1UCL Ear Institute, University College London, London, UK
2Department of Neuroscience, Physiology and Pharmacology, University College London, London, UK
3Laboratory for Neurobiology of Hearing, Faculty of Medicine and Institute of Neuroscience of Castilla y Leon, University of Salamanca, Salamanca, Spain
Neurons in the primary auditory cortex respond more strongly to a rarely presented ”deviant” tone than to the same tone when it is common, or ”standard” (Ulanovsky, Las and Nelken, 2003; Ulanovsky, Las, Farkas and Nelken, 2004). This phenomenon, called ”stimulus-specific adaptation” (SSA), has been proposed as a possible single-neuron correlate of the mismatch negativity (MMN), a cortical evoked potential associated with stimulus novelty. Previous studies in cat have suggested that SSA is absent from single neurons in the auditory thalamus (Ulanovsky et al. 2003, Ulanovsky et al. 2004); however, these reports did not differentiate between the auditory thalamic subdivisions. To explore the possibility of thalamic SSA more completely, we recorded extracellularly from 30 single units and 22 multiunit clusters in the ventral, medial, and dorsal subdivisions of the mouse auditory thalamus, while presenting the anaesthetised animals with sequences of standard and deviant tones. As in the cat studies, standard and deviant tone frequencies were separated by no more than 0.5 octaves, and evoked similar responses in most neurons. Using stimulation rates of 1.25 - 2.5 stimuli/s, we found SSA in neurons in the medial subdivision of the mouse auditory thalamus, but not in the ventral subdivision. The median neuronal stimulus-specific adaptation index (defined as in Ulanovsky et al. 2003) was significantly greater than zero in the medial subdivision at all stimulation rates tested (sign-rank test, p<0.05), but was not significantly different from zero in the ventral or dorsal sudivisions for any of the tested stimulation rates. A smaller sample from the dorsal subdivision (N = 13 dorsal, compared to 20 medial and 19 ventral) also showed no significant SSA. Our results indicate that SSA does occur in the auditory thalamus, but only in the medial subdivision. Together with related findings of SSA in neurons of the ”belt” regions of the inferior colliculus (Perez-Gonzalez, Malmierca and Covey, 2005), the results suggest that SSA is either a general property of some neurons in the non-lemniscal auditory system, or a cortical phenomenon that influences subcortical auditory processing only within the non-lemniscal pathway.
Supported by: The Gatsby Charitable Foundation, Deafness Research UK.