Jack Johnstone

Differential right hemisphere engagement in visuospatial tasks.

Abstract Ten male and ten female normal people performed six spatial tasks and one verbal task while EEG alpha was recorded from parietal and central areas bilaterally. The tasks were controlled to equate gross motor activity and stimulus characteristics across tasks. The spatial tasks engaged the right hemisphere more than the verbal task, withtthe exception of complex mental rotation, which showed an EEG pattern similar to the verbal task. Most of the differences in EEG patterns between tasks were accounted for by differences in right hemisphere engagement. Two unexpected findings were a higher right hemisphere engagement in whole-whole matching compared to part-whole matching, and a high left hemisphere engagement in mental rotation. These unexpected findings may be due to analytic strategies having been used by people performing part-whole matching and mental rotation.

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.

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.

EEG spectra in dyslexic and normal readers during oral and silent reading

EEGs of extensively screened dyslexics and normal readers were recorded while they read easy and difficult texts silently and orally, and during two other verbal tasks which also differed in overt speaking but had no reading component: narrative speaking and listening to a story. Mid-temporal, central and parietal leads were referenced to linked ears and to Cz. Large differences between tasks and between groups were found. With the linked ears reference, power was higher in all bands in oral reading than in silent reading, with the largest change occurring in the temporal leads. In the theta and low beta bands the difference between oral and silent reading was greater for controls than for dyslexics. These effects were not accounted for by differences in reading speed or in difficulty. Similar results were found in two cohorts of subjects. The difference between groups in theta was found only in the reading tasks. In contrast, the group difference in low beta was also found in the change from listening to speaking. This implies that the oral-silent group difference in theta is related to some aspect of the reading tasks other than the presence or absence of overt speaking, and that the low beta group difference is related to some aspect of overt speaking rather than to reading per se. With the Cz reference no group differences were found. It is suggested that the groups differ in the reading strategies they use, and the degree to which they shift strategy between the silent and oral tasks. We hypothesize that these cognitive differences are reflected in the theta activity from the temporal lobe. While there were many differences between the tasks in alpha power and asymmetry, no group differences involving alpha were found.

Choice of Handedness Measures in Studies of Hemispheric Specialization

Left-and right-handers have been reported to differ in lateral hemispheric specialization for cognition. We compared different methods of dividing subjects into handedness groups: preference(a 12-item questionnaire), performance(speed, strength, dexterity), and preference plus performance, and determined which method of handedness classification indicated the greatest group differences on EEG and dichotic measures of lateral specialization. All handedness measures were significantly intercorrelated. These handedness measures were significantly correlated with the dichotic test over the whole population but were significantly correlated with EEG asymmetry only in females. Scoring degree of handedness showed significant relationships to lateral specialization which were not seen when subjects were classified into discrete handedness groups. Ambidextrous subjects performed as well as right- or left-handers on unimanual tasks despite a lack of hand preference. The hand used for writing was shown to be too limited to be used as the sole index of handedness in studies related to cognition.

EEG alpha asymmetry in dyslexics during speaking and block design tasks.

Lateral specialization for speech and spatial construction was investigated in a highly screened group of dyslexic and normal-reading boys, age 9-13, by examining the extent to which their EEG alpha asymmetry changed from one task to the other. EEG was recorded from central, midtemporal, and parietal leads during several minutes of spontaneous narrative speech, and while the child constructed block designs. The dyslexics showed the same strongly task-dependent asymmetry as the normal readers, comparable to that observed in our adult populations. The observations were confirmed with a second independently chosen cohort of subjects. This result is discussed in terms of the constraints it places on models of dyslexia which invoke disorders of lateral specialization and hemisphere integration.

Narrative speech deficits in dyslexics.

Narrative speech of 31 dyslexics and 33 good readers was studied. Subjects were 10- to 12-year-old right-handed boys of normal intelligence, and with normal neurological status, vision, and hearing. The dyslexics used shorter communication units (independent clauses with all their modifiers), and a higher percentage of their words were noncommunications (words which are extraneous to the speakers intended meaning). Rate of speech was the same in both groups. These findings were replicated in an independent second cohort of 21 dyslexics and 21 controls, aged 9 to 13. Our findings support theories of a general language deficit in dyslexia, and demonstrate the involvement of the previously unexamined area of narrative speech.