In multichannel coding an attribute is represented by activation in many pools of neurons (or channels), each tuned to distinct values. In norm-based opponent coding, an attribute is represented by activation in two pools of neurons tuned to opposite ends of the stimulus dimension, with equal activation signaling a neutral point on that dimension. This neutral point is called the
norm, and the coding is described as norm-based because the channels are tuned to deviations from the norm. Both forms of coding are used in the cortical visual system. For example, multichannel coding is used to represent tilt/orientation and spatial frequency (Blakemore & Sutton,
1969; Clifford, Wenderoth, & Spehar,
2000) and norm-based opponent coding to represent color (Webster & Leonard,
2008). For face attributes, norm-based opponent coding appears to be used for spatial configuration, identity-related dimensions, and expression (Burton, Jeffery, Skinner, Benton, & Rhodes,
2013; Jeffery, Read, & Rhodes,
2013; Jeffery et al.,
2010; Jeffery et al.,
2011; Rhodes & Jeffery,
2006; Robbins, McKone, & Edwards,
2007; Skinner & Benton,
2010; Susilo, McKone, & Edwards,
2010), whereas a three-channel system appears to be used for gaze direction and head direction (Calder, Jenkins, Cassel, & Clifford,
2008; Lawson, Clifford, & Calder,
2011, but see Kloth & Schweinberger,
2010).