Characteristic Delay Research Articles

Frequency-invariant representation of interaural time differences

Event Abstract Back to Event Frequency-invariant representation of interaural time differences Hannes Luling1, 2*, Ida Siveke1, Benedikt Grothe1 and Christian Leibold1 1 Ludwig-Maximilians-Universitat, Germany 2 BCCN, Germany The difference in traveling times of a sound from its origin to the two ears is called the interaural time difference (ITD). ITDs are the main cue for low-frequency-sound localization. The frequency of the stimulus modulates the ITD sensitivity of the response rates of neurons in the brain stem. This modulation is generally characterized by two parameters: The characteristic phase (CP) and the characteristic delay (CD). The CD corresponds to a difference in the temporal delays from the ears to a respective coincidence detector neuron. The CP is an additional phase offset the nature of which is still under debate. The two above characteristic quantities hence describe the best ITD at which a neuron responds maximally via (best ITD)=CD+CP/f. Here f describes the frequency of the pure tone stimulus. We recorded neuronal firing rates in the dorsal nucleus of the lateral lemniscus of the mongolian gerbil for pure tone stimuli with varying ITD and frequency. Interestingly, we found that CPs and CDs are strongly negatively correlated. To understand the observed distribution of CPs and CDs among the recorded population, we assessed the mutual information of firing rate and ITD in terms of these two parameters. We therefore computed signal and noise entropies from rate distributions fitted to the experiments. Our results show that the information-optimal distribution of CPs and CDs exhibits a similar negative correlation as the one observed experimentally. Assuming similar rate statistics, we make hypotheses about how CDs and CPs should optimally be distributed for mammals with various head diameters. As expected, the mutual information increases with head diameter. Moreover, for increasing head diameter the two distinct subclusters of high mutual information (peakers and troughers) fuse into one. To reveal correlations in the neural responses, we trained support vector machines (SVMs) and analyzed the resulting weights. We trained one SVM for every ITD interval. As input we used randomly drawn population rate vectors. We found that the error increases with number of different ITDs. For the behavioral localization acuity of 20 microseconds, we find a generalization error of about 0.6 percent. Moreover, ITDs are encoded by neural populations with strongly varying coding characteristics. Conference: Computational and Systems Neuroscience 2010, Salt Lake City, UT, United States, 25 Feb - 2 Mar, 2010. Presentation Type: Poster Presentation Topic: Poster session II Citation: Luling H, Siveke I, Grothe B and Leibold C (2010). Frequency-invariant representation of interaural time differences. Front. Neurosci. Conference Abstract: Computational and Systems Neuroscience 2010. doi: 10.3389/conf.fnins.2010.03.00305 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 07 Mar 2010; Published Online: 07 Mar 2010. * Correspondence: Hannes Luling, Ludwig-Maximilians-Universitat, Munchen, Germany, lueling@bio.lmu.de Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Hannes Luling Ida Siveke Benedikt Grothe Christian Leibold Google Hannes Luling Ida Siveke Benedikt Grothe Christian Leibold Google Scholar Hannes Luling Ida Siveke Benedikt Grothe Christian Leibold PubMed Hannes Luling Ida Siveke Benedikt Grothe Christian Leibold Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Convergent Input from Brainstem Coincidence Detectors onto Delay-Sensitive Neurons in the Inferior Colliculus

Responses of low-frequency neurons in the inferior colliculus (IC) of anesthetized guinea pigs were studied with binaural beats to assess their mean best interaural phase (BP) to a range of stimulating frequencies. Phase plots (stimulating frequency vs BP) were produced, from which measures of characteristic delay (CD) and characteristic phase (CP) for each neuron were obtained. The CD provides an estimate of the difference in travel time from each ear to coincidence-detector neurons in the brainstem. The CP indicates the mechanism underpinning the coincidence detector responses. A linear phase plot indicates a single, constant delay between the coincidence-detector inputs from the two ears. In more than half (54 of 90) of the neurons, the phase plot was not linear. We hypothesized that neurons with nonlinear phase plots received convergent input from brainstem coincidence detectors with different CDs. Presentation of a second tone with a fixed, unfavorable delay suppressed the response of one input, linearizing the phase plot and revealing other inputs to be relatively simple coincidence detectors. For some neurons with highly complex phase plots, the suppressor tone altered BP values, but did not resolve the nature of the inputs. For neurons with linear phase plots, the suppressor tone either completely abolished their responses or reduced their discharge rate with no change in BP. By selectively suppressing inputs with a second tone, we are able to reveal the nature of underlying binaural inputs to IC neurons, confirming the hypothesis that the complex phase plots of many IC neurons are a result of convergence from simple brainstem coincidence detectors.

Response of auditory units in the barn owl's inferior colliculus to continuously varying interaural phase differences

1. We studied the response of single units in the central nucleus of the inferior colliculus (ICc) of the barn owl (Tyto alba) to continuously varying interaural phase differences (IPDs) and static IPDs. Interaural phase was varied in two ways: continuously, by delivering tones to each ear that varied by a few hertz (binaural beat, Fig. 1), and discretely, by delaying in fixed steps the phase of sound delivered to one ear relative to the other (static phase). Static presentations were repeated at several IPDs to characterize interaural phase sensitivity. 2. Units sensitive to IPDs responded to the binaural beat stimulus over a broad range of delta f(Fig. 4). We selected a 3-Hz delta f for most of our comparative measurements on the basis of constraints imposed by our stimulus generation system and because it allowed us to reduce the influence of responses to stimulus onset and offset (Fig. 3A). 3. Characteristic interaural time or phase sensitivity obtained by the use of the binaural beat stimulus were comparable with those obtained by the use of the static technique (Fig. 5; r2 = 0.93, Fig. 6). 4. The binaural beat stimulus facilitated the measurement of characteristic delay (CD) and characteristic phase (CP) of auditory units. We demonstrated that units in the owl's inferior colliculus (IC) include those that are maximally excited by specific IPDs (CP = 0 or 1.0) as well as those that are maximally suppressed by specific IPDs (CP = 0.5; Figs. 7 and 8). 5. The selectivity of units sensitive to IPD or interaural time difference (ITD) were weakly influenced by interaural intensity difference (IID).(ABSTRACT TRUNCATED AT 250 WORDS)

Monaural and binaural response properties of neurons in the inferior colliculus of the rabbit: effects of sodium pentobarbital.

1. We studied the effects of sodium pentobarbital on 22 neurons in the inferior colliculus (IC) of the rabbit. We recorded changes in the sensitivity of these neurons to monaural stimulation and to ongoing interaural time differences (ITDs). Monaural stimuli were tone bursts at or near the neuron's best frequency. The ITD was varied by delivering tones that differed by 1 Hz to the two ears, resulting in a 1-Hz binaural beat. 2. We assessed a neuron's ITD sensitivity by calculating three measures from the responses to binaural beats: composite delay, characteristic delay (CD), and characteristic phase (CP). To obtain the composite delay, we first derived period histograms by averaging, showing the response at each stimulating frequency over one period of the beat frequency. Second, the period histograms were replotted as a function of their equivalent interaural delay and then averaged together to yield the composite delay curve. Last, we calculated the composite peak or trough delay by fitting a parabola to the peak or trough of this composite curve. The composite delay curve represents the average response to all frequencies within the neuron's responsive range, and the peak reflects the interaural delay that produces the maximum response. The CD and CP were estimated from a weighted fit of a regression line to the plot of the mean interaural phase of the response versus the stimulating frequency. The slope and phase intercept of this regression line yielded estimates of CD and CP, respectively. These two quantities are thought to reflect the mechanism of ITD sensitivity, which involves the convergence of phase-locked inputs on a binaural cell. The CD estimates the difference in the time required for the two inputs to travel from either ear to this cell, whereas the CP reflects the interaural phase difference of the inputs at this cell. 3. Injections of sodium pentobarbital at subsurgical dosages (less than 25 mg/kg) almost invariably altered the neuron's response rate, response latency, response pattern, and spontaneous activity. Most of these changes were predictable and consistent with an enhancement of inhibitory influences. For example, if the earliest response was inhibitory, later excitation was usually reduced and latency increased. If the earliest response was excitatory, the level of this excitation was unaltered or slightly enhanced, and changes in latency were minimal. 4. The neuron's response pattern also changed in a predictable way. For example, a response with an inhibitory pause could either change to a response with a longer pause or to a response with an onset only.(ABSTRACT TRUNCATED AT 400 WORDS)

Interaural phase-sensitive units in the inferior colliculus of the unanesthetized rabbit: effects of changing frequency.

We studied the interaural phase sensitivity of 85 units in the inferior colliculus (IC) of the unanesthetized rabbit. We assessed this sensitivity at several frequencies within each unit's responsive range. The interaural phase disparity was varied by delivering tones that differed by 1 Hz to the two ears, resulting in a 1-Hz binaural beat. We analyzed each unit's response to different frequencies by calculating four measures: characteristic delay (CD), characteristic phase (CP), composite peak delay, and mean peak delay. We estimated the CD and CP from the slope and phase intercept, respectively, of the regression line fitted to a plot of the mean interaural phase against stimulating frequency. The composite peak delay was estimated from the peak of a composite delay curve. This was generated by replotting the response to changes in interaural phase, as a function of the equivalent interaural delay and averaging the resultant interaural delay curves. The composite delay curve reflects the unit's average response to interaural delays across frequencies. Last, we calculated a mean peak delay, derived by converting the mean interaural phase of the response at each frequency to an equivalent delay and then averaging these delays. Interaural phase sensitivity was observed to frequencies as high as 2,150 Hz. However, the majority of units showed such sensitivity below 1,500 Hz. For most units, the interaural delay curves measured at several frequencies coincided near the peak discharge. This result is consistent with a neural model, where excitatory inputs from each ear converge upon a binaural cell, evoking maximum discharge only when the two inputs arrive simultaneously. As a first approximation, our data fit this model, indicating that IC neurons can act like coincidence detectors or cross-correlators. The distributions of CD, composite peak delay, and mean peak delay showed that most units preferred ipsilateral stimulus delays, which in the natural situation corresponds to sounds emanating from the contralateral field. Moreover, most units preferred delays that were within the estimated physiological range of the rabbit. These results support the viewpoint that neurons in the IC participate in sound localization. The distributions of CP and CD differ substantially from those found in the IC of the anesthetized cat. These differences may reflect species differences, the effects of anesthesia, or a difference in the population of units sampled. For each unit, we assessed the linearity of the plot of mean interaural phase against frequency of stimulation using a chi 2 method. For most units the plots were significantly nonlinear.(ABSTRACT TRUNCATED AT 400 WORDS)

Selectivity for interaural time difference in the owl's midbrain

The barn owl uses the interaural difference in the timing of sounds to determine the azimuth of the source. When the sound has a wide frequency band, localization is precise. When localizing tones, however, the barn owl errs in a manner that suggests that it is confused by phantom targets. We report a neural equivalent of these phenomena as they are observed in the space-specific neuron of the owl's inferior colliculus. When stimulated with a tone, the space-specific neuron discharges maximally at interaural time differences (ITDs) that differ by one period of the stimulus tone. Changing the stimulus frequency changes the period of the ITD-response functions, but 1 ITD evokes, in most neurons, a maximal response, regardless of frequency. This ITD is the characteristic delay (CD). When stimulated with noise, there is a maximal response only to ITDs at or near the CD. Thus, the space-specific neuron can unambiguously signal the CD, provided that the signal contains more than 1 frequency. The preferential response to a single ITD, which is observed with noise stimuli, was also observed when the summed waveform of the best frequency and another tone, F2, was presented. The response of the space-specific neuron to these 2-tone stimuli could not be accounted for by the summing or averaging of the ITD-response functions obtained with the best frequency or F2 alone, suggesting that nonlinear neural processes are involved.

Binaural interaction in low-frequency neurons in inferior colliculus of the cat. III. Effects of changing frequency.

The effects of changing stimulus frequency on the interaural phase sensitivity of neurons in the inferior colliculus (IC) were studied in barbiturate-anesthetized cats in order to reexamine the issue of characteristic delay (CD). Since the results obtained with the interaural delay and binaural beat stimuli are similar, we used the averaged interaural delay curves and binaural beat period histograms as comparable expressions of a neuron's interaural phase sensitivity. When the averaged interaural delay curves at different frequencies are plotted on a common time axis, for some cells the resulting superimposed delay curves show peaks or troughs that coincide at some CD. For most cells, though, this method of detecting a CD by visual inspection yields ambiguous and uncertain results. Composite curves, computed from the average of all the normalized superimposed delay curves, are also not helpful for showing CD. In order to provide a more objective means of analyzing the data, we plotted the mean interaural phase versus the stimulating frequency and computed the linear regression line, using the mean square error as a measure of linearity. The slope of the regression line is the CD for the neuron, and the phase intercept is referred to as the characteristic phase (CP). Cells that display a CD at the peak discharge have a CP = 0.0 cycles, while those that show a CD at the minimum discharge have a CP = 0.5. Cells that exhibit a CP at any value other than 0.0, 0.5, or 1.0 will have a CD at some relative amplitude other than the peak or trough. For cells that exhibit a CD at the peak or trough, results of the analysis procedure using the phase-frequency plot correspond to those obtained from visual inspection. For cells that do not show a common peak or trough, the analysis procedure not only specifies the location of the CD but also provides a statistical criterion of the linearity. From this analysis about 60% of the runs were identified as satisfying the criteria for CD at the P less than 0.005 level and 71% of these CDs are between +/- 300 micros. Most CD cells do not have the CD at the peak or trough of the response. Our results differ from those found in previous studies but they are in essential agreement with the original concept put forth by Rose et al. (31). Some cels exhibit little change in the CD or CP with variations in intensity, while others display marked systematic shifts in both CD and CP. In general, the peaks and troughs of the composite curves show less variability with intensity than the CD.(ABSTRACT TRUNCATED AT 400 WORDS)