Research has also shown that, under appropriate stimulus conditions, illusory motion smear associated with AM can interfere with the appearance of a stimulus presented along the motion path. This can happen in a number of ways. First, AM can affect the detectability of an object. For example, Yantis and Nakama (
1998) reported that the ability to identify a target stimulus was impaired when placed on the path of motion (relative to performance when the stimulus was off the path of motion), and this finding is indicative of a process in which the neural representation of AM disrupts the coding of another stimulus. Second, AM can bias the spatial registry of an object—the perceived position of an object briefly flashed along the motion path is shifted in the direction of AM with larger distortions noted at shorter temporal intervals. Originally noted by Brigner (
1984), this observation has since been extensively reported in a number of studies (e.g., Eagleman & Sejnowski,
2007; Shim & Cavanagh,
2004,
2006; Watanabe, Nijhawan, & Shimojo,
2002; Whitney,
2002) and mirrors the same effect with real motion (see De Valois & De Valois,
1991; Tsui, Khuu, & Hayes,
2007). This effect likely occurs because the neural representation of AM disrupts the spatial registry of an object and its position code is shifted in the motion direction. Single cell recording studies lend support to this suggestion and show that the receptive field profile of position coding cells in primary visual cortex of the cat, when exposed to image motion, undergoes displacement in the motion direction (e.g., Fu et al.,
2002). Third, AM can affect the appearance of an object briefly presented on the path of motion. MacFarland (
1965) demonstrated that a square briefly flashed in the motion path of AM is not perceived as complete at the point of presentation, but instead appears to be sequentially revealed over time from the leading edge of motion. Together these studies reveal that AM is compelling (see Fahle, Biester, & Morrone,
2001), and that, under specific stimulus conditions, the neural representation of AM can interfere with the coding of objects overlapping with the motion path. However, current understanding of this process is limited, particularly regarding the degree to which form and AM interact, the nature of this interaction, and the conditions under which this interaction occurs.