Proffitt, D. R. (2009). Affordances matter in geographical slant perception. Psychonomic bulletin & review, 16(5), 970-2. doi: 10.3758/PBR.16.5.970.
We argue that the experimental conditions in the Durgin et al. (2009) study were so different from those in Bhalla and Proffitt (1999) that the results of the former study cannot be generalized to the latter. The participants in the Durgin et al. study viewed a 2-m-long ramp; those in Bhalla and Proffitt viewed expansive hills. When drawing generalizations from one study to another, equating experimental conditions is always important; moreover, from an embodied perspective on perception, equating the opportunities for action also matters.