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.

Sensory systems across the brain are specialized for their input, yet some principles of neural organization are conserved across modalities. The pattern of anatomical connections from the primate auditory cortex to the temporal, parietal, and prefrontal lobes suggests a possible division into dorsal and ventral auditory processing streams, with the dorsal stream originating from more caudal areas of the auditory cortex, and the ventral stream originating from more rostral areas. These streams are hypothesized to be analogous to the well-established dorsal and ventral streams of visual processing. In the visual system, the dorsal processing stream shows substantially faster neural response latencies than does the ventral stream. However, the relative timing of putative dorsal and ventral stream processing has yet to be explored in other sensory modalities. Here, we compare distributions of neural response latencies from 10 different areas of macaque auditory cortex, confirmed by individual anatomical reconstructions, to determine whether a similar timing advantage is found for the hypothesized dorsal auditory stream. Across three varieties of auditory stimuli (clicks, noise, and pure tones), we find that latencies increase with hierarchical level, as predicted by anatomical connectivity. Critically, we also find a pronounced timing differential along the caudal-to-rostral axis within the same hierarchical level, with caudal (dorsal stream) latencies being faster than rostral (ventral stream) latencies. This observed timing differential mirrors that found for the dorsal stream of the visual system, suggestive of a common timing advantage for the dorsal stream across sensory modalities.systems neuroscience | neurophysiology | hierarchy

We conducted an experiment to evaluate the effectiveness of spatial audio displays on target acquisition performance. Participants performed a visual search task with and without the aid of a spatial audio display. Potential target locations ranged between plus and minus 180° in azimuth and from -70° to +90° in elevation. Independent variables included the number of visual distractors present (1,5, 10, 25, 50) and the spatial audio condition (no spatial audio, free-field spatial audio, virtual spatial audio). Results indicated that both free-field and virtual audio cues engendered a significant decrease in search times. Potential applications of this research include the design of spatial audio displays for aircraft cockpits and ground combat vehicles.

The scalp-recorded amplitude-modulation following response (AMFR)” is gaining recognition as an objective audiometric tool, but little is known about the neural sources that underlie this potential. We hypothesized, based on our human studies and single-unit recordings in animals, that the scalp-recorded AMFR reflects the interaction of multiple sources. We tested this hypothesis using an animal model, the unanesthetized rabbit. We compared AMFRs recorded from the surface of the brain at different locations and before and after the administration of agents likely to enhance or suppress neural generators. We also recorded AMFRs locally at several stations along the auditory neuraxis. We conclude that the surface-recorded AMFR is indeed a composite response from multiple brain generators. Although the response at any modulation frequency can reflect the activity of more than one generator, the AMFRs to low and high modulation frequencies appear to reflect a strong contribution from cortical and subcortical sources, respectively.

We examined the minimum latency required to locate and identify a visual target (visual search) in a two-alternative forced-choice paradigm in which the visual target could appear from any azimuth (0 degree to 360 degrees) and from a broad range of elevations (from 90 degrees above to 70 degrees below the horizon) relative to a person's initial line of gaze. Seven people were tested in six conditions: unaided search, three aurally aided search conditions, and two visually aided search conditions. Aurally aided search with both actual and virtual sound localization cues proved to be superior to unaided and visually guided search. Application of synthesized three-dimensional and two-dimensional sound cues in the workstations are discussed.

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An experiment was conducted to determine the effects of two types of hearing protectors on auditory localization performance. Six listeners localized a 750-ms broadband noise from loudspeakers ranging in azimuth from -180 degrees to +180 degrees and in elevation from -75 degrees to +90 degrees. Independent variables included the type of hearing protector and the elevation of the source. Dependent measures included azimuth error, elevation error, and the percentage of trials resulting in a front-back confusion. Performance on each of the dependent measures was found to be mediated by one or more of the independent variables. Actual or potential applications include the generation of improved design guidelines for hearing protectors and workplace alarms.