First-order basis of second-order temporal loss in amblyopia

Amblyopia is a neurodevelopmental visual disorder characterized by monocular visual impairment due to abnormal binocular experience in early childhood, with first-order (luminance-defined) and second-order (contrast-defined) processing deficits. While first-order temporal perception deficits in amblyopia are well-documented, the nature of second-order temporal deficits remains unclear, particularly whether they stem from intrinsic extrastriate dysfunction or secondary effects of first-order spatial impairments. This study investigated temporal perception deficits in anisometropic amblyopia by comparing first-order and second-order processing under balanced and unbalanced carrier conditions. Eight anisometropic amblyopes (18–21 years) and ten normal controls (18–23 years) participated. We measured: (1) first-order temporal perception (luminance-modulated gratings) and (2) second-order temporal perception (contrast-modulated noise carriers) under two conditions (identical carriers, contrast = 1.0; balanced carriers, contrast calibrated to equate first-order spatial input between amblyopic eyes [AEs] and fellow eyes [FEs]). A carrier detection task assessed first-order carrier sensitivity. All tasks used a two-alternative forced-choice (2AFC) paradigm with a two-down/one-up staircase procedure to measure thresholds (70.7% accuracy). Statistical analyses included repeated-measures Analysis of Variance (ANOVA) and post-hoc LSD tests, comparing AEs, FEs, and control eyes (CEs). Our experiment demonstrated that amblyopic eyes (AEs) exhibited impaired first-order temporal perception and carrier detection, while their second-order temporal deficits disappeared (AEs/FEs ratio = 1.18 ± 0.28, p > 0.1) when first-order spatial inputs, the carriers, were individually balanced. Fellow eyes (FEs) performed comparably to control eyes (CEs) across all tasks, suggesting preserved temporal and spatial processing in non-amblyopic eyes. These findings indicate that second-order temporal deficits may primarily reflect impaired first-order spatial processing rather than intrinsic extrastriate dysfunction. The study highlights the hierarchical nature of amblyopic visual deficits, where early sensory losses cascade into downstream processing impairments, and emphasizes the importance of distinguishing primary spatial deficits from secondary temporal consequences in both research and clinical assessment of amblyopia.