[Abstract Title]. - Society for Neuroscience

NIH MH064498
Title: A TMS “ping” during fMRI reveals physiological consequences of functional connectivity
and dissociates multivariate from univariate maps of working memory storage
Authors: *E. FEREDOES 1 , A. HELLER 2 , G. TONONI 3 , T. S. WOODWARD 1 , B. R.
POSTLE 2,3 ;
1 Psychiatry, Univ. of British Columbia, Vancouver, BC, Canada; 2 Psychology, 3 Psychiatry,
Univ. of Wisconsin-Madison, Madison, WI
Abstract: It is widely accepted that multivariate approaches to fMRI data analysis can, by
measuring functional connectivity, provide information about distributed functional networks
that cannot be inferred from traditional “massively univariate” analyses. However, most
multivariate methods are nonetheless subject to the same inferential constraints fundamental to
all correlational methods (i.e., neither necessity nor causality can be inferred). We used
transcranial magnetic stimulation (TMS) to evaluate causally a verbal storage network suggested
by a constrained principal components analysis (cPCA) of working memory data. The logic was
that a physiological consequence of functional connectivity should be reflected in the manner in
which a TMS-triggered impulse propagates through the putative functional network. (An analogy
is to “ping” a network of computers by sending a signal from one computer to determine which
other computers are actively connected to the network). In scanning session 1, subjects
performed short-term delayed recognition (DR) of letters at two loads: 2 or 5 items. cPCA
identified a (functionally connected) network of voxels whose delay-period activity was sensitive
to variation of load (an operationalization of “short-term storage”). Scanning session 2 featured a
2 (TMS present; TMS absent) x 3 (Letter DR; Location DR; no task) factorial design. A single
pulse of TMS was delivered in the middle of the delay period to either a node in the cPCAdefined
load-sensitive network, or to a control area. The cPCA map from session 1 was
coregistered into session 2 space and within it the TMS “ping” increased Letter delay-period
activity (i.e., Letter-DR/TMS-present > Letter-DR/TMS-absent), whereas it decreased Location
delay-period activity. Univariate delay-activity maps were also created for Letter-DR/TMSabsent
and Location-DR/TMS-absent trials. Within these univariate masks, TMS had the
uniform effect of decreasing delay-period activity. Notably, within the left and right dorsolateral
prefrontal cortex foci identified by the GLM for Letter-DR/TMS-absent trials, delay-period
activity was greater during Letter-DR/TMS-absent than Letter-DR/TMS-present trials. These
results suggest that a physiological consequence of functional connectivity is facilitation of
synaptic transmission between nodes in the network. They also provide a stark demonstration
that delay-period activity revealed by univariate analyses cannot be assumed a priori to
correspond to the short-term storage of information.
Disclosures: E. Feredoes , None; T.S. Woodward, None; G. Tononi, None; B.R. Postle,
None; A. Heller, None.
Poster