Summary

1. Humans fitted with reversing prisms can learn to use the inverted vision to guide movement, presumably through higher-level processing, but amphibians with reversed vision cannot. However, adult amphibians and fish can regenerate connections from the eye to the brain after a severing of the optic nerve. See Figure 9.1
2. The chemoaffinity hypothesis proposed that the regenerating retinal axons recognized some unique identity of tectal neurons to reestablish the original projections. At least one factor guiding this reinnervation is expression of ephrin receptors by retinal ganglion cells and ephrins by tectal neurons. See Figures 9.2 and 9.3
3. However, experiments resulting in tectal expansion and compression demonstrated some flexibility in the regeneration of retinotectal connections. Likewise, normal growth of the periphery of the retina and the posterior tectum requires continuous shifting of the topographic projection from the retina. See Figures 9.4 and 9.5
4. In three-eyed frog preparations, two eyes innervating the same tectum segregate, each eye projecting to a separate rostral-caudal strip of tectum. This segregation is activity dependent, as visual experience allows neighboring retinal neurons to dominate particular tectal targets, stabilizing those synapses. In normal optic nerve regeneration, this experience-dependent process fine-tunes topographic mapping of visual space onto the tectum. See Figure 9.7
5. In monkeys and cats, an eye silenced by visual deprivation or TTX during a sensitive period in development will show little or no evidence of vision in that eye. Such visual deprivation in adulthood has relatively little effect on visual ability. See Figure 9.9
6. Humans deprived of vision early in life have difficulty identifying objects and faces even after years of visual experience in adulthood. See Figures 9.10 and 9.11
7. Ocular dominance patterns in visual cortex are established and maintained by visual experience, such that the active eye comes to predominate activity of cortical neurons. The competitive nature of this process explains how binocular deprivation may have relatively little effect on ocular dominance. See Figure 9.12
8. Strabismus shifts one eye enough that objects in visual space no longer strike corresponding portions of the two retinas. If this happens early in life, cortical neurons retain connections from one eye or the other, but very few receive binocular input. See Figure 9.13
9. Surgical correction of strabismus must occur in childhood for the person to develop full binocular visual capacity, presumably by preserving binocular innervation of cortical neurons. See Figure 9.14
10. Owlets use experience to align maps of visual space and auditory space in the tectum, in a Hebbian-like process mediated by NMDA receptors. See Figure 9.16
11. Each olfactory sensory neuron (OSN) expresses only one of the thousands of genes for olfactory receptor proteins. Each OSN expressing a particular receptor projects to only two glomeruli in the olfactory bulb. Olfactory experience causes misprojecting OSNs to die, sharpening the map of olfaction onto the bulb. See Figures 9.17 and 9.18
12. The mapping of tactile information from the body to somatosensory cortex is maintained by experience-guided activity of afferents. Removing whiskers in young mice results in expanded input to the cortex from the surviving neighboring whiskers, at the expense of the loss of input from the missing whiskers. See Figure 9.19
13. In monkeys, reducing afferent activity from a digit results in less representation in somatosensory cortex (S1). Increasing activity of a digit causes its representation in S1 to expand. Similarly, in humans who lose a hand in adulthood, the representation of body parts in neighboring S1 expands, a process that can be reversed if a new hand is transplanted. These results indicate that mapping of tactile afferents to S1 is an experience-dependent competitive process going on throughout life. See Figures 9.20–9.22