Effects of Stimulus Information and Number of Stimuli on Sequential Dependencies in Absolute Identification

Abstract Three absolute identification experiments were conducted to examine the effects on sequential dependencies of two factors, namely stimulus information and the number of stimuli used. The data were analysed by multivariate information analysis (Mori, 1991; Mori & Ward, 1995). The results showed that the sequential dependencies became larger as the stimulus information was decreased (Experiments 1 and 3) and as the number of stimuli was increased (Experiments 1 and 2), consistent with previous studies (Gamer, 1953; McGill, 1957; Mori & Ward, 1995). These results are discussed in the context of models of sequential dependencies (e.g. Mori, 1989; Ward & Lockhead, 1971). In addition, multiple regression analysis was performed on the present data, and the results were compared with those of the information analysis. In an absolute identification task, an observer is presented with a set of stimuli and asked to identify the stimuli with a predetermined set of response categories. In sensation and perception research, the absolute identification task is of great use in measuring sensitivity to more than two stimuli (e.g., Braida & Durlach, 1972, 1988). Whereas the measures of sensitivity (e.g., d " information transmission) only reflect the effect of a presented stimulus (Sn) on an observer's response (Rn), it is now well known that Rn is also affected by the immediately preceding stimulus (Sn-1) and response (Rn-1). Such effects of Sn- I and Rn- 1 are called sequential dependencies, and whether the dependencies are positive (assimilative) or negative (contrast) depends on the experimental condition (Mori, 1988; Ward & Lockhead, 1971). There is growing recognition that sequential dependencies on Sn-1 and Rn-I should be studied thoroughly in order to fully understand the observer's performance in the absolute identification task (e.g., Laming, 1984; Mori, 1988, 1989; Mori & Ward, 1995; Treisman, 1985). Previous research has shown that there are at least three factors affecting sequential dependencies in absolute identification: Stimulus information available to the observer, the number of stimuli used, and the presence or absence of trial-by-trial feedback (e.g., Gamer, 1953; McGill, 1957; Mori, 1989; Mori & Ward, 1990; Ward & Lockhead, 1971). The present paper concerns the first two factors, stimulus information available and number of stimuli. The effects of trial-by-trial feedback have been discussed extensively in another paper (Mori & Ward, 1995). STIMULUS INFORMATION AVAILABLE TO AN OBSERVER In the absolute identification task (and also other psychophysical tasks), the observer attempts to obtain information from a presented stimulus, Sn, to make a precise judgment of the stimulus. In applications of information theory to empirical research, information available from Sn to the observer is measured by information transmission from Sn to Rn (Garner, 1962). Numerous experiments have shown that the amount of information transmitted varies with changing stimulus duration (Ward, 1991), stimulus range or spacing (Braida & Durlach, 1972; Luce, Green, & Weber, 1976), viewing or listening conditions (Lockhead, 1966; Mori, 1989; Mori & Ward,1990, 1992; Ward, 1979), and presence or absence of trial-by-trial feedback (Eriksen, 1958; Mori, 1989). Several findings suggest that such a change in stimulus information also affects sequential dependencies. It has been shown that when stimulus information is decreased, the dependencies on Sn-1 and Rn-1 increase (e.g., McGill, 1957; Mori, 1989; Ward & Lockhead, 1971). For example, Mori (1989) systematically varied stimulus information by changing the contrast level of the stimuli for absolute identification of line lengths. Multivariate information analysis (Garner & McGill, 1956; McGill, 1954, 1957) was applied to the data to compute the information measures of dependency on Sn- 1 and on Rn- 1. The results showed that the dependencies on both Sn-1 and Rn-l increased as the stimulus information decreased. …