An impaired ability to disengage attention from a fixation target to a peripheral target, when both targets are visible to the amblyopic eye, is likely responsible for saccadic latency in individuals with amblyopia, according to research published in Vision Research. Oculomotor factors related to saccade generation are likely not responsible for these delays.
Investigators enrolled 10 individuals with amblyopia (mean age 29.4±14.6 years) and 10 age-matched controls (28.2±5.5 years) in a study to determine whether attentional or oculomotor factors increased saccadic latencies in eyes with amblyopia. The team evaluated latency and attention shifts by using a mirror haploscope to present visually-guided saccades to targets for the participants. Overall, they conducted a total of 2 experiments. In the first experiment, researchers presented binocular and monocular targets to measure latency, amplitude gain, and peak velocity of saccades. The second experiment involved presenting a fixation target to one eye while presenting a peripheral target to the contralateral eye and examining attentional shifts.
No participants reported diplopia in the dichoptic setup, and none were unaware that some stimuli were only presented to one eye.
In experiment 1, the percentage of eye movements was 80.5±2.4% and 78.5±2.4% under dominant/fellow eye fixation, 71.2±3.9% and 82.6±2.8% under non-dominant/amblyopic eye fixation, and 79.8±2.4% and 82.7±3.0% under binocular fixation in patients with amblyopia and control individuals, respectively.
Researchers noted that saccadic latencies were significantly longer under amblyopic eye fixation (202.99±8.2 ms) compared with fellow eye fixation (174.5±7.1 ms, P <.001) and binocular fixation (163.4±18.8 ms, P <.001) in the cohort with amblyopia and under all viewing conditions in the control group.
Saccade gain and main sequence rate constraints between study groups did not differ for any viewing conditions (all P >.05).
In experiment 2, the percentage of eye movements for error signal to dominant eye and error signal to non-dominant eye were 76.8±3.5% and 82.8±2.7% and 70.7±4.0% and 77.8±2.8% for patients with amblyopia and control individuals, respectively. Variability in saccadic latency was significantly higher in amblyopic eyes during fellow eye fixation (mean difference [MD], 15.2 ms; P <.001) and binocular fixation (MD, 15.7 ms; P <.001) trials. Investigators noted similarities in mean saccadic latency regardless of which eye received the saccadic error signal for both groups.
Together, these findings indicated that oculomotor factors were unlikely to cause the saccadic latency delays observed in amblyopic eyes.
“Oculomotor factors related to saccade generation are unlikely to be responsible for saccadic latency delays under amblyopic eye fixation,” according to the researchers. “We propose that the inability to shift overt attention from fixation target to a peripheral target when both targets are seen by the amblyopic eye could contribute to increased saccadic latency.”
Study limitations include a small sample size and a mixture of anisometropic and strabismic eyes among participants with amblyopia.
Chow A, Raveendran RN, Erkelens I, Babu R, Thompson B. Increased saccadic latency in amblyopia: oculomotor and attentional factors. Vision Res. Published online May 5, 2022. doi:10.1016/j.visres.2022.108059