Nonlinear Asymmetric Blood Oxygenation Level Dependent Responses in Somatosensory Cortex

Area of Science:

• Neuroscience
• Biophysics
• Functional Magnetic Resonance Imaging (fMRI)

Background:

• Blood oxygenation level dependent (BOLD) signals in fMRI are known to exhibit nonlinear behavior.
• Asymmetry in BOLD responses between stimulus increases and decreases has been observed, potentially due to nonlinear stimulus-response relationships.

Purpose of the Study:

• To investigate the nonlinearity and asymmetry of BOLD responses to vibrotactile stimuli in a monkey model.
• To determine if baseline activation levels influence the asymmetry between positive and negative BOLD responses.
• To correlate neural firing properties with observed BOLD signal characteristics.

Main Methods:

• Monkeys were subjected to varying vibrotactile stimuli (intensity and duration) applied to hand digits.
• BOLD responses were measured using fMRI across different baseline activation levels.
• Electrophysiological recordings from multi-electrode arrays were used to capture neural activity (multi-unit activity).

Main Results:

• BOLD responses demonstrated significant nonlinearity and asymmetry for both increases and decreases in stimulus intensity and duration.
• The observed asymmetry between positive and negative BOLD responses varied with different baseline activation levels.
• Neural recordings revealed nonlinear and asymmetric multi-unit activity patterns, consistent with the fMRI findings.

Conclusions:

• The nonlinear and asymmetric nature of BOLD responses is confirmed, challenging the assumption of a common hemodynamic response function for signal increases and decreases.
• Underestimating the magnitude of decreased activation may occur when using simplified models for BOLD signal analysis.
• Neural firing nonlinearities directly contribute to the observed nonlinear BOLD signal profiles, highlighting the link between neural activity and hemodynamic responses.