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Title: Stimulus-specific Adaptation in Auditory Thalamus of Young and Aged 1 Awake Rats 2 3

20 Novel stimulus detection by single neurons in the auditory system, known as stimulus-specific 21 adaptation (SSA), appears to function as a real-time filtering/gating mechanism in processing 22 acoustic information. Particular stimulus paradigms allowing for quantification of a neuron’s 23 ability to detect novel or deviant stimuli have been used to examine SSA in the inferior 24 colliculus, medial geniculate body (MGB) and auditory cortex of anesthetized rodents. 25 However, the study of SSA in awake animals is limited to auditory cortex. The present study 26 used individually-advanceable tetrodes to record single-unit responses from auditory thalamus 27 (MGB) of awake young-adult and aged Fischer Brown Norway (FBN) rats to 1) examine the 28 presence of SSA in the MGB of awake rats and 2) determine whether SSA is altered by aging in 29 MGB. MGB single units in awake FBN rats displayed SSA in response to two stimulus paradigms: 30 the oddball paradigm and a random blocked/interleaved presentation of a set of frequencies. 31 SSA levels were modestly, but non-significantly increased in the non-lemniscal regions of the 32 MGB and at lower stimulus intensities, where 27 of 57 (47%) young-adult MGB units displayed 33 SSA. The present findings provide the initial description of SSA in the MGB of awake rats and 34 support SSA as being qualitatively independent of arousal level or anesthetized state. Lastly, 35 contrary to previous studies in auditory cortex of anesthetized rats, MGB units in aged rats 36 showed SSA levels indistinguishable from SSA levels in young-adult rats, suggesting that SSA in 37 MGB was not impacted by aging in an awake preparation. 38