Stimulus Specific Adaptation In Rat Auditory System

To survive in an ever-changing environment,humans and animals alike rely on their capacity to identify the abnormal cues rapidly from dynamic surroundings.Stimulus specific adaptation(SSA)has been proposed to be one of the mechanisms that give rise to novelty or change detection.SSA is the reduction in the response to a common stimulus that does not generalize,or only partially generalizes to other rare stimuli.Researchers have extensively investigated SSA using oddball paradigm with pure tones and found the functional topography of SSA to frequency in the auditory system.There have been comprehensive assessments of SSA in the frequency domain but only little attention given to others,such as the spatial domain.Besides,previous work were limited to a certain cue of novelty stimulus while the stimulus emerged abruptly from the natural environment often contain a variety of cues at the same time.It is also unclear whether SSA could integrate different types of cues within a single modality.Moreover,a lot of researches have been done to the SSA,while the exact neuronal mechanisms underlying SSA are still poorly understood.Accordingly,the purpose of this paper here was threefold:1)whether SSA exists for sound spatial cues;2)whether SSA integrates different types of cues within a single modality,such as sound location and sound identity.3)a possible mechanism to explain SSA.We try to address these issues above using extracellular recordings in the auditory system of rats:1)Using spatial oddball paradigm with two sound locations,we found that thalamic reticular nucleus(TRN)neurons exhibited stronger responses to a tone when it was presented rarely as opposed to frequently at a certain spatial location.Subsequently,our results suggest that spatial stimulus specific adaptation facilitates rare location discrimination by sharpening the response gap between two locations.And there is also an effect of inter-stimulus interval(ISI)on spatial SSA.2)Three oddball paradigms are formed by using two natural sounds presented at two different locations:identity oddball(a deviant sound),spatial oddball(the same sound at a deviant location),and integrative oddball(a deviant sound at a deviant location).We recorded single unit in the auditory cortex(AC)of awake rat while simultaneously performing the oddball paradigms.Our data showed that SSA was present in the AC for the natural sounds and the spatial locations in the neuronal population.We also found that the AC response to the double deviant stimulus was stronger than that to a single(either a deviant sound or the same sound at a deviant location).This findings suggest that SSA could integrate sound identity and location cues within the same modality.3)Using the oddball stimulation formed a sequence of electrical stimulation at two sites in AC with tentative monosynaptic projections to the TRN,we found that synaptic adaptation about AC may contribute to adaptation specificity in TRN with respect to the tonotopically organized inputs.These results taken together suggest that synaptic adaptation of AC which is tonotopically organized is a plausible mechanism underlying frequency specific adaptation in the TRN.Collectively,we use oddball paradigm with stimulus combination from frequency to complex natural sound,from single cue to coupling of spatial and identity cues,and simulate the real living environment gradually.Results demonstrate that spatial SSA could play a role along the auditory pathway,which is similar to frequency SSA and detecting the unexpected events benefits from the integration of different cues.Moreover,we observed the SSA from a new perspective and provided data evidence for a possible explanation of the physiological mechanism of SSA.