The Effects Of The Spatial Locations And The Frequency Of Forward Maskers On The Frequency Tuning Of Rat Inferior Collicular Neurons

In natural conditions, human and animals can extract target sound information from a complex acoustical environment, however, its underline neural mechanism is still not fully understood. Previous studies have shown that a forward environmental sound can affect the perception of a lagged target sound, i.e., forward masking effect. In the auditory system, the properties of frequency and level tuning of auditory neurons reflect their functions in encoding sound information. In the present study, using a forward masking paradigm, we study how the frequency and spatial position of a forward masker affect the frequency-level response area (FLRA) of inferior collicular neurons. This will provide experimental evidences for us to understand the mechanism of how neurons in the central auditory system encoding target sound information in a complex acoustical environment.Under quiet and various forward masking conditions, we recorded the FLRAs of auditory neurons in the rat inferior colliculus. We analyzed how the responses in the FLRAs, the minimum thresholds, the characteristic frequency, the bandwidth of FLRAs, and the responses to preferred stimuli and non-preferred stimuli of 70 neurons varied with the spatial locations and the frequency of forward maskers. In the present study, the interval between masker and probe (target sound) was 100 ms, and the level of the masker was set to 40-50 dB above minimum threshold. The probe sound was located in the 45 degree in azimuth and 0 degree in elevation in the contralateral field of auditory space relative to the recording site in the inferior colliculus, and the forward masker was presented either from the same location as the probe or from 45 degree in azimuth and 0 degree in elevation in the ipsilateral field of auditory space relative to the recording site. The results are as follows:1. The forward masker suppressed the responses to the probe sounds of inferior collicular neurons. The forward masker reduced the number of spikes in the FLRA, increased the minimum threshold, and changed the bandwidth of FLRA of auditory neurons. However, the forward masker did not significant change the characteristic frequency of most neurons.2. When the masker frequency was constant, the effects of a forward masker on the FLRA, the minimum thresholds of inferior collicular neurons was stronger when the masker and probe were colocalized than that determined when the masker and probe were separated.3. When the spatial location of the masker and the probe were constant, the effects of a forward masker on the FLRA, the minimum thresholds of inferior collicular neurons was strongest when the masker frequency was at the neuron’s characteristic frequency compared with that determined at other masker frequencies.4. The forward masker had differential influences on the responses to the preferred stimuli and non-preferred stimuli in the FLRA. For most of the neurons analyzed, the effect of a forward maker was less effective in affecting the responses to a neuron’s preferred stimuli than that to the neuron’s non-preferred stimuli.The results in the present study indicated that the frequency tuning of rat inferior collicular neurons were dynamically modulated by the frequency and the spatial locations of a forward masker. However, the masking effect was the least on the responses to the neuron’s preferred stimuli. These response properties were good for maintaining the relative stability in encoding the preferred frequency and level of a central auditory neuron in a complex acoustical environment.