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Sensory Substitution

According to Noe’s account, we should think about the modality of perception- involved brain cells as primarily constituted by dynamic sensorimotor relations with the environment. Noe supports this idea with an early experiment on so-called sensory substitution by Paul Bach-y-Rita (1934-2006). In the experiment (Bach-y-Rita et al. 1969), Bach-y-Rita equips a blind person with a shoulder-carried camera, connected to a belt with a grid of small skin vibration units so the input array of light from the camera is transduced to vibrations on the stomach. Blind people with the belt learn to navigate environments and interact with things surprisingly well—and can even make successful swats at ping-pong balls. Bach-y-Rita claims that this works because we don’t see with our eyes; we see with our brain.

Yet Noe takes Bach-y-Rita’s tactile substitution experiment as demonstrating perceptual plasticity without neural plasticity, and finds this to be evidence for his enactive position:

Bach-y-Rita’s sensory substitution system is perceptual plasticity without neural plasticity. What better reason could there be to acknowledge that we need to look beyond the brain if we want to get a handle on what is bringing about the dramatic changes in the character of experience that we witness? (Noe 2009, p. 58)

However, according to Bach-y-Rita’s own account, his research is all about brain plasticity:

In our studies, tactile vision substitution has been a model of brain plasticity. The brain is a plastic organ, with various mechanisms of information transmission and mechanisms of compensation for damage and sensory loss. (Noe and Thompson 2002, p. 498)

But Noe explains that:

Bach-y-Rita used full grown and therefore relatively nonplastic adults as his subjects. (Noe 2009, p. 58)

Yet Bach-y-Rita notes:

Reorganization of brain function is possible not only in early development, but throughout life, although after certain “critical periods,” specific training or appropriate rehabilitation is necessary. (Noe and Thompson 2002, p. 498)

Noe also notes that:

Moreover, he found that people adapted to his sensory substitution not in weeks or days but in hours and minutes. That’s just not enough time for any significant internal rewiring to occur. (Noe 2009, p. 58)

However, it is common knowledge that significant rewiring can take place in seconds through synaptic changes.[1] Indeed it is difficult to see how the subject could have learned how to use Bach-y-Rita’s system without relying on plasticity, because learning depends on brain plasticity.[2] Altogether, then, it is difficult to see what Noe has in mind when he states that Bach-y-Rita’s system is an example of perceptual plasticity without brain plasticity. Indeed it is unclear why we should rule out Bach-y-Rita’s own account that brain plasticity explains whatever visual experience the subject has.

  • [1] See, for example, Brodal (2010, p. 50), an introductory neuroscience text, where it is stated that“It is customary to distinguish between short-term and long-term synaptic plasticity, without asharp transition. Short-term plasticity lasts from less than a second to some minutes, whereas longterm plasticity can last for at least several weeks.”
  • [2] “Ah learning is likely to involve changes in the properties of existing synapses, formation of newones, and removal of inappropriate ones. Such use-dependent plasticity continues throughout lifeand is the nervous system’s means of adapting to new and changing conditions, in both the bodyitself and the environment” (Brodal 2010, p. 154).
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