[Abstract Title]. - Society for Neuroscience

Poster
286. Imaging the Nervous System
Time: Sunday, November 16, 2008, 1:00 pm - 5:00 pm
Program#/Poster#: 286.7/QQ47
Topic: E.09.e. Functional imaging
Support: NIH grant R01-445671-01
MRC New Investigator Award GO601581
MRC project grant G0100538
Title: Prolonged negative BOLD represents neural inhibition in the deeper cortical layers
Authors: *L. W. BOORMAN, A. KENNERLEY, M. JONES, D. JOHNSTON, Y. ZHENG, J.
MAYHEW, J. BERWICK;
Univ. Sheffield, Sheffield, United Kingdom
Abstract: Blood Oxygen Level Dependent (BOLD) functional magnetic resonance imaging
(fMRI) is used to infer levels of neural activity by examining signal changes associated with
changes in deoxyhemoglobin (Hbr) concentration. The most commonly used mapping signal,
„the positive BOLD response‟, is due to a „washout‟ of Hbr produced by a large increase in blood
flow into an active brain region. However, recently a prolonged negative BOLD signal has been
described in both the primate (Shmuel et al 2006) and human (Smith et al 2004) cerebral cortex.
The cause of this negative BOLD signal is unclear, but it is proposed to be one or a combination
of two mechanisms: (i) a vascular blood „steal effect‟ in which active regions of cortex increase
their blood flow by reducing blood flow to surrounding regions, and ii) inhibitory neural
connections to adjacent regions that lower activity and therefore decrease metabolic demand and
thus decrease blood flow. The current study examines the presence of prolonged negative BOLD
in rat somatosensory cortex. 16s duration electrical stimuli (5Hz, 1.2mA) were presented to the
contra-lateral whisker pad. Cortical hemodynamic responses were measured using 2-dimensional
optical imaging spectroscopy (2D-OIS) and laser Doppler flowmetry (LDF). Neural responses
were measured with multi-channel electrophysiology. Large increases in blood volume,
saturation and flow were seen in the cortical whisker barrel region during the period of
stimulation. Negative changes of a smaller magnitude were seen in regions surrounding the
whisker barrel region. Multi-channel electrodes were placed into both the barrel cortex and the
adjacent regions that displayed „negative‟ hemodynamic responses. Electrophysiological
recordings from the „whisker‟ region displayed large evoked spiking responses with very small
decreases in interim baseline multi-unit activity. Electrophysiological recordings from the
„negative hemodynamic‟ region showed smaller evoked spiking responses, but displayed a far
larger decrease in interim baseline firing. These decreases in baseline spiking were found
predominantly in the deeper layers of the cortex. These data suggest that negative hemodynamic