Inferior colliculus lesions impair eyeblink conditioning in rats

Abstract

The neural plasticity necessary for acquisition and retention of eyeblink conditioning has been localized to the cerebellum. However, the sources of sensory input to the cerebellum that are necessary for establishing learning-related plasticity have not been identified completely. The inferior colliculus may be a source of sensory input to the cerebellum through its projection to the medial auditory thalamus. The medial auditory thalamus is necessary for eyeblink conditioning in rats and projects to the lateral pontine nuclei, which then project to the cerebellar nuclei and cortex. The current experiment examined the role of the inferior colliculus in auditory eyeblink conditioning. Rats were given bilateral or unilateral (contralateral to the conditioned eye) lesions of the inferior colliculus prior to 10 d of delay eyeblink conditioning with a tone CS. Rats with bilateral or unilateral lesions showed equivalently impaired acquisition. The extent of damage to the contralateral inferior colliculus correlated with several measures of conditioning. The findings indicate that the contralateral inferior colliculus provides auditory input to the cerebellum that is necessary for eyeblink conditioning.

Figure 1.

Inferior colliculus lesions. The largest (gray shaded areas) and smallest (black shaded areas) bilateral (left) and unilateral (right) lesions of the inferior colliculus (IC). The unilateral lesions were contralateral to the conditioned eye. Numbers indicate the anterior–posterior stereotaxic coordinates of the sections relative to bregma (Paxinos and Watson 1998).

Figure 2.

Bilateral and unilateral lesions of the inferior colliculus impair eyeblink conditioning. Mean (±) SEM conditioned response (CR) percentage for rats given control surgery (white circles), unilateral inferior colliculus lesions (black circles), and bilateral inferior colliculus lesions (black triangles) across 10 100-trial sessions of eyeblink conditioning.

Figure 3.

Eyeblink conditioning as a function of the relative size of inferior colliculus (IC) lesions. Mean (±) conditioned response (CR) percentage for rats given control surgery (white circles), large IC lesions (black circles), medium IC lesions (black triangles), and small IC lesions (black squares) across 10 100-trial sessions of eyeblink conditioning.

Figure 4.

Scatterplots with regression lines showing the relationship between lesion size and two measures of conditioning. Area of the IC in the rats given lesions is plotted against (A) the overall conditioned response (CR) percentage and sessions to reach (B) a performance criterion of 70% CR.