Distribution and temporal response characteristics of evoked potentials in the visually deprived cat

Suppression of visual cortical evoked responses following deprivation of pattern vision in adult mice

The effect of visual loss on the adult neocortex can have significant impact on the success of a visual implant. Recent research has shown that the adult neocortex retains substantial plasticity following a disruption to its afferent input. The result of these changes may hamper the development of a visual prosthesis if visual sensation cannot be effectively restored by stimulation of the surviving elements of the visual pathway. In order to evaluate further the visual performance of the mammalian adult brain following visual loss, especially the dominant form of blindness in humans, namely loss of pattern vision, we examined the cortical evoked potential of adult mice following 7, 30 and 120 days of visual deprivation via bilateral eyelid suture. Cortical potentials were elicited with a flash visual stimulus or by electrical stimulation of the retina. We found that after 7 days deprivation there was a potentiation of the evoked response while at 30 and 120 days deprivation the visual evoked responses were significantly reduced. Increasing the visual stimulus intensity reduced the effects. The electrical evoked potential demonstrated a corresponding reduction in stimulus threshold at 7 days and a corresponding rise (40-50%) after 30 and 120 days. These findings suggest that the adult brain exhibited significant experience-dependent modifications following visual loss, and the impact depended on the duration of deprivation. Such reduction in visual responsiveness, especially with electrical activation, will need to be taken into account in the development of a visual implant.

Differential effect of visual deprivation on cytochrome oxidase levels in major cell classes of the cat LGN

Cytochrome oxidase histochemistry was used to examine the effects of visual deprivation on the development of neurons in the lateral geniculate nucleus of the kitten. Early postnatal monocular suture results in a decrease in reactivity within the neuropil of visually deprived binocular laminae A, A1, magnocellular C, and medial interlaminar nucleus. Within these regions, monocular suture has a greater effect on the relative numbers of, and the growth of darkly reactive (normally large), presumed Y-cells than on other less reactive geniculate neuronal classes. The decreases in the reactivity of the neuropil may be attributed to the decreases in the number of mitochondria, the number of darkly reactive mitochondria, and/or the number of darkly reactive mitochondria localized within dendrites. Although all classes of dendrites appear to be adversely affected, the decrease in C.O. reactivity was most dramatic within the presumed proximal dendrites of class 1 Y-cells. These dendrites were identified by the type of synaptic contacts they formed with retinal terminals (Rapisardi and Miles, '84, J. Comp. Neurol. 223:515-534; Wilson et al., '84, Proc. R. Soc. Lond. [Biol.] 221:411-436). As with Y-cells, the effects of monocular suture on the large darkly reactive cells were not as dramatic at sites where binocular interactions were either absent or where they had been experimentally eliminated. Based on the present and previously reported findings from several laboratories, it is likely that the selective physiological and morphological effects of monocular suture on Y-cells are accompanied by metabolic deficits involving both dendrites and perikarya. These effects appear to be due more to binocular interactions than to visual deprivation per se.

Critical flicker frequency in monocularly deprived cats

Critical flicker frequency (CFF) was determined for both eyes of long-term monocularly deprived (MD) cats over a wide luminance range. Although MD cats could discriminate flicker before and after lid opening, CFF of the deprived eye (30 Hz) was much lower than CFF of the non-deprived eye (40 Hz) and the CFF of the non-deprived eye was lower than a normal cat's monocular CFF (58 Hz). The CFF deficit of the deprived eye became less pronounced at low luminance levels. The observation (and magnitude) of a CFF deficit for the deprived eye is compatible with the reports of a Y-cell loss in LGNd. The CFF deficit of the non-deprived eye has no obvious explanation.

Pattern processing and slow waves in visual cortex of visually deprived cats

The slow-wave response recorded from electrodes within the visual cortex of the awake cat has been shown to reflect the pattern processing activities of the visual cortex. In a cat deprived of pattern vision by monocular lid closure, the response evoked from the non-deprived eye by patterned and diffuse stimuli show differences similar to those found in a normal cat. However, when these same stimuli are presented to the deprived eye, the differences are not present. These slow-wave data confirm previous data from single unit recordings in showing that pattern processing is altered by deprivation.