Contraction anisocoria: Segregation, summation, and saturation in the pupillary pathway

PURPOSE. To investigate the neural basis of contraction anisocoria and any implications for assessments using pupillary responses, through analysis of topographic variation in amplitudes of direct and consensual pupil responses. METHODS. Direct and consensual pupillary contraction amplitudes were analyzed from six studies in which 120 normal subjects were tested with 24 different stimulus variants. The dichoptically presented multifocal stimulus arrays subtended ± 30° of visual field but varied in color (achromatic or yellow), number of test regions (24 to 60/eye), mean regional presentation interval (0.25 to 16 s), and pulse time-course (33 to 150 ms, flickered or steady). The 290 cd/m² test-regions were displayed on a 10 cd/m² background. Ratios between mean direct and consensual responses were calculated for each region. Results were quantified using multivariate linear analysis. RESULTS. Direct responses within the temporal hemifield were significantly larger than consensual for all stimulus protocols. Across the 24 protocols these differences ranged between 13.8% (t₁₄₁₅ = 3.06, P < 0.01) and 27.0% (t₉₉₀ = 5.72, P < 0.0001). Differences in the nasal field were mostly non-significant. Contraction amplitudes varied systematically across the visual field. By contrast, direct/consensual ratios were markedly uniform within each hemifield. CONCLUSIONS. The distribution of signal from the pretectal olivary nuclei to each Edinger-Westphal nucleus differs depending on the hemifield being stimulated. A simple model incorporating segregation and summation of afferent signals, and differing saturation of midbrain pathways is proposed. This appears to explain inconsistent observations in the literature and predicates the need for separate assessment of direct and consensual responses at hemifield or better resolution.