Shedding light on the frontal hemodynamics of spatial working memory using functional near-infrared spectroscopy.

The dorsolateral prefrontal cortex has been shown to be a key functional network within the middle frontal gyrus in regards to working memory processing. A commonly used paradigm in this line of research is the n-back task. The standard variant of the task requires participants to state whether the current item has been presented n trials prior (or not). Two possible strategies could be used to complete the task. Participants may either actively uphold the last n items in working memory or they may use item familiarity as basis for a decision. Previous functional near infrared spectroscopy studies using this paradigm have reported differing load dependent patterns of middle frontal gyrus activation which might at least in part be attributed to these different strategies. We used a spatial variant of the n-back task in which participants had to reproduce a pattern of locations n trials after presentation. We could thus eliminate the possibility of relying on familiarity for successful task completion. In line with previous functional magnetic resonance imaging studies we found a rise in middle frontal gyrus activity with rising working memory load. This was mainly reflected by a decrease in concentration of deoxygenated blood in this area. In line with previous research utilizing spatial paradigms, we found a lateralization of activity to the right hemisphere. We propose that the forced recall version of the n-back task is a valid alternative to the standard paradigm and can eliminate unwanted variance due to differing strategies, especially in high load conditions.