Binocular competitive interactions and recovery of visual acuity in long-term monocularly deprived cats

Vision restoration after brain and retina damage: the "residual vision activation theory"

Vision loss after retinal or cerebral visual injury (CVI) was long considered to be irreversible. However, there is considerable potential for vision restoration and recovery even in adulthood. Here, we propose the "residual vision activation theory" of how visual functions can be reactivated and restored. CVI is usually not complete, but some structures are typically spared by the damage. They include (i) areas of partial damage at the visual field border, (ii) "islands" of surviving tissue inside the blind field, (iii) extrastriate pathways unaffected by the damage, and (iv) downstream, higher-level neuronal networks. However, residual structures have a triple handicap to be fully functional: (i) fewer neurons, (ii) lack of sufficient attentional resources because of the dominant intact hemisphere caused by excitation/inhibition dysbalance, and (iii) disturbance in their temporal processing. Because of this resulting activation loss, residual structures are unable to contribute much to everyday vision, and their "non-use" further impairs synaptic strength. However, residual structures can be reactivated by engaging them in repetitive stimulation by different means: (i) visual experience, (ii) visual training, or (iii) noninvasive electrical brain current stimulation. These methods lead to strengthening of synaptic transmission and synchronization of partially damaged structures (within-systems plasticity) and downstream neuronal networks (network plasticity). Just as in normal perceptual learning, synaptic plasticity can improve vision and lead to vision restoration. This can be induced at any time after the lesion, at all ages and in all types of visual field impairments after retinal or brain damage (stroke, neurotrauma, glaucoma, amblyopia, age-related macular degeneration). If and to what extent vision restoration can be achieved is a function of the amount of residual tissue and its activation state. However, sustained improvements require repetitive stimulation which, depending on the method, may take days (noninvasive brain stimulation) or months (behavioral training). By becoming again engaged in everyday vision, (re)activation of areas of residual vision outlasts the stimulation period, thus contributing to lasting vision restoration and improvements in quality of life.

Neural activity-dependent brain changes in development: Implications for psychopathology

In this paper we discuss the contribution of neural activity-dependent factors to shaping the brain in normal and pathologic development, and we discuss factors that determine the likelihood of recovery from the experience of abnormal neural activity during development.

Bidirectional modifications of visual acuity induced by monocular deprivation in juvenile and adult rats

Recent electrophysiological studies of rodent visual cortex suggest that, in addition to deprived-eye depression, monocular deprivation (MD) also shifts ocular dominance by potentiation of open-eye responses. We used computer-based, two-choice discrimination tasks to assess the behavioral significance of these findings in rats. As expected, prolonged MD, from postnatal day 21 until adulthood (>150 d) markedly decreased visual acuity through the deprived eye. However, we also found that the acuity through the nondeprived eye was significantly enhanced compared with normally reared controls. Interestingly, when the deprived eye was opened in adults, there was a gradual but incomplete recovery of acuity in the deprived eye preceded by a loss of the enhanced acuity in the nondeprived eye. These changes were reversed by again reclosing the eye. These findings suggest that the bidirectional changes in visually evoked responses after MD are behaviorally meaningful and that significant plasticity is exhibited well into adulthood.

The development of optokinetic nystagmus in strabismic and monocularly enucleated subjects

Assymmetries of monocular optokinetic nystagmus (OKN) following anomalous visual experience are thought to be due to disruption at the cortical level. Visual disruption usually results from eye suture (in animals), unilateral dense and central cataracts or strabismus (in humans). Many form-deprived animals and humans frequently show a residual strabismus after lid opening (animals) or cataract extraction and optical correction (humans). We wanted to determine whether strabismus was unique in causing monocular asymmetries of OKN. Two independent observers rated eye movement videotapes of 20 normal subjects, the non-deviating eye of 25 unilateral strabismic subjects and 29 unilaterally eye-enucleated subjects, who were watching either a nasally directed square wave grating, a temporally directed square wave grating, or a blank field. Observers rated the proportion of trials where OKN occurred, the duration of OKN in each trial and the number of beats of OKN within each trial. Monocular OKN was symmetrical in normal subjects for the proportion and duration measures, but half the normal group showed small but significant asymmetries for the beats measure. Subjects in both enucleate and strabismic groups showed asymmetries of OKN favouring nasally directed stimulation, but only the early onset strabismics (as a group) showed asymmetries that were significantly greater (P less than 0.05) than the normal group. Asymmetry scores correlated significantly with age of diagnosis of strabismus for the strabismic group but not with age of enucleation for the enucleate group. The results are discussed in terms of binocular competition.

The extent of visual recovery from early monocular or binocular visual deprivation in kittens

1. The rate and extent of recovery of vision was studied in the deprived eye of kittens that had been monocularly deprived from near birth for periods that lasted from 6 weeks to 18 months. Recovery was measured in the two situations, where either both eyes were open following the initial deprivation (binocular recovery), or else the non-deprived eye was occluded so as to force the animal to employ its deprived eye (reverse occlusion). 2. Measurements were made of the visual acuity of the deprived eye for gratings at frequent intervals during recovery by means of a simple behavioural technique. 3. The acuity that the deprived eye eventually attained declined with increasing length of deprivation in a manner that could be approximated by a simple exponential decay. Only minimal visual recovery was observed in animals deprived beyond 1 year of age; only two of six animals recovered sufficient vision to enable measurement of visual acuity. In general, animals that were reverse occluded recovered better vision than did those that had both eyes open during recovery. 4. The recovery of vision in the deprived eye of monocularly deprived kittens was compared to that observed following equivalent periods of two forms of binocular deprivation, namely dark-rearing and binocular eyelid suture. 5. The recovery from the two forms of binocular deprivation was quite different. Whereas the extent of recovery from dark-rearing was considerably greater than that observed after equivalent periods of monocular deprivation, the recovery of a limited sample of cats that were binocularly deprived by eyelid suture was worse. 6. These findings suggest that some plasticity remains in the visual pathways for a longer time than indicated by experiments that examine the physiological effects of monocular deprivation on various visual cortical structures.