Robert G. Pachella

Stimulus probability and same-different classification

Three experiments investigated the effect of stimulus probability on same-different classification time. In Experiments I and II, subjects made same responses on the basis of name matches of simultaneously presented letters. Half of the same trials involved letters that were also physically identical. Experiment I showed that the presentation probability of specific letters affected name matches and different responses, but not physical matches. Experiment II varied stimulus contrast as well as probability. Contrast had a main effect but did not interact with probability at any level of processing. In Experiment III, subjects were switched to the physical level of processing. Stimuli that now had the same name but differed in case were called different. In this condition, the probability effects obs(irved in Experiment II disappeared. These results are interpreted as demonstrating that stimulus probability has its effect during the process that derives the name of the stimulus from the visual representation. This process takes place before the name comparison is made, and the name comparison process precedes the determination of the different response.Three experiments investigated the effect of stimulus probability on same-different classification time. In Experiments I and II, subjects made same responses on the basis of name matches of simultaneously presented letters. Half of the same trials involved letters that were also physically identical. Experiment I showed that the presentation probability of specific letters affected name matches and different responses, but not physical matches. Experiment II varied stimulus contrast as well as probability. Contrast had a main effect but did not interact with probability at any level of processing. In Experiment III, subjects were switched to the physical level of processing. Stimuli that now had the same name but differed in case were called different. In this condition, the probability effects obs(irved in Experiment II disappeared. These results are interpreted as demonstrating that stimulus probability has its effect during the process that derives the name of the stimulus from the visual representation. This process takes place before the name comparison is made, and the name comparison process precedes the determination of the different response.

A psychophysical approach to dimensional separability

Combinations of some physically independent dimensions appear to fuse into a single perceptual attribute, whereas combinations of other dimensions leave the dimensions perceptually distinct. This apparent difference in the perceived distinctiveness of visual dimensions has previously been explained by the postulation of two types of internal representations -integral and separable. It is argued that apparent integrality, as well as its intermediate forms, can result from a single type of representation (the separable type), due to various degrees of correspondence between physical and separable psychological dimensions. Three experiments tested predictions of this new conceptualization of dimensional separability. Experiment 1 demonstrated that a physical dimension corresponding to a separable psychological dimension did not produce interference, whereas a physical dimension not corresponding to a separable psychological dimension did produce interference. Experiment 2 showed that the pattern of results obtained in Experiment 1 could not be accounted for by similarity relations between stimuli. Experiment 3 showed that degrees of correspondence could account for different amounts of interference as well as an inverse relationship between interference and condensation time. These findings imply that previous definitions of integrality are inadequate. Two new converging criteria are proposed, based on the invariance of perceived values on psychological dimensions and on the effect of rotating a configuration of stimuli in a multidimensional space. The present findings furthermore raise the possibility that a single type of internal representation may sufficiently account for all phenomena previously believed to arise from integrality.

Encoding processes in memory scanning tasks.

Three experiments are presented that deal with the effect of stimulus probability on the encoding of both alphanumeric characters and nonsense figures. Experiment I replicated a previous finding of an interaction between stimulus probability and stimulus quality in a memory scanning task with numbers as stimuli. Experiments II and III investigated the same paradigm with unfamiliar visual forms as stimuli, and no interaction was found. Results were interpreted as showing that probability affects the encoding mechanism only when the encoding process results in a representation of the name of the stimulus. When stimulus materials are visual forms without names, probability does not appear to affect encoding processes.

Personal values and the value of expert testimony

Psychologists who routinely offer expert testimony to the courts about the problems of eyewitness testimony demonstrate an unwarranted degree of faith in experimental psychology. Although progress in the field ultimately depends on laboratory research, the extrapolation of laboratory research to the real world is fraught with difficulties. Among the difficulties are the following: Laboratory studies are typically not designed with ecological validity in mind, they involve “fixed effects” statistical designs, they do not tell us how individuals (as opposed to mean values) behave under various experimental conditions. Presentation of such studies as relevant to the specific conditions of a court case entails a significant misrepresentation of the results of the research.

The manipulation of stimulus quality and the definition of stimulus encoding operations in memory scanning experiments

Two experiments examine procedures for defining and isolating stimulus encoding processes within the standard memory scanning task. Two manipulations are used to converge on this definition: letter case (i.e., physical vs. name matching of letters) and stimulus quality. Experiment 1 produced equal scanning rates for the name match and the control conditions in which letter case was not varied. The physical match conditions produced scanning rates half as great as the control. None of these rates were greatly affected by the degradation (contrast reduction) of the probe stimulus, although the small difference in rates for the physical match condition was significant. Experiment 2 investigated two modes of stimulus degradation, contrast reduction and the addition of visual noise. All of the results of the first experiment were replicated for both modes of degradation, with the exception of the change in the scanning rates for the physical match condition. In addition, visual noise produced greater differences between positive and negative response times than did contrast reduction, which did not differ from the high contrast control condition. These results indicate that an abstract internal code is derived during the encoding of the probe stimulus from which the effects of stimulus quality have been removed. Thus, factors that interact with stimulus quality in memory scanning tasks can be assumed to have a locus within the encoding stage of processing.

The effect of stimulus probability on the speed and accuracy of naming alphanumeric stimuli

Previous studies that have investigated the effect of stimulus probability on the time to name alphanumeric stimuli have yielded inconsistent results. Experiments purporting to show the independence of naming reaction time from stimulus probability have displayed error rates that were correlated with probability levels. In order to investigate the possible influence of the speed-accuracy trade-off in these results, the effect of stimulus probability on a naming task was studied under different instructional conditions. When accuracy was emphasized, a significant effect of stimulus probability on reaction times was obtained. When speed was emphasized by the use of a response-time deadline procedure, a significant effect of stimulus probability on error rates was observed. It was concluded that stimulus probability does affect the time to name alphanumeric stimuli, that the effect can manifest itself in both reaction times or error rates, and that models predicting naming time to be independent of stimulus probability must be modified, possibly by elaborating the processing operations of the stimulus identification stage.

An analysis of confusion errors in naming letters under speed stress

Three subjects named six visually presented letters under two levels of speed stress. The obtained confusion matrices for each stress condition were fit by Luce’s choice theory, which provided measures of stimulus similarity and response bias. Speed stress produced proportional increases in pairwise similarity measures but had no systematic effect on response biases. In Experiment 2, the same three subjects named the same letters under two levels of stimulus quality and a constant response-time deadline. As with speed stress, degrading the stimulus produced proportional increases in pairwise similarity measures but had no systematic effect on response biases. In Experiment 3, two of the subjects participating in Experiments 1 and 2 named the six letters under conditions where the probabilities of the letters were unequal. The letters toward which the subject had been most biased in Experiments 1 and 2 were assigned low probabilities, and the letters toward which he was least biased were assigned high probabilities. The result of this manipulation was to completely reverse the ordering of the response bias parameters of the Luce choice model. It is suggested that the present methodology provides a means of validating as psychological constructs the parameters of various mathematical models of stimulus recognition.

Psychophysical Compatibility and Information Reduction Tasks1

Two fundamental aspects of performance, information reduction and dimensional integrality, are shown to be two aspects of a single theoretical construct, termed “psychophysical compatibility”. This construct is defined by the mapping of the physical definition of the stimulus space into its perceivable psychological attributes. Experimental data demonstrate the definition of the construct and its application to information processing performance.