Glutamate and BOLD dynamics in a repetition suppression paradigm
Creators: name: Apšvalka, Dace
name: Mullins, Paul G.
name: Gadie, Andrew
Publisher: name: F1000 Research Limited
Year: 2018

description: Repetition suppression (RS) is a reduced neuronal response to a repeated stimulus. It has been widely studied using single-cell recordings, electroencephalography, and functional magnetic resonance imaging (fMRI); however, its neurophysiological foundation remains underexplored. Proton magnetic resonance spectroscopy (MRS) complements other brain research methods by providing measures of neurometabolites noninvasively in a localized brain area. The current study used a novel approach of event-related MRS to investigate blood-oxygen-level dependent signal (BOLD) and glutamate (Glu) dynamics in a RS paradigm. We combined water-suppressed and water-unsuppressed MRS acquisitions to detect BOLD and Glu changes simultaneously and enrich the information about processes underlying brain response to repeated visual stimuli. We hypothesized that with repeated vs. novel stimuli MRS measures will show decrease in BOLD and Glu in the lateral occipital cortex (LOC), an area shown to respond to object stimuli.<br><br><i>Method</i><br><br>MRS spectra were acquired in a 3T MRI scanner using a PRESS sequence (TE/T1/TR = 105/15/1500 ms). An 8cm^3 voxel in the LOC was located using a real-time fMRI localizer scan. Water-suppressed-unsuppressed MRS acquisitions were acquired in an interleaved fashion. Subjects (N=13) were presented with simple line drawings in blocks of 12 (32 blocks in total). Each stimulus block lasted 36 s and was presented as either a non-repetition block, with all novel stimuli, or a repetition block, with 6-8 repeated stimuli and with 1-2 intervening items between repetitions. Rest blocks of 36 s separated all Task blocks. Water peak height and water-scaled Glu concentration was estimated for each condition: novel (P1) and repeated presentations (P2P3), non-repetition (B1) and repetition (B2B3) blocks, Task and Rest blocks.<br><br><i>Results and Discussion</i><br><br>Consistent with the hypothesis, Glu was 11%-13% lower in repetition compared to non-repetition conditions (P2P3 vs. P1 and B2B3 vs. B1), showing that the RS effect can also be seen in Glu dynamics. Interestingly there was no difference in Glu between Rest vs. Task conditions, however this may arise because of the Task including both novel and repeated trials, and so may show reduced Glu due to the RS effect in Task conditions. BOLD also showed decrease with repeated trials (P2P3 vs. P1), as was hypothesised, and consistent with previous fMRI studies on RS. Opposite to Glu, there was also decrease in BOLD in Rest vs. Task conditions, but no difference between B2B3 vs. B1 conditions.<br><br><i>Conclusion</i><br><br>The study complements other methods of RS research and shows that both Glu and BOLD is reduced with repeated visual stimuli, possibly reflecting decrease in neurotransmission. In addition, the simultaneous acquisition of BOLD and Glu changes and the way our experiment was designed revealed possible timing and sensitivity differences between metabolic and haemodynamic processes in the RS paradigm, worthwhile further investigation. The method presented in the current study needs to be further validated and refined and can provide additional means for studying brain functioning in-vivo.