How is the serial order of a spatial sequence represented in short-term memory (STM)? Previous research by Farrell and Lewandowsky (Farrell & Lewandowsky, 2004; Lewandowsky & Farrell, 2008) has shown that 5 alternative mechanisms for the representation of serial order can be distinguished on the basis of their predictions concerning the response times accompanying transposition errors. We report 3 experiments involving the output-timed serial recall of sequences of seen spatial locations that tested these predictions. The results of all 3 experiments revealed that transposition latencies are a negative function of transposition displacement, but with a reduction in the slope of the function for postponement, compared with anticipation errors. This empirical pattern is consistent with that observed in serial recall of verbal sequences reported by Farrell and Lewandowsky (2004), and with the predictions of a competitive queuing mechanism, within which serial order is represented via a primacy gradient of activations over items combined with associations between items and positional markers, and with suppression of items following recall. The results provide the first clear evidence that spatial and verbal STM rely on some common mechanisms and principles for the representation of serial order.
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