The π Mechanisms of W S Stiles: An Historical Review

Abstract

The origins, development, and status of the pi mechanism theory are reviewed. The paper is divided into four sections. In the first section Stiles's general ideas about 'color mechanisms' are examined, and it is concluded that foremost amongst these is a mathematical theory that specifies certain formal rules or laws that should govern a certain class of observations. In the case of pi mechanisms, the class of observations is that of two-color thresholds, and the defining laws are the two well-known displacement laws. Five other laws that two-color increment-threshold observations should obey, if the latter are governed by ideal pi mechanisms, are abstracted from Stiles's writings. In the second section literature pertinent to the testing of the seven Stilesian laws is reviewed, and it is asked whether or not the seven pi mechanisms of Stiles do in fact obey the laws. In the third section the relation of the pi mechanism concept to physiological concepts is examined, and its relation to the 'cone fundamental' is discussed; the evidence pertinent to the question: "Are any of the pi mechanisms of the single-fundamental type?" is then reviewed. The last section is devoted to the evolution of Stiles's ideas in the period after 1959 when Stiles's own investigations and those of others propelled him to reject the initial (1953) pi mechanism theory as an adequate characterization of the data of the two-color threshold.

Essay Concerning Color Constancy

“…This paradigm is the two-color increment threshold tech nique employed in the many exemplary experiments and analyses carried out by W. S. Stiles. Pugh & Kirk (1986) have published a comprehensive historical review of this work, including references to others (among whom Pugh was an important contributor), that outlines the changes in Stiles's own interpretation of such discrimination thresholds and provides the basis for the current interpretation that the mechanism for adaptation to the background light in this paradigm cannot be restricted exclusively to the triplex of retinal light receptors but must also involve postreceptoral adaptation effects in the neural differencing mechanisms-i.e. at the spectrally opponent level of neural color processing.…”

Human gustation and flavour

“…He described several Pi () mechanisms that are today believed to describe the properties of early level signal processing in retinal ganglion cells. 44 This type of work has rarely (if ever) been attempted in taste. 47,49 Individual Difference Studies Individual differences show promise as an area for uncovering potential physiological and qualitative links among different compounds that elicit similar qualities.…”

“…the detection of stimulus A is measured in the presence of background levels of suprathreshold B. 25,44 In taste, relatively little threshold work has been conducted. J. Stevens has reported both additivity and masking effects, 45 -49 i.e.…”

Monogeusia for fructose, glucose, sucrose, and maltose

“…Nonetheless, for his discovery ofthe multiplicity of n mechanisms, Stiles (1967) never doubted the secure evidence from metameric color matching that only three classes ofcone photoreceptors initially encode all photopic stimuli. The 1t mechanisms are now known to be consequences ofthe variety ofpostreceptor pathways that signals-originating in only three classes of cones-follow to the higher visual centers (reviewed in Pugh & Kirk, 1986).…”