Nissen and Bullemer (1987)reported that implicit motor sequence learning was disrupted by the addition of a secondary task. They suggested that this effect was due to the attentionalload that the secondary task adds. Recently it has been suggested that the attentionalload is not critical, but rather that the secondary task affects timing, either by lengthening or by making inconsistent the response-tostimulus interval (RSI)-that is, the delay between when a subject makes a response and when the next stimulus appears. In six experiments we manipulated the RSIand found no support for these two hypotheses. An inconsistent RSIdid not adversely affect implicit motor sequence learning. A long RSI did not affect learning, although under some conditions subjects did not express learning if the RSIwas long. These results are interpreted as reflecting the effects of attention.

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