Onset-Duration Matching of Acoustic Stimuli Revisited: Conventional Arithmetic vs. Proposed Geometric Measures of Accuracy and Precision

How to define thresholds for level and interaural-level-difference discrimination: Insights from scedasticities and distributions

Sensitivity to changes in the stimulus level at one or at both ears and to changes in the interaural level difference (ILD) between the two ears has been studied widely. Several different definitions of threshold and, for one of them, two different ways of averaging single-listener thresholds have been used (i.e., arithmetically and geometrically), but it is unclear which definition and which way of averaging is most suitable. Here, we addressed this issue by examining which of the differently defined thresholds yielded the highest degree of homoscedasticity (homogeneity of the variance). We also examined how closely the differently defined thresholds followed the normal distribution. We measured thresholds from a large number of human listeners as a function of stimulus duration in six experimental conditions, using an adaptive two-alternative forced-choice paradigm. Thresholds defined as the logarithm of the ratio of the intensities or amplitudes of the target and the reference stimulus (i.e., as the difference in their levels or ILDs; the most commonly used definition) were clearly heteroscedastic. Log-transformation of these latter thresholds, as sometimes performed, did not result in homoscedasticity. Thresholds defined as the logarithm of the Weber fraction for stimulus intensity and thresholds defined as the logarithm of the Weber fraction for stimulus amplitude (the most rarely used definition) were consistent with homoscedasticity, but the latter were closer to the ideal case. Thresholds defined as the logarithm of the Weber fraction for stimulus amplitude also followed the normal distribution most closely. The discrimination thresholds should therefore be expressed as the logarithm of the Weber fraction for stimulus amplitude and be averaged arithmetically across listeners. Other implications are discussed, and the obtained differences between the thresholds in different conditions are compared to the literature.

Sound localization in web-based 3D environments

Sound delivery is a key aspect of immersivity in virtual and augmented reality (VR/AR), with studies hinting at a correlation between users' ability to locate sounds around them and the 'feeling of being there'. This is particularly true for WebVR, a method of delivering immersive experiences through a local web browser that has recently captured attention in multiple industries. In WebVR, audio is the main spatial cue. Designers need to select the correct number of sound sources so that users perceive the location of incoming sound correctly. Information on how users localize sound is essential. Sound localization experiments, so far, have been run only in empty spaces or closed rooms, without clear indications for designers in WebVR. Thus, in this study, we investigate sound localization directly through WebVR. To do so, we designed a traditional empty room for training and a city-like virtual environment for testing purposes. In our paper, we also discuss key design parameters, differences in perception for vertical and horizontal directions, the impact of training, and the role of changing virtual environments. In addition, we introduce and test a new sound cue along with the traditional pink noise sound to measure and explore the impact of different sound cues in different environments. The results demonstrate the potential of exploring sound localization using WebVR, and our study will support the development of virtual experiences in human-computer interaction that may be able to reach a large number of participants using a local web browser.

A simplified physiological model of rate-level functions of auditory-nerve fibers

Several approaches have been used to describe the rate-level functions of auditory-nerve fibers (ANFs). One approach uses descriptive models that can be fitted easily to data. Another derives rate-level functions from comprehensive physiological models of auditory peripheral processing. Here, we seek to identify the minimal set of components needed to provide a physiologically plausible account of rate-level functions. Our model consists of a first-order Boltzmann mechanoelectrical transducer function relating the instantaneous stimulus pressure to an instantaneous output, followed by a lowpass filter that eliminates the AC component, followed by an exponential synaptic transfer function relating the DC component to the mean spike rate. This is perhaps the simplest physiologically plausible model capable of accounting for rate-level functions under the assumption that the model parameters for a given ANF and stimulus frequency are level-independent. We find that the model typically accounts well for rate-level functions from cat ANFs for all stimulus frequencies. More complicated model variants having saturating synaptic transfer functions do not perform significantly better, implying the system operates far away from synaptic saturation. Rate saturation in the model is caused by saturation of the DC component of the filter output (e.g., the receptor potential), which in turn is due to the saturation of the transducer function. The maximum mean spike rate is approximately constant across ANFs, such that the slope parameter of the exponential synaptic transfer function decreases with increasing spontaneous rate. If the synaptic parameters for a given ANF are assumed to be constant across stimulus frequencies, then frequency- and level-dependent input nonlinearities are derived that are qualitatively similar to those reported in the literature. Contrary to assumptions in the literature, such nonlinearities are obtained even for ANFs having high spontaneous rates. Finally, spike-rate adaptation is examined and found to be accounted for by a decrease in the slope parameter of the synaptic transfer function over time following stimulus onset.

Defining Glycemic Variability in Very Low-Birthweight Infants: Data from a Continuous Glucose Monitoring System

BACKGROUND

Glucose variability (GV) is a matter of interest for researches in recent years. It is connected with oxidative stress, which is crucial in the development of multiple complication of prematurity. However, glycemic variability in preterm infants was poorly investigated. This study aims to investigate glycemic variability obtained from a continuous glucose monitoring (CGM) system in a cohort of very low-birthweight (VLBW) infants.

METHODS

A prospective, single-center, open cohort study enrolled 74 VLBW infants with a mean birthweight of 1066 g and median gestational age of 28 weeks. A CGM system (Guardian Real-Time CGM^®, Medtronic, Northridge, CA) was used to measure interstitial glucose concentration. The glycemic variability was calculated using EasyGV.

RESULTS

Most glycemic variability indices in VLBW infants showed log-normal distribution and for these, geometric mean ÷/ × geometric standard deviation (GSD) was calculated: M-value 2.28 (÷/ × 1.82), mean amplitude of glycemic excursions (MAGE) 1.89 (÷/ × 1.34), average daily risk ratio (ADRR) 2.22 (÷/ × 2.56), lability index 0.46 (÷/ × 1.71), J-index 0.46 (÷/ × 1.71), low blood glucose index 2.05 (÷/ × 1.66), high blood glucose index 1.11 (÷/ × 2.44), continuous overlapping net glycemic action (CONGA) 5.54 (÷/ × 1.16), mean of daily differences (MODD) 1.23 (÷/ × 1.38), and coefficient of variation 1.15 (÷/ × 1.31). Only SD of glucose concentration showed a normal distribution: arithmetic mean 1.24 (+/-0.37). ADRR, J-index, MODD, CONGA, and MAGE are moderately to strongly correlated with SD.

CONCLUSIONS

In our cohort of VLBW infants, almost all glycemic variability indices showed skewed positive distribution. The natural central tendency measure for the log-normally distributed data is the geometric mean and for statistical variation is the GSD.

A Hierarchy of Time Scales for Discriminating and Classifying the Temporal Shape of Sound in Three Auditory Cortical Fields

Auditory cortex is essential for mammals, including rodents, to detect temporal "shape" cues in the sound envelope but it remains unclear how different cortical fields may contribute to this ability (Lomber and Malhotra, 2008; Threlkeld et al., 2008). Previously, we found that precise spiking patterns provide a potential neural code for temporal shape cues in the sound envelope in the primary auditory (A1), and ventral auditory field (VAF) and caudal suprarhinal auditory field (cSRAF) of the rat (Lee et al., 2016). Here, we extend these findings and characterize the time course of the temporally precise output of auditory cortical neurons in male rats. A pairwise sound discrimination index and a Naive Bayesian classifier are used to determine how these spiking patterns could provide brain signals for behavioral discrimination and classification of sounds. We find response durations and optimal time constants for discriminating sound envelope shape increase in rank order with: A1 < VAF < cSRAF. Accordingly, sustained spiking is more prominent and results in more robust sound discrimination in non-primary cortex versus A1. Spike-timing patterns classify 10 different sound envelope shape sequences and there is a twofold increase in maximal performance when pooling output across the neuron population indicating a robust distributed neural code in all three cortical fields. Together, these results support the idea that temporally precise spiking patterns from primary and non-primary auditory cortical fields provide the necessary signals for animals to discriminate and classify a large range of temporal shapes in the sound envelope.SIGNIFICANCE STATEMENT Functional hierarchies in the visual cortices support the concept that classification of visual objects requires successive cortical stages of processing including a progressive increase in classical receptive field size. The present study is significant as it supports the idea that a similar progression exists in auditory cortices in the time domain. We demonstrate for the first time that three cortices provide temporal spiking patterns for robust temporal envelope shape discrimination but only the ventral non-primary cortices do so on long time scales. This study raises the possibility that primary and non-primary cortices provide unique temporal spiking patterns and time scales for perception of sound envelope shape.

Rate, not selectivity, determines neuronal population coding accuracy in auditory cortex

The notion that neurons with higher selectivity carry more information about external sensory inputs is widely accepted in neuroscience. High-selectivity neurons respond to a narrow range of sensory inputs, and thus would be considered highly informative by rejecting a large proportion of possible inputs. In auditory cortex, neuronal responses are less selective immediately after the onset of a sound and then become highly selective in the following sustained response epoch. These 2 temporal response epochs have thus been interpreted to encode first the presence and then the content of a sound input. Contrary to predictions from that prevailing theory, however, we found that the neural population conveys similar information about sound input across the 2 epochs in spite of the neuronal selectivity differences. The amount of information encoded turns out to be almost completely dependent upon the total number of population spikes in the read-out window for this system. Moreover, inhomogeneous Poisson spiking behavior is sufficient to account for this property. These results imply a novel principle of sensory encoding that is potentially shared widely among multiple sensory systems.