On Constructing a Reality - Understanding Understanding: Essays on Cybernetics and Cognition - page 223

FIGURE 15.
activity is transmitted to deeper regions of the brain via their axons, which
are bundled together to form the optic nerve (7). Computation takes place
within the two layers labeled 3 and 5, that is, where the synapses are located.
As Maturana has shown 3 it is there where the sensation of color and some
clues as to form are computed.
Form computation: Take the two-layered periodic network of Figure 16,
the upper layer representing receptor cells sensitive to, say, “light.” Each of
these receptors is connected to three neurons in the lower (computing)
layer, with two excitatory synapses on the neuron directly below (symbol-
ized by buttons attached to the body) and with one inhibitory synapse (sym-
bolized by a loop around the tip) attached to each of the two neurons, one
to the left and one to the right. It is clear that the computing layer will not
respond to uniform light projected on the receptive layer, for the two exci-
tatory stimuli on a computer neuron will be exactly compensated by the
inhibitory signals coming from the two lateral receptors. This zero response
will prevail under strongest and weakest stimulations as well as for slow or
rapid changes of the illumination. The legitimate question may now arise:
“Why this complex apparatus that doesn't do a thing?”
Consider now Figure 17, in which an obstruction is placed in the light
path illuminating the layer of receptors. Again all neurons of the lower layer
will remain silent, except the one at the edge of the obstruction, for it