Mechanism of the illusory transformation of optic flow fields

As part of our continuing studies of the neuronal correlates of optic flow, we noted their illusory transformation by overlapping patterns of planar motion (Duffy & Wurtz, 1993). We observed that planar motion, overlapping an outward radial pattern, caused the focus of expansion (FOE) in the radial pattern to appear to be shifted in the direction of the planar motion. In their current commentary, Meese, Smith and Harris (1995) confirm our observation of an illusory shift of the FOE in the direction of overlapping planar motion. Further, they suggest that induced motion can account for this phenomenon by way of vector summation with the radial pattern. In our original report (Duffy & Wurtz, 1993), we considered the similarities between the illusory shift of the FOE and induced motion phenomena, recognizing that they may be related. The importance of these similarities is reinforced by the findings of Meese et al. (1995), and we appreciate the significance of a potential relationship betwen the illusory shift and induced motion. However, [he differences between these phenomena suggest that we should not conclude that they necessarily share the same mechanisms. Induced motion consists of the apparent motion of a visual feature in the direction opposite that of overlapping visual motion (Dunker, 1929). In contrast, the illusory transformation consists of an apparent shift of the position of the FOE, not continuous apparent motion of the FOE. In addition, the illusory transformation shifts the apparent FOE in the same direction as the overlapping motion, not the opposite direction. We also have two general comments regarding the findings reported by Meese et al. (1995). First, the illusory shift they observed is more than an order of magnitude smaller than that found in our studies (Duffy & Wurtz, 1993). They suggest that the relatively small magnitude of their effects might be attributable to quantitative difference,.; in stimulus parameters: stimulus