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Abstract:
Abstract: Temporal characterization of brain activity with
fMRI is constrained by the slow and variable hemodynamic response,
and by the challenge in estimating response timing. An
event-related task design was developed leading to increased
stability of temporal estimates. Ten volunteers were imaged
(whole-brain fMRI, 1.5 Tesla scanner, asymmetric spin-echo, TR =
2.68 sec) while viewing an 8-Hz large-field flickering checkerboard
for ~1.5 sec and pressing a key upon stimulus onset with one hand
and upon stimulus offset with the other. Across runs, subjects
alternated the hand used to indicate onset or offset. Sampling was
"interleaved" across runs with the stimulus occurring at either the
beginning or middle of the image acquisition, yielding an effective
sampling rate that was half the TR. To assess reliability, visual
cortex timing estimates were compared across run pairs (r2 = .95
for time-to-peak). To assess sensitivity, motor cortex timing
offsets were explored. Offsets of under 1 second could be reliably
detected. Moreover, relative timing changes between regions (visual
vs. motor cortex) were used to make inferences about which regions
contributed to motor response execution and their relative order in
the processing hierarchy. These methods have broad implications for
the understanding of the relative timing of neural processes
within- and between-regions.
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