opto-fMRI with behavior

COMBINING BIDIRECTIONAL OPTOGENETIC fMRI OF PFC SUBSECTIONS WITH BEHAVIORAL STUDIES

Summary

In our project, we will combine behavioral experiments with fMRI measurements during optogenetic manipulation (opto-fMRI) to understand the role of prefrontal subareas in rats. Opto-fMRI allows for mapping of whole-brain activity while using light to modulate activity of genetically targeted neural populations. While the PFC (prefrontal cortex) has been generally studied with opto-fMRI, comparison of fMRI signals from optogenetic disruption of separate PFC subsections has not been realized.

Optogenetic inhibition plays a crucial role in causal inferring of neural roles in behavioral experiments, but has hardly been studied with fMRI. One reason for this is that continuous light for inhibition via NpHR leads to heating, which produces a pseudo fMRI effect that is difficult to distinguish from true activity (observed in previous experiments). This can be avoided with pulsed light, such as that used for excitation via ChR2, a commonly-used opsin in opto-fMRI (example data shown in Figure 1). We will use BiPOLES, a new optogenetic tool that uses separate wavelengths of pulsed light for excitation and inhibition (Vierock et al., 2021); this allows us to overcome heating issues to measure whole-brain effects from inhibition, and also accommodates direct within-animal comparison of fMRI measurements during bidirectional optogenetic perturbation of the same target area. This is highly desirable as variable baseline activity and targeting specificity between experimental animals can otherwise cause confounding results when comparing activity.

In behavioral experiments, relevant stimulation durations range between 0.3-1s, but in fMRI analyses, durations of 10-20s are usually used due to the low temporal resolution (about 2s), making direct comparisons difficult. In order to study stimulation durations that are comparable to those used in behavioral trials, we will apply a new fMRI protocol with around 0.2s temporal resolution. To achieve this, we plan to assess different strategies to optimize the fMRI acquisition: 1) Use the regular echo-planar-imaging (EPI) sequence and, based on previous results, balance the spatial and temporal resolution, the number and type (gradient and spin echoes) of echoes, the signal-to-noise ratio, the spatial coverage, and the gradient load. 2) Implement a multiband-EPI sequence that inherently provides higher temporal resolution.

This project will optimize an fMRI technique with advanced resolution, creating a platform to compare fMRI measurements to behavioral effects from optogenetic manipulation. We will further apply the novel BiPOLES opsin to target not only excitatory, but the less-investigated inhibitory effects in fMRI. Within-animal comparative analysis of whole-brain activity from bidirectional modulation of individual subsections will be performed, providing novel data on PFC circuit activity.