Comparing effects of the horizontal-vertical illusion on grip scaling and judgment: Relative versus absolute, not perception versus action

The discovery that the prehension component of an open-loop, two-fingered r each is largely immune to certain salient pictorial illusions has been used to suggest that humans possess 2 distinct visual systems, 1 that subserves perceptual judgment and 1 that mediates visually controlled action. In thi s article, the authors present evidence that suggests that the critical dis tinction is not that of reaching and judgment but of relative and absolute perception. Experiment 1 extends the findings of S. Aglioti, J. F. X. DeSou za, and M. A. Goodale (1995) and suggests that the manual prehension compon ent of open-loop reaching is affected by the horizontal-vertical illusion t o a much smaller degree than perceptual size judgments. In Experiments 2 an d 3, however, when perceptual size judgment is directed at a single element of the display, this difference vanishes. Experiment 4 demonstrates that g rip scaling is strongly affected by the illusion when a single reach is sca led to both the horizontal and vertical components of a triangular figure.