A New Context Effect of Human Resolving Power Distinguishes between Perception and Action

We report the discovery of a new effect of context that modulates the human resolving power for an individual stimulus. In particular, we show that the size of the difference threshold or the just noticeable difference around a standard stimulus depends on the range of the other standards tested simultaneously for resolution within the same experimental session. The larger this range, the poorer the resolving power for the given standard. We establish this result for the perception of linear extent, using different psychophysical methods and stimuli. We then proceed to show that this contextual influence is limited to perception and does not affect visually guided action. Notably, the difference threshold remained invariant when the participants were grasping rather than perceiving the same objects. Beyond adding a new member to the class of context effects in psychophysics, the findings lend further support for the idea that computations mediating the visual perception of object size are different from those mediating visually controlled actions toward the same objects. Meeting abstract presented at VSS 2015