Ryder and Martin (1998) have attempted to discredit Dretske's account of belief by offering a counter-example. They have pointed out that the autonomic nervous system (ANS) is capable of associative learning, yet few people would ascribe beliefs to it. Contrary to popular belief, the ANS does not simply follow fixed action patterns, but can be conditioned. The ANS is controlled by the central autonomic network (CAN), which is located in the brainstem, mid-brain and fore-brain. Each individual's CAN contains an indicator which represents whether she is standing, and which compensates for the loss of blood to the brain (caused by gravity) when she stands up, by perfusing her brain with blood. When an astronaut stands up in zero-gravity, her blood rushes to her head, because the ANS is used to counteracting the effects of gravity on earth. However, with time, the astronaut's ANS un-learns this compensating behaviour. When the astronaut returns to earth, her ANS has to re-learn the skill of compensating for the loss of blood to the brain when she stands up. The authors argue that the above example meets Dretske's criteria for genuine learning, and that according to his criteria, the ANS is capable of having beliefs (e.g. about the astronaut's posture) and desires (e.g. to adequately perfuse her brain with blood). On the other hand, the attribution of beliefs to the ANS sounds peculiar, so Ryder and Martin argue that Dretske's criteria for having a belief must be insufficient.

I am not sure that this is a convincing counter-example. In associative learning, an individual has to form an association between two events. However, one could try to explain the behaviour of the astronaut's ANS as a case of non-associative learning: in the absence of gravity, the ANS's compensatory response to standing naturally attenuates, but when the astronaut returns to earth, her body is re-sensitized, and the original response re-appears. Ryder has since conceded that the ANS response in the astronaut may not necessarily be a case of conditioning:

The key is whether the response in microgravity is just a "relaxing" of the normal response, or if something more active is going on. As I recall, there was some evidence for the latter - for one thing, it seems to be linked to the discriminative stimulus, the vestibular inputs (personal email, 4 September 2003).

Case study: conditioned leg withdrawal in headless cockroaches

Cockroach. Clip art licensed from the Clip Art Gallery on DiscoverySchool.com

A more clearcut case of associative learning without a brain is leg withdrawal, which can be conditioned in headless cockroaches or in isolated leg and thoracic ganglion preparations. The thoracic ganglion is a much more complicated cluster of nerves in the cockroach than the brain (Kentridge, 1995). Cockroaches are thus capable of "distributed" learning. Yet it would seem strange to describe their leg withdrawal behaviour as a manifestation of a belief.

Case study: conditioning in the severed spinal cords of rats

Diagram of a laboratory rat, showing its spinal cord.
New research shows that severed spinal cords in rats are capable of undergoing associative learning.
Image courtesy of National Institute of Health, USA.

Another troubling case for Dretske is discussed by Grau (2002). Rats whose spinal cords had been severed at the second thoracic vertebra (T2), leaving them paralysed below their mid-sections, were shown to be capable of undergoing instrumental conditioning within their spinal cords. The rats were placed in an apparatus where a shock was applied to one of their hind legs, whenever it made contact a solution of salt water beneath the rats. The rats soon learned to maintain the leg in a flexed (up) position, thereby avoiding shock. Moreover, the duration of each flexion increased as the training continued, peaking at a little under a minute. Although the rats' hind legs were not controlled by their brains, their spinal cords were capable of being conditioned. The experiment was conducted with yoked control subjects (to exclude the possibility that the shock itself was causing the rats' hind legs to flex), and the response was also observed to occur later under normal conditions, verifying that learning had indeed taken place in the animals' spinal cords. While this case demonstrates that true learning can take place within an animal's nervous system in the absence of a brain, we would not normally explain this by saying that the rats' spinal cords acquired new beliefs. It would be more appropriate to say that they acquired a new response pattern.

Back to: Does associative learning require a mind? *** SUMMARY of conclusions reached References