Image Processing Reference
In-Depth Information
Due to the indirect nature of the BOLD signal as a tool to measure neuronal
activity, in many multimodal experiments a preliminary comparative study is
done first in order to assess the localization disagreement across different modal-
ities. Spatial displacement is often found to be very consistent across multiple
runs or experiments (see
Subsection 8.4.3
for an example). Specifically, observed
differences can potentially be caused by the variability in the cell types and
neuronal activities producing each particular signal of interest [2]. That is why
it is important first to discover the types of neuronal activations that are primary
sources of the BOLD signal. Some progress on this issue has been made. A series
of papers generated by a project to cast light on the relationship between the
BOLD signal and neurophysiology has argued that local field potentials (LFP)
serve a primary role in predicting the BOLD signal ([131], and References 27,
29, 54, 55, and 81 therein). This work countered the common belief that spiking
activity was the source of the BOLD signal (for example, [132]) by demonstrating
a closer relation of the observed visually evoked HR to the local field potentials
(LFP) of neurons than to the spiking activity. This result places most of the
reported nonlinearity between experimental design and observed HR into the
nonlinearity of the neural response, which would benefit a multimodal analysis.
Note that the extracellular recordings experiments described above were
carried out over small ROIs, and therefore they inherit the parameters of under-
lying hemodynamic processes for the given limited area. Thus, even if LFP is
taken as the primary electrophysiological indicator of the neuronal activity caus-
ing the BOLD signal, the relationship between the neuronal activity and the
hemodynamic processes on a larger scale remains an open question.
Because near-infrared optical imaging (NIOI) is capable of capturing the
individual characteristics of cerebral hemodynamics such as total, oxy-, and
deoxyhemoglobin content, some researchers tried to use NIOI to reveal the nature
of the BOLD signal. Rat studies using 2-D optical imaging [133] showed the
nonlinear mapping between the neuronal activity and evoked hemodynamic pro-
cesses. This result should be a red flag for those who try to define the general
relation between neuronal activation and the BOLD signal as mostly linear. The
conjoint analysis of BOLD and NIOI signals revealed the silent BOLD signal
during present neural activation registered by E/MEG modalities [119]. This
mismatch between E/MEG and fMRI results is known as the
sensory motor
paradox
[134]. To explain this effect, the vein and capillary model was used to
describe the BOLD signal in terms of hemodynamic parameters [119]. The
suggested model permits the existence of silent and negative BOLD responses
during positive neuronal activation. This fact, together with an increasing number
of studies [135] confirming that sustained negative BOLD HR is a primary
indicator of decreased neuronal activation, provide yet more evidence that the
BOLD HR generally is not a simple linear function of neuronal activation but
at best is a monotone function that has close-to-linear behavior in a wide range
of nuisance neurophysiologic parameters. We conclude this subsection by noting
that the absence of a generative model of the BOLD response prevents the
development of universal methods of multimodal analysis. Nevertheless, as
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