Neural Substrates of Learning

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Many things that humans and animals learn everyday are predicated on a simple association between two events. Humans learn to avoid hot stoves; a doorbell ringing indicates visitors, and cats learning to approach to the sound of the can opener in the anticipation of receiving food are just a few examples. These events are brought into the neural system by 5 main modality specific pathways, visual, auditory, somatosensory, olfactory and gustatory. Each one of these modalities records very different stimuli and has a unique neural input system. Does learning across different modalities require the same number of neural circuits as types of input? Perhaps there are centrally located neural circuits depending on the type of global information being processed rather than on the individual sensory modality? If similar motor responses were to be learned the same neural learning circuit might be used. The behavioral parameters of simple associative, i.e., classical conditioning, where one stimulus predicts the occurrence of another stimulus, have been well worked out. The neural substrates underlying two specific types of classical conditioning, eyeblink and fear conditioning to tone stimuli, have been verified from input to output with the use of lesion, anatomical tracing and neuronal recording studies. Eyeblink conditioning is similar to limb flexion conditioning. How similar are the neuronal learning circuits for similar types of classical conditioning when using cues from the sample modality? When using cues from different modalities, do these different inputs converge on a common circuit for classical conditioning of a particular motor response? The aim of the proposed research is two-fold: First, map the neural substrates, through measurement of glucose energy metabolism across the whole brain, for a simple classical conditioning paradigm, limb flexion using the commonly used pure tone stimulus. The results will be compared to the known circuitry for eyeblink conditioning. Second, map the neural substrates of this classical conditioned reflex using cues from additional modalities using visual and somatosensory stimuli. Many humans have difficulty learning solely by presentations made in one sensory modality or another. This type of research may allow for the identification of the specific brain regions where these types of deficits are modified.