Psychological refractory delay and response-stimulus interval duration in serial, choice-response tasks

Acta Psychologica 3 A’ttentrionand Performance PI ( W. C. Koster, ed.) 1969, 195-219 @JNorth-Holland Publishing Company, Amsterdam

PATRICK RA

1

IrtsCitute of Experimental Psychology and The Queen’s College, L’it iversit y of Oxford, Oxford, England ABSTIUCT

It has been suggeated that for some fixed period of time after making a response a human subject is unable to process urther information because his single decision chanrel is occupied with an a lysis of kinaesthetic or other LFORD, feedback, validating the successful completion of bis motor act 1952, 1459, 196”‘). On such assumptions it might be predicted that mean correct RT in serial choice tasks would vary inversely with the duration of the time-interval between the com&tion of each sequent response and the moment of onset of the subsequent signal (R-S interval duration). In three experiments performance was compared between conditions in which R-S intervals varied between 20 msec and 20 msec. The experiments also allowed examinations of the possibility of interactions between the effects of R-S interval duration and. signal and response repetition effetis; (BERTELSON, 1965; RABBITT, 1968a). Experiment 3 also examined the Fossibility of interactions between the e:Tfectsof R-S interval duration and tile effects of variations in response information load. In experiments I and 2 data were analyseld to compare delays to re!;ponse following errors with delays to responses preceded by at least two correct responses. The expected inverss relationship of mean correct RT to R-S interval duration ybvasobserved, but the’ data do not allow the interpretation that a fixed refractory delay fc;llows each sequent response. The other intcra&ons examined were not significant. The results allow identification of some complex interactions which result in slow responses following errors in serial choice tasks. 1.

INTRODUCTION

Experiments malde to estimate the duration of the ‘psychological refractory period’ have typically required Ss to make IWO’successive responses (R, and lit,) one to each of two successively presented signals (S, and S,). It has usually been found that when S, and S, are presented 1 The work described. was completed while the author was a member of scientific staff at the MXC!. Applied Psychology Research Unit, 15 Chaucer Road, Cambridge, U.K. 19s

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under cer’za.inconditions1 of close temporal proximity, the reaction time to S, (RT& is slower t&m it is on trials when S, does not occur.

Current theories relatlrz this ‘refractory delay’ to serial informationprocessing in a single, limited-capacity decision-channel (WELFORD, 1959, 1!)67). Such theor& assume that before the decision-channel can begin to analyse information from S,, it must complete at least two successi.ve transactions rlelating to the preceeding signal and response: First, during the time interval S,-R, (i.e. RT,) the decision channel is assumed to be concerned witJh the perceptual identification of S, and the motor organ&&ion of Ii,. However, delays to RT, have also been observed even when S, occurs after R, has been made. To explain this ELHO~ (1959, 1967) has suggested that during some fixed interval time: after the completion of R, the S’s decision-channel is further occu@ed by analysis of kinaesthetic (or other) feedback from the completed response. The time required for this analysis of feedback has II estimated, from various experiments, as approximately 150 msec (WEIMXD, 1967) from the moment of completion of R,. It follows that -when a subject makes a long series of responses to succeadveIy presented signals (i.e. carries out 3 serial choice RT task) he should not be able to analyse each sequent Corral until about 150 ._ msec after making a response to its predecessor. A strong test of Welford’s hypothesis is therefore a comparison between tasks in which the intrzrval bet--r-11 each response and the succeeding signal (R-S interval) is va&d within this critical limit. Apart from the introduction of some novelty into the refractoriness literature such a technique allows emperical examination of possible interactions between refractory delay and two other factors which are very difficult to control in more traditional experimental paradigms: (1) Cln the strict assumption that any given response is followed by a fixed refractory delay, the duration of this delay should vary in precise inverse relationship to the Gme-interval between R, and S,; (i.e. should ve a 45” linear regression when plotted as a function of the R-S rval). Previous failures to demonstrate satisfactory linear regressions s kind hme been attributed to the use of experimental equipment h 2.110~~5 control only of he S,-S2 interval, and which consequently nfourds estimates of the delay to R, with uncontrollable variance in from trial to trial during the course of an experiment 1966, 1967; WELFGRD,1967). Where the duration of the interval& set by the experimental equipment independemly of RT,

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estimates of the duration of the refractory delay should be much less ambiguous. This is a crucial control if it is hoped to examine interactions between duration of refractory delay and response informationload, since both mean RT and trial-to-trial variance in increase as a function of response uncertainty in choic (2) In the absence of appropriate experimental controls is has been assumed that the duration of refractory delay is con rat, irrespective of any relationship between S, and S, or between and R,. This assumption conflicts wi*th results from serial choice tasks in which reduction of R-S intervals has been shown to reduce mean correct when signals (and implicitly responses to them) are immediately repeated on successive trials. Reaction times for disjunctions between signals and responses are not affected in the same: way (BERTELSONand RENKIN,1966). While the R-S intervals examined in such studies have not systematically covered the range in which refractory delays might be predicted (0 msec: to 150 msec), it remains an open quet;tion how repetition-effects and refractoriness are to be reconciled. The most important reason for investigating refractoriness in serial tasks concerns the postulated reasons for the phenomenon rather than calibration of its parameters. Welford’s theoretical analysis assumes at after making a response the S processes feedback from it for some ~~soyt: he wiGes to check whether it was correictly executed. Recent studies have shown that Ss can, indeed, detect from 81 y21to 94 72 of the errors which they make in simple seriaI choice tasks without any source of information from experimental equipment which might help them to do so (RABBITT, 1967). However, the possibility that these errors are detected by feedback-checking is only one OS several viable hyplotheses among which available data do not allow us to choose (RABBITT, 1968b). Any relationships which may be detected between the duration of post-response refractory deIay and the efficiency of error-detection will therefore provide an obvious necessary supplement to the available information. This line of argument has an important corollary als neglected in discussions of refractory delay. A check on feedback from any given response can have two possible outcomes: a ‘match’, when the response is vaIidated as being c:orrectIy executed or a ‘mismatch’ when it is found to be wrong. Empirical data from match/mismatch experiments of other kinds (e.g. POWER and MITCHELL, 1967) and an elegant theoretical analyeis in this volume (NICKERSON, 1969) give strong grounds for

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supposing that match and mismatch judgements take unequal times to complete. There is thus aw unexplored possibility that the time required to discover that a completed response was wrong is not the same as the time required to discover that it was correct. In this case we might expect that the durations of refractory periods following errors would be different from the durations of refractory periods following correctly executed responses. For all these reasons, experiment 1 was made to examine variations in serial choice RT consequent upon variations in the durations of R-S intervals *within the range where refractory delay following responses had been observed. 2, EXPERMIZNT 1 Since pre-vious work had estimated post-response refractory delay as approximately 150 msec three conditions of R-S interval duration (20 msec, 120 msec and 220 kmsec)were compared to cover this range. 2.1. MeHwd and procedure The apparatus used (M.R.C. Applied Psychology Research Unit ‘Sparta’) &.s been pregiously described (e.g. RABBITT, 1968a, 1968b). Ss sat facing a 3’X3’ matt black display screen in the centre of which was mounted a ‘Digitron’ G.R. 10 J display tube. A punched-tape er programmed tEtis t..~ to display the ten Arabic numerals 0 to 9 one at a time in qdences of any desired structure or length. Fifteen sa*uerices, eat-h of 360 signals were made up so that equal numbers of the numerals 1,2, 3 and 4 occured in random order without constraints of any kind. Ss responded to each numeral in turn by pressing one of ‘IOr !z keys inset into a desk on which they could comfortably st hands. They answered the onset of either numeral 1 or era1 2 by pressing the key under their left forefinger, and either 1 3 or numeral 4 by pressing the key under their right fore. A 1.0 mm throw of either key made a contact which triggered events leading to the presentation of the next digit in the programmed es* Three con&ions of R-S interval duration were compared. In CODition R-S 20 a new signal was presented within 20 msec of switchclosure; (a rise-time of approximately 1.5 msec for the display-tube is ed in this estimate). In condition R-S 120 switch-closure started a timer pres~ti a sequent signal 120 msec later. In condition

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19

R-S 220 the timer presented a new signal 220 msec after switch-closure. n all conditions alike Sparta output Frovided punched-tape records ach signal presented, of the key pressed in answer to it and of the d time between the onset of the signal and the moment of switch re (i.e. RT) to within 0.01 sec. ach of six Royal Navy ratings aged from 18 to 20 years ran through all of the fifteen different sequences of 300 signals and responses, experiencing one run at each of the 3 R-S interval, durations on each of five successive weekday mornings. The order of R-S interval conditions experienced by each subject was different every morning. order of conditions was balanced across the group of subjects on each morning of practice. 2.2.

R4?sults

Sparta outpost was collected only on the last morning of practice. Compater analysis of output showed that subjects made betwtcq 1.9 70 and 3.8 $%of errors. The incidence of errors wa:s not significantly different by t-tests betwec:n experimental conditions. The computer programme was written to print out in full records of all errors and of the two correct responses following each error. Included in data anablsed were cases w e arl error was folJowed by w rror (10 cases). Cases where two correct responses and then by anoth ZI error was followed, within two responses, by anot’ller error were identified but disregarded for analysis (6 cases). A?! other correct responses were thus selparated for analysis by the programme only when they were preceeded by two or more other correct responses, or were the initial responses of an experimental run. The class of these responses was further sorted to obtain mea.n and sigmas for three possible transitions between signals and responses (cf. BERTELK, ', 196.5; RABBITT, 1968a). These were identical transitions when C:e same! signal, and so implicitly the same response, OCcurred on immediately successive trials; equivaZent transitions when a new signal occurred but t.he same response wasI made to it and. new transitions when a new signal occurred to which a new response was mme also cojmputed overall mean RT and sigma for all correct responses as; defined above, and gave a corn lete lisl:ing of all correct RTs in each tl.-ansition-class for each subject. Mean RTs and sigmas are set out in table 1.

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PATRICK

RABBKTT

TARLE1 RTs and corresponding standard deviations (0) for a two-;=hoicetask administered at three conditions of R-S interval duration; (experiment 1). Responses are broken down into three classes of transitions between signals and between responses. Cliiss of responses analysed

‘Ne+ transitions ‘EquivaLnt” transitions ‘Identical’ transitions overall mean correct RT

Response-stimulus interval duration 20 rnsefz 424 msec (a = 64 msec) 341 msec (a = 49 msec) 326 msec 4:* =I 34 msec) 381 msec (0 = 79 msec)

120 msec 394 msec (a = 72 msec) 321 msec (a = 32 msec) 296 msec (a = 42 msec) 354 msec (0 = 78 ms~)

223 msec 384 msec (a = 61 msec) 305 msec (0 = 28 msec) 294 msec (0 = 41 msec) 340 msec (0 = 75 msec)

An analysis of variance (AnoVa) was made on the overall mean corrwt mean correct RTs across Ss, showing the effects of R-S interval duration to be significant (p < 0.05). The error-term was used to te S, in order to compare mean RTs betwek:n particular R-S interval conditions. Reaction times for R-S 120 were fis;lsterthan reaction tties for R-S 20 (p < 0.05) and RTs for R-S 220 were also faster than RTs for R-S 120 ~(p < 0.05). These findings were tested by t-tests across tlhe listed correct RTs for each S at each of the three transitionclassy In all cases the same findings were duplicated (at p < 0.05 to p < 0.001). A further extended AnoVa was made over all S’s RTs broken down in terms of the three transition-classes examined. In all cases significant terms emerged for effects or R-S duration i(p < 0.01) and for differences between transition-classes (p < 0.N). The interaction-term for R-S intervals vs. transition-classes was not significant (p > 0.1). Analyses on individual S’s data broadly confirmed these findings. The interaction term for R-S intervals vs. transition-classes was significant only in the case of one subject out of six. Responses following errors were separately a.nalysed. Mean RTs for the two correct responses following each error are given in table 2, On the data for each individual S t-tests showed that the RTs for error -I-1 responses (17 < 0.01) and for error 4-2 responses (p < 0.05 to p < 0.01) twere significantly slower than for all other correct responses. Further t-tests showed that for each S, individually, the RTs

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DI IRATION

2 Results of experiment 1. Overall mean RT and corresponding standard deviations (0) for error + 1 and fDr error + 2 responses in a two-choice :: tsk. Mean correct RTs for all correct responses themselves preceeded by two or I lore correct responses are given for comparison. TABLE

Response-stimulus interval duration

R-S 20 msec

. . .

Overall mean correct RT . . .

R43120msee

. . . . .

R-S 220 msec

. . . . .

381 msec (cs = 79 msec) 354 msec (a = 78 msec) 340 msec (a = 75 msec)

Error + 1 RT

Error +-2 RT

602 msec 422 msec (a = 158 msec) (a = 95 msec) 551 msec 381 msec (0 = 143 msec) (a = 102 msec) 474 msec 354 msec (0 = 162 msec) (0 - 89 msec)

of the error i- 1 responses and of the error +2 responses declined significantly as the R-S imerval duratiorls increased from 20 msec to 120 msec and from 120 msec to 220 mse.: (p < 0.05 to p < 0.01). Because so few responses in eacil possible category were available for examinatior RTs for error -I- 1 responses and for error -I-2 responses were not fu -ther analysed in terms of transitions between specific signals and responses. ‘This, however, later emerged in a crucial variable in the detailed interpretation of these ef c:ts (cf. section 3.2). 2.3. Discussion AS the R-S kterval was increased from 20 msci to 120 msec mean

correct RT wm reduced by some 27 msec. A further reduction of some 14 msec occurred when the R-S interval was increased from 120 msec to 220 msec. There was a slight suggestion that the nature of these relationships was affected by the class of signal and response transition examined. For one S out of six *he reduction in mean correct RT was less for identical and for equivalent transitions than for new transitions. However, considering the limited scale of this experiment, the small size of the effect obtained (i.e. approximately 15 msec difkence between transition-classes) and the time-reso ution of Sparta (i.e. 3_ 10 msec) the data leave ambigucbxs the existence or nature of any relationship between the effects of signal and response transitions and the effects of R-S interval duration. The latter effects can therefore be discussed in relation to overall mean RT without taking the possibility of complicating interactions into consiideration.

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The observed changes in overall mean RT with increasing R-S interval cannot be squared with any hypothesis of a fixed refractory delay followi the completion of each response. On the hypothesis that this refractory delay was shorter than 720 msec no significant differe:nce would be found between the R-S 120 and the R-S 220 conditions. On the alternative possibility, that the refractory delay was 120 msec or longer, the difference in RT between the R-S 20 and the R-S 120 conditions should have been of the order of 100 msec rather than the observed 27 msec. The same arguments apply when error + 1 responses are considered. Error +P responses are very much slower than all other correct res~onse:~ (221 msec, 197 msec and 134 msec in the three R-S interval conditions considered). On a shnplle ypothesis that this slowing occurs because the subject is delayed by detection of his error, analysis of its nature, or by making some: response to its occurrence, we should expect the introduction of ‘dead1 time’ between the response and subsequent signal to reduce the observed slowing of error -I- 1 RT in exact proion a:s the R-S interva.t increases. This evidently does not occur. the R.-S 20 and the I~&-!$ 1120conditions the difference in error s is of the order of 51 msec, and between the -S 120 and R-S 220 conditions of &e order of 77 msec. In both cases, by t-tests for individual Ss data., the observed reduction in error + 1 RT was found to bc sign%cantly less than the corresponding (100 msec) increase in -S interval dura!tions (p < 0.05 - p (= 0.01). These data therefore agG t o%r no support to the simple form of Welford’s hypothesis, that, for some fixed time after the occurrence of a response the S cannot process ne v information lbecarmehe is concerned with validation of his motor act. since there is no clean evidence that a fixed ‘refractory delay’ follows an error the data do not allow us to speculate whether the duration of the processes undertaken when an error is detected [‘misat&’ test) differs from the duration of processes which validate the suc~~ssful completion of a response (‘match’ test on feedback). e other factor 1iLdy to contribute to slowing of error -I- 1 tentatively indentified from these results. In the R-S 20 slowing of error -II-1 respolnses relative to all other correct s is of the order of 221 msec. In the R-S 220 condition, (when ve, presumably, had 200 msez of ‘dead-time’ to recover from ts of making an error) Sis slowing is still of the order of 134 are forq& to infer tlhat some factors resulting in slowing of

TORY

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to operate until after the moment in been presented. on is suggested by exa e responses are also sig e equivalent conditions of his slowing cannot, of course, be attributed to mediately following an error s nce the error -+ vened. A possi le explanation is that, having mad to guard against the possibility of further mistakes by taking longer to respond to subsequent signals: in simplistic terms, they respond more The slowing of error + 1 responses may therefore partly and may represent an interaction. by the same tende between at least two factors ocesses concerned with the validation of the error, or with some res se to it (which may endure from 51 msec to 129 msec), and a transient change in response strategy to mini= mise the possibility of further errors. Unfortunately dearth of o servations made it impossible to adequately test whether this strategy was successful: i.e. whether the probability of an error was indeed lower during the two responses immediin the sequence of signals ately following an zrror than at other p n 3.2) there are also other and responses. As will be seen later (cf. tion of any such analysis complicating factors which make the int dubious. It was apparent from a detailed survey of errors ;9zid error -t 1 responses that the interactions nvolved were too complex for adequate analysis from the small samp of data obtained. This provideri an adn extended replication of experiment 1. ditional reason for undertaki riment 1 that there is no evidence that We mulct conclude: from e the S monitors each response hich he makes for some fixed time in order to check whether it was correct or wrong. Similar failures to detect post-response refractory delays have been re n discussin; these results WELFO ARILL I(1957). arses lthe caveat t t highly practised subjects may fin it unnecessary to monitor their responses for accuracy, and so learn to respond without post-response refractory delays. In the present experiment Ss attained levels of practice seldom discussed in the literature. An additional reason for replicating t’,lis study was therefore the analysis of data from early stages in practice, where maximum refr would be predicted o.n Welford’s hypothesis. error + 1 respo

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3. ExPER~~~PENT 2 Method and procedure were identical with those described for experiment 1 with the exception that allarger group of Ss was tested (ten Royal Navy ratings aged from I81to t23years).

On this occasion Sparta output was cohected and processed for each S’s first and for his final run in each condition of R-S interval duration, The same programmes were again used. Data from correct RTs and from RTs following errors were separately analysed as before. Mean f:orrect RTs from the first and from the fiil run in each condition, broken Idown in terms of transition-classes, are set out in table 3. TABLE 3 Mean correct RTs and corresponding standard deviations (0) in the first and final, sessions of practice on a 2-choxce serial task, at three durations of wponse stimulpls interval (experiment 2). Data are presented. separately for three classes of transitL)ns between s.ignals and responses. overall &me,an correct RTs are given for comparison, --

ResghDnse-stimulus interval duration

_--. First xssion of practice ‘New’ tbansiltions ‘Equivabnt’ transitions ‘Identical’ transitions 0ver. J correct RT Fina!~s&on of practice ‘New’ transitions ‘Equivak~.nt transitions ‘Idea&a c’transit ions

L-

R-S %Omsec

R-S 120 msec

R-S 220 msec

501 msec 505 msec (ct = 124 msec) (a = 1132msec) 450 ms&ec 451 msec (a = 14Ximsec)(a= 111 msec) 359 msec 376 msec (G = 112 msec) (Q= 134 msec) 458 msec 460 msec (a = I138msec) (a = 142 msec)

(a

432 msec (0 = 51 msec) 352 mwc (a = 42 msec) 31.8msec (0 = 34 msec) 385 msec (a = 62msec)

394 msec !a = 72 msec) 317 msec (a = 68 msec) 289 msec (a = 29 msec) 349 m!Nx (a= 78msec)

405 msec (CI= 62 msec) 320 msec (CJ= 50 msec) 301 msec (B = 58 msec) 360 msec (:a= 69msec)

(a (a (a

491 msec = 140 msec) 457 msec = 151 msec) 358 msec = 127 msec) 450 msec = 161 msec)

rate analyses were made for each level of practice. For the first practice runs an AnoVa Iwas made on the RTs broken down in terms

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of transitions between signals and re significant effect of ss emeqed (p < 0.01) effects of R-S i duration were not significant (p > 0.1). for the final practice runs were similarly analysed. cant terms for t itions-class (p < 0.0 1) and for duration (p < O.Ol),, e interaction term between thes ificant (p > 0.1). Thus in spite of t stion that the decline correct RT may be slightly less ntical or eq trivalent transitions than for new transitions (table 3) we can ignore t.he possibility of this co lplication in the present discuss Ss made 3.4% errors in the R-S 20 condition, 4.2% of urors in the R-S 120 condition and 3.9% of errors in rhe R-S 220 condition. etween conditions were not significant by AnoVa (p > 0.1). s for the two correct res onses following each error are set separaltely for tk fir2 and for the final practice runs in each condition of R-S interval duration. An AnoVa across Ss for the first practice run gave a significant effect of response-class. (i.e. error + 1 vs. error + 2; p < 0.01). term for R-S interval duration was not significant (p > 0.1). Both error --I-1 responses (p < 0.01) and error -I-2 responses (p < 0.01) were significantly slower when compared by t-test against mean correct for all other responses. Data from the final practice run were nalysed in the same way. this occassion AnoaJa gave a significant effect of response-class (p < 0.01) and of R-S interval duration (p < O.OIje The interaction term between these effects was also significant (p > 0.05). We must infer that the reduction in RT with increasing R-S interval duration is significantly greater for error -I- 1 responses t.han for error -t-2 responses. An additional analysis was made to compare the absolute magnitudes of the slowing of error -I- 1 responses relative to mean overall correct RTs in the first and in the final practice runs (i.e. the difference scores mean error -I- 1 RT - mean overall correct RT were obtained for each subject, in each of tlx two sessions at each R-S interval duration and compared by t-tests). At each R-S interval condition, considered separately, these difference-scores were significantly larger during the first than during the final practice run. Slowing of error -t- 1 RT relative to overall mean correct IRT thus appears to bc reduced by practice.

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PATRICK RABBITT TABLJ!4

Mean RT and corresponding standard deviations (a) for the first and far the second wrrect responses immediately following errors in a 2-choice serial task (data from experiment 2). Reaction times for all other correct responses ar*egiven for comparison+ Data are separately analysed for the first,, and for the liftlh morning of practice. Data from tit

session of practice

Overall mean correct RT

Response-stimulus interval duration R-S 20 msec

......

R-S120msec

.

R-S 220 msec

......

. . . . .

Error + 1 responses

Error +2 responses

791msec 543 msec 458msec (0 = 138 msec) (o = 211 rnsec) (a = 150 msec) 871 msec 539 msec 460 msec (0 = 142 msec) (a = 240 msec) (a = 163 msec) 763 msec 40 msec 661 msec (0 -’ 161 mszc) (a = 232 msec) (a = 172 msec)

Data from &al session of practice Response-stimulus interval duration R-S 20 msec

. . o . . .

R-S 120 msec

. . . . I .

R-S 220 msec

. . . . . .

3.2.

0ve rail mean correct RT 318 msec (a = 34 msec) 310 msec (a = 58 msec) 289 msec (oi = 29 msec)

Error + 1 responses

Error +2 responses

469 msec 679 msec (a = 160 msec) (0 = 114 msec) 419 msec 620 msec (a = 158 msec) (CT= 98 msec) 554 msec 3910msec (a = 149 msec) (a = 79 rnsec)

Discussion

Early in practice variations in R-S interval duration have no apparent effect on overall mean correct RT ‘or upon RT for error -I- 1 and for error 4-2 responses. This finding makes it unlikely that the conclusions of experiment 1. can be d.ismissed on the grounds that they refer only to a level of practice at \F&&zhSs may have learned to dispense with monitoring of feedback from each response which they make. A more likely explanation of the d&appearance of this effect early in practice is that the effects of R-S ir rval duration are never very large a~qdthat, in the first practice runs, they are swamped by order effects in the data. Analyses were made in an attempt to detect and partial out such effects, but these were not successful. The possibility that the effects exist, and affect the data, nevertheless remains.

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the h;rpothes?s of a he difference in maw terval conditions ws

interval O_< 0.01

difference in error -t-

between the R-S

R-S 220 conditions are (approximately 63 msec as against 100 msec p < 0.01 by Gtest). It is again appa rf%t, that the slowing of error + 1 responses must interpreted as partly due to the operation of factors i(perhaps increased caution among t!rem) which only come into operation after the presentation of the error -I-1 signal. A close inspection ~4% all error + 1 responses recorded in experiment 2 made it obvious that error + 1 RT was strongly affected by another factor which could not be analysed bet se of the limited number of observations possible. It will be recalled that Sparta cannot distinguish errors from correct responses. Thus, whether a S makes an error or not, the next signal in the programme is automatically triggered by his response. From the S’s point of view this signal might fall into two categories: it might require him to make the response which he should have made to the preceding signal (i.e. ‘correct’ his error) or it might require him to make the alternative response (i.e. ‘repeat’ the response wihich had been wrong on the preceding trial). When sequences of the former kinc? (“correct error’) were examined, it was evident that error -I- 1 responses might be very fast indeed (occassionally of the order of 150 msec to 250 msec that is, iaster than mean correct RT. In contrast, for the second type of sequence (‘repeat wrong response’) RTs were always very slow (e.g. 600 msec to 900 msec), Since there was considerable imbalance in the trance of such sequences between subjects and between R-S conditions a relatively crude analysis of error -l- 1 RI’s in terms of overall means was all that could be attempted on these data.

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greatly increased variance attributable ao use operation of this sequential effect may, however, be seen in table 2 and in table 4. The appearance of this ef1Eec.tis pre
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load of serial choice-res onse tasks interact with variations in interval duration. As has been pointed out, the resent technique is rticularly apt for this p rpose, since ciifferences in mean to-trial riation in consequent upon changes in info confound es tes of tne delays observed as w 1alone is under experimental control. xperiments 3a and 3b were accordingly made 0 pursue this question. 4.

XPERIMENT

3

his experiment was esigned to compare variations in interval duration between conditions in which the subjects selected between 2 and between 4 responses to a constant set of 8 signals. ethod and procedure Both conditions of this experiment shared a common series of 200 ammes of 300 signals each. These were random sequences of the numerals 1 to 8. In the 2 response condition subjects responded with the left forefinger to digits 1, 2, 3 and 4 and with the right forefinger to digits 5,6, 7 and 8; (2R /8S conditio In the comparison condition Ss respo with the left second-finger to numerals 1 and 2, with the left foref to numerals 3 and 4, with the right forefinger to numerals 5 and 6 and with the right second-finger to numerals 7 and 8; (4R/8S condition). On each of 5 weekday mornings subjects experienced two different runs in each of these conditions. During one of these runs the R-S interval was 20 msec and during the other it was 220 msec. The order of conditions, and of R-S interval durations across conditions was balanced, as far as possible across subjects. Ten Royal Navy ratings aged from 17 to 22 years served as Ss in this experiment.

4.1.

4.2, Results Experiments 1 and 2 had shown no interaction between response transition-class and the effects of R-S interval duration. he data were not broken down to further examine this question. Ss made 3.2% of errors in the 2R/8S condition and 4.9% of errors in the 4R/8S condition (significzint by t-test across subjects at p < 0.05). The incidence of errors in the R-S 20 and in the conditions were sot significantly different at either ccildition of re-

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210

sponse-choice b(pl> 0.1). Errors, and the two correct responses following each error weTIenot analysed. Mean overall correct RT for all responses preceded by at least two other correct responses were computed for each condition at each R-S interval duraticsn. Thes.e data are set aut in table:5. T’ABLE 5

EfWs of response-choice and R-S interval duration on mean correct RT and on mrresponding standard deviation (a) in serial choice t’asks in which subjects selected between 2 or among 4 responses to 8 signals (Arabic digits). Condition of response-choice

2Rl8S condition

. .

4R/B condition

. . . . . .

. . . .

R-S interval duration R-S interval 20 msec 456 msec (a = 59 msec) 631 msec (0 = 98 msec)

R-S interval 220 msec 422 msec (a = 65 msec) 579 msec (a = 85 msec)

An AnoVa for mean correct RTs across all subjects showed significant differences between conditions (p < 0.01) and between R-S interval durations (12< 0.01). The interaction-term between these effects was not significant (p > 0.1). Thus the experiment again confirms that overall mean RT decreases as the R-S interval duration increases, but we cannot state that the magnitude of this decrease (approximately 34 msec 51 the 2RJ8S condition and approximately 52 msec in the 4R /$S con&ion) differs between the conditions of Iresponse-choice compared. 5. EXPERIMENT 3b

This experiment was made to compare tlhe effects of R-S interval duration between conditio:hs in livhich subjects selected between 2 and between 8 different responses. Because the nature of the experiment required very exten.ded practice only four Ss were tested. 5.1. Method

larui

procedure

Thirty prog-amnes of signals; and responses, including those made up for the previous experiment presenki ,random sequences of 300 digits (1 to 8). Qne condition wa!s a replication of the 2R/ 8s condition descri for experiment 3a. Tl.le remaining condition was an 8R/SS task in which subjects responded to each of the digits 1 to 8 with a

REFRACTORY

DELAY AND R-S INTERVAL

DURATION

211

different finger ( ing 1 to 8 from left to right across hands from left little-finger to rig,ht We-finger). ach condition of respons+choice was experienced alt three co -S interval duration (i.e. three in each coninterval duration, ch of 5 weekday mornings. The order of conditions mornings. Ss were 4 1 Navy ratings aged 18 and 19 years.

5.2.

exults

nly the final practice runs in each condition were analysed as ribed for experiment 3a. Data for each individual subject are set

TABLE 6

Reaction times of four individual Ss as a funr:ion of reqonse stimuiuts duration. Comparison between conditions in which xbjects selected oetween 2 and among 8 different responses to a set of S possible signals (Ar,\bic numerals). -2 response / 8 signal condition Response-stimulus interval duration Subject 1 . . . . . Subject 2 _ . . . , Subject 3 . . . . . Subject4 . . . . .

R-S 120 msec

R-S 220 msec

329 msec 355 msec 321 msec

348 msec 371 msec 324 msec

348 msec 364 msec 309 msec

R-S 20 msec 705 msec 624 msec 641 msec 602 msec

R-S 120 msec 654 msec 609 msec 639 msec 612 msec

R-S 220 msec 652 msec 550 msec 587 msec 609 msec

. . . .

. .

. . . . . .

,

Response-stimulus interval durakn Subject 1 . . . . . . . . Subject 2 . . . . . . . Subject 3 . . . . . . . . Subject4 II.......

I

l

A separate AnoVa was made on each S’s data for each condition. In the 2R/8S condition no S showed any significant effect of R-S interval duration on his mean correct RT. In the 8R/8S condition Ss 1, 2

212

PATRICK RABBlTT

and 3 showed significant effects of R-S interval. duration upon overall

mean RT (S 1, p < 0.05; S 2, p < 0.01and S 3,~ < 0.01). S 4 did not show this effect (p >> 0.1) Since significant order effects for R-S interva,l duration conditions were observed for all subject!; fp < 0.05)a possible interpretation of these results is that small variations in RT between R-S interval conitions were masked by this source of variance. A corrollary t:o this interpretation would be that the variations in mean RT with R-S interval duration are lariger in the 8K /8S condition than in the 2R/8S condition, and so are evident from the mean RTs only in this former conion, However, in no case did An&as reveal significant interactions n the effects of response-choice and the e:ffects of R-S interval duration.

The results of e:xperiment 3a do not demonstrate that the magnitude of the variation in overall mean RT with 13-S inte:rval duration is affeeti by the information load of the task which the !i has to perform. There is, howeve.r, a slight trend in this direction. In experiment 3b there is again a wggestion that the e:ffects of R-S interval duration are more marked in a
REFRACT0Q.Y DELAY AYD R-S INTERVAL

DURATION

213

ossible that the effects of -S interval duration not only require some practice before they begin to appear (experiment 2) that they may Once again dwindle and vanish as practice is extended y valid corn -S interval effects etween conditions of different information-load (i.e. tasks of di ent difficulty) therefore requires that the poin of emergence, the asy ote and the hypothetical ishing point of this effect are available for comparison. course, very unlikely that these practice effects will follow precisely the same course for tasks of different difficulty. he selection of sing points along the bractice-curve is therefore likely (as rhaps in the present experiments) to lead to ambiguous conclusions. of better data it can only be said that if the effects duration and the e fects of response information-load interact, the conis likely to tribution of this interaction to the observed variance in be *verysmal . If it is to be detected, the use of apparatus with a timeresolution better thlan 0.01 set seems desirable. The main effect of R-S interval duration is, in contrast, well within ths time resolution the equipment used (t_ 10 msec). The observed differences between Ts for R-S 20 and R-S 220 conditions were as follows: experiment 1, 41 msec; experi 2, 36 msecp experiment ?a, xperiment 3b no variation 34 msec (2R/8S)
214

PATRICK RABBITT

%~ervals experienced during one or another run. No direct test has made of this specific hypothesis, but the relationship between e-uncertainty effects and R-S interval effects has been examined in another context: In collaboration with other workers2 the author has been able to preset Ss with runs of signals during which R-S intervals were systematically varied according to various statistical paradigms. It has so far ome apparent that the size of R-S interval effects is greater when they are statistically varied than when they are constant, as in the present studies. However, no suggestion has been found that Ss are a& to make use of heavy bias to lone or another R-S interval, nor that 3% transitions between une ual R-S intervals have any systematic effzcts. A somewhat different line of explanation derives from consideration of the slowing observed in simple RT +asks when the duration of the tween a warning-signal and a signal to respond is reduced msec (BOTWINNXX, 1965; BERTELSON and TNEYRE 1968). en fore-period durations are kept constant, Bertelson and Tissevre find increases in simple R.T of the order of 25 msec between re-period durations of 200 msec and 20 msec (BERTELWN and TISSEPRE, 2968; fig 1, p. 298). The: parameters of this effect are therefore strikingly similar to those obiserved in the present experiments. Also relevant to such an analogy is a comparison between the present nd the results of ‘refractoriness’ e,uperiments in which delays to have been observed consequent upon the reduction of the i%:rval :n a S, to which the 5’ does not respond (i.e. logically-spe&ing, a fore-signal) aud an S, to which a response ia made (RRAISSE, 1957; IS, 1.959).The delays observed are again similar to those shown in e present experiments, v here other parameters ar? comparable. For IS (1959) presents data for two Ss from separate tasks al fore-signal is followed by a visual signal to respond visuai task) and in which a visual fore-signal is followed a~ auditory signal to respond (visual/auditory task). Co i of fore-signal durations of 50 msec and 200 msec, as th rameters employed in the present experiments9 Davis’ Ss timately 78 msec ([C) and 77 nsec (D) slowing in the Ivisual task and approximately 59 msec (C) and 56 msB= (D) bison, Max Syrtle:and Neville Moray, u&ng the Elli& 903 partment of Psych~otogy, Universityof Sheffield.

REFRACTORY

DELAY A.ND R-S INTERvAL

DURATION

215

slowing in the visual auditory task ( AVIS, 1957; table 1: table 2, p. 214). he striking coincidences between the parameters of effects obtaine in these three different kinds of experiments makes it likely that the further pursuit of this an gy offers the best hope for future etation of the effects of S interval duration in serial choice ble that R-S interval effects relate more to the timeeparation for successive signals and responses than to any process of response-monitoring or ‘refractory delay’. The effects of R-S interval duration upora s of responses immediately following errors cannot be discussed in the same terms, As already stated in section 3.2 the slowing of responses following errors appears to be mainly affected by the ype of response required of the whether or not this response is the one which he should have m the preceding trial). Another factor in the relative slowing of error +l and of error -f-2 responses appears to be a change in the strategy of responding following recognition of the occurrence of an error. detecting his error the S appears to begin to respond more slowly, (and hence, by extrapolation from recent theoretical statements perhaps more ‘cautiously’; viz. J,AMING, 1968; WOLFENDALE, 1967). There is considerable evidence that in serial choice-r-es TT, 1966; LAMING, tasks are unusually fast responses; I(. PEW, this volume). There is also evidt t the probability that any given response will be an error is an inverse function of its speed (SCHOUTEN and BEKKER, 1967; PEW, this volume). This introduces the intriguing suggestion that a S may operate in a serial choice task by ‘tracking’ between upper limits of RT where he may be accurate (but may be penalised for going slowly) and lower limits of T where the probability of errors may be higher than the experimenter tolerates, On such a model the only source of information which a S has about the limits of his performance in a serial choice task may well commission of e:rrors and their subsequent detection (with subsequent avoidance of the lower RT limit identified as ‘risky’). When faced with the usual (highly ambiguous!) experimental instruction ‘Respond as fast as you can and make as few errors as possible’ a S therefore canno/: possibly comply unless he makes errors in order to discover just how fast he can re”i&. On such assumptions a very rational resbonse to the commission and detection of an error would be slowing of subsequent responses in order to regain a. ‘safe’ strategy of respondi

PATRICK RABBITT

216

The preFent data do little more than raise these possibilities fo- 3 ,ure iririririrtigation.These experiments are best considered as initiat vahdatib9s elf a technique which (allows insNightinto a range of factors which evidentIy contribute to the variance in N’s observed in choice-response tasks. An incidental benefit is &at no model which attempts to explain “refectory delays” in such tasks solely in terms of the operation of a singIe factor or process (such as monitoring of response f#eedback) need any longer be seriously conside:red. hXNOWLEDGEMENT

It is a pleasure to acknowledge that without programmer contributed s. Diary Monroe and Mrs. Patricia Altham, of M.R.C. Applied hology Research Unit, Cambtiidge, and without the other facilities these experiments of that organisation (where this work was don would have been impossibLe.

BERTELSON, B., __I_-

1%5. Serial &nice reaction-time as a function of response versus signal-and-response repetition. Nature, 206, 217-218. 1366. Central intermittenqr twenty years later. Quart. J. exp. Psychol. 18,153-163. 19G. The reflX&i;)i ~,ri XI of choice reactions with regular and irr<:ular interstimuil. mtervals. In: Attention anld performance, A. F. ?Ianders (ed.), Acta Psychol ?7,45-56. and A. REMZ~J,1966. Reaction times to new versus repeated signals in a serial task as a function of response-signaL time interval. Acta Psychol. 25, 132-l 36. and P, ?%SEYRE, 1968. The time-course of pre,paration with regular and irregular foreperiods. Quart. J. exp. Psycho!. 20, 297-300. imc:% J., 1%5. Theories of antecedent conditions of speed of response. In: Al T. Welford and J. IZ. Birren (eds), Behaviour, aging and the nervous system. Springfield, III.: Charles C. Trlt.)mlas,67-87. A-W, Ii?.. 1956. Ihe limits elf the ‘psy&ologicatl refractory’ period. Quart. J. exp. I?a;grcbol.8, H-38. 1959. The role of ‘attention’ ii the psychological refractory period. Quart- J. exp. Psydlol. l&211-220. ZSSE,P., 1957. La pkiode rkfractaire psychologique. An.& psychol. 57, 9

--

9

-

-II

-__

_-we

b.1

9

E. T., 1956. Time uncertainty in simple reaction time. J. exp. Psychol.

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AND R-S INTERVAL

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LAMING, D. R. J., 1968. Information

theory of choice-reaction time. London: Academic Press. MWLL, T., 1957. The psychological refractory phase. Brit. J. Psychol. 93-97. NKKERSON, R. S., 1968. ‘Same’-‘d ifferent’ response times: A model and a preliminary test. This volume, 257-275. POSNER,M. I. and R. F. TCHELL, 1967. Chronometric analysis of classification. Psychol. Rev. 392-409. I.968. The speed-accuracy operating characterititic. ‘,“his volume, 16. RABBITT, P. IU. A., 1966. Errors and error correction in choice-response tasks. J. exp. Psychol. 71, 264-272. -, 1967. Time to detect errors as a function of factors affecting choiceresponse time. n: Attention and performance, A. F. Sanders (ed.), Acta Psychol. responses in a serial-choice s 1968a. Three kinds of error-signalling ta&. Quart. J. exp. Psychol. 20, 179-188. effects and signal classification strategies 9 1968b. Repetition gies in serial choice-response tasks. Quart. J, exp Psychol. 24r3. SCHOUTEN,J. F. and J. A. M. BEKKER, 1967. Reaction time and accuracy. In: Attention and performance, A. F. Sanders (ed.), Acta Psychol. 27, 143-153. WELFORD, A. T., 1952. The ‘Psychological refractory period’ and the timing of high-speed performance. A review and a theory. Brit. J. Psychol. 2-l 9. decision mechanism 1imit * of a single9 1959. Evidence k. Quart. J. exp. Psychol. performance in a serial reac 193-210. operation in the brain. In: Attention and perI 1967. Single-channel formance, A. F. Sanders (eo.), Acta Psychol. 27, 5-22. WOLFENDALE,G. L., 1967. Decision times in signal detection. In: Attention and performauce, A. F. Sanders (ed.), Acta Psychol= 27, 154-159.

DISCUSSION

Falmugrte: This question

is related

to the increase

of tht: mean reaction

time

after an error. It might be of interest to analyse the data in more detail. There is evidence that the probability of an error is not constant in the course of successive trials and is highly dependent on the type of sequence that you have. YOU might select difficult sequences and then analyse whether a long reaction time is due to that type of sequence or to an error made. Rabbirt: Your argument,

in its simplest

terms, would be that errors may occur

for a variety of reasons, and occasionally perhaps reflect transitional properties within a sequence of signals and responses. In this case one might get different post-error

effects depending

on the nature

of a particular

error. Recently

I haIre

218

PATRICK RhBBITT

been examining 2: ta& in which I hive been able to v;in stimuli

di~tilinability. Here, because of variations in the number of errors between conditions of di&minability one: can pick out those errors which are attributable to systematic confusiom between srimuG, and those which reflect other effects. In this task there is no evidence that people can correct errors specifically due to perceptual confusion!3, Iwhile they can, apparently, correct some errors of other kinds. IIowever, as far as my cl.assification of errors allows, have not yet been able to find any systematic difference in post-error effects. Thii task, it sh.ould be said, is a discrete rather than a continuous response task. an admission of :rour point for the present experiment lies in data which I have c~~~#~lly conmlle:d from you. The magnitude of the post-error increase in RT turns out to be contingent on the properties of the particular sequence of signals and ‘responses involved. With the apparatus which I used, when you present. a sign:31 to the subject, and he responds to it, a new si whether this response IS correct or not. When the subject responds orvronglyto a signal, arid the next sighal requires a res)ponse which is, in effect,, the appropriate response to t;!le previous signal, (i.e. the response which the subject should have made) you get almost no post-error increase in RT. In the caise where a new signal requires a response which is different from that which he should have made, you get a much-enhanced effect. I hold this up as further evidence that error-cotrection is faster and more efficient when you are allowed to correct an error by making the response wbilch you :should have made (cf. &iBBI’TT, P. M. A., 1968 Three kinds of error-sfgnalling responses in a serial choice task’. Quart. J. exp. Psycbol. 20, 179488). In other words, I hold this up as evidence tbqt you correct an error lbecause you know ,what you ought to have done. I beJzome increasingly convinced that you either know exactly what you should have clone, or you do not know that you have made an error. Thus you have complete, but a~, far as I lcnow, never partial, irlformatlon about the accuracy of a response which you have made. Knlmagr;re: You observation of very short corrc&on later&s seems to me a very important result. It could favour the idea that the greaentation of a stimulus would evoke lmore than one response at a time. If you do not allow correction reqxmse~ them the appearance of a response might inhibit the other responses. If you allow the subject t.o make correction responses, what you would expect is that there are two responses that appear almost at the same time. Rabhitt: You are suggestiiig, in fact, that there may be two or more evidence accumulators, which accumulate Q’r+dencein ,.r;rrallel, and which are plugged in to an equivalent number of different response-programming units. Each of these accumulators might have jits own, Cvdependent, criterion of adequacy of evidence. This is a feasible system, and I know OHnothing in my work to contradict such a possibility. Another possibility is sequential processing. I think that I have got some evidence: for it. Instead of several processes in parallel one might assume a single one accumulating evidence over time. At any given point in time you may apply a criterion of adequacy of e:vidence, and emit a reqpnse based on the state of the procesr at that moment. Although a particular criterion, momen-

CTORY DELAY AND R-S INTERVAL

DURATION

2

tarily applied, may be satisfied, it is a possiVe assumption that accumulation of evidence proceeds beyond the moment in time when the response is made, fore, at some later p&t in time, you may have better evidence upon which zlt sponse may, perha s be based, I think that I have some evidencs of this type. at is the probability of an error on the-error + l-response3 Rczbbitt:

is higher than normal because the subject tends to make a correction 0 the preceding error.

It

h;at would you consider when the stimulus was switched off before the response was made? This would help to decide whether the error correction is on the basis of a parallel calculation on the original sample compared with a new sample taken of the stimulus he has in front. R&bitt: There is an additional constraint we have also done, i.e. that the stimulus when it appears is not only switched of but is succeeded by a masking petual confusion errors you will get a reduction in the field. In the case of percentage of corrections. You will also find an increase in false positives. i.e. that he signals errors which he has not committed. This seems to be due to a. sort of serial recomputation.