Forward masking in the superior paraolivary nucleus of the rat

Processing of Paired Click-Tone Stimulation in the Mice Inferior Colliculus

The inferior colliculus (IC) is known as a neuronal structure involved in the integration of acoustic information in the ascending auditory pathway. However, the processing of paired acoustic stimuli containing different sound types, especially when they are applied closely, in the IC remains poorly studied. We here firstly investigated the IC neuronal response to the paired stimuli comprising click and pure tone with different inter-stimulus (click-tone) intervals using in vivo loose-patch recordings in anesthetized BALB/c mice. It was found that the total acoustic evoked spike counts decreased under certain click-tone interval conditions on some neurons with or without click-induced supra-threshold responses. Application of click could enhance the minimum threshold of the neurons responding to the tone in a pair without changing other characteristics of the neuronal tone receptive fields. We further studied the paired acoustic stimuli evoked excitatory/inhibitory inputs, IC neurons received, by holding the membrane potential at -70/0 mV using in vivo whole-cell voltage-clamp techniques. The curvature and peak amplitude of the excitatory/inhibitory post-synaptic current (EPSC/IPSC) could be almost unchanged under different inter-stimulus interval conditions. Instead of showing the summation of synaptic inputs, most recorded neurons only had the EPSC/IPSC with the amplitude similar as the bigger one evoked by click or tone in a pair when the inter-stimulus interval was small. We speculated that the IC could inherit the paired click-tone information which had been integrated before reaching it.

Nonuniform impacts of forward suppression on neural responses to preferred stimuli and nonpreferred stimuli in the rat auditory cortex

In natural conditions, human and animals need to extract target sound information from noisy acoustic environments for communication and survival. However, how the contextual environmental sounds impact the tuning of central auditory neurons to target sound source azimuth over a wide range of sound levels is not fully understood. Here, we determined the azimuth-level response areas (ALRAs) of rat auditory cortex neurons by recording their responses to probe tones varying with levels and sound source azimuths under both quiet (probe alone) and forward masking conditions (preceding noise + probe). In quiet, cortical neurons responded stronger to their preferred stimuli than to their nonpreferred stimuli. In forward masking conditions, an effective preceding noise reduced the extents of the ALRAs and suppressed the neural responses across the ALRAs by decreasing the response strength and lengthening the first-spike latency. The forward suppressive effect on neural response strength was increased with increasing masker level and decreased with prolonging the time interval between masker and probe. For a portion of cortical neurons studied, the effects of forward suppression on the response strength to preferred stimuli was weaker than those to nonpreferred stimuli, and the recovery from forward suppression of the response strength to preferred stimuli was earlier than that to nonpreferred stimuli. We suggest that this nonuniform forward suppression of neural responses to preferred stimuli and to nonpreferred stimuli is important for cortical neurons to maintain their relative stable preferences for target sound source azimuth and level in noisy acoustic environments.