Repetitive visual stimulation enhances recovery from severe amblyopia

Abstract

Severe amblyopia, characterized by a significant reduction in visual acuity through the affected eye, is highly resistant to reversal in adulthood. We have previously shown that synaptic plasticity can be reactivated in the adult rat visual cortex by dark exposure, and the reactivated plasticity can be harnessed to promote the recovery from severe amblyopia. Here we show that deprived-eye visually evoked responses are rapidly strengthened in dark-exposed amblyopes by passive viewing of repetitive visual stimuli. Surprisingly, passive visual stimulation rapidly enhanced visually evoked responses to novel stimuli and enhanced the recovery from severe amblyopia driven by performance of active visual discriminations. Thus a series of simple, noninvasive manipulations of visual experience can be used in combination to significantly guide the recovery of visual response strength, selectivity, and spatial acuity in adult amblyopes.

Figure 1.

Chronic monocular deprivation induces severe amblyopia. (A) Experimental timeline: chronic monocular deprivation (MD) was initiated at eye opening (postnatal day 14 [P14]) and maintained into adulthood (postnatal day 120 [P120]). (B) Polar plots of a representative layer IV neuron reveal a decrease in the strength and orientation selectivity of spiking output evoked by visual stimulation of the deprived eye (Dep Eye, gray) relative to the nondeprived eye (non-Dep Eye [black] 25 cycles of 0.05 cycles/degree, 100% contrast full-field vertical gratings reversing at 0.5 Hz). (C) Post-stimulus time histograms of spiking output from a representative layer IV neuron evoked by visual stimuli of preferred (black) and orthogonal (gray) orientations presented to the nondeprived eye. (D) Post-stimulus time histograms of spiking output from a representative layer IV neuron evoked by visual stimuli of preferred (black) and orthogonal (gray) orientations presented to the deprived eye. (E) The cumulative distributions of single unit activity reveal a significant decrease in neuronal spiking rates evoked by stimulation of the deprived eye (gray) relative to the nondeprived eye (black) (KS test, P = 0.004, n = units, subjects). (F) The cumulative distributions of single unit activity reveal significantly less orientation tuning in the neuronal spiking evoked by stimulation of the deprived eye (gray) relative to the nondeprived eye (black). (G) Chronic monocular deprivation induced a significant decrease in the spatial acuity of the deprived eye (gray) (paired t-test, P < 0.001); (inset) nondeprived eye spatial acuity assessed with a vertical (trained) stimulus (black) and a novel stimulus rotated 45° from vertical (gray) reveals a significant decrease in spatial acuity (paired t-test, P < 0.05) after a change in stimulus orientation.

Figure 2.

Passive viewing of repetitive visual stimuli induces a rapid, noninput specific enhancement of visual responses in dark-exposed amblyopes. (A) Experimental timeline: following chronic monocular deprivation (MD; from P14 to P110), one cohort received 10 d of dark exposure (DE). Subsequently, all subjects received reverse deprivation, and passively viewed repetitive visual stimuli (100 cycles of 0.05 cycles/degree, 100% contrast full-field vertical gratings reversing at 0.5 Hz) with the previously deprived eye (Dep Eye). (B) Passive visual stimulation (Stim) induced a rapid enhancement of layer IV VEP amplitudes in dark-exposed amblyopes (average ± SEM, one-way ANOVA, F_(2,18) = 18.3033, P < 0.0001, [*] P < 0.05 vs. baseline, Tukey Kramer HSD post hoc); (inset) representative deprived eye VEPs (Dep Eye) at time 0 (baseline) and 240 min post passive visual stimulation. (C) Passive visual stimulation (Stim) did not induce a rapid enhancement of layer IV VEP amplitudes in amblyopes that did not receive dark exposure (average ± SEM); (inset) representative deprived eye VEPs (Dep Eye) at time 0 (baseline) and 240 min post passive visual stimulation. (D) Visual stimuli with novel spatial frequencies evoked enhanced VEPs in dark-exposed amblyopes that received passive visual stimulation (black) (average amplitude [norm to max] ± SEM). (E) Visual stimuli with novel orientations evoked enhanced VEPs in dark-exposed amblyopes that received passive visual stimulation (black) (average ± SEM, repeated measures ANOVA F_(1,10) = 6.544, P = 0.028. (F) Polar plots of representative layer IV neurons demonstrate that passive repetitive visual stimulation (Stim [black]) increased the strength of visually evoked spiking across all orientations. (G) No improvement in orientation selectivity of visually evoked spiking output following passive repetitive visual stimulation in dark-exposed (DE) amblyopes (black) versus control amblyopes without dark exposure (gray; KS test, P = 0.908; n = units, subjects). (H) Passive visual stimulation (Stim) did not improve spatial acuity of the deprived eye of dark exposed (DE) amblyopes (average monocular spatial acuity ± SEM, DE + Stim [black] 0.035 ± 0.013, n = 7; DE [gray], 0.036 ± 0.011, n = 6; t-test, P = 0.955). Scale bars: 50 μV; 50 msec.

Figure 3.

Passive viewing of repetitive visual stimuli does not induce a rapid enhancement of visual responses in dark-exposed binocular subjects. (A) Experimental timeline: following normal binocular visual experience (12-h light/12-h dark/day, from P14 to P110), one cohort received 10 d of dark exposure (DE). Subsequently, all subjects passively viewed repetitive visual stimuli (Stim, 100 cycles of 0.05 cycles/degree, 100% contrast full-field vertical gratings reversing at 0.5 Hz) with their dominant, contralateral eye. (B) Passive visual stimulation (Stim) did not induce a rapid enhancement of layer IV VEPs in dark-exposed binocular subjects (average ± SEM); (inset) representative contralateral eye VEPs at time 0 (baseline) and 240 min post visual stimulation. (C) Passive visual stimulation did not induce a rapid enhancement of layer IV VEPs in binocular subjects that did not receive dark exposure (average ± SEM); (inset) representative contralateral eye VEPs at time 0 (baseline) and 240 min post visual stimulation. (D) Visual stimuli with novel spatial frequencies did not evoke VEPs with enhanced amplitudes in binocular subjects that received passive visual stimulation (average amplitude [norm to max] ± SEM). (E) Visual stimuli with novel orientations did not evoke VEPs with enhanced amplitudes in binocular subjects that received passive visual stimulation (average ± SEM). Scale bars: 50 μV; 50 msec.

Figure 4.

Passive visual stimulation improves the recovery of spatial acuity. (A) Experimental timeline: chronically deprived subjects were trained to perform a visual discrimination task with the nondeprived eye. Following assessment of nondeprived eye spatial acuity, subjects received dark exposure (DE at ∼P110), reverse deprivation, and began trials of active visual discriminations (Disc) with the chronically deprived eye. One cohort received passive visual stimulation prior to active visual discrimination (Vis Stim + Disc). (B) Passive visual stimulation followed by active visual discrimination enhances the recovery of spatial acuity (one-way ANOVA, F_(2,25) = 5.6914, P = 0.010, Stim only [black] versus Stim + Disc [gray]; [*] P < 0.05, Tukey-Kramer HSD post hoc). (C) Polar plots of a representative layer IV neuron revealed an increase in the strength and orientation selectivity of visually evoked spiking output in the previously deprived eye. (D) Post-stimulus time histograms of spiking output of a representative layer IV neuron evoked by visual stimuli of preferred (black) and orthogonal (gray) orientations presented to previously deprived eye. (E) The cumulative distribution of single unit activity revealed a significant increase in the strength of visually evoked spiking output in the previously deprived eye (Dep Eye [gray] 56,5, DE Stim + Disc [black]) 63,5, KS test, P = 0.029, n = units, subjects). (F) The cumulative distribution of single unit activity reveals a significant increase in the orientation selectivity of neuronal spiking output in the previously deprived eye (DE [gray]) vs. DE Stim + Disc [black]), KS test, P = 0.007, n = units, subjects).