fMRI of the conscious rabbit during unilateral classical eyeblink conditioning reveals bilateral cerebellar activation

Abstract

The relative contributions of the ipsilateral and contralateral cerebellar cortex and deep nuclei to delay eyeblink conditioning have been debated and are difficult to survey entirely using typical electrophysiological and lesion techniques. To address these issues, we used single-event functional magnetic resonance imaging (fMRI) in the conscious rabbit to visualize the entire cerebellum simultaneously during eyeblink conditioning sessions. Examination of the blood oxygenation level-dependent (BOLD) response to a visual conditioning stimulus early in training revealed significant bilateral learning-related increases in the BOLD response in the anterior interpositus nucleus (IPA) and significant bilateral deactivation in hemispheric lobule VI (HVI) of the cerebellar cortex. Later in training, the BOLD response remained bilateral in the cortex and predominantly ipsilateral in the IPA. Conditioning stimulus-alone trials after conditioning revealed that both sides of HVI were affected similarly but that only the ipsilateral interpositus nucleus was activated. These results suggest that both sides of HVI normally influence the side of the IPA being conditioned and illustrate how fMRI can be used to examine multiple brain regions simultaneously in an awake, behaving animal to discover more rapidly the neural substrates of learning and memory.

Figure 1.

Behavioral response during eyeblink conditioning. Eyeblink responses are shown for a single animal during pseudoconditioning and training stages (40 and 80% CRs), as well as during CS-alone trials after training. Responses were recorded with an infrared reflectance sensor, with a voltage increase indicating a blink. Note the earlier onset of the responses as learning occurs. The dashed lines represent the timing of the stimuli.

Figure 2.

Functional activation in cerebellum during eyeblink conditioning. Functional maps are shown (a) for a representative animal during control and training stages, as well as during CS-alone memory trials after training. Colors denote the peak magnitude of the averaged response for each cluster. Selected anatomical regions corresponding to HVI (1) and IPA (2) were averaged across subjects (n = 4) to characterize the temporal response of each region on the ipsilateral (b) and contralateral (c) sides. The dashed line represents the onset of the stimuli; the error bars represent the SEM.