The effects of short periods of strabismus on cortical binocularity

Background: Previously we reported that only 14 days of optically induced strabismus around 4 weeks of age were sufficient to disrupt the disparity sensitivity of neurons in V1 and to increase the prevalence of interocular suppression (Kumagai et al, 2000; Mori et al, 2002). In this study we investigated whether shorter durations of misalignment could disrupt the binocular response properties of V1 neurons. Methods: Strabismus was optically simulated in 4 infant rhesus monkeys using a prism-rearing procedure. Two infant monkeys were reared with prisms for 7 days and two additional monkeys experienced optical strabismus for only 3 days. The onset age was fixed at 4 weeks of age for all subjects. The microelectrode recording experiments were conducted immediately after the end of the rearing period (i.e., no recovery). Results: Seven days of strabismus (roughly equivalent to 4 weeks in humans) resulted in a high prevalence of binocularly suppressive neurons and a decrease in the average degree of binocular disparity sensitivity. However, these deficits were not as severe as those that occurred after two weeks of misalignment. Three days of optical strabismus had no obvious effects on the degree of disparity sensitivity of individual neurons. In contrast, the prevalence and magnitude of interocular suppression were greatly increased in the monkeys that experienced just 3 days of strabismus. Conclusions: The present results indicate that the first binocular response alteration in V1 that emerges following an ocular misalignment is interocular suppression, which is closely followed by a breakdown of binocular disparity sensitivity.