Jurnal f Persnality and Scial Psychlgy 1994. Vl. 67. N. 2, 334-344 Cpyright 1994 by the American Psychlgical Assciatin, Inc. 0022-35I4/94/$3.00 Impulsivity and Time f Day: Is Rate f Change in Arusal a Functin f Impulsivity? Kristen Jan Andersn and William Revelle Impulsivity has been interpreted as a stable mediatr f rate f change in arusal states. T test this hypthesis, 129 Ss differing in impulsivity were given placeb r caffeine at 9:00 a.m. r 7:30 p.m. Recgnitin memry was tested fr the last 20 items frm 2 lists f 24 items and 2 lists f 80 items. Scres frm this paradigm reflect sustained attentin and are thus sensitive t changes in arusal. A 4-way interactin amng impulsivity, time f day, drug, and prir stimuli (p <.05) indicated that fr thse given placeb, recgnitin memry fr lng and late lists was prer the higher the impulsivity in the mrning; this pattern reversed in the evening. Caffeine reduced recgnitin errrs. These results indicate that impulsivity is nt a stable predictr f rate f change in arusal states. Instead, susceptibility t attentinal lapses is mediated by impulsivity-related phase differences in diurnal arusal rhythms. Impulsivity is ne f the dimensins f individual differences frequently identified by therists cncerned with the bilgical bases f persnality. Althugh the apprpriate theretical interpretatin f impulsivity is a matter f nging debate, many mdels either explicitly r implicitly psit a relatinship t arusal (e.g., Barratt & Pattn, 1983; H. J. Eysenck & Eysenck, 1985; Gale, 1987; Revelle, Andersn, & Humphreys, 1987; Schalling, Edman, & Asberg, 1983; Strelau, 1987; Zuckerman, 1983). Arguments that impulsivity is linked t arusal can be traced largely t Hans Eysenck (1967), wh prpsed that (a) there are genetically influenced differences in basal arusal levels, (b) all individuals experience maximally psitive hednic tne at intermediate arusal levels, and (c) individuals wh are chrnically underarused develp patterns f behavir designed t increase their arusal. Because scial, spntaneus, and risky behavirs (fr example) typically affrd greater arusal ptential than slitary, planned, r safe endeavrs, Eysenck prpsed that phentypically extraverted behavir patterns tend t develp amng thse whse basal arusal levels are lw. This thery prvided an explanatry link between evidence f the heritability f extraversin and data suggesting differences between intrverts and extraverts in a variety f labratry phenmena. Kristen Jan Andersn and William Revelle, Department f Psychlgy, Nrthwestern University. This research was supprted in part by cntracts MDA903-90-C- 0108 and MDA9O3-93-K-OOO8 frm the United States Army Research Institute t William Revelle and Kristen Jan Andersn. The views, pinins, and findings cntained in this article are urs and shuld nt be cnstrued as an fficial Department f the Army psitin, plicy, r decisin, unless s designated by ther fficial dcumentatin. We wish t thank Rbert Caldern, Debra Janiszewski, Kimberly My, Kathryn Rsenthal, Tina Rvick, Vicky Singh, Sara Wilcx, and Peggy Wu fr their assistance in cllecting and scring data. Crrespndence cncerning this article shuld be addressed t Kristen Jan Andersn, Department f Psychlgy, Nrthwestern University, Evanstn, Illinis 60208. In brief, Eysenck prpsed that the basal arusal level f intrverts is higher than that f extraverts and, as a crllary, that the tw primary subtraits f extraversin, sciability and impulsivity, are als negatively related t arusal. The term arusal is used here t refer t a state invlving nnspecific physilgical activatin and the nndirectinal cmpnent f alertness. Reflected in experiences f alertness, peppiness, and liveliness, it varies within individuals frm very lw levels characteristic f sleep t very high levels assciated with great excitement r panic. Mre specifically, we use the term arusal t dente a hypthetical cnstruct representing the net result f a variety f prcesses that mediate activatin, alertness, and wakefulness. As an abstractin, it reflects several electrcrtical, autnmic, and behaviral mechanisms, but it is nt synnymus with any ne f them. Despite the difficulties that fllw frm the cncept f a generalized nndirectinal energizer, as a hypthetical cnstruct, arusal has pragmatic usefulness in rganizing a wide array f empirical phenmena (cf. Andersn, 1990; Claridge, 1987; Gale & Eysenck, 1992; Hebb, 1955; Humphreys* Revelle, 1984). Substantial evidence has been presented in supprt f the argument linking intrversin t arusal (see, e.g., reviews by H. J. Eysenck & Eysenck, 1985, and Stelmack, 1981), but recent research suggests that revisins t Eysenck's mdel are necessary. First, there is munting evidence that it is impulsivity, rather than sciability r extraversin, that is arusal related (e.g., Andersn, in press; Bwyer, Humphreys, & Revelle, 1983; Campbell, 1992; H. J. Eysenck & Levey, 1972; Frcka & Martin, 1987; L, 1980; O'Grman & Llyd, 1987; Revelle, Humphreys, Simn, & Gilliland, 1980; Schalling et al., 1983; Stenberg, 1992). Mrever, research suggests that the persnality-arusal relatinship is mediated by phase differences in diurnal arusal rhythms (e.g., Blake, 1967; Revelle et al., 1980). In general, arusal increases during the mrning, levels ff r decreases slightly thrugh the afternn, and then declines during the evening; extraverts lag behind intrverts in this pattern. Again, al- 334
IMPULSIVITY AND TIME OF DAY 335 thugh sme incnsistent findings have been reprted (e.g., Larsen, 1985; Wilsn, 1990), data suggest a greater rle fr impulsivity than sciability in mediating this relatinship between persnality and diurnal arusal rhythms (e.g., M. W. Eysenck &Flkard, 1980; Matthews, 1987b; Neubauer, 1992; Revelleet al., 1980; Zuber & Ekehammar, 1988). Thus, the available data suggest that high-impulsive subjects are less arused than lwimpulsive subjects during the mrning, but that the reverse is true in the evening, when high-impulsive subjects are mre arused than lw-impulsive subjects. Taken tgether, these data present serius difficulties fr H. J. Eysenck's theretical mdel, an issue discussed mre fully elsewhere (e.g., Revelle et al., 1987; Revelle & Andersn, 1992; H. J. Eysenck & Eysenck, 1985; Gray, 1981). Central t the present study are questins regarding the basis fr the apparently cmplex relatinship between impulsivity and arusal. These prir findings frce rejectin f any simple ntin f a direct relatinship between impulsivity (r extraversin) and basal arusal levels. Instead, it is pssible that impulsivity is a stable predictr f rate f change in arusal states (Revelle, 1983: Revelle et al., 1987). Such a relatinship culd be fund if arusability is a negative functin f impulsivity (s that lw impulsives becme mre arused than high impulsives t equivalent stimuli) 1 r if rate f decay f arusal is a psitive functin f impulsivity (s that high impulsives lse arusal mre rapidly than lw impulsives). We previusly argued that it is this rate f change in arusal states that leads t bth impulsivity and t diurnal rhythm shifts: Nnimpulsives wuld build up arusal faster than impulsives, becming alert sner in the mrning. After several hurs, high and lw impulsives wuld achieve the same arusal level, but impulsives wuld seek new stimulatin cnstantly t maintain the arusal. By evening, nnimpulsives, wh have been highly arused fr much f the day, wuld be fatigued and cease t seek arusal. Arusal wuld decay, and the nnimpulsive wuld retire fr the evening. Impulsives wuld nt have been as highly arused fr as lng and wuld nt be fatigued yet. Thus impulsives wuld still want t maintain a high arusal level and cntinue t seek stimulatin. Eventually fatigue wuld set in and even impulsives wuld call it a night. (Revelle et al., 1987, p. 22) Replacing the Eysenckian cncept f stable differences in basal arusal with the prpsitin that impulsivity reflects stable differences in rate f change in arusal states wuld thus allw accmmdatin f the available evidence indicating impulsivity-related differences in diurnal arusal rhythms. At least tw lines f research are relevant t this hypthesis: studies linking persnality t sustained attentin and thse addressing individual differences in rienting respnses. As reviewed belw, this evidence ffers sme indirect, albeit incmplete, supprt fr this interpretatin f impulsivity. Arguments linking persnality t sustained attentin fllw frm a brad base f empirical evidence invlving a variety f tasks in which attentinal prcesses are central, that is, tasks such as simple reactin time tasks, simple arithmetic, r letter cancellatin, in which subjects are required t prcess a stimulus, assciate an arbitrary respnse t that stimulus, and execute that respnse. Studies f perfrmance n such tasks have yielded cnverging results frm manipulatins including drugs, exercise, heat, and nise, as well as state and trait assessments with presumed relevance t arusal. These parallels led several investigatrs t prpse, as part f their brader mdels f mtivatin and perfrmance, that arusal facilitates attentinal prcessing, and sustained attentin in particular (e.g., Bradbent, 1971; H. J. Eysenck & Eysenck, 1985; Humphreys & Revelle, 1984). If impulsivity is linked t rate f change in arusal states, then in cmparisn with lw impulsives, high impulsives shuld have greater difficulty sustaining attentin. As nted abve, there are tw versins f this hypthesis linking impulsivity t arusal states, ne psiting a direct relatinship t rate f decay f arusal and the ther psiting a negative relatinship t rate f increase in arusal. If arusal is crucial t sustaining attentin and if high impulsives lse arusal mre quickly, as suggested by the rate f decay f arusal hypthesis, then high impulsives will experience mre attentinal lapses ver time than lw impulsives. Similar predictins fllw frm the hypthesis linking impulsivity t rate f increase in arusal states: If in cmparisn with high impulsives, lw impulsives are mre arused by stimuli, r if their arusal increases mre rapidly, then their greater arusal wuld facilitate sustained attentinal prcessing and, again, attentinal lapses wuld be mre frequent amng high impulsives than amng lw impulsives. Thus, althugh there are imprtant distinctins between these mdels in terms f the mechanism identified as respnsible fr individual differences in rate f change in arusal, the implicatins f either psitin fr sustained attentin are equivalent. A number f investigatrs have examined the influence f extraversin n sustained attentin in vigilance r cntinuus perfrmance tasks. A recent meta-analysis f the vigilance literature by Kelega (1992), which highlights methdlgical difficulties with much f the available research, cncluded that the assciatin between extraversin and vigilance decrements, althugh reliable, is very weak: The estimated effect size f the relatinship between extraversin and vigilance decrements as indexed by change in hit rates was r =.05. N ther perfrmance measure was reliably assciated with extraversin ver time. We argue that a central reasn fr the apparent weakness f the relatinship between persnality and vigilance decrements is that the crucial variable is impulsivity, nt extraversin. If impulsivity is indeed the critical individual-difference dimensin, then studies cmparing intrverts and extraverts wuld smetimes (but nt always) indicate reliable differences (because impulsivity and sciability are mderately crrelated); mrever, the differences that are bserved wuld tend t be weaker than thse that wuld be evident if subjects were differentiated n the basis f impulsivity. Kelega's bservatin that an assciatin between extraversin and vigilance decrements was strngest when extreme grups were used (r =.27) culd simply reflect the greater pwer f that type f design, but it is als true that selecting grups wh are extreme in terms f extraversin simultaneusly selects fr impulsivity, even if the 1 Several extraversin researchers have similarly prpsed that intrverts and extraverts may differ nt in basal arusal levels, but rather in arusability (e.g., Smith, 1983;Stelmack&Geen, 1992).
336 KRISTEN JOAN ANDERSON AND WILLIAM REVELLE measure f extraversin des nt include impulsivity cntent (again, because sciability and impulsivity are psitively crrelated). Kelega's review als addressed evidence regarding time f day effects n this relatinship. The mst glaring prblem is that very few researchers have cnsidered the pssible rle f diurnal arusal variatins, neither cntrlling fr nr even reprting the time at which testing ccurred. A ntable exceptin is a study by Clquhun (1960) that included assessment at several times f day and suggested that intrverts were better at vigilance than extraverts in the mrning, but extraverts were better than intrverts in the afternn. Kelega stressed several criticisms f Clquhun's prject and cncluded that time f day des nt mediate the extraversin-vigilance relatinship. Given bth the paucity f available evidence and the ptentially critical distinctin between impulsivity and extraversin, the questin f the impact f time f day n the persnality-vigilance relatinship must be cnsidered unanswered. Similar methdlgical limitatins pertain t research n the rienting respnse. H. J. Eysenck's mdel (1967) predicted that intrverts wuld shw mre prnunced rienting respnses t stimuli and wuld als take lnger t habituate t them. Research n rienting respnses thus bears n hyptheses linking persnality t arusability, rate f decay f arusal, r bth. As reviewed by H. J. Eysenck and Eysenck (1985), O'Grman (1983), and Stelmack (1981), a cnsiderable bdy f evidence is cnsistent with the hypthesis that extraverts habituate mre rapidly than intrverts, althugh incnsistent findings have been reprted. Data regarding the magnitude f rienting respnses suggests that with mderate-intensity stimuli, intrverts shw larger skin cnductance respnses than extraverts; this pattern des nt typically emerge with lw- r high-intensity stimuli (Stelmack, 1981; Stelmack & Geen, 1992). As a rule, these studies f rienting respnses have examined extraversin rather than impulsivity, and time f day has rarely been reprted r cntrlled. Thus, again, the questin f the relatinship f impulsivity t rate f change in arusal states remains unanswered. Taken tgether, results f research n vigilance decrements and rienting respnses prvide sme limited, but bviusly indirect, supprt fr a relatinship between impulsivity and rate f change in arusal states. Insfar as this evidence derives primarily frm studies cnducted during mrning r afternn hurs, it is nt clear whether these findings will generalize t the evening. A strng test f the hypthesis regarding impulsivity and rate f change in arusal states requires a cmparisn f high and lw impulsives at bth mrning and evening times n a task sensitive t changes in arusal ver time. One such task requires sustained attentin t stimulus materials tested later in a recgnitin memry paradigm. Previus research by Bwyer et al. (1983) demnstrated the ptential usefulness f this task, which they adapted frm Underwd (1978). Intrigued by Schulman's (1974) results, which indicated that the psitive assciatin between list length and errrs in recgnitin memry was nt due t retractive interference, Underwd (1978) explred the pssible rle f practive interference. Subjects cmpleted frced-chice recgnitin memry tasks after lists f 24,40,60, and 80 items. In each case, subjects were tested n nly the last 20 items, thus equating retentin intervals; subjects were nt infrmed f this restrictin. Theretical analyses f practive interference suggested that with increasing list length, the resultant increase in ptential assciative links (bth semantic and rthgraphical/acustical) wuld prduce a crrespnding increase in recgnitin errrs. Underwd's methds allwed careful analyses f errr patterns, and althugh errr rates did increase reliably with list length, his results failed t supprt the hypthesis that the higher errr rates fr lnger lists were due t interitem assciatins. Instead, Underwd advcated an attentinal explanatin, prpsing that attentinal lapses were mre likely as the number f previus stimuli increased. If attentinal lapses are respnsible fr the assciatin between list length and errr rates in supraspan recgnitin memry paradigms, and if arusal facilitates sustained attentin, then increases in arusal shuld enhance recgnitin memry. Bwyer et al. (1983) pursued that pssibility in a replicatin and extensin f Underwd's study. Arusal was manipulated thrugh administratin f caffeine. In additin, impulsivity was assessed; because all f their subjects were tested in the mrning (9:00 a.m.), it was assumed that high impulsives were less arused than lw impulsives. Subjects cmpleted tw-alternative frced-chice recgnitin memry tasks after each f fur lists f wrds; these lists included, in rder, 24, 80, 80, and 24 wrds. Fllwing Underwd (1978), Bwyer et al. tested subjects fr nly the last 20 wrds f each list. Results indicated that errr rates increased acrss the fur lists. Mre crucial was the predictedfinding that this decrement in perfrmance ver time was reliably greater fr high-impulsive subjects than fr lw-impulsive subjects. Caffeine was assciated with fewer errrs verall and als with a reduced susceptibility t increasing errrs acrss trials, but the interactin between drug and trial was nly marginally significant. Bwyer et al. argued that the parallels between the effects f impulsivity and caffeine n recgnitin memry within their study and the parallels between their results andfindingsf ther research n attentinal perfrmance suggested that a cmmn mechanism was invlved, namely, that arusal facilitates sustained attentin. As they nted, hwever, the pattern f results was nt fully cnsistent with their predictins. In particular, parallel effects f arusal variables n list length had been expected, but nt cnfirmed. The present study is a replicatin and extensin f the experiment reprted by Bwyer et al. (1983): High- and lw-impulsive subjects were tested in the mrning r in the evening after cnsuming caffeine r placeb. Recgnitin memry was examined as a functin f number f previus stimuli, defined by bth list length (24 r 80 items) and number f prir lists. This paradigm permitted a test f tw hyptheses regarding impulsivity and arusal. As reviewed abve, prir research led t a rejectin f the hypthesis f a stable relatinship between impulsivity and arusal states. The trait f impulsivity culd, hwever, be related t either stable differences in rate f change in arusal states r t stable differences in diurnal arusal rhythms. 1. The first pssibility is that regardless f phase differences in diurnal arusal rhythms, impulsivity is assciated with stable
IMPULSIVITY AND TIME OF DAY 337 differences in rate f change in arusal states. In this case, highimpulsive subjects shuld be less able t sustain arusal and hence less able t sustain attentin ver time than lw-impulsive subjects, and this difference shuld be fund at all times f day. As already nted, this predictin fllws whether the difference in rate f change in arusal states invlves rate f increase r rate f decay f arusal. Thus, each mdel regarding rate f change in arusal states predicts that in bth mrning and evening sessins, high-impulsive subjects shuld make mre recgnitin errrs than lw-impulsive subjects as the number f prir stimuli increases. 2. An alternative hypthesis is that impulsivity is related t phase differences in diurnal arusal rhythms, and it is these diurnal variatins in arusal (and nt a stable attribute f impulsivity) that determine the capacity t sustain attentin. This mdel predicts that perfrmance n tasks that are facilitated by arusal will reveal a crssver interactin amng time f day, impulsivity, and demands fr sustained attentin: Recgnitin memry fr lng and later lists wuld thus be expected t be prer fr high impulsives than fr lw impulsives in the mrning, but prer fr lw impulsives than fr high impulsives in the evening. Bth f these hyptheses psit an assciatin between impulsivity and arusal and between arusal and perfrmance n this task. T strengthen interpretatin in terms f arusal thrugh cnvergent validatin (cf. Ck & Campbell, 1979; Crnbach & Meehl, 1955), arusal was als manipulated thrugh administratin f caffeine. Extensive research with this central nervus system stimulant (reviewed by Gilbert, 1976, James, 1991, Lieberman, 1992, and Rail, 1980) is cnsistent with an arusal interpretatin f the effects f the dse we used. It was predicted that caffeine wuld facilitate sustained attentin, and wuld thus be assciated with better recgnitin memry than placeb as the number f prir stimuli increases. T summarize ur design, subjects differing in impulsivity were given either a placeb r caffeine at either 9:00 a.m. r 7:30 p.m. Fllwing Bwyer et al. (1983), they were then shwn fur lists f wrds with (in rder) 24,80,80, and 24 wrds. After each list, recgnitin memry was tested fr the last 20 wrds nly, thus equating retentin intervals. Several predictins were tested: 1. It was assumed that recgnitin scres frm this paradigm wuld reflect the subject's ability t sustain attentin during the stimulus presentatin phase f each trial and acrss trials. Recgnitin scres were thus expected t be lwer fr 80-item and later lists in cmparisn with thefirst,shrt list. 2. It was assumed that increased arusal wuld facilitate sustained attentin. Caffeine, which prduces an increase in arusal, was therefre expected t be assciated with fewer recgnitin memry errrs than placeb. Beneficial effects f caffeine were expected t be mst prnunced when the number f previus stimuli (and thus the demand fr sustained attentin) was high, that is, fr the lng and late lists. 3. The primary purpse f this study was t examine tw different hyptheses regarding impulsivity: If impulsivity is related t stable differences in rate f change in arusal states, then high impulsives given placeb wuld shw prer recgnitin fr 80-item and later lists than lw impulsives given placeb, and this wuld be fund in bth mrning and evening sessins. In cntrast, if impulsivity is related t phase differences in diurnal arusal rhythms, but nt t stable differences in rate f change in arusal states, then impulsivity and time f day wuld interact: In the mrning, high-impulsive subjects given placeb wuld recgnize fewer items frm lng and later lists than their lw-impulsive cunterparts. In the evening, the reverse wuld be true. Thus, these tw hyptheses abut impulsivity yield similar predictins fr subjects given placeb and tested in the mrning, but ppsite predictins fr subjects given placeb and tested in the evening. Subjects Methd Subjects included 157 students taking intrductry psychlgy at Nrthwestern University; they received partial curse credit fr participating. Individual differences were assessed with the Eysenck Persnality Inventry (EPI; H. J. Eysenck & Eysenck, 1964), using the impulsivity subscale identified by Revelle et al. (1980). Scres n this scale, which emphasizes spntaneity and rapidity f cgnitive and behaviral activity, have repeatedly been shwn t mediate perfrmance as expected n the basis f predictins frm arusal thery (e.g., Andersn, in press; Andersn & Revelle, 1982; Bwyer et al., 1983; Revelle et al., 1980). The distributin f impulsivity scres (M = 4.61, SD = 2.07) was cmmensurate with ther samples. Alternative EPI impulsivity scres were available fr many f the subjects included in analyses. Cmparisn with scres btained under cnditins in which the subjects spent an hur cmpleting varius persnality questinnaires yielded a test-retest reliability f.72 (n = 124); cmparisn with scres btained in ther experiments in ur lab indicated a test-retest reliability f.82 («= 95). Equipment malfunctins prevented use f data frm 13 f these subjects. In additin, 1 subject became ill during the experimental sessin and 5 had cnsumed caffeine shrtly befre participating. One subject grssly vilated task directins and 8 subjects displayed excessively lw recgnitin rates (less than 56%, 3 SD belw mean levels), suggesting failure t cmply with experimental instructins. Data analyses are thus based n 129 subjects. 2 Assignment t time f day and drug cnditins was randm and was determined befre the experimental sessin began. Of the subjects included in analyses and tested at 9:00 a.m., 29 received placeb and 34 caffeine; f thse tested at 7:30 p.m., 32 received placeb and 34 caffeine. Impulsivity scres did nt differ reliably acrss experimental cnditins. Materials Caffeine versus placeb. Subjects received either placeb (flat quinine water) r caffeine citrate (4 mg caffeine per kg bdy weight) disslved in an range-flavred breakfast drink. Memry task. The stimulus set, which supplemented the list used by Underwd (1978) by fur wrds, included 288 fur-letter wrds that varied greatly in frequency. Fur lists f wrds, tw with 24 items and tw with 80 items, were randmly selected and randmly rdered, with the remaining items used as distractrs in tw-alternative, frcedchice recgnitin tests. Stimuli were presented in the same rder t all 2 With the exceptin f the ne subject wh clearly vilated task instructins by circling bth items in sme pairs and neither item in thers, the pattern f means was nt affected by exclusin f these subjects.
338 KRISTEN JOAN ANDERSON AND WILLIAM REVELLE subjects. The recgnitin tests prbed nly the last 20 items f each list; wrd pairings and pair rder fr the recgnitin tests were randmly determined. Prcedure Subjects were scheduled by telephne fr a sessin at the time f day t which they had been randmly assigned; they were nt infrmed that sessins were als being run at an alternative time. All subjects were asked t avid caffeine r any ther psychactive drug fr at least 6 hr befrehand. On their arrival, subjects were prvided with a cnsent frm that indicated pssible side effects f caffeine and screened fr cntraindicated medical cnditins. All subjects cnsented and were given the beverage cntaining either caffeine r placeb using duble-blind prcedures. Subjects then cmpleted several questinnaires, including the EPI and a set f questins regarding typical and immediately prir caffeine cnsumptin. When 23 min had elapsed since drug administratin, subjects cmpleted several state measures, including the 40 items frm Thayer's Activatin-Deactivatin Adjective Checklist (AD-ACL; 1986) rated n a 0 t 3 scale. Instructins fr the recgnitin task were then given: Subjects were infrmed that they wuld see several lists f wrds that varied in length, that after each list they wuld be shwn a set f pairs f wrds and asked t indicate which wrd frm each pair was n the list; and that nt all items n the list wuld be tested. They were tld nt t leave any pairs unmarked, guessing if necessary. Subjects were nt tld hw many items were in any list, hw many items wuld be prbed, r that prbed items were always frm the end f the list. Furthermre, t minimize the pssibility that anticipatin f the end f the task wuld influence perfrmance n the last list (cf. Andersn & Revelle, 1983), tw extra trays f slides were placed with the fur that were actually used t create the impressin that further lists wuld be presented. Once any questins were answered, we presented the first list f 24 wrds using a slide prjectr. The slides, each having a single wrd in lwercase letters in the center, were presented at a 2.7-s rate. Subjects were then given 90 s t cmplete a printed tw-alternative frced-chice recgnitin task. The tw 80-wrd lists and the final 24-wrd list were presented and tested in like manner. This rdering f lists directly replicated that used by Bwyer et al. (1983). The AD-ACL and ther state measures were then readministered. Finally, subjects were thanked and debriefed. They were urged t refrain frm discussing the experiment with ther ptential subjects. Manipulatin Check Results AD-ACL scres were subjected t a general linear mdel analysis with trial (befre r after the memry task) as a withinsubjects factr, time f day (9:00 a.m. r 7:30 p.m.) and drug (caffeine r placeb) as categrical between-subjects factrs, and impulsivity as a cntinuus between-subjects factr. Althugh the predicted main effect f caffeine was evident, F(l, 121) = 11.99, MS C = 160.89, p <.001, this analysis als indicated an unexpected three-way interactin amng trial, time f day, and drug, F(\, 121) = 9.47, MS C = 36.44, p <.01. (See Figure 1.) Inspectin f the means indicates that in the evening, caffeine was, as expected, assciated with higher levels f self-reprted activatin than placeb bth befre and after cmpleting the recgnitin task. In cntrast, caffeine was nt assciated with higher levels f self-reprted activatin than placeb immediately befre the recgnitin task amng subjects tested in the mrning. Their self-reprted activatin scres were higher with caffeine than with placeb after the task. Recgnitin Memry Scres Number f crrect respnses n the recgnitin tests was subjected t a general linear mdel analysis with time f day (9:00 a.m. r 7:30 p.m.) and drug (caffeine r placeb) as categrical between-subjects factrs and impulsivity as a cntinuus between-subjects factr. Within-subjects effects were analyzed using planned rthgnal cntrasts selected n the basis f ur hyptheses. 1. We expected that susceptibility t attentinal lapses wuld increase as bth number f prir lists and number f prir items increased. Thus, recgnitin memry shuld be prer fr the lnger and later lists than fr the initial list f 24 items. Our first cntrast cmpared perfrmance n thefirstlist with that n the three later lists (which included bth 80-item lists); we refer t this cntrast as the effect f prir stimuli. 2. This same lgic led t the predictin that perfrmance n the secnd list f 80 items shuld be wrse than n the first list f 80 items. Our secnd cntrast thus cmpared these tw lists and is referred t as the set effect fr lng lists. 3. The remaining rthgnal cntrast tested the effect f list length by cmparing the tw 80-item lists with thefinal24-item list. We did nt have a specific predictin fr this cntrast, because perfrmance n the final list reflects bth increasing time n task (rendering it mre susceptible t attentinal lapses than earlier lists) and shrter list length (rendering it less susceptible t attentinal lapses than the 80-item lists). 3 List effects. Results indicated a significant effect f prir stimuli, F(l, 121) = 59.40, MS t = 25.60, p<.001. Means fr the fur lists in the rder f their presentatin (i.e., the first list f 24, thefirstlist f 80, the secnd list f 80, and the secnd list f 24) were 19.22, 17.81, 18.21, and 18.37. Thus, as expected, average recgnitin memry scres were lwer fr the lng and late lists than fr the first, shrt list. The set effect fr lng lists was als reliable, F(\, 121) = 4.63, MS C = 1.85, p <.05. Cntrary t expectatin, average scres fr the secnd list f 80 3 Tw alternative analyses wuld have been feasible, ne subjecting the list effects t trend analysis, and the ther t a factrial design crssing list length with set (first r secnd). Neither ptin directly addresses ur central hyptheses. Mrever, bth ignre the ambiguity f predictins regarding the final list relative t the 80-item lists. As a result, any f several utcmes frm either f these alternative analyses culd have been viewed as cnsistent with ur hyptheses. That is, interactins f arusal variables with either linear r quadratic trends culd have been interpreted as cnsistent with ur predictins; alternatively, interactins f arusal variables with list length, set, r their interactin culd have been interpreted as supprting ur mdel. Recgnizing that a priri specificatin f particular patterns f results is nt always pssible and that, as a result, a prbabilistic bias in favr f ne's hyptheses can accrue, we have previusly argued (Andersn, in press; Revelle & Andersn, 1992) fr the use f techniques that reduce the number f utcmes that can be cnstrued as cnsistent with predictins. We thus rejected bth the trend and the factrial analytic strategies in favr f cmparisns that prvided unambiguus tests f ur predictins.
IMPULSIVITY AND TIME OF DAY 339 32 T 30 2 u 28 - - - - - a.m.; placeb ACL Q 26 24 22 - - - - - a.m.; caffeine p.m.; placeb p.m.; caffeine 20 18 Befre After Trial (Relative t Memry Task) Figure I. Activatin-Deactivatin Adjective Checklist (AD-ACL) scres as a functin f time f day (9:00 a.m. r 7:30 p.m.), drug (placeb r caffeine), and trial (befre r after the recgnitin memry task). items were higher than fr the first list f 80 items. List length als significantly influenced recgnitin scres, F(l, 121) = 6.00, MS C = 8.60, p <.05. Average scres were higher fr the final list f 24 items than fr the tw 80-item lists. Caffeine effects. It was predicted that caffeine wuld enhance sustained attentin, thus resulting in better recgnitin memry fr 80-item lists and lists late in the experimental sessin. The main effect f caffeine, F( 1,121) = 11.52, MS e = 6.66, p <.001, was significant. Mre imprtant, caffeine interacted with the prir stimulus effect, F(l,12\) = 9.68, MS C = 25.60,p <.01. As shwn in Figure 2, the pattern f means indicates that subjects given caffeine recgnized mre items frm the last three lists than did thse given placeb. Impulsivity effects. The crucial questin addressed by this study cncerned the effects f impulsivity n recgnitin memry. If the persnality dimensin f impulsivity is related t stable differences in rate f change in arusal states, then regardless f time f day, high-impulsive subjects given placeb wuld be mre likely than lw-impulsive subjects given placeb t shw vigilance-like decrements in recgnitin memry. Thus, impulsivity wuld interact with prir stimuli, but time f day wuld nt mediate that interactin. In cntrast, if the relatinship between impulsivity and rate f change in arusal states is nt temprally stable, instead depending n diurnal arusal rhythms (which are in turn linked t impulsivity), then susceptibility t vigilance-like decrements wuld be an interactin f impulsivity with time f day: In the mrning, high impulsives given placeb wuld be mre susceptible t vigilance-like decrements; in the evening, it wuld be the lw impulsives given placeb wh wuld be mre likely t shw this susceptibility. The crucial finding was f a fur-way interactin between impulsivity, time f day, drug, and the prir stimulus effect, F(l, 121) = 4.07, MS e = 25.60, p <.05. (See Figure 2.) Inspectin f the means cnfirms the already nted effect f caffeine: At bth times f day, subjects given placeb perfrmed mre prly n the three later lists than did thse given caffeine. Mst imprtant, fr thse given placeb, a crssver interactin f impulsivity with time f day was evident: Amng thse given placeb in the mrning, the mre impulsive the subject, the lwer the recgnitin scres fr the lng and late lists (r = -.25). In the evening, the impulsivity effects fr placeb subjects reversed: At this time f day, it was the mre impulsive subjects wh had the least difficulty with the three later lists (r =. 12). Fr the subjects given caffeine, the crrelatins between impulsivity and average scres fr the lng and late lists were. 17 in the mrning and -.24 in the evening. Simple effects tests cnfirmed the reliability f the 3-way interactin amng impulsivity, time f day, and caffeine fr the later three lists, simple F{\, 121) = 4.64, MS t = 7.27, p<.05. T further clarify this interactin, scres fr each list were analyzed separately: On the first list f 24 items, recgnitin memry scres did nt vary reliably as a functin f caffeine, time f day, impulsivity, r any f their interactins. On the first list f 80 items, the effect f caffeine apprached significance, F(\, 121) = 3.18, MS C = 3.81, p <.08. On each f the last tw lists, the triple interactin f drug, impulsivity, and time f day was reliable, F(l, 121) = 3.93, MS t = 3.45, p <.05, fr the secnd 80- item list and F(l, 121) = 3.95, MS e = 3.29, p <.05, fr the secnd 24-item list. Similarly, the effect f caffeine was reliable fr each f these later tw lists, F( 1, 121) = 11.40, MS e = 3.45, p <.001, fr the secnd 80-item list, and F(l, 121) = 16.31, MS; = 3.29, p <.001, fr the secnd 24-item list. Finally, results als indicated a lwer-rder interactin between time f day and prir stimuli, F(l, 121) = 4.05, MSe = 25.60, p <.05. Althugh there was little difference n the first list f 24 items between subjects tested in the mrning (M =
340 in 20 -i KRISTEN JOAN ANDERSON AND WILLIAM REVELLE 9:00 a.m. E 0) 5 c 'c u CD 0C 19-18 - 17-16 1st 24 1st 80 2nd 80 2nd 24 List placeb; lw imp placeb; high imp - caffeine; lw imp caffeine; high imp (0 u 20-1 7:30 p.m. 19- E 0) c v^ c 18-17- placeb; lw imp placeb; high imp - caffeine; lw imp caffeine; high imp 0) DC 16 1 st 24 1 st 80 2nd 80 2nd 24 List Figure 2. Recgnitin memry scres as a functin f time f day (9:00 a.m. r 7:30 p.m.), impulsivity (imp), drug (placeb r caffeine), and list. Subjects with impulsivity scres greater than 5 were cnsidered high impulsive; thse with scres less than 4, lw impulsive. 19.30) and the evening (M = 19.15), subjects tested in the evening recgnized mre items frm the lng and late lists (M = 18.29) than did thse tested in the mrning (M = 17.95). N ther effects, main r interactive, reached standard levels f statistical significance. Other persnality effects. The crucial hyptheses tested in this experiment invlved impulsivity. As nted abve, we believe that ne reasn fr apparent incnsistencies in the available literature linking persnality t perfrmance is that many researchers fcus n extraversin r sciability rather than impulsivity. T examine this argument, we als analyzed ur data as a functin f these persnality dimensins. When sciability (as measured by the EPI subscale identified by Revelle et al., 1980; M = 7.60, SD = 2.63) replaced impulsivity as a cntinuus variable in the analysis, ne effect invlving this persnality dimensin reached standard levels f statistical significance: Sciability interacted with caffeine and the list-length effect (i.e., the cmparisn f the tw 80-item lists t thefinallist f 24 items), F( 1,121) = 4.82, MS C = 8.25, p <.05. (See Figure 3.) Fr the subjects given placeb, the crrelatin between sciability and recgnitin scres was slightly psitive:.10 fr the 80-item lists and.06 fr the secnd list f 24 items. Fr the subjects given caffeine, the crrelatin was slightly negative fr the 80-item lists (r = -.10), but psitive fr the secnd 24-item list (r =.20). With extraversin (M = 13.57, SD = 4.25) replacing impulsivity as the persnality variable included in the design, the furway interactin amng extraversin, time f day, caffeine, and prir stimuli was statistically significant, JF(1, 121) = 4.38, MS C = 25.46, p <.05. The interactin f extraversin, caffeine, and the list length effect was als significant, F(l, 121) = 4.25, M5 e = 8.39, p <.05. N ther reliable effects invlved extraversin.
IMPULSIVITY AND TIME OF DAY 341 20-i 0) 0) u CO E placeb; lw sc placeb; high sc caffeine; lw sc caffeine; high sc O U V 19-18- 17-16 1st 24 1st 80 2nd 80 List 2nd 24 Figure 3. Recgnitin memry scres as a functin f sciability (sc), drug (placeb r caffeine), and list. Subjects with sciability scres greater than 9 were cnsidered high sciable; thse with scres less than 7, lw sciable. Discussin The central purpse f this study was t clarify the relatinship between impulsivity and arusal. Althugh a stable relatinship between the trait f impulsivity and states f arusal had been rendered implausible by previus research, questins remained regarding the relatinship f this persnality dimensin t arusal states. The trait f impulsivity culd be related t stable differences in rate f change in arusal states r, alternatively, t phase differences in diurnal arusal rhythms. Our keyfindingwas a fur-way interactin amng impulsivity, time f day, drug, and prir stimuli. The pattern f means indicates that (a) regardless f time f day, subjects given placeb recgnized fewer wrds frm the 80-item lists and frm the secnd 24-item list than subjects given caffeine and (b) in the absence f caffeine, recgnitin memry fr these lng r late lists was prer the higher the impulsivity in the mrning, but better the higher the impulsivity in the evening. This crssver interactin between impulsivity and time f day cntradicts the hypthesis that impulsivity is related t stable individual differences in either basal arusal levels r in rate f change in arusal states. Instead, this interactin is cnsistent with the alternative hypthesis that impulsivity is linked t phase differences in diurnal arusal rhythms, which in turn determine ability t sustain attentin. Relative t placeb, caffeine reliably facilitated recgnitin memry fr lng and late lists, as anticipated if arusal enhances sustained attentin. Results f ur manipulatin check unexpectedly indicated that the placeb and caffeine subjects in the mrning sessin did nt differ reliably in self-reprted activatin states in the minutes immediately preceding the first memry task. Thus, it is pssible that the beneficial effect f caffeine n later trials in the mrning merely reflected an increase in the extent t which the drug had taken effect. It is wrth nting, hwever, that the effects f caffeine n recgnitin memry acrss the fur lists in the evening are similar t the effects bserved in the mrning, and data frm the evening sessin indicated large, reliable differences in self-reprted activatin bth befre and after the memry task. It is thus difficult t attribute the bserved effects f caffeine n the later lists slely t a delayed respnse t the drug during the mrning. It is als imprtant t stress that despite sme lack f clarity regarding the interactin f caffeine with the prir stimuli effect, interpretatin f the higher rder crssver interactin amng impulsivity, time f day, caffeine, and prir stimuli is nt ambiguated by the self-reprt data. Interpretatin f this higher rder interactin relies n the bserved crssver between impulsivity and time f day fr late and lng lists amng the subjects given placeb, which demnstrates that the impact f impulsivity n recgnitin errrs reverses frm mrning t evening. This pattern suggests that while engaging in a relatively mntnus task in the mrning, high impulsives are mre susceptible than lw impulsives t a reductin in arusal level ver time, but in the evening, it is the lw impulsives wh are mre susceptible t this effect. That caffeine minimized recgnitin memry errrs n late lists (when self-reprt measures suggested reliable differences between placeb and caffeine subjects) prvides cnvergent validatin f ur interpretatin f this higher rder interactin in terms f arusal. One intriguing aspect f the bserved effect f caffeine was that in bth mrning and evening sessins, it was mst beneficial t thse subjects wh, in the absence f caffeine, had the mst difficulty with the task. This utcme was nt expected.
342 KRISTEN JOAN ANDERSON AND WILLIAM REVELLE (We had predicted that caffeine wuld be assciated with fewer recgnitin errrs, but had nt predicted disrdinal interactins invlving caffeine.) We have argued elsewhere (e.g., Humphreys & Revelle, 1984; Revelle et al., 1987) that arusal interferes with shrt-term memry prcesses and have demnstrated that caffeine increases the time required t scan items in shrt-term memry within a Sternberg paradigm (Andersn, Revelle, & Lynch, 1989). Althugh we believe that the task used in this experiment reflects primarily the influence f attentinal prcesses, it is pssible that a weak detrimental effect f arusal n shrt-term memry is cunteracting its beneficial effects n attentin fr the mre arused subjects, that is, the lw-impulsive subjects given caffeine in the mrning and the high-impulsive subjects given caffeine in the evening. 4 We had expected that susceptibility t vigilance-like decrements wuld increase with bth number f previus lists and number f previus items within a list. Thus, we had predicted that perfrmance n the secnd list f 80 items wuld be wrse than that n thefirstsuch list, particularly amng subjects given placeb. Our findings were cntrary t this hypthesis. Spurius differences in list difficulty, changes in the subject's expectatins abut the task, r practice effects culd have cunteracted a detrimental impact f time n task. The design f this experiment des nt permit evaluatin f these pssibilities, althugh cmparisn with the pattern f results frm Bwyer et al. (1983), wh fund that errrs were smewhat mre cmmn n the secnd than thefirstlist f 80 items, tends t supprt the list-difficulty interpretatin. As reviewed abve, ur interpretatin f results in terms f arusal and sustained attentin draws n previus research: First, Underwd (1978) strngly suggested that recgnitin memry errrs fr supraspan lists in the paradigm we used reflect attentinal lapses during stimulus presentatin. Secnd, the parallels between perfrmance in this paradigm and in ther cntinuus-perfrmance r vigilance tasks, and cnvergences acrss a range f manipulated and bserved variables in their impact n perfrmance n such tasks, are mst parsimniusly explained by psiting a cmmn mechanism, specifically, that arusal facilitates sustained attentin (cf. Bwyer et al., 1983; Bradbent, 1971; Humphreys & Revelle, 1984). These data thus bear n the questin f the relatinship between persnality and vigilance decrements. As discussed abve, Kelega's (1992) recent review suggested at best a weak relatinship between extraversin and vigilance decrements and questined the imprtance f time f day effects. This prject jins thers (e.g., Matthews, 1987a; Revelle et al., 1980) in suggesting that time f day is a crucial mderatr f persnality effects n cgnitive tasks, with theretically meaningful arusal-related reversals ccurring between mrning and evening sessins. Mrever, as we have argued befre, impulsivity appears mre clsely assciated with arusal variatins and hence perfrmance than sciability r extraversin. Our analyses in terms f alternative persnality measures highlight these cnclusins: When we reanalyzed the data with sciability replacing impulsivity as the persnality variable, sciability was fund t interact with drug and list length. This result indicated that amng subjects given placeb, higher sciability was assciated with slightly better recgnitin memry. Amng thse given caffeine, high sciability was assciated with better recgnitin memry fr the secnd 24-item list but prer recgnitin memry fr the tw 80-item lists. Accrding t Eysenck's mdel, sciability and arusal are negatively assciated. Thefindingthat lw sciability was assciated with better perfrmance nly amng subjects given caffeine and nly n 80-item lists is incnsistent with an arusal interpretatin. Our measure f extraversin (frm the EPI) reflects bth impulsivity (r =.81) and sciability (r =.84). Thus, it is nt surprising that urfindingsinvlving extraversin captured the results f analyses based n each dimensin separately. Analyses in terms f impulsivity yielded predicted and interpretable results; analyses in terms f sciability yielded unexpected and uninterpretable results; analyses in terms f extraversin yielded a mixture f interpretable and uninterpretable findings. This pattern is cnsistent with ur argument that impulsivity is the crucial persnality variable in arusal research, and further, that reliance n measures f extraversin will yield weak r incnsistent evidence regarding underlying relatinships. That many measures f extraversin tap sciability but nt impulsivity (see, fr example, Rcklin & Revelle, 1981) raises bvius difficulties fr research in this dmain. 5 In cnfirming previus suggestins that (a) impulsivity, nt sciability, is the persnality dimensin that mediates arusalrelated perfrmance effects and (b) impulsivity is related t phase differences in diurnal arusal rhythms, the results f this prject raise serius difficulties fr several theretical interpretatins f the individual-difference dimensin f impulsivity. Mdels linking impulsivity t stable differences in basal arusal, stable differences in general rate f decay f arusal r habituatin, r stable differences in general rate f increase in arusal r arusability are all incnsistent with the bserved pattern f results. That is, any mdel psiting a temprally cnsistent relatinship between impulsivity and arusal states r between impulsivity and patterns f change in arusal states is questined by these data. Tw particular mdels deserve attentin here. First, these data bviusly cntradict ur wn previus arguments (e.g., Revelle et al., 1987; Revelle & Andersn, 1992) that impulsivity is linked t stable differences in rate f change in arusal states. Secnd, these findings discnfirm interpretatin f impulsive 4 This lgic wuld suggest that the effects f arusal might vary acrss the serial psitin curve. Althugh the experiment was nt designed t test serial psitin effects, an additinal analysis was cnducted t explre this pssibility. The 20 wrds tested frm each list were gruped int fur sublists in rder f stimulus presentatin. A Serial Psitin X Trial interactin, F(9, 1089) = 6.08, MS C = 0.36, p <.001, indicated that serial psitin effects varied frm list t list. Examinatin f the pattern f means failed t suggest a recency effect fr any list. The interactin between serial psitin, trial, impulsivity, and caffeine was als significant, F(9, 1089) = 2.15, MS C = 0.36, p <.05. The pattern f crrelatins between impulsivity and recgnitin memry scres varied acrss lists and serial psitins and was thus nt amenable t theretical interpretatin. 5 Changes in the measurement f extraversin frm inclusin f bth sciability and impulsivity cntent t emphasis nly n sciability reflects a change in Eysenck's theretical cnceptualizatin f the structure f persnality (e.g., H. J. Eysenck & Eysenck, 1985).
IMPULSIVITY AND TIME OF DAY 343 behavirs in terms f states f arusal per se, thus creating substantial difficulties fr H. J. Eysenck's (1967) theretical explanatin f impulsive behavir (cf. Revelle et al., 1987; Revelle & Andersn, 1992; Gray, 1981). Specifically, ur data suggest that temprally cnsistent patterns f impulsive behavir cannt be attributed t effrts t cmpensate fr either lw basal arusal levels r stable tendencies tward lw arusal levels. That is, if recgnitin memry in this supraspan paradigm des reflect the influence f arusal, then ur data suggest that high-impulsive subjects are (a) mre (nt less) arused than lw impulsives in the evening and (b) mre (nt less) able t sustain arusal than lw impulsives in the evening. It is thus difficult t attribute impulsive behavir, which presumably ccurs mre frequently amng high impulsives than lw impulsives n matter what the time f day, t either lw arusal r a greater rate f change in arusal. Instead, the directin f causality may be frm impulsivity t arusal states, rather than frm arusal states t impulsivity. We cannt reject the pssibility that impulsivity is related t differences in preferred arusal level, an issue that, as Gale (1981) nted, has received virtually n serius investigatin. Similarly, Gray's (1981) hypthesis that impulsivity reflects sensitivity t signals f reward was nt directly addressed by this prject. Althugh this study did nt ffer a test f these mdels f impulsivity, the evidence fr a link between impulsivity and diurnal arusal rhythms suggests that a cmplete understanding f this persnality dimensin will require sme cnsideratin f arusal states. That impulsivity is arusal related is strngly supprted by the pattern f results frm this and ther studies f cgnitive perfrmance, which have yielded predictable and replicable interactins amng impulsivity, arusal manipulatins, and task characteristics. Mrever, as discussed elsewhere (e.g., Revelle & Andersn, 1992), the higher rder interactins bserved in these studies have eliminated a variety f alternative (i.e., nnarusal related) explanatins fr the perfrmance effects. Fr example, althugh impulsivity is assciated with a preference fr speed ver accuracy, such strategic differences cannt explain the full pattern f results. (Nte that the reversal f impulsivity differences frm mrning t evening in the present study likewise argues against a stable strategic explanatin fr the bserved perfrmance effects.) Similarly, hyptheses f differential sensitivity t caffeine r f perfrmance disruptin due t mnitring f unusual states f arusal are eliminated by interactins f impulsivity with time f day. T summarize, ur data suggest that impulsivity is linked t arusal and thereby t perfrmance, but in a cmplex way: Susceptibility t lapses in attentin is a functin f arusal, with impulsivity bearing a cnsistent relatinship nt t basal arusal levels, and nt t rate f change in arusal states, but rather t phase differences in diurnal arusal rhythms. Thus, high impulsives are mre susceptible t vigilance-like decrements than lw impulsives in the mrning, but less susceptible in the evening. This interactin f impulsivity with time f day cntradicts hyptheses linking impulsivity t stable differences in basal arusal level (e.g., H. J. Eysenck, 1967) r in rate f change in arusal states (e.g., Revelle et al., 1987) and raise serius difficulties fr any mdel prpsing that individual differences in arusal states cause impulsive behavir. Instead, these findings strngly supprt an assciatin between impulsivity and phase differences in diurnal arusal rhythms. References Andersn, K. J. (1990). 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