Oscillatory motion but not pattern reversal elicits monocular motion VEP biases in infantile esotropia

Patients with early disruptions of binocularity show cortical directional a symmetries in their steady state monocular VEP response to oscillatory moti on. The VEP directional asymmetry is characterized by significant first har monic components that show a 180 degrees difference in the response phase b etween the two eyes. By contrast, the normal response is dominated by even- order response harmonics, although some normal observers also have measurab le responses at the first harmonic. Experiments and simulations were conduc ted to determine if the first harmonic in patients could reasonably be attr ibuted to direction selective mechanisms. A secondary goal was to determine whether the first harmonic response of normals was also due to imbalances in direction selective mechanisms. Monocular steady state VEPs were elicite d by oscillating 3 c/deg gratings presented at 6 and 10 Hz in normal observ ers and observers with infantile esotropia. Responses were also obtained to phase-reversing gratings of the same spatial and temporal frequencies. Pha se reversal eliminated the majority of first harmonic responses which were recorded for normal observers to oscillatory motion. However, phase reversa l did not elicit the cortical motion asymmetry in infantile esotropia. Mode ling results suggest that the first harmonic response to oscillatory motion arises due to non-linearities in both direction selective and non-directio n-selective mechanisms, with the latter being dominant in patients with ear ly onset strabismus. (C) 1999 Elsevier Science Ltd. All rights reserved.