The triphasic intrinsic signal: implications for functional imaging

Area of Science:

• Neuroimaging
• Optical Imaging
• Functional Neuroscience

Background:

• Intrinsic signal optical imaging with red illumination (ISOI) is a high-resolution technique mapping stimulus-evoked hemodynamic signals to infer neuronal activity.
• The typical ISOI signal comprises an initial dip, followed by a stronger, more sustained overshoot.
• Blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) also detects hemodynamic signals, exhibiting an initial dip and subsequent overshoot, with a later undershoot.

Purpose of the Study:

• To investigate the potential of the ISOI undershoot phase for functional mapping.
• To compare the spatiotemporal characteristics of the ISOI initial dip, undershoot, and overshoot phases.
• To correlate ISOI signal phases with electrophysiological recordings to validate their neuronal origin.

Main Methods:

• ISOI with 635 nm illumination was employed to image hemodynamic responses for 13.5 seconds after a 1-second stimulus.
• Spatiotemporal attributes (e.g., areal extent, peak magnitude) were quantified for the initial dip, undershoot, and overshoot phases.
• Simultaneous electrophysiology recordings were conducted to assess neuronal responses during the ISOI signal phases.

Main Results:

• The ISOI undershoot phase was successfully detected and utilized for functional mapping.
• Signal magnitude and spatial extent were largest for the overshoot, followed by the undershoot, then the initial dip.
• Peak activity locations varied across phases, with modest within-phase correlations and sparse cross-phase correlations; electrophysiology did not support a neuronal basis for the undershoot.

Conclusions:

• The ISOI undershoot phase provides a distinct signal for functional mapping, though its spatiotemporal characteristics differ from the dip and overshoot.
• Discrepancies in peak activity location and signal attributes across phases suggest complex hemodynamic contributions.
• Further research is needed to fully elucidate the physiological underpinnings of ISOI signal phases and their relationship to BOLD fMRI.