Zhiyan Wang¹, Antonia Wittman¹, Sinah Wiborg¹, Markus Becker¹, Mark W. Greenlee¹, Sebastian M. Frank¹; ¹Institute for Psychology, University of Regensburg

Functional magnetic resonance imaging (fMRI) studies have provided compelling evidence that the content of working memory can be successfully decoded in early visual areas (EVA). However, it remains elusive whether fMRI-measured hemodynamic response patterns following neural activity in EVA support the successful recognition of targeted information, during which a rapid comparison of current sensory input with representations held in working memory is necessary. Using event-related functional proton magnetic resonance spectroscopy (1H-fMRS), it is possible to measure transient changes in excitatory glutamatergic processing in EVA. Therefore, to characterize the transient processes underlying the recognition of targets, we measured participants’ (n = 30) glutamatergic responses in EVA while they performed a working memory task with low (1-back) and high (2-back) working memory load. Participants were presented a series of letters in the center of the screen and responded by button press when the letter in the current trial matched the previous trial (low load, 1-back) or when it matched the letter two trials before (high load, 2-back). Low and high load conditions were presented in blocks using a pseudorandom interleaved order. Each block contained 16 trials, where on each trial a letter was presented for 700 ms, following by an intertrial interval jittered between 1.05-1.25 s. On average, 4 targets were presented during each block. The glutamate response was calculated for the time periods corresponding to target and non-target presentations. We found a significantly greater glutamate-response during target compared with non-target presentations. Furthermore, as mental loads increased from 1-back to 2-back, the target-related glutamate-response also increased. No such effects in glutamate response were found in a separate control experiment with 1H-fMRS in dorsolateral prefrontal cortex. Our study suggests that a transient boost of excitatory neuronal processing in EVA, modulated by working memory load, may underly the dynamic recall of target information.

Acknowledgements: Alexander von Humboldt Foundation. Julitta und Richard Müller Stiftung; Deutsche Forschungsgemeinschaft (DFG): Emmy Noether Grant (Project Number 491290285)