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81stimulus. This pattern is seen for many kinds of learning and will be discussed in greater detail in Chapters 4 and 8.
Animals given sessions of multiple exposures to stimuli in which the expo-sures are separated by short intervals will typically show habituation after fewer exposures than animals given sessions in which the same number of exposures are spread out over time (Rankin & Broster, 1992; Thompson & Spencer, 1966):
more rapid repetition of a stimulus generally leads to more rapid habituation.
Exposures that are repeated closely together in time are called massed, whereas exposures that are spread out over time are called spaced. If your goal is to habit-uate your response to some repeated stimulus as rapidly as possible, your best bet is to find a way to make that stimulus as nonarousing as possible, to expose yourself to closely spaced repetitions of that stimulus, and to repeat the process frequently (as you do when you wear shirts with tags in the back).
The effects of habituation may last for a few minutes or several hours and under some circumstances may last a day or more. But they generally do not last forever and are especially likely to dissipate if the stimulus is absent for a while.
Habituation that goes away in seconds or minutes is called short-term habitu-ation; habituation that lasts longer is called long-term habituation. If a rat has gotten used to a loud noise and then goes through a period of an hour or so in which the noise does not occur, the rat is likely to startle anew when the noise is played again. The reappearance or increase in strength of a habituated response after a period of no stimulus presentation is called spontaneous recovery.
The factors that determine how quickly an individual’s response habituates also affect how long the effects of habituation last. Animals that experience massed exposures to a stimulus learn to ignore that stimulus faster than animals given spaced exposures, but if they are retested after a relatively long break, the ani-mals given massed exposures are also more likely to show spontaneous recovery.
When exposures are spaced in time, it takes longer for responding to habituate, but once habituation occurs, it lasts for a longer time (Gatchel, 1975; Pedreira, Romano, Tomsic, Lozada, & Maldonado, 1998).
This finding makes intuitive sense, because animals that have gotten used to the intermittent occurrence of a stimulus should find the recurrence of the stim-ulus after a long interval to be familiar. As a student reading this chapter, you are easily able to detect that repetition is occurring, even when that repetition occurs after moderately long intervals. If your repeated experiences are spread out over time, the likelihood is greater that you will continue to recognize repeating events farther into the future. So, if you want habituation to last for as long as possible, your best bet is to repeatedly expose yourself to the relevant stimulus after longer and longer stretches of time.
Although spontaneous recovery might seem to suggest that habituation is a temporary effect, habituation effects accumulate over time. So, if an infant is shown a donut shape 20 times during a single session, its orienting response to that image will likely habituate. If a day or two later the infant is shown the donut shape again, spontaneous recovery will probably have occurred, and the infant’s fixation time will be as long as if the image were completely novel. However, this time it may only take eight trials before the infant’s orienting response becomes habituated: the effects of repeated experiences have been potentiated by the prior repetitions. This shows that the effects of earlier repeated experiences have not simply faded away. Furthermore, the mechanisms underlying habituation continue to change with repeated exposures, even when behavioral responses are no longer changing. For instance, a rat exposed to a loud sound many times might stop showing any indication that it even hears the sound—its response has decreased to the point at which there no longer is a response. Nevertheless,
if the sound continues to be repeated many times after this point, the amount of time required before spontaneous recovery occurs will increase. In this case, the learning associated with repeated exposures is latent, because there are no observable changes in the rat’s behavior associated with the increased number of repetitions. The additional effects of repeated exposures after behavioral responding to a stimulus has ceased are only evident when subsequent tests show delayed spontaneous recovery (Thompson & Spencer, 1966).
The Process of Sensitization
Several times a year, news reports appear of a celebrity attacking a member of the paparazzi. The actor Sean Penn was charged with attempted murder after he grabbed a photographer by the ankles and held him over a ninth-floor bal-cony. Rapper Kanye West was arrested for attacking a reporter at Los Angeles International Airport and destroying his camera. Given that celebrities have lots of photos taken of them by lots of people on a daily basis, you might expect that they would eventually become used to all the attention and take no notice of photographers. What causes some celebrities to become so aggressive when confronted with paparazzi? Is it just a case of bad tempers?
One possible explanation is that celebrities have had negative experiences involving photographers in the past that are affecting their responses to new interactions with random members of the paparazzi. Sensitization is a phenom-enon in which experiences with an arousing stimulus lead to stronger responses to a later stimulus. In some cases, a single, very intense stimulus can produce sensitization, whereas in others, repeated exposures are required. In some ways, sensitization seems to be almost the opposite of habituation. Whereas in habituation repeated experiences can attenuate a rat’s acoustic startle reflex, in sensitization repeated experiences can heighten it. As described above, when rats are subjected to a loud noise over and over again, their startle response often habituates (Figure 3.2, green line). But if some of these rats are given an electric shock (Figure 3.2, red line) and then the loud noise is played again, their startle response will be much greater than that of the rats who did not receive a shock (Davis, 1989). In other words, the strong electric shock sensitizes the rats, increasing their startle response to a subsequent loud noise stimulus. Such sensitization is usually short-lived, however. It may persist for 10 or 15 minutes after the shock, but beyond that, the startle response drops back to normal levels. You may notice that the effect of shock in this experiment is very similar to dishabituation. In fact, some researchers have argued that dishabituation is the result of introducing a sensitizing stimulus (Thompson &
Spencer, 1966).
Humans too show sensitization of startle reflexes. This is most easily revealed using the skin conductance response (SCR), also known as the galvanic skin response (GSR). The SCR is a change in the skin’s electrical conductivity associated with emotions such as anxiety, fear, or surprise. Fluctuations in electrical conductance can be recorded by electrodes similar to those used for an electroencephalogram (EEG; see Chapter 2 for details). Lie detector tests usually measure a person’s SCR, because the emotions evoked by attempts at deception can alter the SCR. (Unfortunately, other emotions—such as nervousness or excitement—can also alter the SCR, which is why lie detec-tor tests are not perfectly reliable indicadetec-tors of truthfulness.)
Exposure to an unexpected loud noise (say, an explosion or a yell) causes a pronounced startle response in humans,
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Figure 3.2 Sensitization of the rat acoustic startle reflex When a startle-provoking noise is presented repeatedly over a 20-minute period, rats’ startle reflex habituates (green line). If a foot shock is then administered to a subset of the rats (at minute 21—red line), the amplitude of their startle reflex to a subsequent noise (at minute 22) is then greater than in the unshocked rats.
(Adapted from Davis, 1989.)
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83accompanied by a sharp SCR. A neutral musical tone may cause a mild startle response, which produces a small SCR. If the loud noise is played before pre-sentation of the tone, the participant’s SCRs to the tone are stronger than they would be without the loud noise (Lang, Davis, & Ohman, 2000). The loud noise sensitizes the person’s startle response to the tone, just as electric shock sensitizes the startle response in rats.
Like habituation, sensitization is seen in a wide range of species, including bullfrogs, sea slugs, and humans (Bee, 2001; Eisenstein, Eisenstein, & Bonheim, 1991; Marcus, Nolen, Rankin, & Carew, 1988). Also like habituation, sensitiza-tion can rapidly dissipate in some situasensitiza-tions and can lead to longer-lasting learn-ing in others (Borszcz, Cranney, & Leaton, 1989; Davis, 1972, 1980). However, fewer exposures are typically necessary to produce sensitization than to produce habituation, and whereas habituation is stimulus specific, sensitization is not.
For example, an animal’s startle response may habituate to one loud tone that is repeated over and over; but if a different loud noise is presented, the startle response reappears in full force—habituation doesn’t transfer to the new sound.
By contrast, exposure to a sensitizing stimulus (such as an electric shock) can amplify the startle response to any stimulus that comes later: tone, loud noise, butterfly, or anything else. Similarly, a celebrity who catches a photographer peering into her house might be responding not just to that particular photog-rapher but, as an aftereffect, to other even more annoying photogphotog-raphers that she has previously encountered. Interpreted this way, disproportionate attacks on random photographers are as much the fault of the overeager paparazzi as they are the fault of the “hot-tempered” celebrities.
Interestingly, whether or not a sequence of repeated events leads to habitua-tion or sensitizahabitua-tion does not depend only on what those events are or how they are distributed in time. A series of events that might normally lead to habitua-tion can in a sick animal lead to sensitizahabitua-tion (Domjan, 1977). In other words, the state of the observer can play a large role in what he or she learns about repeated events. So, a celebrity who repeatedly encounters paparazzi while being sleep deprived, hung over, or ill may be more likely to become sensitized to annoying intrusions of privacy and therefore more likely to strike back.
Dual Process Theory
If repeated events can potentially lead to either habituation or sensitization, how can anyone predict what an organism will learn from repeated exposures? Novel stimuli are often arousing, but what is it that determines whether an event will generate increasing arousal with repetition as opposed to becoming boring? One popular theory, called dual process theory, suggests that, in fact, repeated events always lead to the processes underlying both sensitization and habituation (Groves &
Thompson, 1970; Thompson, 2009). Here’s how it works. Imagine that a stimu-lus S is presented, evoking some chain of neural responses that eventually leads to activation of a motor response R and also activates a state system that signals detection of a stimulus (Figure 3.3). Habituation after repeated exposures to the stimulus (let’s say 10 repetitions) can then be modeled as a weakening of the con-nection between S and R (Figure 3.3b), combined with mild arousal of the state system. The weaker connection decreases the likelihood of activity within motor neurons, making the response to S weaker or less likely to occur. If the stimu-lus is not very arousing, then the weakened connection essentially determines responding.
In dual process theory, both sensitization and habituation processes occur in response to every stimulus presentation, and it is the summed combination of these two independent processes that determines the strength of responding
(Groves & Thompson, 1970). The actual outcome—the strength of the response to S on a given presentation—depends on such factors as how often S has been repeated and how intense and recent was the sensitizing event. It can also depend on whether other stimuli have activated the state system. For stimuli that lead to little arousal, decreases in connection strengths associated with processes of habituation will be the main determinants of how an organism’s responses change over time, leading to the behavioral phenomenon that researchers call habituation. When stimuli are highly arousing, global effects of sensitization will be more evident in responses, leading to the behavioral phenomenon known as sensitization. In dual process theory, both sensitization and habituation processes change over time such that the largest effects of repetition always occur in early exposures.
The role of arousal in learning about repeated events is most evident in the habituation and sensitization of emotional responses (as discussed in “Learning and Memory in Everyday Life” on page 80). Studies of emotional responses to extreme events (say a roller-coaster ride) suggest that there are multiple phases of emotional responding—an initial phase that is scary followed by a rebound effect of exhilaration. After repeated experiences, the initial fear responses may become weaker, whereas the rebound responses grow stronger (such that what
Stimulus S habitu-ation Dual process theory suggests that both habituation and sensitization processes occur in parallel during every presentation of a stimulus and that the final response after repeated presentations results from the combination of both processes. (a) Initially, a stimu-lus such as S activates sensory neurons that lead to a motor response R and activates a separate state system signaling detection of the stimulus. (b) In habituation, repeated presen-tations of S can weaken the connections between neurons (smaller arrow), thus reducing the strength of R or the likeli-hood that S leads to R. (c) In sensitization, exposure to an arousing stimulus increases the likelihood that subsequent pre-sentations of S lead to R.
[(a) Adapted from Groves and Thompson, 1970.]
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85was once scary can become fun). One model of this process, called opponent process theory, explains this effect as a way that organisms maintain emotional stability. Opponent process theory is similar to dual process theory in that it assumes that an experienced event leads to two independent processes—in this case, two emotional processes, one that is pleasurable and one that is less pleasant. The overall emotion that one experiences in response to an event is the combined result of these two independent processes. Repeated experiences have different effects on the initial reaction versus the “rebound” reaction, such that over time the initial response habituates faster than its counterpart. Thus, your first time bungee jumping may not be nearly as much fun as your fifth time. Both dual process theory and opponent process theory suggest that the outcomes of repeated experiences are not as simple as they might at first appear.
A “simple” decrease or increase in responding to an increasingly familiar event may reflect multiple learning processes occurring in parallel. Additionally, repeated experiences can change not only how an individual reflexively responds to familiar events but also how the person perceives and interprets those events, as described in the following section.
Test Your Knowledge
Maximizing Habituation
In some cases, such as in romantic relationships, it makes sense to try and minimize habituation. In contrast, if you discover that the shirt you are wearing is itchy, you’d be better off if you could maximize habituation and avoid sensitization. So what can you do (aside from changing your shirt)? Try to come up with at least three strategies that could help you to minimize your scratching. (Answers appear in the Answers section in the back of the book.)