Neuropsychobiology 5 : 301 308 (1979)
Interhemispheric and Sex Differences in the Visual Evoked Response Recovery Cycle Fernando Lolas Department o f Physiology and Biophysics, University o f Chile, Santiago and Department o f Neurological Sciences, Rush University, Chicago, 111.
Key Words. Recovery cycle • Visual evoked potential • Hemispheric asymmetry • Psychiatric
electrophysiology
Since the early work o f Gastaut et al. (19) on the ‘cycle o f cortical excitability’ to paired flashes o f light in the human unprocessed EEC record, many authors have studied the recovery function o f average evoked responses (AER) to somatosensory, visual, and auditory stimulation (1, 2, 6, 7, 3 0 -3 4 , 37, 38). The most widely employed procedure involves the sequential presentation o f a conditioning (S ,) and a test (S2) stimulus at different interstimulus intervals
(ISI). The ratio between the second and the first AER (R 2/R,) is thought to represent an index of ‘ recovery’ in response parameters such as latency or amplitude o f different AER com ponents. A ratio o f 1 indicates full recovery, while higher and lower ratios indicate super normal and subnormal excitability periods alternating after the delivery o f S 2. A modified recovery function procedure in which the ISI is held constant and only the intensity o f S! is
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Abstract. The recovery cycle o f the flash-evoked response was studied in 6 female and 6 male right-handed subjects o f comparable age. The aim o f the study was to evidence possible interhemispheric differences between occipital leads. Two flashes, presented at graded time intervals between 20 and 150 msec, allowed the computation o f an amplitude ratio between the second and the first evoked response, considering the III—IV peak-to-peak amplitude. A faster recovery was found for the left hemisphere, with more pronounced interhemispheric differences in male than in female subjects. Women showed a more reduced amplitude recovery than men and a somewhat different time-course of the recovery function. Women also showed less waveshape similarity between evoked responses elicited by the second of a pair o f flashes in the right and left hemisphere. These findings are discussed in terms of differences in speed o f information processing by both cerebral hemispheres. It is suggested that recovery cycle studies may contribute to brain laterality research and be o f particular relevance to psychiatric studies involving assumptions about differential hemispheric excitability.
302
psychiatric patients or normals subject to various experimental manipulations.
Method Subjects 12 nonpaid volunteers (6 women and 6 men) participated in this study. The age range was 2 2 -4 0 years, with a median o f 28. A ll o f them were righthanded, as adjudged by a series o f simple laterality tests (45), and with no previous neurological or psychiatric impairment.
Procedure Each subject attended one afternoon recording ses sion lasting about 30 min, during which he or she was seated in a comfortable armchair within a darkened room. All subjects were instructed to keep their eyes closed during the test and to report any discomfort. Aside from this, no specific instruction regarding the task was provided, although they were told that on some occasions a double flash would be presented. Chlorided silver cup electrodes (Grass) were at tached by means o f collodion-impregnated gauze patches al positions O , - A , and 0 : A : o f the 1 0 -2 0 system, with a ground on the left mastoid process. Although additional electrodes were positioned in a midsagittal derivation (Pz- F z), data to be discussed here were obtained from the occipital leads, the mid line derivation aiding in the identification o f relevant evoked response components. Brain electrical activity was recorded by means o f Grass P511 H amplifiers with frequency cutoffs at 0.1 and 100 Hz, and aver aged on-line with a Nicolet MED-80 computer using a resolution o f 10 bits in the analog-to-digital converter and a sampling epoch o f 350 msec spanned by 256 data points. Each session was subdivided into three blocks, each block consisting o f four averaging sequences o f 100 trials each. An interblock rest period o f 1 min was used. Single (S ,) and paired (S, S j) 10-Msec flashes at intensity 8 o f the Grass PS 22 were delivered 50 cm in front o f the subject at a rate o f 1.5/sec. The eight 1SI employed (20. 40, 60, 80, 100, 120, 140 and 150 msec) were presented in ascending or descending order to any given subject. The averaging sequences o f single flashes interspersed between the paired-flash runs served to monitor the degree o f alertness during
Downloaded by: Karolinska Institutet, University Library 130.237.122.245 - 1/12/2019 4:00:49 PM
varied, introduced by Sliagass et al. (36, 40), has not been used for modalities other than the somatosensory. Studies involving the visual modality have been aimed at defining the behavioral correlates o f the recovery function that might be relevant to psychiatric diagnosis, drug effects, and the exploration o f organismic variables (15, 16, 23, 35, 39, 42, 44). Most o f this work has been done employing midsagittal derivations or elec trodes placed over only one hemisphere. In one o f the few studies that have addressed the ques tion o f interhemispherical asymmetries in the AER to the second o f a pair o f flashes, Shipley (41) reports that amplitude shows a relatively larger drop in the right hemisphere than in the left, thus failing to confirm his own hypothesis that the right hemisphere 'is inherently faster resolving for visual stimuli’ than is the left. Although employing only 4 subjects and one 1S1 (500 msec), that study shows the potential usefulness o f recovery cycle data in the assess ment o f hemispheric asymmetry. Electrocortical indices, derived both from spontaneous background rhythms and from event-related potentials, have been employed in studies o f hemispheric asymmetry in relation to cognitive style, information processing, task demands, and affective states (4, 5, 12, 13, 17, 18, 22, 28). The A E R excitability cycle may add an other dimension to these studies. The aim o f this work was to gather informa tion on amplitude recovery cycle o f flash-AER recorded from homologous areas over both cerebral hemispheres in male and female sub jects o f comparable age and handedness. Bilat eral recovery cycle was studied without the compounding effects o f any specifically in structed task or set in order to assess the ‘basal’ reactivity in a supposedly neutral situation. These data should then serve as a baseline that could be later compared to those obtained from
Lolas
303
Hemispheric Asymmetry, Sex, and Visual Recovery Cycle
Data Analysis Since R , (response to the second flash o f a pair) was always incorporated into an R , R , composite waveshape, its isolation was accomplished by subtract ing from this composite the response to the single flash ( R ,) . The main peaks in R , and R , were identi fied by means o f an ‘hysteresis peak define’ routine that selected all peaks larger than 10% o f the total vertical excursion on any given average and printed its latency and peak-to-baseline amplitude. Attention was devoted to a postitive component that appeared consistently on occipital records (though not so prominently as in the midline deriva tion) and corresponds roughly to Ciganek's (8) com ponent IV , or 5a according to Gastaut et al. (19). This selection was based on evidence that indicates that this peak is reliably recorded in man (10, 24). In some instances it was not easily discernible from a I V - V VI complex. Mean latency to this wave across all subjects under the single-flash condition was 117.80 ± 22.2 msec with a mean I I I - I V peak-to-peak amplitude o f 11.98 ± 3.45 mV (see Results). Criterion measure was the R : /R, ratio o f the III IV peak-to-pcak amplitude. Data were analyzed within the framework o f a three-dimensional design, factors being sex, hemisphere, and IS1 (2 X 2 X 8), so that the interactions between them could be explored. Analyses o f variance and t tests were employed. As an additional parameter, the degree o f similarity between left and right R , waveshape was assessed by computing off-line Pearson correlation coefficients. These were then transformed to z scores for statistical analyses. The similarity o f average R , from left and right was explored in relation to sex and ISI.
Results
A E R obtained under the single-flash con dition were not influenced either by sex or hemisphere. Two-way analyses o f variance per formed on latency to component IV and III/IV peak-to-peak amplitude yielded nonsignificant
F ratios, although women showed a tendency toward higher amplitudes (F = 2.053; d.f. = 1.16; p
Interhemispheric and Sex Differences in the Visual Evoked Response Recovery Cycle Fernando Lolas Department o f Physiology and Biophysics, University o f Chile, Santiago and Department o f Neurological Sciences, Rush University, Chicago, 111.
Key Words. Recovery cycle • Visual evoked potential • Hemispheric asymmetry • Psychiatric
electrophysiology
Since the early work o f Gastaut et al. (19) on the ‘cycle o f cortical excitability’ to paired flashes o f light in the human unprocessed EEC record, many authors have studied the recovery function o f average evoked responses (AER) to somatosensory, visual, and auditory stimulation (1, 2, 6, 7, 3 0 -3 4 , 37, 38). The most widely employed procedure involves the sequential presentation o f a conditioning (S ,) and a test (S2) stimulus at different interstimulus intervals
(ISI). The ratio between the second and the first AER (R 2/R,) is thought to represent an index of ‘ recovery’ in response parameters such as latency or amplitude o f different AER com ponents. A ratio o f 1 indicates full recovery, while higher and lower ratios indicate super normal and subnormal excitability periods alternating after the delivery o f S 2. A modified recovery function procedure in which the ISI is held constant and only the intensity o f S! is
Downloaded by: Karolinska Institutet, University Library 130.237.122.245 - 1/12/2019 4:00:49 PM
Abstract. The recovery cycle o f the flash-evoked response was studied in 6 female and 6 male right-handed subjects o f comparable age. The aim o f the study was to evidence possible interhemispheric differences between occipital leads. Two flashes, presented at graded time intervals between 20 and 150 msec, allowed the computation o f an amplitude ratio between the second and the first evoked response, considering the III—IV peak-to-peak amplitude. A faster recovery was found for the left hemisphere, with more pronounced interhemispheric differences in male than in female subjects. Women showed a more reduced amplitude recovery than men and a somewhat different time-course of the recovery function. Women also showed less waveshape similarity between evoked responses elicited by the second of a pair o f flashes in the right and left hemisphere. These findings are discussed in terms of differences in speed o f information processing by both cerebral hemispheres. It is suggested that recovery cycle studies may contribute to brain laterality research and be o f particular relevance to psychiatric studies involving assumptions about differential hemispheric excitability.
302
psychiatric patients or normals subject to various experimental manipulations.
Method Subjects 12 nonpaid volunteers (6 women and 6 men) participated in this study. The age range was 2 2 -4 0 years, with a median o f 28. A ll o f them were righthanded, as adjudged by a series o f simple laterality tests (45), and with no previous neurological or psychiatric impairment.
Procedure Each subject attended one afternoon recording ses sion lasting about 30 min, during which he or she was seated in a comfortable armchair within a darkened room. All subjects were instructed to keep their eyes closed during the test and to report any discomfort. Aside from this, no specific instruction regarding the task was provided, although they were told that on some occasions a double flash would be presented. Chlorided silver cup electrodes (Grass) were at tached by means o f collodion-impregnated gauze patches al positions O , - A , and 0 : A : o f the 1 0 -2 0 system, with a ground on the left mastoid process. Although additional electrodes were positioned in a midsagittal derivation (Pz- F z), data to be discussed here were obtained from the occipital leads, the mid line derivation aiding in the identification o f relevant evoked response components. Brain electrical activity was recorded by means o f Grass P511 H amplifiers with frequency cutoffs at 0.1 and 100 Hz, and aver aged on-line with a Nicolet MED-80 computer using a resolution o f 10 bits in the analog-to-digital converter and a sampling epoch o f 350 msec spanned by 256 data points. Each session was subdivided into three blocks, each block consisting o f four averaging sequences o f 100 trials each. An interblock rest period o f 1 min was used. Single (S ,) and paired (S, S j) 10-Msec flashes at intensity 8 o f the Grass PS 22 were delivered 50 cm in front o f the subject at a rate o f 1.5/sec. The eight 1SI employed (20. 40, 60, 80, 100, 120, 140 and 150 msec) were presented in ascending or descending order to any given subject. The averaging sequences o f single flashes interspersed between the paired-flash runs served to monitor the degree o f alertness during
Downloaded by: Karolinska Institutet, University Library 130.237.122.245 - 1/12/2019 4:00:49 PM
varied, introduced by Sliagass et al. (36, 40), has not been used for modalities other than the somatosensory. Studies involving the visual modality have been aimed at defining the behavioral correlates o f the recovery function that might be relevant to psychiatric diagnosis, drug effects, and the exploration o f organismic variables (15, 16, 23, 35, 39, 42, 44). Most o f this work has been done employing midsagittal derivations or elec trodes placed over only one hemisphere. In one o f the few studies that have addressed the ques tion o f interhemispherical asymmetries in the AER to the second o f a pair o f flashes, Shipley (41) reports that amplitude shows a relatively larger drop in the right hemisphere than in the left, thus failing to confirm his own hypothesis that the right hemisphere 'is inherently faster resolving for visual stimuli’ than is the left. Although employing only 4 subjects and one 1S1 (500 msec), that study shows the potential usefulness o f recovery cycle data in the assess ment o f hemispheric asymmetry. Electrocortical indices, derived both from spontaneous background rhythms and from event-related potentials, have been employed in studies o f hemispheric asymmetry in relation to cognitive style, information processing, task demands, and affective states (4, 5, 12, 13, 17, 18, 22, 28). The A E R excitability cycle may add an other dimension to these studies. The aim o f this work was to gather informa tion on amplitude recovery cycle o f flash-AER recorded from homologous areas over both cerebral hemispheres in male and female sub jects o f comparable age and handedness. Bilat eral recovery cycle was studied without the compounding effects o f any specifically in structed task or set in order to assess the ‘basal’ reactivity in a supposedly neutral situation. These data should then serve as a baseline that could be later compared to those obtained from
Lolas
303
Hemispheric Asymmetry, Sex, and Visual Recovery Cycle
Data Analysis Since R , (response to the second flash o f a pair) was always incorporated into an R , R , composite waveshape, its isolation was accomplished by subtract ing from this composite the response to the single flash ( R ,) . The main peaks in R , and R , were identi fied by means o f an ‘hysteresis peak define’ routine that selected all peaks larger than 10% o f the total vertical excursion on any given average and printed its latency and peak-to-baseline amplitude. Attention was devoted to a postitive component that appeared consistently on occipital records (though not so prominently as in the midline deriva tion) and corresponds roughly to Ciganek's (8) com ponent IV , or 5a according to Gastaut et al. (19). This selection was based on evidence that indicates that this peak is reliably recorded in man (10, 24). In some instances it was not easily discernible from a I V - V VI complex. Mean latency to this wave across all subjects under the single-flash condition was 117.80 ± 22.2 msec with a mean I I I - I V peak-to-peak amplitude o f 11.98 ± 3.45 mV (see Results). Criterion measure was the R : /R, ratio o f the III IV peak-to-pcak amplitude. Data were analyzed within the framework o f a three-dimensional design, factors being sex, hemisphere, and IS1 (2 X 2 X 8), so that the interactions between them could be explored. Analyses o f variance and t tests were employed. As an additional parameter, the degree o f similarity between left and right R , waveshape was assessed by computing off-line Pearson correlation coefficients. These were then transformed to z scores for statistical analyses. The similarity o f average R , from left and right was explored in relation to sex and ISI.
Results
A E R obtained under the single-flash con dition were not influenced either by sex or hemisphere. Two-way analyses o f variance per formed on latency to component IV and III/IV peak-to-peak amplitude yielded nonsignificant
F ratios, although women showed a tendency toward higher amplitudes (F = 2.053; d.f. = 1.16; p
