In a discrete-trials, two-key choice situation, probability-learning by pigeons was studied under a variety of training conditions. Matching was found in simultaneous and in successive problems, but a spatial problem produced only maximizing. In the simultaneous problem, noncorrection produced maximizing, while correction produced matching. Guidance produced maximizing when the animals were required to earn each opportunity for choice by pecking a center key on FR-5, but matching when the center key was not used. In a discrete-trials one-key situation, with latency as the measure, frequency and probability of reinforcement were varied independently. Differences in probability produced differences in latency of response, but differences in frequency did not.

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Background: Many nonbiological variables PsychobiologyFellowship grant MH 18399 (Michael Irwin, M.D.), and are reported to predict treatment response for UCSD ClinicalResearchCentergrant M01 RR00827 (Edward W.Holmes, major depression; however, there is little agree-M.D.). merit about which variables are most predictive. A preliminary analysis of these data was presented at the Young Method: Inpatient subjects (N = 59) diagnosed Investigatorsposter session at the 151st annual meeting of the American with current DSM-IV major depressive disorder PsychiatricAssociation;May 30--June4, 1998; Toronto,Ontario, Canada. The authors thank Marla sinith, R_N.; Kathy Resovsky, R.N.; and completed weekly depressive symptom ratings Debbie Greenfield,M.A.,for assistance with data collection and Lesley with the Hamilton Rating Scale for Depression Wetherell,B.S.,for data-managementand statistical support.