Abstract
BackgroundIn a natural environment, contextual noise frequently occurs with a signal sound for detection or discrimination in a temporal relation. However, the representation of sound frequency by auditory cortical neurons in a noisy environment is not fully understood. Therefore, the purpose of this study was to explore the impact of contextual noise on the cortical tuning to signal sound frequency in order to better understand the mechanism of cortical frequency coding in a complex acoustical environment.ResultsWe compared the excitatory frequency-level receptive fields (FLRFs) of neurons in the rat primary auditory cortex determined under both quiet and preceding noise conditions. Based on the changes of minimum threshold and the extent of FLRF of auditory cortical neurons, we found that the FLRFs of a cortical neuron were modulated dynamically by a varying preceding noise. When the interstimulus interval between noise and the probe tone was constant, the modulation of the FLRF increased as the level of noise was increased. If the preceding noise level was constant, the modulation decreased when the interstimulus interval was increased. Preceding noise sharpened the bandwidth of the FLRFs of 47.6% tested neurons. Moreover, preceding noise shifted the CFs of 47.6% neurons by more than 0.25 octaves, while the CFs of the rest of the neurons remained relatively unchanged.ConclusionsThe results indicate that the cortical representation of sound frequency is dynamically modulated by contextual acoustical environment, and that there are cortical neurons whose characteristic frequencies were resistant to the interference of contextual noise.
Highlights
In a natural environment, contextual noise frequently occurs with a signal sound for detection or discrimination in a temporal relation
The detection of the effect of a forward masker on the frequency-level receptive field (FLRF) of an auditory cortex (AI) neuron relied on a change in minimum threshold (MT), characteristic frequency (CF), area change of FLRF, and the bandwidth of FLRF
The CFs and the frequency tuning ranges of these neurons in control conditions were quiet different (Figure 2A-1, B-1, and C-1), the FLRF data in Figure 2 demonstrated that preceding noise had monotonic suppressive effects on the frequency tuning of the neurons with increasing noise level
Summary
Contextual noise frequently occurs with a signal sound for detection or discrimination in a temporal relation. Previous neurophysiological studies have demonstrated cortical neural processing of contextual stimuli in auditory cortex [7,8], visual cortex [9,10] and somatosensory cortex [11] These findings suggest that the contextual masking effect on the response to a target stimulus is common in different sensory modalities. In a dynamic acoustic environment, an effective preceding sound (i.e., masker) can elevate the detection threshold of a following target sound (i.e., probe). This forward masking effect has been demonstrated in both human [1,3] and animal [12] behavioral studies. These studies demonstrated that the forward masking effect is dependent on the acoustical parameter (e.g., frequency, level, and time) of both the masker and the probe
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