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Australian Journal of Psychology, Vol 18, No. 3, 1966, pp.255-261
Reaction_Time66.doc
REACTION TIME AS AN INDEX OF MASKING
AND THE EFFECT OF CHECK TRIALS ON
THOUGHTFUL SUBJECTS1
J. J. FUREDY
Indiana University
Reaction time (RT) of 28 Ss to a tone preceded by an air-puff was measured for
40 trials. Absence v. presence of puff-tone overlap and short (410 msec.) v. long
(620 msec.) intervals between puff and tone onsets were varied orthogonallv
within Ss and randomly over the trial series, which also included 6 check
(toneless puff) trials in order to discourage anticipations. RT was significantly
faster with no overlap, indicating a differential masking effect of the tone by the
puff. Additional significant effects were interpreted as indicating that the
introduction of check trials complicated the short-long interval comparisons
through some “thoughtful” Ss recognizing and acting on certain differential
probabilities in the situation.
Reaction time (RT), because of its
sensitivity to differences in stimulus
intensity (Woodworth & Schlosberg, 1955,
p. 19), seems to provide a useful index of
masking, an example of which occurs
when a tone is preceded by a noisy puff of
air. The experiment to be reported sought
to establish whether any differences in
degree of masking would emerge in this
situation, if the following two-level factors
were orthogonally varied within subjects
(Ss): (a) whether or not the puff overlapped
the tone onset, and (b) the interval between
the onsets of the puff and the tone. The
problem and the specific conditions here
used arose from a study in classical
appetitive conditioning (Furedy, 1965) in
which the unconditioned stimulus (UCS)
preceded the conditioned stimulus (CS). A
possible source of confounding in this arrangement was that a multiple-pulse
1
The author is grateful to R. A. Champion,
K. T. Ng, and I. Gormezano for their help at
various stages of this report, but hastens to add
that they are unlikely to approve of the final
version.
air-puff UCS may have had a differential
(greater) masking effect on the tone CS
than did a single-pulse UCS. In using RT
to investigate this possibility, anticipation
of the tone on the basis of the puff was discouraged partly by instructions and partly
by introducing check trials (puff without
tone) into the trial series. It will be argued
below that the check trials complicated the
experiment through their effect on the RT
of Ss who were “thoughtful” enough to
recognize, and skilful enough to act on, a
state of differential probabilities which was
introduced into the situation by the
addition of these trials.
METHOD
Subjects
The Ss were 29 students from an
introductory psychology course at the
University of Sydney.
Apparatus
The S was seated in a sound-reduced
room in which the temperature was kept at
approximately
256
J .J. Furedy
850 F, and which was separate from the
room containing E and the main apparatus.
The tone, delivered through earphones,
was 2000 cps, 83 db spl, the 100-msec
duration being controlled by an electronic
timer.
To allow presentation of the air-puff on
the forehead, S wore an aluminum band
lined with sponge rubber around the head,
with an adjustable strap at the back. The
air was blown through two rubber tubes
(1/8 in. internal and 3/8 in. external
diameter) attached to the sides of the
headband near S’s right and left temples.
The open ends of the tubes were
approximately ^ in. from S’s head, and
were placed so as to blow the air from two
sides onto a circular area of the forehead
of approximately I-in. diameter. The
centre of this area was located at the
middle of the forehead about 1 in. above
the eyebrows.
The air-puff originated in the air system
at a pressure of 15 lbs/sq. in. The onset
and offset of the puff were controlled by
an AC solenoid valve, in turn controlled
by electronic timers which allowed the
delivery of either the single 300-msec.
pulse (S) or the double 300-msec. pulses
separated by the 200-msec. interval (D).
Between the solenoid valve and the outlets
at S’s head the air passed through a series
of connected pipes of approximately 28 ft.
(to left outlet) or 29 ft. (to right outlet)
total length. There was an 80-msec.
interval between the onset of the timer and
the arrival of the air at S’s forehead, a
delay caused by the mechanical lag of the
solenoid operation (40 msec.) and the time
taken for the
air to travel from the solenoid to S (40
msec). Hence the puff-tone intervals of joo
msec, and 700 msec, set on the timer
resulted in actual intervals of 420 msec,
and 620 msec, respectively between the
arrival of the air at S and tone onset (Fig.
1). With both intervals, however, the S
puff always (no overlap) and the D puff
never (overlap) terminated before tone
onset.
At S’s right hand there was a flat
rectangular key (5 in. x 2 in.), the
depression of which stopped a clock
which had been started by tone onset. RT
to the tone was measured to the nearest 10
msec, but since the clock stopped 1 sec.
after tone onset, the maximum measurable
RT was 1000 msec.
Procedure
The four conditions representing the
combinations of the overlap and interval
factors were each presented 10 times to all
Ss in a series of 40 trials made up of 10
blocks (I...X), where each condition
occurred once and only once in each
block. Within these restrictions the order
of conditions was randomized by selecting
10 sequences at random from the 24
possible ways of ordering four unspecified
but different events: I, J, K, and L. This
basic order was: JKIL; LKJI; IJKL; LIKJ;
ILJK; IKLJ; JILK; KLJI; KLIJ; JLKI. To
counterbalance for ordinal position within
trials, four orders were generated from the
above basic order by specifying I, J, K,
and L in terms of the four conditions used
in the experiment. The four orders were
presented to four equal groups of (7) Ss, an
arrangement which allowed any of
257
the four conditions to be presented once
and only once on any trial (1-40) within
each set of 4 Ss.
To minimize anticipations three single
(S) and three double (D) check trials (no
tone) were presented. Random numbers
were used both to place these check trials
within the trial series and to determine
their nature; the 7th, 9th, 15th, 29th, 31st,
and 36th trials
coming. Sometimes there will only be one
puff; at other times there will be two puffs,
so that the tone will come on while you are
still receiving the puff. I have said that the
puff onset will precede the tone by about
half a second, but this interval will vary
from trial to trial. So, although you should
use the puff onset to get ready for the tone,
make sure that you do not try to predict or
anticipate the tone before it actually comes
on.
FIG. 1. Durations of and intervals between puff and
tone under the four combinations of the overlap
(single v. double puff) and interval (430 v. 620
msec.) factors. (Numbers give time in msec.)
were followed by check S, S, D, S, D, and
D trials respectively. The instructions to all
Ss were:
In this experiment your task will be to
press this key as soon as you hear the brief
tone through the headphones. This tone,
which will be quite brief, sounds
something like the time-pips you hear on
the wireless. About half a second before
the tone, you will receive a cool puff of air
on your brow to warn you that the tone is
Make sure that you press only after you
have actually heard the tone. To ensure
that you do not press before the tone comes
on, I shall sometimes present the puff or
puffs without presenting the tone, so make
sure you press only when you have heard
the tone itself. Once you have heard it,
however, you should press the key as
quickly as you can. Finally, when you
press the key, hold it down for a couple of
seconds, like this, before releasing it.
258
J .J. Furedy
The tone was taken to have been
anticipated on non-check trials if the time
on the clock was less than ioo msec, when
S was warned not to anticipate, his
apologies were accepted, and a reading of
1000 msec, (maximum measurable RT)
was re-
RESULTS
Fig. 2 shows the mean performance of
a8 Ss under the four conditions over the
whole period of training, with mean
reciprocal RT (response speed) plotted on
the ordinate.
Since
performance
appeared
FIG. 2. Mean speed of key-pressing to a tone by 38
Ss over tea blocks of trials under the four
combinations of the overlap (single v. double puff)
and interval (4ao v. 610 msec.) factors. (A block
contains each of the four combinations of trials.)
corded. Except for one S, who was
eliminated for anticipating on the 26th
trial, no anticipation occurred after the end
of block II (8th trial). The inter-trial
interval was varied unsystematically
between 10 and 15 sec, except when E
spoke to S about anticipations.
to be near-asymptotic by Block VI, a triple
classification analysis of variance without
replication (McNemar, 1955) with overlap,
interval, and Ss as the three factors was
applied to the scores summed over the last
five blocks (trials 21-40), in order to
examine the effects
Reaction Time as an Index of Masking
learning had become less important. Both
the overlap (F (I, 27)= 15.00) and the
interval (F (I, 27)= 23.81) were significant
(p < .001) with slower mean RT under the
D and long-interval conditions than under
the S and short-interval conditions
respectively. The only other significant
effect was an interaction between the
interval and Ss factors (F (27, 27)= 2.27, p
< .05). To investigate the nature of this
interaction (a type which is not readily
inter-pretable because of the many levels
in the Ss factor), algebraic difference
scores between the short and long levels of
the interval factor were obtained for each
S. When the scores were grouped into 10
classes with a class interval of 12 and midpoints ranging from 153.5 to -44.5, the
obtained frequencies were 1, 0, 3, 2, 7, 3,
4, 5, 2 and 1, with modes at 65.5 and –0.5.
The apparent bimodality of this
distribution suggests that Ss tended to fall
into two classes as regards the effect of the
interval factor, although the departure of
the distribution from normality was not
significant (p>.2) as tested by the
Kolmogorov-Smirnov one-sample technique (Siegel, 1956).
To provide further evidence concerning
an interpretation of the interval and
interval x Ss interaction effects to be put
forward below, algebraic difference scores
between trials immediately following (F)
and preceding (P) the first and last check
trials were computed for each S. It will be
noted that at the latter as compared to the
former check trial, S had already been presented with a series of isolated and
infrequent check trials, and this
259
was assumed to reduce the subjective
probability that the immediately following
(F) trial would itself be a check trial. The
algebraic values of the (F-P) scores for the
last check trial were significantly larger
than the values of the (F-P) scores for the
first check trial (t (27)= 2.29, p< .05). That
is, using an immediately preceding (P)
non-check trial as a baseline to control for
acquisition differences, RT was relatively
faster to a trial following a check trial at
the end than at the beginning of the series.
There was also a significant rank
correlation of 0.33 (p< .05) over the 28 Ss
between the (F-P) scores for the last check
trial and the difference scores between the
short and long levels of the interval factor
over trials 21.40.
DISCUSSION
The slower RT under the D condition,
together with the absence of interaction of
the overlap factor with the Ss factor,
clearly suggest that the D condition
introduced a differential masking effect on
the tone; which in turn suggests that the
classical conditioning experiment referred
to at the outset may have been confounded
by such a differential masking effect. If the
effect included as a component some
specific auditory masking of the tone by
the noise of the puff, an interaction
between the overlap and interval factors
might have been expected, since it was
only with the long interval that the puff
under the D condition was actually present
at tone onset (Fig. 1, lower right cell).
However, since a second stimulus during
the latent period of reaction can
260
J .J. Furedy
increase RT (Helson & Steger, 1962) the
expected overlap-interval interaction
could have been obscured by the shortinterval D condition also being uniquely
unfavourable for RT speed in that this was
the only case where a puff onset occurred
during such a latent period after the tone
(Fig. 1, 2nd pulse of puff in upper right
cell).2 The failure to find an overlapinterval interaction, then, does not rule out
the presence of a specific auditory
component in the masking effect of the
puff.
If the puff onset is regarded as a ready
signal, the significant interval effect,
though not the interaction between the
interval and Ss factors, might be accounted
for in terms of a difference in foreperiod,
the optimum for which has been estimated
to be 2 sec. (Woodworth & Schlosberg,
1955, p. 30). However, unless the range of
this estimate includes values as low as .5
sec, such an account fails to predict the
inferiority of the long-interval (620-msec.)
condition. The direction of the effect is
also not consistent with more recent
investigations which have included
foreperiods from 400 to 700 msec, and
which generally find a negative
relationship between RT and foreperiod
(e.g. Davis, 1965; Drazin, 1961; Klemmer,
1956). Both the direction of the interval
effect and its interaction with the Ss factor
can be accounted for, however, by the
speculative, though not unsupported,
assumptions: (a) the expectation that the
next trial will be a check trial inhibits the
key pressing response, and (b) the in2
The author is grateful to one of the referees
of this paper for drawing his attention to this
point.
elusion of check trials introduces
differential probabilities which are related
to the interval condition but are realized
and acted upon only by some Ss.
The stress laid on not getting “caught”
on check trials lends plausibility to the
first assumption. In addition, the finding
of a relative increase in response speed on
trials immediately following (F) a late
check trial in the series is consistent with
the assumption: S’s expectancy that such
an P trial is a check trial is low, so that his
response is facilitated. Finally, that these
expectancies appear to be related to the
interval effect is indicated by the
significant correlation over Ss between the
relative facilitation scores, as given by the
(F-P) scores for the last check trial, and
the difference scores between the short
and long levels of the interval factor.
The second assumption involves
recognizing that there is a greater range of
possibilities open to S at the point of time
just before the onset of the tone under the
short-interval than under the long-interval
condition. In the first case the trial may be
a short-interval, a long-interval, or a check
trial; in the second case, provided that 420
msec, has elapsed since puff onset, the
trial can only be a long-interval or a check
trial. Hence the probability that a given
trial is a check trial is greater in the case of
the long-interval than of the short-interval
trials. Those Ss who both recognize this
discrepancy and can make sufficiently fine
temporal discriminations to detect (at a
better-than-chance level) the elapsing of
Reaction Time as an Index of Masking
the short interval before the elapsing of the
long interval will give slower responses
under the long-interval than under the
short-interval condition. Other Ss will not
be influenced by the check trials in this
way, so that there will be an interaction
between the interval and Ss factors, the
nature of this interaction being such as to
suggest a bimodal distribution of Ss as regards the interval effect. While the
departure from normality in the
distribution of interval differences was not
significant, it is at least suggestive both
that the obtained distribution was bimodal
and that one mode (- .05) was close to
zero. Finally, it will be noted that the
failure of the overlap factor to interact with
the Ss factor is consistent with these
speculations about the check-trials effect;
the puff-tone overlap, being a simple
masking phenomenon, would not be
affected by differences in Ss’ abilities to
recognize and act on a rather subtle state of
differential probabilities.
The check trials were originally
introduced to enable the key-pressing
response to be made specific to the tone, in
the sense of forcing S to respond to it
rather than to certain characteristics of the
puff. As regards testing the masking
effects of puff-tone overlap, the check
trials served their purpose, but if the
interpretation of the interval and interval x
Ss interaction effects is correct, it seems
that the use of
261
check trials introduces complexities into
the RT experiment. In particular, any
comparison of different foreperiods which
employs check trials to eliminate
anticipations has to contend with the
confounding
influence
of
certain
thoughtful
Ss
whose
temporal
discriminations are sufficiently acute for
them to turn thought into action.
REFERENCES
DAVIS, R. Expectancy and intermittency.
Quart. J. exp. Psychol., 1965, 17,
75-78.
DRAZIN, D. H. Effects of foreperiod,
foreperiod variability, and probability
of stimulus occurrence on simple
reaction time. J. exp. Psychol, 1961, 62,
43-50.
FUREDY, J. J. Reinforcement in classical
aversive and appetitive conditioning.
Unpublished Ph.D. thesis, Univer. of
Sydney, 1965.
HELSON, H. & STEGER, J. A. On the
inhibitory effects of a second stimulus
following the primary stimulus to react.
J. exp. Psychol., 1962, 64, 201-205.
KLEMMER, E. T. Time uncertainty in
simple reaction time. J. exp. Psychol.,
1956, 51, 179-184.
MCNEMAR, Q. Psychological statistics.
New York: Wiley, 1955.
SIEGEL, S. Nonparametric statistics. New
York: McGraw-Hill, 1956.
WOODWORTH, R. S. SCHLOSBERGH.
Experimental
psychology. London:
Methuen, 1955.
(Manuscript received 12 April 1966)