David Galin

Asymmetry in evoked potentials as an index of lateralized cognitive processes: Relation to EEG alpha asymmetry ☆

Abstract Flash evoked potentials and background EEG were recorded from left and right temporal and parietal areas while subjects performed verbal and spatial tasks. Changes in asymmetry of evoked potential power and peak-trough amplitudes paralleled the task-dependent asymmetry of the EEG alpha power. Both the EEG and evoked potential asymmetry measures reflected the hemispheric specialization for these cognitive tasks.

Lateral eye movement and cognitive mode

The direction of lateral eye movement elicited by reflective questions is strongly modified by the cognitive demands of the question; verbal and arithmetical questions elicit more movements to the right than do spatial and musical questions. This is consistent with the lateral specialization of the human brain for these cognitive processes. These results support the hypothesis that the direction of the lateral eye movement indicates the activation of the contralateral cerebral hemisphere.

Individual differences in cognitive style—I. Reflective eye movements☆

Abstract Reflective eye movements were studied in subjects whose vocations emphasize either verbal-analytic or spatial-holistic cognitive modes: lawyers and ceramicists (N=35). Questions demanding verbal or spatial thought were administered, and the first eye movement following the question was scored in both vertical and lateral directions. Lawyers and ceramicists differed only in the vertical direction; more up movements for ceramicists than for lawyers. The effects of question type were studied in this population and in a group of non-specialized subjects (N=19). The results confirm our previous findings: verbal questions evoke more right movements than spatial questions. In addition, verbal questions evoked more down movements than spatial questions.

Neurometrics does not detect ‘pure’ dyslexics ☆

Thirty-eight severely dyslexic boys and 38 good readers were evaluated with neurometrics, a diagnostic procedure based on the application of numerical taxonomy to EEG spectra obtained during resting conditions, supplemented by selected evoked potential features. This procedure generates deviance scores for the EEG spectra by comparing each individuals values to those obtained from a normative population and has been reported to discriminate learning disabled children from normal controls (Ahn et al. 1980). In the present study, all subjects, dyslexic and control, passed stringent screening to assure normal intellectual, neurological, sensory and emotional status. The false positive rate obtained in our control group was comparable to that reported earlier. However, none of the deviance scores significantly discriminated dyslexics from controls; most subjects from both groups were classified as normal. Severe dyslexia per se is thus not associated with the specific neurometric abnormalities reported previously in more heterogeneous learning disabled populations.

EEG power spectra in normal and dyslexic children. I. Reliability during passive conditions

Abstract Three minutes of passive eyes-closed (EC) and eyes-open (EO) EEG were recorded before and after 4–5 h of behavioral tasks in 10–12-year-old boys of normal intelligence and neurological status. Half were severely reading disabled; half were reading normally. Bilateral central, parietal, and mid-temporal EEG referenced both to vertex and to linked ears was recorded. Averaged FFT power spectra of artifact-free 1 sec epochs were computed within each recording condition. Test-retest reliabilities were computed for each band and for the entire spectrum using intra-class correlations (ICC). Reliabilities were assessed separately for each group, lead and reference condition, and for absolute power and relative power. The results reveal excellent reliability in the normal reading group. Reliabilities are higher with the Cz reference than the A1A2 reference, primarily in the delta and beta2 bands. Temporal recordings have lower reliabilities than central or parietal leads. These effects summate to yield poor reliabilities for the delta and beta2 bands for temporal-A1A2 recordings. Reliabilities for the dyslexic group are lower than control group values, yet are still acceptably high. Beta2 ICCs were markedly reduced in the dyslexics, possibly reflective of increased EMG artifact. Our finding that absolute power is a reliable as relative power is at odds with the report of John et al. (1980) that absolute power was not reliable enough to be useful in automated EEG assessment procedures. Use of absolute power is warranted whenever possible, since the interpretation of findings based solely upon relative power can be ambiguous. Our results indicate that under properly controlled conditions, excellent reliability of both absolute and relative power even for the passive EC and EO conditions can be obtained. These findings support the utility of EEG power spectra as a reliable index of brain function for studies of normal and learning disabled children.

EEG spectra in dyslexic and control boys during resting conditions

We studied the resting eyes open and eyes closed EEG in carefully screened samples of 9-13-year-old dyslexic and control boys within a 2-cohort cross-validation design with repeat testing 1-3 years later. We found: no difference between groups in delta or theta activity in either cohort or in repeat testing 1-3 years later; in the dyslexics, decreased beta (19-24 c/sec) activity at bilateral central, parietal and mid-temporal leads referenced to vertex; inconsistent group differences in the alpha band: the first cohort dyslexics had lower alpha power than did the controls, but there was no group difference in the second cohort; and significant relative delta differences between groups in the first cohort, which were shown to be a statistical artifact of the use of relative power. We conclude that: dyslexia per se is not associated with increased absolute power in the delta and theta bands; lower power in the high beta band is reliably found in these samples of dyslexics without other disorders; and alpha power levels are not consistently lower in the dyslexic group.

EEG spectra in 9–13-year-old boys are stable over 1–3 years

Good reliability was found in EEG spectra recorded during resting conditions in two studies 1-3 years apart in groups of adolescent dyslexic and control boys who were well screened for neurological, sensory and emotional disabilities. The studies used different equipment and analysis (FFT in study 1 vs. iterative digital bandpass filters in study 2). The central leads (C3 and C4) referenced to vertex were recorded in both studies. Pearson correlation coefficients were computed as reliability estimates for each spectral band separately for each group, for absolute and relative power and for eyes open and eyes closed recording conditions. For both groups and for all conditions, the absolute power reliabilities were above 0.71 for all bands except delta for which they were above 0.54. Reliabilities were more variable for relative power measures.

Regional brain activity in dyslexic and control children during reading tasks: Visual probe event-related potentials ☆

Event-related potentials (ERPs) elicited by task-irrelevant visual stimuli were recorded from 34 control and 32 dyslexic 10- to 12-year-old boys while they performed silent and oral reading at two levels of difficulty. All subjects were extensively screened for neurological problems, IQ, and sensory acuity. Specific features of the ERPs were affected in amplitude and latency by the experimental variables: group membership, oral vs. silent reading, difficulty level, and recording site (within and between hemispheres). A specific effect of difficulty on the central and parietal ERP was seen in the dyslexics but not the controls. Different patterns of asymmetry were found for the two groups in silent vs. oral reading at midtemporal placements. A marked asymmetry (R greater than L) was found at the midtemporal region for both groups.

Tracking Eye Movements Are Normal in Dyslexic Children

ABSTRACT We measured saccadic and smooth eye tracking movements in 34 dyslexic and 35 control subjects and were unable to show differences in any of the tasks or parameters measured. Subjects were carefully screened for neurological, optometric, audiological, and behavioral problems before admission to the study. We suggest that differences between our results and those of previous investigators are based on sampling differences. We conclude that while there may be children with reading disorders secondary to visual or oculomotor deficits, dyslexia can and does exist independent of such deficits. Because, in this and other studies, we have ruled out peripheral involvement, we feel that dyslexia is primarily a deficit of central information processing of written or spoken material.

Sex and handedness differences in EEG measures of hemispheric specialization

Abstract EEG alpha asymmetry was studied in 90 normal adults: right-handed, left-handed, and ambidextrous, male and female. Recordings were made from homologous central, parietal, and occipital leads, referenced to vertex, while subjects engaged in writing, speaking, reading, listening to speech, singing, and block design construction. These data confirm our previous findings that alpha asymmetry is task-dependent and extend them to a broader range of tasks, subjects, and leads. Among right-handers significant differences were found between the language tasks and the musical and spatial tasks: the R L alpha ratio is higher in the language tasks. In addition, significant ordering of R L alpha ratios was found among the language tasks themselves: WRITE a SPEAK > READ > LISTEN. No one “verbal” task can be considered representative of all language behaviors. Task differences in asymmetry were greater at the central than at the parietal leads, and no differences were found at the occiput. Differences among the handedness groups were found in R L alpha ratio in specific tasks, in the relationship among tasks, and in alpha power level. Non-right-handers showed less task-dependent asymmetry. On some measures ambidexters appear to be a distinct group, not simply representing the middle range of a left-handed/right-handed continuum. Reversal of the “expected” right-handed pattern (SPEAK R L ratio > BLOCKS R L ratio) was seen in 10% of right-handers, and in 36% of left-handers, particularly among left-handed females (46%), suggesting a possible sex difference among non-right-handers. No sex difference was found among right-handers on any task with any measure at any lead.