Defective decision making in patients with acquired VMPC damage has been discussed in the framework of the somatic marker hypothesis
, which posits that decision making is often assisted by emotional processes and somatic “markers,” originating not only from the body itself, but also from several large-scale cortical and subcortical brain networks, including the VMPC, amygdala, insular cortices/somatosensory cortices, and possibly the basal ganglia, as well as signals from the peripheral nervous system.30,36,37
In previous studies, electrodermal activity, specifically the skin conductance response (SCR), has been used in our laboratory as a dependent measure of somatic state activation and somatic “signaling” activity.38
In previous work, we have shown that healthy, non-elderly individuals generate anticipatory SCRs prior to a Bad deck selection, while age-matched VMPC patients (mean age
44 years) fail to generate such SCRs.31
In other words, young participants generate discriminatory anticipatory SCRs during the IGT, with the largest SCRs observed just prior to a Bad deck selection, and smaller SCRs just prior to a Good deck selection. By contrast, VMPC patients generate small and relatively equivalent SCRs to both types of selections, and thus do not display such discrimination.39
These findings have been taken as evidence that the somatic “signaling” that normally facilitates decision making under conditions of uncertainty and risk is disrupted in VMPC patients.
In another study in the current series, our objective was to add an investigation of the psychophysiological correlates of decision making in older adults, to determine whether the integrity of anticipatory SCRs might be compromised in the subset of older individuals that demonstrates impaired IGT performance. We hypothesized that the “somatic signaling” process would be attenuated in the impaired decision makers, but not in the unimpaired ones. Specifically, we predicted that (1) Older-Impaired participants would not generate discriminatory anticipatory SCRs during the IGT; and (2) Older-Unimpaired participants would generate discriminatory anticipatory SCRs during the IGT.
Using the same rationale and procedures as in the previous study (Study 1: Behavioral Correlates of Defective Decision Making
), 40 new older adult participants were recruited. Thus, the overall sample comprised 80 healthy, community-dwelling older adults, aged 56–85 (40 previous particiants, 40 new participants). The two samples did not differ with respect to demographic variables, such as age, education, and gender distribution (P
s > 0.05).
The IGT was administered according to the standard protocol of our laboratory, involving computer administration and psychophysiological (i.e., SCR) measurement. As before, we carried out the following analysis of the IGT behavioral data. Under the assumption that random behavior on the IGT would yield a score of zero for the formula, [(C + D)−(A + B)], we categorized each older adult participant as “Unimpaired” or “Impaired,” based on whether their IGT performance score collapsed across the five Trial Blocks differed significantly from zero, and in which direction, using the binomial test. Participants who had overall performance scores that were significantly different from zero in the positive direction were categorized as “Unimpaired” on the IGT, and participants who had performance scores that were significantly different from zero in the negative direction were categorized as “Impaired” on the IGT. (As in the previous study, this left a middle group of participants whose scores did not significantly differ from zero in either direction, and we refer to this group as “Borderline.”)
While playing the IGT, participants were connected to a polygraph. SCRs were recorded from two Ag/AgCl electrodes attached to the thenar and hypothenar eminences of each hand. Every turn of a card from any deck coincided with a mark on the SCR polygram. The inter-trial interval was set to 6 s, although, given time for deliberation, the average time between card choices was approximately 10 s.39
For the present study, we were interested in anticipatory SCRs generated during the IGT, and this corresponds to the time window between the end
of the 5-s period following the choice of a card and before
the next click of a card (i.e., the time period during which participants are pondering their choice).
The SCR data were acquired through an MP100 WS system (BIOPAC Systems, Inc., Santa Barbara, CA) at the rate of 100 samples per second. The IGT SCR data were analyzed using AcqKnowledge III software (BIOPAC Systems, Inc.) for the MP100 WS system. Quantification of the SCR wave involved elimination of the downward drift using a mathematical transformation function named “Difference,” followed by visual inspection of the wave for experimental artifacts. The primary dependent SCR variable was “area under the curve” in microSiemens per second (µS/s), during the “anticipation phase”; again, this was the time window between the end of the 5-s period following choice of a card and before the next click of a card.
Initial descriptive statistics on the SCR data revealed that, within participant group, the means and standard deviations were similar in magnitude (as is common for electrodermal data of this type).40
We also noted that, between participant groups, the variances were unequal (i.e., heterogeneity of variance). For these reasons, the SCR data were analyzed with non-parametric techniques. Specifically, we utilized the Wilcoxon matched-pairs signed-ranks test and the Mann–Whitney U
test as non-parametric alternatives to the t
test and F
test, respectively, to analyze the SCR data.
The results of the binomial test revealed 24 Older-Impaired participants and 36 Older-Unimpaired participants. The data from six participants were excluded, in three cases secondary to a lack of measurable SCRs (2 men, 1 woman), and in three cases secondary to experimenter error (3 men). One of those excluded was “Impaired” and five were “Unimpaired,” which left 23 Older-Impaired decision makers and 31 Older-Unimpaired decision makers in the final sample of psychophysiological data.
The first prediction was confirmed. Using Wilcoxon paired samples signed-ranks test, we found that the Older-Impaired participants failed to generate discriminatory anticipatory SCRs (P = 0.93); in fact, their anticipatory SCRs were nearly identical for the advantageous and disadvantageous decks. The second prediction was also confirmed. The Older-Unimpaired participants demonstrated discriminatory anticipatory SCRs (P < 0.05). Specifically, this Group generated larger amplitude (i.e., greater area under the curve) SCRs to the advantageous decks compared to the disadvantageous decks, as shown in .
Mean (±SEM) anticipatory SCRs in microSiemens (µS) as measured during the IGT. Data are presented by Group (Older-Unimpaired versus Older-Impaired) and by Deck Type (Good versus Bad).
It is important to explore whether these findings can be explained by a basic between-Group difference in overall SCR responsivity. A Mann–Whitney U test failed to demonstrate any reliable between-Group differences (Ps > 0.05), as the magnitude of SCRs was generally comparable between the Impaired and Unimpaired participants.
This study provides an extension of our previous work exploring the nature of decision making in healthy older adults.1
Specifically, we demonstrated that the decision-making defect has a psychophysiological correlate; namely, Older-Impaired participants lacked discriminatory SCRs to advantageous versus disadvantageous choices, whereas Older-Unimpaired participants demonstrated reliable anticipatory psychophysiological discrimination of good and bad choices.
The psychophysiological findings supported our basic hypothesis regarding the absence of somatic “signaling” in the impaired participants. Specifically, the Older-Impaired participants failed to generate discriminatory anticipatory SCRs. It was interesting, though, that the pattern of results was different from that observed previously in patients with bilateral VMPC damage.39
Those patients failed to acquire anticipatory SCRs to either the advantageous or disadvantageous choices. By contrast, the Older-Impaired participants did
acquire anticipatory SCRs, although those responses did not discriminate good from bad choices.
Interestingly, the Older-Unimpaired participants generated discriminatory anticipatory SCRs, consistent with our second prediction. However, the direction of the anticipatory SCR discrimination was reversed in the Older-Unimpaired participants compared to that found in previous studies involving healthy, non-elderly (young) participants. That is, the Older-Unimpaired participants produced higher-amplitude SCRs to the advantageous decks, while young participants produce higher-amplitude SCRs to the disadvantageous decks. Taken at face value, the pattern of anticipatory discrimination during successful IGT performance differs in important ways for young and older adults.
In conclusion, older adults with strong decision-making abilities, as measured by the IGT, show discriminatory anticipatory SCRs, and it appears that positive (rather than negative) somatic markers play a significant role in shaping their advantageous decisions. By contrast, older adults with poor decision-making abilities do not appear to be differentially influenced by either positive or negative somatic markers, although they may be influenced by both types of markers in a manner unlike patients with acquired VMPC lesions.