The role of the cerebral cortex during classical eyeblink conditioning in the rabbit

Abstract

Learning is defined as the acquisition or modification of specific behaviors that allow the individual to adapt to changes occurring in the surrounding environment. The primary focus of many investigations has been the characterization of neural bases underlying learning. Thus, the motor system of the eyelid and the nictitating membrane has been extensively used as an experimental model to study the mechanisms and neural structures determining associative learning, mainly by the use of different paradigms of classical eyeblink conditioning. Over several decades, the cerebellum and the hippocampus have been proposed as the brain structures responsible for processing the neural codes underlying the generation of learned eyeblink responses. However, some researchers raised the hypothesis of the participation of other brain regions. Specifically, reported changes in neuronal activity in various cortical structures during learning suggest the involvement of sensory and motor pathways in the generation of conditioned responses. The main approach of the experimental work presented in this Doctoral Thesis was to investigate the role of the cerebral cortex during the classical conditioning of eyeblink responses. First, projections from the motor cortex to the facial nucleus were characterized by injecting an anterograde tracer in the cortical region corresponding to the orbicularis oculi muscle. Hence, monosynaptic neural projections from the rabbit’s motor cortex were identified in the facial nucleus. On the other hand, extracellular unitary activity in the palpebral region of the motor cortex corresponding to the orbicularis oculi was recorded during eyeblink conditioning. As a result, motor cortex neurons activated antidromically from the red nucleus as well as the facial nucleus showed an increase of their firing rates well in advance (≥ 50 ms) with regard to the conditioned response onset. With the aim to verify the importance of the motor cortex during the acquisition process of associative learning it was examined whether modulating excitability of motor cortex neurons by means of transcranial current stimulation would be able to modify the learning process. Similarly, the contribution of the sensory cortex during the acquisition of eyeblink conditioning – using light stimulation as conditioned stimulus – was studied by applying transcranial electrical currents to the primary visual cortex during learning. The induced modulation of neuronal excitability in the motor and visual cortex resulted in a significant impact on learning consisting of a polarity-dependent change in the percentage of learned responses. Specifically, an increase in the number of learned responses was observed when stimulation with anodal polarity was applied to the motor cortex, whereas cathodal polarity decreased the number of conditioned responses when applied to the visual cortex. Regarding the quality of responses as observed for the motor cortex, anodal stimulation promoted increased magnitude, whereas cathodal stimulation reduced magnitude and slightly delayed the beginning of learned responses. Finally, thermal changes of the brain tissue were measured by means of an epidurally implanted thermistor placed under the transcranial current stimulation location to rule out that tissue heating of the stimulated site interfered with the observed effects on motor learning. No significant changes in brain temperature were induced either during or after the application of transcranial stimulation. In brief, this Doctoral Thesis reveals that the participation of the motor cortex during classical eyeblink conditioning is essential for the correct acquisition of learned responses. In addition, the results support the hypothesis of the involvement of the sensory cortex in this type of associative motor learning. Finally, the data presented in this thesis shows for the first time experimental evidence supporting the absence of thermal changes in the brain tissue due to the transcranial current stimulation.