Attentional load asymmetrically affects early electrophysiological indices of visual orienting.

Recent behavioral studies suggest that asymmetries in visuospatial orienting are modulated by changes in the demand on nonspatial components of attention, but the brain correlates of this modulation are unknown. We used scalp-recorded event-related potentials to examine the influence of central attentional load on neural responses to lateralized visual targets. Forty-five participants were required to detect transient, unilateral visual targets while monitoring a stream of alphanumeric stimuli at fixation, in which the target was defined either by a unique feature (low load) or by a conjunction of features (high load). The earliest effect of load on spatial orienting was seen at the latency of the posterior N1 (190-240 ms). The commonly observed N1 enhancement with contralateral visual stimulation was attenuated over the right hemisphere under high load. Source analysis localized this effect to occipital and inferior parietal regions of the right hemisphere. In addition, we observed perceptual enhancement with increasing load within the focus of attention (fixation) at an earlier stage (P1, 90-140 ms) than has previously been reported. These data support the view that spatial asymmetries in visual orienting are modulated by nonspatial attention due to overlapping neural circuits within the right hemisphere

Two types of action error: electrophysiological evidence for separable inhibitory and sustained attention neural mechanisms producing error on Go/No-go tasks.

Disentangling the component processes that contribute to human executive control is a key challenge for cognitive neuroscience. Here, we employ Event-Related Potentials to provide electrophysiological evidence that action errors during a Go/No-go task can result either from sustained attention failures, or from failures of response inhibition, and that these two processes are temporally and physiologically dissociable, even though the behavioural error ? a non-intended response ? is the same. Thirteen right-handed participants performed a version of a Go/No-go task in which stimuli were presented in a fixed and predictable order thus encouraging attentional drift and a second version in which an identical set of stimuli were presented in a random order thus placing greater emphasis on response inhibition. Electro-cortical markers associated with goal maintenance (late positivity, alpha synchronisation) distinguished correct and incorrect performance in the fixed condition while errors in the random condition were linked to a diminished N2/P3 inhibitory complex. In addition, the amplitude of the Error-Related Negativity did not differ between correct and incorrect responses in the fixed condition consistent with the view that errors in this condition do not arise from a failure to resolve response competition. Our data provide an electrophysiological dissociation of sustained attention and response inhibition

Internal and external influences on the rate of sensory evidence accumulation in the human brain.

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 ?-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

Linking time-on-task, spatial bias and hemispheric activation asymmetry: a neural correlate of rightward attention drift.

Biases of spatial attention may be moderated by non-spatial factors such as attentional load and time-on-task. Although these effects are thought to arise from depletion of right hemisphere processing resources, their neurophysiological bases have yet to be confirmed. We recorded posterior ?-band EEG ? a marker of cortical excitability linked to spatial attention orienting ? from 66 non-clinical participants who detected transient, unilateral visual targets while also monitoring stimuli at fixation. Asymmetry indices were derived for both lateral target reaction times and hemispheric differences in ?-activity before and after lateral target onsets. Pre-target ? became more prominent over the right, relative to left, hemisphere as the task progressed over 48-min, and this change was correlated with a significant rightward shift in spatial bias. Contrary to past studies of posterior ?-asymmetry and orienting, here participants did not receive pre-target cues. Thus we show that asymmetries in the hemispheric distribution of anticipatory ? are not only apparent during externally-cued attention orienting, but are also sensitive to decreasing alertness over time. These data are the first to link rightward attention drift over time with change in hemispheric activation asymmetry, providing important implications for our understanding of interacting spatial attention and non-spatial alertness networks

Ocular exposure to blue-enriched light has an asymmetric influence on neural activity and spatial attention

Brain networks subserving alertness in humans interact with those for spatial attention orienting. We employed blue-enriched light to directly manipulate alertness in healthy volunteers. We show for the first time that prior exposure to higher, relative to lower, intensities of blue-enriched light speeds response times to left, but not right, hemifield visual stimuli, via an asymmetric effect on right-hemisphere parieto-occipital ?-power. Our data give rise to the tantalising possibility of light-based interventions for right hemisphere disorders of spatial attentio

Characterisation of executive deficits in attention-deficit / hyperactivity disorder and avenues for their remediation

THESIS 8178Attention-Deficit/Hyperactivity Disorder (ADHD) is a common neurodevelopmental disorder that affects both children and adults and is associated with a range of behavioural and cognitive difficulties. This thesis aims to advance our understanding of this disorder by exploring two main themes. The first is that the neural abnormalities underlying executive dysfunction in ADHD can be elucidated by combining electrophysiological measures of brain function with cognitive paradigms for which valid brain-behaviour relationships have been established. The second part of this thesis is concerned with exploring the possibility that our knowledge of the neural instantiation of cognitive functions can provide a new avenue for the development of effective cognitive training strategies for ADHD. Chapter 1 begins by providing a broad overview of ADHD across the lifespan and outlines the theoretical basis for the experiments conducted in this thesis. Chapter 2 discusses the potential value of electrophysiological parameters in ADHD research

Transcranial direct current stimulation over right dorsolateral prefrontal cortex enhances error awareness in older age

The ability to detect errors during cognitive performance is compromised in older age and in a range of clinical populations. This study was designed to assess the effects of transcranial direct current stimulation (tDCS) on error awareness in healthy older human adults. tDCS was applied over DLPFC while subjects performed a computerized test of error awareness. The influence of current polarity (anodal vs cathodal) and electrode location (left vs right hemisphere) was tested in a series of separate single-blind, Sham-controlled crossover trials, each including 24 healthy older adults (age 65-86 years). Anodal tDCS over right DLPFC was associated with a significant increase in the proportion of performance errors that were consciously detected, and this result was recapitulated in a separate replication experiment. No such improvements were observed when the homologous contralateral area was stimulated. The present study provides novel evidence for a causal role of right DLPFC regions in subserving error awareness and marks an important step toward developing tDCS as a tool for remediating the performance-monitoring deficits that afflict a broad range of populations

The effects of a self-alert training (SAT) program in adults with ADHD

Attention-Deficit/Hyperactivity Disorder (ADHD), a neuropsychiatric condition characterized by attention and impulsivity problems, is one of the most common behavioral disorders. The first line of treatment for ADHD is psychostimulant medication, but this has limited effectiveness, particularly in adults, and is often associated with adverse side-effects. Thus, it is imperative that new non-pharmaceutical approaches to treatment are developed. This study aims to evaluate the impact of a non-pharmacological Self-Alert Training (SAT) intervention on ADHD symptom prevalence, psychological and cognitive functioning, and on everyday functional impairment in adults with ADHD. Fifty-one adult participants with a current diagnosis of ADHD were randomized to either SAT or a Control Training (CT) program. They were assessed at baseline, immediately following the 5-week training period, and after 3-months using ADHD symptoms scales, as well as a series of neuropsychological tests and psychological questionnaires. Subjective ratings of everyday life attention and memory problems were also collected. The SAT group showed significant improvements in ADHD inattentive and impulsive symptoms, depressive symptoms and in self-efficacy ratings compared to the CT group at both post-training and at the 3-month assessment. Pre-post improvements in SAT participants on untrained cognitive tasks measuring selective attention and executive functions were also observed. Finally, the SAT group reported improved subjective ratings of everyday life attention at both assessment points. This pattern of results suggests that SAT may be beneficial in treating ADHD symptoms as well as psychological and cognitive impairments in adult ADHD. A large-scale randomized controlled trial (RCT) is neede

An electrophysiological signal that precisely tracks the emergence of error awareness.

Recent electrophysiological research has sought to elucidate the neural mechanisms necessary for the conscious awareness of action errors. Much of this work has focused on the error positivity (Pe), a neural signal that is specifically elicited by errors that have been consciously perceived. While awareness appears to be an essential prerequisite for eliciting the Pe, the precise functional role of this component has not been identified. Twenty-nine participants performed a novel variant of the Go/No-go Error Awareness Task (EAT) in which awareness of commission errors was indicated via a separate speeded manual response. Independent component analysis (ICA) was used to isolate the Pe from other stimulus- and response-evoked signals. Single-trial analysis revealed that Pe peak latency was highly correlated with the latency at which awareness was indicated. Furthermore, the Pe was more closely related to the timing of awareness than it was to the initial erroneous response. This finding was confirmed in a separate study which derived IC weights from a control condition in which no indication of awareness was required, thus ruling out motor confounds. A receiver-operating-characteristic (ROC) curve analysis showed that the Pe could reliably predict whether an error would be consciously perceived up to 400 ms before the average awareness response. Finally, Pe latency and amplitude were found to be significantly correlated with overall error awareness levels between subjects. Our data show for the first time that the temporal dynamics of the Pe trace the emergence of error awareness. These findings have important implications for interpreting the results of clinical EEG studies of error processing