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Running head: EFFECTS OF HABITUATION TO AN UNCONDITIONED STIMULUS
Effects of Habituation to an Unconditioned Stimulus
Jessica Ivison
Algoma University
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Abstract
Psychologists have long argued over whether classical conditioning involves a stimulus-stimulus
(SS) connection or a stimulus-response (SR) connection. In 1973 Rescorla found a SS connection in
rats. The present study examines whether classical conditioning in humans involves a SS or a SR
connection. Using a computer, participants learned that a flash of light signaled that a loud noise
was about to occur. After training, half of the participants were presented with the noise alone (to
break that noise + response connection; habituation). Finally, all participants were presented with
just the flash of light. If a SS connection exists, participants that were habituated to the noise should
not show a startle response following presentation of the flash of light (as the noise + response
connection has been broken). Neither group showed a change in responding, suggesting that
conditioning may not have occurred in the first phase. Further research could be directed towards
using a more aversive stimulus and forming a stronger level of initial conditioning.
Keywords: classical conditioning, habituation, suppression, conditioned stimulus, unconditioned
stimulus, unconditioned response
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Effects of Habituation to an Unconditioned Stimulus
Classical conditioning is an association between two stimuli, quite often understood as an
association between an unconditioned stimulus and a conditioned stimulus (Domjan, 2010).
When a stimulus consistently produces an unlearned response it is called an unconditioned
stimulus (US). Through pairing it can cause a second stimulus, a conditioned stimulus (CS), to
produce that same response (Domjan, 2010). This was accidentally discovered by Ivan Pavlov
(1928) while he was studying the digestive processes of dogs. A buzzer was presented (the CS)
just before presenting the dog with food (the US). After repeated trials of this the dog began to
salivate (the response) after hearing the buzzer, even without the presence of food (Pavlov, 1928).
A question that developed from this finding is whether the learning involved in classical
conditioning is due to an association between the stimulus and its response (SR), or an
association between the two stimuli (SS) (Byrne, 2003).
This question has caused learning theorists to debate which association is actually taking
place (Byrne, 2003). Hull (1943) and Spence (1950) were two supporters of the SR approach to
learning. Spence (1950) indicates that a main difference between the two learning theories is that
one suggests an association between a sensory process and a motor process (the SR approach),
while the other suggests an association between two sensory processes (the SS approach). He
also acknowledges that it is possible for both types of learning to co-exist, but that he is sure that
all types of learning are not just SS connections. One of the leading theorists of the SS approach
was Tolman (Byrne, 2003). He argued that learning is more cognitive than the SR theorists were
accounting for (Tolman, 1932). Tolman (1948) compares the SS theory of learning to a map and
the SR theory of learning to a telephone switchboard. The SS theory is like a map because it
indicates that as an individual encounters stimuli in his environment he establishes a relationship
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between each stimulus. Consequently, he creates a cognitive map that he can use to navigate the
environment (Tolman, 1948). The SR theory is analogous to a telephone switchboard because
when the individual encounters a stimulus (incoming calls), a response is elicited (outgoing calls).
In simply reacting to stimuli the individual is moving through the environment as if he or she has
no purpose or direction (Tolman, 1948). SS and SR learning theorists often went back and forth
in this way, however neither side had evidence that could prove their theory while
simultaneously disproving the other’s theory (Byrne, 2003; Spence, 1950).
Rescorla (1973) designed a study to provide insight to this debate through examining the
relationship between a US and a CS. Using rats as subjects Rescorla studied the effects of
habituation to a US following conditioning to a CS. Before the experiment began the rats were
trained to press a bar for food. The level of conditioning was measured throughout the
experiment by counting the number of times each rat pressed the bar. In the first phase of this
experiment the rats were conditioned to the light. All rats were placed in a Skinner box and
presented with a light (the CS) followed by a noise (the US). In the second phase the rats were
split into two groups and each group received a different treatment; Group E was habituated to
the US through repeated presentations of the noise, Group C was placed in the Skinner box with
no presentations of the noise. By the end of this phase Group E no longer startled when presented
with the noise, showing that they have become habituated to the noise. In the third phase both
groups were presented with the light only. This was to test each rat’s level of suppression to the
CS. The suppression ratio compares the number of bar presses that occurred before the CS,
spanning the same amount of time as the CS, with the number of bar presses that occurred during
the CS. In Group C the suppression ratio shows the level of conditioning to the light. The less the
rats press the bar during the CS, the closer the suppression ratio is to 0. A suppression ratio of 0
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indicates that the rats have been conditioned to expect to the CS (Rescorla, 1973). Likewise, the
more the rats press the bar during the CS, the closer the suppression ratio is to .5. A suppression
ratio of .5 indicates no conditioning to the CS (Rescorla, 1973). In Group E the suppression ratio
shows whether the rats are making a SS connection or a SR connection. A suppression ratio of 0
indicates that the rats are making a SR connection. A suppression ratio of .5 indicates that the
rats are making a SS connection. The results of this experiment were indicative of a SS
connection because Group C had a suppression ratio closer to 0, indicating that conditioning had
occurred, and Group E had a suppression ratio closer to .5, indicating that a SS connection was
being made by the rats because the presence of the light did not affect the rate of bar pressing
(Rescorla, 1973). This conclusion was supported in similar studies by Taylor (1956) and
Rescorla (1974). They found that changes to a US after conditioning altered the response
produced by the CS, which indicates a connection between the CS and the US (Taylor, 1956;
Rescorla, 1974).
While non-humans are typically the focus of classical conditioning experiments,
conditioning in humans has not been studied as thoroughly. This leaves in question how
important human awareness is to learning. Lovibond and Shanks (2002) examined many
different studies concerning the connection between awareness and conditioning. The general
consensus was that awareness is needed for conditioning to occur (Lovibond & Shanks, 2002).
However, Lovibond and Shanks (2002) also came across evidence demonstrating that awareness
is not always needed for conditioning to occur. They suggest that the role of human awareness in
conditioning needs to be further examined, and that an effective way to do this would be to
present a CS and assess how expectant the subject is of the US (Lovibond & Shanks, 2002). The
purpose of the present study is to examine the role that human awareness plays in classical
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conditioning, and more specifically whether classical conditioning in humans is a connection
between two stimuli or between a stimulus and a response. Since classical conditioning and
habituation are similar in human and non-human animals (Byrne, 2003), it was hypothesized that
participants would make a SS connection.
Methods
Participants and Design
Ten adults participated in this study. They were randomly assigned to either the
habituation group or the control group.
Materials
A pc-based computer, a computer program, and headphones were the only materials used
for this study. The computer program was created by Jon Natale, a computer science major at
Algoma University, for this study. The opening screen of the computer program listed the
options to input how long the program would run (in seconds), the individual times at which
each flash of light would occur (in seconds), and the individual times each noise would occur (in
seconds). The flash of light was made by changing the screen from a continuous black screen to
a white screen for 0.5 seconds and then changing back to black. The noise was a white noise
created by the program designer which lasted 0.4 seconds. The noise had a delay on it so that it
would overlap with the flash by 0.2 seconds instead of occurring at the exact same time (e.g. if
the flash and the noise were both set to occur at 15 seconds, the flash would occur at 15.0
seconds while the noise would occur at 15.4 seconds). Once the times were inputted into the
program a start button appeared. Once the start button was pressed the program would begin
recording the time of each space bar press. After the set amount of time expired a done button
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would appear. When the done button was pressed it would display the time of each bar press, as
well as the interval of time between each bar press.
Procedure
There were three phases to this study. Participants were asked to wear headphones and
press the space bar throughout all three phases. In phase 1 each participant received ten
presentations of the flash and noise together, set at random intervals. Phase 1 lasted 10-min. At
the end of phase 1 participants were given a 5-min break.
In phase 2 participants were randomly assigned to one of two groups: habituation or
control. The habituation group received ten presentations of the noise at random intervals. The
control group received no presentations of the noise or the flash. Phase 2 lasted 10-min. At the
end of phase 2 participants were given a 5-min break.
In phase 3 both groups received ten presentations of the flash set at random intervals.
Phase 3 lasted 10-min. Participants were verbally debriefed following completion of phase 3.
Results
There were 5 participants in the control group, and 5 participants in the habituation group.
A suppression ratio was used to measure the difference between the number of bar presses before
the CS, spanning the same amount of time as the CS, and the number of bar presses during the
CS for each participant. Then the average suppression ratio was found for each group. In phase 1
both the control group and the habituation group had a suppression ratio of .5 (SD = 0). In phase
2 both groups had a suppression ratio of .5 (SD = 0). In phase 3 both groups had a suppression
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ratio of .5 (SD = 0). This shows there was no change in bar pressing due to condition, and that
the rate of bar pressing did not change when either stimuli were presented.
Discussion
Previous research on the association made in classical conditioning has demonstrated that
rats form a SS connection in associative learning (Rescorla, 1973; Taylor, 1956; Rescorla, 1974).
Since classical conditioning and habituation work similarly in humans and non-human animals
(Byrne, 2003), the present study tested the hypothesis that humans would also make a SS
connection in classical conditioning. The results of this study indicated that participants may not
have been conditioned to the light because the number of bar presses did not change for either
group when the light was presented. Therefore, it cannot be determined whether participants
were making a SS or a SR connection. In order to identify which connection participants were
making there must first be evidence that the participants had been conditioned to the light; i.e.
there needs to be evidence that participants had learned that the light predicts the noise. There
was no evidence that participants had learned this because if they had the control group would
have had a different rate of bar pressing during the CS than before the CS. The results of this
study do not confirm or deny the hypothesis.
The lack of conditioning may have occurred due to phase 1 being too short to establish an
association between the noise and the light. Participants were conditioned to the light for ten
minutes with ten presentations of the light and noise together. This may not have been enough
time or enough pairings of the stimuli to establish a strong level of classical conditioning.
Rescorla (1973) presented the rats with the noise-light pairing over two sessions in two days.
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Perhaps increasing the length of phase 1 or having multiple sessions of phase 1 is needed to
establish conditioning to the light.
Alternatively, the noise the participants heard may not have been aversive enough to
startle participants and cause them to change their rate of bar pressing. If this were true,
participants may have learned that the light predicts the noise, however since the noise did not
cause them to startle their learning could not be observed because there was no change in bar
pressing when the light was presented. Consequently, even if learning did occur there is no way
to tell from the data collected in this study if participants had been conditioned. Similarly, the
participants may have been expecting a sound to come through the headphones due to the
knowledge that it is common for sound to come through headphones. Expecting a sound to occur
most likely made the sound less startling to the participants. A sound that is more unexpected
may be needed to produce the startle response from the participants so that there is a way to
measure their learning.
In future studies, a longer period of conditioning to the light with more pairings of the
two stimuli may be needed to establish conditioning. Also, a more startling noise could be used
and presented to participants in a way that makes the noise unexpected. For example, the sound
could come through hidden speakers placed near the participant rather than through headphones.
The current study indicates that, possibly due to human awareness, it may be necessary to set up
a situation where the participants are not expecting the stimuli that are going to be presented to
them.
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