NeurobiologyofAging, Vol. 11, pp. 193-200. Pergamon Press plc, 1990. Printed in the U.S.A.
0197-4580/90 $3.00 + .00
Age and Fitness Effects on EEG, ERPs, Visual Sensitivity, and Cognition R. E. D U S T M A N , * ? : ~ 1 R. Y. E M M E R S O N , * ? § R. O. R U H L I N G , ¶ D. E. S H E A R E R , * L. A. S T E I N H A U S , * S. C. J O H N S O N , # H. W. B O N E K A T * * A N D J. W. S H I G E O K A t t
*Neuropsychology Research, Veterans Administration Medical Center, Salt Lake City, UT "kDepartment of Psychology, University of Utah, Salt Lake City, UT ~:Department of Neurology, School of Medicine, University of Utah §Department of Psychiatry, School of Medicine, University of Utah ¶Department of Health, Sport and Leisure Studies, George Mason University, Fairfax, VA #Department of Physical Education, College of Health, University of Utah **Pulmonary Department, Veterans Administration Outpatient Clinic, Sacramento, CA ffPulmona©' Department, Veterans Administration Medical Center, Salt Lake City, UT R e c e i v e d 14 M a r c h 1988; A c c e p t e d 14 N o v e m b e r 1989
DUSTMAN, R. E., R. Y. EMMERSON, R. O. RUHLING, D. E. SHEARER, L. A. STEINHAUS, S. C. JOHNSON, H. W. BONEKAT AND J. W. SHIGEOKA. Age andfitness effectson EEG, ERPs,visualsensitivity,and cognition. NEUROBIOL AGING 11(3) 193-200, 1990.--Measures of EEG, event-related potentials (ERPs), visual sensitivity, and cognition were obtained from 30 young (20-31 years) and 30 older (50-62 years) healthy men. Age groups were evenly divided between subjects with low and high fitness levels documented by VOzmaxduring a maximal exercise test. Age comparisons revealed that, compared to young adults, the older men had reduced visual sensitivity, delayed ERP latencies, greater homogeneity of EEG activity across recording sites, more positive visual-evoked potential (VEP) amplitude-intensity (A/I) slope, and poorer performance on a battery of neurocognifive tests. The EEG and VEP A/I slope findings are believed to reflect weakened central inhibition for the older men. In general, the measures that differentiated groups on the basis of age were also sensitive to differences in aerobic fitness. Compared to low fit men, the physically active men had shorter ERP latencies, stronger central inhibition, better neurocognitive performance, and better visual sensitivity. We speculate the performance superiority of the physically active men was, at least in part, the result of more oxygen being available for cerebral metabolism. Aerobic fitness
Aging
Amplitude-intensity slope
Cognition
PHYSICAL fitness has emerged as a major concern in the lifestyles of many Americans. Especially popular for developing and maintaining fitness are "aerobic exercises," such as fast walking, jogging, bicycling, and swimming. Technically, aerobic exercises are those which involve continuous and rhythmic use of large muscles for at least 15 minutes on three or more occasions a week, and which increase resting heart rate by at least 60% of heart rate reserve (1,24). Improved cardiovascular fitness following aerobic training has been documented by increased oxygen utilization (VO2ma×), indicating more efficient transport and delivery of oxygen to consumer cells (24,60). Well-known benefits of aerobic training include increased life expectancy (52) and improvements in cardiovascular efficiency, body composition, and muscle function (24). Somewhat less anticipated are reports of positive influences at a cognitive/affective level, including reports of antidepressant and anxiety reducing properties (38,66), and improved cognitive performance (29, 37, 54).
EEG
Event-related potentials
VO2max
We previously investigated the effects of aerobic and nonaerobic exercise on neuropsychological function of sedentary community volunteers aged 55-70 y e u s (29). After a four-month program of supervised exercise VO2maxhad increased by an average of 27% for the aerobically trained subjects and 9% for subjects doing nonaerobic (strength and flexibility) exercises. Results from a battery of neuropsychological tests administered before and after the experimental period indicated that performance of both groups had improved. However, performance improvement for the aerobically trained individuals was significantly greater than that for the strength and flexibility group. In general, the results suggested that the effects of aerobic exercise included improved central nervous system (CNS) function and that such effects were widespread. For example, significant improvement occurred on recall and reproduction of verbal and auditory memory items, critical flicker fusion (CFF) threshold, and measures which reflect visuo-motor speed and the ability to quickly
~Requests for reprints should be addressed to Robert E. Dustman, Director, Neuropsychology Research (151A), Veterans Administration Medical Center, 500 Foothill Drive, Salt Lake City, UT 84148.
193
DIlSTMAN ET AL
194
shift perceptual set (29). As performance on all of these measures has been reported to decline with advancing age (10, 46, 471, we suggested that the improved performance accompanying increased aerobic fitness may reflect alteration of basic neurobiological processes that are also subject to age-related decline. The notion that physical activity level may influence the rate of age-related decline and that many of the changes thought to be associated with aging are more closely linked to sedentary lifestyles of older individuals has been discussed (8,24). Noninvasive measures of the brain's electrical activity, i.e., the electroencephalogram (EEG) and event-related potentials (ERPs), also provide evidence of CNS aging. These measures show a slowing of dominant brain rhythms (EEG) as well as slowing of signal processing at brain stem, cortical sensory receiving areas, and at cognitive-integrative levels (51,55). Other characteristic changes in EEG and ERPs that occur during adult aging have been interpreted as reflecting a diminution of inhibitory strength. For example, relative to younger adults, EEG and ERPs of elderly subjects show greater homogeneity across recording sites and their ERP amplitudes are more reactive to changes in stimulus intensity (amplitude-intensity slopes are larger) (27, 28, 32). The extent to which measures of the brain's electrical activity reflect physical fitness level is not clear. We did not find reliable changes in EEG and ERP measures of our subjects who participated in a four-month aerobic exercise program, even though significant improvement in cognitive functioning was observed (29). Because behavioral measures of CNS function appear to be more sensitive to health status (62) and to a variety of motivational and peripheral variables and because researchers have generally reported a low degree of correspondence between behavioral and electrophysiological measures (30, 36, 53, 57, 67), these results were not unexpected. Fitness levels may have to be raised to higher levels than those achieved by our aerobically trained subjects for measureable changes in EEG and ERPs to occur. Even though mean VO2m~x had increased by 27% at the conclusion of the program, the age-corrected fitness level for these subjects was still no better than poor-fair (mean "~O2~nax = 25 ml/kg/min). This level of fitness was little more than half of that considered to reflect a superior fitness level for individuals of that age (44 ml/kg/min) (20). To provide a more powerful test of the hypothesis that aerobic fitness affects basic CNS processes associated with aging, we compared EEG and ERP measures of young and older subjects whose lifestyles resulted in VO2,~x levels which placed them in poor-fair or excellent-superior fitness categories. We predicted that for older people, electrophysiological measures from highly fit subjects would show smaller age-related differences than would occur for sedentary subjects. In addition, we were interested in whether any electrophysiological advantages would be observed for younger highly fit individuals. Such findings would suggest a more direct relationship between aerobic fitness and CNS function, i.e., a relationship between these variables which exists independently of aging. METHOD
Subject Selection and Procedures Healthy, nonsmoking males aged 20--31 years and 50-62 years with high or low fitness levels were solicited by newspaper and other advertisement to participate in this study. Fitness level was documented by ~O2max obtained during a maximal exercise test on a Hayden Pacer R-9 motor driven treadmill utilizing a modified Batke Multistage Progressive Treadmill protocol (3.3 m.p.h. speed for all but young men who reported they were physically very active and were tested at a speed of 3.7 m.p.h.; slope was
TABLE 1 MEANS AND STANDARD DEVIATIONS OF AGE, VO2m~, EDUCATION AND VOCABULARY FOR YOUNG AND OLDER MALES WITH HIGH AND LOW FITNESS LEVELS Young
High Fit
Age (years) VOz~,~~ (ml/kg/min) Education (years) Vocabulary (raw score)
Older
Low Fit
High Fit
Low Fit
Mean
SD
Mean
SD
Mean
SD
Mean
SD
24.1 60.5
2.9 5.9
26.3 38.4
2.6 5.5
53.8 49.8
3.0 5.5
55.9 29.1
3.2 3.8
16.3
2.3
15.5
2.3
18.5
3.7
14.9
2.0
59.8
5.9
61.3
5.2
64.7
4.6
60.1
5.6
Each age-fitness group contained 15 subjects. increased 1% per minute) (3). Fitness of prospective subjects was measured until each age group comprised 15 individuals with excellent or superior fitness levels and t5 with poor or fair fitness levels (20). An additional 19 men (12 young and 7 older) had fitness levels that were intermediate to our accepted criteria and were not studied further. The four Age x Fitness level groups were designated as young high fit (YHF), young low fit (YLF) and older high and low fit (OHF, OLF). Mean age, VO 2. . . . years of education, and vocabulary scores for the groups are given in Table I. The vocabulary measure (subtest of the Wechsler Adult Intelligence Scale) provided an assessment of verbal and general mental abilities (46,68). All subjects were administered a battery of tests that provided measures of sensitivity of somatosensory and visual systems to low intensity stimuli as well as measures of EEG, ERPs and cognitive performance.
Somatosensory and Visual Sensitivity Brief (0.5 msec) shocks were applied to the median nerve of the right wrist at a rate of about one every two seconds by a Grass S10SCM stimulator. Somatosensory sensitivity was defined as the voltage which elicited a just noticeable thumb twitch. Two measures of visual sensitivity were obtained 1) Visual Threshold. Subjects sat in a dark room facing a viewing box that contained one or more Wratten neutral density filters and a stimulus slide displaying a narrow black diagonal line (33). Visual threshold was defined as the lowest flash intensity at which subjects could correctly report the orientation (45 degrees left or right from vertical) of the black lines. Intensity was adjusted with the filters. 2) Critical Flicker Fusion (CFF) Threshold. CFF threshold was measured with a Lafayette Instrument Flicker Fusion Control Unit (Model 12025) with attached viewing chamber (Model 12026) which presented a flashing light to the dominant eye. The light/dark ratio of the stimuli was 1:1. CFF threshold was the frequency (Hz) at which the flashes appeared to fuse into a continuous light (29).
EEG and ERPs EEG and ERPs were recorded via disc electrodes attached to the scalp at midline (Fz, Cz, Pz, and Oz) and left central (C3) sites and to the outer canthus of one eye (for monitoring of eyeblinks). Electrodes were referred to linked earlobes. The electrophysiological signals were amplified by a Grass Model 78B 8-channel
HUMAN AGING AND AEROBIC FITNESS SEP
195
pf~p
c', I
I
I
I
100
VEP
I
200
100
N130
P300
N65 5uV
l
~
P95 I
200
I
~ I
100
'-/200 I
1
I
1
100
t
I
300
-
I
l
J
500
Meoc
FIG. 1. Group averaged somatosensory (SEP), pattern reversal (PREP), visual (VEP) and P300 event-relatedpotentials(ERPs). Each group ERP was averaged from ERPs of the 60 individualsparticipatingin this study.
EEG/polygraph and stored on magnetic tape for off-line analyses (band pass was 0.1-100 Hz for P300 and 1-100 Hz for other recordings). EEG cortical coupling. Cortical coupling measures similarity of EEG patterns from pairs of electrodes (15,71). Cortical coupling was computed on "eyes closed" EEG for all pairings (N = 6) of the Fz, Cz, Pz and Oz electrodes. Detailed descriptions of cortical coupling procedures may be found elsewhere (28,71). Factor analysis of EEG coupling data for the six electrode pairs showed that the pairs could be conceptually separated into two sets of three pairs: pairings among the frontal, central and parietal electrodes, i.e., Fz-Cz, Fz-Pz and Cz-Pz, and pairings of these electrodes with the occipital electrode, i.e., Fz-Oz, Cz-Oz and Pz-Oz. Coupling values for each of the three electrode pairs within a set were converted to standard scores (mean = 100; S.D. = 15). The mean of each subject's standard scores was his coupling value for that set. Somatosensory-evoked potential (SEP) latency. Middle latency SEPs were elicited by shock stimuli that were 1.5 times the intensity that produced a just noticeable thumb twitch (see procedures described above for somatosensory sensitivity). SEPs were averaged from 75 artifact-free single responses. The latency of P50 in SEPs from C3 was measured. Figure 1 illustrates a SEP as well as other sensory-evoked and event-related potentials reported in this paper. Pattern reversal-evoked potential (PREP) latency. PREPs were averaged from 200 individual responses recorded from Oz. Subjects viewed a video screen on which appeared a checkerboard pattern that alternated at a 2/sec rate (45). Two sizes of checks were employed: 15' and 30' of retinal arc. Stimulation was monocular (best eye--determined by a test of visual acuity). As small checks are more sensitive to age differences than larger checks (59) we report findings for latency of P100 in PREPs evoked by the 15' checks (see Fig. 1). Visual-evoked potential (VEP) latency. VEPs to three intensities of patterned flashes were recorded from Fz, Cz, Pz and Oz. Stimulus intensities were 1.5, 2.5 and 3.5 log~o steps above
individually determined visual thresholds (33). Flashes were generated by a Grass PS-22 photostimulator set at an intensity of PS8 and delivered by a lamp enclosed in a sound attenuating container. VEPs were averaged from 50 artifact-free individual responses. Latency data for two electrode sites are reported, Oz and Cz. Two middle latency, P45 and N65, and two long latency components, P95 and N135, in VEPs from Oz, and three long latency components in Cz VEPs, N85, P105 and N130, were analyzed (see Fig. 1). As bright flashes are needed to elicit middle latency occipital VEP components, Oz data are for the highest intensity. Subjects' data for Cz VEP components were the means of latencies associated with the three flash intensities. Data were combined across components so that three latency measures were analyzed: 1) Oz--middle latency (P45 and N65); 2) O Z - l o n g latency (P95 and N135); 3) Cz--long latency (N85, P105 and N130). Component latencies were converted to standard scores (mean = 100, S.D. = 15); standard scores were meaned across the appropriate components for each subject. P300 event-related potential. Two hundred intermixed target (the letter X, 16%) and background (the letter O, 84%) stimuli of 50 msec duration were presented on a video monitor with an interstimulus interval ranging from 1.0 to 1.5 sec. Subjects were asked to count the target stimuli. P300s were recorded from Fz, Cz and Pz and averaged from 25 artifact-free individual responses. Latency of P3, a positive component occurring about 400 msec following stimulation, was measured for P300s recorded from Cz and Pz sites (see Fig. 1). The mean of the two P3 latencies was used in data analyses. VEP amplitude~intensity (A/1) slope. A/I slope was computed using a least squares solution (32) for the sum of the amplitudes of VEP components N85-P105 and P105-N130 from Cz. Y-coordinates were component amplitudes associated with the three intensities used to elicit VEPs. A positive A/I slope indicates that amplitude becomes larger as stimulus intensity is increased; a negative slope reflects a reduction in amplitude. An inverse relationship between VEP A/I slope and central inhibition has been reported (32,44).
Cognitive Performance Factor analysis identified four interrelated cognitive tests that were selected as a measure of cognition: Sternberg reaction time, Stroop Color Interference, Symbol Digit Modalities, and Trails B (2, 58, 64, 65). These tests reflect abilities in the areas of response speed, attention, visual scanning and tracking, and the capacity to quickly shift perceptual set (46), abilities that we previously found to be related to aerobic fitness level (29). Results from these measures will be described in detail elsewhere; for this paper findings for a composite value derived from the four measures are reported. Each subject's composite value was the mean of his four test scores after they had been transformed to standard scores (mean = 100; S.D. = 15).
RESULTS Vocabulary and Education ANOVA revealed that neither age nor fitness had a direct effect on vocabulary performance (p>0.10). The interaction of age with fitness, however, was significant, F(1,56)=4.98, p=0.03. A comparison of mean differences by t-test showed that vocabulary scores were higher for the OHF group than for the OLF and YHF groups (p0.05). Fitness, F(1,56)= 10.94, p0.10), was related to years of education. In addition,
I)[rSTMAN ET ,.'iL,
196
the Fitness x Age interaction was significant, F(1,56) = 4.17, p
0197-4580/90 $3.00 + .00
Age and Fitness Effects on EEG, ERPs, Visual Sensitivity, and Cognition R. E. D U S T M A N , * ? : ~ 1 R. Y. E M M E R S O N , * ? § R. O. R U H L I N G , ¶ D. E. S H E A R E R , * L. A. S T E I N H A U S , * S. C. J O H N S O N , # H. W. B O N E K A T * * A N D J. W. S H I G E O K A t t
*Neuropsychology Research, Veterans Administration Medical Center, Salt Lake City, UT "kDepartment of Psychology, University of Utah, Salt Lake City, UT ~:Department of Neurology, School of Medicine, University of Utah §Department of Psychiatry, School of Medicine, University of Utah ¶Department of Health, Sport and Leisure Studies, George Mason University, Fairfax, VA #Department of Physical Education, College of Health, University of Utah **Pulmonary Department, Veterans Administration Outpatient Clinic, Sacramento, CA ffPulmona©' Department, Veterans Administration Medical Center, Salt Lake City, UT R e c e i v e d 14 M a r c h 1988; A c c e p t e d 14 N o v e m b e r 1989
DUSTMAN, R. E., R. Y. EMMERSON, R. O. RUHLING, D. E. SHEARER, L. A. STEINHAUS, S. C. JOHNSON, H. W. BONEKAT AND J. W. SHIGEOKA. Age andfitness effectson EEG, ERPs,visualsensitivity,and cognition. NEUROBIOL AGING 11(3) 193-200, 1990.--Measures of EEG, event-related potentials (ERPs), visual sensitivity, and cognition were obtained from 30 young (20-31 years) and 30 older (50-62 years) healthy men. Age groups were evenly divided between subjects with low and high fitness levels documented by VOzmaxduring a maximal exercise test. Age comparisons revealed that, compared to young adults, the older men had reduced visual sensitivity, delayed ERP latencies, greater homogeneity of EEG activity across recording sites, more positive visual-evoked potential (VEP) amplitude-intensity (A/I) slope, and poorer performance on a battery of neurocognifive tests. The EEG and VEP A/I slope findings are believed to reflect weakened central inhibition for the older men. In general, the measures that differentiated groups on the basis of age were also sensitive to differences in aerobic fitness. Compared to low fit men, the physically active men had shorter ERP latencies, stronger central inhibition, better neurocognitive performance, and better visual sensitivity. We speculate the performance superiority of the physically active men was, at least in part, the result of more oxygen being available for cerebral metabolism. Aerobic fitness
Aging
Amplitude-intensity slope
Cognition
PHYSICAL fitness has emerged as a major concern in the lifestyles of many Americans. Especially popular for developing and maintaining fitness are "aerobic exercises," such as fast walking, jogging, bicycling, and swimming. Technically, aerobic exercises are those which involve continuous and rhythmic use of large muscles for at least 15 minutes on three or more occasions a week, and which increase resting heart rate by at least 60% of heart rate reserve (1,24). Improved cardiovascular fitness following aerobic training has been documented by increased oxygen utilization (VO2ma×), indicating more efficient transport and delivery of oxygen to consumer cells (24,60). Well-known benefits of aerobic training include increased life expectancy (52) and improvements in cardiovascular efficiency, body composition, and muscle function (24). Somewhat less anticipated are reports of positive influences at a cognitive/affective level, including reports of antidepressant and anxiety reducing properties (38,66), and improved cognitive performance (29, 37, 54).
EEG
Event-related potentials
VO2max
We previously investigated the effects of aerobic and nonaerobic exercise on neuropsychological function of sedentary community volunteers aged 55-70 y e u s (29). After a four-month program of supervised exercise VO2maxhad increased by an average of 27% for the aerobically trained subjects and 9% for subjects doing nonaerobic (strength and flexibility) exercises. Results from a battery of neuropsychological tests administered before and after the experimental period indicated that performance of both groups had improved. However, performance improvement for the aerobically trained individuals was significantly greater than that for the strength and flexibility group. In general, the results suggested that the effects of aerobic exercise included improved central nervous system (CNS) function and that such effects were widespread. For example, significant improvement occurred on recall and reproduction of verbal and auditory memory items, critical flicker fusion (CFF) threshold, and measures which reflect visuo-motor speed and the ability to quickly
~Requests for reprints should be addressed to Robert E. Dustman, Director, Neuropsychology Research (151A), Veterans Administration Medical Center, 500 Foothill Drive, Salt Lake City, UT 84148.
193
DIlSTMAN ET AL
194
shift perceptual set (29). As performance on all of these measures has been reported to decline with advancing age (10, 46, 471, we suggested that the improved performance accompanying increased aerobic fitness may reflect alteration of basic neurobiological processes that are also subject to age-related decline. The notion that physical activity level may influence the rate of age-related decline and that many of the changes thought to be associated with aging are more closely linked to sedentary lifestyles of older individuals has been discussed (8,24). Noninvasive measures of the brain's electrical activity, i.e., the electroencephalogram (EEG) and event-related potentials (ERPs), also provide evidence of CNS aging. These measures show a slowing of dominant brain rhythms (EEG) as well as slowing of signal processing at brain stem, cortical sensory receiving areas, and at cognitive-integrative levels (51,55). Other characteristic changes in EEG and ERPs that occur during adult aging have been interpreted as reflecting a diminution of inhibitory strength. For example, relative to younger adults, EEG and ERPs of elderly subjects show greater homogeneity across recording sites and their ERP amplitudes are more reactive to changes in stimulus intensity (amplitude-intensity slopes are larger) (27, 28, 32). The extent to which measures of the brain's electrical activity reflect physical fitness level is not clear. We did not find reliable changes in EEG and ERP measures of our subjects who participated in a four-month aerobic exercise program, even though significant improvement in cognitive functioning was observed (29). Because behavioral measures of CNS function appear to be more sensitive to health status (62) and to a variety of motivational and peripheral variables and because researchers have generally reported a low degree of correspondence between behavioral and electrophysiological measures (30, 36, 53, 57, 67), these results were not unexpected. Fitness levels may have to be raised to higher levels than those achieved by our aerobically trained subjects for measureable changes in EEG and ERPs to occur. Even though mean VO2m~x had increased by 27% at the conclusion of the program, the age-corrected fitness level for these subjects was still no better than poor-fair (mean "~O2~nax = 25 ml/kg/min). This level of fitness was little more than half of that considered to reflect a superior fitness level for individuals of that age (44 ml/kg/min) (20). To provide a more powerful test of the hypothesis that aerobic fitness affects basic CNS processes associated with aging, we compared EEG and ERP measures of young and older subjects whose lifestyles resulted in VO2,~x levels which placed them in poor-fair or excellent-superior fitness categories. We predicted that for older people, electrophysiological measures from highly fit subjects would show smaller age-related differences than would occur for sedentary subjects. In addition, we were interested in whether any electrophysiological advantages would be observed for younger highly fit individuals. Such findings would suggest a more direct relationship between aerobic fitness and CNS function, i.e., a relationship between these variables which exists independently of aging. METHOD
Subject Selection and Procedures Healthy, nonsmoking males aged 20--31 years and 50-62 years with high or low fitness levels were solicited by newspaper and other advertisement to participate in this study. Fitness level was documented by ~O2max obtained during a maximal exercise test on a Hayden Pacer R-9 motor driven treadmill utilizing a modified Batke Multistage Progressive Treadmill protocol (3.3 m.p.h. speed for all but young men who reported they were physically very active and were tested at a speed of 3.7 m.p.h.; slope was
TABLE 1 MEANS AND STANDARD DEVIATIONS OF AGE, VO2m~, EDUCATION AND VOCABULARY FOR YOUNG AND OLDER MALES WITH HIGH AND LOW FITNESS LEVELS Young
High Fit
Age (years) VOz~,~~ (ml/kg/min) Education (years) Vocabulary (raw score)
Older
Low Fit
High Fit
Low Fit
Mean
SD
Mean
SD
Mean
SD
Mean
SD
24.1 60.5
2.9 5.9
26.3 38.4
2.6 5.5
53.8 49.8
3.0 5.5
55.9 29.1
3.2 3.8
16.3
2.3
15.5
2.3
18.5
3.7
14.9
2.0
59.8
5.9
61.3
5.2
64.7
4.6
60.1
5.6
Each age-fitness group contained 15 subjects. increased 1% per minute) (3). Fitness of prospective subjects was measured until each age group comprised 15 individuals with excellent or superior fitness levels and t5 with poor or fair fitness levels (20). An additional 19 men (12 young and 7 older) had fitness levels that were intermediate to our accepted criteria and were not studied further. The four Age x Fitness level groups were designated as young high fit (YHF), young low fit (YLF) and older high and low fit (OHF, OLF). Mean age, VO 2. . . . years of education, and vocabulary scores for the groups are given in Table I. The vocabulary measure (subtest of the Wechsler Adult Intelligence Scale) provided an assessment of verbal and general mental abilities (46,68). All subjects were administered a battery of tests that provided measures of sensitivity of somatosensory and visual systems to low intensity stimuli as well as measures of EEG, ERPs and cognitive performance.
Somatosensory and Visual Sensitivity Brief (0.5 msec) shocks were applied to the median nerve of the right wrist at a rate of about one every two seconds by a Grass S10SCM stimulator. Somatosensory sensitivity was defined as the voltage which elicited a just noticeable thumb twitch. Two measures of visual sensitivity were obtained 1) Visual Threshold. Subjects sat in a dark room facing a viewing box that contained one or more Wratten neutral density filters and a stimulus slide displaying a narrow black diagonal line (33). Visual threshold was defined as the lowest flash intensity at which subjects could correctly report the orientation (45 degrees left or right from vertical) of the black lines. Intensity was adjusted with the filters. 2) Critical Flicker Fusion (CFF) Threshold. CFF threshold was measured with a Lafayette Instrument Flicker Fusion Control Unit (Model 12025) with attached viewing chamber (Model 12026) which presented a flashing light to the dominant eye. The light/dark ratio of the stimuli was 1:1. CFF threshold was the frequency (Hz) at which the flashes appeared to fuse into a continuous light (29).
EEG and ERPs EEG and ERPs were recorded via disc electrodes attached to the scalp at midline (Fz, Cz, Pz, and Oz) and left central (C3) sites and to the outer canthus of one eye (for monitoring of eyeblinks). Electrodes were referred to linked earlobes. The electrophysiological signals were amplified by a Grass Model 78B 8-channel
HUMAN AGING AND AEROBIC FITNESS SEP
195
pf~p
c', I
I
I
I
100
VEP
I
200
100
N130
P300
N65 5uV
l
~
P95 I
200
I
~ I
100
'-/200 I
1
I
1
100
t
I
300
-
I
l
J
500
Meoc
FIG. 1. Group averaged somatosensory (SEP), pattern reversal (PREP), visual (VEP) and P300 event-relatedpotentials(ERPs). Each group ERP was averaged from ERPs of the 60 individualsparticipatingin this study.
EEG/polygraph and stored on magnetic tape for off-line analyses (band pass was 0.1-100 Hz for P300 and 1-100 Hz for other recordings). EEG cortical coupling. Cortical coupling measures similarity of EEG patterns from pairs of electrodes (15,71). Cortical coupling was computed on "eyes closed" EEG for all pairings (N = 6) of the Fz, Cz, Pz and Oz electrodes. Detailed descriptions of cortical coupling procedures may be found elsewhere (28,71). Factor analysis of EEG coupling data for the six electrode pairs showed that the pairs could be conceptually separated into two sets of three pairs: pairings among the frontal, central and parietal electrodes, i.e., Fz-Cz, Fz-Pz and Cz-Pz, and pairings of these electrodes with the occipital electrode, i.e., Fz-Oz, Cz-Oz and Pz-Oz. Coupling values for each of the three electrode pairs within a set were converted to standard scores (mean = 100; S.D. = 15). The mean of each subject's standard scores was his coupling value for that set. Somatosensory-evoked potential (SEP) latency. Middle latency SEPs were elicited by shock stimuli that were 1.5 times the intensity that produced a just noticeable thumb twitch (see procedures described above for somatosensory sensitivity). SEPs were averaged from 75 artifact-free single responses. The latency of P50 in SEPs from C3 was measured. Figure 1 illustrates a SEP as well as other sensory-evoked and event-related potentials reported in this paper. Pattern reversal-evoked potential (PREP) latency. PREPs were averaged from 200 individual responses recorded from Oz. Subjects viewed a video screen on which appeared a checkerboard pattern that alternated at a 2/sec rate (45). Two sizes of checks were employed: 15' and 30' of retinal arc. Stimulation was monocular (best eye--determined by a test of visual acuity). As small checks are more sensitive to age differences than larger checks (59) we report findings for latency of P100 in PREPs evoked by the 15' checks (see Fig. 1). Visual-evoked potential (VEP) latency. VEPs to three intensities of patterned flashes were recorded from Fz, Cz, Pz and Oz. Stimulus intensities were 1.5, 2.5 and 3.5 log~o steps above
individually determined visual thresholds (33). Flashes were generated by a Grass PS-22 photostimulator set at an intensity of PS8 and delivered by a lamp enclosed in a sound attenuating container. VEPs were averaged from 50 artifact-free individual responses. Latency data for two electrode sites are reported, Oz and Cz. Two middle latency, P45 and N65, and two long latency components, P95 and N135, in VEPs from Oz, and three long latency components in Cz VEPs, N85, P105 and N130, were analyzed (see Fig. 1). As bright flashes are needed to elicit middle latency occipital VEP components, Oz data are for the highest intensity. Subjects' data for Cz VEP components were the means of latencies associated with the three flash intensities. Data were combined across components so that three latency measures were analyzed: 1) Oz--middle latency (P45 and N65); 2) O Z - l o n g latency (P95 and N135); 3) Cz--long latency (N85, P105 and N130). Component latencies were converted to standard scores (mean = 100, S.D. = 15); standard scores were meaned across the appropriate components for each subject. P300 event-related potential. Two hundred intermixed target (the letter X, 16%) and background (the letter O, 84%) stimuli of 50 msec duration were presented on a video monitor with an interstimulus interval ranging from 1.0 to 1.5 sec. Subjects were asked to count the target stimuli. P300s were recorded from Fz, Cz and Pz and averaged from 25 artifact-free individual responses. Latency of P3, a positive component occurring about 400 msec following stimulation, was measured for P300s recorded from Cz and Pz sites (see Fig. 1). The mean of the two P3 latencies was used in data analyses. VEP amplitude~intensity (A/1) slope. A/I slope was computed using a least squares solution (32) for the sum of the amplitudes of VEP components N85-P105 and P105-N130 from Cz. Y-coordinates were component amplitudes associated with the three intensities used to elicit VEPs. A positive A/I slope indicates that amplitude becomes larger as stimulus intensity is increased; a negative slope reflects a reduction in amplitude. An inverse relationship between VEP A/I slope and central inhibition has been reported (32,44).
Cognitive Performance Factor analysis identified four interrelated cognitive tests that were selected as a measure of cognition: Sternberg reaction time, Stroop Color Interference, Symbol Digit Modalities, and Trails B (2, 58, 64, 65). These tests reflect abilities in the areas of response speed, attention, visual scanning and tracking, and the capacity to quickly shift perceptual set (46), abilities that we previously found to be related to aerobic fitness level (29). Results from these measures will be described in detail elsewhere; for this paper findings for a composite value derived from the four measures are reported. Each subject's composite value was the mean of his four test scores after they had been transformed to standard scores (mean = 100; S.D. = 15).
RESULTS Vocabulary and Education ANOVA revealed that neither age nor fitness had a direct effect on vocabulary performance (p>0.10). The interaction of age with fitness, however, was significant, F(1,56)=4.98, p=0.03. A comparison of mean differences by t-test showed that vocabulary scores were higher for the OHF group than for the OLF and YHF groups (p0.05). Fitness, F(1,56)= 10.94, p0.10), was related to years of education. In addition,
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the Fitness x Age interaction was significant, F(1,56) = 4.17, p
