Topography of brain electrical activity dissociates the sequential order transformation of verbal versus spatial information in humans

Slow event related potentials (ERPs) of the electroencephalogram were recorded while subjects performed a short-term memory task. They had to mentally transform the order of sequentially presented words (verbal condition) or positions within a grid (spatial condition). The difficulty of the transformation process was varied. The amplitudes of the slow ERPs elicited during the transformation process varied with task difficulty. The topography of this amplitude modulation turned out to be information specific: The maximal effect during transformation of the sequential order of verbal information was found over the left-frontal cortex and during the transformation of spatial information over the parietal cortex. These results suggest that the manipulation of short-term memory contents seems to take place in information specific cortical areas which are thought to be also involved during on-line processing and long-term storage of these informations.