If training can lead to cortical change that helps people recover motor functions after a brain injury, might it also enable people to develop perceptual abilities they don’t have? For example, could discrimination training that leads to cortical change help someone who is deaf learn to hear? This is indeed possible and has in fact already been accomplished with sensory prostheses. Sensory prostheses are electromechanical devices that interface with the brain areas that normally process sensory information.
Figure 3.18 Overcoming learned non-use Patients with cortical lesions affecting one limb (e.g., an arm) often start using the unaffected limb in preference—a learned non-use of the affected limb. The graph shows results for two groups of patients who initially (pre-training) had very little use of one arm. One group under-went constraint-induced move-ment therapy, which forced them to use the affected arm for daily activities. These patients showed dramatic improvements in function of the injured arm compared with control patients not receiving this therapy.
(Adapted from Taub et al., 2002.)
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113To date, the most extensively developed sensory prosthetic technology is the cochlear implant (Figure 3.19). This device electrically stimulates auditory nerves to produce hearing sensations in profoundly deaf individuals, primarily to assist them in processing speech. Multiple electrodes implanted in the cochlea (the part of the ear that converts sounds into neural firing patterns) modify responses in the auditory nerve in ways that roughly simulate the neural activ-ity normally produced by sounds. This technology is most effective in young children and in adults who have only recently lost their hearing. Conventional hearing aids amplify external sounds, but cochlear implants re-create the effects of sounds within the brain, generating “virtual sounds” from information about environmental sound that has been electronically detected and processed. The virtual speech sounds generated by cochlear implants are quite different from normal speech, so people using the implants must learn to discriminate between the new sounds before they can being to understand what they hear, an example of perceptual learning. Like most practice-based learning, speech perception by individuals with cochlear implants shows
initial rapid improvement in the early months of use, followed by more gradual improvement over years (Clarke, 2002; Tajudeen, Waltzman, Jethanamest, & Svirsky, 2010).
It is likely that changes in speech-processing abilities after installation of a cochlear implant are the result of cortical plasticity, but this has yet to be experimentally demonstrated in humans. Neural activity generated by the implant may lead to changes in many areas of cortex, because the implant provides the brain with access not only to new sounds but also to all of the abilities that these sounds provide (such as the ability to engage in spoken conver-sations). Researchers have found that cochlear implants in deaf cats lead to massive changes in auditory cortex (Klinke, Kral, Heid, Tillein, &
Sensory prosthesis of the future?
Researchers at Boston University are developing retinal implants that may provide blind users with virtual sight.
Courtesy of Boston Retinal Implant Project
Electrode array Cochlea Receiver
stimulator Transmitter
coil Microphone
Behind-the-ear speech processor
Body-worn speech processor
Auditory nerve
Figure 3.19 Cochlear implant Cochlear implants use electricity to stimulate neurons in the auditory system, thereby creating virtual speech sounds in the brain.
(Adapted from Clarke, 2002.)
Hartmann, 1999). After implantation, auditory cortex becomes organized differ-ently from that of what is seen in deaf cats without implants and also from that of hearing cats, suggesting that the virtual sounds these cats hear are driving the observed changes in cortex.
Most current sensory prostheses are designed to replace lost abilities, but in principle, it should also be possible to use such devices to enhance existing capabilities or create new sensory abilities. No one knows how well cortical neurons would be able to process inputs from sensors detecting stimuli such as infrared light or ultrasonic sounds that humans are normally unable to perceive.
Given how easily deaf people have learned to process novel inputs from cochlear implants, however, it seems likely that the human brain could accommodate a wide range of machine-provided inputs through perceptual learning.
Interim Summary
■ Learned non-use of a desensitized limb can be overcome by constraint-induced movement therapy, which forces the individual to use the limb.
Learned non-use can be viewed as a type of habituation, whereas constraint-induced movement therapy is an applied form of discrimination training that may lead to recovery through cortical plasticity.
■ Sensory prostheses, such as cochlear implants, provide individuals with new sensory-processing capabilities. Training with a cochlear implant leads to perceptual learning that improves the user’s ability to discriminate simulated speech sounds and also can lead to receptive field changes in auditory cortex.
SYNTHESIS
Habituation, perceptual learning, and other phenomena that result from repeated exposures to stimuli are often described as the simplest forms of learning. This is partly because even the most primitive animals show habituation (even amoe-bas!). Moreover, neither perceptual learning nor habituation demands any obvi-ous effort from the learner, although in the case of perceptual learning, practice can be beneficial. Nervous systems have evolved to collect information about what’s new in the world and to recognize the familiar.
The processes involved in learning about repeated events can, however, be highly complex, involving the combined activities of multiple brain regions inter-acting in many different ways. A good example is the learning of landmarks for spatial navigation. Complex combinations of stimuli, including both visual pat-terns and specific movement patpat-terns, determine how place cells in the hippocam-pus respond. Spatial learning can happen independent of observable responses, which means it is difficult for an observer to determine what another individual is learning about any particular set of landmarks. Who would guess that as you sit in a car staring out of the window, your brain is recording the locations of certain landmarks that can help you to find your way back to that location later?
Repeated experiences can slow down your ability to learn (in the case of habituation) or speed it up (in the case of priming and perceptual learning).
They can also affect your responses to other, seemingly unrelated stimuli (as in the case of sensitization). For example, reading about the cannibalistic murderer Jeffrey Dahmer in the beginning of this chapter was probably novel in your experiences of reading psychology textbooks. If you have gotten used to expe-riencing the relatively predictable material within college textbooks (in other words, if you get bored reading textbooks), then this disturbing example may have caused your learned response to textbooks to dishabituate, which may have
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115in turn increased your capacity to encode and remember the subsequent mate-rial, despite the fact that this material may have little to do with psychopathic killers. How is it that repeated exposures to stimuli can generate such a wide range of learning phenomena? Part of the answer, at least in mammals, is the contribution made by the cerebral cortex, one of the most complex structures in the brain. Experience-dependent changes in how cortical neurons respond to repeated events constitute one of several powerful mechanisms that seem to contribute to both perceptual learning and habituation.
Changes in behavior stemming from memories of repeated events have important implications for our daily lives, especially when the brain is not pro-cessing information the way it should. Understanding the mechanisms by which brains learn from repeated experiences can help clinicians interpret the effects of cortical damage and take steps to alleviate sensory deficits. The ability of brains to adapt in the ways described in this chapter may be the key to overcoming many mental deficits for which there are currently no cures. Thus, although habituation and perceptual learning can sometimes lead to negative outcomes (as in the circumstances surrounding Jeffrey Dahmer and in learned non-use), these processes also point to ways of rehabilitating patients and expanding people’s perceptual abilities.
Key Terms
acoustic startle reflex, p. 78 Aplysia, p. 97
cochlear implant, p. 113 comparator model, p. 89 constraint-induced movement
therapy, p. 112 cortical plasticity, p. 103 differentiation theory, p. 90 discrimination training, p. 87 dishabituation, p. 79
dual process theory, p. 83 familiarity, p. 95 habituation, p. 77 Hebbian learning, p. 105 heterosynaptic, p. 100 homosynaptic, p. 99 learned non-use, p. 111 learning specificity, p. 88 mere exposure learning, p. 86 multimodal, p. 104
novel object recognition, p. 94 orienting response, p. 78 perceptual learning, p. 85 place cell, p. 108 place field, p. 108 priming, p. 95 receptive field, p. 102 sensitization, p. 82 sensory prosthesis, p. 112
skin conductance response (SCR), p. 82
spatial learning, p. 92 spontaneous recovery, p. 81 stroke, p. 111
synaptic depression, p. 99 word-stem completion
task, p. 95
read the chapter again, you may learn even more.
Is this an example of learning from repeated events?
4. When structural MRIs of London taxi drivers were compared with those of control participants who did not drive taxis, researchers discovered that the average size of the hippocampus in the taxi drivers was larger than in the control partici-pants and was correlated with the number of years they had been driving a taxi (Maguire et al., 2000).
Why might that be?
Concept Check
1. A weight lifter repeatedly lifts a barbell. After sev-eral repetitions, he begins lifting it more slowly, until eventually he stops. Is this habituation?
2. A common example of sensitization is the experi-ence of walking down a dark alleyway at night. The setting may produce feelings of nervousness, which lead to heightened arousal: you’ll jump if you hear a noise behind you. Can you think of any situations in which people are intentionally sensitized?
3. After reading this chapter, you’ll have learned at least some of the information it contains. If you
117 Behavioral Processes
Basic Concepts of Classical Conditioning
Error Correction and the Modulation of US Processing Modulation of CS Processing Further Facets of Conditioning
Brain Substrates Mammalian Conditioning of
Motor Reflexes
Brain Rhythms and Conditioning Invertebrates and the Cellular
Basis of Learning
Clinical Perspectives Learning and Memory in Everyday
Life: Extinguishing a Drug Habit