Adaptive changes in perceptual responses and visuomanual coordination during exposure to visual metrical distortion

The abilities of the human visual system to perform metrical judgements (comparison of lengths, estimation of angles...) involve the existence of some geometrical structure in the visual perceptual space. The question arises whether this geometrical structure is rigidly determined or is subject to adaptive changes. We have tried to answer the question by using a paradigm in which subjects are exposed to a vertically lengthened visual world and then asked to compare simultaneously presented lengths and to evaluate angles between two lines. Their perceptual responses clearly indicate a plastic adaptation to the deformed environment, though the adaptation was never complete after several days of continuous exposure to strong (25%) lengthening. For a maximum time of exposure of 7 days the rate of adaptation was found to be roughly independent of the initial degree of optical distortion. Visuomanual coordination was also investigated in these subjects, but the responses were less conclusive in this case, because of the high inter-subject variability.