An apparent compression cannot explain the difference between the original and the induced Roelofs effect

When a rectangular frame is presented with one of its edges on the subject's objective median plane, that edge is reported to be shifted to the side opposite to the rest of the frame (Roelofs, 1935). This effect is called the (original) Roelofs effect. When a target dot is presented within the same rectangular frame, an offset of the frame to one side of the subject's objective median plane causes a bias in the perceived target position in the opposite direction. This has been called the induced Roelofs effect (Bridgeman, Peery, & Anand, 1997). Both Roelofs effects could be caused by a perceived shift in straight-ahead. We rejected this hypothesis in a previous paper (De Grave, Brenner, & Smeets, 2002). Our main argument was that if our subjects did not know in advance what to report (position of frame or target), their responses showed a Roelofs effect for the frame, without an induced Roelofs effect for the target. Dassonville and Bala (2004) question this conclusion. They assume that the original Roelofs effect contains two components: an apparent compression and a shift in straight-ahead. The apparent compression is a general phenomenon that applies to the target as well as the frame, and should be present regardless of the temporal order of the frame and the target. It could be a response bias or an error in converting the perceived position to a value in centimetres. Such an apparent compression is indeed evident in the target gains of our target estimation tasks (these gains do not equal 1). The other component is a shift in the perceived straight-ahead. As our data showed clear signs of apparent compression, we agree with Dassonville and Bala that this is a better framework to analyse our data. The question is: can we reject the hypothesis that the induced Roelofs effect and the shift component of the original Roelofs effect are both caused by the same shift in perceived straight-ahead? In other words, does our conclusion still hold if we take the apparent compression into ac-count? The effect of a combination of apparent compression and shift in straight-ahead can be formalised as follows: the estimated frame position ðP 0 f Þ depends on a shift in perceived straight-ahead that is proportional to the frame position ðb 1 à P f Þ and a gain of the apparent compression of positions ðg à P f Þ. It is …