Adaptation in the auditory midbrain of the barn owl (Tyto alba) induced by tonal double stimulation

Spatial-dependent suppressive aftereffect produced by a sound in the rat’s inferior colliculus is partially dependent on local inhibition

In a natural acoustic environment, a preceding sound can suppress the perception of a succeeding sound which can lead to auditory phenomena such as forward masking and the precedence effect. The degree of suppression is dependent on the relationship between the sounds in sound quality, timing, and location. Correlates of such phenomena exist in sound-elicited activities of neurons in hearing-related brain structures. The present study recorded responses to pairs of leading-trailing sounds from ensembles of neurons in the rat’s inferior colliculus. Results indicated that a leading sound produced a suppressive aftereffect on the response to a trailing sound when the two sounds were colocalized at the ear contralateral to the site of recording (i.e., the ear that drives excitatory inputs to the inferior colliculus). The degree of suppression was reduced when the time gap between the two sounds was increased or when the leading sound was relocated to an azimuth at or close to the ipsilateral ear. Local blockage of the type-A γ-aminobutyric acid receptor partially reduced the suppressive aftereffect when a leading sound was at the contralateral ear but not at the ipsilateral ear. Local blockage of the glycine receptor partially reduced the suppressive aftereffect regardless of the location of the leading sound. Results suggest that a sound-elicited suppressive aftereffect in the inferior colliculus is partly dependent on local interaction between excitatory and inhibitory inputs which likely involves those from brainstem structures such as the superior paraolivary nucleus. These results are important for understanding neural mechanisms underlying hearing in a multiple-sound environment.

Intrinsic and Synaptic Dynamics Contribute to Adaptation in the Core of the Avian Central Nucleus of the Inferior Colliculus

The reduction of neuronal responses to repeated stimulus presentation occurs in many sensory neurons, also in the inferior colliculus of birds. The cellular mechanisms that cause response adaptation are not well described. Adaptation must be explicable by changes in the activity of input neurons, short-term synaptic plasticity of the incoming connections, excitability changes of the neuron under consideration or influences of inhibitory or modulatory network connections. Using whole-cell recordings in acute brain slices of the embryonic chicken brain we wanted to understand the intrinsic and synaptic contributions to adaptation in the core of the central nucleus of the inferior colliculus (ICCc). We described two neuron types in the chicken ICCc based on their action potential firing patterns: Phasic/onset neurons showed strong intrinsic adaptation but recovered more rapidly. Tonic/sustained firing neurons had weaker adaptation but often had additional slow components of recovery from adaptation. Morphological analysis suggested two neuron classes, but no physiological parameter aligned with this classification. Chicken ICCc neurons received mostly mixed AMPA- and NMDA-type glutamatergic synaptic inputs. In the majority of ICCc neurons the input synapses underwent short-term depression. With a simulation of the putative population output activity of the chicken ICCc we showed that the different adaptation profiles of the neuron classes could shift the emphasize of stimulus encoding from transients at long intervals to ongoing parts at short intervals. Thus, we report here that description of biophysical and synaptic properties can help to explain adaptive phenomena in central auditory neurons.

Combination of Interaural Level and Time Difference in Azimuthal Sound Localization in Owls

A function of the auditory system is to accurately determine the location of a sound source. The main cues for sound location are interaural time (ITD) and level (ILD) differences. Humans use both ITD and ILD to determine the azimuth. Thus far, the conception of sound localization in barn owls was that their facial ruff and asymmetrical ears generate a two-dimensional grid of ITD for azimuth and ILD for elevation. We show that barn owls also use ILD for azimuthal sound localization when ITDs are ambiguous. For high-frequency narrowband sounds, midbrain neurons can signal multiple locations, leading to the perception of an auditory illusion called a phantom source. Owls respond to such an illusory percept by orienting toward it instead of the true source. Acoustical measurements close to the eardrum reveal a small ILD component that changes with azimuth, suggesting that ITD and ILD information could be combined to eliminate the illusion. Our behavioral data confirm that perception was robust against ambiguities if ITD and ILD information was combined. Electrophysiological recordings of ILD sensitivity in the owl’s midbrain support the behavioral findings indicating that rival brain hemispheres drive the decision to orient to either true or phantom sources. Thus, the basis for disambiguation, and reliable detection of sound source azimuth, relies on similar cues across species as similar response to combinations of ILD and narrowband ITD has been observed in humans.

Response adaptation in the barn owl's auditory space map

Response adaptation is the change of the firing rate of neurons induced by a preceding stimulus. It can be found in many sensory systems and throughout the auditory pathway. We investigated response adaptation in the external nucleus of the inferior colliculus (ICX) of barn owls ( Tyto furcata), a nocturnal bird of prey and specialist in sound localization. Individual neurons in the ICX represent locations in auditory space by maximally responding to combinations of interaural time and level differences (ITD and ILD). Neuronal responses were recorded extracellularly under ketamine-diazepam anesthesia. Response adaptation was observed in three double stimulation paradigms. In two paradigms, the same binaural parameters for both stimuli were chosen. A variation of the level of the second stimulus yielded a level increase sufficient to compensate for adaptation around 5 dB. Introducing a silent interstimulus interval (ISI) resulted in recovery from adaptation. The time course of recovery was followed by varying the ISI, and full recovery was found after an ISI of 50 ms. In a third paradigm, the ITD of the second stimulus was varied to investigate the representation of ITD under adaptive conditions. We found that adaptation led to an increased precision and improved selectivity while the best ITD was stable. These changes of representation remained for longer ISIs than were needed to recover from response adaptation at the best ITD. Stimuli with non-best ITDs could also induce similar adaptive effects if the neurons responded to these ITDs. NEW & NOTEWORTHY We demonstrate and characterize response adaptation in neurons of the auditory space map in the barn owl's midbrain with acoustic double-stimulation paradigms. An increase of the second level by 5 dB compensated for the observed adaptive effect. Recovery from adaptation was faster than in upstream nuclei of the auditory pathway. Our results also show that response adaptation might improve precision and selectivity in the representation of interaural time difference.

A novel relay nucleus between the inferior colliculus and the optic tectum in the chicken (Gallus gallus)

Processing multimodal sensory information is vital for behaving animals in many contexts. The barn owl, an auditory specialist, is a classic model for studying multisensory integration. In the barn owl, spatial auditory information is conveyed to the optic tectum (TeO) by a direct projection from the external nucleus of the inferior colliculus (ICX). In contrast, evidence of an integration of visual and auditory information in auditory generalist avian species is completely lacking. In particular, it is not known whether in auditory generalist species the ICX projects to the TeO at all. Here we use various retrograde and anterograde tracing techniques both in vivo and in vitro, intracellular fillings of neurons in vitro, and whole-cell patch recordings to characterize the connectivity between ICX and TeO in the chicken. We found that there is a direct projection from ICX to the TeO in the chicken, although this is small and only to the deeper layers (layers 13-15) of the TeO. However, we found a relay area interposed among the IC, the TeO, and the isthmic complex that receives strong synaptic input from the ICX and projects broadly upon the intermediate and deep layers of the TeO. This area is an external portion of the formatio reticularis lateralis (FRLx). In addition to the projection to the TeO, cells in FRLx send, via collaterals, descending projections through tectopontine-tectoreticular pathways. This newly described connection from the inferior colliculus to the TeO provides a solid basis for visual-auditory integration in an auditory generalist bird. J. Comp. Neurol. 525:513-534, 2017. © 2016 Wiley Periodicals, Inc.

ON and OFF inhibition as mechanisms for forward masking in the inferior colliculus: a modeling study

Masking effects of a preceding stimulus on the detection or perception of a signal have been found in several sensory systems in mammals, including humans and rodents. In the auditory system, it has been hypothesized that a central "OFF-inhibitory" mechanism, which is generated by neurons that respond after a sound is terminated, may contribute to the observed psychophysics. The present study constructed a systems model for the inferior colliculus that includes major ascending monaural and binaural auditory pathways. The fundamental characteristics of several neuron types along the pathways were captured by Hodgkin-Huxley models with specific membrane and synaptic properties. OFF responses were reproduced with a model of the superior paraolivary nucleus containing a hyperpolarization-activated h current and a T-type calcium current. When the gap between the end of the masker and the onset of the signal was large, e.g., >5 ms, OFF inhibition generated strong suppressive effects on the signal response. For smaller gaps, an additional inhibitory source, which was modeled as ON inhibition from the contralateral dorsal nucleus of the lateral lemniscus, showed the potential of explaining the psychophysics. Meanwhile, the effect of a forward masker on the binaural sensitivity to a low-frequency signal was examined, which was consistent with previous psychophysical findings related to sound localization.

Influence of double stimulation on sound-localization behavior in barn owls

Barn owls do not immediately approach a source after they hear a sound, but wait for a second sound before they strike. This represents a gain in striking behavior by avoiding responses to random incidents. However, the first stimulus is also expected to change the threshold for perceiving the subsequent second sound, thus possibly introducing some costs. We mimicked this situation in a behavioral double-stimulus paradigm utilizing saccadic head turns of owls. The first stimulus served as an adapter, was presented in frontal space, and did not elicit a head turn. The second stimulus, emitted from a peripheral source, elicited the head turn. The time interval between both stimuli was varied. Data obtained with double stimulation were compared with data collected with a single stimulus from the same positions as the second stimulus in the double-stimulus paradigm. Sound-localization performance was quantified by the response latency, accuracy, and precision of the head turns. Response latency was increased with double stimuli, while accuracy and precision were decreased. The effect depended on the inter-stimulus interval. These results suggest that waiting for a second stimulus may indeed impose costs on sound localization by adaptation and this reduces the gain obtained by waiting for a second stimulus.

Estimating characteristic phase and delay from broadband interaural time difference tuning curves

Characteristic delay and characteristic phase are shape parameters of interaural time difference tuning curves. The standard procedure for the estimation of these parameters is based on the measurement of delay curves measured for tonal stimuli with varying frequencies. Common to all procedures is the detection of a linear behavior of the phase spectrum. Hence a reliable estimate can only be expected if sufficiently many relevant frequencies are tested. Thus, the estimation precision depends on the given bandwidth. Based on a linear model, we develop and implement methods for the estimation of characteristic phase and delay from a single broadband tuning curve. We present two different estimation algorithms, one based on a Fourier-analytic interpretation of characteristic delay and phase, and the other based on mean square error minimization. Estimation precision and robustness of the algorithms are tested on artificially generated data with predetermined characteristic delay and phase values, and on sample data from electrophysiological measurements in birds and in mammals. Increasing the signal-to-noise ratio or the bandwidth increases the estimation accuracy of the algorithms. Frequency band location and strong rectification also affect the estimation accuracy. For realistic bandwidths and signal-to-noise ratios, the minimization algorithm reliably and robustly estimates characteristic delay and phase and is superior to the Fourier-analytic method. Bandwidth-dependent significance thresholds allow to assess whether the estimated characteristic delay and phase values are meaningful shape parameters of a measured tuning curve. These thresholds also indicate the sampling rates needed to obtain reliable estimates from interaural time difference tuning curves.

Coding space-time stimulus dynamics in auditory brain maps

Sensory maps are often distorted representations of the environment, where ethologically-important ranges are magnified. The implication of a biased representation extends beyond increased acuity for having more neurons dedicated to a certain range. Because neurons are functionally interconnected, non-uniform representations influence the processing of high-order features that rely on comparison across areas of the map. Among these features are time-dependent changes of the auditory scene generated by moving objects. How sensory representation affects high order processing can be approached in the map of auditory space of the owl's midbrain, where locations in the front are over-represented. In this map, neurons are selective not only to location but also to location over time. The tuning to space over time leads to direction selectivity, which is also topographically organized. Across the population, neurons tuned to peripheral space are more selective to sounds moving into the front. The distribution of direction selectivity can be explained by spatial and temporal integration on the non-uniform map of space. Thus, the representation of space can induce biased computation of a second-order stimulus feature. This phenomenon is likely observed in other sensory maps and may be relevant for behavior.

Effects of forward masking on sound localization in cats: basic findings with broadband maskers

Forward masking is traditionally measured with a detection task in which the addition of a preceding masking sound results in an increased signal-detection threshold. Little is known about the influence of forward masking on localization of free-field sound for human or animal subjects. Here we recorded gaze shifts of two head-unrestrained cats during localization using a search-coil technique. A broadband (BB) noise masker was presented straight ahead. A brief signal could come from 1 of the 17 speaker locations in the frontal hemifield. The signal was either a BB or a band-limited (BL) noise. For BB targets, the presence of the forward masker reduced localization accuracy at almost all target levels (20 to 80 dB SPL) along both horizontal and vertical dimensions. Temporal decay of masking was observed when a 15-ms interstimulus gap was added between the end of the masker and the beginning of the target. A large effect of forward masking was also observed for BL targets with low (0.2-2 kHz) and mid (2-7 kHz) frequencies, indicating that the interaural timing cue is susceptible to forward masking. Except at low sound levels, a small or little effect was observed for high-frequency (7-15 kHz) targets, indicating that the interaural level and the spectral cues in that frequency range remained relatively robust. Our findings suggest that different localization mechanisms can operate independently in a complex listening environment.

The representation of sound localization cues in the barn owl's inferior colliculus

The barn owl is a well-known model system for studying auditory processing and sound localization. This article reviews the morphological and functional organization, as well as the role of the underlying microcircuits, of the barn owl's inferior colliculus (IC). We focus on the processing of frequency and interaural time (ITD) and level differences (ILD). We first summarize the morphology of the sub-nuclei belonging to the IC and their differentiation by antero- and retrograde labeling and by staining with various antibodies. We then focus on the response properties of neurons in the three major sub-nuclei of IC [core of the central nucleus of the IC (ICCc), lateral shell of the central nucleus of the IC (ICCls), and the external nucleus of the IC (ICX)]. ICCc projects to ICCls, which in turn sends its information to ICX. The responses of neurons in ICCc are sensitive to changes in ITD but not to changes in ILD. The distribution of ITD sensitivity with frequency in ICCc can only partly be explained by optimal coding. We continue with the tuning properties of ICCls neurons, the first station in the midbrain where the ITD and ILD pathways merge after they have split at the level of the cochlear nucleus. The ICCc and ICCls share similar ITD and frequency tuning. By contrast, ICCls shows sigmoidal ILD tuning which is absent in ICCc. Both ICCc and ICCls project to the forebrain, and ICCls also projects to ICX, where space-specific neurons are found. Space-specific neurons exhibit side peak suppression in ITD tuning, bell-shaped ILD tuning, and are broadly tuned to frequency. These neurons respond only to restricted positions of auditory space and form a map of two-dimensional auditory space. Finally, we briefly review major IC features, including multiplication-like computations, correlates of echo suppression, plasticity, and adaptation.