Visuo-motor rotation influences representational acuity but not space representation

Prism adaptation is a well-known experimental procedure to study sensorimotor plasticity. It has been shown that following prism exposure, after-effects are not only restricted to the sensorimotor level but extend as well to spatial cognition. In the present study, we used a visuo-motor rotation task which approaches the perturbations induced by prism exposure. We induced either leftward or rightward 15-degree rotations and we presented the perturbation either abruptly (from one trial to the next) or gradually (over a 34-trial transition). First, we found that none of the conditions produced cognitive after-effects in perceptive line bisection task. This result has a strong methodological impact for prospective investigations focusing on sensorimotor plasticity while sparing space cognition; it is particularly relevant when investigating sensorimotor plasticity in patients with specific representational feature to preserve from aggravation. Second, another interesting result was the increase of the sensitivity with which we discriminate the center of the line, that we propose to call representational acuity. It improved following the perturbation more particularly after gradual exposure and persisted for some time after the sensorimotor adaptation. These innovative results are discussed in terms of sensorimotor processes underpinning the transfer of visuomotor plasticity to spatial cognition.

Short-term effects of aligning prisms on the objective and subjective fixation disparity in far distance

Fixation disparity (FD) refers to a suboptimal condition of binocular vision. The oculomotor aspect of FD refers to a misadjustment in the vergence angle between the two visual axes that is measured in research with eye trackers (objective fixation disparity, oFD). The sensory aspect is psychophysically tested using dichoptic nonius lines (subjective fixation disparity, sFD). Some optometrists use nonius tests to determine the prisms for constant wear aiming to align the eyes. However, they do not (yet) use eye trackers. We investigate the effect of aligning prisms on oFD and sFD for 60 sec exposure duration of prisms determined with the clinically established Cross test in far distance vision. Without prisms, both types of FD were correlated with the aligning prism, while with prisms the FD was close to zero (these analyses included all base-in and base-out cases). The effect of base-in prisms on oFD was proportional to the amount of the aligning prism for the present 60 sec exposure, similar as for the 2- 5 sec exposure in Schmid et al. (2018). Thus, within 1 minute of prism exposure, no substantial vergence adaptation seems to occur in the present test conditions. Further studies may investigate intra- individual responses to different exposure times of aligning prisms in both prism directions.

Three timescales in prism adaptation

It has been proposed that motor adaptation depends on at least two learning systems, one that learns fast but with poor retention and another that learns slowly but with better retention (Smith MA, Ghazizadeh A, Shadmehr R. PLoS Biol 4: e179, 2006). This two-state model has been shown to account for a range of behavior in the force field adaptation task. In the present study, we examined whether such a two-state model could also account for behavior arising from adaptation to a prismatic displacement of the visual field. We first confirmed that an “adaptation rebound,” a critical prediction of the two-state model, occurred when visual feedback was deprived after an adaptation-extinction episode. We then examined the speed of decay of the prism aftereffect (without any visual feedback) after repetitions of 30, 150, and 500 trials of prism exposure. The speed of decay decreased with the number of exposure trials, a phenomenon that was best explained by assuming an “ultraslow” system, in addition to the fast and slow systems. Finally, we compared retention of aftereffects 24 h after 150 or 500 trials of exposure: retention was significantly greater after 500 than 150 trials. This difference in retention could not be explained by the two-state model but was well explained by the three-state model as arising from the difference in the amount of adaptation of the “ultraslow process.” These results suggest that there are not only fast and slow systems but also an ultraslow learning system in prism adaptation that is activated by prolonged prism exposure of 150–500 trials.