Procedural differences in processing intact and degraded stimuli

Different impact of task switching and response-category conflict on subsequent memory

The impact of cognitive control demands on long-term memory is mixed, with some conflicts leading to better, others leading to worse subsequent memory. The current study was designed to investigate how different types of cognitive control demands modulate the effects on memory. At study, participants had to switch between two classification tasks and later, free recall performance was assessed. The stimuli consisted of two interleaved words, one word had to be categorized and the other word had to be ignored. In four experiments, the congruency between target and ignored words was manipulated by changing the distractor category. This allowed us to investigate the impact of different types of conflict (i.e., task switching, perceptual load, response-category conflict, stimulus-category conflict). The results revealed that task switching impaired memory in all experiments. In Experiment 1, higher perceptual load also impaired memory. Experiments 2–4 showed that the co-activation of two words which required different responses (i.e., response-category conflict) enhanced memory performance but only when the conflict stimuli were presented in pure blocks. Overall, memory performance seems to depend on attentional policies. Withdrawing attention from target encoding results in lower memory performance. In contrast, focusing attention on the target results in enhanced memory performance.

The Parallel Episodic Processing (PEP) model 2.0: A single computational model of stimulus-response binding, contingency learning, power curves, and mixing costs

The current paper presents an extension of the Parallel Episodic Processing model. The model is developed for simulating behaviour in performance (i.e., speeded response time) tasks and learns to anticipate both how and when to respond based on retrieval of memories of previous trials. With one fixed parameter set, the model is shown to successfully simulate a wide range of different findings. These include: practice curves in the Stroop paradigm, contingency learning effects, learning acquisition curves, stimulus-response binding effects, mixing costs, and various findings from the attentional control domain. The results demonstrate several important points. First, the same retrieval mechanism parsimoniously explains stimulus-response binding, contingency learning, and practice effects. Second, as performance improves with practice, any effects will shrink with it. Third, a model of simple learning processes is sufficient to explain phenomena that are typically (but perhaps incorrectly) interpreted in terms of higher-order control processes. More generally, we argue that computational models with a fixed parameter set and wider breadth should be preferred over those that are restricted to a narrow set of phenomena.

Identifying stimuli of different perceptual categories in pure and mixed blocks of trials: evidence for stimulus-driven switch costs

Responding to stimuli of different perceptual categories is usually faster when the categories are presented isolated from each other, in pure blocks, than when they are presented randomly intermixed, in mixed blocks; a difference denoted as perceptual mixing costs. The present study examined the contribution of strategic and stimulus-driven factors to these costs. The first two experiments showed that perceptual mixing costs were not reduced when participants were informed at the start of each trial in mixed blocks about the impending category. Furthermore, Experiments 1 and 3 showed that mixing costs were concentrated on those trials of mixed blocks where the perceptual category was different from that of the preceding trial. These results support the view that perceptual mixing costs derive from stimulus-driven trial-by-trial adjustments in processing. Some general implications for processing models are discussed.

Stimulus control of visual fixation duration in a single saccade paradigm

Subjects performed a task in which they successively inspected two stimuli presented at an equal distance of the left (SL) and the right (SR) of the visual meridian and subtending a 45 degrees visual angle. This was followed by a joint response on the basis of the status of both stimuli. The instruction was to fixate the position of SL at the start of a trial, followed by a single saccade to SR. In earlier studies on this paradigm it was suggested that the fixation time of SL (TL) may serve as a modern version of the (d)-reaction in which the time for achieving perceptual identification is measured uncontaminated by decision. This suggestion has the implicit assumption that, at least in this single saccade paradigm, the saccade from SL to SR is triggered when perceptual identification has been completed. There is the potential alternative that the duration of TL is programmed in advance, depending on the general processing demands of the stimuli in a block of trials. These two options were tested in two experiments. In the first, presentation of SL was delayed with a fixed or with a variable interval (200-400 ms) during a block of trials. In the second, the stimulus quality of SL was varied between and within blocks of trials. The results of either manipulation argue against preprogramming TL, and they are consistent with the hypothesis that the saccade is triggered upon completion of perceptual processing. Thus, TL in the single saccade paradigm appears a serious candidate for the (d)-reaction.