The n-back task was hypothesized to be a dual task, permitting the imposition of parametrically increasing attentional and working memory demands, while keeping constant the demands of an embedded matching subtask. Visual targets were presented for 200 ms every 2.2 s at pseudorandomly varying positions on a computer screen. Participants were required to remember the most recent 0, 1, 2, or 3 positions and responded with a choice button push to whether the current target position matched the position presented n items previously. P300 peak latency was constant across n-back tasks, reflecting constant perceptual and cognitive demands of the matching subtask. P300 peak amplitude decreased with increasing memory load, reflecting reallocation of attention and processing capacity away from the matching subtask to working memory activity. These data support a dual-task nature of the n-back, which should be considered when employing this paradigm.

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Why is it so difficult to do two things at once? Many accounts of dual-task performance have suggested that the answer lies in the concept of a central processing bottleneck-a stage of processing that can only be performed for one task at a time. Using variations of the psychological refractory period (PRP) paradigm, most authors have converged on Pashler and colleagues' (Pashler, 1984(Pashler, , 1994Pashler & Johnston, 1998) response-selection bottleneck (RSB) theory. RSB theory relies on a discretestage processing assumption in modeling PRP reaction time data using the locus-of-slack logic. This assumes that the processes involved in the bottleneck-canonically the response-selection stages for both tasks in a PRP experiment-are discrete and serial. Specifically, RSB theory states that Task 2 response selection (RS) cannot begin until Task 1 RS is complete.This article investigates whether response-selection processes of both tasks of a dual-task pair may in fact operate in parallel, by directly testing this discreteness assumption. Instead of the more traditional locus-of-slack approach, we looked for evidence of priming of Task 1 RS processes from Task 2 RS information in a typical PRP paradigm. Such priming would imply that Task 2 RS had begun before Task 1 RS was complete-that both RS processes were operating in parallel. Such evidence would constitute a violation of the discrete-stage processing assumption and pose a problem for RSB theory. The present study goes beyond previous investigations (e.g., Hommel, 1998;Logan & Delheimer, 2001;Logan & Gordon, 2001;Logan & Schulkind, 2000) by directly assessing the influence of Task 2 response information on Task 1 response selection, independently and separately from the semantic relationship between Tasks 1 and 2. The Psychological Refractory Period ParadigmTelford (1931) described the "psychological refractory period" effect in human subjects required to make responses to two stimuli in quick succession. Subjects' reaction times to the second stimulus became increasingly longer as the stimuli were presented closer together in time. Telford's analogy to the refractory behavior of action potential generation in neurons was less than ideal, but the terminology has persisted. In a typical PRP experiment, subjects are presented with two stimuli, S1 and S2, separated in time by a stimulus onset asynchrony (SOA) of 0-1,000 msec. Subjects make response R1 to S1 to produce reaction time RT1 (altogether termed Task 1) and make R2 to S2 to produce RT2 (altogether termed Task 2), typically performing the tasks in this order. The basic results from this paradigm are extremely robust. RT2 becomes longer as SOA decreases, typically approaching a slope of Ϫ1 as SOA approaches zero, but RT1 is relatively unaffected by SOA. Locus-of-Slack Logic and Response-Selection Bottleneck TheoryIn 1984, Harold Pashler described a series of experiments using the PRP paradigm that used a serial processing stage model of task performance to identify potential "bottlenecks" in dual-task PRP pe...

The N170 event-related potential component is currently under investigation for its role in face identity processing. Using a location-matching paradigm, in which face identity is task irrelevant, we observed a progressive decrease in N170 amplitude to multiple repetitions of upright faces presented at unattended locations. In contrast, we did not observe N170 habituation for repeat presentations of inverted faces. The findings suggest that the N170 repetition effects reflect early face identity processes that are part of familiarity acquisition of new faces.

The present study investigates the effect of practice in a psychological refractory period (PRP) paradigm on the backward compatibility effect (BCE), in order to determine the locus of this response priming effect on Task1 performance. In two experiments, we show that the size of the BCE is closely associated with the duration of the response selection stage in Task1. When this stage is shortened through PRP practice, the magnitude of the BCE decreases. Subsequently increasing the duration of Task1 response selection results in a larger BCE, but manipulating the same stage in Task2 does not. Our results suggest that the BCE reflects crosstalk of unattended response information for Task2 acting on the response selection stage in Task1, and that response information for two tasks may be activated simultaneously.