Vergence adaptation and the order of clinical vergence range testing

When measuring horizontal relative vergence ranges, the conventional clinical procedure is to assess the base-in (or divergence) range before the base-out (or convergence) determination. The rationale for this order of testing is that the convergence responses stimulated during the base-out measurements may produce vergence adaptation (i.e., a fusional aftereffect), which may temporarily bias the subsequent base-in values in the base-out direction. However, the effect of reversing this order of testing (i.e, base-out before base-in) has not been directly evaluated. Accordingly, the present study compared distance base-in vergence ranges measured both before and after base-out testing at separate test sessions. Additionally, tonic vergence (TV) was assessed before and after each vergence range measurement. The results indicated that base-out vergence range testing stimulated vergence adaptation, and furthermore produced a statistically significant reduction in the subsequently measured base-in recovery value. However, the magnitude of this shift was less than the estimated degree of repeatability of the vergence range measurement. Therefore, a more appropriate order of testing would be to assess the heterophoria compensating range first in order to obtain accurate results during this more critical measurement.