Neurons in the inferior colliculus (IC) change their firing rates with sound pressure level. Some neurons maintain monotonic increases in firing rate over a wide range of sound intensities, whereas other neurons are monotonic over limited intensity ranges. We examined the conditions necessary for monotonicity in this nucleus in vitro in rat brain slices and in vivo in the unanesthetized rabbit. Our in vitro recordings indicate that concurrent activation of GABA A synapses with excitatory inputs facilitates monotonic increases in firing rate with increases in stimulus strength. In the absence of synaptic inhibition, excitatory input to IC neurons causes large depolarizations that result in firing block and nonmonotonicity. In vivo, although GABA A synapses decrease the firing rate in all IC neurons, they can have opposing effects on rate-level functions. GABAergic inputs activated by all sound intensities maintain monotonicity by keeping the postsynaptic potential below the level at which depolarization block occurs. When these inputs are blocked, firing block can occur and rate-level functions become nonmonotonic. High-threshold GABAergic inputs, in contrast, cause nonmonotonic responses by decreasing the firing rate at high intensities. Our results suggest that a dynamic regulation of the postsynaptic membrane potential by synaptic inhibition is necessary to allow neurons to respond monotonically to a wide range of sound intensities.

Processing dynamic changes in the stimulus stream is a major task for sensory systems. In the auditory system, an increase in the temporal integration window between the inferior colliculus (IC) and auditory cortex is well known for monaural signals such as amplitude modulation, but a similar increase with binaural signals has not been demonstrated. To examine the limits of binaural temporal processing at these brain levels, we used the binaural beat stimulus, which causes a fluctuating interaural phase difference, while recording from neurons in the unanesthetized rabbit. We found that the cutoff frequency for neural synchronization to the binaural beat frequency (BBF) decreased between the IC and auditory cortex, and that this decrease was associated with an increase in the group delay. These features indicate that there is an increased temporal integration window in the cortex compared to the IC, complementing that seen with monaural signals. Comparable measurements of responses to amplitude modulation showed that the monaural and binaural temporal integration windows at the cortical level were quantitatively as well as qualitatively similar, suggesting that intrinsic membrane properties and afferent synapses to the cortical neurons govern the dynamic processing. The upper limits of synchronization to the BBF and the bandpass tuning characteristics of cortical neurons are a close match to human psychophysics.

While a number of strategies have been developed to reduce data collection requirements for multidimensional NMR based on non-Fourier methods of spectrum analysis, there is an increasing awareness that the principal differences in the performance of these methods is attributable to the sampling strategies employed, and not the method of spectrum analysis per se. The ability of maximum entropy reconstruction to utilize essentially arbitrary sampling schemes makes it a useful platform for comparative analysis of sampling strategies. Here we use maximum entropy reconstruction to demonstrate that artifacts characteristic of sparse sampling result from regularity in the sampling pattern, and that they can be substantially reduced by introducing a degree of randomness to an otherwise regular sampling scheme, without requiring additional sampling.

In neonates, transposed tones are more effective than sinusoidal amplitude modulated tones in evoking the AMFR, modulation frequencies between 41 to 88 Hz are almost equally effective in evoking the AMFR, and band-pass noises are more effective in evoking the AMFR than tones. These three stimulus factors all add incrementally to the efficiency of evoking the AMFR. The short detection times indicate that the AMFR could be an effective tool for hearing screening.