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Several eye movement properties develop in synchrony with the sensory parts of the visual system. The ability to fixate improves as the fovea and acuity develop. The ability to make smooth pursuit eye movements, as opposed to saccades that keep the eyes tracking the stimulus, improves as the ability to detect the velocity of a moving stimulus improves. Vergence movements improve as depth perception becomes more acute. On the other hand, saccadic eye movements show changes that are not related to sensory properties. These are the establishment of a 200-ms delay between saccades, the disappearance of oscillatory movements away from the object and back again, and the establishment of a single large saccade, as opposed to several smaller saccades, to small targets far away. The crucial component in the development of these properties is most likely in the brainstem, rather than in the afferent pathways.

The initial events in the development of the visual system, which occur before the connections are formed, are under genetic and molecular control. These include the generation and migration of cells, the projection of their axons to their targets, the formation of topographic projections, the general organization of the terminals within the targets, the crossing of fibers in the chiasm, the position of areas within the cerebral cortex, and the formation of cytochrome oxidase blobs in the cortex. Most of this takes place before birth. Later events, occurring after the connections are formed, and in most cases after the eyes open, are under the control of electrical activity. This includes the refinement of maps within a nucleus, the refinement of columns within the cortex, and the formation of layers within the lateral geniculate nucleus. Some development is affected by both molecular cues and electrical activity. This includes the specificity of layers within the cortex, including the specificity of afferent connections, efferent connections, and connections within the cortex, also the formation of pinwheels around the cytochrome oxidase blobs. Given the complexity of the system, it is amazing that the various cues work together to produce the final result.

The summary of all this work is that some elements of the organization of the visual system are present at birth, that there is very substantial development over the first 4 weeks of age in parallel with the large increase in the number of synapses during this period, and that there is a further refinement of visual properties over the next 3 months or so. These ages apply to the cat and are different in the primate, but the same general sequence of events occurs in both. The elements of a system of orientation columns are there at birth, but the selectivity of individual cells within this system of columns develops after birth. Some cells are definitely responsive to direction of movement at birth. On the other hand, the input from the two eyes to the cortex initially overlaps, and only segregates into eye-specific columns after birth. Sensitivity to disparity develops over the same period as segregation of ocular dominance. Acuity improves as the size of the center of the receptive fields of the sustained cells gets smaller and the eyeball gets larger. Movement sensitivity improves as the temporal properties of the Y cell system improves. The development of physiological properties, therefore, tallies with the development of psychophysical properties. In both cases, most properties, with the exception of stereopsis, exist at birth, but substantial refinement and tuning occurs postnatally.

This short discussion shows that a number of different problems can lead to disruption of the signals reaching the visual cortex. On the sensory side, there is diffusion of the image on the retina (cataract), poor focus of the image on one retina compared with the image on the other (anisometropia), poor focus along one axis (astigmatism), and excessive growth of the eyeball (myopia). On the motor side, there are a number of causes of misalignment of the two eyes and mismatch of the information from the two retinas (strabismus). As emphasized in the chapter on the development of the visual system, there is an interaction between sensory and motor systems so that deficits in one will lead to deficits in the other. In all cases there is a danger that the connections in the visual cortex will become rewired to compensate for the deficit, and that the rewiring will become permanent if the underlying deficit is not treated.