Electrophysiological Evidence for Parallel Response Selection in Skilled Typists

Abundant behavioral and psychophysiological evidence suggests that people cannot select more than one response at a time, as if performance were limited by an immutable bottleneck in which responses must be chosen serially (Osman & Moore, 1993; Pashler, 1994). This observation has been the cornerstone of prominent theories of attention for 60 years (Broadbent, 1958; Pashler & Johnston, 1989; Welford, 1952). Most of the supporting evidence comes from a dual-task procedure known as the psychological refractory period (PRP), in which subjects respond rapidly to two successive stimuli. Researchers sought exceptions to serial response selection by examining practice effects and judiciously choosing stimuli and responses (Hazeltine, Ruthruff, & Remington, 2006; Hazeltine, Teague, & Ivry, 2002), but exceptions are rare, and serial processing of successive tasks is still the best explanation of the PRP effect (Tombu & Jolicoeur, 2004). Behavioral studies identified the locus of the bottleneck in the process of response selection that chooses between alternative responses (McCann & Johnston, 1992; Pashler & Johnston, 1989). Psychophysiological studies focused on the lateralized readiness potential (LRP), which is recorded from electrode sites on the scalp over the primary motor cortex in the left and right hemispheres. The LRP reflects the difference in activation of responses in subjects’ two hands and has been linked directly to response selection (Coles, 1989). Several studies have shown that the LRP is delayed in the PRP procedure, as is response selection (Lein, Ruthruff, Hsieh, & Yu, 2007; Osman & Moore, 1993; Sommer, Leuthold, & Schubert, 2001). We took a different approach to the question of serial response selection, examining 16 skilled typists who had a mean of 11 years of practice (SD = 6.5 years), reported typing for a mean of 2.5 hr per day (SD = 0.6 hr per day), and typed at a mean of 50.3 words per minute (SD = 14.7 words per minute) on the typing test devised by Logan and Zbrodoff (1998). Behavioral evidence from priming studies (Crump & Logan, 2010; Logan, 2003) and video recordings of finger movements (Flanders & Soechting, 1992) suggest parallel activation of responses in skilled typists, but these effects could occur before or after response selection (Salthouse, 1986). To address response selection, we examined LRPs recorded while typists typed words with keystrokes distributed between their left and right hands. We used three sets of 60 threeto fiveletter words taken from Logan (2003). In each set, half of the words began with a left-hand keystroke and half began with a right-hand keystroke. One set (LEFT/right) consisted of words (e.g., rest) that required all keystrokes to be made by one hand. The second set (LEght/riFT) consisted of words (e.g., swim) that required the first two keystrokes to be made by one hand and the remaining keystrokes to be made by the other hand. The third set (Light/rEFT) consisted of words (e.g., dump) that required the first keystroke to be made by one hand and the remaining keystrokes to be made by the other hand. The LRP measures the difference in activation of responses in the two hands. If typists select keystroke responses in parallel, then the amplitude of the LRP for the first keystroke should decrease systematically from LEFT/right to LEght/riFT to Light/rEFT words, as progressively more keystrokes are activated in the opposite hand. If typists select keystrokes serially, then the LRP for the first keystroke should not differ between word sets. Words were presented one at a time, and the typists typed them quickly and accurately. Mean response time was short and did not vary much between word sets (LEFT/right: M = 662 ms, SD = 80 ms; LEght/riFT: M = 666 ms, SD = 84 ms; Light/rEFT: M = 656, SD = 84 ms). Mean interkeystroke interval (IKSI) was also short (LEFT/right: M = 197 ms, SD = 30 ms; LEght/riFT: M = 171 ms, SD = 41 ms; Light/rEFT: M = 163 ms, SD = 37 ms). Accuracy was high (LEFT/right: M = 89.7%, SD = 7%; LEght/riFT: M = 90.5%, SD = 7%; Light/rEFT: M = 92.6%, SD = 5%). LRPs locked to the first keystroke were computed for correct responses as the difference between C3 and C4 electrode sites (international 10-20 system; Jasper, 1958; C4 – C3 for first keystrokes in the left hand, and C3 – C4 for first keystrokes in the right hand). These LRPs are plotted in Figure 1.