The autonomic response (galvanic skin response) to a noxious stimulus (shock) is reduced when the stimulus is preceded by a warning signal. The greatest reduction, 53%, was obtaqined with a warning interval of 1 second. Warning also reduces variance of the response over trials, a decrease of over 90% for the optimum 1-second interval

Lykke, David T.

English

Cognitive Sciences ePrint Archive

ReprintedfromScience,August31, 1962,Vol. 137,No. 3531,pages665-666
Copyright 0 1962 by the American Association for the Advancement of Science
Preception in the Rati  Autonomic
Response to Shock as Function of
+ngth  of Warning Interval
A bstruct. The autonomic response
(galvanic skin response) to a noxious
stimulus (shock) is reduced when the
stimulus is preceded by a warning signal.
The greatest reduction, 53 percent, was
obtained with a warning interval of 1
second. Warning also reduces variance of
the response over trials, a decrease of over
90 percent for the optimum l-second
\ interval.
It has been shown with human sub-
jects that the galvanic skin response
(GSR) to a painful electric shock is
smaller and less variable when the
shock is preceded by a brief warning
signal than when presented unexpected-
ly (I). It is known that the mammalian
nervous system contains elaborate mech-
anisms for preliminary analysis and
modulation of sensory input prior to
arrival at the highest neural destination,
mechanisms which presumably subserve
the functions of sensory adaptation,
selective attention, and the like. It may
be, therefore, that a warning signal
which presages a brief noxious stimulus
can produce an fferent “set” which
r@7
rves to attenuate selectively the sen-
jr-y representation of the noxious stim-
ulus when it occurs. Since the relative
amplitude of the galvanic skin response
is known to covary with other indicants
of subjective stimulus intensity (2), such
a hypothesis would account for the re-
sults mentioned above and would also
explain why many subjects volunteered
that the shock felt less strong when pre-
ceded by the warning tone.
This hypothetical effect of the warn- ’
ing signal upon the “admittance” of the
tierent system for the expected stim-
ulus was called the “preception re-
sponse.”When the expected stimulus
is noxious and unavoidable, the warning
appears to generate a negative precep-
tion response which attenuates the af-
ferent result of the stimulus. When, as
in an ordinary sensory threshold ex-
periment, the expected stimulus is non-
noxious or weak, or both, the warning
signal may perhaps elicit a positive pre-
ception response, which then selectively
amplifies the signal initiated by the ex-
pected stimulus. The fact that sensory
thresholds are lower for short warning
mtervals and higher for variable int r-
als (3) would seem to square with such
an interpretation.
These considerations suggest that the
negative preception phenomenon should
vary as a function of the length of the
warning interval. Suppose an animal is
given a series of brief shocks at varying
inter-trial intervals and that half of these,
at random, are preceded by a warning
signal lasting exactly t seconds and
terminating with the onset of the shock.
If the galvanic skin responses to the
shock on the tone-shock (TS) trials are
smaller on the average than those pro-
duced by the shock-alone (S) trials, this
relative diierence is a measure of the
negative preception produced by the
t-second warning. Longer intervals (for
example t= 10 set) should yield less
preception since the time of occurrence
of the shock cannot be predicted as
accurately as it can when the interval
is short. On the other hand, too short
an interval would not allow time for
the preception response to occur. (Thus,
th  shock alone can be thought of as
providing its own warning signal with
t = 0.) Therefore, one might expect
decreased negative preception with long
and very short intervals and a maximum
at some interval of intermediate length.
The 20 animals used in this experi-
ment were rats of the Wistar strain, all
about 90 days old. Each animal was
lightly anesthetized with ether and then
bound to a special restraining platform
(4) for GSR recording. Small zinc GSR
electrodes were attached to the plantar
surfaces of the two hind feet with thin
trips of tape, after the footpads had
been daubed with a zinc-sulfate paste
(5). TWO shock electrodes, made of lead,
were taped about 1 inch apart near the
proximal end of the tail. The animal
was then placed in a sound-insulated
1.00 -
0
4 .00-
z
i .60-
Y
k!
n 40-
I 2 3 4 5 6 7 8
BLOCKS OF FIVE TRIALS
Fig. 1. Preception ratio is GSR to shocks
preceded by warning tone divided by GSR
to interspersed shocks given without warn-
ing. Curves show attenuation of response
to the more predictable shocks found in
five groups of animals run on different
constant warning intervals.
1
TS-GSR Variance
S-GSR Variance
WARNING INTERVAL IN SECONDS
Fig. 2. Solid curve shows ratio of GSR
amplitude on tone-shock (TS) trials to
amplitude on shock-alone (S) trials as a
function of the duration of the tone or
warning interval. Broken curve shows
similar ratio of the variances (across
trials) or GSR amplitudes.
box containing a compressed air inlet
which provided both ventilation and a
“white” masking sound. Skin conduct-
ance was measured directly (6) b  con-
necting a 3-volt mercury battery in
series with the GSR electrodes and a
small resistor (100 to 500 ohms); the
signal across this resistor, which varied
linearly with skin conductance, was fed
to a Sanbom 150-1500 chopper pre-
amplifier and its associated chart re-
corder. With this arrangement, apparent
skin conductance varied from about 2
to 11 @ho and current from about 6
to 33 pa. The shock stimulus was the
discharge of a 1-pf capacitor charged
to 300 volts. The warning signal was a
1000 cy/sec tone delivered through a
speaker situated about 6 inches over the
animal’s head.
Each animal was given 40 shocks (S-
trials) randomly intermixed with 40
tone-shocks (TS-trials) . The inter-trial
interval varied unpredictably from 15
to 60 seconds with a mean of 35 sec-
onds. Four animals were run on each
of the five warning intervals, 0.5, 1, 2,
5, and 10 seconds.
Since, on the TS-trials, the galvanic
skin response to the warning signal fre-
quently merges into the response to the
shock itself, it is necessary to devise a
means of determining what portion of
the total response following shock is
actually elicited by the shock. First, the
GSR latency was measured for each
shock-alone trial, that is, the time from
the onset of shock until the subsequent
response had reached 10 percent of its
total amplitude (such “10 percent la-
tencies” can be measured considerably
more accurately than can the time to
the absolute start of the response).
These S-trial latencies were then aver-
aged for each five successive trials. The
shock-GSR for any TS-trial was then
found by laying off one latency period
from the start of the shock (by use of
the mean for the five S-trials nearest
that TS-trial) and measuring the ampli-
tude from that point to the peak of the
shock-GSR. This value was taken as
being 0.9 of the total galvanic skin re-
sponse actually elicited by the shock on
that TS-trial. Since most of these ani-
mals showed a moderate negative p-
type correlation between GSR amplitude
and latency, this scoring procedure tends
to err in the direction of reducing the
expected differences. The preception hy-
pothesis predicts that the galvanic skin
responses on the TS-trials shall be small-
er than those on the S-trials, hence their
latencies should be somewhat longer
also; the scoring procedure used here
would therefore tend to overestimate
GSR amplitudes for the TS-trials.
Mean GSR amplitudes were com-
puted for successive blocks of five S-
trials and five TS-trials. For each
such block of five pairs of trials, a pre-
ception score was obtained by dividing
the mean TS-GSR by the mean S-GSR;
this ratio represents the shock response
on the TS-trials as a fraction of the re-
sponse shown on the interpolated S-
trials. These values are plotted for each
warning interval group in Fig. 1. The
TS-GSR was always smaller than the
S-GSR except for one point on the lo-
second curve and another on the S-sec-
ond curve. Thus, the expected negative
prec ption effect is apparent. Moreover,
the effect appears to be strongest for
the l-second warning interval and least
for the two longest intervals. The over-
ll mean preception ratios for the five
interval groups are plotted in Fig. 2.
For the l-second group, the TS-GSR
averaged only 47 percent of the S-GSR
amplitude but was about 65 percent for
the 0.5 and 2-second intervals and over
85 percent for the 5- and IO-second
intervals.
Th  preception hypothesis also pre-
dicts that the responses to the more
predictable shocks should be less vari-
able from trial to trial than those pro-
duced by the shocks given alone. There-
fore, the warning interval which is
optimum for preception should yield
the smallest ratio of TS-GSR variance
to .S-GSR variance (these variances be-
ing computed over the eight blocks of
five trials each). This variance ratio is
also plotted in Fig. 2 and shows the
xpected result. For the optimum l-sec-
ond interval, the variance of the TS-
trials is only about 4 percent of the
variance on the S-trials: for the 0.5
and 2-second intervals, the values are
32 and 53 percent, rising to 94 and 74
percent for the 5- and lo-second
groups, respectively.
An immediate practical implication
of these findings concerns the wide-
spread use of shock or other brief
noxious stimuli in behavioral studies of
str ss, emotional learning, and the lik
qThe data make clear that the effectiv.
int nsity of a stimulus having a con-
tant physical intensity may vary great-
ly from trial to trial unless the time of
ccurrence of the stimulus is made pre-
dictable for the subject. Whether the
optimum warning interval is the same
for the human and other species as for
the rat remains to be determined (7).\*
DAVTD  T. LYKKEN I
Center for Personality Research,
University of Minnesota, Minneapolis
References and Notes
1. D. Lykken, Psychophysiol. Meas. Newsletter
5, 2 (1959).
2. Measurement problems complicate the com-
~arison  of galvanic skin resoonses from dif-
%ent indivihuals, b u t  t h e  &neral truth o f
this interpretation rests upon an abundance
of evidence-for example, see H. G. Mc-
Curdy, Psychol. Rev. 57, 322 (1960).
3 .  C .  Howarth a n d  M .  Treisman, Q u a r t .  1.
Exptl. Psychol. 10, 130 (1958).
4. D. Lykken and R. Rose, Am. I. Psychol. 72,
621 (1959).
5. D. Lykken,  3. Camp. Physiol. Psychol.  52,
629 (1959).
6. D. Lykken and N. Roth, Am. 3. Psychol.
293 (1961).
n
7. This study was supported in part by grant
No. M-4697 from the National Institute of
Mental Health. I  gratefully acknowledge
the skilled help of Sheridan Stevens in con-
ducting the experiment.
18 July 1962
.
2


Measurement problems complicate the com~arison of galvanic skin resoonses from dif%ent indivihuals, b u t t h e &neral truth o f this interpretation rests upon an abundance of evidence-for example,

This study was supported in part by grant No. M-4697 from the National Institute of Mental Health. I gratefully acknowledge the skilled help of Sheridan Stevens in conducting the experiment.

Preception in the rat: Autonomic response to shock as a function of length of the warning interval.

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