Forward Masking Induced By Noise Modulates The Frequency Tuning Of Neurons In The Rat Auditory Cortex And Inferior Colliculus

Human and animals can perform target sound detection and discrimination very well in a noisy acoustical environment,however,the correlated neural mechanism is still not fully understood.Here we used a forward masking paradigm to study how the forward masking induced by white noise affected the frequency tuning characteristics of neurons in the auditory cortex and inferior colliculus.This study will provide experimental evidences for further studying the neural mechanisms of target sound detection and discrimination in the central auditory system.Using white noise as the forward masker and pure tone as the probe sound(or target sound),we measured the frequency-level response areas(FLRA)of neurons in the rat auditory cortex and inferior colliculus under quiet conditions and forward masking conditions.We studied the level impacts and the temporal impacts of forward masking noise on the FLRA by varying the level of the noise and the interstimulus interval between the noise and the lagging probe tone.The results were as follows:1)The forward masking induced by noise decreased the response strength of auditory neurons,reduced the extent of the FLRAs,and increased the minimum thresholds of auditory neuron.The inhibitory impacts were dependent on the level of the noise and the inter-stimulus interval(ISI)between noise and the probe tone.The inhibitory impacts increased with increasing noise level and decreased with lengthening the ISI.2)Under quiet conditions,auditory neurons respond strongly to the stimuli of preferred frequency-level combinations but weakly to the stimuli of non-preferred frequency-level combinations in the FLRA.Under forward masking noise conditions,for a portion of auditory cortex neurons,the inhibitory impacts of forward masker on the responses to the preferred stimuli were weaker than the inhibitory impacts on the responses to the non-preferred stimuli.This non-uniform inhibitory impacts are useful for auditory neurons to maintain their preference to particular sound parameters in a noisy acoustical environment.This non-uniform inhibitory impacts were more evident in the auditory cortex than in the inferior colliculus.3)After the offset of forward masking noise,the response strength in the FLRA of auditory neurons gradually recovered to that determined under quiet conditions,however,the recovery time for auditory cortex neurons was longer than the recovery time for inferior collicular neurons.4)The forward masking induced by noise had no significant impacts on the characteristic frequency and the best frequency of the majority of the neurons tested in the auditory cortex and inferior colliculus whereas it widened or narrowed the bandwidth of FLRAs for a portion of neurons tested.The results in the present study indicated that,in a noisy acoustical environment,the frequency tuning of auditory neurons in the auditory cortex and inferior colliculus were dynamically changed with varying environments sounds,however,a portion of auditory neurons can maintain their the preferences for sound frequency and level in the noisy environment.