Academic journal article The Psychological Record

Mutual Selection and Membership in Open-Ended Classes: Variant-to-Base and Base-to-Variant Testing

Academic journal article The Psychological Record

Mutual Selection and Membership in Open-Ended Classes: Variant-to-Base and Base-to-Variant Testing

Article excerpt

Behavior that is controlled by categories or classes of stimuli is of great adaptive utility because it enables an individual to respond effectively to the new stimuli that are inevitably encountered in real world settings (Bruner, Goodnow, & Austin, 1965; Medin & Smith, 1984; Rosch & Mervis, 1975; Smith, 1989). Some categories or stimulus classes are said to be open-ended because they contain an infinite number of exemplars that resemble each other (Herrnstein, 1990). Open-ended classes have also been called dimensional, perceptual (Fields & Reeve, 2000), feature-based (Stromer & Mackay, 1997), similarity-based (Wasserman, Keidinger, & Bhatt, 1988), fuzzy (Rosch & Mervis, 1975), ill-defined (Homa & Chambliss, 1985; Homa & Little, 1985), and probabilistic (Medin & Smith, 1984). Because open-ended classes are ubiquitous, it is important to understand the properties that characterize class members. The present experiment investigated an unexplored property of open-ended classes: mutual selectivity.

An open-ended class consists of contiguous stimuli along some continuum, all of which occasion the same response that has been trained to one or a few of the stimuli in that range (Cook, Wright, & Krendrick, 1990; Fields & Reeve, 2001; Goldiamond, 1966; Hull, 1920; Keller & Schoenfeld, 1950; Lea, 1984; Reeve & Fields, 2001; Wasserman et al., 1988; Wright, Cook, Rivera, Sands, & Delius, 1988). This definition implies that all of the stimuli in a class should be substitutable or interchangeable with each other (Goldiamond, 1962; Sidman, 1994). In tests conducted in a conditional discrimination format, interchangeability would be demonstrated by the selection of any class member used as a comparison stimulus in the presence of any member of the same class presented as a sample. That is, all of the stimuli in a class should result in the mutual selection of each other. Mutual selection, then, might be viewed as a defining property of stimuli that are members of open-ended classes. Indeed, mutual selection of phys ically disparate stimuli is the basis for demonstrating the formation of another type of category called an equivalence class (Fields & Verhave, 1987; Sidman, 1994; Sidman & Tailby, 1982).

Of necessity, the evaluation of mutual selectivity requires the presentation of tests conducted in variant-to-base and base-to-variant formats (Reeve & Fields, 2001). These can be illustrated by considering straight lines that vary in length, with the shortest and longest lines serving as the base stimuli for the classes of short and long lines, respectively. When tests are conducted in the variant-to-base format, intermediate length lines called variants are presented as sample stimuli along with the longest and shortest lines (base stimuli) as comparisons. With these tests, the class of long lines would include all variants that occasioned the selection of the long line comparison with the same high likelihood. When the base-to-variant tests are conducted, either the longest or shortest lines (the base stimuli) are presented as samples with the variants as comparisons. In these tests, the class of long lines would include all variants selected with the same high likelihood in the presence of the long line p resented as a sample (Fields & Reeve, 2000).

According to mutual selection, the variants that are members of an open-ended class would be those that occasion the selection of the base stimulus during the variant-to-base tests and are also selected in the presence of the same base stimulus during the base-to-variant tests. In addition, the variants that are not class members would include those variants that were selected in the presence of a given base stimulus in base-to-variant tests but did not occasion the selection of the same base stimulus in variant-to-base tests, or vice versa. Notwithstanding the plausibility of assuming that the stimuli in an open-ended class should have the functional property of mutual selection, data obtained from many studies of stimulus generalization and psychophysics suggest otherwise. …

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