Adaptive Synaptic Connections Formed in the Visual Pathways in Response to Congenitally Aberrant Inputs

In Siamese cats, some of the axons that pass from the retina to the brain are misrouted in the optic chiasm. The abnormal fibers, which arise in a well-defined and limited part of the temporal retina, pass to the wrong side of the brain. This simple developmen-tal aberration, originating in the eye or in the optic chiasm, produces a number of complex and intriguing secondary changes: the development of neural path-ways and synaptic onnections i modified in the ab-normal brains so that the animal, even though it is not receiving a normal, orderly mapping of the outside world, can still respond appropriately tovisual stimuli. We will consider the central modifications that occur in Siamese cats and will show that they are adaptive. There are rules of synaptic development on the basis of which cortical connections can be modified. Two strategies are used within the developing brain to produce the adaptive modifications: one is to suppress sequences of neuronal activation that contain internal inconsistencies and that do not represent the outside world in a coherent manner; the other is to correct such sequences. Each strategy can be used on either the input to the cortex or upon its output o the motor centers. Experimental observations of Siamese cats have dem-onstrated all four types of central modification. They have shown that a part of the cortical input can be either suppressed or rearranged as a sequence that represents the visual world in an orderly manner. Similarly, the cortical output can be suppressed or rearranged. The experimental observations upon which these conclusions are based have been pub