The boundary condition for observing compensatory responses by the elderly in a flanker-task paradigm.

Biological Psychology 103 (2014) 69–82

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The boundary condition for observing compensatory responses by the elderly in a flanker-task paradigm Shulan Hsieh a,b,∗ , Yu-Chi Lin a a b

Cognitive Electrophysiology Laboratory, Department of Psychology, National Cheng Kung University, Taiwan Institute of Allied Health Sciences, National Cheng Kung University, Taiwan

a r t i c l e

i n f o

Article history: Received 5 March 2014 Accepted 12 August 2014 Available online 26 August 2014 Keywords: Age-related flanker effect Compensatory response Event-related potential Reversal response rule

a b s t r a c t This study aimed to establish a baseline condition to observe ERP responses for older adults in a conventional flanker-task paradigm, in which neither a reversal response rule toward a target nor a color-coded target was employed. In addition, this study aimed to examine whether the previous finding of the compensatory responses reflected on event-related potential (ERP) for older adults in performing a flanker task was due to the specific demand of the reversal response rule toward a target or simply due to the pop-out effect with a singleton target manipulation. The results of the current study showed that (1) some of the previously thought-to-be compensatory ERP responses might not really reflect compensatory responses; (2) the previous finding of age-related ERP compensatory responses was mainly due to the manipulation of the reversal response rule condition; and (3) in some scenarios of flanker-task paradigms, older adults were just as capable as younger adults in conquer with the flanker interference even though no ERP compensatory responses were found. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Flanker task developed by Eriksen and Eriksen (1974) has been widely used to tap inhibitory function for decades. In this paradigm, an individual is required to respond to a centrally positioned target and ignore simultaneously presented distracters that flank the target, and the flanking distractors are either associated with a response that is congruent to the target (congruent-flanker), in conflict with the target (incongruent-flanker), or neutral. It is typically found that behavioral performance (reaction time (RT), accuracy) is better in the congruent-flanker conditions, and worse in the incongruent-flanker conditions, relative to the neutral conditions. The performance difference between congruent- and incongruentflanker conditions is termed ‘flanker effect’. The magnitude of the flanker effect has often been associated with inhibitory function (Salthouse, 2010). As people aged, various cognitive processes such as memory and executive functions decline along with neural degeneration (e.g., Angel, Fay, Bouazzaoui, & Isingrini, 2010; Cona, Arcara, Amodio, Schiff, & Bisiacchi, 2013; Friedman, Nessler, Johnson,

∗ Corresponding author at: Cognitive Electrophysiology Laboratory, Department of Psychology, Institute of Allied Health Sciences, National Cheng Kung University, 1 University Road, Tainan 701, Taiwan. Tel.: +886 6 2008703; fax: +886 6 2008703. E-mail address: [email protected] (S. Hsieh). http://dx.doi.org/10.1016/j.biopsycho.2014.08.008 0301-0511/© 2014 Elsevier B.V. All rights reserved.

Ritter, & Bersick, 2008; Jurado & Rosselli, 2007; Kopp, Lange, Howe, & Wessel, 2014; Luszcz, 2011; Salthouse, Atkinson, & Berish, 2003; Whitson et al., 2014; Zanto, Hennigan, Östberg, Clapp, & Gazzaley, 2010). Among these cognitive functions, the older adults’ ability to resist distractor’s interference has received much research attention. Laboratory studies have shown that older adults are more susceptible to distraction and interference by irrelevant-information, especially when the irrelevant-information is incongruent with the target goals (Weeks & Hasher, 2014), than are younger adults, as indexed by increased reaction times and error rates (see Kok, 1999 for an overview). A widely known age-related inhibitory deficit theory has thus been proposed to account for older adults’ increased susceptibility to distractors (Hasher & Zacks, 1988; Hasher, 2007). However, a closer examination of the literature reveals that the general theory of age-related inhibitory deficit has received mixed support from empirical studies using the Eriksen flanker task (Eriksen & Eriksen, 1974). While some studies have demonstrated that older adults are less able to inhibit flanker interference, which supports inhibitory deficit theory (Colcombe, Kramer, Erickson, & Scalf, 2005; Machado, Devine, & Wyatt, 2009; Shaw, 1991; Zeef & Kok, 1993; Zeef, Sonke, Kok, Buiten, & Kenemans, 1996), some studies of the flanker interference effect have failed to find significant differences between younger and older adults (Falkenstein, Hoormann, & Hohnsbein, 2001; Fernandez-Duque & Black, 2006; Gunter, Jackson, & Mulder, 1996; Jennings, Dagenbach, Engle, & Funke,

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S. Hsieh, Y.-C. Lin / Biological Psychology 103 (2014) 69–82

2007; Kramer, Humphrey, Larish, Logan, & Strayer, 1994; Madden & Gottlob, 1997; Nieuwenhuis et al., 2002; Wild-Wall, Falkenstein, & Hohnsbein, 2008), whereas others have found the opposite pattern in which older adults exhibit less interference than younger adults (Kamijo et al., 2009; Madden & Gottlob, 1997; Mathewson, Dywan, & Segalowitz, 2005; for a more comprehensive review, see Guerreiro, Murphy, & Van Gerven, 2010)1 . Hence, some mediating factors contributing to the observed discrepant age-related flanker interference effect may not yet have been identified. In one of our previous studies, we have indicated a potential mediating factor, i.e., the age-related brain compensatory responses, which might partially contribute to the observed discrepant age-related flanker interference effect (Hsieh & Fang, 2012). In that study, the flanker-task paradigm was modified by introducing a color-coded target in which different target colors (e.g., red and green) represented either a compatible (PRO) response or an incompatible (ANTI)2 response with respect to the target pointing direction (Hsieh & Fang, 2012; Hsieh, Liang, & Tsai, 2012; Van’t Ent, 2002). For example, when participants in the “PRO” condition, they were instructed to respond to a green (or red, counterbalanced across participants) target arrow as quickly and accurately as possible by indicating the compatible direction of the target arrow with their left or right index finger. In contrast, during the “ANTI” condition, they responded to a red (or green, counterbalanced across participants) target arrow by indicating the opposite (incompatible) direction. The results of Hsieh and Fang’s (2012) study showed that the elderly did not exhibit an increased behavioral (i.e., response time (RT) and accuracy) flanker effect, nevertheless, the elderly was found to utilize compensatory responses reflected on ERP components to overcome their deficient inhibitory function. These ERP compensatory responses for older adults as compared to younger adults include an increased stimulus-locked N1 at the occipital sites (attention-related processes), a decreased (or even the absence of) N2 at the frontal site (conflict-related processes), a delayed P3b at the parietal site (stimulus evaluation time), an equivalent response-locked error-related negativity (ERN) at the fronto-central site (error-related processes), and an increased correct-related negativity (CRN) at the frontocentral site (correction-related processes). Despite the robustness, findings in Hsieh and Fang’s (2012) study regarding the unimpaired behavioral flanker effect accompanied by ERP compensatory responses for the elderly, it remains unclear whether the observed the so-called ERP compensatory responses for the elderly were authentic due to their lack of baseline-control condition, and whether their findings were specifically due to the manipulation of the ANTI (incompatible response) condition which required more active control for the elders thus inducing their compensatory responses in performing the flanker task, or due to the aid of the color-coded target which acted like a “singleton” and thus resulting in more passive stimulusdriven compensatory processes for the elderly. In the visual search research domain, a singleton target has been demonstrated to automatically attract visual attention (Horstmann, 2002). Hence, the color-coded target in the flanker task paradigm might act like a color singleton, thus facilitating the target processing and reducing the influence from flankers, and such facilitation in target

1 The counterintuitive findings that older adults are more capable of inhibiting irrelevant information or thoughts than younger adults have also been reported in a different context with different approaches (such as comprehension, memory, problem solving, and sustained attention; Giambra, 1989; Jackson & Balota, 2012; Smallwood et al., 2004; Staub et al., 2014). 2 Please note, the terms “PRO” and “ANTI” are used to refer to the compatibility of the response with the target which are different from the terms “congruent” and “incongruent” referring to the compatibility of the responses between target and flankers.

processing might be somehow more effective for the elderly, thus resulting in an equivalent amount of flanker effect between the younger and older groups. Since the previous study by Hsieh and Fang (2012) was not designed to directly test these two hypotheses, we thus set out this study to address the issue. Therefore, we conducted three experiments in the present study. The first experiment served as a baseline-control condition, in which a regular flanker-task paradigm was employed; that is, both the target and flankers were shown in the same color (i.e., white) and only compatible response (i.e., PRO) trial conditions were incorporated. The purpose of incorporating this baselinecondition experiment was to provide a performance baseline for us to see if the elderly without any aid of active control (due to the manipulation of ANTI trial condition) or passive perceptual facilitation (due to the manipulation of a color-coded singleton target) would thus exhibit deficient inhibitory function resulting in an increased flanker effect on behavioral measures. Another purpose of incorporating this baseline-condition experiment is to yield what the elders’ brain responses (in terms of ERPs) naturally are in the context of a conventional flanker-task paradigm, since previously there was no such baseline information regarding how the original ERP responses for the elderly should be because all of the previous experiments had incorporated ANTI trial condition. Such ERP baseline information is important if we wish to verify the authenticity of the observed the so-called ERP compensatory responses for the elderly. Finally, the ERP baseline information derived from this baseline-control experiment may additionally serve to clarify the issue if the observed similar amount of the RT flanker effect between the two age groups necessarily involves ERP compensatory responses for the elderly as previously found. The second experiment differing from the first experiment used color-coded (either in a red or green color) central targets, yet likewise incorporated only compatible response (i.e., PRO) trial conditions as in Experiment 1; whereas the third experiment not only used color-coded central targets, but also incorporated both PRO and ANTI trial conditions (50% of the trials for each condition)3 . The rationale of designing these experiments is as follows. If the previous finding of age-related compensatory responses was specifically due to the manipulation of the ANTI trial condition per se, then we would expect to observe the ERP compensatory responses in overcoming inhibition deficiency only in the experiment with the mixture of the PRO and ANTI conditions (i.e., the current Experiment 3), but not in the experiment without the manipulation of the ANTI condition but simply incorporating a color-coded target (i.e., the current Experiment 2). Furthermore, we would expect to observe little or null age-related increased flanker effect on behavioral measures in the current Experiment 3, but significant age-related increased flanker effect on behavioral measures in the current Experiment 2. On the other hand, if the previous finding of age-related compensatory responses was primarily due to the manipulation of a color-coded target, then we would expect to also observe the ERP compensatory response in the current Experiment 2. Furthermore, we would then expect to observe little or null age-related increased flanker effect on behavioral measures in both the current Experiments 2 and 3. Finally, as aforementioned, by incorporating the information derived from the first baseline-control experiment, we probably would observe that some of previously thought-to-be ERP compensatory responses for the elderly turn out to be independent of the behavioral flanker effect, and/or in some flanker-task scenarios, the elderly might be

3 Please note, in order to make a fair comparison between these two experiments, only trials in the PRO conditions in the PRO-ANTI experiment (the current Experiment 3) were retrieved for analyses.


just as capable as younger adults in conquer with flanker interference even without involving ERP compensatory responses. 2. Method 2.1. Overview of Experiments 1–34 All three experiments conducted in this study employed the same flanker stimulus and trial procedure, except the nature of the central target. In Experiment 1, the central target was the same color (being neutral white) as the flankers, signaling compatible response rule (PRO only). In Experiment 2, the central target could be either red or green, yet both signaling compatible response rule (PRO only). In Experiment 3, the central target could be either red or green, signaling either compatible (PRO) or incompatible response rule (ANTI) respectively (color-rule mapping was counterbalanced across participants). It should be noted that in the present study, in Experiments 1 and 2, only the manipulation of three types of target–flanker compatibility (congruent, neutral, incongruent) was employed while keeping the target and response always compatible (i.e., PRO condition) (see Fig. 1a and b), whereas in Experiment 3, the manipulation of PRO vs. ANTI compatibility between the target and response was introduced and which was independent of the flanker effect manipulation of the congruent, neutral, or incongruent compatibility between the target and flankers. There were three types of target–flanker compatibility: congruent flankers that pointed in the same direction as the target; neutral flankers that did not include direction information; and incongruent flankers that pointed in the opposite direction from the target. These three types of target–flanker compatibility occurred randomly and with equal probability across the PRO and ANTI trials (see Fig. 1c). Because “compatibility” could be understood to refer to either the target–response or target–flanker relationships, the terms “PRO” and “ANTI” are used to refer to the compatibility of the response with the target, and the terms “congruent,” “neutral,” and “incongruent” are used to refer to the compatibility of the flankers with the target in the remainder of the paper. Consequently, to yield a fair comparison across three experiments, only PRO (target–flanker compatible) trials (specifically refer to Experiment 3) were analyzed. For all three experiments, electrophysiological and behavioral data on RT and accuracy were collected, and event-related potentials (ERPs), which included N1, N2, P3b, ERN, and CRN, were measured. 2.2. Participants A total of 192 individuals were recruited through Internet and local community advertisements; each of the three experiments included 64 participants (32 younger adults and 32 older adults). In Experiment 1 (with a neutral-white target), the 32 young adults (12 females) had a mean age of 21.66 ± 2.04 years (range 18–26 years) and an average of 15.63 ± 1.29 years of education; the 32 elderly adults (11 females) had a mean age of 67.72 ± 5.74 years (range 60–82 years) and an average of 12.53 ± 4.30 years of education. In Experiment 2 (with a color-coded target yet only PRO trial manipulation), the 32 young adults (17 females) had a mean age of 21.59 ± 1.31 years (range 20–24 years) and an average of 15.53 ± 1.34 years of education; the 32 elderly adults (15 females) had a mean age of 65.84 ± 4.42 years (range 60–78 years) and an average of 11.88 ± 2.57 years of education. In Experiment 3 (with a color-coded target and PRO-ANTI trial manipulation), the 32 young adults (18 females) had a mean age of 21.09 ± 1.30 years (range 19–24 years) and an average of 14.97 ± 1 years of education; the 32 elderly adults (16 females) had a mean age of 64.09 ± 2.98 years (range 60–70 years) and an average of 13.22 ± 2.59 years of education. The 2-way analysis of variance (ANOVA) on years of age with two betweensubjects factors of experiment and age group showed significant main effects of experiment, F(2, 186) = 6.12, p < .005, and of age group, F(1, 186) = 8249.66, p < .001. There was also a significant interaction between experiment and age group, F(2, 186) = 3.30, p < .05. Follow-up simple effect tests showed that only the elderly group yielded a significant main effect of experiment, F(2, 186) = 9.15, p < .001, with the elderly group in Experiment 1 being older (67.72 ± 5.74) than those in Experiment 3 (64.09 ± 2.98). In addition, the 2-way ANOVA on years of education with two between-subjects factors of experiment and age group showed a significant main effect of age group, F(1, 186) = 89.33, p < .000, and a significant interaction between experiment and age group, F(2, 186) = 3.85, p < .05. Follow-up simple effect tests showed that only the elderly group showed a significant main effect of experiment, F(2, 186) = 3.53, p < .05, with the elderly group in Experiment 3 had longer educational years (13.22 ± 2.59) than those in Experiment 2 (11.88 ± 2.57). Given the fact that the elderly groups across the three experiments were not perfectly equated in terms of their age and years of education, we sub-sampled 28 participants for each age group and experiment. A series of the ANOVAs on the flanker-task performance

4 A between-subjects design for the three experiments was adopted due to time constraints. Requiring individuals to participate in all of the three experiments would have been too tiring for the older participants because each experiment required from 1 to 2 h to complete in addition to the 2-h preparation time required for the electrophysiological recording.

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data for these sub-samples yielded the same patterns as those for the original 32 participants for each age group and experiment reported in this manuscript. All participants provided their written informed consent, and the study protocol was approved by the Institutional Review Board (IRB) of the National Cheng Kung University Hospital, Taiwan. All participants were paid NT $500–1000 (US $15–30) for approximately 3–4 h of participation. All participants were right-handed, free of neurological and psychological disorders, and had normal or corrected-to-normal vision. The Mini-Mental State Examination (MMSE; Folstein, Folstein, & McHugh, 1975) screened all participants for dementia based on the following screening criteria: 25–30 points = normal; 21–24 points = mild dementia; 14–20 points = moderate dementia; and ≤13 points = severe dementia. In Experiment 1, the mean MMSE score was 29.38 ± 0.91 for the younger adults and 27.78 ± 1.34 for the older adults. In Experiment 2, the mean MMSE score was 29.25 ± 1.14 for the younger adults and 27.41 ± 1.58 for the older adults. In Experiment 3, the mean MMSE score was 29.44 ± 0.76 for the younger adults and 28.28 ± 1.49 for the older adults. The ANOVA on MMSE scores with two betweensubjects factors of experiment and age group (young, old) showed no significant main effects of experiment and of age group (all ps > .5). In addition, there was no significant interaction effect of experiment and age group. 2.3. Apparatus and stimuli Stimuli were presented on a 17-in. monitor (1024 × 768 resolution). The stimuli were generated using E-Prime 2.0 software (Psychology Software Tools, Inc., Pittsburgh, PA) operating on an IBM-PC computer with a Pentium-4 3 GHz processor. Participants were seated in a comfortable chair facing a computer screen at a distance of 89 cm and pressed the left “Z” key or the right “/” key on the keyboard to indicate their responses. All of the stimuli were on a dark background, which consisted of a 3 × 3 array of arrows or a combination of an arrow and rectangles; the stimulus array subtended 0.86◦ × 1.03◦ of visual angle. The target stimulus—an arrowhead pointing to the left or right—was displayed in the center of the screen (see Fig. 1). In Experiment 1, only PRO conditions, in which participants were instructed to respond to the target arrow by indicating the direction of the target arrow with their left or right index finger, and only targets with a neutral color (e.g., the same as flankers in white) were used (see Fig. 1a). The central target was surrounded by white flanker arrows of the same size, and the side-to-side distance between the target and flanker arrows was 0.1◦ . Congruent-flanker arrows pointed in the same direction as the target arrow; incongruent-flanker arrows pointed in the opposite direction from the target; and neutral flankers were rectangles rather than arrows. In Experiment 2, likewise only PRO conditions were employed, yet the color of the central arrow was introduced, being either green (R:0, G:127, B:0) or red (R:127, G:0, B:0). Participants were instructed to respond to the target arrow regardless of its color by indicating the direction of the target arrow with their left or right index finger (see Fig. 1b). In Experiment 3, the color (red or green) of the central arrow now signals the PRO or ANTI condition respectively. For example, when participants in the PRO trial condition, they were instructed to respond to the green (or red in a counterbalanced condition) target arrow appeared on the screen as quickly and accurately as possible by indicating the direction of the target arrow with their left or right index finger. In contrast, during the ANTI trial condition, they responded to a red (or green in a counterbalanced condition) target arrow by indicating the opposite direction (see Fig. 1c). 2.4. Design and procedure In each experiment, participants performed the flanker task individually in a dimly lit, soundproof test room, and were continuously observed via a video monitor connected to an infrared charge-coupled device camera. At the beginning of each trial, a fixation cross was presented at the center of the screen for 500 ms followed by the presentation of an array of flanker stimuli; after 100 ms, the target stimulus was also presented and displayed for 50 ms; finally, both the target and flanker stimuli were removed. Participants were instructed to respond immediately after the appearance of the central target arrow. The onset of the next trial was 2000 ms after the participant’s response to the target letter; during the inter-trial interval, a blank screen was presented for 1000 ms, followed by presentation of the symbol “#” in the center of the screen for 1000 ms. If the participant did not respond to the presentation of the target stimulus, there would be a waiting time of 3000 ms followed by the blank screen of 1000 ms and the symbol “#” of 1000 ms, hence the inter-trial interval ranged between the shortest response time plus 2000 ms and the longest time of 5000 ms (see Fig. 1d). In each experiment, participants completed 10 blocks of trials, with 120 trials in each block. In each block of trials, the congruent, neutral, and incongruent-flanker arrays were presented in random order with an equal probability of occurrence. In both Experiments 1 and 2, only PRO trial conditions were required, in which participants were instructed to respond to the target arrow, regardless of its color, by indicating the direction of the target arrow with their left or right index finger. Nevertheless, while the color of target arrow was the same as flankers being white in Experiment 1, the target arrow could be either red or green color differing from flankers’ color (white) in Experiment 2. In Experiment 3, the proportions of ANTI and PRO trials in each block were 50% and 50%, respectively. The occurrence of ANTI and PRO trials with red or green target stimuli was also randomized with equal

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Fig. 1. (a) Experimental setup for Experiment 1. The central target was presented in white, and only the manipulation of three types of target–flanker compatibility (congruent, neutral, incongruent) was employed, while keeping the target and response always compatible (all PRO condition). (b) Experimental setup for Experiment 2. The central target was colored green or red, yet only the manipulation of three types of target–flanker compatibility (congruent, neutral, incongruent) was employed while keeping the target and response always compatible (all PRO condition regardless of target’s color). (c) Experimental setup for Experiment 3. The visual stimulus consists of a central target arrow surrounded by task-irrelevant flanker arrows. The flankers are associated with the same response as the target (congruent trials), the opposite response (incongruent trials), or neither (neutral trials). The central target was colored green or red. Green target stimuli signaled a movement of the hand corresponding to the target arrow (PRO conditions), and red target stimuli signaled a hand response in opposition to the target arrow (ANTI conditions). (d) Schematic illustration of a typical trial sequence.

probability within a block, and the color associated with the ANTI and PRO trial conditions was counterbalanced across participants. Between trial blocks, participants received 1 min or longer to close their eyes and relax. Prior to the experimental trials, the participants completed a practice session of two 48-trial blocks to become familiar with the task procedures. Participants were instructed to respond to the targets quickly and accurately. During the practice session, participants received onscreen feedback regarding their performance at the end of each trial. 2.5. Electrophysiological recording In each experiment, electroencephalographic (EEG) activity was continuously recorded using a Neuroscan Q-cap AgCl-32 electrode cap at 32 scalp locations (Neuroscan Inc., El Paso, TX, USA). The horizontal and vertical electrooculograms (hEOGs and vEOGs) were measured by electrodes placed at the outer canthi of both eyes as well as above and below the left eye. The EEGs from all electrode sites were initially referenced to the left mastoid and then re-referenced offline to the mean activity of the left and right mastoids. Electrode impedances were maintained below 5 k. The EEG and EOG were amplified using Synamps1 amplifiers and Scan 4.3 acquisition software (Neuroscan, Inc.) at a bandpass of 0.05–50 Hz and digitized at 500 Hz. The

EEG and EOG epochs permitted the offline removal of artifacts and excessive EOG amplitude. 2.6. Event-related potential (ERP) analysis In each experiment, stimulus-locked epochs were taken from the continuous EEG signal and time-locked to the onset of the flanker stimuli from 200 ms before to 822 ms5 after the target stimulus onset for all recording channels. All stimuluslocked epochs, except for those derived from the O1 and O2 sites, were baseline corrected by obtaining the mean level of activity in the period from 200 to 100 ms prior to the target onset (from 100 to 0 ms preceding the flanker stimuli onset) for each channel and then subtracting that average from the level of activity at each channel sample point. The epochs derived from O1 and O2 sites were baseline corrected by subtracting the average from 100 to 0 ms prior to the target onset. Response-locked epochs for each experiment were taken from the continuous EEG signal and time-locked to the response onset from 200 ms before to 822 ms after the

5

The total interval of 1022 ms could yield 512 data points.

S. Hsieh, Y.-C. Lin / Biological Psychology 103 (2014) 69–82 response onset for all recording channels. All response-locked epochs were baseline corrected by taking the average from 200 to 100 ms prior to the response onset for each channel and subtracting that average from each channel sample point. An algorithm provided by the Neuroscan Scan 4.3 analysis software rejected any epoch that exhibited a signal below −50 ␮V or higher than 50 ␮V, an EEG drift from the baseline greater than 50 ␮V, or in which the A/D converters became saturated. If residual horizontal or vertical eye movement was present, the epochs were visually inspected, and epochs that were contaminated with EOG activity were eliminated manually. On average, the total rejection rate for all kinds of artifacts (including drifting, EMG (electromyogram), EOG, skin potential) across different experiments was 14.64% ± 8.31% for younger adults and 13.23% ± 11.81% for older adults and was approximately equally distributed across conditions. 2.7. Target stimulus-locked ERPs: N1, N2 and P3b The N1 peak amplitudes were defined as the negative peak values in a time window of 50–200 ms after the onset of the target stimulus at the O1 and O2 electrode sites. The N2 component was maximal at the Fz site on incongruent trials for younger adults. The mean amplitude of N2 was measured 50 ms before and after the N2 component peak (i.e., a fixed time-window of 275–375 ms after the onset of the target stimulus for younger adults in all three Experiments; a fixed time-window of 350–450 ms for older adults in Experiments 1 and 2, and of 275–375 ms for older adults in Experiment 3).6 The P3b component was most pronounced at the Pz site. The peak latency of P3b was defined as the time value where the most positive peak occurred in a time-window of 300–600 ms after the onset of the target stimulus at the Pz site.7 2.8. Response-locked ERPs: ERN and CRN The ERN and CRN amplitudes were defined as the difference between the most negative value in a time-window of 0–150 ms following the onset of the incorrect or correct response, respectively, and the mean value of the preceding positivity (peak-to-peak amplitude) at the FCz site in the response-locked ERP.

3. Results

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Table 1 Mean accuracy (% ± SD) and reaction time (ms ± SD) separately for congruent, neutral, and incongruent trials for young and old groups in three experiments. Young Experiment 1 Performance accuracy (ACC) Congruent Neutral Incongruent Reaction time (RT) Congruent Neutral Incongruent Experiment 2 Performance accuracy (ACC) Congruent Neutral Incongruent Reaction time (RT) Congruent Neutral Incongruent Experiment 3 Performance accuracy (ACC) Congruent Neutral Incongruent Reaction time (RT) Congruent Neutral Incongruent

Old

98.98 ± 1.30 98.10 ± 1.55 83.30 ± 13.54

97.41 ± 2.63 96.80 ± 4.22 88.93 ± 10.89

337.33 ± 31.15 371.54 ± 26.75 414.31 ± 27.01

462.08 ± 60.49 488.27 ± 55.32 554.47 ± 58.61

98.53 ± 1.64 96.68 ± 2.42 81.41 ± 9.75

95.66 ± 3.98 93.17 ± 6.47 86.74 ± 10.85

342.24 ± 31.33 381.08 ± 31.44 420.26 ± 33.73

490.79 ± 78.43 525.85 ± 77.98 567.43 ± 68.32

92.45 ± 5.89 91.84 ± 6.59 90.63 ± 7.19

93.69 ± 4.86 92.67 ± 5.13 93.13 ± 5.00

501.50 ± 61.74 512.40 ± 60.83 521.59 ± 62.63

647.79 ± 122.00 657.64 ± 116.11 673.45 ± 119.85

3.1. Behavioral data Trials with reaction times (RTs) that were less than 100 ms or greater than 3000 ms were rejected. Because data screening tests indicated that the data did not violate the assumption of normality, the RT and response accuracy data across all trials and the three experiments were analyzed in the 3-way ANOVAs with two between-subjects factors of experiment (Experiments 1, 2, 3) and age group (young, old), and one within-subjects factor of flanker type (congruent, neutral, incongruent). Table 1 presents the behavioral data and Table 2 presents a summary of the ANOVA results on RTs and performance accuracy (ACC). 3.1.1. Reaction time (RT) The 3-way ANOVA on RT (Table 1) revealed significant main effects of experiment, age group, and flanker type. Tukey’s post hoc tests showed that the mean RT was significantly shorter in both the neutral-color-target experiment (Experiment 1: 437.99 ± 85.60 ms) and the colored-target experiment (Experiment 2: 454.61 ± 97.90 ms) than in the PRO-ANTI experiment (Experiment 3: 585.73 ± 119.56 ms), p < .01; that younger adults (422.47 ± 80.73 ms) responded more quickly than older adults

6 Generally speaking, due to different task demands across the three experiments, the onset of some ERP components, especially the N2, varied across the three experiments, hence, we measured the components from different time windows. In addition, the reason why we retrieved the N2 from a later time-window of 350–450 ms for older adults specifically in Experiments 1 and 2 but not in Experiment 3 was because the older adults compared to younger adults showed a delayed onset of N2 in Experiments 1 and 2, yet showed no presence of N2 in Experiment 3. Hence, the time-window of N2 for older adults in Experiment 3 was accommodated to that for younger adults. 7 The discrepant time-windows for the N1, N2, and P3 between the current study (N1: 50–200 ms; N2: 275–375 ms; P3: 300–600 ms) and our previous study (N1: 50–150 ms; N2: 250–350 ms; P3: 300–500 ms in Hsieh and Fang’s (2012) study) was possibly due to the exclusion of ANTI trials in the current study and different populations between the two studies.

(563.09 ± 114.60 ms). Tukey’s post hoc tests comparing the effects of the flanker types found a conventional flanker effect, with congruent-flanker trials (463.62 ± 126.61 ms) exhibiting faster RTs than neutral-flanker trials (489.47 ± 117.51 ms; p < .001), and neutral-flanker trials exhibiting faster RTs than incongruentflanker trials (525.25 ± 112.03 ms; p < .001), p < .01. There were significant 2-way interactions of experiment and flanker-type, and of age group and flanker type. There was also a significant 3-way interaction between experiment, age group, and flanker-type (see Table 2). Only post hoc analyses that involved significant interactions between age group and flanker type factors are reported here. The significant 2-way interaction of age group and flanker type indicated that age influenced the effect of flankers on RT. For this 2-way interaction, simple effect tests indicated that the flanker effect was significant for both younger, F(2, 372) = 531.05, p < .0001 (incongruent–congruent = 58.36 ± 31.80 ms), and older

Table 2 Summary table for the 4-way analyses of variance conduced on accuracy (as percentage of correct responses) and mean reaction time (RT, ms) with Experiment (Experiments 1, 2, 3) and age group (young vs. old) as between-subject factors, and Flanker type (congruent-flanker, neutral-flanker, incongruent-flanker) as withinsubject factors. Effect

df

Experiment Group E*G

2, 186 1, 186 2, 186

2.29 0.84 0.52

Flanker type F*E F*G F*E*G

2, 372 4, 372 2, 372 4, 372

166.84** 33.85** 20.00** 2.89*

* **

p < .05. p < .01.

ACC F-value

RT F-value 89.09** 201.73** 0.46 1193.90** 123.78** 9.25** 3.59**

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adults, F(2, 372) = 672.10, p < .0001 (incongruent–congruent = 64.90 ± 35.55 ms). The significant 3-way interaction between experiment, age group, and flanker type was examined to determine whether the flanker effect exhibited by the two age groups was modulated by experiments with different manipulation of target conditions (PRO-ANTI, color-coded target, neutral-white target experiments). Follow-up simple interaction tests indicated that there was a significant interaction between age group and flanker type only in Experiment 1, F(2, 372) = 14.73, p < .0001, but not in Experiment 2, F < 1 and Experiment 3, F(2, 372) = 1.32, p = .28, suggesting that there was an increased age-related RT flanker effect (incongruent–congruent) only in Experiment 1 with a conventional neutral-white target (young: 76.98 ± 18.66 ms vs. old: 92.39 ± 23.63 ms, t(63) = −2.89, p < .005).

3.1.2. Response accuracy (ACC) The 3-way ANOVA on response accuracy (ACC) (Table 1) revealed a significant main effect of flanker type. Tukey’s post hoc tests comparing the effects of the flanker types found a conventional flanker effect, with congruent-flanker trials (96.12 ± 4.44%) exhibiting higher accuracy than neutral-flanker trials (94.88 ± 5.29%; p < .001), and neutral-flanker trials exhibiting higher accuracy than incongruent-flanker trials (87.35 ± 10.58%; p < .001), p < .01. Similar to the RT data, there were significant 2way interactions of experiment and flanker-type, and of age group and flanker-type. There was also a significant 3-way interaction between experiment, age group, and flanker-type on mean accuracy (see Table 2). Only post hoc analyses that involved significant interactions between age group and flanker factors are reported here. The significant 2-way interaction of age group and flanker type indicated that age influenced the effect of flankers on ACC. For this 2-way interaction, simple effect tests indicated that the flanker effect was significant for both younger, F(2, 372) = 150.35, p < .0001 (incongruent–congruent = −11.55 ± 12.05%), and older adults, F(2, 372) = 36.49, p < .0001 (incongruent–congruent = −5.99 ± 8.76%). The significant 3-way interaction between experiment, age group, and flanker type was examined to determine whether the flanker effect exhibited by the two age groups was modulated by the target conditions (neutral-color target, color-coded target, PROANTI experiments). Follow-up simple interaction tests indicated that there was a significant interaction between age group and flanker type in Experiment 1, F(2, 372) = 10.29, p < .0001 (flanker effect: young: −15.69 ± 13.71% vs. old: −8.48 ± 9.15%, t(63) = −2.47, p < .01), and Experiment 2, F(2, 372) = 15.02, p < .0001 (flanker effect: young: −17.13 ± 9.65% vs. old: −8.92 ± 9.22%, t(63) = −3.48, p < .001), but not in Experiment 3, F < 1; however, such an age effect on the accuracy flanker effect appeared to be in the opposite direction to the RT flanker effects in which it was now the older adults who exhibited a decreased flanker effect on ACC in both Experiments 1 and 2. The decreased flanker effect on ACC for older adults was due to the increased accuracy for the incongruent-flanker trials but decreased accuracy for both the neutral and congruent-flanker trials for older adults compared to younger adults in both Experiments 1 and 2. Hence, there appeared to be speed-accuracy tradeoff for the older adults in both Experiments 1 and 2.

8 A conventional way of dealing with the confounding of the speed-accuracy tradeoff is to equate RTs rather than ACCs. Yet, in the current study, since our main research interest was to observe the changes in RTs (especially the RT flanker effect) for older adults as compared to younger adults, we decided to control the performance accuracy between the two age groups, and then observe if there was still an age-related RT flanker effect in both Experiments 1 and 2.

3.1.3. RT data of the 16 participants from each age group with equated ACC for Experiments 1 and 28 In order to preclude the possible confounding of the speedaccuracy tradeoff on the observed RT flanker effect patterns, we sampled 16 participants from each age group for Experiments 2 and 3 respectively. Two 2-way ANOVAs on RTs with factors of age and flanker type for Experiments 1 and 2 were performed respectively. The results of the ANOVA on RTs for Experiment 1 showed a significant main effect of age, F(1, 30) = 38.61, p < .0001, indicating that younger adults (386.79 ± 38.80 ms) responded more quickly than older adults (483.63 ± 66.75 ms), and of flanker type, F(2, 60) = 893.31, p < .0001, indicating that congruent-flanker trials (399.18 ± 63.88 ms) exhibiting faster RTs than neutral-flanker trials (427.25 ± 61.01 ms; p < .001), and neutral-flanker trials exhibiting faster RTs than incongruent-flanker trials (479.22 ± 69.81 ms; p < .001), p < .01. There was also a 2-way interaction of age and flanker type, F(2, 60) = 18.06, p < .0001, suggesting that older adults exhibited larger flanker effect than younger adults (flanker effect: young: 70.89 ± 13.45 ms; old: 89.20 ± 14.32; F(1, 30) = 13.03, p < .001). The results of the ANOVA on RTs for Experiment 2 showed a significant main effect of age, F(1, 30) = 74.19, p < .0001, indicating that younger adults (389.56 ± 39.91 ms) responded more quickly than older adults (520.74 ± 61.62 ms), and of flanker type, F(2, 60) = 413.90, p < .0001, indicating that congruent-flanker trials (419.34 ± 78.83 ms) exhibiting faster RTs than neutral-flanker trials (453.82 ± 76.19 ms; p < .001), and neutral-flanker trials exhibiting faster RTs than incongruent-flanker trials (492.29 ± 79.43 ms; p < .001), p < .01. There was no interaction between age and flanker type, F(2, 60) = 1.20, p = 0.31, suggesting that both younger and older adults exhibited near equivalent effect of flanker type. To summarize, further statistical results on these 16 subsamples for each age group for Experiments 1 and 2 showed almost identical RT patterns as those on the original 32 participants, hence, the speed-accuracy tradeoff did not change the conclusion regarding the older adults’ RT flanker effect in Experiments 1 and 2. Therefore, the overall results of behavioral data showed that older adults exhibited an increased RT flanker effect in Experiment 1, regardless of whether the older adults’ performance accuracy were superior than or equal to the younger adults, and a near equivalent RT flanker effect as younger adults in both Experiments 2 and 3. 3.2. Electrophysiological data 3.2.1. Stimulus-locked ERPs: N1, N2, and P3b 3.2.1.1. N1 Peak amplitude. The 4-way ANOVA on the N1 peak amplitude at O1 and O2 (see Fig. 2) revealed significant main effects of electrode, F(1, 186) = 20.04, p < .0001, and flanker type, F(2, 372) = 39.22, p < .0001. Tukey’s post hoc analyses indicated that the N1 peak amplitudes for both congruentflanker trials (−1.34 ± 1.39 ␮V) and incongruent-flanker trials (−1.29 ± 1.41 ␮V) were larger than those of the neutral-flanker trials (−0.90 ± 1.22 ␮V)(all ps < .01). There were significant 2-way interactions of experiment and age group, of electrode and experiment, of electrode and flanker type, and of age group and flanker type. Of the main interest, simple effect tests on the interaction of experiment and age group showed that the N1 amplitude was significantly larger for the younger than older adults in Experiment 1 (young: −1.65 ± 1.55 ␮V; old: −0.62 ± 1.05 ␮V; F(1, 186) = 16.90, p < .001), and no significant difference between the two groups in Experiment 2 (young: −1.18 ± 1.29 ␮V; old: −0.85 ± 1.28 ␮V; F(1, 186) = 1.77, p = .19), but on the contrary was significantly larger for the older than younger adults in Experiment 3 (young: −1.11 ± 1.20 ␮V; old: −1.65 ± 1.40 ␮V; F(1, 186) = 4.61, p < .05). Furthermore, the N1 amplitude was significantly larger in Experiment 3 than in both


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Fig. 2. Stimulus-locked event-related potentials (ERPs) recorded from electrode sites of O1 and O2 were plotted to depict N1 component for the younger (thick lines) and older (thin lines) participants for congruent (solid), neutral (dashed), and incongruent (dotted) flanker types of correct trials in three experiments (upper panel: Experiment 1; middle panel: Experiment 2; lower panel: Experiment 3).

Experiments 1 and 2, F(2, 186) = 9.20, p < .001, specifically only for the older adults but not for the younger adults. In summary, the N1 amplitude was smaller for the older adults than younger adults in Experiment 1 (neutral-white target) where older adults exhibited an increased behavioral flanker effect on RT. On the other hand, the N1 amplitude was larger for the older adults than younger adults in Experiment 3 (PRO-ANTI experiment) where older adults did not exhibit increased RT flanker effect (see Fig. 3). The patterns appeared to suggest that N1 amplitude might reflect a compensatory response for the elderly in conquer with the flanker interference. However, the augmentation of N1 amplitude might not be a prerequisite for observing equivalent RT flanker effect between older and younger adults, since in Experiment 2, although there was likewise no age-related increased RT flanker effect observed for the elderly, the N1 amplitude was not increased for the elderly compared to younger adults. It is likely that in some flanker-task scenarios, such as in Experiment 2 with a

Fig. 3. Grand mean N1 peak amplitudes collapsed over the three flanker types (congruent, neutral, incongruent) and the two electrode sites of O1 and O2 for the younger (filled bar) and older (open bar) participants in three experiments respectively. Error bars represent standard errors.

color singleton target, the older adults did not need to initiate the compensatory response that function to enhance the processing of the central target as reflected on N1 amplitude while performing a flanker task. 3.2.1.2. Frontal N2 mean amplitude. The three 2-way ANOVAs on frontal N2 amplitude for each experiment (Fig. 4) revealed that in all three experiments, there were significant main effects of age group (Experiment 1: F(1, 62) = 34.10, p < .001; Experiment 2: F(1, 62) = 55.58, p < .001; Experiment 3: F(1, 62) = 12.91, p < .001), of flanker type (Experiment 1: F(2, 124) = 74.66, p < .001; Experiment 2: F(2, 124) = 55.67, p < .001; Experiment 3: F(2, 124) = 3.61, p < .05), and a significant 2-way interaction of age group and flanker type (Experiment 1: F(2, 124) = 4.04, p < .05; Experiment 2: F(2, 124) = 6.67, p < .001; Experiment 3: F(2, 124) = 4.67, p < .05). Simple effect tests following these significant interactions showed that in both Experiments 1 and 2, significant main effect of flanker type occurred for both younger and older adults (Experiment 1: young: F(2, 124) = 56.67, p < .001; old: F(2, 124) = 22.03, p < .001; Experiment 2: young: F(2, 124) = 50.41, p < .001; old: F(2, 124) = 11.93, p < .001), whereas in Experiment 3, the significant main effect of flanker type occurred only for younger adults, F(2, 124) = 8.24, p < .001, but not for older adults, F(2, 124) < 1. In summary, the N2 results suggested that in both Experiments 1 and 2, the older adults exhibited a conventional N2 flanker effect, albeit with a smaller magnitude and a later onset than that for the younger adults (all ps < .05; see Fig. 5), whereas in Experiment 3, the older adults exhibited the absence of the flanker effect on N2. Likewise, since the older adults in Experiment 2 did not show an increased RT flanker effect as those in Experiment 3, yet the N2 was present on incongruent trials as those seen in Experiment 1, the results seemed to suggest that the absence of N2 on incongruent trials for older adults was not a prerequisite for observing a near equivalent RT flanker effect between older and younger adults. 3.2.1.3. P3b peak latency. The three 2-way ANOVAs on P3b peak latency (Fig. 6) for each experiment revealed that in all three experiments, there were significant main effects of age group

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Fig. 4. Stimulus-locked event-related potentials (ERPs) recorded from electrode site Fz were plotted to depict N2 component for the younger (thick lines) and older (thin lines) participants for congruent (solid), neutral (dashed), and incongruent (dotted) flanker types of correct trials in three experiments (upper panel: Experiment 1; middle panel: Experiment 2; lower panel: Experiment 3).

(Experiment 1: F(1, 62) = 202.61, p < .001; Experiment 2: F(1, 62) = 176.89, p < .001; Experiment 3: F(1, 62) = 25.96, p < .001) showing a larger P3b peak latency overall for older adults compared to younger adults, of flanker type (Experiment 1: F(2, 124) = 363.44, p < .001; Experiment 2: F(2, 124) = 278, p < .001; Experiment 3: F(2, 124) = 8.16, p < .001) showing shorter P3b peak latency for congruent-flanker trials than neutral-flanker trials, and further than incongruent-flanker trials (p < .01), and a significant 2-way interaction of age group and flanker type (Experiment 1: F(2, 124) = 9.98, p < .001; Experiment 2: F(2, 124) = 8.72, p < .001; Experiment 3: F(2, 124) = 3.82, p < .05). In summary, the results of the P3b peak latency suggested that the older adults exhibited an overall (i.e., collapsed over all three types of flanker trials) greater P3b peak latency than the younger adults in all three experiments (Fig. 7). Hence, the results suggested

Fig. 5. The flanker effect (incongruent-flanker–congruent-flanker) on the N2 mean amplitudes for the younger (filled bar) and older (open bar) participants in three experiments respectively. Error bars represent standard errors.

that the increased P3b peak latency for older adults might not reflect compensatory responses for the elderly in performing a flanker task, since the increased P3b peak latency did not guarantee an equivalent RT flanker effect for the elderly compared to younger adults.

3.2.2. Response-locked ERPs: ERN and CRN 3.2.2.1. ERN amplitude (n = 18). Because participants were instructed to respond as quickly and accurately as possible, some participants made no errors in some experimental conditions. For the ERN analyses, participants who responded in error in at least five trials in any condition were identified. Furthermore, given the error trials occurred mainly on incongruent trials, only ERN for incongruent trials was analyzed. Finally, to ensure that the number of participants was the same across the experiments and matched the number of participants meeting the selection criteria, data from 18 out of 32 participants for each age group in each of the experiments were retrieved and analyzed (see Fig. 8). The 2-way ANOVA on ERN amplitudes for incongruent trials only based on the data from this subset of participants revealed significant main effects of experiment, F(2, 102) = 6.88, p < .005, age group, F(1, 102) = 66.32, p < .0001, and an interaction of experiment and age group, F(2, 102) = 12.64, p < .0001. Tukey’s post hoc tests indicated that the ERN for Experiment 1 (−6.81 ± 3.40 ␮V) and Experiment 2 (−7.35 ± 5.17 ␮V) was larger than Experiment 3 (−5.03 ± 1.79 ␮V), p < .01, p < .01. The ERN was greater in the younger adults (−8.57 ± 4.02 ␮V) than in the older adults (−4.22 ± 2.00 ␮V). Further simple effect tests on the 2-way interaction of experiment and age group showed that in Experiment 3, there was no significant difference on the ERN amplitude for incongruent trials between the two age groups, F(1, 102) = 1.79, p = .19, whereas in Experiments 1 and 2, there was a significant main effect of age group on ERN amplitude for incongruent trials (Experiment 1: F(1, 102) = 18.87, p < .0001; Experiment 2: F(1, 102) = 70.94, p < .0001), suggesting that younger adults exhibited a larger ERN than older adults in Experiments 1 and 2.


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Fig. 6. Stimulus-locked event-related potentials (ERPs) recorded from electrode site Pz were plotted to depict P3b component for the young (thick lines) and older (thin lines) participants for congruent (solid), neutral (dashed), and incongruent (dotted) flanker types of correct trials in three experiments (upper panel: Experiment 1; middle panel: Experiment 2; lower panel: Experiment 3).

In summary, there was larger ERN amplitude for younger adults than older adults in Experiments 1 and 2, whereas the ERN amplitude for incongruent trials was similar between the two age groups in Experiment 3 (Fig. 9). Since the older adults in Experiment 2 did not exhibit an increased RT flanker effect as in Experiment 3, yet showed a decreased ERN compared to younger adults, the results seemed to suggest that the near equivalent ERN amplitude for the older adults compared to the younger adults was not a prerequisite for observing a near equivalent RT flanker effect between the two age groups. 3.2.2.2. CRN amplitude. The 3-way ANOVA analysis on CRN amplitudes (Fig. 10) revealed significant main effects of age group,

Fig. 7. Grand mean P3b peak latencies collapsed over three flanker types (congruent, neutral, incongruent) for the younger (filled bar) and older (open bar) participants in three experiments respectively. Error bars represent standard errors.

and flanker type. The CRN was greater in the older adults (−3.82 ± 2.39 ␮V) than in the younger adults (−2.12 ± 1.34 ␮V). Tukey’s post hoc tests indicated that the CRN amplitude was significantly smaller for congruent-flanker trials (−2.47 ± 2.06 ␮V) than neutral-flanker trials (−2.89 ± 2.12 ␮V) which was further smaller than incongruent-flanker trials (−3.54 ± 2.04 ␮V; p < .01), p < .01. There were significant 2-way interactions of experiment and age group, of experiment and flanker type, and of age group and flanker type. Simple effect tests on the significant experiment and age group indicated that for all three experiments, there was a significant main effect of age (Experiment 1: F(1, 186) = 28.62, p < .0001; Experiment 2: F(1, 186) = 30.08, p < .0001; Experiment 3: F(1, 186) = 4.00, p < .05) in which older adults exhibited larger CRN amplitude than younger adults. Simple effect tests on the significant 2-way interaction of age group and flanker type indicated that there was a significant main effect of flanker type for the younger adults, F(2, 372) = 36.58, p < .0001, and for the older adults, F(2, 372) = 11.97, p < .0001. There was also a significant 3-way interaction between experiment, age group, and flanker type. Simple interaction effects showed that in all three experiments, there was a significant interaction between age group and flanker type (Experiment 1, F(2, 372) = 3.05, p < .05; Experiment 2, F(2, 372) = 5.06, p < .01; Experiment 3: F(2, 372) = 7.52, p < .0001). Further simple effect tests on the interaction separately for the three experiments indicated that in both Experiments 1 and 2, both age groups exhibited a conventional flanker effect on CRN (all ps < .001), whereas in Experiment 3, only younger adults but not older adults exhibited a conventional flanker effect on CRN (young: F(2, 372) = 13.63, p < .001; old: F < 1). In summary, the older adults exhibited overall (i.e., collapsed over different types of flanker trials) greater CRN amplitude than

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Fig. 8. Response-locked event-related potentials (ERPs) recorded from electrode site FCz were plotted to depict ERN component for the younger (thick lines) and older (thin lines) participants for congruent (solid), neutral (dashed), and incongruent (dotted) flanker types of error trials in three experiments (upper panel: Experiment 1; middle panel: Experiment 2; lower panel: Experiment 3).

the younger adults in all three experiments (see Fig. 11). Hence, these results seemed to suggest that the increased CRN amplitude for older adults might not reflect compensatory responses for the elderly in performing a flanker task since the increased CRN did not guarantee an equivalent RT flanker effect for the elderly compared to younger adults.

Fig. 9. Grand mean ERN peak-to-peak amplitudes of the incongruent-flanker trials for the younger (filled bar) and older (open bar) participants in three experiments respectively. Error bars represent standard errors.

4. Discussion This study aimed to establish a baseline condition to observe ERP responses for older adults in a conventional flanker-task paradigm (i.e., Experiment 1 with a regular flanker-task paradigm of this study). Such ERP baseline information is important for one to verify the authenticity of the observed so-called ERP compensatory responses for the elderly, since it is likely that some age-related ERP changes found in the previous study were unrelated to compensatory responses. In addition, the baseline information could also serve the purpose to clarify if the observed equivalent RT flanker effect between the two age groups was necessarily accompanied with ERP compensatory response. It is likely that in some flankertask scenarios, the elderly was as just capable as younger adults in conquer with flanker interference even without involving ERP compensatory responses. More critically, this study aimed to examine whether the previous finding of the equivalent RT flanker effect yet accompanied by ERP compensatory responses for older adults compared to younger adults in performing a flanker task (Hsieh & Fang, 2012) was due to the specific demand of the reversal mapping of response rule to a target (i.e., the additional ANTI trial manipulation in Experiment 3 of this study) or simply due to the pop-out effect with a singleton target manipulation (i.e., the manipulation of a color-coded target in Experiment 2 of this study). The behavioral results of this study showed that while in Experiment 1 with a regular flanker task paradigm, there was an age-related increased flanker effect on RT, yet there was no


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Fig. 10. Response-locked event-related potentials (ERPs) recorded from electrode site FCz were plotted to depict CRN component for the younger (thick lines) and older (thin lines) participants for congruent (solid), neutral (dashed), and incongruent (dotted) flanker types of correct trials in three experiments (upper panel: Experiment 1; middle panel: Experiment 2; lower panel: Experiment 3).

age-related increased flanker effect on RT in either Experiment 2 with a color-coded target or Experiment 3 with the ANTI trial manipulation. As for the performance accuracy, the current results showed that there was an age-related flanker effect on accuracy, yet such an age effect on the accuracy flanker effect appeared to be in the opposite direction to the RT flanker effects in which it was now the older adults who exhibited a decreased flanker effect on accuracy in both Experiments 1 and 2. Such a decreased flanker effect on accuracy for older adults was due to the increased accuracy for the incongruent-flanker trials but decreased accuracy for both the neutral and congruent-flanker trials for older adults compared to younger adults in both Experiments 1 and 2. Converging

Fig. 11. Grand mean CRN peak-to-peak amplitudes for the younger (filled bar) and older (open bar) participants in three experiments respectively, collapsed over the three flanker types (congruent, neutral, incongruent). Error bars represent standard errors.

both RT and accuracy results, it seemed to suggest that there was a speed-accuracy tradeoff in Experiments 1 and 2. To preclude the possible confounding of speed-accuracy tradeoff while comparing RT flanker performance between the older and younger adults in Experiments 1 and 2, we further sampled 16 participants from each age group of these two experiments whose performance accuracy were equated, and then re-examined if the two age groups differed in terms of their behavioral RT flanker effect. The results of these sub-samples turned out to show the same RT patterns as the original samples in that there was no agerelated increased RT flanker effect for the elderly in Experiment 2, yet remained an age-related increased RT flanker effect in Experiment 1. The results suggested that the speed-accuracy tradeoff did not change the original RT patterns of the flanker effect for the elderly. Of the main interest, the ERP results of this study showed that while the current Experiment 3 (with ANTI manipulation) likewise exhibited similar age-related ERP changes associated with equal-to-young adults RT flanker effect as previously reported by Hsieh and Fang (2012), Experiment 1 (with a regular flanker task) also exhibited some age-related ERP changes yet associated with an increased age-related flanker effect. Hence, it appears that not all of the previously though-to-be age-related ERP compensatory responses really reflected compensatory responses. As aforementioned, the results of the current Experiment 1 showed an age-related increased RT flanker effect even after the correction of speed-accuracy tradeoff phenomenon, yet there were still some ERP responses by older adults in Experiment 1 similar to those seen in Experiment 3. For example, there was seen age-related increased P3b peak latency as well as increased CRN amplitude for all three experiments. Therefore, these two ERP responses appeared to be less likely to really reflect compensatory responses for the older adults, since these two ERP changes were independent of whether there was age-related increased RT flanker effect (increased for Experiment 1; no increased for Experiments 2 and 3).

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Nevertheless, another possibility is that the older adults in Experiment 1 although initiated compensatory responses as reflected on ERPs, these compensatory responses still failed to conquer the flanker interference. We think these two hypotheses were equally likely and reasoned that the prolonged P3b peak latency might not reflect compensatory but a general slowing phenomenon, whereas the increased CRN amplitude reflect compensatory response yet contribute mainly to performance accuracy rather than RT performance. Previous research has indicated that the prolongation of P3b peak latency might reflect general performance slowing by the elderly (Polich, 1996, 2007), hence the current observed prolonged P3b peak latency is likely to reflect the behavioral outcome of the slower responses by the elderly in all three experiments. On the other hand, the current findings that older adults exhibited overall larger CRN amplitude might suggest that older adults strategically adopted a more conservative response criterion that emphasized accuracy. The previous literature supports this view by demonstrating that adopting a more controlled response strategy enhanced the CRN (Schreiber, Pietschmann, Kathmann, & Endrass, 2011). These results are also consistent with the behavioral data in the current study: older adults’ lower error rates and longer response times revealed that they responded more cautiously than younger adults. Yet, such an enhanced response criterion by the elderly did not guarantee the success of conquering flanker interference as reflected on the RT data, especially in Experiment 1. After precluding the P3b and CRN, there remained age-related N1, N2, and ERN changes that might be related to compensatory responses. Yet, among them, the N1 appears to be the most discriminatory factor that reflects compensatory response. The observing of an increased N1 associated with a decreased RT flanker effect for the older adults than younger adults in Experiment 3 with the ANTI trial manipulation, whereas a decreased N1 associated with an increased RT flanker effect for the older adults than younger adults in Experiment 1 with a regular flanker task, suggested that the N1 augmentation for the older adults reflected a compensatory response, in which the older adults engaged in increased top–down visual processing of the central target (hence increasing in attention and accuracy; Haider, Spong, & Lindsley, 1964; Natale, Marzi, Girelli, Pavone, & Pollmann, 2006) compared to the younger adults. The findings were in accordance with those by Wild-Wall et al.’s (2008) and our previous findings (Hsieh & Fang, 2012). As to the age-related N2 changes, the observing of the absence of the N2 on the incongruent trials for the older adults associated with an equal to younger adults’ RT flanker effect in Experiment 3, and a conventional albeit smaller magnitude and later onset of the N2 on the incongruent trials for the older adults associated with an increased RT flanker effect in Experiment 1, suggested that the older adults experienced less conflict on incongruent trials (because N2 amplitude reflects the extent of flanker conflict processes; Bartholow et al., 2005), specifically in Experiment 3, hence resulting in an equal-to younger adults’ RT flanker effect. The lack of a clear N2 in the older adults seems to support the claim that the older adults’ emphasis on performance accuracy led them to intensify central target processing to reduce flanker interference. Yet, there is an alternative interpretation regarding the age-related N2 changes. It is possible that the larger N1 essentially precluded N2 appearance, such as in Experiment 3, whereas the reduced N1 was related to the subsequent N2 when no aid of the color-coded target was given, such as in Experiment 1. Hence, the age-related N2 changes might play a role mere in response to the N1 compensatory response.9

9 We would like to thank one of the reviewers, Ashwini K. Pandey, for providing this alternative interpretation.

Regarding the age-related ERN changes, previous research has reported findings of attenuated ERN in the elderly that suggest that they experience deficits in error monitoring (Band & Kok, 2000; Coles, Scheffers, & Holroyd, 2001; Falkenstein et al., 2001; Gehring & Knight, 2000; Mathewson et al., 2005; Nieuwenhuis et al., 2002), yet the current findings showed that while the older adults exhibited a decreased ERN in both Experiments 1 and 2, the older adults in Experiment 3 exhibited an ERN equal to that of younger adults in Experiment 3. Some researchers have suggested that ERN attenuation in the elderly is due to a diminished ability to detect errors (e.g., Falkenstein et al., 2001), others have suggested that it is due to the uncertainty regarding the correct response (e.g., Band & Kok, 2000). Hence the current result seemed to suggest that in Experiment 3 the ANTI trial manipulation might cause older adults experience the task as more difficult, which in turn induced the older adults to exert more cognitive control. Turning to the critical issue if the previous finding of age-related compensatory responses was specifically due to the manipulation of the ANTI condition per se, or the manipulation of a color-coded target, we compared the ERP results between Experiments 2 and 3. As summarized, the results of the current Experiment 3 replicated our previous findings (Hsieh & Fang, 2012), that is, there was no age-related increased RT flanker effect, yet shown several ERP compensatory responses specifically for older adults. On the other hand, the results of the current Experiment 2 with a color-coded target showed a rather different pattern from those of Experiment 3, that is, now while there was likewise no age-related increased RT flanker effect and an even decreased flanker effect on performance accuracy, there were no ERP compensatory responses, as seen in Experiment 3, for the older adults. Hence, we can conclude that the previous finding of age-related ERP compensatory responses in Hsieh and Fang’s (2012) study was mainly due to the manipulation of the ANTI trial condition rather than simply due to the manipulation of the color-coded target, since only the current Experiment 3 exhibited no deficit in flanker effect yet along with ERP compensatory responses for the elderly. Nevertheless, an additional finding in the current study is that the ERP compensatory responses might be initiated only when the task demand was high, such as in the scenario where a reversal response was required (Experiment 3). Conversely, in the experiment condition where there was no reversal response required yet the central target was color coded (Experiment 2), older adults might still be able to allocate more attention to the central target and adopt a more conservative response criterion specifically for incongruent trials. This was based on the finding that although there was no age-related increased RT flanker effect observed, there were no expected ERP compensatory responses as seen in Experiment 3 for the older adults in Experiment 2. The current data on N1 amplitudes (a decreased N1 in Experiment 1, equivalent N1 in Experiment 2, and an increased N1 in Experiment 3 for older adults compared to younger adults), N2 amplitudes (a decreased and delayed N2 in both Experiments 1 and 2, and the absence of N2 in Experiment 3 for older adults compared to younger adults), and ERN amplitudes (a decreased ERN in both Experiments 1 and 2, and equivalent ERN in Experiment 3 for older adults compared to younger adults) provided evidence in support of this view. Therefore, the current results suggested that in some scenarios of flanker-task paradigms, such as the current Experiment 2, older adults were just as capable as younger adults in conquer with the flanker interference by the aid of color coding for the central target even though no ERP compensatory responses were involved. Before closing, it is worth relating the current ERP findings to some existing age-related compensatory models, such as the models of “posterior-anterior shift in aging (PASA)” (Dennis & Cabeza, 2008) and “hemispheric asymmetry reduction in order


adults (HAROLD)” (Cabeza, 2002).10 The PASA model was proposed to account for the findings of age-related reduction in occipitotemporal activity coupled with age-related increase in frontal activity across a variety of cognitive domains. The HAROLD model was proposed to account for the findings that while young adults typically engage left-lateralized brain activity (e.g., verbal working memory), older adults show bilateral brain activity. At first blush, the current finding of the ERP compensatory response, e.g., an increased N1 at the occipital sites associated with a decreased RT flanker effect for the older adults than younger adults, seems to be contradictory to the PASA model’s prediction. In addition, no hemispheric changes (in terms of changes in the ERP scalp topography) were found for the older adults in the current study, which also seems to be inconsistent with the HAROLD model’s prediction. Yet, both the PASA and HAROLD models were developed based on the results obtained by the functional magnetic resonance imaging (fMRI) technique, whereas the current study used the ERP technique. Therefore, it is difficult to make direct comparisons among these studies, since the fMRI has a high spatial resolution but a low temporal resolution, whereas ERP has a high temporal resolution but a low spatial resolution. Future aging studies combining these different neuroimaging techniques may be promising to obtain a more comprehensive picture of the age-related compensatory responses. 5. Conclusions To conclude, the current study demonstrated that (1) some of the previously thought-to-be compensatory ERP responses did not really reflect compensatory responses; (2) the previous finding of age-related ERP compensatory responses in Hsieh and Fang’s (2012) study was mainly due to the manipulation of the ANTI trial condition rather than simply due to the manipulation of the colorcoded target; and (3) in some scenarios of flanker-task paradigms, such as in the flanker-task paradigm with a color-coded target, older adults were just as capable as younger adults in conquer with the flanker interference even though no compensatory responses reflected on ERPs were found to involve. One important implication of this study is that coloring the central target in a flanker task can effectively help the elderly in conquering the flanker interference without exerting compensatory responses. Acknowledgments We would like to thank the Ministry of Science and Technology (MOST) of the Republic of China, Taiwan for financially supporting this research (Contract No. 101-2410-H-006-046-MY3). In addition, this research received funding from the Headquarters of University Advancement at the National Cheng Kung University, which is sponsored by the Ministry of Education, Taiwan, R.O.C. We also thank Ashwini K. Pandey and two anonymous reviewers for their constructive comments on the manuscript. References Angel, L., Fay, S., Bouazzaoui, B., & Isingrini, M. (2010). Individual differences in executive functioning modulate age effects on the ERP correlates of retrieval success. Neuropsychologia, 48, 3540–3553. Band, G. P. H., & Kok, A. (2000). Age effects on response monitoring in a mentalrotation task. Biological Psychology, 51, 201–221. Bartholow, B. D., Pearson, M., Dickter, C., Sher, K. J., Fabiani, M., & Gratton, G. (2005). Strategic control and medial frontal negativity in the event-related brain potential: Beyond errors and response conflict. Psychophysiology, 42, 33–42. Cabeza, R. (2002). Hemispheric asymmetry reduction in old adults: The HAROLD model. Psychology and Aging, 17, 85–100.

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