Deviance detection in auditory subcortical structures: what can we learn from neurochemistry and neural connectivity?

[EN] A remarkable ability of animals that is critical for survival is to detect and respond to to unexpected stimuli in an ever-changing world. Auditory neurons that show stimulusspecific adaptation (SSA), i.e., a decrease in their response to frequently occurring stimuli while maintaining responsiveness when different stimuli are presented, might participate in the coding of deviance occurrence. Traditionally, deviance detection is measured by the mismatch negativity (MMN) potential in studies of evoked local field potentials.We present a review of the state-of-the-art of SSA in auditory subcortical nuclei, i.e., the inferior colliculus and medial geniculate body of the thalamus, and link the differential receptor distribution and neural connectivity of those regions in which extreme SSA has been found. Furthermore, we review both SSA and MMN-like responses in auditory and non-auditory areas that exhibit multimodal sensitivities that we suggest conform to a distributed network encoding for deviance detection. The understanding of the neurochemistry and response similarities across these different regions will contribute to a better understanding of the neural mechanism underlying deviance detection

Differences in the strength of cortical and brainstem inputs to SSA and non-SSA neurons in the inferior colliculus

[EN] In an ever changing auditory scene, change detection is an ongoing task performed by the auditory brain. Neurons in the midbrain and auditory cortex that exhibit stimulus-specific adaptation (SSA) may contribute to this process. Those neurons adapt to frequent sounds while retaining their excitability to rare sounds. Here, we test whether neurons exhibiting SSA and those without are part of the same networks in the inferior colliculus (IC). We recorded the responses to frequent and rare sounds and then marked the sites of these neurons with a retrograde tracer to correlate the source of projections with the physiological response. SSA neurons were confined to the non-lemniscal subdivisions and exhibited broad receptive fields, while the non-SSA were confined to the central nucleus and displayed narrow receptive fields. SSA neurons receive strong inputs from auditory cortical areas and very poor or even absent projections from the brainstem nuclei. On the contrary, the major sources of inputs to the neurons that lacked SSA were from the brainstem nuclei. These findings demonstrate that auditory cortical inputs are biased in favor of IC synaptic domains that are populated by SSA neurons enabling them to compare top-down signals with incoming sensory information from lower areas

Macaque monkeys and humans sample temporal regularities in the acoustic environment

Many animal species show comparable abilities to detect basic rhythms and produce rhythmic behavior. Yet, the capacities to process complex rhythms and synchronize rhythmic behavior appear to be species-specific: vocal learning animals can, but some primates might not. This discrepancy is of high interest as there is a putative link between rhythm processing and the development of sophisticated sensorimotor behavior in humans. Do our closest ancestors show comparable endogenous dispositions to sample the acoustic environment in the absence of task instructions and training? We recorded EEG from macaque monkeys and humans while they passively listened to isochronous equitone sequences. Individual- and trial-level analyses showed that macaque monkeys’ and humans’ delta-band neural oscillations encoded and tracked the timing of auditory events. Further, mu- (8–15 Hz) and beta-band (12–20 Hz) oscillations revealed the superimposition of varied accentuation patterns on a subset of trials. These observations suggest convergence in the encoding and dynamic attending of temporal regularities in the acoustic environment, bridging a gap in the phylogenesis of rhythm cognition