Acta Neurol. Scandinav. 51, 380-384, 1975
EEG Department, Section of Neurological Sciences, The London Hospital, London, England.
THE DEGREE OF ALPHA ASYMMETRY AND ITS RELATION TO HANDEDNESS IN NEUROPSYCHIATRIC REFERRALS
R. CABRALand D. F. SCOTT ABSTRACT While alpha rhythm asymmetry among normals is thoroughly documented, t h e significance of this finding i n patients is not well known. The occurrence of alpha amplitude asymmetry i n 82 clinical referrals of neuropsychiatric patients was studied i n this research. Two separate means of quantifying the alpha asymmetry were used: The first by visual inspection; and the second by measurement i n a standard manner. A handedness questionnaire was administered and indicated t h a t 77 of the sample were right- and five were left-handed patients. By measurement, the mean overall differences i n the alpha amplitude between the hemisphere was small ( 4 per cent) and non-significant. Seventeen patients showed a difference of 15 per cent or greater, and in two t h e difference reached 33 per cent. I t was concluded that alpha asymmetry must be interpreted with care because it was found that there was a n association between left-temporal abnormalities and ipsilateral alpha enhancement, and the greater degrees of asymmetry were found i n normal EEGs.
It is widely held that the alpha rhythm may be asymmetrical (Kiloh ef al. 1972). While this is thoroughly documented among normals (Cornil & Gastauf 1947), the significance of this finding in patients is not well known. The present study was set up to examine alpha asymmetry in detail, using both visual inspection and careful measurement of the alpha activity in a seriLs of patient referrals using the EEG recording techniques in routine use. In addition, we assessed handedness (Oldfield 1971) and attempted to relate this to the degree of alpha asymmetry. METHOD AND MATERIALS I t was initially intended t h a t the project should embrace all referrals to the EEG department during a single month. It proved necessary to exclude those patients
38 1 who were incapable, f o r a variety of reasons, of answering questions o n handedness; also those whose routine EEG showed no alpha activity because of disturbance in level of arousal o r major EEG abnormalities. The sample obtained was of 82 patients: 49 males and 33 females, with an average age of 39 years. Twenty of the sample were referred with a diagnosis of epilepsy, 19 f o r undiagnosed loss of consciousness, 12 with possible space-occupying lesions, and 11 for assessment prior to cardiac surgery. The remainder had a variety of psychological and neurological disorders. The recordings were taken i n a standard manner, using Schonander 16 channel apparatus with 10/20 electrode placements, and a sensitivity of 50 p / c m . Bipolar and common-average derivations, a s routinely employed i n the department, were used. On routine assessment, it was found that 26 patients had normal tracings; 20 had left-temporal and 3 right-temporal abnormalities, generally of minor episodic type; 12 had generalized disturbance; and, i n 15, a variety of other features were seen. A handedness questionnaire was administered by t h e technician. I t consisted of 1 2 simple questions concerning the hand used f o r daily tasks, such as writing and eating. Subsequently the laterality quotient was calculated (Oldfield 1971) and showed t h a t t h e sample consisted of 77 right- and five left-handed patients. Two independent EEG assessments were made. The first was a routine visual assessment by D.F.S. The second was by measurement using a cursor by R.C. Altogether, 20 measurements of peak-to-peak alpha amplitude were obtained for each tracing. They were based on a series of 1-sec epochs, a t least 2 sec before or after eye opening o r closure i n which t h e alpha could be seen rhythmically. Sections of the EEGs i n which drowsiness occurred were excluded. The recordings made during overbreathing and photic stimulation were also excluded. For common average reference recording, two measurements were made i n each of t h e occipital, parietal and posterior-temporal leads on each side making a total of 12. On bipolar recording eight measurements were taken, two from each of the parieto-occipital and posterior-temporo-occipital channels on each side. Alpha frequency was also noted for all these 20 measured epochs.
RESULTS
The alpha rhythm was judged by visual assessment on bipolar recording to be symmetrical in 47 of the 7 7 right-handed patients. It was of greater amplitude over the right hemisphere in 22, and over the left hemisphere in 8 patients (see Table 1 ) . Of the five left-handed patients, two had a symmetrical alpha rhythm and two an alpha of greater amplitude over the left hemisphere. By measurement, on bipolar recording, 35 of the 77 right-handed patients had an alpha rhythm of greater amplitude over the right hemisphere, and for 12 it was found to be symmetrical. Among the right-handers, the mean alpha amplitude measured 26 pv (SD = 7.4) over the right, and 25 pv ( SD= 6.7) over the l e f t hemisphere. For the left-handers comparable f i g u r e s were
26.5 pv (SD = 5.6) and 27.6 pv (SD = 6.5), respectively. Common average reference yielded similar results. The overall mean asymmetry 25
ACTA NEUROL.
SCARD.
51,5
Table 1. Alpha asymmetries on bipolar recording in 77 right-handed and 5 left-handed patients.
Symmetrical alpha By visual assessment R. handed L. handed By measurement R. handed L. handed
Greater amplitude Greater amplitude on the right on the left
47 (57 Yo) 2 ( 2.5 %) 12 ( 1 4 1 ( 1
Yo) %)
8 (10 %) 2 ( 2.5 %)
35 (43 %) 2 ( 2.5 %)
30 (37 % ) 2 ( 2.5 %)
22 (27 1 ( 1
%) 70)
of alpha in the whole group was 1 pv (4 per cent), a difference which did not prove to be statistically significant. The mean frequency over both hemispheres was approximately the same at 9.1 Hz (SD = 0.5). A sub-group of 17 right-handed patients presented an asymmetry of alpha which was 5 pv ( 15 % ) or greater; in two, the asymmetry was of 33 per cent. Twelve of the patients in this sub-group had an alpha which was of greater amplitude over the right hemisphere, and in the remainder it was symmetrical. Comparison between the two types of assessment was then made, and there was agreement in 12 of the 17. In these, the mean alpha asymmetry was 6.3 pv (18 per cent), while in those with a discrepant assessment, the mean asymmetry was found to be 5.5 pv, 15.6 per cent. When the relationship between the measured alpha asymmetry and EEG abnormality (see Table 2 ) was considered, it was found that of Table 2. The relationship of EEG abnormalitu to alpha r h y t h m , amplitude determined bgj measurement in 77 right-handed patPenfs.
No. of subjects
Alpha greater over right hemisphere
Alpha greater over left hemisphere
26 (34 %)
1 4 (54 Yo)
9 (34 %)
3 (12 %)
%)
4 (33 %)
2 (17 %)
5 (25 %)
11 (55 %)
4 (20 %)
4 ( 5 To)
2 (50 To)
2 (50 %)
Alpha
Normal Generalized abnormality Left-temporal abnormality Right-temporal abnormality Other abnormalities
20 (25
16 (20 %)
8 (53 %)
4 (27 %)
3 (20 %)
Total sample
7 7 (100%)
35 (46 %)
30 (38 % )
12 (16 9%)
12 (16 %)
70)
6 (50
383 the 26 patients with normal EEGs, in 14 the alpha rhythm was greater over the right, and in 9 it was greater over the left hemisphere. I n the remainder, the alpha was symmetrical. This contrasted with the 20 patients who had a left temporal abnormality, of whom only 5 had an alpha amplitude greater over the right hemisphere. In 11, the alpha was greater over the left hemisphere, and in the remainder this activity was symmetrical. It is of interest that out of 17 patients who had an alpha asymmetry of greater than 15 per cent, 9 had normal EEGs and 4 had a left-sided abnormality. Both EEGs which showed discrepancy in alpha betwecn the hemisphere of 33 per cent were normal.
DISCUSSION
Many studies have been carried out on alpha asymmetry in normal subjects (Cobb 1963, Lachaud e f al. 1972). Further, the effects of various tasks on the alpha rhythm in normals have also been studied. (Butler & Glass 1974, Cabral & S c o f f 1974). However, the aim of the present investigation was a different one: to study the degree of alpha asymmetry which could be expected in clinical EEG referrals using bipolar and common-average derivations as routinely employed. Another part of the study involved a comparison between judgement of alpha asymmetry by the routine visual inspection and by measurement in a standard manner. In addition, the alpha asymmetry was related to the patient’s handedness, as tested by an inventory (Oldfield 1971). Taking all the patients together, whether left- or right-handed, the difference between the hemispheres in alpha amplitude by measurement was non-significant. Neither of the two methods of assessment showed that the majority of right-handed subjects had alpha activity which was of greater amplitude over the right side, a view commonly held for normal subjects (Riloh e f al. 1972). Within the sub-group of about 20 per cent of patients who showed an alpha asymmetry of 15 per cent or greater, the situation was somewhat different in that the majority of these right-handed patients showed alpha which was of greater ampliture over the right hemisphere. It is of interest that only in this sub-group was there a clear degree of concordance between the two methods of assessment; and we were able to determine that an assymmtry between the hemisphere of about 16 per cent was needed for agreement. The general view is that in patients with unilateral, posteriorly placed lesions the alpha is asymmetrical o r less prominent on the affected side (e.g. Fischer-Williams e f al. 1962). However, this effect is 25.
384
not clear-cut because, in some instances, it is said that ipsilateral alpha augmentation occurs (e.g. Van d e r Drift 1957, Kiloh e t al. 1972). In this connection, in more than 50 per cent of the patients reported here who showed left-temporal abnormilities in the EEG, the alpha was augmented on that side. This is one reason for interpreting alpha asymmetries with caution. Another is the fact that (in the present series) the majority of patients with greater degrees of alpha asymmetry the EEGs were found to be normal.
REFERENCES Butler, S. R. & A. Glass (1974) : Asymmetries in the electroencephalogram associated with cerebral dominance. Elcctroenceph. clin. Neurophysiol. 36, 481-491. Cabral, R . & D. F. Scott (1974): Handedness and alpha asymmetry i n the EEG. Electroenceph. clin. Neurophysiol. 37, 325. Cobb, W. A. (1963): The normal adult EEG; t h e EEG of lesions i n general; the EEG of specific lesions. Electroencephalography, ed. D. Hill & G. Parr, p. 509. Macdonald, London. Cornil, L. & H. Gastaut (1947) : Btude Clectroenckphalographique de la dominance sensorielle d’un hkmisphtke cCr6brale. Presse m6d. 37, 421-422. Fischer-Williams, R., S. L. Last, G. Lybcri & D. W. C. Northfield (1962): ClinicoEEG study on 128 gliomas and 50 intracranial metastatic tumours. Brain 85, 1-46. Kiloh, L. G., A. J. McComas & J. Osselton (1972) : Clinical Electroencephalography, pp. 53 and 134. Butterworths, London. Lachaud, J. C., J. H. Lavoisy & H. Dupont (1972) : Absence of statistical correlation between alpha rhythm amplitude and handedness. Electroenceph. clin. Neurophysiol. 33, 534. Margerison, J. H., P. St. J. Loe & C. D. Binnie ( 1 9 6 7 ) : Electroencephalography. Manual of Psychophysiological Methods, ed. P. H. Venables & I. Martin, pp. 387-388. North Holland Publishing Co., Amsterdam. Oldfield, R. C. (1971) : The assessment and analysis of handedness. The Edinburgh inventory. Ncuropsychologia 9, 97-113. Van der Drift, J. H. A. (1957) : The Significance of t h e Electroencephalography for Diagnosis and Localization of Cerebral Tumours, p. 22. H. E. Stenfert Kroese. N.V, Leiden. Received November 22, 1974.
D . F . S c o t t , M.R.C.P., D.P.M. The EEG Department The London Hospital Whitcchapel London E 1 1 BB England
EEG Department, Section of Neurological Sciences, The London Hospital, London, England.
THE DEGREE OF ALPHA ASYMMETRY AND ITS RELATION TO HANDEDNESS IN NEUROPSYCHIATRIC REFERRALS
R. CABRALand D. F. SCOTT ABSTRACT While alpha rhythm asymmetry among normals is thoroughly documented, t h e significance of this finding i n patients is not well known. The occurrence of alpha amplitude asymmetry i n 82 clinical referrals of neuropsychiatric patients was studied i n this research. Two separate means of quantifying the alpha asymmetry were used: The first by visual inspection; and the second by measurement i n a standard manner. A handedness questionnaire was administered and indicated t h a t 77 of the sample were right- and five were left-handed patients. By measurement, the mean overall differences i n the alpha amplitude between the hemisphere was small ( 4 per cent) and non-significant. Seventeen patients showed a difference of 15 per cent or greater, and in two t h e difference reached 33 per cent. I t was concluded that alpha asymmetry must be interpreted with care because it was found that there was a n association between left-temporal abnormalities and ipsilateral alpha enhancement, and the greater degrees of asymmetry were found i n normal EEGs.
It is widely held that the alpha rhythm may be asymmetrical (Kiloh ef al. 1972). While this is thoroughly documented among normals (Cornil & Gastauf 1947), the significance of this finding in patients is not well known. The present study was set up to examine alpha asymmetry in detail, using both visual inspection and careful measurement of the alpha activity in a seriLs of patient referrals using the EEG recording techniques in routine use. In addition, we assessed handedness (Oldfield 1971) and attempted to relate this to the degree of alpha asymmetry. METHOD AND MATERIALS I t was initially intended t h a t the project should embrace all referrals to the EEG department during a single month. It proved necessary to exclude those patients
38 1 who were incapable, f o r a variety of reasons, of answering questions o n handedness; also those whose routine EEG showed no alpha activity because of disturbance in level of arousal o r major EEG abnormalities. The sample obtained was of 82 patients: 49 males and 33 females, with an average age of 39 years. Twenty of the sample were referred with a diagnosis of epilepsy, 19 f o r undiagnosed loss of consciousness, 12 with possible space-occupying lesions, and 11 for assessment prior to cardiac surgery. The remainder had a variety of psychological and neurological disorders. The recordings were taken i n a standard manner, using Schonander 16 channel apparatus with 10/20 electrode placements, and a sensitivity of 50 p / c m . Bipolar and common-average derivations, a s routinely employed i n the department, were used. On routine assessment, it was found that 26 patients had normal tracings; 20 had left-temporal and 3 right-temporal abnormalities, generally of minor episodic type; 12 had generalized disturbance; and, i n 15, a variety of other features were seen. A handedness questionnaire was administered by t h e technician. I t consisted of 1 2 simple questions concerning the hand used f o r daily tasks, such as writing and eating. Subsequently the laterality quotient was calculated (Oldfield 1971) and showed t h a t t h e sample consisted of 77 right- and five left-handed patients. Two independent EEG assessments were made. The first was a routine visual assessment by D.F.S. The second was by measurement using a cursor by R.C. Altogether, 20 measurements of peak-to-peak alpha amplitude were obtained for each tracing. They were based on a series of 1-sec epochs, a t least 2 sec before or after eye opening o r closure i n which t h e alpha could be seen rhythmically. Sections of the EEGs i n which drowsiness occurred were excluded. The recordings made during overbreathing and photic stimulation were also excluded. For common average reference recording, two measurements were made i n each of t h e occipital, parietal and posterior-temporal leads on each side making a total of 12. On bipolar recording eight measurements were taken, two from each of the parieto-occipital and posterior-temporo-occipital channels on each side. Alpha frequency was also noted for all these 20 measured epochs.
RESULTS
The alpha rhythm was judged by visual assessment on bipolar recording to be symmetrical in 47 of the 7 7 right-handed patients. It was of greater amplitude over the right hemisphere in 22, and over the left hemisphere in 8 patients (see Table 1 ) . Of the five left-handed patients, two had a symmetrical alpha rhythm and two an alpha of greater amplitude over the left hemisphere. By measurement, on bipolar recording, 35 of the 77 right-handed patients had an alpha rhythm of greater amplitude over the right hemisphere, and for 12 it was found to be symmetrical. Among the right-handers, the mean alpha amplitude measured 26 pv (SD = 7.4) over the right, and 25 pv ( SD= 6.7) over the l e f t hemisphere. For the left-handers comparable f i g u r e s were
26.5 pv (SD = 5.6) and 27.6 pv (SD = 6.5), respectively. Common average reference yielded similar results. The overall mean asymmetry 25
ACTA NEUROL.
SCARD.
51,5
Table 1. Alpha asymmetries on bipolar recording in 77 right-handed and 5 left-handed patients.
Symmetrical alpha By visual assessment R. handed L. handed By measurement R. handed L. handed
Greater amplitude Greater amplitude on the right on the left
47 (57 Yo) 2 ( 2.5 %) 12 ( 1 4 1 ( 1
Yo) %)
8 (10 %) 2 ( 2.5 %)
35 (43 %) 2 ( 2.5 %)
30 (37 % ) 2 ( 2.5 %)
22 (27 1 ( 1
%) 70)
of alpha in the whole group was 1 pv (4 per cent), a difference which did not prove to be statistically significant. The mean frequency over both hemispheres was approximately the same at 9.1 Hz (SD = 0.5). A sub-group of 17 right-handed patients presented an asymmetry of alpha which was 5 pv ( 15 % ) or greater; in two, the asymmetry was of 33 per cent. Twelve of the patients in this sub-group had an alpha which was of greater amplitude over the right hemisphere, and in the remainder it was symmetrical. Comparison between the two types of assessment was then made, and there was agreement in 12 of the 17. In these, the mean alpha asymmetry was 6.3 pv (18 per cent), while in those with a discrepant assessment, the mean asymmetry was found to be 5.5 pv, 15.6 per cent. When the relationship between the measured alpha asymmetry and EEG abnormality (see Table 2 ) was considered, it was found that of Table 2. The relationship of EEG abnormalitu to alpha r h y t h m , amplitude determined bgj measurement in 77 right-handed patPenfs.
No. of subjects
Alpha greater over right hemisphere
Alpha greater over left hemisphere
26 (34 %)
1 4 (54 Yo)
9 (34 %)
3 (12 %)
%)
4 (33 %)
2 (17 %)
5 (25 %)
11 (55 %)
4 (20 %)
4 ( 5 To)
2 (50 To)
2 (50 %)
Alpha
Normal Generalized abnormality Left-temporal abnormality Right-temporal abnormality Other abnormalities
20 (25
16 (20 %)
8 (53 %)
4 (27 %)
3 (20 %)
Total sample
7 7 (100%)
35 (46 %)
30 (38 % )
12 (16 9%)
12 (16 %)
70)
6 (50
383 the 26 patients with normal EEGs, in 14 the alpha rhythm was greater over the right, and in 9 it was greater over the left hemisphere. I n the remainder, the alpha was symmetrical. This contrasted with the 20 patients who had a left temporal abnormality, of whom only 5 had an alpha amplitude greater over the right hemisphere. In 11, the alpha was greater over the left hemisphere, and in the remainder this activity was symmetrical. It is of interest that out of 17 patients who had an alpha asymmetry of greater than 15 per cent, 9 had normal EEGs and 4 had a left-sided abnormality. Both EEGs which showed discrepancy in alpha betwecn the hemisphere of 33 per cent were normal.
DISCUSSION
Many studies have been carried out on alpha asymmetry in normal subjects (Cobb 1963, Lachaud e f al. 1972). Further, the effects of various tasks on the alpha rhythm in normals have also been studied. (Butler & Glass 1974, Cabral & S c o f f 1974). However, the aim of the present investigation was a different one: to study the degree of alpha asymmetry which could be expected in clinical EEG referrals using bipolar and common-average derivations as routinely employed. Another part of the study involved a comparison between judgement of alpha asymmetry by the routine visual inspection and by measurement in a standard manner. In addition, the alpha asymmetry was related to the patient’s handedness, as tested by an inventory (Oldfield 1971). Taking all the patients together, whether left- or right-handed, the difference between the hemispheres in alpha amplitude by measurement was non-significant. Neither of the two methods of assessment showed that the majority of right-handed subjects had alpha activity which was of greater amplitude over the right side, a view commonly held for normal subjects (Riloh e f al. 1972). Within the sub-group of about 20 per cent of patients who showed an alpha asymmetry of 15 per cent or greater, the situation was somewhat different in that the majority of these right-handed patients showed alpha which was of greater ampliture over the right hemisphere. It is of interest that only in this sub-group was there a clear degree of concordance between the two methods of assessment; and we were able to determine that an assymmtry between the hemisphere of about 16 per cent was needed for agreement. The general view is that in patients with unilateral, posteriorly placed lesions the alpha is asymmetrical o r less prominent on the affected side (e.g. Fischer-Williams e f al. 1962). However, this effect is 25.
384
not clear-cut because, in some instances, it is said that ipsilateral alpha augmentation occurs (e.g. Van d e r Drift 1957, Kiloh e t al. 1972). In this connection, in more than 50 per cent of the patients reported here who showed left-temporal abnormilities in the EEG, the alpha was augmented on that side. This is one reason for interpreting alpha asymmetries with caution. Another is the fact that (in the present series) the majority of patients with greater degrees of alpha asymmetry the EEGs were found to be normal.
REFERENCES Butler, S. R. & A. Glass (1974) : Asymmetries in the electroencephalogram associated with cerebral dominance. Elcctroenceph. clin. Neurophysiol. 36, 481-491. Cabral, R . & D. F. Scott (1974): Handedness and alpha asymmetry i n the EEG. Electroenceph. clin. Neurophysiol. 37, 325. Cobb, W. A. (1963): The normal adult EEG; t h e EEG of lesions i n general; the EEG of specific lesions. Electroencephalography, ed. D. Hill & G. Parr, p. 509. Macdonald, London. Cornil, L. & H. Gastaut (1947) : Btude Clectroenckphalographique de la dominance sensorielle d’un hkmisphtke cCr6brale. Presse m6d. 37, 421-422. Fischer-Williams, R., S. L. Last, G. Lybcri & D. W. C. Northfield (1962): ClinicoEEG study on 128 gliomas and 50 intracranial metastatic tumours. Brain 85, 1-46. Kiloh, L. G., A. J. McComas & J. Osselton (1972) : Clinical Electroencephalography, pp. 53 and 134. Butterworths, London. Lachaud, J. C., J. H. Lavoisy & H. Dupont (1972) : Absence of statistical correlation between alpha rhythm amplitude and handedness. Electroenceph. clin. Neurophysiol. 33, 534. Margerison, J. H., P. St. J. Loe & C. D. Binnie ( 1 9 6 7 ) : Electroencephalography. Manual of Psychophysiological Methods, ed. P. H. Venables & I. Martin, pp. 387-388. North Holland Publishing Co., Amsterdam. Oldfield, R. C. (1971) : The assessment and analysis of handedness. The Edinburgh inventory. Ncuropsychologia 9, 97-113. Van der Drift, J. H. A. (1957) : The Significance of t h e Electroencephalography for Diagnosis and Localization of Cerebral Tumours, p. 22. H. E. Stenfert Kroese. N.V, Leiden. Received November 22, 1974.
D . F . S c o t t , M.R.C.P., D.P.M. The EEG Department The London Hospital Whitcchapel London E 1 1 BB England
