Amblyopia, a neurodevelopmental disorder, is commonly assessed through amblyopic eye visual acuity (VA) deficits, but recent studies also highlight abnormalities in the fellow eye. This study quantified binocular and fellow/dominant eye VA in individuals with amblyopia and strabismus without amblyopia and examined factors influencing these measures, including fixation eye movement (FEM) abnormalities. Identifying which subsets of patients—such as those with nystagmus, concurrent strabismus, or greater fixation instability—exhibit more pronounced deficits in binocular visual acuity and binocular summation can enhance clinical decision-making by enabling tailored interventions and aiding patient counseling. Sixty-eight amblyopic, seventeen strabismic without amblyopia, and twenty-four control subjects were assessed using an adaptive psychophysical staircase procedure and high-resolution video-oculography to evaluate FEMs and fixation instability (FI). Binocular and fellow eye VA were significantly lower in amblyopia, regardless of type or nystagmus presence, whereas binocular and dominant eye VA in strabismus without amblyopia did not differ from the controls. Despite reduced binocular acuity, amblyopic and strabismic subjects exhibited binocular summation, with binocular VA exceeding fellow/dominant eye VA. Reduced binocular VA correlated with greater fellow eye VA deficits, diminished binocular summation, and increased FI in the amblyopic eye. Fellow eye VA deficits were linked to greater amblyopic eye VA deficits, an increased degree of anisometropia, higher FI, and stronger nystagmus correlation. These findings suggest amblyopia affects both visual sensory and motor systems, impacting binocular function and fixation stability, with potential consequences for everyday visuomotor tasks like reading.

Fixation instability (FI) and vergence instability (VI) in amblyopia and strabismus are associated with disrupted physiologic fixation eye movements (FEMs). This study examined how viewing conditions affect FEM patterns in strabismic subjects with and without amblyopia. FEMs of the non-dominant/amblyopic and dominant/fellow eyes were recorded using video-oculography during both-eye viewing (BEV), fellow/dominant-eye viewing (FEV/DEV), and amblyopic/non-dominant-eye viewing (AEV/NDEV) in strabismic subjects with amblyopia (SA, n = 56), without amblyopia (S, n = 19), and controls (C, n = 25). FI, VI, fast FEM amplitudes, slow FEM velocities, and time-based control of eye deviation were analyzed. The SA group showed the greatest FI in the amblyopic eye during AEV compared with the fellow eye during FEV, whereas minimal inter-ocular FI differences were observed in the S group and controls. Under monocular viewing, both SA and S groups exhibited increased FI in the non-viewing eye and higher VI than controls. Regression analyses indicated that visual acuity deficits primarily influenced viewing-eye FI and FEM dynamics, while strabismus mainly affected non-viewing-eye FI and slow FEMs. C and S groups showed the least eye deviation during BEV, whereas the SA group showed the least eye deviation—but the highest VI—during AEV, indicating a distinct pattern of incomitance. Distinct FEM patterns shaped by viewing conditions may reflect underlying visuomotor control mechanisms and serve as biomarkers for AI (artificial intelligence)-based classification.