Stimulus probability and motor response in young and old adults: An ERP study

Bmlogicul Psychology North-Holland

125

29 (1989) 125-148

STIMULUS PROBABILITY AND MOTOR RESPONSE AND OLD ADULTS: AN ERP STUDY * Huib LOOREN Psychophysiologv

IN YOUNG

DE JONG

Section, Psychology Department,

Free University, Amsterdam,

Psychophysiology The Netherlands

Section, PsychologV Department,

University of Amsterdam,

John C.G.M.

VAN ROOY

Psychophysiology

Section, Psychology Department,

Albert

The Netherlands

KOK Amsterdam,

Free University, Amsterdam,

The Netherlands

The effects of responding hand and stimulus probability were investigated in young and old subjects in an RT task in which both rare and frequent stimuli required a response. It was found that the effect of stimulus probability was less pronounced in old subjects than in young, and that the latency of P3 was longer in the elderly, although their RTs were not different from young subjects. ERPs for right hand responses were larger than for left hand responses; this difference was discernible already in the P2 and N2 peaks of the ERP. A tentative explanation is offered for these large and unexpected hand differences. An interpretation in terms of an age-related decrease in resources is proposed for the increased P3 latency and the decreased probability effect on P3 amplitude in old subjects.

1.

Introduction

This study reports differences in information processing in old and young subjects in an oddball paradigm. This type of task involves two stimuli of unequal probability, presented in random order, to which the subject is required to respond by pressing a button or by counting. This task has become a standard paradigm for eliciting P3 potentials, yielding consistent results, that is, a larger amplitude and longer latency for the more improbable stimuli than for the probable (e.g., Donchin, Ritter, & McCallum, 1978; Duncan-Johnson & Donchin, 1977; Fitzgerald & Picton, 1981). A common interpretation of P3 * This research was supported by a grant from the Dutch Organisation for Fundamental Research ZWO (No. 560-265-013). We wish to thank Tony Gaillard for his comments on a previous version of this paper. Address reprint requests to: Albert Kok, University of Amsterdam, Psychology Department, Weesperplein 8, 1018 XA Amsterdam, The Netherlands.

0301-0511/89/$3.50

0 1989, Elsevier Science Publishers

B.V. (North-Holland)

126

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Looren

de Jong et ai / Stimulus pmbabr fity, motor response and

age

amplitude is that it reflects the amount of processing invested in memory updating (see, for example, Donchin, 1981; Donchin et al., 1978; DuncanJohnson & Donchin, 1977; Johnson, 1988; Kok, 1988a; Kok & Looren de Jong, 1980; Ruchkin & Sutton, 1983). The oddball paradigm has been proposed as a test for cognitive decline as a result of aging: usually, a decrease in amplitude and increase in latency of the P3 with advancing age is found (e.g., Pfefferbaum, Ford, Roth, & Kopeil, 1980; Pfefferbaum, Ford, Wenegrat, Roth, & Kopell, 1984; Picton Stuss, Champagne, & Nelson, 1984). P3 latency is considered as a measure of stimulus-evaluation time, not contaminated by response requirements and speed-accuracy trade-off (e.g., Kutas, McCarthy, & Donchin, 1977; Magliero, Bashore, Coles, & Donchin, 1984). The relative timing of P3 and RT can be interpreted as an indication of speed-accuracy trade-off; a short RT combined with a late P3 indicates premature responding (“jumping the gun”). Picton et al. (1984) found that in old subjects P3 is late relative to RT, as compared with young subjects, indicating that old subjects tend to respond before completely having evaluated the stimulus. Most oddball studies require a response (a button press or counting) only to the rare stimulus, thus confounding probability and target effects. It has been consistently found that when the stimulus is a target, the amplitude of the P3 evoked by it increases (Ford, 1978; Johnson, 1988; Pritchard, 1987). Johnson (1988) attributes this to increased processing required by targets (i.e., increased task complexity). It has further been found that P3 is smaller and later when stimuli are counted than when a motor response is required (Barrett, Neshige, & Shibasaki, 1987; Johnson, 1988). Also, considerable differences in ERPs associated with Go and Nogo responses have been reported (Kok, 1986; Pfefferbaum, Ford, Weller, & Kopell, 1985; Pfefferbaum & Ford, 1988). This may result from overlap with motor potentials (Kok, 1988b), or alternatively, P3 to Go stimuli may be an entirely different component than P3 to Nogo stimuli (Pfefferbaum & Ford, 1988). It seems advisable then to separate response requirements and stimulus probability in this type of tasks. Thus, we used a bimanual choice RT task, in which both frequent and rare stimuli required a motor response. In this study, the following issues will be addressed. Firstly, it is investigated whether differences in P3 amplitude between frequent and infrequent signals are smaller in old subjects than in young. It has been suggested that old people have a shorter memory span, and consequently base their expectancies on a shorter sample of preceding stimuli (Griew, 1968; Polich, Howard, & Starr, 1983; Rabbitt & Vyas, 1980), which would result in a less pronounced expectancy effect on P3. Secondly, both P3 latency and RT of young and old subjects are measured to investigate whether relative timing of stimulus evaluation and response production, and consequently speed-accuracy tradeoff differs between age groups. It will be noted that an additional advantage of

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the bimanual motor response design is that P3 latencies can be compared with response latencies for both rare and frequent stimuli. Thirdly, age-related difference in the rate of adaptation to the probability structure of the task in early and late components of the ERP are investigated, by comparing ERPs evoked by frequent stimuli at the beginning and at the end of a block of trials. It is conceivable that old subjects may need more time to adapt to the task. Additionally, psychometric test scores were obtained for memory span and processing speed (Digit Span (DS) and Digit Symbol Substitution (DSS) of the WAIS respectively), to replicate results reported by Polich et al. (1983) who found a correlation between P3 latency and DS, and Picton et al. (1984), who found a correlation between DSS and P3 latency.

2. Method 2.1. Subjects Twenty-four young male subjects (aged 18-24, mean = 21, s.d. = 1.6) and 24 old males (aged 65-75, mean = 70, s.d. = 3.3) served as paid volunteers. All were in good health, right-handed, and had normal or corrected to normal vision (minimal 0.72 as determined by a Landolt chart). None of them reported a history of neurological disorders, and with the exception of some of the elderly who were under treatment for light cardiovascular disorders, none of them was under medication. Young subjects were students, the elderly had at least college education. All subjects were administered a common Dutch intelligence test (GIT) and the DSS and DS subtests of the WAIS (as adapted and normalized for the Dutch population). Average performance IQ (not corrected for age) was 125 for the old group (s.d. = 10.7) and 125 for the young (s.d. = 7.2). 2.2. Stimuli and apparatus Stimuli were two upper-case letters (L or R) presented at the centre of a CRT display (Hewlett Packard 1311B), which was controlled by a graphics translator (Hewlett Packard 1350A) and a DEC PDP11/24 computer. Each letter subtended an angle of 1.43” vertically and 1.03” horizontally. The average luminance of the screen was 0.08 cd/m2. The subject was comfortably seated in a dimly lit, electrically shielded and sound-attenuated cabin, with a response panel under his right and left hand. The display was positioned approximately 1 m in front of the subject’s head. Stimulus duration was 200 ms, and the interstimulus interval was 3000 ms (onset to onset). EEG was recorded according to the lo-20 system from Fz, Cz, Pz, Oz, C3, and C4 referred to linked mastoids using Ag-AgCl electrodes. Horizontal and

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vertical bipolar EOG was recorded using Beckman Ag-AgC1 pellet electrodes, placed at the outer canthi of the eyes and above and below the right eye, respectively. Impedance of the EEG electrodes was kept below 4 kti, and EOG electrodes below 10 kQ. The signal was amplified by a Beckman R711 amplifier with time constant set at 10 s, high-frequency cut-off at 35 Hz (- 3 dB, roll-off -6 dB/octave), recorded on paper and digitized on-line by a DEC PDP11/24 computer. RTs were measured in milliseconds by the computer and checked for errors, omissions and premature responses. EEG and EOG were monitored on-line for EOG artifacts and saturation of the A-D converter. 2.3. Procedure The subject was instructed to respond as quickly and accurately as possible with a right hand button press to the letter R, and with a left hand button press to the letter L. There were four blocks of 100 stimuli, 20 of which were rares and 80 frequents, in randomized order. In blocks 1 and 3 the R (right hand response) was frequent and the L (left hand response) rare, while in blocks 2 and 4 L was frequent and R was rare. That is, responding hand is varied between blocks. The second half of the experiment (blocks 3 and 4) thus constitutes a replication of the first half, and rephcation was analysed as a main factor in ANOVA. Each block was preceded by a warning tone, and followed by a I-min pause, during which the subject was requested to remain seated, relax and wait for the next block. The experiment was preceded by a training period in which R and L were equiprobable, and which was continued until performance was almost error-free and RTs seemed stable (usually about 40 trials). Subjects were requested to assume a relaxed posture and to minimize head movements, eye movements and blinks. 2.4. Data analysis EEG and EOG were sampled at a rate of 100 Hz for an epoch of 1280 ms, starting at 300 ms prestimu~us. Trials with incorrect, premature (< 100 ms) or too late (> 900 ms) reactions, and trials in which EEG or EOG saturated the A-D converter, were excluded from further analysis. The remaining EEG data were corrected for horizontal and vertical eye movements and blinks, using the method of regression analysis in the frequency domain introduced by Woestenburg, Verbaten, and Slangen (1983). Subsequently EEG was averaged separately for all eight conditions (four blocks, and frequent or rare stimulus) and the prestimulus baseline was subtracted. The averaged ERPs were reduced to 43 data points by omitting the prestimulus period, and by discarding every second data point and the last 10% (130 ms) of the epoch. The resulting database consisted of 2,304 waveforms (2 age groups X 24 subjects X 2 hands

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x 2 probability levels X 2 replications X 6 electrode sites). Additionally, to study adaptation to the probability structure, averaged ERPs were computed over the first and the last ten frequents of each block, resulting in a second data base of 2,304 waveforms (2 age groups X 24 subjects X 2 hands X 2 begin versus end trials x 2 replications X 6 electrode sites). These two sets of data were then subjected to principal component analysis (PCA), using the BMDP4M program (Dixon, 1983). The covariance matrix was factored, and the first five factors were rotated according to the Varimax criterion. Additionally, these analyses were also carried out for both age groups separately (i.e., each on 1,152 waveforms). PCA has recently been criticized as a method for analysing ERPs. Wood and McCarthy (1984) found in an extensive series of simulations that experimental variance is sometimes misallocated between components, that is, component scores show statistically significant effects of experimental conditions which were not present in the original (simulated) component. PCA is nevertheless surprisingly reliable in reconstructing the underlying component structure, even when the database does not completely fulfil the requirements for PCA (Hunt, 1985). Thus, we decided on the following procedure: we used PCA to determine the underlying component structure of the ERP in our database, and computed area measures (average amplitudes) in the time windows where component loadings were maximal (all components had a single, unambiguous peak in the plot of component loadings). These were then used for statistical testing. Thus we hoped to obtain an objective way of partitioning the ERP in meaningful components, while keeping the probability of spurious experimental effects as low as possible. The area measures were subjected to a series of ANOVAs, using the BMDP2V program (Dixon, 1983). For the conventional averages, the factors were: age (old and young, between groups) X responding hand (left and right) X probability (frequent and rare) X replication (first and second half of the experiment) X electrode (Fz, Cz, Pz, Oz, C3 and C4). For the begin-end frequents the factors were: age X responding hand X begin - end X replication X electrode. When appropriate, separate ANOVAs for young and old subjects are reported, to facilitate interpretation. According to the modified Bonferroni procedure proposed by Holm (1979), alpha levels in these follow-up analyses were set at 0.025. Additionally, separate follow-up ANOVAs were carried out on C3 and C4 to evaluate effects of left and right hand motor responses. Latencies were computed for visually identifiable peaks and troughs in the ERP at the electrode location where the peak was most pronounced (see section 3.2.1) and subjected to ANOVA. RTs were averaged per condition, allowing direct comparison with ERPs. Finally, correlations of DS and DSS were computed with averaged RTs and peak latencies of N2 of P3 for each age group and for all eight conditions (2 frequent/rare x 2 left/right x 2 replications).

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For all main factors and interactions with more than two levels, the Geisser-Greenhouse adjustment of degrees of freedom was applied to correct for inflated Type I error due to heterogeneity of covariances between levels (Winer, 1971: p. 523). Since only minor effects were found for the replication factor (see also Polich et al., 1983) these will be listed, but not further discussed. ERP waveforms in the figures are pooled over replications; the amplitudes of area measures in table 2 are presented separately for first and second presentation.

3. Results 3. I. Behavioural measures RTs: Frequent stimuli were associated with faster reactions than infrequent stimuli in both age groups (see fig. l), F(1, 46) = 148.41, p < .OOl. Right hand responses were significantly faster than left hand responses, F(1, 46) = 15.75, p < ,001. The hand X probability interaction was not significant. Replication and replication x age were not significant. A significant interaction between hand and replication, F(1, 46) = 9.7, p < .004, indicates a decrease in RT for right hand responses, and an increase for left hand. Although the elderly seemed to have slower RTs than the young, this effect was not significant, F(1, 46) = 2.02, p < .17. None of the above effects had a significant interaction with age, suggesting a basically identical performance between age groups. Errors were more frequent in old subjects than in young (see table l), F(1, 46) = 11.49, p < .002, more errors were made for rare stimuli than for frequent, F(1, 46) = 68.64, p < .OOl, and mostly so by old subjects, F(1, 46) = 5.65, p < .03. Test results: For old subjects, the mean score for DSS was 48.6 (sd. 10.0); its correlation with IQ (unadjusted for age) was 0.43 (p -c .04), its correlation with age - 0.5 (p < .02). For young subjects, the mean score for DSS was 64.0

Table 1 Error percentages for left and right hand responses and frequent and rare stimuli Old Left Frequent Rare Right Frequent Rare

Young

4.7 9.6

1.4 3.4

2.3 10.3

1.1 5.8

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(s.d. 6.9); its correlation with IQ (unadjusted for age) was 0.65 ( p < .OOl), its correlation with age 0.30 (p < .15). For old subjects, the mean score for DS was 15.5 (s.d. 3.1); its correlation with IQ (unadjusted for age) was 0.28 (p < .20), its correlation with age - 0.13 (p < .54). For young subjects, the mean score for DS was 16.3 (s.d. 3.4); its correlation with IQ (unadjusted for age) was 0.33 ( p < .12), its correlation with age 0.20 ( p < .35). Correlations ofDS andDSS with RTs and N2 and P3 latencies: DS did not show any significant correlation with RTs or peak latencies in either age group. DSS was found to have significant negative correlations with all RTs in the old group (right hand frequent r = - 0.62 (p < .OOl) for the first replication, and Y= - 0.54 ( p < .006) for the second, right hand rare r = - 0.57 ( p < .004) and Y= - 0.43 (p < .04), left hand frequent r = - 0.61 ( p < .002) and r= - 0.67 (p < .OOl), and left hand rare r = - 0.57 ( p < .OO3) and r = -0.45 ( p c: .03)). Negative correlations between DSS and P3 latency were found in the old in all eight conditions (ranging from - 0.29 to - 0.49), two of which reached significance. No significant correlations were found with N2 latency in old subjects. No significant correlations between DSS and either RT or P3 latency were found for the young group. Consistently positive correlations between DSS and N2 latency were found in the young (between 0.11 and 0.42), but these reached significance in only one condition. 3.2. ERPs:

The effects

of probability and responding hand

As can be seen in fig. 2, the ERPs of both age groups showed a consistent pattern of P2, N2, P3 and slow wave at all electrode locations, with the exception of Oz, where P2 was obscured by Nl (most conspicuously in old subjects). 3.2.1. Latency measures P.? latency ~maximum 100-250 ms at Cz) was not significantly different for young and old subjects. It was earlier for right hand responses in both age groups, F(1, 46) = 4.99, p < .03 (see fig. 2). The probability effect was not significant, but a significant interaction with age, F(1, 46) = 6.23, p < .02, indicated that rare stimuli were associated with longer P2 latencies than frequents only in old subjects (217.2 and 206.7 ms), whereas in young subjects the reverse was true (215.3 and 228.0 ms). No other main effects or interactions were significant. N2 latency (minimum 200-350 at Cz) did not significantly differ between age groups (see fig. 1). Frequent stimuli evoked earlier N2 peaks than rares, F(1, 46) = 13.19, p < .OOl, and mostly so in right hand responses (probability X hand, Ffl, 46) = 6.33, p < .02). Replication and replication X age were not significant, but a hand X replication X age interaction, F{l, 46) = 4.65, p <

133

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Table

motor response and age

2

Amplitudes

of area measures (~JV)

P2

Young

Old Fz

Cz

Pz

Oz

c3

c4

Fz

Cz

Pz

oz

c3

c4

Left Frequent 1st

4.5

2.0

0.9

-1.8

2.0

2.5

3.7

3.6

3.6

0.9

2.5

2.8

2nd

4.2

2.3

1.2

-1.6

2.0

2.6

3.6

3.4

3.4

0.6

2.5

2.1

Rare 1st

4.3

1.8

0.4

- 2.6

1.5

2.1

4.9

4.6

3.6

2.9

3.6

2nd

5.1

2.4

0.8

- 2.1

2.2

3.1

4.2

3.4

3.2

0.1

2.2

2.2

-0.2

Right Frequent 1st

5.8

3.4

2.1

-1.3

2.9

4.2

4.2

4.4

4.9

2.3

2.9

4.0

2nd

6.3

4.0

3.2

-0.9

3.3

4.8

4.3

4.9

5.2

2.4

3.4

3.9

1st

1.3

4.5

3.1

- 0.7

3.8

5.1

6.3

6.4

6.4

2.0

4.4

5.4

2nd

7.5

5.0

3.7

-0.7

4.1

5.0

5.9

6.3

6.4

2.2

4.5

5.0

Rare

N2 Young

Old Fz

Cz

Pz

Oz

c3

c4

Fz

Cz

Pz

oz

c3

c4

Left Frequent 1st

5.0

2.9

3.8

2.3

4.0

3.7

2.9

4.7

8.4

5.2

4.2

4.3

2nd

4.5

3.0

3.8

2.1

3.7

3.6

1.9

3.8

7.8

5.0

3.8

3.6

1st

4.2

1.0

2.5

1.9

2.3

2.9

3.4

5.5

9.4

6.0

5.4

5.1

2nd

5.6

1.8

2.4

1.2

2.6

3.8

2.1

4.0

8.3

5.3

3.9

3.6

Rare

Right Frequent 1st

4.3

2.9

4.8

3.0

2.5

5.4

1.6

4.9

9.6

6.8

3.0

6.0

2nd

5.0

4.1

5.9

3.6

3.4

6.4

1.9

5.2

9.6

6.8

3.2

5.1

1st

6.2

3.4

5.0

2.9

3.4

6.2

2.9

4.7

9.5

6.4

3.2

6.4

2nd

5.6

3.1

4.8

2.9

3.0

5.4

2.0

4.2

8.9

6.2

3.0

5.4

Rare

P3 Young

Old Fz

Cz

Pz

c3

Oz

c4

Fz

Cz

Pz

oz

c3

c4

Left Frequent 1st

8.2

9.1

9.5

4.3

8.2

7.5

3.9

6.9

10.9

5.5

5.6

6.2

2nd

7.1

8.7

9.0

3.6

7.8

7.1

2.3

5.6

9.9

5.0

4.9

5.0

Rare 1st

7.8

1.3

9.8

4.8

8.0

6.8

5.2

10.4

15.4

8.2

9.1

9.1

2nd

8.4

1.9

10.4

5.4

8.8

7.7

5.3

9.3

14.1

7.7

8.2

7.3

135 Table 2 (continued)

Young

Old FZ Right Frequent 1st 2nd Rare 1st 2nd

cz

Pz

02

c3

C4

Fz

Cz

Pz

02

c3

c4

8.6 9.0

9.4 9.9

10.9 10.6

5.1 5.0

7.1 7.3

9.4 9.8

4.7 3.8

9.3 8.2

13.1 12.3

7.0 6.8

6.3 5.1

8.9 7.8

10.8 9.8

10.6 10.2

12.2 12.6

6.4 6.6

8.2 8.0

12.3 11.9

7.1 5.4

12.1 11.4

16.2 15.7

8.6 8.6

8.0 7.6

12.2 10.9

Pz

02

C3

c4

Cz

Pz

oz

CJ

c4

SW Young

OId FZ Left Frequent 1st 2nd Rare 1st 2nd Right Frequent 1st 2nd Rare 1st 2nd

cz

Fz

0.7

4.4 4.4

6.5 6.4

6.0 5.8

2.5 2.1

5.5 5.3

5.1 5.3

-0.4

3.5 2.7

6.0 5.2

2.7 2.4

2.9 2.5

4.0 3.2

3.5 4.9

4.6 5.7

6.7 7.3

2.9 3.4

5.3 6.1

5.3 5.9

-0.3 0.7

4.9 5.0

10.5 11.0

5.8 6.3

5.2 5.2

5.2 4.9

3.9 5.0

6.1 6.8

6.3 6.5

2.8 2.4

4.5 5.0

6.3 6.9

0.4 -0.3

4.4 3.9

7.2 6.7

3.6 3.6

3.7 2.9

5.3 4.3

6.3 5.7

7.0 6.9

8.3 8.5

4.0 3.9

5.9 6.0

8.4 8.3

2.3 1.9

7.3 7.5

12.6 11.8

6.8 6.6

5.4 5.4

8.5 7.8

.04, suggests that N2 latency decreases in left hand responses, but increases in right hand responses with replication in old subjects. P_? latency (ma~mum 350-700 at Pz) was longer for old than for young subjects, F(1, 46) = 72.19, p -c .OOl. Frequent stimuli evoked earlier P3s than rares, F(1, 46) = 25.27, p < .OOl. No significant main effect or interaction of replication was found. As can be seen in fig. 1, RT follows P3 in young subjects, but precedes it in the elderly.

3.2.2. Area measures A first PCA was performed on the database pooled over old and young groups. Inspection of the component structure suggested that latency differences in P3 between age groups might have caused PCA to produce two components related to P3 (see Donchin & Heffley, 1978: p. 565; Vaughan, Herrmann, & Bell, 1987). Thus, we ran separate PCAs for young and old

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subjects. In both old and young, the first (largest) component had its maximum loadings around the P3 peak of the ERP for that group, and could thus be identified as P3. The first rotated component identified by PCA in old subjects explained 54% of the total variance, and was maximally active between 380 and 520 ms, the second 16% (580-680 ms), the third 8% (160-240 ms), the fourth 7% (260-320 ms) and the fifth 6% (740-840 ms). In young subjects the first component explained 46% (320-420 ms), the second 16% (660-840 ms), the third 11% (460-560 ms), the fourth 7% (160-240 ms) and the fifth 5% (260-320 ms). Hence, the following area measures were computed: P2 (averaged amplitude in the 160-240 ms window), N2 (260-320 ms), P3 (380-520 ms in old, in 320-420 ms in young), SW (580-680 ms in old, and 460-560 ms in young). The SW resolution component (660-840 ms and 740-840 ms) will not be discussed. P2 area (160-240 ms) was not significantly different for young and old (see table 2). The P2 was a predominantly fronto-central component (electrode: F(5, 230) = 32.57, p < .OOl), and a significant electrode x age interaction, F(5, 230) = 5.79, p < .005, suggests that P2 was more frontally located in the elderly and more evenly distributed over frontal, central and parietal sites in the young (see fig. 2 and table 2). P2 amplitude was significantly larger for right hand responses, F(l, 46) = 152.81, p < ,001, and larger for infrequent stimuli, F(l, 46) = 12.82, p < ,001. A significant interaction between hand and probability, F(1, 46) = 16.00, p < ,001, indicates that the probability difference was confined to right hand responses (see table 2). Age X probability, age x hand, and age x probability X hand were not significant. A significant interaction between electrode and hand, F(5, 230) = 8.38, p -c .OOl, indicates that the hand effect was somewhat larger in old subjects than in young at all electrode sites, with the exception of Oz, where the hand effect was minimal for old subjects (age X electrode X hand: F(5, 230) = 5.81, p ( .002). Significant interactions between electrode and probability, F(5, 230) = 23.83, p -C ,001, and between age, electrode and probability, F(5, 230) = 3.33, p -c .03, indicate that the probability effect was most pronounced at fronto-central electrodes in the young, and was frontally located in the elderly. The replication effect was not significant; neither were interactions of replication with electrode, probability or hand. A significant interaction with age, F(1, 46) = 4.57, p < .04, suggests an increase in amplitude in old subjects with replication. A separate ANOVA on C3 and C4 electrodes showed again a highly significant hand x electrode effect, indicating that the P2 was much more positive on C4 than on C3 for right hand responses, and only slightly more positive for left hand responses, F(l, 46) = 26.94, p < .OOl. There was no significant interaction between hand and age.

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N2 area (260-320 ms) was not significantly different between age groups. It was a fronto-central component (main effect electrode: F(5, 230) = 9.92, p -c .OOl), with a steep anterior-posterior gradient in the young and a more diffuse distribution in the old (age X electrode: F(5, 230) = 12.78, p < .OOl). Right hand responses evoked smaller (more positive or less negative) N2s, F(1, 46) = 18.03, p -c .OOl, and a significant interaction with age, F(l, 46) = 5.10, p < .03, suggests that this difference was only present (was only significant) in the old age group. A significant hand X electrode interaction, F(5, 230) = 39.43, p < .OOl, indicates that the hand effect was most pronounced at the central and parietal electrodes both for young and old subjects (age x hand x electrode not significant). Replication was not significant; a significant interaction with age, F(l, 46) = 4.44, p < .05, indicated that N2 tends to decrease (become less negative or more positive) in old subjects, but increases in the young. No significant interactions between replication and either probability, hand or electrode were found. In the separate ANOVA with only C3 and C4 as electrodes the hand x electrode interaction was highly significant, F(1, 46) = 109.04, p < .OOl: N2s at C3 and C4 electrodes practically coincide for left hand responses, but N2 at C4 was smaller (more positive) than at C3 for right hand responses for young and old subjects (the interaction between age, hand and electrode was not significant). The probability effect was not significant, neither was there a significant interaction with age. The probability x electrode interaction, F(5, 230) = 7.55, p < .OOl, was difficult to interpret. The age x probability x electrode interaction was not significant. The interaction between hand and probability was not significant, but a significant interaction with age (age X hand X probability: F(l, 46) = 6.18, p < .02) suggests that in the elderly (but not in the young) the probability effect was largest for left hand responses (see table 2). P3 area (380-520 ms and 320-420 ms) was not significantly different between age groups. It was a parietal component (electrode: F(5, 230) = 26.69, p < .OOl), with a steep anterior-posterior gradient in the young and a more diffuse distribution in old subjects (electrode X age: F(5, 230) = 8.93, p < .OOl). Right hand responses evoked a larger (more positive) P3 than left hand responses, F(l, 46) = 38.74, p < .OOl), in both age groups (see fig. 2). Rare stimuli evoked more positive P3s than frequents, F(l, 46) = 36.08, p < .OOl, but a significant interaction with age, F(l, 46) = 10.86, p < .002, suggests that this was not the case in the elderly (in a separate ANOVA for the old age group probability was not significant; in the young group it was highly significant, F(1, 23) = 40.67, p < .OOl). The probability x hand was not significant, but a significant age X probability X hand interaction, F(l, 46) = 5.94, p < .002, suggests that in the elderly the probability effect was confined to right hand responses, while it was present in both hands in the young (see

138

table 2) (separate ANOVA for the old group probability X hand: F(l, 23) = 10.47, p < .004, separate ANOVA in the young probability X hand not significant). The probability effect was largest at the Pz and Cz electrodes, but only in the young (probability x electrode: F(5, 230) = 5.40, p < .004; age x probability x electrode: F(5, 230) = 6.66, p -c.OOl; age X hand X probability X electrode not significant). The hand effect was strongest at Cz and C4 and practically absent at C3 for both age groups (hand X electrode: F(5, 230) = 43.89, p < .OOl; age X hand X electrode not significant). RepIication was associated with a decrease in amplitude, F(1, 46) = 4.87, p < .04, but only in young subjects (age X replication: F(1,46) = 6.02, p < .02). No significant interactions between replication and either probability, hand or electrode were found. A separate ANOVA with only C3 and C4 as levels for the factor electrode revealed that the hand X electrode interaction was highly significant, F(1, 46) = 129.83, p < .OOl: the C3-C4 difference, which was quite pronounced (C4 more positive than C3) in right hand responses, disappeared in left hand responses. This effect was not different for old and young subjects (hand X age, electrode X age and hand X electrode X age not significant). Slow Wuue (area 580-680 ms and 460-560 ms): The effect of age was not significant. SW had a parietal maximum (electrode: F(5, 230) = 28.55, p < .OOl); in the young it had a frontal ~nimum and a steep ~terior-posterior gradient, in old subjects it had an occipital minimum and was more evenly distributed over the scalp (age X electrode: F(5, 230) = 11.42, p < .OOl). Right hand responses evoked larger SWs than left hand in both age groups, F(1, 46) = 14.22, p -c.OOl. This effect was largest at Cz among the midline electrodes; besides, while C3 was less positive than C4 for right hand responses, the difference disappeared for left hand responses (hand X electrode: F(5, 230) = 10.40, p < .OOl). Rare stimuli were associated with larger SWs than frequents, F(1, 46) = 52.72, p -=z,001, and mostly so in young subjects at centro-parietal sites (age x probability: F(1, 46) = 16.90, p < .OOl; probability X electrode: F(5, 230) = 28.69, p < .OOl; age x probability X electrode: F(5, 230j = 12.07, p < .OOl). A significant hand X probability interaction indicated that the probability effect was largest for the right hand in both old and young subjects (hand x probability: F(l, 46) = 7.08, p < .Ol), and mostly so at central and parietal sites (hand X probability X electrode: F(5, 230) = 9.59, p -c.OOl, both age groups). Replication was not significant; neither were interactions of this factor with age, hand, probability or electrode. A separate ANOVA for only C3 and C4 as electrodes indicated that the hand X electrode interaction was significant, F(l, 46) = 33.31, p < .OOl; while the SW at C4 was more positive than at C3 for right hand responses, the difference disappeared for left hand responses. This C3-C4 difference was not

H. Looren de Jong et nl. / Stimulus probability,

significantly different for young electrode were not significant). 3.3. ERPs: Begin-end

motor response and age

and old (age, age X hand,

139

and age X hand X

effects

Figure 3 presents ERPs averaged over the first and last ten frequent stimuli of each block; as can be seen, the begin-end effect (the difference between first and last frequent trials of a block) is manifested in an overall reduction of positivity in the ERP. Separate PCAs for young and old age groups on this database yielded the following components: P2 (160-240 ms), P3 (400-500 ms and 280-420 ms for old and young respectively), SW (560-680 ms and 520-600 ms). No consistent N2 component could be identified, which is probably due to the practical absence of experimental variance in this epoch (see fig. 3). Since this database overlaps with the conventional averages, only effects not reported in the previous section will be described. P3 latency (maximum positivity 350-700 ms at Pz) was later in old subjects than in young (490 ms and 400 ms respectively), F(1, 46) = 62.51, p < .OOl. Frequents at the begin of a block had shorter latencies than those at the end, F(1, 46) = 5.39, p < .03. P2 area (160-240 ms) was not significantly different for young and old. Frequents at the beginning of a block were larger than at the end, F(1, 46) = 12.30, p < .OOl, and this begin-end effect was largest in young subjects (age X begin-end: F(1, 46) = 6.96, p < .02), mostly at frontal and central electrodes (begin-end x electrode: F(5, 230) = 37.12, p < ,001; age x begin-end X electrode: F(5, 230) = 3.01, p < .04). P3 urea (400-500 ms and 280-420 ms): The main effect of age was not significant. P3 at the beginning of a block was larger than at the end, F(1, 23) = 62.15, p < .OOl, and this begin-end effect was largest for young subjects (age X begin-end: F(1, 46) = 11.37, p < .0015). It was not significantly different in right and left hand (hand x begin-end and age X hand x begin-end not significant). A significant interaction between electrode and begin-end, F(5, 230) = 21.50, p < .OOl, indicates that the begin-end effect was largest at Cz, and this was most pronounced for young subjects (age X begin-end X electrode: F(2, 230) = 5.11, p < .006). Slow Wave (560-680 ms and 520-600 ms) was more positive for old than for young subjects. Frequents at the beginning of a block were associated with larger (more positive or less negative) SWs than those at the end of a block, F(1, 46) = 10.02, p < .003, mostly at frontal and central electrodes (electrode X begin-end: F(5, 230) = 5.03, p < .OOl), for both young and old (age x begin-end, age X begin-end X electrode, age X hand X begin-end, and age x hand X begin-end X electrode not significant).

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141

4. Discussion This study replicates in several respects the effects of age in visual oddball tasks found in other studies (Mullis, Holcomb, Diner, & Dykman, 1985; Pfefferbaum et al., 1984; Picton et al., 1984). P3 amplitude was more evenly distributed over the scalp in old subjects than in young, and P3 at Fz was larger in old than in young subjects; no reliable overall decrease in amplitude with age was found. As in Picton et al.‘s (1984) results, P3 latency was longer in the elderly, but this effect was not accompanied by equal slowing of RT; in young subjects P3 precedes or coincides with RT, whereas in the elderly P3 follows RT. The absence of significant correlations between DS (memory span) scores and each of the behavioural and ERP latencies is at variance with the results of Polich et al. (1983). A major difference between Polich et al.‘s (1983) subject pool and ours was that they did not control for differences in educational level and general intelligence between young and old subjects. DS is known to correlate highly with general intelligence (about 0.66; see Matarazzo, 1972: p. 206). Our finding that controlling for differences in intelligence eliminates the correlation between P3 latency and DS suggests that Polich et al.‘s (1983) results may be attributed to a general intelligence component in DS, not to memory span. The presumed test for central slowing (DSS) correlated significantly with RT (and to a lesser extent with P3 latency) in old subjects, but not in young. If DSS were a pure index of central slowing (as distinguished from perceptual-motor processes (Cerella, 1985)), it would be expected to correlate more strongly with P3 or N2 latencies than with RT. Probably, DSS contains a motor-speed component in in old subjects, but not in young (Laux & Lane, 1985). Apparently, in spite of claims to the contrary (Salthouse, 1985a), DSS is not a valid test for central speed in the elderly. Interesting features of our data, to be discussed below, were strong hand and probability effects on early components (P2 and N2) in both age groups, and a smaller probability effect on P3 amplitude and a longer P3 latency in the elderly. 4. I. Age and stimulus evaluation

time

The P2 was influenced by both probability and responding hand; it was larger and earlier for right hand responses and larger for rare stimuli. N2 was earlier for frequent trials and smaller (more positive relative to the baseline) for right hand responses. The usually observed increase in N2 amplitude in response to rare stimuli (e.g., Sams, Paavilainen, Ahlo, & N$igt;inen, 1985) seems largely absent in our data. Although this is clearly at variance with the

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interpretation of N2 as an automatic mismatch detector (N&it$inen & Gaillard, 1983; Ritter, Simson, & Vaughan, 1983) the shorter N2 latency for frequent stimuli might be indicative of a match-mismatch evaluation in the N2 epoch. These hand and probability effects found in P2 and N2 in our data suggest that stimulus evaluation and response programming can occur before P3. Ritter, Simson, Vaughan, and Macht (1982), and Ritter et al. (1983) found that N2 latency accounted for most of the variance in RT in a visual cIassification task. Renault, Ragot, Lesevre, and Remond (1982) found that N2 rather than P3 latency was related to RT, and concluded that the timing of information processing leading to the onset of motor programming is reflected primarily in N2, not in P3. It seems reasonable then to suppose that the early components reflect automatic registering of deviant stimuli (NBatanen & Gaillard, 1983; Ritter et al., 1983; see also Hasher & Zacks, 1984) and response initiation (Ritter et al., 1983). Our results suggest that response initiation takes place concurrent with or before evaluation of probability. Furthermore, since N2 latency, P2 latency and RT are not different between age groups, it can be concluded that the automatic registration of probability and response triggering occur at about the same time for young and old subjects. This casts some doubt upon the use of P3 latency as an index of age-related changes in stimulus evaluation time in oddball studies (e.g., Ford & Pfefferbaum, 1985). P3 is much later relative to RT in old subjects than in the young (in old subjects P3 follows RT; however, P3 latency is no absolute measure, and therefore cannot be compared with RT on the same time scale). A possible explanation for this difference in P3-RT lag between age groups is the tendency to premature responding (“jumping the gun”) in old subjects (Kutas et al., 1977; see also Ford & Pfefferbaum, 1985). This is supported by the decreased accuracy found in old subjects. Furthermore, the bias towards frequent responses reflected in the higher error rate for rare stimuli is most prominent in the elderly. Alternatively, it is possible that the processes associated with P3 and RT, that is, memory updating and motor programing respectively, can occur in parallel (Picton et al., 1984). Presumably, RT can be triggered using evidence from early stimulus classification stage (reflected in P2 and N2), bypassing the memory update reflected in P3. This is consistent with Sanders’ (1980) notion of immediate arousal, which causes, under conditions of stress or high processing load, bypassing of central computational stages, feeding stimulus information directly into motor processes. If this is accepted, it could explain the different RT-P3 lag in young and old subjects: the elderly are able to emit a fast and reasonably accurate response, using probability and response information from processes reflected in N2 and P2, while memory updating (indexed by P3) occurs later than in young subjects.

H. Looren de Jong et al. / Stimulus probability,

4.2. P3 amplitude

motor response and age

143

and stimulus probability

The probability effect on P3 amplitude was less pronounced in the elderly, and practically absent for left hand responses. This result is unexpected in view of the majority of the literature, reporting robust probability effects on P3 (see Donchin, 1981; Johnson, 1988). Furthermore, it is at variance with Picton et al. (1984), who found that probability determinants remain the same across the lifespan, and with Ford, Duncan-Johnson, Pfefferbaum, and Kopell (1982) who found no age-related differences in sequential expectancies (i.e., local probabilities). Barrett et al. (1987) and Pfefferbaum et al. (1980, 1984) found no appreciable P3 to frequents in either old or young subjects in oddball tasks. In contrast with these studies, we found a rather pronounced P3 to frequent stimuli. This presumably represents the target effect: both frequents and rares had target quality in our study (cf. Johnson, 1988; Pritchard, 1987). While in the common oddball paradigm target effects are confounded with probability, in our design probability information is not related to releasing the correct response (which is not to say that probability is not registered - see Hasher & Zacks, 1984). Since the probability effect on P2 and N2 was not different in old and young subjects (in P2 latency it was even more pronounced in the elderly than in the young), the reduced probability effect on P3 in the elderly cannot be attributed to impaired registering of deviant stimuli per se. It has been found that subjects show no P3 when context updating does not contribute to task performance (Klein, Coles, & Donchin, 1984). Likewise, the absence of a probability effect in P3 in the elderly can be interpreted as an indication that they to some extent omit to keep track of long-term prediction and anticipation and confine themselves to information directly related to the current response. This would also explain the apparently reduced memory span in the elderly as a result of a less precisely updated model of the environment (cf. Rabbitt & Vyas, 1980; Griew, 1968). The reduced (almost absent) probability effect in left hand responses is difficult to explain. The locus of S-R probability effects is usually placed in the response programming stage (Sanders, 1980) more precisely, in differential predetermination of the decision threshold in the response identification process (Requin, Lecas, & Bonnet, 1984). The increased positivity and earlier P2 latency in right hand responses could, speculatively, be interpreted as a manifestation of preferential activation of responses with the dominant hand. Its interaction with probability could then indicate that both affect the same processing stage, namely, response programming. The probability effect reflected in P3 amplitude, which is less pronounced in the elderly, is presumably a sign of a controlled, effort-demanding process of updating expectancies about the environment (Riissler, 1983; Shiffrin & Schneider, 1977) using the information available from the automatic mis-

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match register. It might be speculated that the reduced probability effect on P3 in old subjects results from diminished processing resources (cf. Craik & Byrd, 1982). 4.3. Adaptation

to probabilities

The difference between beginning and end of a block appeared as an overall decrease in positivity in all components, which is visible in the ERP from about 100 ms onwards. This decrease was largest in young subjects, and was most pronounced at fronto-central electrodes, and practicafly absent at Oz in both young and old subjects. Presumably, the subject learns to differentiate between frequent and rare stimuli in the course of a block, and it seems plausible that this can be considered as a kind of tuning in to the probability structure of a block (cf. Donald, 1983; Hansen & Hillyard, 1988). The smaller difference between beginning and end of a block in P3 and SW in old subjects corresponds to the smaller probability effect found in the elderly in the same components of the conventional averages. It should be noted, however, that the increase in P3 latency with longer exposure to the task runs counter to the usually observed shorter latency for more familiar stimuli (e.g., Kok & Looren de Jong, 1980), and an interpretation in terms of habituation or fatigue remains possible. 4.4. The effects of responding hand It is rather surprising that right hand responses produced larger and earlier ERP components than left hand responses; no such effects were found by Ragot (1984). It might be argued that the hand effect is a result of overlap with (negative) motor potentials and readiness potentials. This is supported by the C3-C4 difference, which indicated an increase in negativity (N2) or a decrease in positivity (P2, P3, SW), contralateral to the responding hand. This difference was asymmetrical, that is, the C3-C4 difference for right hand responses was larger (more negative) than C4-C3 for left hand response (the latter being practically zero or even slightly positive). A similar asymmetry in motor potentials in volunta~ hand movement has been reported by Rutas and Donchin (1977). However, it seems that motor potentials cannot explain the entire hand effect in the present data, and that it must be to some extent a cognitive process. Firstly, the hand effect on the C3-C4 difference in P2, N2, P3 and SW did not interact with age, while motor potentials are known to decrease with advancing age (Deecke, 1980). Secondly, there was also a hand effect on SW and P3 in the elderly although these occurred after the RT, and consequently after the motor potential associated with it.

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Although the nature of the amplitude difference between right and left hand ERPs remains unclear, the time course of this divergence might provide some insight in the process of response preparation. Continuous flow models (Coles, Gratton, Bashore, Eriksen, & Donchin, 1985; Eriksen & Schulz, 1979; Gratton, Coles, Sirevaag, Eriksen, & Donchin, 1988) assume that activation in response centres builds up gradually, partial output of previous stages being continuously available to all later stages. It is conceivable that the time course of the hand effect, starting in the P2 area, reflects this gradual build-up of activation in response channels.

5. Conclusion To sum up, it was found that old subjects’ responses were as fast as young subjects’, although somewhat less accurate. In combination with the prolonged P3 latency relative to RT, this might indicate a speed-accuracy shift in the elderly; such an interpretation rests on the assumption that complete stimulus evaluation is not reflected in the ERP before P3. However, since the effects of detection of deviants and response choice were discernible remarkably early in the ERP, and equally so in both age groups, it was concluded that RT is coupled to N2, rather than to P3, and that the (probably automatic) processes of frequency detection and response initiation in this task are unimpaired with age. In contrast, the expectancy effect on P3 amplitude was reduced, and P3 had a longer latency in all conditions in old subjects, without accompanying effects on RT; it was concluded that the controlled memory updating reflected in the expectancy effect on P3 amplitude runs parallel to response initiation, and that old subjects invest less effort in this updating, presumably as a result of diminishing processing resources. The latter conclusion illustrates how ERPs can provide insight in age-related changes in performance that are not directly manifest in behavioural measures.

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