Empirical Laws of Pavlovian Responding 361Predispositions: Genetic and Experiential. The con-
struct of salience speaks to the ease with which a cue will
come to control behavior, but it does not take into account the
nature of the outcome. In fact, some stimuli more readily be-
come cues for a specific outcome than do other stimuli. For
example, Garcia and Koelling (1966) gave thirsty rats access
to flavored water that was accompanied by sound and light
stimuli whenever they drank. For half the animals, drinking
was immediately followed with foot shock, and for the other
half it was followed by an agent that induced gastric distress.
Although all subjects received the same audiovisual-plus-
flavor compound stimulus, the subjects that received the foot
shock later exhibited greater avoidance of the audiovisual
cues, whereas the subjects that received the gastric distress
exhibited greater avoidance of the flavor. These observations
cannot be explained in terms of the relative salience of the
cues. Although Garcia and Koelling interpreted this cue-
to-consequence effectin terms of genetic predispositions re-
flecting the importance of flavor cues with respect to gastric
consequences and audiovisual cues with respect to cutaneous
consequences, later research suggests that pretraining experi-
ence interacts with genetic factors in creating predispositions
that allow stimulus control to develop for some stimulus
dyads more readily than for others. For example, Dalrymple
and Galef (1981) found that rats forced to make a visual dis-
crimination for food were more apt to associate visual cues
with an internal malaise.
Spatiotemporal Contiguity (Similarity). Stimulus
control of acquired behavior is a strong direct function of the
proximity of a potential Pavlovian cue to an outcome in space
(Rescorla & Cunningham, 1979) and time (Pavlov, 1927).
Contiguity is so powerful that some researchers have sug-
gested that it is the only nontrivial determinant of stimulus
control (e.g., Estes, 1950; Guthrie, 1935). However, several
conditioning phenomena appear to violate the so-called law
of contiguity. One long-standing challenge arises from the
observation that simultaneous presentation of a cue and out-
come results in weaker conditioned responding to the cue
than when the cue slightly precedes the outcome. However,
thissimultaneous conditioning deficithas now been recog-
nized as reflecting a failure to express information acquired
during simultaneous pairings rather than a failure to encode
the simultaneous relationship (i.e., most conditioned re-
sponses are anticipatory of an outcome, and are temporally
inappropriate for a cue that signals that the outcome is al-
ready present). For example, Matzel, Held, and Miller (1988)
demonstrated that simultaneous pairings do in fact result in
robust learning, but that this information is behaviorally
expressed only if an assessment procedure sensitive to simul-
taneous pairings is used.
A second challenge to the law of contiguity has been
based on the observation that conditioned taste aversions
yield stimulus control even when cues (flavors) and outcome
(internal malaise) are separated by hours (Garcia, Ervin, &
Koelling, 1966). However, even with conditioned taste aver-
sions, stimulus control (i.e., aversion to the flavor) decreases
as the interval between the flavor and internal malaise
increases. All that differs here from other conditioning prepa-
rations is the rate of decrease in stimulus control as the inter-
stimulus interval in training increases. Thus, conditioned
taste aversion is merely a parametric variation of the law of
contiguity, not a violation of it.
Another challenge to the law of contiguity that is not so
readily dismissed is based on the observation that the effect
of interstimulus interval is often inversely related to the aver-
age interval between outcomes (e.g., an increase in the
CS-US interval has less of a decremental effect on condi-
tioned responding if the intertrial interval is correspondingly
increased). That is, stimulus control appears to depend not so
much on the absolute interval between a cue and an outcome
(i.e., absolute temporal contiguity) as on the ratio of this in-
terval to that between outcomes (i.e., relative contiguity; e.g.,
Gibbon, Baldock, Locurto, Gold, & Terrace, 1977). A further
challenge to the law of contiguity is discussed in this chap-
ter’s section entitled “Mediation.”
According to the British empiricist philosophers, associa-
tions between elements were more readily formed when the
elements were similar (Berkeley, 1710/1946). More recently,
well-controlled experiments have confirmed that develop-
ment of stimulus control is facilitated if paired cues and out-
come are made more similar (e.g., Rescorla & Furrow, 1977).
The neural representations of paired stimuli seemingly in-
clude many attributes of the stimuli, including their temporal
and spatial relationships. This is evident in conditioned re-
sponding reflecting not only an expectation of a specific out-
come, but the outcome occurring at a specific time and place
(e.g., Saint Paul, 1982; Savastano & Miller, 1998). If tempo-
ral and spatial coordinates are viewed as stimulus attributes,
contiguitycan be viewed as similarityon the temporal and
spatial dimensions, thereby subsuming spatiotemporal conti-
guity within a general conception of similarity. Thus, the law
of similarity appears able to encompass the law of contiguity.Objective Contingency. When a cue is consistently
followed by an outcome and these pairings are punctuated by
intertrial intervals in which neither the cue nor the outcome
occurs, stimulus control of behavior ordinarily develops over