Octopus visual system: a functional MRI model for detecting neuronal electric currents without a blood-oxygen-level-dependent confound

Abstract

Purpose: Despite the efforts that have been devoted to detecting the transient magnetic fields generated by neuronal firing, the conclusion that a functionally relevant signal can be measured with MRI is still controversial. For human studies of neuronal current MRI (nc-MRI), the blood-oxygen-level-dependent (BOLD) effect remains an irresolvable confound. For tissue studies where hemoglobin is removed, natural sensory stimulation is not possible. This study investigates the feasibility of detecting a physiologically induced nc-MRI signal in vivo in a BOLD-free environment.

Methods: The cephalopod mollusc Octopus bimaculoides has vertebrate-like eyes, large optic lobes (OLs), and blood that does not contain hemoglobin. Visually evoked potentials were measured in the octopus retina and OL by electroretinogram and local field potential. nc-MRI scans were conducted at 9.4 Tesla to capture these activities.

Results: Electrophysiological recording detected strong responses in the retina and OL in vivo; however, nc-MRI failed to demonstrate any statistically significant signal change with a detection threshold of 0.2° for phase and 0.2% for magnitude. Experiments in a dissected eye-OL preparation yielded similar results.

Conclusion: These findings in a large hemoglobin-free nervous system suggest that sensory evoked neuronal magnetic fields are too weak for direct detection with current MRI technology.

Keywords: cephalopod; electroretinogram; functional MRI; local field potential; optic lobe; retina.

Fig. 1

MRI slice through the Octopus bimaculoides visual system acquired with a T₂-weighted RARE sequence. Retinal fibers project topographically across the surface of the optic lobe, with most ending in a superficial plexiform layer (arrows), but some traveling deeper to a radial layer (r) of irregular columns of cells and fibers. The optic lobe’s center (c) is composed of islands of cells enmeshed in a network of axons and dendrites. Output fibers of the lobe travel by the optic tract (asterisk) to the central brain. VL, vertical lobe of the central brain.

Fig. 2

In vivo measurement of electrical response in the octopus visual system. (A) ERG responses to light flashes of varying durations. (B) LFPs in the OL elicited by light flashes of differing durations. Measurements in A and B were made immediately after electrode implantation. (C) ERG responses in one octopus before and after two hours of nc-MRI scanning. (D) LFP recorded in the OL after two hours nc-MRI scanning in the same octopus presented in C. For all plots, the light stimulus began at 0 ms. In C and D the stimulus duration was 50 ms, which is indicated by the black boxes on the bottom axis. The grey boxes on the top axis show the acquisition windows of the two MR slices. All curves were obtained by low passing the raw data at 100 Hz and averaging it over 10 – 15 repetitions.

Fig. 3

In vitro measurement of electrical response in dissected octopus visual system. (A) ERG responses to light flashes of varying durations. (B) LFP recordings in the OL induced by light flashes of differing durations. Measurements in A and B were made immediately after electrode implantation. (C) ERG responses in one octopus before and after one hour of nc-MRI scanning. (D) LFP recorded in the OL after one hour nc-MRI scanning in the same explant presented in C. In all the plots, the light stimulus began at 0 ms. In C and D the stimulus duration was 100 ms, which is indicated by the black boxes on the bottom axis. The grey boxes on the top axis show the acquisition windows of the three MR slices. All curves were obtained by low passing the raw data at 100 Hz and averaging over 10 – 15 repetitions.

Fig. 4

In vivo ERG (A) and OL LFP (B) responses to repeated stimulations. Four ISIs from 0.2 s to 2 s were tested using a stimulation duration of 50 ms. An ISI of 2 s was chosen for the nc-MRI scans of both structures.

Fig. 5

(A) RARE images from an in vivo nc-MRI scan. The color map shows the statistical test results using the GLM thresholded at the p < 0.01 (uncorrected) level. (B) EPI images for the nc-MRI scans of the two corresponding slices. The color maps show the statistical test results from the permutation test, thresholded at the p < 0.01 (uncorrected) level. The acquisition windows of the first and second slices corresponded to the peak activity in the retina and the OL respectively (see Fig. 2C and D). The B₀ field was perpendicular to the slices and hence also perpendicular to the photoreceptor cells in the retina.

Fig. 6

(A) RARE images from an in vitro nc-MRI scan. The color map shows the statistical test results using the GLM, thresholded at the p < 0.01 (uncorrected) level. (B) EPI images for the nc-MRI scans of the three corresponding slices. The color maps show the statistical test results from the permutation test, thresholded at the p < 0.01 (uncorrected) level. The acquisition window of the second slice corresponded to the peak activity in the retina and the OL. The first and third slices covered the rising and falling period of the electrical activity (see Fig. 3C and D). The B₀ field was perpendicular to the slices, and hence also perpendicular to the photoreceptor cells in the retina.