Five experiments are reported which assessed the effect on unskilled and skillod keying performance of delaying by a small and irregular amount tho activation of the print unit attached to a teletype keyboard. The first three experiments involving unskilled subjocts showed: a significant decrease in the speed of performance; tho persistence of tho impairment over extended testing; and the occurrence of the effect even when a modified transcription task permitted faster keying. The remaining experiments involving skilled subjects showed: a decrease in the speod of performance; an association of the impairment with an increase in the longer interrosponse times; the elimination of tho effect after an initial exposure to tho delay. Those experimental outcomes are not consistent either with the open-loop or the closed-loop hypothesis of keying performance suggested by theories of motor learning. A modified version of each hypothesis is proposed, tormod #x2019; adaptation #x2019; and #x2019; substitution #x2019; respectively. Both modified hypotheses are consistent with the overall effects of delay. Further examination of the data, however, suggests that#x2019; substitution #x2019; is more consistent with the results reported for unskilled subjects and #x2019; adaptation #x2019; nioro consistent with the results for skilled subjects.
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