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Response Modulation to Ongoing Broadband Sounds in Primary Auditory Cortex It is well-known that in certain forms, background noise does not interfere with one's ability to attend to a specific auditory stream. Driven by previous reports of adaptation to persistent stimuli in other brain regions, we are investigating short term (1-2 sec) adaptive effects in the primary auditory cortex of ferrets (Mustela putorius). We obtain electrophysiological data in response to the presentation of structured narrowband and broadband sounds, and are looking to characterize the temporal response to these stimuli. Neuronal Response Characteristics in the Inferior Colliculus of the Ferret Much of what is known about the neuronal response of inferior colliculus (IC) comes from anesthetized preparations; however, it is unclear to what extent the effects of anesthesia change the response properties. We report here on the first in a series of experiments characterizing IC neurons in the ferret--an auditory generalist. Classical stimuli (pure tones, clicks, and AM tones) and dynamic ripples were presented to the animal in order to compute response measures and derive spectro-temporal receptive fields (STRFs). With these measures, we assess the degrees of linearity in the neuronal response and identify possible sub-populations amongst the sample set. We compare and contrast our results with those obtained from anesthetized ferrets, and with relevant studies in other species. Spectro-Temporal Representation of Feature Onsets in Primary Auditory Cortex We are investigating how feature onsets of broadband stimuli such as speech are represented in the firing patterns of cortical neurons. However, a feature onset can be confounded by a stimulus onset, since there is a considerable change in level from silence to the level of the stimulus played. A level change of this magnitude tends to drive cortical neurons to respond no matter to which features they respond best. To circumvent this issue, we use stimuli which contain transitions in features without a change in the mean level. This allows an examination of the encoding of feature onsets and offsets, or transients within a stimulus, by themselves - disentangled from the stimulus onset and offset responses (i.e. the abrupt and large change in level). In our experiments, we present these special broadband sounds to an ferret and use a multi-electrode array to record neuronal responses from the primary auditory cortex. Targeting the changes in inferior colliculus induced by tinnitus There is a strong correlation between tinnitus and altered neural activity in the inferior colliculus, a brain structure essential for the perception of sounds. This makes the inferior colliculus a natural place to measure the effect that various therapeutic interventions might have. In a novel approach, we will implant the inferior colliculus of animals with chronic multi-electrode arrays and measure neural activity both before and after noise-induced trauma. We will couple the neural recordings in animals with simultaneous behavioral measurements that will indicate the presence of tinnitus. We will also measure the change in neural coding of complex sounds such as speech and music before and after noise trauma. Lidocaine is known to alleviate tinnitus in many patients but has serious side-effects. We will perform the above experiments with intravenous Lidocaine post-trauma and understand its effect on abnormal neural activity. This will give important clues about the mechanisms that allows Lidocaine to reduce tinnitus and will allow the choice of other pharmacological agents that might similarly affect collicular activity but with fewer side-effects. | |