From Lu & Sperling (2001). The data of Seiffert and Cavanagh69 can be explained more parsimoniously when we recognize that their proce- dure does not differentiate spatiotemporal filtering before motion detection from frequency selectivity in the motion detector itself or from the nature of the motion algorithm. It is well known that the temporal tuning function for first-order motion has a bandpass characteristic.14,108,109 This would tend to produce a U-shaped curve for displace- ment threshold versus frequency. The temporal tuning function for second-order motion is low pass41,80 in the range in which it has been studied. This would tend to produce a flat curve for displacement threshold versus frequency and lead to a false conclusion that second-order motion perception was based on a displacement mecha- nism when, in fact, second-order motion perception is based on a motion-energy type of algorithm (see below).