Additivity in prism adaptation as manifested in intermanual and interocular transfer

Level of adaptation was assessed in both exposed and unexposed eye and/or hand for visual shift (VS), proprioceptive shift (PS), and the eye-hand coordination, negative after effect (NA) measure of both visual and proprioceptive change, following 15-min and 20-diopter base-right displacement viewing of the active hand, under conditions of unconstrained head movement and terminal exposure feedback. Transfer was complete for the VS test, and significant, but incomplete for the PS and NA tests. For both exposed and unexposed eye/hand situations, level of adaptation was greater for the NA than for the PS test, which in turn showed greater adaptation than the VS test. Additivity was virtually perfect for the unexposed eye/hand (VS+PS = NA), but underadditivity appeared for the exposed eye/hand (VS+PS < NA). This underadditivity was approximately equal in magnitude to the amount that transfer on the NA test was less than on the PS test, suggesting that underadditivity was due to a nontransferable, assimilated corrective response in the NA test with the exposed eye/hand. Possible explanations for intermanual transfer are discussed.