BOLD fMRI of visual and somatosensory-motor stimulations in baboons

Abstract

Baboon, with its large brain size and extensive cortical folding compared to other non-human primates, serves as a good model for neuroscience research. This study reports the implementation of a baboon model for blood oxygenation level-dependent (BOLD) fMRI studies (1.5 x 1.5 x 4 mm resolution) on a clinical 3T-MRI scanner. BOLD fMRI responses to hypercapnic (5% CO(2)) challenge, 10 Hz flicker visual, and vibrotactile somatosensory-motor stimulations were investigated in baboons anesthetized sequentially with isoflurane and ketamine. Hypercapnia evoked robust BOLD increases. Paralysis was determined to be necessary to achieve reproducible functional activations within and between subjects under our experimental conditions. With optimized anesthetic doses (0.8-1.0% isoflurane or 6-8 mg/kg/h ketamine) and adequate paralysis (vecuronium, 0.2 mg/kg), robust activations were detected in the visual (V), primary (S1) and secondary (S2) somatosensory, primary motor (M cortices), supplementary motor area (SMA), lateral geniculate nucleus (LGN) and thalamus (Th). Data were tabulated for 11 trials under isoflurane and 10 trials under ketamine on 5 baboons. S1, S2, M, and V activations were detected in essentially all trials (90-100% of the time, except 82% for S2 under isoflurane and 70% for M under ketamine). LGN activations were detected 64-70% of the time under both anesthetics. SMA and Th activations were detected 36-45% of the time under isoflurane and 60% of the time under ketamine. BOLD percent changes among different structures were slightly higher under ketamine than isoflurane (0.75% versus 0.58% averaging all structures), but none was statistically different (P>0.05). This baboon model offers an opportunity to non-invasively image brain functions and dysfunctions in large non-human primates.

Fig. 1

(A) BOLD fMRI “activation” maps under isoflurane and (B) averaged time courses of hypercapnic (5% CO₂) inhalation under isoflurane (blue line) and ketamine (red dash line). Paralytic was used for both anesthetics. Color bar indicates Z scores.

Fig. 2

BOLD fMRI activation maps responding to simultaneous visual and somatosensory–motor stimulation from a representative subject under (A) isoflurane and (B) ketamine. Paralytic was used for both anesthetics. Color bar indicates Z scores. (C) Regions of interests (ROIs) used in subsequent quantitative analysis include the primary (S1) and secondary (S2) somatosensory cortex, primary motor cortex (M), supplementary motor area (SMA), thalamus (Th), visual cortex (V), and lateral geniculate nuclei (LGN).

Fig. 3

Individual time courses of all stimulation trials from the S1 under isoflurane and ketamine. Thick dash lines indicate the means of all traces. ROI used is shown in Fig. 2C.

Fig. 4

Averaged BOLD fMRI time courses from ROIs shown in Fig. 2C under isoflurane (blue line) and ketamine (red dotted line). Paralytic was used for both anesthetics. Error bars indicate the standard errors of the means for 11 trials under isoflurane and 10 trials under ketamine.