Volume effect of localized injection in functional MRI and electrophysiology

The local injection of neurotransmitter agonists and antagonists to modulate recorded neurons in awake animals has long been an important and widely used technique in neuroscience. Combined with functional magnetic resonance imaging (fMRI) and simultaneous electrophysiology, local injection enables the study of specific brain regions under precise modulations of their neuronal activity. However, localized injections are often accompanied by mechanical displacement of the tissue, known as volume effect (VE), which can induce changes in electrophysiological recordings as well as artifacts that are particular to fMRI studies.

We characterize the changes produced by VE in an agarose phantom as well as during stimulus-evoked and resting-state fMRI and simultaneously acquired electrophysiology in awake rabbits.

Our results demonstrate that localized injection can produce significant intensity changes in fMRI data, even while effects on electrophysiological recordings are minimized. These changes are localized to the vicinity of the injection needle and diminish over time due to diffusion of the injected volume.

Sufficient time should be allowed for drug diffusion to ensure stable results, particularly for resting-state fMRI experiments.