Mind, Body, World- Foundations of Cognitive Science, 2013a

was able to represent the actual distance between all city pairs with a high degree
of accuracy.
The discovery of coarse coding in navigational networks has important theoretical
implications. Since the discovery of place cells in the hippocampus (O’Keefe
& Dostrovsky, 1971), it has been thought that one function of the hippocampus is to
instantiate a cognitive map (O’Keefe & Nadel, 1978). One analogy used to explain
cognitive maps is that they are like graphical maps (Kitchin, 1994). From this, one
might predict that the cognitive map is a metric, topographically organized, twodimensional
array in which each location in the map (i.e., each place in the external
world) is associated with the firing of a particular place cell, and neighbouring place
cells represent neighbouring places in the external world.
However, this prediction is not supported by anatomical evidence. First, place
cells do not appear to be topographically organized (Burgess, Recce, & O’Keefe, 1995;
McNaughton et al., 1996). Second, the receptive fields of place cells are at best
locally metric, because one cannot measure the distance between points that are
more than about a dozen body lengths apart because of a lack of receptive field
overlap (Touretzky, Wan, & Redish, 1994). Some researchers now propose that the
cognitive map doesn’t really exit, but that map-like properties emerge when place
cells are coordinated with other types of cells, such as head direction cells, which fire
when an animal’s head is pointed in a particular direction, regardless of the animal’s
location in space (McNaughton et al., 1996; Redish, 1999; Redish & Touretzky, 1999;
Touretzky, Wan, & Redish, 1994).
Dawson et al. (2000) observed that their navigational network is also subject to
the same criticisms that have been levelled against the notion of a topographically
organized cognitive map. The hidden units did not exhibit topographic organization,
and their inaccurate responses suggest that they are at best locally metric.
Nevertheless, the behaviour of the Dawson et al. (2000) network indicated that
it represented information about a metric space. That such behaviour can be supported
by the type of coarse coding discovered in this network suggests that metric,
spatial information can be encoded in a representational scheme that is not isomorphic
to a graphical map. This raises the possibility that place cells represent
spatial information using a coarse code which, when its individual components are
inspected, is not very map-like at all. O’Keefe and Nadel (1978, p. 78) were explicitly
aware of this kind of possibility: “The cognitive map is not a picture or image which
‘looks like’ what it represents; rather, it is an information structure from which
map-like images can be reconstructed and from which behaviour dependent upon
place information can be generated.”
What are the implications of the ability to interpret the internal structure of
artificial neural networks to the practice of connectionist cognitive science?
186 Chapter 4