Abstract
In classic Psychological-Refractory-Period (PRP) dual-task paradigms, decreasing stimulus onset asynchronies (SOA) between the two tasks typically lead to increasing reaction times (RT) to the second task and, when task order is non-predictable, to prolonged RTs to the first task. Traditionally, both RT effects have been advocated to originate exclusively from the dynamics of a central bottleneck. By focusing on two specific electroencephalographic brain responses directly linkable to perceptual or motor processing stages, respectively, the present study aimed to provide a more detailed picture as to the origin(s) of these behavioral PRP effects. In particular, we employed 2-alternative forced-choice (2AFC) tasks requiring participants to identify the pitch of a tone (high versus low) in the auditory, and the orientation of a target object (vertical versus horizontal) in the visual, task, with task order being either predictable or non-predictable. Our findings show that task order predictability (TOP) and inter-task SOA interactively determine the speed of (visual) perceptual processes (as indexed by the PCN timing) for both the first and the second task. By contrast, motor response execution times (as indexed by the LRP timing) are influenced independently by TOP for the first, and SOA for the second, task. Overall, this set of findings complements classical as well as advanced versions of the central bottleneck model by providing electrophysiological evidence for modulations of both perceptual and motor processing dynamics that, in summation with central capacity limitations, give rise to the behavioral PRP outcome.
Highlights
In classic Psychological-Refractory-Period (PRP) dual-task paradigms, the time taken to respond to the stimulus of the second task typically increases with decreasing inter-task interval, whereas there is no influence of inter-task stimulus onset asynchronies (SOA) on reaction times (RT) to the stimulus of the first task (e.g., Welford, 1952; Pashler and Johnston, 1989)
While both perceptual and motor stages are generally assumed to operate in parallel, the commonly advocated view (e.g., Pashler, 1984; Luck, 1998; Schubert, 1999) is that the effect of inter-task SOA on RTs to the second task may originate exclusively from a processing bottleneck located at the central stage, in particular: central-stage processing of the second task is delayed until central processing of the first task has been completed
When the auditory task was performed first, we found RTs to be modulated interactively by task order predictability (TOP) and SOA
Summary
In classic Psychological-Refractory-Period (PRP) dual-task paradigms, the time taken to respond to the stimulus of the second task typically increases with decreasing inter-task interval (i.e., stimulus onset asynchrony, SOA), whereas there is no influence of inter-task SOA on reaction times (RT) to the stimulus of the first task (e.g., Welford, 1952; Pashler and Johnston, 1989) This well-established and extensively studied effect has traditionally been explained in terms of a sequential processing model consisting of three stages: (1) a perceptual stage, which selects the task-relevant stimulus (e.g., based on a spatial characteristic: left versus right positioning of the stimulus relative to the vertical midline of the display) and, if required, extracts the response-critical stimulus attribute (e.g., exact featural identity: red versus green) required for subsequent response decisions; (2) a central stage which decides upon the appropriate motor response (e.g., left versus right index finger press) on the basis of a pre-specified task setting (i.e., stimulus-response, S-R, and mapping); and (3) a motor stage, which produces and executes this response. Both task control processes are scheduled within the central system (involving the operation of executive control); again assuming solely central processes as origin of the RT cost associated with unpredictable, relative to predictable, task orders
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