Abstract
BackgroundThe prefrontal cortex (PFC) is an anatomically and functionally heterogeneous area which influences cognitive and limbic processing through connectivity to subcortical targets. As proposed by Alexander et al. (1986) the lateral and medial aspects of the PFC project to distinct areas of the striatum in parallel but functionally distinct circuits. The purpose of this preliminary study was to determine if we could differentially and consistently activate these lateral and medial cortical-subcortical circuits involved in executive and limbic processing though interleaved transcranial magnetic stimulation (TMS) in the MR environment.MethodsSeventeen healthy individuals received interleaved TMS-BOLD imaging with the coil positioned over the dorsolateral (EEG: F3) and ventromedial PFC (EEG: FP1). BOLD signal change was calculated in the areas directly stimulated by the coil and in subcortical regions with afferent and efferent connectivity to the TMS target areas. Additionally, five individuals were tested on two occasions to determine test-retest reliability.ResultsRegion of interest analysis revealed that TMS at both prefrontal sites led to significant BOLD signal increases in the cortex under the coil, in the striatum, and the thalamus, but not in the visual cortex (negative control region). There was a significantly larger BOLD signal change in the caudate following medial PFC TMS, relative to lateral TMS. The hippocampus in contrast was significantly more activated by lateral TMS. Post-hoc voxel-based analysis revealed that within the caudate the location of peak activity was in the ventral caudate following medial TMS and the dorsal caudate following lateral TMS. Test-retest reliability data revealed consistent BOLD responses to TMS within each individual but a large variation between individuals.ConclusionThese data demonstrate that, through an optimized TMS/BOLD sequence over two unique prefrontal targets, it is possible to selectively interrogate the patency of these established cortical-subcortical networks in healthy individuals, and potentially patient populations.
Highlights
There are many ways to parse the primate prefrontal cortex (PFC) anatomically and functionally, one of the most frequently cited divisions include the dorsal-lateral (DLPFC) and the ventral-medial portions (MPFC) of the PFC [1]
Spatial Topography of Peak BOLD Signal As described above, the EEG coordinate system was used as a guide for transcranial magnetic stimulation (TMS) coil placement
Given individual variability in the shape of the skull, amount of cerebrospinal fluid between the skull and cortex, and in gyral folding, there is likely to be some variability in the location of peak BOLD signal subsequent to TMS
Summary
There are many ways to parse the primate prefrontal cortex (PFC) anatomically and functionally, one of the most frequently cited divisions include the dorsal-lateral (DLPFC) and the ventral-medial portions (MPFC) of the PFC [1]. Alexander and colleagues (1986) proposed that these regions have parallel architecture, but remain functionally and anatomically distinct as they project to the striatum and onto the thalamus These parallel functionally segregated pathways from the DLPFC and the MPFC have unique roles in shaping executive and limbic processing [2,3]. The primary subcortical targets of the DLPFC in primates include the dorsal striatum and hippocampus These areas are typically activated during executive processing tasks including logical decision making [4] and working memory [5,6,7]. The purpose of this preliminary study was to determine if we could differentially and consistently activate these lateral and medial corticalsubcortical circuits involved in executive and limbic processing though interleaved transcranial magnetic stimulation (TMS) in the MR environment
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