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 lev-els. 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).
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, sensiti-zation can rapidly dissipate in some situations and can lead to longer-lasting learning in others (Borszcz, Cranney, & Leaton, 1989; davis, 1972, 1980; poon
& young, 2006). 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 sensitiz-ing 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 his house might be responding not just to that particular photographer but, as an aftereffect, to other even more annoying photographers that he has previ-ously encountered.
one way that researchers study sensitization experimentally in humans is using the skin conductance response (SCR).
the sCr is a rapid change in the skin’s electrical conductivity that is caused by the nervous system and associated with
80 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.
Data from Davis, 1989.
sensitization. A phenomenon in which a salient stimulus (such as an electric shock) temporarily increases the strength of responses to other stimuli.
skin conductance response (SCR).
A change in the skin’s electrical conductivity associated with emotions such as anxiety, fear, or surprise.
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79anxiety, fear, or surprise. In the laboratory, researchers record sCrs with electrodes similar to those used for recording electroencephalograms (eeg; see Chapter 2 for details). exposure to an unexpected loud noise (say, an explosion or a yell) causes a pronounced startle response in humans, accompanied by a sharp sCr. a neutral musical tone may cause a mild orienting response as well as a small sCr. If the loud noise is played before presentation of the tone, the participant’s sCrs to the tone are stronger than they would be without the loud noise (Lang, davis, & ohman, 2000). portable sensors are now commercially available that make it possible for you to record your sCrs 24-7 (figure 3.3a). In principle, you could use such sensors to track and monitor events in your life that are leading to sensitization (figure 3.3b; poh, swenson, & picard, 2010). access to such information might make it easier to avoid or mitigate some psychological disorders (discussed in more detail in the Clinical perspectives section).
Loud noises can sensitize a person’s response to tones, just as electric shock sensitizes the startle response in rats. surprisingly, if the order of stimuli is reversed, then the effects on responding can also reverse. specifically, if a rela-tively quiet tone is presented just before a much louder and more startling noise, then the startle response to the louder noise may be reduced relative to the response that would have occurred if only the startling sound had been heard.
this effect is called prepulse inhibition. prepulse inhibition is similar to habituation in that some stimuli are “tuned out” based on past experience, leading to reduced responses. however, it is also similar to sensitization in that (1) the initial weak stimulus can affect responding to a wide range of subsequent stimuli, including stimuli in other modalities; and (2) a single presentation of the weak stimulus can produce the effect (Braff, geyer, & swerdlow, 2001). these properties have led some researchers to describe prepulse inhibition as a case of desensitization, in which past experiences reduce responses to a wide range of stimuli (poon &
young, 2006; poon, 2012). you are probably familiar with the idea of people becoming “desensitized” to violence or to events that are unpleasant (like chang-ing dirty diapers). It is difficult to distchang-inguish desensitization from habituation in these natural contexts, because both involve decreased responses to specific kinds of stimuli. Laboratory studies of the neural processes engaged by habituation and desensitization provide new ways of disentangling these two closely related phenomena (discussed further, below, in the Brain substrates section).
Interestingly, the nature of repeated events and their distribution in time are not the only factors that determine whether they will lead to habituation, desensitization, or sensitization. a series of events that might normally lead to habituation can, in a sick animal, lead to sensitization (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 sen-sitized to annoying intrusions of privacy and therefore more likely to strike back.
Homework Sleep
Class
Time
9:00am 12:00pm 3:00pm 6:00pm 9:00pm 12:00am 3:00am 06:00am 9:00am (a)
(b) Figure 3.3 Continuous recording of skin conductance responses (a) Wrist-worn sen-sors make it possible to record measurements of nervous sys-tem activity throughout the day and night. (b) Skin conductance responses (here shown as vertical deflections) can provide indica-tions of how repeated experiences lead to increases or decreases in physiological responses.
(b) Data from Poh, Swenson, & Picard, 2010.
Empatica
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 when a stimulus s is presented, it evokes a chain of neural responses that (1) eventually leads to activation of a motor response r and (2) also activates a state system that signals detection of a stimulus (figure 3.4a). habituation after repeated expo-sures to the stimulus (let’s say 10 repetitions) can then be modeled as a weakening of the connection between s and r (figure 3.4b), combined with only 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 stimulus is not very arousing, then the weakened connection essen-tially determines responding. sensitization can be modeled as an increase in the effect of the state system on sensory-evoked responses (figure 3.4c), such that even harmless stimuli may be able to produce strong responses.
Stimulus S Figure 3.4 The dual process
theory of habituation Dual process theory suggests that both habituation and sensitiza-tion processes occur in parallel during every presentation of a stimulus and that the final response after repeated pre-sentations results from the combination of both processes.
(a) Initially, a stimulus such as S activates sensory neurons that lead to a motor response R and also activate a separate state system signaling detection of the stimulus. (b) In habituation, repeated presentations of S can weaken the connections between neurons (thinner arrow), thus reducing the strength of R or the likelihood that S leads to R. (c) In sensitization, exposure to an arousing stimulus increas-es the likelihood that subsequent presentations of S lead to R.
(a) Information from Groves and Thompson, 1970.
dual process theory. The theory that habituation and sensitization are independent of each other but operate in parallel.
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81In dual process theory, both sensitization and habituation processes occur in response to every stimulus presentation, and it is the summed combi-nation 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 the intensity and recency of highly arousing events. 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.
Opponent Process Theory
studies of emotional responses to extreme events—to a roller-coaster ride, for example—suggest that there are multiple phases of emotional responding. an initial phase that is scary is 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 was 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 learning resulting from repeated experiences is not as simple as 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 expe-riences can change not only how an individual reflexively responds to famil-iar 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 wearing a different shirt)? Try to come up with at least three strategies that could help you to maximize your habituation to the repeated tactile stimulation produced by the shirt. (Answers appear in the back of the book.)