Decision-Making and Learning: The Peak Shift Behavioral Response

In a typical peak shift experiment, control subjects aretrained to respond (e.g., button-press) to a positively rein-forced stimulus (Sþ, for example a line of a particularorientation or a color of a particular hue). Treatmentsubjects are trained in a manner identical to control sub-jects with respect to Sþ and also to withhold response toan unreinforced or punished stimulus (S–, a line or huesimilar but not identical to Sþ). Both groups of subjectsare then tested without reinforcement on a continuum ofstimuli comprising various line orientations or hues,including the training stimuli. Figure 1 depicts the com-mon finding of such discrimination learning experiments:during the test, control subjects respond most frequentlyto the Sþ stimulus. Treatment subjects, however, respondmost frequently to a stimulus they have never encoun-tered before.The treatment subjects’ expression of a preference foran unrewarded and novel stimulus over Sþ is somewhatparadoxical. If all test stimuli are discriminable, why arethe treatment subjects (Sþ/S trained) not like the con-trol subjects (Sþ training only), responding most stronglyto the stimulus they have learned is rewarding? Thephenomenon that treatment subjects display in this typeof experiment is known as peak shift. When the subjects’frequency of response is plotted as a function of stimulusvalue,datashowapeakedresponsegradient.Thestimulusreceiving the maximum, or ‘peak,’ response by the treat-ment subjects is said to be ‘shifted’ relative to that of thecontrol subjects.An area shift is often noticeable, even in experimentsthatdonotresultinasignificantpeakshift.Anareashiftischaracterized by an elevation of the rates of response tothe novel stimuli on the side of Sþ away from S–. Areashift often co-occurswith peak shift. In addition, a shift ofthemoststronglyavoided stimulus isalsoproduced,offofS– in a direction away from Sþ.Peak shift is considered to be a general outcome ofgeneralization accompanying discrimination learning. Ina typical peak shift experiment, the stimuli are simplesensory perceptions, monotonically increasing in valueon the stimulus domain. The experiment consists of acontrol group and a treatment group each undergoingtraining (discrimination) and testing (generalization) phases.Stimuli are presented one at a time, for instance, on alighted button that the subject presses to indicate itsresponse. The button is lit for a set length of time, andpresses to the buttonwhile it is lit bya given color are thedependent variable. No reward is given during the testingphase. A variable intermittent reward schedule with all-or-nonerewardquantitiesmaybegivenforcorrectresponsesduring the training phase to minimize extinction duringthe testing phase. During training, incorrect responses toSþ and S– are followed by mild punishment (e.g., lightsturn off and a delay is imposed prior to the next trial).Intertrial intervals last 2–3s, during which the responsebutton is not lit. The training phase lasts until the treatmentgroup reliably responds to the stimuli (e.g., 80% correct).The number of Sþ training trials may be balanced acrossboth groups, and a variety of reinforcement schedules,stimulus dimensions, and species have been used. As aphenomenonoflearning,themagnitudeofshiftisafunctionof stimulus presentation parameters such as Sþ andS– similarity, reward value, and stimulus encounter rates.