A note on the measurement of stimulus discriminability in conditional discriminations

Abstract

In a conditional discrimination, pigeons' responses on a right key were reinforced in the presence of a 0° line orientation, and responses on a left key were reinforced in the presence of any of five other orientations, 15° through 75°. Variable-interval schedules of reinforcement for right and left responses were changed over five experimental conditions. Values for a measure of discriminability of 0° from other orientations increased as orientation difference increased. Sensitivity of the choice between right and left in the different orientations, to reinforcers for right and left responses, changed with orientation difference. The effect of reinforcers in one component of a conditional discrimination on responding in another may therefore be modulated by the discriminability of the stimuli signaling the components.In a simple successive discrimination, two stimuli alternate in succession, each associated with a reinforcement schedule. A discrimination may develop as long as the stimuli and their associated schedules differ. The most obvious evidence that the discrimination depends on differential reinforcement is the absence of differential responding when the reinforcer rates are the same in the presence of the two stimuli. We have argued that if the reinforcer differential is varied over several experimental conditions, it is possible to obtain an index of the extent of control by the discriminative stimuli independently of the effects of the reinforcer differentials that otherwise maintain the discrimination (White, Pipe, & McLean, 1983, 1984. This index is the exponent of the power function relating ratios of responses in the two stimuli to ratios of reinforcers.Traditional methods for obtaining measures of discriminability of successively presented stimuli independently of reinforcement effects rely on successive conditional discriminations. For example, standard detection procedures involve discrete trials in which the stimuli to be discriminated are presented successively across trials and the choice or detection response is made within trials. Because the choice response involves a simultaneous discrimination (e.g., between left and right, or red and green, or yes and no), the detection procedure introduces an additional discrimination. That is, detection and other conditional procedures require discriminations between successive stimuli and also between the simultaneous stimuli signaling the choice response (White et al., 1984). The advantage of the additional simultaneous discrimination is that it allows errors to occur. In the context of traditional treatments of detection performance, the occurrence of errors is essential if analyses are designed to separateThe authors' mailing address is: Department of Psychology. Victoria University of Wellington. Wellington. New Zealand.153 the effects of stimulus discriminability from response bias toward reporting one or another stimulus.In our earlier work (White et al., 1983), we had found that, as stimulus separation between successively presented line or...

Conjoint Control of Performance in Conditional Discriminations by Successive and Simultaneous Stimuli

“…The present data are consistent with a result reported earlier by White, Pipe, and McLean (1985), who varied VI schedules in a conditional discrimination in the same way as in the present Experiment 2. Right responses were reinforced in the presence of a 00 line on both keys and left responses were reinforced when any of 15°, 30°, 450, 600, or 750 lines were presented.…”

Effects of Differences Between Stimuli, Responses, and Reinforcer Rates on Conditional Discrimination Performance

“…It is, however, consistent with the results of discretetrial choice experiments employing multiple stimulus values within a single session, where there is an inverse relation between a and log d (Davison & McCarthy, 1987). It is also consistent with the results of free-operant conditional discrimination research (White, 1986;White, Pipe, & McLean, 1985). Most recently, Alsop (1991) and Alsop and Davison (1991) reported a U-shaped relation between sensitivity to reinforcement and stimulus discriminability (using measures that are described below), so perhaps the discrepant results on this question depend on the choice of stimulus values.…”

Psychophysics of Remembering

“…The result that the biasing effect of the reinforcer ratio is weak at short delays when discriminability is high and is strong at long delays when discriminability is weak has a parallel in an earlier result for conditional discriminations (White, 1986;White, Pipe, & McLean, 1985). Here, reinforcer control of the choice was weak for an easy line-tilt discrimination and strong for a difficult discrimination.…”