Tutorial Stimulus Control – Savvy Essay Writers | savvyessaywriters.net
Tutorial Stimulus Control – Savvy Essay Writers | savvyessaywriters.net
The Behavior Analyst 1995, 18, 253-269 No. 2 (Fall)
Tutorial Stimulus Control: Part II
James A. Dinsmoor Indiana University
The second part of my tutorial stresses the systematic importance of two parameters of discrimi- nation training: (a) the magnitude of the physical difference between the positive and the negative stimulus (disparity) and (b) the magnitude of the difference between the positive stimulus, in par- ticular, and the background stimulation (salience). It then examines the role these variables play in such complex phenomena as blocking and overshadowing, progressive discrimination training, and the transfer of control by fading. It concludes by considering concept formation and imitation, which are important forms of application, and recent work on equivalence relations. Key words: stimulus control, disparity, salience, blocking, overshadowing, transfer, fading, con-
cept formation, imitation, equivalence relations
The first part of this tutorial dealt with the basic principles that account for the acquisition of stimulus control under what is conventionally known as discrimination training. Among the sa- lient points that were raised were sug- gestions that (a) control by antecedent stimuli is just as important in operant as it is in respondent behavior; (b) Pav- lov’s conditional stimulus is a discrim- inative stimulus; (c) stimulus general- ization is not a behavioral process in- dependent of and antagonistic to dis- crimination but is simply another way of describing control by antecedent stimuli; and (d) the increases in control that occur during discrimination train- ing can be attributed to more frequent and more prolonged observation of the relevant stimuli accompanied, it is pre- sumed, by concomitant changes in at- tention.
Note that in conventional discrimi- nation training, the subject is exposed to repeated alternations between the positive stimulus (S+), which is ac- companied by a schedule of reinforce- ment, and an alternative stimulus (S-), which is not accompanied by rein- forcement. This alternation establishes
Address correspondence to James A. Dins- moor, Department of Psychology, Indiana Uni- versity, Bloomington, Indiana 47405.
a positive correlation between the S+ and the primary reinforcer that selects the one relevant stimulus out of the many that impinge upon the organism and transforms it into a conditioned re- inforcer of observing behavior. Note also that if the subject is exposed ex- clusively to S+ training or to a single period of S+ training followed by a single period of S- training rather than to a continued alternation, control is less adequate (Honig, Thomas, & Gutt- man, 1959; Yarczower & Switalski, 1969). Similarly, in a compound dis- crimination like that studied by Blough (1969), if one dimension is left contin- uously at its positive value for a num- ber of sessions, control by that dimen- sion is reduced and control by the other dimension enhanced; when the proce- dure returns to the previous alternation between positive and negative stimuli, the discriminative performance returns to its normal level. The second part of my tutorial will
necessarily be less tightly integrated. Taking off from the foundation laid in Part I, it extends the treatment of stim- ulus control to a discussion of two of its most important parameters and to several more complex patterns that seem to hold special significance for basic theorizing and practical applica- tion.
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254 JAMES A. DINSMOOR
STIMULUS PARAMETERS There are a number of variables that
influence the rate at which the subject learns to discriminate between stimuli and the level of performance that will ultimately be attained. However, some of these, like the physical quality of the stimulus (e.g., wavelength of light, en- ergy of sound), the topography of the response, and individual and species differences among different subjects, do not readily lend themselves to the formulation of general laws of behav- ior. Of greater interest from a system- atic point of view are those parameters that enter into a variety of behavioral paradigms. There are two parameters, in particular, that will appear and reap- pear in subsequent accounts of topics like overshadowing, blocking, the easy-to-hard effect, fading, and con- cept formation. These are (a) the mag- nitude of the difference in physical units between the positive and the neg- ative stimulus, sometimes subsumed by the phrase stimulus disparity, and (b) the magnitude of the difference be- tween the discriminative stimuli and the background stimulation, subsumed as stimulus salience.
Stimulus Disparity Clearcut evidence for the role played
by the disparity between the two stim- uli may be found in a series of early studies, using rats as subjects, con- ducted by Rosemary Pierrel and her as- sociates at Brown University. Bar pressing was reinforced on a variable- interval schedule in the presence of the positive stimulus but not in the pres- ence of the negative stimulus. In Pier- rel, Sherman, Blue, and Hegge (1970), for example, the discriminative stimuli were pulsed tones of 4 kHz that dif- fered in intensity. The difference be- tween the positive and the negative stimulus was set at 10, 20, 30, or 40 dB, with the higher intensity serving as the positive stimulus for half the groups and as the negative stimulus for the other half. Also, for half the ani- mals, independently assigned, the low-
er intensity was set at 60 dB, with the higher intensity determined by the magnitude of the difference between the two; for the other half, the higher intensity was set at 100 dB, with the lower intensity determined by the mag- nitude of the difference. (Because there was only one 60-100 group and one 100-60 group, there were 14 rather than 16 groups in all, plus a special control group.)
It seems obvious that very small physical differences between the posi- tive stimulus (SD or S+) and the neg- ative stimulus (S^ or S-) must be dif- ficult to discriminate. Up to some limit, at least, larger differences should pro- mote faster acquisition and a larger ul- timate difference between the two per- formances. This expectation is borne out by a series of plots tracing the course of the discrimination index (multiplied by 10) as a function of the hours of training. In all four panels of Figure 1, the discrimination is slowest to develop and attains the lowest final level in groups for which the stimuli differ by only 10 dB; groups for which the difference is 20 dB do somewhat better; groups for which the difference is 30 or 40 dB differ less during ac- quisition and tend to converge, but at still higher levels of performance. With pigeons as subjects, Hanson (1959) found a similar relation between the magnitude of the difference in wave- length and the time required for the de- velopment of a discrimination. Again the slope of the function was steep at small values but decreased as larger values were approached.
Stimulus Salience
Further examples of the effects of stimulus disparity would not be hard to find, but the magnitude of the differ- ence between the discriminative stim- uli and their background stimulation (salience) poses more of a problem. This is a dimension that does not come up for consideration within the older response-strengthening-and-weakening theories of discrimination learning.
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Figure 1. Percentage of responding in SD (S+), multiplied by 10, as a function of hours of training. Each point represents the mean of 4 rats during an 8-hr session. The stimuli for each group are specified in decibels. (Reproduced from Pierrel et al., 1970; copyright Society for the Experimental Analysis of Behavior.)
However, when investigators are guid- ed by an interest in the role of observ- ing or attention, it becomes a more likely candidate. In my laboratory, we became aware of the importance of sa- lience while studying the effects of
stimulus disparity on the relative rates of pecking two observing keys (Dins- moor, Sears, & Dout, 1976). In our first experiment, the birds pecked a key that produced large increases or decreases in illumination as discriminative stim-
256 JAMES A. DINSMOOR
uli at higher rates than they pecked a key that produced smaller increases or decreases in illumination. In our sec- ond experiment, however, we varied the magnitude of change for the posi- tive and the negative stimuli indepen- dently. We discovered that the positive relation between magnitude of change and rate of pecking stemmed entirely from the positive stimulus: For the negative stimulus, the function was negative in slope. That is, larger changes in stimulation reduced the rate. On returning to the experimental literature, we were led to the conclu- sion that in most previous examina- tions of stimulus difference, the mag- nitude of the difference between the two stimuli had been confounded with the magnitude of the difference be- tween the discriminative stimuli and their background. That is, the disparity betwen the stimuli had been confound- ed with their salience.
At the time, we were not aware that a study of discrimination in which the salience of the stimuli had indeed been varied independently of other factors had already been conducted by John- son (1970). A student of Cumming, Johnson was interested in the role of attention in discrimination learning, and one of the parameters he had var- ied, as part of a larger study of selec- tive control, was the brightness of a white line displayed on the pigeon’s key. Pecks when the line was vertical (S+) were reinforced on a random-in- terval schedule, but pecks when the line was…
