Separate modifiability, mental modules, and the use of pure and composite measures to reveal them

How can we divide a complex mental process into meaningful parts? In this p aper, I explore an approach in which processes are divided into parts that are modular in the sense of being separately modifiable. Evidence for separ ate modifiability is provided by an instance of selective influence: two fa ctors F and G (usually experimental manipulations) such that part A is infl uenced by F but invariant with respect to G, while part B is influenced by G but invariant with respect to F. Such evidence also indicates that the mo dules are functionally distinct. If we have pure measures MA and Mg, each o f which reflects only one of the parts, we need to show that MA is influenc ed by F but not G, while Mg is influenced by G but not F. If we have only a composite measure MAB Of the entire process, we usually also need to confi rm a combination rule for how the parts contribute to MAB I present a taxonomy of separate-modifiability methods, discuss their infer ential logic, and describe several examples in each category, The three cat egories involve measures that are derived pure (based on different transfor mations of the same data; example: separation of sensory and decision proce sses by signal detection theory), direct pure (based on different data; exa mple: selective effects of adaptation on spatial-frequency thresholds), and composite (examples: the multiplicative-factor method for the analysis of response rate; the additive-factor method for the analysis of reaction time ). Six of the examples concern behavioral measures and functional processes , while four concern brain measures and neural processes. They have been ch osen for their interest and importance; their diversity of measures, specie s, and combination rules; their illustration of different ways of thinking about data; the questions they suggest about possibilities and limitations of the separate-modifiability approach; and the case they make for the frui tfulness of searching for mental modules. (C) 2001 Elsevier Science B.V. Al l rights reserved.