Intertrial unconditioned stimuli differentially impact trace conditioning

Three experiments assessed how appetitive conditioning in rats changes over the duration of a trace conditioned stimulus (CS) when unsignaled unconditioned stimuli (USs) are introduced into the intertrial interval. In Experiment 1, a target US occurred at a fixed time either shortly before (embedded), shortly after (trace), or at the same time (delay) as the offset of a 120-s CS. During the CS, responding was most suppressed by intertrial USs in the trace group, less so in the delay group, and least in the embedded group. Unreinforced probe trials revealed a bell-shaped curve centered on the normal US arrival time during the trace interval, suggesting that temporally-specific learning occurred both with and without intertrial USs. Experiments 2a and 2b confirmed that the bulk of the trace CS became inhibitory when intertrial USs were scheduled, as measured by summation and retardation tests, even though CS offset evoked a temporally precise conditioned response. Thus, an inhibitory CS may give rise to new stimuli specifically linked to its termination, which were excitatory. A modification to the micostimulus temporal difference model is offered to account for the data

Visual similarity effects in immediate serial recall and (sometimes) in immediate serial recognition

Words that sound dissimilar are recalled better than otherwise comparable words that sound similar on both immediate serial recall and immediate serial recognition tests, the so-called acoustic similarity effect. Although studies using immediate serial recall have shown an analogous visual similarity effect, in which words that look dissimilar are recalled better than words that look similar, this effect has not been examined in immediate serial recognition. We derived a prediction from the Feature Model that a visual similarity effect will be observed in immediate serial recognition only when the items are acoustically dissimilar; the model predicts no effect when the items are acoustically similar. Experiments 1 and 2 used visually dissimilar and visually similar stimuli that were all acoustically similar and replicated the visual similarity effect in serial recall but revealed no effect in serial recognition. Experiments 3 and 4 used a second set of stimuli that were acoustically dissimilar and found a visual similarity effect in both serial recall and serial recognition. The experiments confirm the Feature Model’s predictions and add to earlier findings that the two tests, serial recall and serial recognition, may show quite different results because the two tests are not as similar as previously thought