Background noise exerts diverse effects on the cortical encoding of foreground sounds.

In natural listening conditions, many sounds must be detected and identified in the context of competing sound sources, which function as background noise. Traditionally, noise is thought to degrade the cortical representation of sounds by suppressing responses and increasing response variability. However, recent studies of neural network models and brain slices have shown that background synaptic noise can improve the detection of signals. Because acoustic noise affects the synaptic background activity of cortical networks, it may improve the cortical responses to signals. We used spike train decoding techniques to determine the functional effects of a continuous white noise background on the responses of clusters of neurons in auditory cortex to foreground signals, specifically frequency-modulated sweeps (FMs) of different velocities, directions, and amplitudes. Whereas the addition of noise progressively suppressed the FM responses of some cortical sites in the core fields with decreasing signal-to-noise ratios (SNRs), the stimulus representation remained robust or was even significantly enhanced at specific SNRs in many others. Even though the background noise level was typically not explicitly encoded in cortical responses, significant information about noise context could be decoded from cortical responses on the basis of how the neural representation of the foreground sweeps was affected. These findings demonstrate significant diversity in signal in noise processing even within the core auditory fields that could support noise-robust hearing across a wide range of listening conditions.NEW & NOTEWORTHY The ability to detect and discriminate sounds in background noise is critical for our ability to communicate. The neural basis of robust perceptual performance in noise is not well understood. We identified neuronal populations in core auditory cortex of squirrel monkeys that differ in how they process foreground signals in background noise and that may contribute to robust signal representation and discrimination in acoustic environments with prominent background noise.

Effects of auditory stimulus context on the representation of frequency in the gerbil inferior colliculus.

Prior studies of dynamic conditioning have focused on modulation of binaural localization cues, revealing that the responses of inferior colliculus (IC) neurons to particular values of interaural phase and level disparities depend critically on the context in which they occur. Here we show that monaural frequency transitions, which do not simulate azimuthal motion, also condition the responses of IC neurons. We characterized single-unit responses to two frequency transition stimuli: a glide stimulus comprising two tones linked by a linear frequency sweep (origin-sweep-target) and a step stimulus consisting of one tone followed immediately by another (origin-target). Using sets of glide and step stimuli converging on a common target, we constructed conditioned response functions (RFs) depicting the variability in the response to an identical stimulus as a function of the preceding origin frequency. For nearly all cells, the response to the target depended on the origin frequency, even for origins outside the excitatory frequency response area of the cell. Results from conditioned RFs based on long (2-4 s) and short (200 ms) duration step stimuli indicate that conditioning effects can be induced in the absence of the dynamic sweep, and by stimuli of relatively short duration. Because IC neurons are tuned to frequency, changes in the origin frequency often change the "effective" stimulus duty cycle. In many cases, the enhancement of the target response appeared related to the decrease in the "effective" stimulus duty cycle rather than to the prior presentation of a particular origin frequency. Although this implies that nonselective adaptive mechanisms are responsible for conditioned responses, slightly more than half of IC neurons in each paradigm responded significantly differently to targets following origins that elicited statistically indistinguishable responses. The prevailing influence of stimulus context when discharge history is controlled demonstrates that not all the mechanisms governing conditioning depend on the discharge history of the recorded neuron. Selective adaptation among the neuron's variously tuned afferents may help engender stimulus-specific conditioning. The demonstration that conditioning effects reflect sensitivity to spectral as well as spatial stimulus contrast has broad implications for the processing of a wide range of dynamic acoustic signals and sound sequences.

Role of synaptic inhibition in processing of dynamic binaural level stimuli.

We have recently discovered a paradoxical aftereffect associated with inhibition in the gerbil auditory midbrain. Single neurons in the inferior colliculus (IC) were assessed for sensitivity to a virtual motion stimulus produced by modulating the interaural level difference (ILD), a major cue for sound localization. The class of neuron studied was predominantly excited by contralateral stimulation and inhibited by ipsilateral stimulation. Sound pressure level was modulated trapezoidally at the ipsilateral "inhibitory" ear, whereas the contralateral "excitatory" level remained constant. When the inhibitory stimulus was decreased within a range of sound levels that maintained suppression under static conditions, an unexpected discharge was often elicited, apparently because of an aftereffect of synaptic inhibition. In contrast, when the inhibitory stimulus was increased within a range of sound levels that produced only modest suppression under static conditions, neuronal discharge was often profoundly suppressed. In many cases the "conditioned enhancement" or "conditioned suppression" persisted for several seconds after the modulation of ILD, and such conditioned responses were influenced by the modulation depth and rate. To test the effect of inhibition in the IC directly, glycine and GABA were pulsed from a glass recording pipette during a constant monaural excitatory stimulus. The acoustically elicited discharge rate was potentiated markedly if preceded immediately by the brief (0.5-10 sec) application of inhibitory transmitter. Collectively, these results revealed unusually long-lasting effects of inhibition that may establish a new range of acoustic cues to which the neuron responds best. This may have broad implications for processing ensuing auditory stimuli.