THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 1994, 47A (1) 201-205 Comment on McLeod and Hume, Overlapping Mental Operations in Serial Performance with Preview: Typing Harold Pashler University of California San Diego, La Jolla, California, U.S.A. This paper deals with the claim made by McLeod and Hume (this issue) that, for a skilled typist, the rate of responding is controlled by a limit on the number of responses that can be executed simultaneously, not by a limit on response selection. I point out that McLeod and Hume's observations are consistent with various interpretations, including one in which response rates are limited by response selection. I also note the importance of trying to distinguish structural and strategic bottlenecks in speeded responding. Pashler (this issue) concluded that responses are selected one at a time in an unpractised serial choice reaction-time task, even when the subject was permitted to preview stimuli ahead of the ones to which they had responded. This was demonstrated by showing that the preview caused the duration of response selection to become rate-limiting, whereas the duration of perception and response execution ceased to be rate-limiting. McLeod and Hume (this issue) questioned whether such a claim could apply to the behaviour of skilled copy-typists, who produce a sequence of keypresses separated by extremely short interresponse intervals. They reported that a skilled typist made finger movements that anticipated the spatial position of keypresses well ahead of those currently being produced. For example, when the typist typed PINING, the finger on the left hand that would type the G often began moving into position around the time the first N was typed. If keypress responses were selected one at a time, McLeod and Hume reasoned, then anticipatory behaviour of this sort should not have occurred (see also Rumelhart & Norman, 1982). Details of the anticipations suggested to McLeod and Hume that response rate Requests for reprints should be sent to Harold Pashler, Department of Psychology C-0109, University of California San Diego, La Jolla, California 92093, U.S.A. The author is grateful to Clark Fagot and James C. Johnston for useful comments. 1994 The Experimental Psychology Society
202 PASHLER was limited by a bottleneck in execution of the keypresses, and they inferred that the proposed response selection bottleneck could not apply in typing. McLeod and Hume's observations are of great interest, but they are consistent with various interpretations other than those they mention. Logically speaking, what can be inferred from the fact that a hand moves into position to produce the response to Character N + 3 before the response to Character N has been produced? It would seem that the only thing one can infer with certainty is that the response to Character N + 3 was selected before the response to Character N was produced. This does not rule out the possibility that the response to Character N was selected before the response to Character N + 3 was selected. McLeod and Hume appear to assume that execution of a response follows its selection without delay. It is logically possible, however, that selection operates rapidly and sequentially, character by character, inserting response specifications into a buffer from which responses are sequentially produced (at a slightly slower rate) but after a significant further delay. This account appears to be consistent with all of McLeod and Hume's observations. It may also be problematic to assume that what amounts to response selection in typing if such a process exists in that task operates one character at a time. This assumption underlies McLeod and Hume's argument. There is another possibility, however: that the response selection process handles "chunks" larger than a single keypress. This is an old idea that has been discussed for a long time in the typing literature (see e.g., Salthouse 1984; Shaffer & Hardwick, 1970). On this view, the output of the response selection stage might be a relatively abstract (perhaps spatial) specification of a sequence of several keypress responses. This process might leave the details of particular finger movements to be determined by subsequent motor programming mechanisms. The fact that typists type random letter strings more slowly than they type words has been cited as evidence for this idea (Fendrick, 1937). The hypothesis that response selection at the level of these larger chunks proceeds sequentially would be quite consistent with anticipatory behaviour of the sort observed by McLeod and Hume. Several kinds of evidence might shed further light on these various possibilities. One interesting question is whether the anticipatory movements observed by McLeod and Hume still occur when a typist types letter strings that have extremely low transitional frequencies designed to discourage chunking at the word or even bigram level. Another experiment that might be informative would combine speeded typing of a word with another task, such as a vocal response to a tone presented at a short delay. Suppose there really is a response selection process in typing that takes input in word-sized chunks and constitutes a bottleneck, as suggested above. In that case the typing task ought to produce a PRP effect (delaying
CONTROL OF SERIAL ORDER IN TYPING 203 the vocal response), but that delay ought to depend very little on the length of the word being typed. Pashler and Christian (submitted) found roughly this pattern of RTs with a first task that required subjects to produce one of several arbitrary keypress sequences. The sequences consisted of a variable number of keypresses; variation in the number of keypresses required in the first task had little effect on how much the subsequent vocal response was delayed. One final experimental approach would be to seek factorial manipulations that would define translation and execution stages in typing and observe their effects on IRIs (with and without preview), as in the Pashler (this issue) study. In short, McLeod and Hume's observations are intriguing, but they are by no means inconsistent with the claim that there is a response selection process that proceeds sequentially in skilled copytyping. This process might select individual keypress responses very rapidly well before they are actually produced or, perhaps more plausibly, larger-sized response "chunks" at a more modest rate (Salthouse, 1984). Either of these possibilities would be consistent with response execution being rate-limiting, as McLeod and Hume argue, and further investigations of typing ought to be able to distinguish among these hypotheses. If it should turn out that selection of individual keypress responses proceeds in parallel as McLeod and Hume suppose this would raise the interesting question of why typing would differ from the serial choice-rt tasks investigated by Pashler (this issue). One obvious difference is the amount of practice the subjects have had with a particular S-R mapping low in the case of the serial RT task, and high for the typist. Response selection operations that have been consistently practised may simply operate in parallel with other activities. Although intuitively plausible, there is little objective evidence for this idea. The psychological refractory period (PRP) effect is not readily eliminated with practice (Gottsdanker & Stelmach, 1971), but as there is no specific evidence that response selection remains a bottleneck after extensive practice, that does not completely resolve the issue. McLeod and Hume offer a different sort of explanation for why response selection proceeds sequentially in the serial RT situation (Pashler, this issue), even though it might and, in their view, it does operate in parallel in typing. Their suggestion is that the variability of response selection duration in choice RT is so great that if responses were selected in parallel, responses would often come out in the wrong order. Therefore, they suggest, the subject adopts a strategy of selecting them one at a time in order to prevent this from occurring (whereas in typing, subjects do not need to do this). This is an interesting possibility. I am not aware of any serial RT experiments that would test it, but recent data collected using the PRP task does not support this idea. Carrier and Pashler (submitted) found evidence of a central bottleneck when the first task was quick (button
204 PASHLER press to the pitch of a tone), the second task was time-consuming (paired associate retrieval with a vocal response), and the instructions simply told the subject to perform each task as quickly as possible. The data showed the usual PRP effect, and association strength slowed the second response to the same degree, regardless of SOA. This evidence indicates queueing of central stages. It is difficult to see how this queueing could have been adopted as a strategy to ensure that responses came out in the right order, as (1) the responses would almost always have come out in the usual order, even if they had been selected independently, and (2) the instructions to the subjects did not require responding in any particular order in any case. Nonetheless, McLeod and Hume are surely correct that the issue of structural versus strategic bottlenecks is important and needs to be investigated further. In summary, McLeod and Hume's commentary achieves several useful purposes. It documents a very interesting effect in copytyping behaviour, an effect that might seem to make sequential selection of keypresses an unlikely possibility (although, it was argued above, that possibility actually remains viable); their commentary also raises the important issue of whether response selection bottlenecks are structural features of the human mind or strategies questions in need of further investigation. The present reply suggests that McLeod and Hume's copytyping data are compatible with the idea of a structural bottleneck in response choice operating even in the case of a skilled copytypist. Perhaps the most important conclusion to be drawn from the present exchange is this: some cognitive models may be difficult or impossible to test with psychological data, but hypotheses about the scheduling of relatively time-consuming mental operations (such as selecting responses and programming motor sequences) seem to be particularly suited to being empirically tested with the techniques of the experimental psychologist. These hypotheses also have fundamental implications about human cognitive architecture. REFERENCES Carrier, M., & Pashler, H. (submitted). Attentional demands of memory retrieval. Fendrick, P. (1937). Hierarchical skills in typewriting. Journal of Educational Psychology, 28, 609-620. Gottsdanker, R., & Stelmach, G.E. (1971). The persistence of psychological refractoriness. Journal of Motor Behavior, 3, 301-312. McLeod, P., & Hume, M. (1994, this issue). Overlapping mental operations in serial performance with preview: Typing. Quarterly Journal of Experimental Psychology, 47A, 193-199. Pashler, H.E. (1994, this issue). Overlapping mental operations in serial performance with preview. Quarterly Journal of Experimental Psychology, 47A, 161-191. Pashler, H.E., & Christian, C. (submitted). Dual-task interference and the production of manual and vocal responses.
CONTROL OF SERIAL ORDER IN TYPING 205 Rumelhart, D.E., & Norman, D.A. (1982). Simulating a skilled typist: A study of skilled cognitive-motor performance. Cognitive Science, 6, 1-36. Salthouse, T.A. (1984). The skill of typing. Scientific American, 250, 128-135. Shaffer, L.H., & Hardwick, J. (1970). The basis of transcription skill. Journal of Experimental Psychology, 84, 424-440.