Comparisons of interocular suppression in v1 neurons of normal neonatal and infant strabismic monkeys

Background: V1 neurons in neonatal monkeys (6-14 days of age) show a higher prevalence of interocular suppression than in adults to both interocularly matched (iso-oriented) and unmatched (orthogonally-oriented) gratings. However, the prevalence of these suppressive interactions rapidly decreases to normal adult levels by 8 weeks of age (Endo et al, 2001). In this study we investigated how early onset strabismus influenced this normal maturation of binocular signal interactions. Methods: Strabismus was optically simulated in 8 infant rhesus monkeys using a prism-rearing procedure. The onset of strabismus was at 2 weeks of age (before the know onset age for stereopsis), and 4 or 6 weeks of age (after stereopsis onset), and the duration was 14 days (short) and 4 or 8 weeks (long). Immediately after the end of the rearing period, we conducted the microelectrode recording experiments. Results: In all strabismic infants, the binocular signal interactions in V1 neurons were very similar to those that were found in normal neonatal monkeys. Specifically, the strabismic monkeys exhibited a higher than normal prevalence of interocular suppression and the prevalence of interocular suppression for the orthogonally oriented gratings was nearly identical to that for binocularly matched gratings. Conclusions: These findings suggest that the higher than normal prevalence of interocular suppression in V1 in both strabsimic and normal neonatal monkeys has similar underlying causes. One possibility is that the effectiveness of excitatory binocular connections, both local and long-range, is reduced in strabsimic subjects due to early conflicting binocular inputs or in normal neonates due to retinal and/or cortical immaturities, while inhibitory inputs are largely spared or, at least relatively, more mature (Sepigel et al, 1996; Smith et al, 1997; Kumagami et al, 2000).