William Revelle and Debra A. Loftus
in S.A. Christianson (Ed.) (1992) Handbook of Emotion and Memory. Erlebaum.
Although our emphasis upon the arousal or intensity aspect of motivation is common in the individual differences and motivational literature, we suspect that our arousal oriented approach to the problem of how affect or emotion relates to memory differs from those with a more cognitive interpretation of affect (e.g. Ortony, Clore & Collins, 1990). Our orientation is quite different from those who view affect as just another memory code, the effects of which can be modeled in the same way as the type face or the color of the stimuli. We prefer to emphasize the important role that motivational and affective intensity play upon the very way in which information is detected, processed, and stored for later retrieval.
We have previously suggested that the motivational construct of arousal has a critical impact upon memory and that any study of the relationships between affect, mood and memory needs to take individual differences and situational sources of arousal into account (Revelle and Loftus, 1990). In the following pages we first review arousal as a useful psychological construct, discuss some of the controversy in its measurement and manipulation, consider how the construct of arousal has proven to be helpful in discriminating among competing models of personality, and then propose four different ways in which arousal affects cognitive performance. We then summarize some of the findings relating affect to memory and suggest that many of the manipulations used to change affect are in fact manipulations of arousal as well. We propose that some of the confusion relating affective state to memory performance is due to a lack of concern for the direct effects of arousal upon memory. Based upon our review we propose a model of how arousal and affect modulate memory. Finally we suggest what steps need to be taken in the study of mood and memory if the implications of the effect of arousal on memory are to be taken seriously.
In all of our discussion we emphasize that individual differences in arousal need to be taken into account if particular effects are expected to replicate across various situational manipulations. This partly reflects our own biases, but also reflects a good deal of prior research (reviewed in Anderson, 1989; Revelle, 1987, 1989) that has shown that what seems at first to be a morass of conflicting findings relating arousal manipulations to performance is actually much more orderly when the effects of several personality variables are taken into account.
It is difficult to find an area of psychology where the construct of arousal has not played an important role. For instance, in real time theories of conditioning, arousal is necessary for some neural associations to form (Grossberg, 1987). In psychophysiology, arousal is the cause of desynchronization of the electroencephalogram (EEG), is a non-specific response to orienting stimuli (O'Gorman, 1977), and is a response to increases in task complexity (Berlyne, 1960). In some personality theories arousal level is thought to be maintained at an intermediate value as part of a homeostatic mechanism believed to cause differences in stimulation seeking and hyperactivity (H.J. Eysenck, 1967, 1981; Zuckerman, 1979). In theories of human performance, arousal has been discussed as a determinant of sustained performance in vigilance situations (c.f., Davies and Parasuraman, 1982; Mackie, 1977), and has been used as a way of relating individual differences in personality to the effects of stimulant drugs and time of day on cognitive performance (Revelle, Humphreys, Simon and Gilliland, 1980).
Theories of generalized arousal are not without their critics (e.g., Hockey, 1979, 1984; Neiss, 1988; Venables, 1984) or more accurately, the supporters of theories of generalized arousal are sometimes hard to find (but see Anderson, 1989; Gale, 1986, 1987; Humphreys & Revelle, 1984; Revelle, 1987, 1989). While many theorists recognize the appeal of a unified construct of intensity, the experimental evidence for the dissassociation of various arousal manipulations seems to some to be quite compelling. Indeed, some disillusioned arousal theorists now refer to the study of the intensity aspects of performance with the less theory-laden term of "energetics" (Hockey, Gaillard & Coles, 1986).
Although the evidence is quite clear that there are many different biological responses to increased demands for energy expenditure, it is less clear that functionally these responses are not serving a similar purpose. Following Corcoran's very compelling definition of arousal as the inverse probability of falling asleep (1965, 1981), or the related hypothesis that arousal level is a positive function of energetic requirements, we propose that the low levels of energy expenditure associated with sleeping are a means of conserving physiological resources for more demanding activities. Conversely, high levels of arousal are associated with higher rates of metabolic response to task demands. That is, variations in arousal may be seen as serving the function of varying the resources available for information processing: "cardiovascular changes during information processing reflect the regulation of the organism's energetic economy in support of the specific processes required by the task at hand" (pg. 641, Jennings, Nebes & Brock, 1988).
Compelling demonstrations of how separate arousal systems can serve the same function are the studies by McGaugh and his colleagues who have examined how various stimulants, depressants, and neurotransmitters have similar effects upon memory modulation (McGaugh, 1990). The peripheral stimulant epinepherine, the central stimulant, norepinepherine, and the opiate antagonist, naxolone, all can be shown to modulate memory by their effect on the amygdala. Similarly, arousal changes associated with such widely disparate sources as stable personality traits (Introversion/Extraversion), time of day, stimulant drugs, white noise, evaluation by others and exercise have resulted in highly similar effects on memory (Revelle & Loftus, 1990).
Even if all variations in arousal might be serving the same purpose, it is not necessary that they are caused by similar mechanisms. That is, for primates accustomed to gathering fruits and seeds in the daytime while looking out for predators, conserving energy by sleeping at night is a very useful adaptation to lowered visual accuity and associated higher risk of predation at night. It is functionally more efficient to reduce one's rate of metabolic activity at times when demands are low than it is to keep a constant rate. Consequently, a diurnal variation in arousal is a useful means of coping with the diurnal variation in task demands from the environment. An organism with a very strong diurnal metabolic system, however, will be particularly sensitive to predation at low points in its metabolic cycle. (Monkey's that fail to wake up when attacked by pythons late at night will have a lower reproductive fitness than those that do wake up and are able to flee.) Thus, there is a need to have a separate system that can override or compensate for the normally low nocturnal levels of arousal - a system more capable of responding in an acute sense to immediate demands within the environment. It is possible to argue for a cascade of such control systems, each of which may have evolved separately to compensate for under or over control of the previous level of energy allocation. The function of all of these systems, however, may be seen as controlling the current level of energy expenditure. It is the functioning of this complex of systems that we choose to describe in terms of arousal.
Arousal and the psychological spectrum. Psychological phenomena range across a temporal spectrum of at least 12 orders of magnitude: from the milleseconds used to index firing rates of neurons, to the seconds of a verbal learning study, to the hours of a vigilance experiment, and finally to the decades that make up a lifetime. Different psychological phemenona typically are measured at different temporal frequencies (or durations) across the spectrum. For example, Event Related Potentials (ERPs) have durations of 100-600 ms, priming effects for reaction time persist over only a few seconds, changes in affect takes tens of seconds to occur, stimulants such as caffeine have effects only after 30-40 minutes (~2 x 103 sec), and cognitive performance changes can range over the day (~105 sec), the menstrual cycle (~106 sec), a year of schooling (~3 x 107 sec), and over the life span (~3x109 sec).
Measures of arousal may be classified in terms of this psychological spectrum. At the shortest intervals are measures that include indices of cortical activity such as the EEG, both event related and resting frequency. At somewhat longer intervals are the autonomic measures of Skin Conductance (SC) and Heart Rate (HR). At even longer levels are endrocrine measures such as the level of Mono-Amine Oxidase (MAO) and general metabolic measures such as core Body Temperature (BT). Other measures with fine temporal resolution include psychophysical sensitivities to light and sound. At a less fine resolution are measures of activity level. In addition to the direct physiological measures are measures of self report of arousal. By asking subjects how peppy, active and vigorous they feel, it is possible to show general effects with durations from a few minutes to a few hours. In fact, as Robert Thayer (1986, 1989) has shown, self report measures seem to reflect the general factor of many of the finer grain physiological measures (see also Clements, Hafer & Vermillion, 1976 and Matthews, 1987, 1989).
Time course of arousal measures: growth, level, and decay of arousal. Arousal changes over time. Therefore, we need to be concerned with the rates of growth and decay, as well as the average level of arousal. Variables that affect one rate of change need not affect the others. Consider the various patterns of arousal that can occur as a result of different patterns of stimulation duration (both the on and off times) if change in arousal (dA) is represented as a function of excitatory sensitivity (e) to stimulation (S) and habituation which is a function of arousal level and some decay parameter (c): dA = eS - cA. Average arousal level is a function of e, c, and the length of time the stimulation is on or off. When measuring arousal level it is important to remember that these parameters of arousal are logically, and frequently empirically, independent.
Latency and sampling frame. A classic problem in the measurement of arousal is how to relate measures with different onset latencies after the stimulus. Each index of arousal has different temporal parameters, with different delays and sampling rates, so that different indices reflect arousal averaged over different lengths of time and with different delays (e.g., EEG activation will occur within milliseconds of a stimulus onset, skin conductance changes after several seconds, self reports of activity and energy reflect arousal sampled over a longer period than either EEG or SC, and body temperature seems to indicate average activity over a period of minutes to hours). Depending upon the fineness of the temporal resolution of the stimulus, these different measures will respond differently. A missing stimulus in the P300 oddball paradigm that elicits a change in EEG evoked potentials, and perhaps an Orienting Response as indexed by a change in SC or HR, should not be expected, however, to affect ratings of alertness or core body temperature. These broader measures reflect changes in the system that are so slow as to be undetectable or are discarded by those who record ERPs or GSRs.
Within versus between subject measures. Several different questions arise with respect to the problem of measurements taken within subjects versus between subjects. Arousal is normally thought of as something which varies within subjects - a stimulus induces an orienting response, we become excited while giving a talk, exercise increases heart rate. While some measures (such as self report) are very useful indicators of arousal changes within subjects, they are of questionable validity between subjects. For instance, it is difficult to know what different people mean by the term low arousal. What person A reports as being calm, person B might well report as excited and peppy. When one of us feels exhausted and thinks that he is talking very slowly, students report that his rate of talking in a lecture is finally at a normal level. This problem of calibration between subjects is not just for self report; for example, skin conductance differences can reflect skin thickness between subjects as well as differences in excitement within and between subjects.
Finally, it is important to note that when indices covary within individuals, they do not necessarily covary between individuals. Some individuals will respond to stimuli with greater changes in heart rate than in skin conductance, while others will have large increases in skin conductance but only small increases in heart rate. Within subjects, changes in SC and HR can correlate positively even though there is zero or a negative correlation between subjects. The question of how to aggregate data across subjects is particularly important for any consideration of the effect of arousal upon performance.
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An alternative to physiological measures is to assess the subjective feel of alertness and vigor by self report. As Thayer (1967, 1978, 1989) and Clements et al. (1976) have shown convincingly, when measures are taken within subjects, self reports of arousal correlate more highly with each of the separate physiological measures than the measures do with each other. This is, of course, just the pattern that would be expected if the self-report measures represented either the common factor of arousal, or the sum of activity in each of several independent systems.
Each manipulation of arousal has specific effects as well as a general effect. It is important when examining the relationships between various manipulations and measures to take into account the irrelevancies of each manipulation as well as the common effects (Anderson, 1989). For instance, the behavioral consequences of lowering arousal through sleep deprivation or sustained performance are quite different than those associated with the effects of alcohol. Similarly, noise and caffeine both increase arousal, but their effects on performance differ, at least partly because high levels of noise are distracting and can mask inner speech while high levels of caffeine can cause muscle tremor. Much of the arguement over the utility of the arousal construct centers on the relative importance of the specific versus common effects of arousal manipulations (Anderson, 1989).
Within the arousal domain, Thayer (1967, 1978, 1986, 1989) has explicated two orthogonal factors that can be related to dimensions of affect: energetic arousal and tense arousal. Energetic arousal and positive affect covary strongly as do tense arousal and negative affect. Thayer (1989) found that energetic arousal varies diurnally along with positive affect. Loftus (1990) found that an exercise induction (as compared to a relaxation induction) tended to increase positive affect while negative affect remained unaffected. Both energetic and tense arousal increased as a result of the exercise induction also. This result was in contrast to an earlier study performed by Thayer that suggested exercise increased energetic arousal and decreased tense arousal (Thayer, 1989).
Although the exact nature of the relationship between positive/negative affect
and low/high or energetic/tense arousal has yet to be clearly delineated, it
is clear that many manipulations of affect also are manipulations of arousal.
The negative affect experienced while watching a horror movie is accompanied by
an increase in tense arousal. The positive affect associated with a lively
party is also associated with an increase in energetic arousal. It is this
confounding of affective manipulations with arousal manipulations that we
believe may account for at least part of the inconsistent findings (discussed
later) within the study of mood-state dependent effects. In addition, it is
likely that the so-called mood intensity effects are more attributable to the
effects of the arousal levels present in the experiments than the affect.