Trends in Cognitive Sciences
Research updateAction does not resist visual illusions
Section snippets
Standard perceptual measures provide no evidence for a smaller motor illusion
The perceptual effects of visual illusions have been assessed in different ways. In Table 1, I distinguish between standard perceptual measures and non-standard perceptual measures. Standard perceptual measures are usually used in the investigation of visual illusions. For example, participants adjust the size of a comparison stimulus to match the size of a target stimulus.
Aglioti et al. (Ref. 4, and replicated in Refs 5, 9) found that the Titchener/Ebbinghaus illusion affected standard
Non-standard perceptual measures show larger illusion effects than both perception and action
A number of studies used non-standard perceptual measures. Usually, participants estimated target size by opening index finger and thumb, either seeing or not seeing their hand and the stimulus during performance of the task5, 8, 12, 13, 14, 16. I see two problems with these non-standard perceptual measures (see also Ref. 17). First, it is unclear whether these measures can be interpreted as perceptual measures. As Bruno pointed out2, it is difficult to find a priori criteria for classifying
Is it possible to reconcile the action vs. perception hypothesis with these data?
There have been different attempts to reconcile the action vs. perception hypothesis with the finding of equal motor and perceptual illusions in the Titchener/Ebbinghaus display6, 7. Some authors have argued that the motor illusion might be generated in the motor system independently of the perceptual illusion1, 8, 18, 19. This could be the case if the motor system treated the context circles as potential obstacles for the fingers and tried to avoid them. In my opinion, this is an important
Conclusions
At first, the finding of Aglioti et al. that the motor system largely resisted the Titchener/Ebbinghaus illusion seemed to provide convincing evidence for the Milner and Goodale action vs. perception hypothesis. Today, this finding is in doubt and the accumulated evidence suggests that the effects of the illusion on grasping might well be similar to the effects on perception. As directions for future research, I suggest that more attention be paid to the problem of matching the task demands of
Acknowledgements
This work was supported by the grant FA 119/15–3 from the Deutsche Forschungsgemeinschaft (DFG) and by the Max Planck Society.
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