OKN, PERCEPTUAL AND VEP DIRECTION BIASES IN STRABISMUS

The present study quantified nasalward/temporalward biases in monocula r optokinetic nystagmus (MOKN) and perceived velocity in patients with either early onset esotropia, late onset esotropia and in normals. MO KN was measured with low spatial frequency, small-field gratings drift ing at 9.4 degrees/s. MOKN bias was quantified as the ratio of nasalwa rd slow-phase velocity divided by the sum of temporalward and nasalwar d slow-phase velocities (N/(N + T)). Observers also rated the perceive d velocity of gratings moving in nasalward and temporalward directions (3 or 9.4 degrees/s) using a two interval forced choice task. MOKN an d perceived velocity biases were correlated negatively in both early o nset and late onset groups in the perceptual task-nasalward moving tar gets were rated as slower than temporalward targets, but in the MOKN t ask, slow-phase gain was higher for nasalward than for temporalward ta rgets. Oscillatory-motion, visual evoked potentials (VEPs), were recor ded in response to 1 c/deg gratings undergoing apparent motion at 10 H z in a subset of the observers. VEP direction biases were quantified b y calculating the ratio of first harmonic response amplitudes to the s um of first and second harmonic amplitudes. Significant correlations w ere found between the direction biases obtained on all three measures. Perceived velocity and MOKN bias measures were also correlated negati vely. Patients with early onset esotropia (infantile esotropia) had la rger biases than late onset esotropes or normals on each measure and t he biases were more frequently bilateral in the early onset patients. The pattern of results is consistent with early critical periods for t he mechanism(s) underlying MOKN, perceived velocity and cortical respo nsiveness. A. single site model for all three asymmetries is unlikely, at least in simple form, because of the negative correlation between MOKN and perceived velocity biases and because of the differences in r elative magnitude between the perceptual and MOKN biases. (C) 1998 Els evier Science Ltd. All rights reserved.