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George D Pollak

Publications and source records attributed to George D Pollak.

10 recordsLinked to original sources
PubMed2005

Differing roles of inhibition in hierarchical processing of species-specific calls in auditory brainstem nuclei.

Here we report on response properties and the roles of inhibition in three brain stem nuclei of Mexican-free tailed bats: the inferior colliculus (IC), the dorsal nucleus of the lateral lemniscus (DNLL) and the intermediate nucleus of the lateral lemniscus (INLL). In each nucleus, we documented the response properties evoked by both tonal and species-specific signals and evaluated the same features when inhibition was blocked. There are three main findings. First, DNLL cells have little or no surround inhibition and are unselective for communication calls, in that they responded to approximately 97% of the calls that were presented. Second, most INLL neurons are characterized by wide tuning curves and are unselective for species-specific calls. The third finding is that the IC population is strikingly different from the neuronal populations in the INLL and DNLL. Where DNLL and INLL neurons are unselective and respond to most or all of the calls in the suite we presented, most IC cells are selective for calls and, on average, responded to approximately 50% of the calls we presented. Additionally, the selectivity for calls in the majority of IC cells, as well as their tuning and other response properties, are strongly shaped by inhibitory innervation. Thus we show that inhibition plays only limited roles in the DNLL and INLL but dominates in the IC, where the various patterns of inhibition sculpt a wide variety of emergent response properties from the backdrop of more expansive and far less specific excitatory innervation.

Acoustic Stimulation↗
PubMed2005

Serotonin shifts first-spike latencies of inferior colliculus neurons.

Many studies of neuromodulators have focused on changes in the magnitudes of neural responses, but fewer studies have examined neuromodulator effects on response latency. Across sensory systems, response latency is important for encoding not only the temporal structure but also the identity of stimuli. In the auditory system, latency is a fundamental response property that varies with many features of sound, including intensity, frequency, and duration. To determine the extent of neuromodulatory regulation of latency within the inferior colliculus (IC), a midbrain auditory nexus, the effects of iontophoretically applied serotonin on first-spike latencies were characterized in the IC of the Mexican free-tailed bat. Serotonin significantly altered the first-spike latencies in response to tones in 24% of IC neurons, usually increasing, but sometimes decreasing, latency. Serotonin-evoked changes in latency and spike count were not always correlated but sometimes occurred independently within individual neurons. Furthermore, in some neurons, the size of serotonin-evoked latency shifts depended on the frequency or intensity of the stimulus, as reported previously for serotonin-evoked changes in spike count. These results support the general conclusion that changes in latency are an important part of the neuromodulatory repertoire of serotonin within the auditory system and show that serotonin can change latency either in conjunction with broad changes in other aspects of neuronal excitability or in highly specific ways.

Acoustic Stimulation↗
PubMed2005

Serotonin modulates responses to species-specific vocalizations in the inferior colliculus.

Neuromodulators such as serotonin are capable of altering the neural processing of stimuli across many sensory modalities. In the inferior colliculus, a major midbrain auditory gateway, serotonin alters the way that individual neurons respond to simple tone bursts and linear frequency modulated sweeps. The effects of serotonin are complex, and vary among neurons. How serotonin transforms the responses to spectrotemporally complex sounds of the type normally heard in natural settings has been poorly examined. To explore this issue further, the effects of iontophoretically applied serotonin on the responses of individual inferior colliculus neurons to a variety of recorded species-specific vocalizations were examined. These experiments were performed in the Mexican free-tailed bat, a species that uses a rich repertoire of vocalizations for the purposes of communication as well as echolocation. Serotonin frequently changed the number of recorded calls that were capable of evoking a response from individual neurons, sometimes increasing (15% of serotonin-responsive neurons), but usually decreasing (62% of serotonin-responsive neurons), this number. A functional consequence of these serotonin-evoked changes would be to change the population response to species-specific vocalizations.

Acoustic Stimulation↗
PubMed2002

Roles of inhibition for transforming binaural properties in the brainstem auditory system.

This review is concerned with the operation of circuits in the central auditory system, how they transform response features and what functional significance may be attributed to those transformations. We focus on the role that GABAergic inhibition plays in processing interaural intensity disparities (IIDs), the principal cues for localizing high frequencies, and the transformations of IID coding that occur between the superior olivary complex and the inferior colliculus (IC). IIDs are coded by excitatory-inhibitory (EI) cells, so called because they are excited by one ear and inhibited by the other. EI neurons are first created in the lateral superior olive (LSO), but they also dominate the dorsal nucleus of the lateral lemniscus (DNLL) and regions of the IC. The three nuclei are intimately linked through a complex arrangement of excitatory and inhibitory connections. One of these is a crossed excitatory projection from the LSO to both the DNLL and IC. The binaural properties of EI neurons in LSO, DNLL and IC appear strikingly similar, suggesting that the EI properties created in the LSO are simply imposed on the DNLL and IC through the crossed excitatory projections. Recent studies support the idea that EI properties created in lower centers are imposed on some IC cells. However, other studies show that the circuitry linking LSO, DNLL and IC generates a number of response transformations in many IC cells. These transformations include marked changes in EI properties with stimulus duration, the generation of highly focused spatial receptive fields, shifts in sensitivity to IIDs, and the de novo creation of the EI response property. All of these transformations are produced by inhibitory innervation of the IC. An additional emergent property is also imposed on IC cells that receive GABAergic innervation from the DNLL. That property is a change in the binaural features of the IC cell, a change produced by the reception of an earlier sound whose IID is strongly excitatory to the IC cell. We illustrate each of these transformations, propose circuitry that could account for the observed properties and suggest some functional relevance for each. In the final section, we discuss some of the inherent uncertainties associated with attributing functional consequences to response features and then consider whether the transformations found in some mammals are species-specific or are universal features of all mammals.

Acoustic Stimulation↗
PubMed2002

Serotonin in the inferior colliculus.

It has been recognized for some time that serotonin fibers originating in raphe nuclei are present in the inferior colliculi of all mammalian species studied. More recently, serotonin has been found to modulate the responses of single inferior colliculus neurons to many types of auditory stimuli, ranging from simple tone bursts to complex species-specific vocalizations. The effects of serotonin are often quite strong, and for some neurons are also highly specific. A dramatic illustration of this is that serotonin can change the selectivity of some neurons for sounds, including species-specific vocalizations. These results are discussed in light of several theories on the function of serotonin in the IC, and of outstanding issues that remain to be addressed.

Acoustic Stimulation↗
PubMed2002

Response selectivity for species-specific calls in the inferior colliculus of Mexican free-tailed bats is generated by inhibition.

Here we show that inhibition shapes diverse responses to species-specific calls in the inferior colliculus (IC) of Mexican free-tailed bats. We presented 10 calls to each neuron of which 8 were social communication and 2 were echolocation calls. We also measured excitatory response regions: the range of tone burst frequencies that evoked discharges at a fixed intensity. The calls evoked highly selective responses in that IC neurons responded to some calls but not others even though those calls swept through their excitatory response regions. By convolving activity in the response regions with the spectrogram of each call, we evaluated whether responses to tone bursts could predict discharge patterns evoked by species-specific calls. The convolutions often predicted responses to calls that evoked no responses and thus were inaccurate. Blocking inhibition at the IC reduced or eliminated selectivity and greatly improved the predictive accuracy of the convolutions. By comparing the responses evoked by two calls with similar spectra, we show that each call evoked a unique spatiotemporal pattern of activity distributed across and within isofrequency contours and that the disparity in the population response was greatly reduced by blocking inhibition. Thus the inhibition evoked by each call can shape a unique pattern of activity in the IC population and that pattern may be important for both the identification of a particular call and for discriminating it from other calls and other signals.

Acoustic Stimulation↗
PubMed2002

Spectral determination of responses to species-specific calls in the dorsal nucleus of the lateral lemniscus.

This study evaluated how neurons in the dorsal nucleus of the lateral lemniscus (DNLL) in Mexican free-tailed bats respond to both tone bursts and species-specific calls. Up to 20 calls were presented to each neuron, of which 18 were social communication and 2 were echolocation calls. We also measured excitatory response regions (ERRs): the range of tone burst frequencies that evoked discharges at a fixed intensity. Neurons were unselective for one or another call in that each neuron responded to any call so long as the call had energy that encroached on its ERR. Additionally, responses were evoked by the same set of calls, and with similar spike counts, when they were presented normally or reversed. By convolving activity in the ERRs with the spectrogram of each call, we showed that responses to tones accurately predicted discharge patterns evoked by species-specific calls. DNLL cells are remarkably homogeneous in that neurons having similar BFs responded to each of the species-specific calls with similar response profiles. The homogeneity was further illustrated by the ability to accurately predict the response profiles of a particular DNLL cell to species-specific calls from the ERR of another similarly tuned DNLL cell. Thus DNLL neurons tuned to the same or similar frequencies responded to species-specific calls with latencies and temporal discharge patterns that were so similar as to be virtually interchangeable. What this suggests is that DNLL responses evoked by complex sounds can be largely explained by a simple summation of the excitation in each neuron's ERR. Finally, superimposing the spectrograms of each call on the responses evoked by that call revealed that the DNLL population response re-creates both the spectral and the temporal features of each signal.

Acoustic Stimulation↗