360 Conditioning and Learning
has sought quantitative relations between the frequency of a re-
sponse and its (prior) environmental consequences. Readers
interested in the preparations that have traditionally been used
to study acquired behavior should consult Hearst’s (1988) ex-
cellent review, which in many ways complements this chapter.
EMPIRICAL LAWS OF PAVLOVIAN RESPONDING
Given appropriate experience, a stimulus will come to elicit
behavior that is not characteristic of responding to that stim-
ulus, but is characteristic for a second stimulus (hereafter
called an outcome). For example, in Pavlov’s (1927) classic
studies, dogs salivated at the sound of a bell if previously the
bell had been rung before food was presented. That is, the
bell acquired stimulus controlover the dogs’ salivation.
Here we summarize the relationships between stimuli that
promote such acquired responding, although we begin with
changes in behavior that occur to a single stimulus.
Single-Stimulus Phenomena
The simplest type of learning is that which results from expo-
sure to a single stimulus. For example, if you hear a loud
noise, you are apt to startle. But if that noise is presented
repeatedly, the startle reaction will gradually decrease, a
process calledhabituation.Occasionally, responding may in-
crease with repeated presentations of a stimulus, a phenome-
non calledsensitization.Habituation is far more common
than sensitization, with sensitization ordinarily being ob-
served only with very intense stimuli. Habituation is regarded
as a primitive form of learning, and is sometimes studied ex-
plicitly because researchers thought that its simplicity might
allow the essence of the learning process to be observed more
readily than in situations involving multiple stimuli. Consis-
tent with this view, habituation exhibits many of the same
characteristics of learning seen with multiple stimuli
(Thompson & Spencer, 1966). These include (a) decelerating
acquisition per trial over increasing numbers of trials; (b) a
so-called spontaneous loss of habituation over increasing re-
tention intervals; (c) more rapid reacquisition of habituation
over repeated series of habituation trials; (d) slower habitua-
tion over trials if the trials are spaced, but slower spontaneous
loss of habituation thereafter (rate sensitivity); (e) further ha-
bituation trials after behavioral change over trials has ceased
retard spontaneous loss from habituation (i.e., overtraining
results in some sort of initially latent learning); (f ) general-
ization to other stimuli in direct relation to the similarity of
the habituated stimulus to the test stimulus; and (g) temporary
masking by an intense stimulus (i.e., strong responding to a
habituated stimulus is observed if the stimulus is presented
immediately following presentation of an intense novel stim-
ulus). As we shall see, these phenomena are shared with
learning involving multiple events.
Traditionally, sensitization was viewed as simply the op-
posite of habituation. But as noted by Groves and Thompson
(1970), habituation is highly stimulus-specific, whereas sen-
sitization is not. Stimulus specificity is not an all-or-none
matter; however, sensitization clearly generalizes more
broadly to relatively dissimilar stimuli than does habituation.
Because of this difference in stimulus specificity and be-
cause different neural pathways are apparently involved,
Groves and Thompson suggested that habituation and sensi-
tization are independent processes that summate for any test
stimulus. Habituation is commonly viewed as nonassocia-
tive. However, Wagner (1978) has suggested that long-term
habituation (that which survives long retention intervals) is
due to an association between the habituated stimulus and the
context in which habituation occurred (but see Marlin &
Miller, 1981).Phenomena Involving Two Stimuli:
Single Cue–Single OutcomeFactors Influencing Acquired Stimulus Control
of BehaviorStimulus Salience and Attention. The rate at which stim-
ulus control by a conditioned stimulus (CS) is achieved
(in terms of number of trials) and the asymptote of control
attained are both positively related to the salienceof both
the CS and the outcome (e.g., Kamin, 1965). Salience here
refers to a composite of stimulus intensity, size, contrast with
background, motion, and stimulus change, among other
factors. Salience is not only a function of the physical stimu-
lus, but also a function of the state of the subject (e.g., food is
more salient to a hungry than a sated person). Ordinarily, the
salience of a cue has greater influence on the rate at which
stimulus control of behavior develops (as a function of num-
ber of training trials), whereas the salience of the outcome
has greater influence on the ultimate level of stimulus control
that is reached over many trials. Clearly, the hybrid construct
of salience as used here has much in common with what is
commonly called attention, but we avoid that construct be-
cause of its additional implications. Stimulus salience is not
only important during training; conditioned responding is di-
rectly influenced by the salience of the test stimulus, a point
long ago noted by Hull (1952).