This study examined relations between eye movements and accuracy scores in a delayed matching-to-sarnple procedure with multiple sample stimuli. Four adult humans responded with high accuracy when there were 2 samples per trial. When the number of samples was increased to 4 per trial, accuracy scores fell to intermediate levels for 2 of the subjects, and then recovered to high levels following practice. Eye-tracking analyses showed that subjects with high and low accuracy on the initial 4-sample trials made similar numbers of observations per trial , but high-accuracy subjects exhibited longer sample-stimulus observing durations. Practice and improvement to high accuracy was accompanied by relatively small changes in observing frequencies and large increases in sample-stimulus observing durations. The results are discussed in terms of independent stimulus control of different aspects of observing behavior topography. One topic of continuing interest in analyses of stimulus control concerns the range, breadth, or number of stimuli that can exert control simultaneously. This question has been addressod in such diverse areas as, for example, breadth of attention in animal cognition (e.g., Maki & Leuin, 1972); the effects of task complexity (number of stimuli) on reaction times in aging populations (e.g., Baron & Menich, 1985); and stimulus overselectivity, an atypically restricted breadth of stimulus control related to learning problems of some individuals with dHvelopmental disabilities Data collection and manuscript preparation were supported by NICHD Grants HD25995 and HD37055, and CNPq Grant 200552/96-1. The contents of this paper are solely the responsibility of the authors and do not necessarily represent the official views of NICHD. Some of the data were presented at the annual conference of the Southeastern Association for Behavior Analysis, Chapel Hill, NC, October, 1997. We thank Kevin Farren for assistance in data analysis, Bill Mcllvane for advice on the project, and Bill Mcllvane and Richard Serna for comments on the manuscript. For their generosity with technical support and assistance, we thank Rikki Razdan and

Restricted stimulus control refers to discrimination learning with atypical limitations in the range of controlling stimuli or stimulus features. In the study reported here, 4 normally capable individuals and 10 individuals with intellectual disabilities (ID) performed two-sample delayed matching to sample. Sample-stimulus observing was recorded with an eye-tracking apparatus. High accuracy scores indicated stimulus control by both sample stimuli for the 4 nondisabled participants and 4 participants with ID, and eye tracking data showed reliable observing of all stimuli. Intermediate accuracy scores indicated restricted stimulus control for the remaining 6 participants. Their eye-tracking data showed that errors were related to failures to observe sample stimuli and relatively brief observing durations. Five of these participants were then given interventions designed to improve observing behavior. For 4 participants, the interventions resulted initially in elimination of observing failures, increased observing durations, and increased accuracy. For 2 of these participants, contingencies sufficient to maintain adequate observing were not always sufficient to maintain high accuracy; subsequent procedure modifications restored it, however. For the 5th participant, initial improvements in observing were not accompanied by improved accuracy, an apparent instance of observing without attending; accuracy improved only after an additional intervention that imposed contingencies on observing behavior. Thus, interventions that control observing behavior seem necessary but may not always be sufficient for the remediation of restricted stimulus control.

Overselective stimulus control refers to discriminative control in which the number of controlling stimuli is too limited for effective behavior. Experiment 1 included 22 special-education students who exhibited overselective stimulus control on a two-sample delayed matching task. An intervention added a compound identity matching opportunity within the sample observation period of the matching trials. The compound matching functioned as a differential observing response (DOR) in that high accuracy verified observation and discrimination of both sample stimuli. Nineteen participants learned to perform the DOR and two-sample delayed matching accuracy increased substantially for 16 of them. When the DOR was completely withdrawn after 10 sessions, accuracy declined. In Experiment 2, a more gradual withdrawal of DOR requirements showed that highly accurate performance could be maintained with the DOR on only a proportion of trials for most participants. The results show that DOR training may lead to a general improvement in observing behavior.