Processes contributing to forward and backward span: an ERP investigation.

Reversal of the order in which items are recalled reduces verbal span more than visuospatial span. We investigated behavioral performance and event-related potential components elicited during novel recognition span tasks with verbal (digits) and visuospatial (blocks) stimuli. Reversing item order reduced behavioral span and the amplitudes of the P3a and P3b event-related potential components in the verbal but not in the visuospatial tasks. P3a is thought to reflect detection of deviance from expectation, and hence the smaller P3a suggests expectancy of the stimulus sequence is attenuated in the digits backward task. P3b is thought to reflect allocation of attention to updating memory representations, hence the smaller P3b suggests attenuation of these attentional resources when the demanding operation of verbal order reversal is required in the digits backward span task.