Task Engagement Improves Neural Discriminability in the Auditory Midbrain of the Marmoset Monkey.

While task-dependent changes have been demonstrated in auditory cortex for a number of behavioral paradigms and mammalian species, less is known about how behavioral state can influence neural coding in the midbrain areas that provide auditory information to cortex. We measured single-unit activity in the inferior colliculus (IC) of common marmosets of both sexes while they performed a tone-in-noise detection task and during passive presentation of identical task stimuli. In contrast to our previous study in the ferret IC, task engagement had little effect on sound-evoked activity in central (lemniscal) IC of the marmoset.

Rapid Task-Related Plasticity of Spectrotemporal Receptive Fields in the Auditory Midbrain.

Previous research has demonstrated that auditory cortical neurons can modify their receptive fields when animals engage in auditory detection tasks. We tested for this form of task-related plasticity in the inferior colliculus (IC) of ferrets trained to detect a pure tone target in a sequence of noise distractors that did not overlap in time. During behavior, responses were suppressed at the target tone frequency in approximately half of IC neurons relative to the passive state.

Alignment of sound localization cues in the nucleus of the brachium of the inferior colliculus.

Accurate sound localization is based on three acoustic cues (interaural time and intensity difference and spectral cues from directional filtering by the pinna). In natural listening conditions, every spatial position of a sound source provides a unique combination of these three cues in "natural alignment." Although neurons in the central nucleus (ICC) of the inferior colliculus (IC) are sensitive to multiple cues, they do not favor their natural spatial alignment.

Linear processing of interaural level difference underlies spatial tuning in the nucleus of the brachium of the inferior colliculus.

The spatial location of sounds is an important aspect of auditory perception, but the ways in which space is represented are not fully understood. No space map has been found within the primary auditory pathway. However, a space map has been found in the nucleus of the brachium of the inferior colliculus (BIN), which provides a major auditory projection to the superior colliculus.

Cutting-edge science and coffee: Auditory System Gordon Research Conference and Seminar 2012 report.

At the third Gordon Research Conference and Gordon Research Seminar on the Auditory System (2012), investigators from all career stages reported on emerging research in a broad range of sub-fields. A distinguishing feature of these conferences is their attention to junior investigators, and their experience is the focus of this conference report.

Acoustic basis of directional acuity in laboratory mice.

The acoustic basis of auditory spatial acuity was investigated in CBA/129 mice by relating patterns of behavioral errors to directional features of the head-related transfer function (HRTF). Behavioral performance was assessed by training the mice to lick a water spout during sound presentations from a "safe" location and to suppress the response during presentations from "warning" locations. Minimum audible angles (MAAs) were determined by delivering the safe and warning sounds from different locations in the inter-aural horizontal and median vertical planes.

Information conveyed by inferior colliculus neurons about stimuli with aligned and misaligned sound localization cues.

Previous studies have demonstrated that single neurons in the central nucleus of the inferior colliculus (ICC) are sensitive to multiple sound localization cues. We investigated the hypothesis that ICC neurons are specialized to encode multiple sound localization cues that are aligned in space (as would naturally occur from a single broadband sound source). Sound localization cues including interaural time differences (ITDs), interaural level differences (ILDs), and spectral shapes (SSs) were measured in a marmoset monkey.

Tonotopic tuning in a sound localization circuit.

Nucleus laminaris (NL) neurons encode interaural time difference (ITD), the cue used to localize low-frequency sounds. A physiologically based model of NL input suggests that ITD information is contained in narrow frequency bands around harmonics of the sound frequency. This suggested a theory, which predicts that, for each tone frequency, there is an optimal time course for synaptic inputs to NL that will elicit the largest modulation of NL firing rate as a function of ITD.

Sound localization cues in the marmoset monkey.

The most important acoustic cues available to the brain for sound localization are produced by the interaction of sound with the animal's head and external ears. As a first step in understanding the relation between these cues and their neural representation in a vocal new-world primate, we measured head-related transfer functions (HRTFs) across frequency for a wide range of sound locations in three anesthetized marmoset monkeys. The HRTF magnitude spectrum has a broad resonance peak at 6-12 kHz that coincides with the frequency range of the major call types of this species.

Diversity of gain modulation by noise in neocortical neurons: regulation by the slow afterhyperpolarization conductance.

Neuronal firing is known to depend on the variance of synaptic input as well as the mean input current. Several studies suggest that input variance, or "noise," has a divisive effect, reducing the slope or gain of the firing frequency-current (f-I) relationship. We measured the effects of current noise on f-I relationships in pyramidal neurons and fast-spiking (FS) interneurons in slices of rat sensorimotor cortex.

Two-dimensional time coding in the auditory brainstem.

Avian nucleus magnocellularis (NM) spikes provide a temporal code representing sound arrival times to downstream neurons that compute sound source location. NM cells act as high-pass filters by responding only to discrete synaptic events while ignoring temporally summed EPSPs. This high degree of input selectivity insures that each output spike from NM unambiguously represents inputs that contain precise temporal information.