Jeanne Townsend

Removal of eye activity artifacts from visual event-related potentials in normal and clinical subjects

OBJECTIVES Electrical potentials produced by blinks and eye movements present serious problems for electroencephalographic (EEG) and event-related potential (ERP) data interpretation and analysis, particularly for analysis of data from some clinical populations. Often, all epochs contaminated by large eye artifacts are rejected as unusable, though this may prove unacceptable when blinks and eye movements occur frequently. METHODS Frontal channels are often used as reference signals to regress out eye artifacts, but inevitably portions of relevant EEG signals also appearing in EOG channels are thereby eliminated or mixed into other scalp channels. A generally applicable adaptive method for removing artifacts from EEG records based on blind source separation by independent component analysis (ICA) (Neural Computation 7 (1995) 1129; Neural Computation 10(8) (1998) 2103; Neural Computation 11(2) (1999) 606) overcomes these limitations. RESULTS Results on EEG data collected from 28 normal controls and 22 clinical subjects performing a visual selective attention task show that ICA can be used to effectively detect, separate and remove ocular artifacts from even strongly contaminated EEG recordings. The results compare favorably to those obtained using rejection or regression methods. CONCLUSIONS The ICA method can preserve ERP contributions from all of the recorded trials and all the recorded data channels, even when none of the single trials are artifact-free.

Impairment in shifting attention in autistic and cerebellar patients

MRI and autopsy evidence of early maldevelopment of cerebellar vermis and hemispheres in autism raise the question of how cerebellar maldevelopment contributes to the cognitive and social deficits characteristic of autism. Compared with normal controls, autistic patients and patients with acquired cerebellar lesions were similarly impaired in a task requiring rapid and accurate shifts of attention between auditory and visual stimuli. Neurophysiologic and behavioral evidence rules out motor dysfunction as the cause of this deficit. These findings are consistent with the proposal that in autism cerebellar maldevelopment may contribute to an inability to execute rapid attention shifts, which in turn undermines social and cognitive development, and also with the proposal that the human cerebellum is involved in the coordination of rapid attention shifts in a fashion analogous to its role in the coordination of movement.

Analysis and visualization of single-trial event-related potentials

In this study, a linear decomposition technique, independent component analysis (ICA), is applied to single‐trial multichannel EEG data from event‐related potential (ERP) experiments. Spatial filters derived by ICA blindly separate the input data into a sum of temporally independent and spatially fixed components arising from distinct or overlapping brain or extra‐brain sources. Both the data and their decomposition are displayed using a new visualization tool, the “ERP image,” that can clearly characterize single‐trial variations in the amplitudes and latencies of evoked responses, particularly when sorted by a relevant behavioral or physiological variable. These tools were used to analyze data from a visual selective attention experiment on 28 control subjects plus 22 neurological patients whose EEG records were heavily contaminated with blink and other eye‐movement artifacts. Results show that ICA can separate artifactual, stimulus‐locked, response‐locked, and non‐event‐related background EEG activities into separate components, a taxonomy not obtained from conventional signal averaging approaches. This method allows: (1) removal of pervasive artifacts of all types from single‐trial EEG records, (2) identification and segregation of stimulus‐ and response‐locked EEG components, (3) examination of differences in single‐trial responses, and (4) separation of temporally distinct but spatially overlapping EEG oscillatory activities with distinct relationships to task events. The proposed methods also allow the interaction between ERPs and the ongoing EEG to be investigated directly. We studied the between‐subject component stability of ICA decomposition of single‐trial EEG epochs by clustering components with similar scalp maps and activation power spectra. Components accounting for blinks, eye movements, temporal muscle activity, event‐related potentials, and event‐modulated alpha activities were largely replicated across subjects. Applying ICA and ERP image visualization to the analysis of sets of single trials from event‐related EEG (or MEG) experiments can increase the information available from ERP (or ERF) data. Hum. Brain Mapping 14:166–185, 2001.

Imaging human EEG dynamics using independent component analysis.

This review discusses the theory and practical application of independent component analysis (ICA) to multi-channel EEG data. We use examples from an audiovisual attention-shifting task performed by young and old subjects to illustrate the power of ICA to resolve subtle differences between evoked responses in the two age groups. Preliminary analysis of these data using ICA suggests a loss of task specificity in independent component (IC) processes in frontal and somatomotor cortex during post-response periods in older as compared to younger subjects, trends not detected during examination of scalp-channel event-related potential (ERP) averages. We discuss possible approaches to component clustering across subjects and new ways to visualize mean and trial-by-trial variations in the data, including ERP-image plots of dynamics within and across trials as well as plots of event-related spectral perturbations in component power, phase locking, and coherence. We believe that widespread application of these and related analysis methods should bring EEG once again to the forefront of brain imaging, merging its high time and frequency resolution with enhanced cm-scale spatial resolution of its cortical sources.

Electroencephalographic brain dynamics following manually responded visual targets.

Scalp-recorded electroencephalographic (EEG) signals produced by partial synchronization of cortical field activity mix locally synchronous electrical activities of many cortical areas. Analysis of event-related EEG signals typically assumes that poststimulus potentials emerge out of a flat baseline. Signals associated with a particular type of cognitive event are then assessed by averaging data from each scalp channel across trials, producing averaged event-related potentials (ERPs). ERP averaging, however, filters out much of the information about cortical dynamics available in the unaveraged data trials. Here, we studied the dynamics of cortical electrical activity while subjects detected and manually responded to visual targets, viewing signals retained in ERP averages not as responses of an otherwise silent system but as resulting from event-related alterations in ongoing EEG processes. We applied infomax independent component analysis to parse the dynamics of the unaveraged 31-channel EEG signals into maximally independent processes, then clustered the resulting processes across subjects by similarities in their scalp maps and activity power spectra, identifying nine classes of EEG processes with distinct spatial distributions and event-related dynamics. Coupled two-cycle postmotor theta bursts followed button presses in frontal midline and somatomotor clusters, while the broad postmotor “P300” positivity summed distinct contributions from several classes of frontal, parietal, and occipital processes. The observed event-related changes in local field activities, within and between cortical areas, may serve to modulate the strength of spike-based communication between cortical areas to update attention, expectancy, memory, and motor preparation during and after target recognition and speeded responding.

The brain in infantile autism Posterior fossa structures are abnormal

Infantile autism is a neurologic disorder of social, cognitive, and language development. Earlier MRI studies found hypoplasia of posterior vermal lobules VI and VII and cerebellar hemispheres in the majority of autistic patients, and recent autopsy analyses find severe Purkinje neuron loss in the posterior vermis (lobules VI and VII and VIII to X) and hemispheres. A second type of cerebellar pathology in infantile autism was recently found: hyperplasia (excessive enlargement) of posterior vermal lobules VI and VII. If the autistic samples in some MRI studies that did not detect cerebellar hypoplasia were actually composed of both the hypoplasia and hyperplasia subtypes, then the autistic mean size reported in such studies would have appeared to be near the normal mean size only because it would be the sum of the two opposite subtypes. To test this possibility, we statistically reanalyzed previously published vermal area measures of 78 autistic patients from four separate studies. The results revealed that the autistic patient samples from these four studies were indeed composed of both the hypoplasia subtype (87%, 92%, 89%, and 84% of patients) and the hyperplasia subtype (13%, 8%, 11%, and 16% of patients). Cerebellar abnormalities have been found in 15 autopsy and quantitative MRI reports from nine laboratories involving a total of 226 autistic eases. Autism may be one of the first developmental neuropsychiatric disorders for which substantial concordance exists among several independent microscopic and macroscopic studies as to the location and type of neuroanatomic maldevelopment. Onset might be as early as the second trimester. Discovery of the etiologies underlying cerebellar maldevelopment may be the key to uncovering some of the causes of infantile autism.

Prenatal, Perinatal, and Neonatal Factors in Autism, Pervasive Developmental Disorder-Not Otherwise Specified, and the General Population

Objectives. To examine various pre-, peri-, and neonatal factors in autistic participants and in pervasive developmental disorder-not otherwise specified (PDD-NOS) participants and to compare the incidence of each factor to that of the normal population. Methods. Seventy-four participants (66 males, 8 females) were diagnosed with autism at 2.5 through 4 years of age using the most accurate and up-to-date methods, including the Diagnostic and Statistical Manual of Mental Disorders and the Autism Diagnostic Interview-Revised. At age 5, all participants were reevaluated using the Diagnostic and Statistical Manual of Mental Disorders, the Autism Diagnostic Interview-Revised, the Childhood Autism Rating Scale, and the Autism Diagnostic Observation Schedule-Revised, resulting in 61 autistic and 13 PDD-NOS participants. Twenty-eight pre-, peri-, and neonatal factors were examined in these 2 groups using both medical records and parental interviews. Incidences were compared with those of the US population as reported in the Report of Final Natality Statistics, 1995. This grand scale population group was used to closely approximate comparison to a normal, unbiased population. Results were analyzed using the binomial probability test, with a P value of <.05, constituting a significant difference in incidence. A Bonferroni correction was applied to the data to adjust for the number of factors investigated. Results. Although most of the factors showed comparable incidences between the index and control groups, several factors showed statistically significant differences. Following the Bonferroni correction, the autism group was found to have a significantly higher incidence of uterine bleeding, a lower incidence of maternal vaginal infection, and less maternal use of contraceptives during conception when compared with the general population. Similarly, the PDD-NOS group showed a higher incidence of hyperbilirubinemia when compared with the general population. Conclusions. The results of this study support previous findings suggesting a consistent association of unfavorable events in pregnancy, delivery, and the neonatal phase and the pervasive developmental disorders. However, interpretation of the meaningfulness of these results is difficult, as the specific complications that carried the highest risk of autism and PDD-NOS represented various forms of pathologic processes with no presently apparent unifying feature. Additional studies are needed to corroborate and strengthen these associations, as well as to determine the possibility of an underlying unifying pathological process. This studys analysis of obstetric and neonatal complications in combination with the use of participants diagnosed at an early age provides some interesting concepts to consider. Perhaps future research will confirm certain pre-, peri-, and neonatal associations that could be used to generate a high-risk historical profile with which to use in conjunction with currently employed diagnostic tools. This may, in turn, help to determine the reliability of a diagnosis of autism in younger children, leading to earlier intervention and assistance for an improved outcome in long-term functionality and quality of life.

Neuroanatomic contributions to slowed orienting of attention in children with autism

Previous research has demonstrated that adult autistic patients are abnormally slow to orient attention, with degree of slowed orienting associated with severity of cerebellar hypoplasia. This research was extended to children who, at ages two through six, met diagnostic criteria for autism and underwent magnetic resonance imaging (MRI). An average of 3 years later, when old enough to participate in behavioral experiments, the children returned to the laboratory and completed a spatial attention paradigm. Degree of slowed attentional orienting to visual cues was significantly correlated with degree of cerebellar hypoplasia, but not with size of other neuroanatomic regions. Additionally, there was a trend for orienting speed to differ between diagnostic outcome subgroups; children with confirmed diagnoses of autism at time of behavioral testing had larger orienting deficits than those who no longer met diagnostic criteria for autism. This research is among the first to establish a specific brain-behavior link in autistic children.

Visual attention abnormalities in autism: Delayed orienting to location

These studies provide evidence for slowed spatial orienting of attention in autism. A group of well-defined adult autistic subjects and age-matched normal controls performed a traditional spatial cueing task in which attention-related response facilitation is indexed by speed of target detection. To address the concern that motor impairment may interfere with interpretation of response time measures in those with neurologic abnormality, we also used a new adaptation of the traditional task that depended on accuracy of response (target discrimination) rather than speed of response. This design allowed separation of time to process and respond to target information from the time to move and engage (orient) attention. Results from both tasks were strikingly similar. Normal subjects oriented attention very quickly, and showed maximal performance facilitation at a cued location within 100 ms. Autistic subjects oriented attention much more slowly and showed increasing benefits of a spatial cue with increasing cue-to-target delays. These results are consistent with previous reports that patients with autism, the majority of whom have developmental abnormalities of the cerebellum, as well as those with acquired damage to the cerebellum, are slow to shift attention between and within modalities. This paper also addresses the variability in behavioral findings in autism, and suggests that many of the apparently contradictory findings may actually reflect sampling differences in patterns of brain pathology.

Atypical attentional networks and the emergence of autism.

The sociocommunicative impairments that define autism spectrum disorder (ASD) are not present at birth but emerge gradually over the first two years of life. In typical development, basic attentional processes may provide a critical foundation for sociocommunicative abilities. Therefore early attentional dysfunction in ASD may result in atypical development of social communication. Prior research has demonstrated that persons with ASD exhibit early and lifelong impairments in attention. The primary aim of this paper is to provide a review of the extant research on attention in ASD using a framework of functionally independent attentional networks as conceptualized by Posner and colleagues: the alerting, orienting and executive control networks (Posner and Petersen, 1990; Petersen and Posner, 2012). The neural substrates and typical development of each attentional network are briefly discussed, a review of the ASD attention literature is presented, and a hypothesis is proposed that links aberrant attentional mechanisms, specifically impaired disengagement of attention, with the emergence of core ASD symptoms.