RFs

[Jim Bower]

Just a brief note on the question of receptive fields from a biological 
perspective. Classically, receptive fields are defined within neurobiology as
those regions of the stimulus space that, when activated, obviously alter
the firing pattern of the neuron in question (either excite or inhibit, or
both).  Traditionally, the issue of the actual anatomical connectivity 
underlying the formation of the receptive field has not been considered in
much detail (often it is not known).  However, physiologists have recently
discovered that neuronal receptive fields can be far more complicated
than previously assumed.  The most famous current examples are the so-
called nonclassical receptive fields of neurons in visual regions of cerebral 
cortex.  In this case, it has been discovered that peripheral stimuli
that do not by themselves activate a neuron are capable of significantly 
modifying neuronal responses to stimulation of classical receptive
fields.  These effects are probably a result of the understudied and 
underemphasized horizontal connectivity of cerebral cortical networks.  The
neglect of this important network feature is, in fact, largely due to the
emphasis on the restricted "locality" of classical receptive fields and the
resulting (faulty in my view) notion of cortical columnar organization.
The point with respect to the current discussion is that it IS important to
carefully define what is meant by a receptive field and especially to take
into account the anatomical organization of the network involved.
Operational definitions related to the simple response properties of the
neuron can obscure important network properties.  It could be that this 
attention to detail is less important in the case of the simple connectionist
networks currently being constructed.  However, as connectionist models
inevitably become more complex in the pursuit of real usefulness, it will
become increasingly important to be careful about what is meant by a receptive
field. 

Jim Bower