List memory of pigeons, monkeys, and humans was tested with lists of four visual items (travel slides for animals and kaleidoscope patterns for humans). Retention interval increases for list-item memory revealed a consistent modification of the serial-position function shape: a monotonically increasing function at the shortest interval, a U-shaped function at intermediate intervals, and a monotonically decreasing function at the longest interval. The time course of these changes was fastest for pigeons, intermediate for monkeys, and slowest for humans.

Pigeons were trained in delayed matching-to-sample with two postsample stimuli. A postsample R-cue signaled that a matching choice phase would follow. A postsample F-cue signaled that a matching choice phase would not follow. Previous research found reduced matching accuracy on F-cued probe trials when comparison stimuli were presented in the choice phase. The present four experiments systematically varied the events following an F-cue to determine the conditions under which the F-cue reduces delayed-matching accuracy. When F-cues and R-cues controlled different behavior, matching on probe trials was poor. When both cues controlled the same behavior, matching on probe trials was good. This result is best explained by the theory that comparison stimuli retrieve the sample representation, but only in the behavioral context established by the R-cue. The present research supports the view that response-produced stimuli serve a contextual role in animal short-term memory.

Theoretical accounts of retroactive interference in delayed matching-to-sample attribute decrements in performance to disruption of rehearsal or retrieval of a sample stimulus representation or an instruction of which test stimulus to peck. However, only two sources of retroactive interference have been identified so far: changes in illumination and electric shock. The present experiments sought evidence of additional sources of retroactive interference. Stimuli associated with variable interval (Vl), extinction (EXT), and differential reinforcement of other behavior (DRO) schedules were interpolated into the delays of delayed matching. The results of three experiments demonstrated that a stimulus associated with a VI schedule of reinforcement interferes with delayed matching but that an EXT-associated stimulus does not. Experiment 2 showed that a DRO-associated stimulus interferes with delayed matching. Additional analysis indicated that interference with delayed matching was a function of the degree to which the interpolated stimuli disrupted delay-interval behavior associated with accurate matching. These findings support the behavioral-context hypothesis, which states that response-produced stimuli arising from delay behavior provide the context necessary for accurate matching. Retroactive interference thus occurs to the extent that interpolated stimuli interfere with baseline delay behavior. 391Short-term memory in animals has been extensively studied in the delayed matching-to-sample paradigm. The successive delayed matching procedure is composed of four successive events: presentation ofthe sample stimulus, a delay interval, presentation of the test stimulus, and an intertrial interval. Responses during the test stimulus are correct or incorrect, that is, reinforced or not, conditional upon the relation between the previously presented sample stimulus and the current test stimulus (Wasserman, 1976). The task for the animal is to remember at the time of the test stimulus information based on the sample stimulus.Retroactive interference may be produced by interpolating events into the delay period. Retroactive interference is observed when matching accuracy on the interpolated test trials declines relative to matching accuracy on the baseline trials

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Three experiments showed stimulus control by either the absolute properties of probe stimuli, relational properties of the probe-list relationship, or both in a serial probe recognition memory task in which a four-item memory list was followed by a single probe (test) item. In Experiment 1, 3 rhesus monkeys received 39 to 75 repetitions of the same 24-trial stimulus sequence. Special tests showed stimulus control by the absolute properties of the probe stimuli. Retention of previous relational control was demonstrated by the good transfer (83%) to novel list and probe stimuli at the beginning of Experiment 2. During Experiment 2, control by absolute properties of the probe stimuli gradually reoccurred. Only a small measure of control by list stimuli could be detected or promoted. In Experiment 3, 4 monkeys were shown to have largely lost their ability to perform on the basis of the list-probe relationship, and were performing primarily on the basis of the absolute properties of the probe stimuli. Over the next 15 weeks, these monkeys were transferred to new stimuli at the beginning of each week. Control by the relational aspects of the task gradually returned. As transfer performance increased, control by the absolute properties of the probe stimuli was eliminated. The results are discussed in terms of stimulus control and performance strategies used by the monkeys.Key words: memory, stimulus control, absolute stimulus control, relational stimulus control, strategies, relational learning, picture stimuli, monkeys A recurrent issue in animal discrimination learning has been control of behavior by the absolute properties of individual stimuli or control by the relational properties between stimuli (Gonzalez, Gentry, & Bitterman, 1954;Hanson, 1959;Hearst, 1968;Honig & Urcuioli, 1981;Hull, 1943;Kendler, 1950; K6h-ler, 1918;1925;Lashley, 1942;Lawrence & DeRivera, 1954;Premack, 1978Premack, ,1983aPremack, , 1983bRiley, 1968;Rilling, 1977;Spence, 1936Spence, ,1937Staddon, 1983; Wright, in press;Wright, Cook, Rivera, Sands, & Delius, 1988;Zeiler, 1963;Zentall & Hogan, 1974). Kohler (1918Kohler ( /1939 proposed that animals were capable of learning relations between stimuli. He trained chickens on a brightness comparison and gave them transposition tests to other brightness comparisons; the results supported the proposition that the brightness relation controlled performance. Counterproposals followed in which these transposition results were ac-