Cortical Activation in fMRI during a Working Memory Task Depends on Attended Stimuli Features and Encoding Strategies.

Abstract:
This study was designed to assess whether and how cortical activation in a working memory task depends on the stimuli features attended to. Differently colored letters in various locations were shown to 12 subjects using a blocked design. Subjects were instructed to remember either the identity of the letter, or its colour, or its location. In the corresponding control conditions subjects had to react to a prespecified feature of the respective domain. Functional magnetic resonance images were obtained with an EPI-sequence. The images were analyzed with the Statistical Parametric Mapping package (Wellcome Department of Cognitive Neurology, London) and a cross correlation method developed by our group. All subjects showed a widespread bilateral activation of frontal and parietal cortices. Significant increased activation was found in the left prefrontal cortex during the letter and color condition and in the right superior frontal and left superior parietal cortex in the location condition. No differences were found between letter and color condition. All subjects reported that they remembered the colors by silent rehearsal of the color names. We conclude that the cortical activation differs with respect to the stimulus features attended to but also with respect to the encoding strategy.