Effects of differences between stimuli, responses, and reinforcer rates on conditional discrimination performance.
Stimulus clarity and response topography carry conditional discrimination; reinforcer ratios only nudge bias when the other cues are already strong.
01Research in Context
What this study did
Pigeons pecked keys in a conditional-discrimination task. The birds had to tell two colors apart and peck left or right depending on the color. The team changed three things at once: how different the colors looked, how different the left and right pecks had to be, and how often each choice paid off.
They wanted to know which factor—stimulus difference, response form, or reinforcer rate—really drove good discrimination.
What they found
Clear color differences and clear peck differences both helped the birds choose correctly. Surprisingly, the payoff ratio hardly mattered. When the colors looked alike, the birds stayed accurate only if the pecks were very different. When the colors were easy to tell apart, the birds could use similar pecks and still score high.
Reinforcer rates did shift bias, but only when the pecks were already different. The three variables did not trade off in a tidy, equal way.
How this fits with other research
Reberg et al. (1979) showed that changing payoff rates creates response bias while leaving true discrimination untouched. Sisson et al. (1993) agree that payoff moves bias, but add that the effect disappears when response topography is held constant—an extra layer of detail.
Jones et al. (1992) found that longer delays between sample and choice act like making the stimuli look more alike. The 1993 paper mirrors this idea: small stimulus differences force the learner to lean on response differences instead.
Lie et al. (2010) used punishment instead of food. Punisher ratios biased human clicks without changing how well people saw the signals. The pigeon data now show the same split: consequences steer side bias, but stimulus and response clarity set the accuracy ceiling.
Why it matters
When you write a conditional-discrimination program, first make the stimuli easy to tell apart. If you can’t, then build in clear response differences—distinct motions or locations. After that, keep reinforcer rates equal across choices; tweaking payoff alone will not fix poor discrimination. Check both visual clarity and response form before you touch the schedule.
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Arrange teaching materials so the S⁺ and S⁻ look very different; if they must look alike, require clearly different learner responses before delivering the reinforcer.
02At a glance
03Original abstract
In a discrete-trial conditional discrimination procedure, 4 pigeons obtained food reinforcers by pecking a key with a short latency on trials signaled by one stimulus and by pecking the same key with a long latency on trials signaled by a second stimulus. The physical difference between the two stimuli and the temporal separation between the latency values required for reinforcement were varied factorially over four sets of conditions, and the ratio of reinforcer rates for short and long latencies was varied within each set of conditions. Stimulus discrimination varied directly with both stimulus and response differences and was unaffected by the reinforcer ratio. Sensitivity to reinforcement, estimated by generalized-matching-law fits to the data within each set of conditions, varied directly with the response difference but inversely with the stimulus difference arranged between sets of conditions. Because variations in stimulus differences, response differences, and reinforcer differences did not have equivalent effects, these findings question the functional equivalence of the three terms of the discriminated operant: antecedent stimuli, behavior, and consequences.
Journal of the experimental analysis of behavior, 1993 · doi:10.1901/jeab.1993.59-147