The Effects of Stimulus Structure and Familiarity on Same-Different Comparison* HOWARD S
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Perception & Psychophysics 1973. Vol. 14. No. 3.413-420 The effects of stimulus structure and familiarity on same-different comparison* HOWARD S. HOCK+ Florida A tlantic University. Boca Raton. Florida 33432 It was hypothesized that performance in the same-different comparison task is based on two modes of processing: (I) structural processes that organize the detailed parts of a stimulus into a well-formed whole, and (2) analytic processes that decompose the stimulus into features. This hypothesis was supported, but with the unexpected finding of individual differences in the mode of processing underlying "same" responses. Those Ss in the "same" condition whose reaction times were faster for symmetrical than for asymmetrical patterns supported the hypothesis for structural processes. The remaining Ss in the "same" condition, as well as all the Ss in the "different" condition, were unaffected by symmetry. These Ss supported the hypothesis for analytic processes. Although familiarity effects were obtained for both structural and analytic Ss, the rotation of the familiar patterns into an unfamiliar orientation virtually eliminated familiarity effects for the structural Ss, bu t left them intact for analytic Ss. In recent years, there has been extensive research "different" response can be based on the detection of using the same-different comparison task, which involves any differentiating feature, while a correct "same" the measurement of reaction times for "same" and response must be the result of an exhaustive comparison "different" responses to pairs of simultaneously (or of all the features of the stimuli. On this basis, "same" sequentially) presented stimuli. For the most part, this responses could be as fast as "different" responses, but research has been based on the assumption that feature they should never be faster. analysis is the fundamental process underlying both Although it remained conceivable that a sufficiently "same" and "different" comparison. In his recent review elaborate feature-analysis model could account for all of this literature, however, Nickerson (1972) discusses the discrepancies described above, the purpose of the two problems which suggest that this assumption may be research reported in this paper was to explore the invalid. possibility that two fundamentally different processes The first problem is based on evidence that "same" underlie "same" and "different" comparison. and "different" responses are differentially affected by the same stimulus manipulations. On the one hand, only A Two-Process Hypothesis "different" reaction times are affected by the number of relevant stimulus attributes (Hawkins, 1969), the size of Since a "different" response can be based on the an irrelevant bracket (Krueger, 1970), and the number detection of a single differentiating feature, it was of letters being compared (Beller, 1970). On the other hypothesized that "different" comparison is based on hand, only "same" reaction times are affected by the analytic processes that decompose the stimulus meaningfulness of letter trigrams and the orientation of information into features. This hypothesis was familiar alphabet letters (Egeth & Blecker, 1971). If the supported most directly by Hawkins's (1969) finding assumption that feature analysis underlies both "same" that the speed of "different" responses decreased as the and "different" responses is valid, the evidence described number of features that differentiated pairs of above indicates that the features are being processed in a "different" stimuli was increased. Since "same" very different manner for the two kinds of responses. responses require the comparison of all the information The second problem involves the fact that "same" in each stimulus, it was hypothesized that "same" responses are often faster than "different" responses comparison involves a process of structuring the stimulus (e.g., Bamber, 1970). The latter finding is a source of information into well-organized units, or chunks. The difficulty for feature-analysis models because a correct two-process hypothesis, then, was that "same" comparison is based on structural processes and *This paper is based on a doctoral dissertation submitted to "different" comparison on analytic processes. The Johns Hopkins University. October 1972. The invaluable An important qualification to this two-process model assistance of Howard Egeth, the author's thesis advisor, is is that it does not require that "same" responses be gratefully acknowledged. Thanks are also extended to Bert Green, William Bevan. Warren Torgerson, and John Jonides, faster than "different" responses. The latter would whose suggestions were of great benefit to the author. The depend on the nature of the stimuli; the speed of experimental research was supported by a contract between the "same" responses would be determined by how well Engineering Psychology Programs, Office of Naval Research, and each stimulus could be structured or organized into a The Johns Hopkins University. fRequests for reprints should be sent to Howard 5. Hock. whole, while the speed of "different" responses would Department of Psychology, Florida Atlantic University. Boca depend on the number of features that differentiate the Raton, Florida 33432. stimuli constituting "different" pairs (as well as the 413 414 HOCK discriminability of these features). That is. the relative stimuli, Egeth and Blecker found that familiarity speeds of "same" and "different" responses would, affected reaction times for "same" but not for according to the two-process hypothesis. depend on the "different" responses. This finding, in conjunction with "goodness of form" of each stimulus relative to the the two-process hypothesis, implied that past experience discriminability of the total set of stimuli (there was, influenced structural, but not analytic, processes in the however, no attempt to manipulate the relative speeds of same-different comparison task. Given the importance of "same" and "different" responses in the present study). this implication, it was necessary to determine whether In order to examine the hypothesis that "same" and Egeth and Blecker's results depended on their use of "different" responses are based on different processes, rotation as a manipulation of familiarity. To this end, half the stimuli used in the present research were the experiment reported in this paper was designed so symmetrical and half were asymmetrical. The possibility that a more direct manipulation of familiarity could be that symmetry could function as a feature that would introduced into the same-different comparison task. This differentiate between two "different" stimuli was was done by providing Ss with preliminary training at eliminated by designing the experiment so that: (1) all reproducing stimuli that were initially unfamiliar and the symmetrical stimuli had the same axis of symmetry, designing the experiment so that the familiar (as a result and (2) the stimuli forming "different" pairs were either of training) and unfamiliar (no training) stimuli were both symmetrical or both asymmetrical. In this way, matched in complexity. The posttraining same-different differences in response time between pairs of comparison task then included pairs of familiar stimuli, symmetrical and pairs of asymmetrical stimuli could not pairs of unfamiliar stimuli, and pairs of rotated-familiar be attributed to the use of symmetry as a stimuli. If a difference in response time between the response-relevant feature. familiar and unfamiliar stimuli were obtained, it would For "same" comparison, it was hypothesized that provide direct evidence that past experience affects responses would be faster for pairs of identical "same" and/or "different" comparison. symmetrical stimuli than for pairs of identical To summarize, it was predicted that symmetry would asymmetrical stimuli. This hypothesis was based on the affect reaction times for "same" but not for "different" assumption that: (1) the structural processes underlying responses. This would support the hypothesis that "same" responses are concerned with the organization of "same" comparison is based on structural processes, the detailed parts of the stimulus into a well-formed while "different" comparison is based on analytic whole, and (2) symmetry would facilitate structural processes. Furthermore, with familiarity manipulated in processes, and thereby "same" responses, by providing a a more direct manner than in Egeth and Blecker's simple rule for putting the detailed parts of a stimulus experiment, familiarity effects were hypothesized for into a structured relationship. 1 Since "different" both "same" and "different" responses. Whereas a responses were hypothesized to depend on feature familiarity effect for "different" responses could be analysis and the experiment was designed so that interpreted in terms of the acquisition of distinctive symmetry was not a response-relevant feature, a features, the manner in which familiarity might difference in reaction time between pairs of symmetrical influence structural processes is essentially unknown. and pairs of'asymmetrical stimuli was not predicted for For this reason, there was no basis for predicting "different" responses. whether familiarity and symmetry would have independent, or interactive, effects on "same" responses. Familiarity and the Two-Process Hypothesis It was hoped that the data obtained in the present study, together with the results of future research, would If, as hypothesized, same-different