Linearity as the basic law of psychophysics: Evidence from studies of the neural mechanisms of roughness magnitude estimation

Summary

The search for general laws governing our perceptual representation of the external world goes back at least to Fechner and is no less important now than it was then. Fechner used human discrimination performance to infer that the basic relationship between subjective intensity and the intensity of a physical stimulus was logarithmic. A hundred years later, Stevens demonstrated that direct subjective report frequently results in a power law rather than a logarithmic law. However, it was soon appreciated that power laws and logarithmic laws are indistinguishable without peering inside the mechanisms leading to a subject’s responses. After doing so, Mountcastle and his colleagues proposed that the basic law is linearity between the neural representation of an intensive continuum and its subjective magnitude. We were drawn to the same conclusion in a study of the neural coding mechanisms of roughness perception that was designed to be independent of any assumptions about the basic laws of psychophysics. We adopted consistency as the test of neural coding hypotheses. The single, consistent neural coding hypothesis that survived all challenges resulted in a linear relationship between the neural coding measure and subjects’ reports of intensity. That hypothesis was that roughness perception depends on spatial variation in the SAI afferent population response. This study and other similar studies support the idea that once the internal neural representation of intensity is established, the fundamental laws are linear, as Mountcastle proposed over thirty years ago.