On the Mechanics of Immediate Corrections and Aftereffects in Prism Adaptation.

Prisms laterally shifting the perceived visual world cause arm movements to deviate from intended targets. The resulting error-the direct effect-both for pointing and throwing movements, usually corresponds to only around half of the prism's optical power due to an "immediate correction effect". We investigated the mechanisms of this immediate correction effect. In three experiments with 73 healthy subjects we find that the immediate correction effect is associated with a head and/or eye rotation. Since these rotations are subconscious they are not taken into account by the participants. These subconscious rotations compensate for a large portion of the prism's optical effect and change the subjective straight ahead. These movements seem to be induced only in a rich visual environment and hence do not take place in the dark. They correspond to the difference between the direct effect and the optical power of the prisms and seem to cause the immediate correction effect. Hence, eye-hand adaptation only adapts to the prism's optical power minus unconscious head rotation and hence is much smaller than the optical power of the prisms.

Influence of Visual Prism Adaptation on Auditory Space Representation.

Prisms shifting the visual input sideways produce a mismatch between the visual versus felt position of one's hand. Prism adaptation eliminates this mismatch, realigning hand proprioception with visual input. Whether this realignment concerns exclusively the visuo-(hand)motor system or it generalizes to acoustic inputs is controversial. We here show that there is indeed a slight influence of visual adaptation on the perceived direction of acoustic sources. However, this shift in perceived auditory direction can be fully explained by a subconscious head rotation during prism exposure and by changes in arm proprioception. Hence, prism adaptation does only indirectly generalize to auditory space perception.

How to Get the Full Prism Effect.

We investigate how the immediate correction effect decreases mispointing under prisms. Subjects performed rhythmic pointing movements under different conditions with horizontally shifting prisms. Even the first (initial) pointing error is much smaller than the prismatic shift, a phenomenon called the immediate correction effect. Knowledge about the structure of the room and of objects in the room obtained before the prisms were worn may limit the amount of the prismatic displacement perceived. We therefore compared the direct prism effect as well as prismatic adaptation with room illumination switched on versus switched off. Our 44 subjects participated in two experiments, with varying amounts of information about room structure available. The results show a direct effect corresponding to the optical power of the prisms in the dark condition, when in addition body position was slightly rotated in direction of the prismatic shift. But even in the dark, a significant immediate correction effect arises with the fixed body position. The largest immediate correction amounting to almost half of optical displacement arose in the standard condition of bright light and fixed body position.