Stimulus control in a two-choice discrimination procedure.
Reinforcement balance during training sets how sharply learners later tell stimuli apart.
01Research in Context
What this study did
Pigeons pecked two keys in a simple discrimination task.
The researcher varied how often each correct choice paid off.
After training, the birds were tested with new stimuli to see how sharp their stimulus-control gradients looked.
What they found
When one choice paid off more often, the pigeons later showed a steeper gradient around that stimulus.
The reinforcement balance during training shaped how sharply the birds discriminated later.
How this fits with other research
Reberg et al. (1979) later showed the same bias effect across three signal-detection tasks.
Their work broadens the 1973 finding: reinforcement ratio skews responding even when the task changes.
Jensen et al. (1973) seemed to disagree—they saw contrast effects that did not track reinforcement frequency.
The difference is focus: C et al. looked at contrast peaks, while Kelly (1973) mapped whole gradients.
Both can be true; contrast and gradient steepness are separate measures.
Why it matters
When you set up conditional-discrimination programs, keep reinforcement rates close across choices.
If one option pays more, the learner may lock onto that cue and miss finer differences.
Check your data for response bias—steep gradients can hide weak stimulus control on the non-preferred side.
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02At a glance
03Original abstract
The relation between performance during discriminative training and subsequently obtained measures of stimulus control was investigated. Pigeons served as experimental subjects. In the discriminative training phase, a single peck on the center key, transilluminated by a bright or dim white light, resulted in the onset of the side keys, one red and one green. If the center key was brightly lighted, a response on the red side key was correct. A response on the green side key was correct if the center key was dimly lighted. Correct responses were reinforced on independently arranged variable-interval schedules. Following discriminative training, tests of stimulus control were administered during which white light of 11 intensities was projected on the center key and responses on the red and green side keys recorded. The proportion of correct responses in the presence of a bright or dim center-key stimulus decreased with decreases in the frequency of reinforcement of correct red or correct green responses, respectively. The slopes of the stimulus control gradients were related to the extent of response bias during training. The greater the bias to respond on the green key, the flatter the gradient showing the proportion of green-key responses to each stimulus and the steeper the corresponding gradient of red-key responses.
Journal of the experimental analysis of behavior, 1973 · doi:10.1901/jeab.1973.20-473