The cortical pathway responsible for motion processing is relatively well documented, but the mechanisms that underlie the encoding of
speed are still poorly understood. Early visual processing is characterized by neurones whose receptive fields are spatio-temporally separable (e.g., Foster, Gaska, Nagler, & Pollen,
1985; Tolhurst & Movshon,
1975), and thus their responses cannot provide unambiguous speed information. A unifying characteristic of most mechanistic models of speed processing is the assumption that the code for speed is extracted by combining the responses of these early spatio-temporally separable units in some manner (e.g., Adelson & Bergen,
1985; Hammett, Champion, Thompson, & Morland,
2007; Smith & Edgar,
1994; Thompson, Brooks, & Hammett,
2006; Watson & Ahumada,
1985). There is now considerable behavioral evidence that is consistent with such a scheme (see Burr & Thompson,
2011 for a comprehensive review). For instance, the perceived speed of lower contrast gratings is underestimated at temporal frequencies below around 8 Hz (Brooks,
2001; Gegenfurtner & Hawken,
1996; Hürliman, Kiper, & Carandini,
2002; Müller & Greenlee,
1994; Stocker & Simoncelli,
2006; Stone & Thompson,
1992; Thompson,
1982; Thompson, Brooks, & Hammett,
2006). These distortions in perceived speed are readily accommodated in a simple two-channel model whereby the code for speed is derived from the ratio of temporal mechanisms tuned for low and high frequencies. Other distortions in perceived speed induced by both adaptation and luminance have also been found to be consistent with such a ratio model (e.g., Hammett, Bedingham, & Thompson,
2000; Hammett, Champion, Morland, & Thompson,
2005; Hammett et al.,
2007; Smith & Edgar,
1994; Thompson,
1981; Vaziri-Pashkam & Cavanagh,
2008).