The effect of prism adaptation on state estimates of eye position in the orbit.

Prism adaptation (PA) after-effects are assessed using tests that measure changes in sensorimotor systems. After-effects on pointing without feedback to a visual target (open loop pointing - OLP) are traditionally described as being larger than those measured by straight ahead pointing (SAP) with eyes closed, and the difference between them is attributed to a shift in visual localisation. However, neither differences between OLP and SAP, nor shifts in perceptual judgement of visual straight ahead (VSA), are consistently reported. Moreover, since very few studies have directly recorded direction of gaze, an effect of PA on the state estimate of gaze direction has not been reliably documented. The current research aimed to isolate the effects of PA on state estimates of eye position. We measured sensorimotor after-effects through common (OLP, SAP, and VSA) measures, and also recorded eye position and additional after-effect measures to interrogate changes to the oculomotor system and how these might relate to other measures of sensorimotor change. To ascertain if PA's effects on estimates of eye position could be attributed to eye muscle potentiation, we compared the effects of PA to sustained gaze deviation without adaptation. PA induced no effect on visual straight-ahead and no change in direction of gaze, when measured while positioning a target, looking straight ahead in the dark, or looking toward the passively positioned and occluded unexposed hand. We also found that after-effects measured by SAP with the eyes open were larger than SAP with the eyes closed and equal to those observed with OLP. The findings challenge the concept that total adaptation after-effect is a direct sum of arm proprioceptive and visual after-effects as conventionally measured, and suggest that the oculomotor system is altered by prism adaptation only in interaction with an arm motor command when vision is available.