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

Abstract

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.

Figure 1.

Visual acuity of normally reared adult animals assessed using two different forced-choice discrimination tasks. A, Visual acuity obtained binocularly using visual task 1. A group acuity threshold of 0.89 cyc/° was observed (n = 6). B, Visual acuity obtained monocularly is comparable with that obtained binocularly in normally reared rats. Using visual task 2, acuity obtained when subjects (n = 7) performed the task using both eyes (0.89 cyc/°) was comparable with those obtained when performing the task using only the left (0.91 cyc/°) or right (0.89 cyc/°) eye.

Figure 2.

Long-term MD produces a profound decrease in deprived-eye acuity and an enhancement of nondeprived-eye function. A, Monocular deprivation beginning at P21 and maintained for >150 d results in a significant decrease in deprived-eye acuity (MD-DE; 0.20 cyc/°, n = 10) compared with the acuity obtained monocularly from normally reared animals of a comparable age (NR-LE; 0.91 cyc/°, n = 7). Data from normally reared animals is the same data as presented in Figure 1B (left eye). B, Visual acuity is enhanced in the nondeprived eye of long-term MD animals. Monocular visual acuity (MD-NDE; 1.20 cyc/°, n = 10) obtained before opening of the deprived eye of long-term MD animals is significantly greater than the monocular acuity of normally reared animals (NR-RE; 0.89 cyc/°, n = 7) of a comparable age. Data from normally reared animals is the same data as presented in Figure 1B (right eye).

Figure 3.

Enhanced visual acuity of the nondeprived eye of long-term MD animals decreases as acuity in newly opened eye increases. A, Individual data obtained from six long-term MD animals. B, Group summary of individual data presented in A demonstrating that acuity thresholds of the nondeprived eye decrease before the onset of visual recovery in the deprived eye. Asterisks indicate time at which a significant change in acuity threshold is first observed. NDE, Nondeprived eye; DE, deprived eye.

Figure 4.

Changes in deprived and nondeprived eye acuity of long-term MD animals are bidirectional. A–D, Individual cases demonstrating that, after instatement of binocular vision, acuity thresholds of the nondeprived eye decrease as acuity thresholds of the deprived eye increase. Resuturing the initially deprived eye results in a return of enhanced visual acuity in the nondeprived eye, which once again declines after reinstatement of binocular vision. Data in D are abbreviated because of subject mortality.