Temporal characteristics of working memory for spatial relations: an ERP study.

Spatial relations can be represented categorically, by means of abstract labels, or coordinately, in metric, absolute measures. These representations have been associated to the left and the right hemispheres respectively (Kosslyn, 1987). Recent studies have focused on the temporal dynamics of spatial relation processing, with working memory task designs. In this light, we examined the suggested lateralization effect in an ERP study incorporating a visual half field match-to-sample design, in which two sequentially presented stimuli were compared. By manipulating the length of the retention intervals between the two stimuli (500 ms, 2000 ms, and 5000 ms), spatial working memory effects were studied at three separate stages of working memory; encoding, memorization, and retrieval. The hypothesized interaction of instruction and visual field was found in the behavioural data, restricted to the 2000 ms retention interval. The EEG data indicate a strong overall right hemisphere effect, which is likely related to spatial working memory in general. Categorical and coordinate processing appears to already differentiate during the encoding stage in the P300 complex (300-500 ms after presentation of the first stimulus), where instruction interacts significantly with hemisphere in the parietal area. We found a clear right hemisphere advantage for coordinate processing and no lateralization for categorical processing. We argue that the outcome indicates qualitative rather than quantitative differences between categorical and coordinate processing.