Acta Psychologica 39 (1975), 321-328

0 North-Holland Publishing Company

SEQUENTIAL EFFECTS OF FOREPERIOD DURATION AND

CONDITIONAL PROBABILITY OF THE SIGNAL IN A CHOICE

REACTION TIME TASK*

Jesus ALEGRIA and Martine DELHAYE-REMBAUX

Universitt! libre de Bruxelles, Lab. de Psychologie expkimentale, II 7, av. Ad. Buy& 1050 Bruxelles, Belgium

Received February 1975

The aim of the present study was to determine whether the negative relationship

usually found between reaction time and foreperiod duration in the variable foreperiod

paradigm is entirely due to sequential foreperiod effects. It has been shown that when a

particular foreperiod has been preceded by a longer one on the previous trial, reaction

time is longer than when the preceding foreperiod was equal or shorter. This may be

sufficient to explain the increase in reaction time observed for short foreperiods in

variable foreperiod conditions.

The present result’s show that, when sequential effects were controlled by the

elimination of all trials where the foreperiod was shorter than the preceding one, the negative slope of the reaction time-foreperiod function diminished but did not disappear.

The results suggest an interpretation of the role of conditional probability of stimulus arrival in terms of variation in the tendency to reprepare when the moment initially

chosen for preparation appears to fall short of the moment at which the stimulus is

actually presented.

Introduction

Some of the effects found m reaction time (RT) studies where the foreperiod (FP) duration varies in an unpredictable manner from trial to trial seem to be related to the strategies used by the subjects. These consist of systematic biases in the way in which subjects choose a particular moment for which to prepare themselves on each trial. One

* This work has been carried out under the direction of Professor P. Bertelson. It has been

supported by the Belgian ‘Fonds de la Recherche Fondamentale Collective’ under contracts 6 12 and 10.152.

322 J. Alegria, M. DelhayefR T and foreperiod duration

of these effects is that the shortest foreperiods yield the longest RTs (Karlin 1959; Requin and Granjon 1969; N2itinen 1970; Stilitz 1972). This relationship between RT and FP duration will here be called the mgutiw slope. A second factor which can be interpreted in terms of strategical bias is the sequential effects of FP. It has often been reported that, when a particular FP has been preceded by a longer one on the previous trial, RT is longer than when the preceding FP (PFP) has been equal or shorter (Woodrow 1914; Karlin 1959; Zahn and Rosenthal 1966; Possamai’et al. 1973).

The negative slope can be related to the conditional probability of the signal, which increases as time elapses from the beginning of the trial. It has been supposed that the subject uses this variation in the momentary probability of delivery of the signal, such that the tendency of preparing to react increases towards the end of the trial. Some authors have provided support for this interpretation of the negative slope with studies using FP distributions where the conditional prob-

. . . ability of the stimulus supposedly does not vary in time (Naatanen 1971; Granjon et al. 1973). The method consists of an increase in the frequency of the shorter FPs and a decrease in the frequency of the longer ones. The results mainly show that the negative slope disappears under these conditions.

The sequential effects of FP have also been interpreted in terms of the subject’s strategy. It has been suggested that subjects tend to expect the arrival of the stimulus on each trial after the same FP as on the preceding trial (Karlin 1959; Drazin 196 1; Alegria 1974a). As stated above, sequential effects of FP are non-symmetrical, i.e. increases in RT are observed solely when the FPF was longer than the actual one, not when it was shorter. This has led some authors to suggest that when subjects expect the stimulus too soon, they can extend their prepara- tion until the arrival of the stimulus (Karlin 1959; Thomas 1967). This interpretation of the asymmetry is hard to reconcile with Karlin’s ( 1966) and Alegria’s (1974a) data suggesting that after reaching a peak, preparation dissipates very rapidly. A more likely explanation is that under some circumstances, subjects can produce more than one peak of preparation during a single trial, but that intervals of about one second are needed to allow this kind of performance (Alegria 1974a).

All the work cited above showing negative slopes has used experi- mental conditions where multiple preparations within each trial were possible. It is probable that under these circumstances the negative

J. Alegti, M. DeIhaye/R T and foreneriod duration 323

slope was at least partially due to the asymmetry of the sequential effects of FP (short FPs often follow longer ones). The main aim of the present study was to establish whether or not the negative slope can be wholly attributed to such sequential effects. The elimination of the negative slope obtained when increasing the frequency of short FPs is compatible with this view.

The FPs used in the present experiment were separated from each other by a sufficient time interval to allow the subject to develop a new peak of preparation when he had expected the signal too soon. Time estimation effects were eliminated by giving subjects a continuous cue to the passage of time. Requin and Granjon (1969) have shown that this method increases the negative slope. In addition, a choice RT situation was used in an attempt to control anticipatory responses. Some authors (Snodgrass 1969; Kornblum 1973) have claimed that all the effects of FP on RT can be interpreted in terms of variations of the tendency to make anticipations. Indeed all the experiments cited showing negative slopes have been made under simple RT conditions.

If the negative slope of RT is entirely due to sequential effects, then the slope should disappear when all cases where the FP is shorter than the preceding one are eliminated. If it does not, the procedure used should permit inferences concerning the role played by conditional signal probability in the tendency to produce more than one peak of preparation on a particular trial.

Method

Apparatus

The S was seated in front of an oscilloscope (Philips PM 3220) on which a spotmoved

horizontally from left to right, at a speed of 2 cm/set. One cm under and 1 cm above the spot’s trajectory there was a piece of white cardboard on which four vertical lines were marked. The

first line indicated the starting point of the course. The other three indicated the three points in time at which the stimulus could occur. The total course of the spot was 10 cm in length. The time interval between two successive points was 1.5 set so the FPs used (time intervals between

the starting point and the three points where the stimulus to react could occur) were 1.5, 3.0

and 4.5 sec. This stimulus consisted in a displacement of the spot up or down its horizontal

trajectory at the point corresponding to the FP chosen by the E. The displacement was 9 mm high and had no apparent width. Two response keys were used, one for the downward, and one

for the upward displacement. The S kept his right index finger permanently on one of the keys and his right middle finger on the other. As soon as the displacement occurred, he was to push

the appropriate key. A pressure of about 100 gr was sufficient to register the response. The

324 J. Alegrib, M. Delhaye/R T and foreperiod duration

beginning of a trial was initiated by the S himself. He activated with the left hand a starting

switch which set the spot in motion. A lamp situated 10 cm above the screen of the oscilloscope was lit at the beginning of each trial to indicate to the S that he could activate the

starting switch.

Two Tektronix units controlled the situation, a type 161 Pulse Generator and a type 162

Waveform Generator. The Waveform Generator controlled the movement of the spot on the

screen and triggered the Pulse Generator at a predetermined moment. This latter had a double

function: it switched off a relay which presented the stimulus to the subject and started an

electronic timer (Advance TC-12 Timer-Counter).

Subjects

Twelve university students participated in the experiment. They were paid a fixed rate per

session, plus bonus and penalties depending on speed and accuracy (see below).

Procedure

A trial began when the E switched on the lamp above the screen of the oscilloscope. The S

then pushed the starting switch. The spot, when it arrived just between the lines corresponding

to the FP programmed by the experimenter, jumped up or down. As soon as the S saw the

jump he pushed the corresponding key, which registered his RT. There were two conditions:

- In the Experimental Condition the stimulus occurred unpredictably after one of the three

FPs. In this condition, a block consisted of 72 trials, 24 of each FP.

- In the Control Condition, before each trial, the E informed the S after which FP the

stimulus would be presented. In this condition blocks were of 21 trials, 7 of each FP. The

two stimuli always had identical relative frequency (0.50).

Each session involved three experimental and three control blocks. Half of the Ss worked in

the Experimental-Control order, the other half in the opposite order. Each S participated in

three sessions on two consecutive days. The fist one was devoted to training and was discarded

from the analysis.

After each response, the E informed the S as to whether hrs RT had been ‘fast’ or ‘slow’,

meaning faster or slower than the mean correct RT in the Control Condition on the previous

session. When an error was made, the S was told so without any further information regarding

speed of the response. For.each fast RT the S was awarded 0.5 Belgian francs (about 0.5 new

pence). For each error he lost 2 francs. In order to stress the pay-off system a breakdown of

rewards and penalties was made immediately after each session.

Results

Mean correct RTs per condition and FP averaged over subjects and session are given in

table 1. Experimental RT is classified by FP and PFP duration. The row means in this condition

represent RT as a function of FP irrespective of PFP. The results clearly confirm the classical

observations: on the one hand, RT decreases when FP duration increases; on the other hand, RT decreases when the FP was shorter than the PFP, compared to the cases where it was equal

J. M. Delhaye/R and foreperiod duration 325

Table 1 Mean correct RT (ms) per Condition and Foreperiod. RTs in the Experimental Condition have

been classified as a function of the Foreperiod and the Preceding Foreperiod duration.

Foreperiod

duration

kc)

Control

Condition

Experimental Condition

Preceding Foreperiod duration (set)

1.5 3.0 4.5 Mean

1.5 270 283 294 302 293 3.0 271 291 288 300 293 4.5 271 269 272 273 272

or longer. Control RTs were independent of the length of the FP. It seems clear that the Ss

efficiently utilized the temporal information given by the spot.

Mean RTs per condition have been plotted as a function of FP duration in fig. 1. This figure

also includes experimental RTs with the trials where the FP was shorter than the preceding one

eliminated. It appears that when these RTs are eliminated the negative slope diminishes but

does not disappear.

0

290 -

270- .

Forepermd (WC)

Fig. 1. Mean correct RTs per condition as a function of the FP duration. Experimental Condition: open circles, Control Condition: filled circles. The dashed line represents experimen- tal RTs observed when the FP was either longer than or equal to the one at the preceding FP.

326 J. Alegria, M. DelhayefR T and foreperiod duration

Analysis of variance applied to RTs per 5, FP, and Condition (in the Experimental

Condition those trials where FP was shorter than PFP have not been considered) shows significant effects of FP (F = 3.59, df= 2,22, p < O.OS), of Conditions (F= 16.41, df = l,ll, p < 0.005) and of the FP by Conditions interaction (F= 6.00, df = 2,22, p < 0.01). The

significant interaction between Conditions and FP indicates that the negative slope remains

when sequential effects were controlled. None of the interactions between Ss and the other

factors approaches the level of significance (F = 1.53, df = 22,22 and F = 1.41, df= 11,22 for

the subjects X FP and subjects X Conditions interactions respectively).

Scheffe multiple comparisons were used to compare each of the three experimental RTs

against mean control RT. The confidence limit found at the 0.05 level was 7.9 msec. Mean

experimental RTs obtained for the 1.5 and 3.0 set FPs were both significantly different from

mean control RT. Those obtained for the 4.5 set FP were not.

The total percent of errors was about 6%. It varied between 5.5% and 7.2% with FP and

Conditions, but no systematic variations were revealed across conditions.

Discussion

The results clearly show that the negative slope relating RT and FP duration cannot be entirely explained by sequential FP effects. The strategic factors used to account for sequential effects and the negative slope seem to play independent roles in determining subject’s be- haviour.

The tendency to expect repetition of the PFP clearly appears in the present experiment, i.e. when the FP was shorter than the preceding one, RT increases. In those cases, the signal often finds the response system in an unprepared state because it was expected later. It is interesting to analyse what happens when the FP was longer than the preceding one. In those cases, following the reasoning developed to explain sequential effects of FP, the subject tends to prepare himself before the arrival of the signal. The interval separating two successive potential times of signal occurrence (1.5 set) gives him enough time to reprepare for the next potential arrival time (Alegria 1974a). If he exploits this possibility, RTs for the 3.0 set FP when presented after the 1.5 set FP would be similar to the Control RTs. The results show that mean RT under this condition was clearly longer than Control RT. This fact seems to show that any tendency to reprepare for the 3.0 set FP is weak or non-existent. The examination of the results obtained for the 4.5 set FP shows that it was, on one hand, independent of the PFP duration, and, on the other hand, as rapid as control RT. This seems to indicate that when the signal was presented at this moment, the subjects were always prepared to react. This means that when the signal

J. Alegria, h4. Delhaye/R T and foreperiod duration 321

has been initially expected at an earlier point, subjects always prepare themselves again. This tendency is quite systematic as shown by the similarity between control and experimental RT for the 4.5 set FP. It is interesting to note that, when the experimental conditions do not allow more than one preparation on each trial, RTs for the longest FP clearly depend on the PFP duration (Alegria 1974b)

The present results allow a more analytical interpretation of the role played by the conditional probability of signal presentation than that usually found in the literature. It seems to affect the tendency to develop a new peak of preparation when, on a particular trial, the subject has prepared too soon, and this in an all-or-nothing fashion. That is, the subject develops a new peak of preparation if, and only if, he is certain that his preparation will be reinforced by the presentation of the signal. The present results clearly suggest this interpretation. It would be interesting to test this hypothesis in an experiment where a more gradual evolution of the conditional probability could be examined. In the present case, this probability suddenly changes from 0.5 to 1.0 when the subject became aware that the signal had not been presented at the end of the 3.0 set FP. It is not impossible that the all-or-nothing relationship between conditional probability and the tendency to re-prepxe is due to the particular values used in this study.

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