the peak amplitude of the P300-like component and on EROs. This study provides evidence that AEP P300-like responses can be elicited by rats engaged in attentive and memory processing of target stimuli and outline the relevance of the cholinergic system in stimulus discrimination processing. The findings highlight the sensitivity of this translational index for investigating brain circuits and/or novel pharmacological agents, which modulate cholinergic transmission associated with increased allocation of attentional resources.

The Impact of Performance-Based Pay and Competition on Rare Target Search Performance

The Impact of Performance-Based Pay and Competition on Rare Target Search Performance

Musicians at the Cocktail Party: Neural Substrates of Musical Training During Selective Listening in Multispeaker Situations.

Musical training has been demonstrated to benefit speech-in-noise perception. It is however unknown whether this effect translates to selective listening in cocktail party situations, and if so what its neural basis might be. We investigated this question using magnetoencephalography-based speech envelope reconstruction and a sustained selective listening task, in which participants with varying amounts of musical training attended to 1 of 2 speech streams while detecting rare target words. Cortical frequency-following responses (FFR) and auditory working memory were additionally measured to dissociate musical training-related effects on low-level auditory processing versus higher cognitive function. Results show that the duration of musical training is associated with a reduced distracting effect of competing speech on target detection accuracy. Remarkably, more musical training was related to a robust neural tracking of both the to-be-attended and the to-be-ignored speech stream, up until late cortical processing stages. Musical training-related increases in FFR power were associated with a robust speech tracking in auditory sensory areas, whereas training-related differences in auditory working memory were linked to an increased representation of the to-be-ignored stream beyond auditory cortex. Our findings suggest that musically trained persons can use additional information about the distracting stream to limit interference by competing speech.

Inter-trial variability in brain activity as an indicator of synergistic effects of HIV-1 and drug abuse

BackgroundThe objective of this investigation was to detect evidence of the synergism in the effects of HIV-1 and drug abuse on brain function that has been hypothesized but rarely shown. The investigation incorporated several noteworthy improvements in the approach. It used urine toxicology tests to exclude participants complicated by recent methadone use and illicit drug use. Also, it defined drug abuse on a scale that considered symptom severity. Most importantly, it examined inter-trial variability in brain activity as a potentially more sensitive indicator of group differences and functional impairment than the across-trial average. Methods173 participants were assigned to groups defined by their HIV-1 serostatus and Drug Abuse Screening Test score (DAST < vs. > = 6). They completed a simple letter discrimination task including rare target and rare nontarget stimuli. Event-related electroencephalographic responses and key press responses were measured on each trial. During a separate assessment, posturographic measures were recorded. ResultsThe inter-trial standard deviation of P300-like activity was superior to the mean amplitude of this activity in differentiating the groups. Unlike the mean, it revealed synergistic statistical effects of HIV and drug abuse. It also correlated significantly with static ataxia. ConclusionsInter-trial variability in P300-like activity is a useful marker for detecting subtle and episodic disruptions in brain function. It demonstrates greater sensitivity than the mean amplitude for detecting differences across groups. Also, as a putative indicator of a disruption in the attentional monitoring of behavior, it predicts subtle impairments in gross motor function.