Sharpness Of Tuning Curves Research Articles

Multiple sounds degrade the frequency representation in monkey inferior colliculus.

How we distinguish multiple simultaneous stimuli is uncertain, particularly given that such stimuli sometimes recruit largely overlapping populations of neurons. One commonly proposed hypothesis is that the sharpness of tuning curves might change to limit the number of stimuli driving any given neuron when multiple stimuli are present. To test this hypothesis, we recorded the activity of neurons in the inferior colliculus while monkeys made saccades to either one or two simultaneous sounds differing in frequency and spatial location. Although monkeys easily distinguished simultaneous sounds (~90% correct performance), the frequency selectivity of inferior colliculus neurons on dual‐sound trials did not improve in any obvious way. Frequency selectivity was degraded on dual‐sound trials compared to single‐sound trials: neural response functions broadened and frequency accounted for less of the variance in firing rate. These changes in neural firing led a maximum‐likelihood decoder to perform worse on dual‐sound trials than on single‐sound trials. These results fail to support the hypothesis that changes in frequency response functions serve to reduce the overlap in the representation of simultaneous sounds. Instead, these results suggest that alternative possibilities, such as recent evidence of alternations in firing rate between the rates corresponding to each of the two stimuli, offer a more promising approach.

The influence of probe level on the tuning of stimulus frequency otoacoustic emissions and behavioral test in human

BackgroundFrequency selectivity (FS) of the auditory system is established at the level of the cochlea and it is important for the perception of complex sounds. Although direct measurements of cochlear FS require surgical preparation, it can also be estimated with the measurements of otoacoustic emissions or behavioral tests, including stimulus frequency otoacoustic emission suppression tuning curves (SFOAE STCs) or psychophysical tuning curves (PTCs). These two methods result in similar estimates of FS at low probe levels. As the compressive nonlinearity of cochlea is strongly dependent on the stimulus intensity, the sharpness of tuning curves which is relevant to the cochlear nonlinearity will change as a function of probe level. The present study aims to investigate the influence of different probe levels on the relationship between SFOAE STCs and PTCs.MethodsThe study included 15 young subjects with normal hearing. SFOAE STCs and PTCs were recorded at low and moderate probe levels for frequencies centred at 1, 2, and 4 kHz. The ratio or the difference of the characteristic parameters between the two methods was calculated at each probe level. The effect of probe level on the ratio or the difference between the parameters of SFOAE STCs and PTCs was then statistically analysed.ResultsThe tuning of SFOAE STCs was significantly positively correlated with the tuning of the PTCs at both low and moderate probe levels; yet, at the moderate probe level, the SFOAE STCs were consistently broader than the PTCs. The mean ratio of sharpness of tuning at low probe levels was constantly around 1 while around 1.5 at moderate probe levels.ConclusionsProbe level had a significant effect on the sharpness of tuning between the two methods of estimating FS. SFOAE STC seems a good alternative measurement of PTC for FS assessment at low probe levels. At moderate probe levels, SFOAE STC and PTC were not equivalent measures of the FS in terms of their bandwidths. Because SFOAE STCs are not biased by higher levels auditory processing, they may represent cochlear FS better than PTCs.

Empirical and biophysical estimations of human cochlea’s psychophysical tuning curve sharpness

Despite the advances in cochlear research, the estimation of auditory nerve fiber frequency tuning of human cochlea is mostly based on psychophysical measurements. Although efforts had been made to estimate human frequency tuning sharpness from various physiological measurements which are less species dependent such as the compound action potential and stimulus-frequency otoacoustic emission delay, conclusions on the relative frequency tuning sharpness compared with that of other mammals vary. We simulated the biophysical human cochlea’s tuning curve based on physiological measurements of human cochlea and compared the human frequency tuning sharpness with results from empirical methods as well as experimental data of other mammalian cochleae. The compound action potential are more accurate at frequencies below 3 kHz while the stimulus frequency-otoacoustic emission delay are more accurate at frequencies above 1 kHz regions. The results from mechanical cochlear models, with support from conclusions of the other two empirical methodologies, suggest that the human frequency tuning sharpness at frequencies below 1 kHz is similar to common laboratory mammals but is exceptionally sharp at higher frequencies.

Sample Skewness as a Statistical Measurement of Neuronal Tuning Sharpness

We propose using the statistical measurement of the sample skewness of the distribution of mean firing rates of a tuning curve to quantify sharpness of tuning. For some features, like binocular disparity, tuning curves are best described by relatively complex and sometimes diverse functions, making it difficult to quantify sharpness with a single function and parameter. Skewness provides a robust nonparametric measure of tuning curve sharpness that is invariant with respect to the mean and variance of the tuning curve and is straightforward to apply to a wide range of tuning, including simple orientation tuning curves and complex object tuning curves that often cannot even be described parametrically. Because skewness does not depend on a specific model or function of tuning, it is especially appealing to cases of sharpening where recurrent interactions among neurons produce sharper tuning curves that deviate in a complex manner from the feedforward function of tuning. Since tuning curves for all neurons are not typically well described by a single parametric function, this model independence additionally allows skewness to be applied to all recorded neurons, maximizing the statistical power of a set of data. We also compare skewness with other nonparametric measures of tuning curve sharpness and selectivity. Compared to these other nonparametric measures tested, skewness is best used for capturing the sharpness of multimodal tuning curves defined by narrow peaks (maximum) and broad valleys (minima). Finally, we provide a more formal definition of sharpness using a shape-based information gain measure and derive and show that skewness is correlated with this definition.

Altered Traveling Wave Propagation and Reduced Endocochlear Potential Associated with Cochlear Dysplasia in the BETA2/NeuroD1 Null Mouse

The BETA2/NeuroD1 null mouse has cochlear dysplasia. Its cochlear duct is shorter than normal, there is a lack of spiral ganglion neurons, and there is hair cell disorganization. We measured vertical movements of the tectorial membrane at acoustic frequencies in excised cochleae in response to mechanical stimulation of the stapes using laser doppler vibrometry. While tuning curve sharpness was similar between wild-type, heterozygotes, and null mice in the base, null mutants had broader tuning in the apex. At both the base and the apex, null mice had less phase lag accumulation with increasing stimulus frequency than wild-type or heterozygote mice. In vivo studies demonstrated that the null mouse lacked distortion product otoacoustic emissions, and the cochlear microphonic and endocochlear potential were found to be severely reduced. Electrically evoked otoacoustic emissions could be elicited, although the amplitudes were lower than those of wild-type mice. Cochlear cross-sections revealed an incomplete partition malformation, with fenestrations within the modiolus that connected the cochlear turns. Outer hair cells from null mice demonstrated the normal pattern of prestin expression within their lateral walls and normal FM 1-43 dye entry. Overall, these data demonstrate that while tonotopicity can exist with cochlear dysplasia, traveling wave propagation is abnormally fast. Additionally, the presence of electrically evoked otoacoustic emissions suggests that outer hair cell reverse transduction is present, although the acoustic response is shaped by the alterations in cochlear mechanics.

Dopamine modulation of prefrontal delay activity-reverberatory activity and sharpness of tuning curves

Recent electrophysiological experiments have shown that dopamine (D1) modulation of pyramidal cells in prefrontal cortex reduces spike frequency adaptation and enhances NMDA transmission. Using four models, from multicompartmental to integrate-and-fire, we examine the effects of these modulations on sustained (delay) activity in a reverberatory network. We find that D1 modulation may enable robust network bistability yielding selective reverberation among cells that code for a particular item or location. We further show that the tuning curve of such cells is sharpened, and that signal-to-noise ratio is increased. We postulate that D1 modulation affects the tuning of “memory fields” and yield efficient distributed dynamic representations.

Psychophysical tuning curves in subjects with tinnitus suggest outer hair cell lesions

A study by Penner (J Speech Hear Res 1980;23:779-86) found evidence for impaired lateral suppression in subjects with tinnitus and sensorineural hearing loss. If lateral suppression is related to tuning curve sharpness and lateral suppression is impaired, the shape of the tuning curve should be affected. The purpose of this study was to determine whether subjects with tinnitus have psychophysical tuning curves that are different from those of subjects without tinnitus. Psychophysical tuning curves and hearing thresholds were obtained from 18 subjects, 7 with tinnitus and 11 without tinnitus. Only subjects with normal audiograms (through 8 kHz) were selected for this study. In subjects with tinnitus psychophysical tuning curves were obtained in the region pitch-matched to their tinnitus. In nontinnitus subjects psychophysical tuning curves were determined at the same frequencies as for the tinnitus subjects in a yoked-control design. The slopes of the tails and tips and the Q10 and other measures were calculated for each tuning curve. The psychophysical tuning curves in subjects with tinnitus were significantly different (0.01 level) from those of control subjects and often had hypersensitive tails and some elevated tips. These shapes of tuning curves are consistent with cochlear lesions involving the loss of outer hair cells without damage to the inner hair cells or nerve fibers. (OTOLARYNGOL HEAD NECK SURG 1995;113:223-33.)

Changes in auditory sensitivity during and following exposure to intense sound in the young chick

Tone‐burst‐evoked potentials recorded from electrodes placed in the cochlear nuclei were used to measure auditory function in groups of chicks, at different time intervals during and after exposure to an intense pure tone. Threshold sensitivity, defined as the stimulus SPL that produced a just‐detectable response in a quiet environment, was measured at test frequencies between 0.1 and 4.5 kHz. A simultaneous masking paradigm obtained masked evoked response thresholds, and from these masked tuning curves were constructed at five different probe‐tone frequencies. Lastly, the relation between evoked response magnitude and stimulus SPL was measured to explore the presence of evoked‐response recruitment. The growth of threshold shift and tuning curve selectivity loss exhibited a dual process. The recovery of threshold sensitivity, following exposure, was more rapid than the recovery of tuning curve sharpness. Evoked‐response recruitment was severe immediately after exposure, but completely recovered within 3 days. These results are considered with respect to cochlear pathology and the regeneration of receptor cells on the chick basilar papilla. [Work supported by the Deafness Research Foundation, Pennsylvania Lions Hearing Research Foundation, and the NIDCD.]

Recovery of auditory function following intense sound exposure in the neonatal chick

We report the changes in auditory function that occurred at selected intervals following exposure to an intense pure-tone stimulus. One day old chicks were exposed to a 0.9 kHz tone for 48 h at 120 dB. At 0, l, 3, 6, 9, 12 and 15 days after exposure, cochlear nucleus sound-evoked potentials were used to assess threshold sensitivity and frequency selectivity. Immediately after removal from the pure tone a threshold shift of 60 dB relative to age-matched controls was measured. The sharpness of tuning curves, as measured by Q 10 dB. decreased by over 50%. By post-exposure day 15, near complete recovery of function was seen, with the greatest recovery occurring within the first three days. We relate these results to recent reports of structural recovery on the basilar papilla of the chick.

Age-related changes in the C57BL/6J mouse cochlea. I. Physiological findings

The development and degeneration of cochlear output was studied in C57BL/6J mice from the time of hearing onset (12 days of age) through adulthood (50 days of age) using the eighth nerve compound action potential (N 1). Mice were stimulated in free-field, using short rise-time tone bursts (2–35 kHz). Sensitivity to tones and threshold tuning curve sharpness (Q 10) increased markedly between 12 and 20 days of age. Response latencies changed, in a complex way, to attain minimum values by 20 days of age. The form of intensity functions was essentially identical in 12- and 16-day-old mice. In 30- and 40-day-old mice signs of both hearing development and degeneration were seen. For example, while N 1 threshold intensities to low-frequency tones continued to decline, those to high frequencies began to increase. The results indicate that some of the previously observed age-related changes in central auditory function in C57BL/6J mice can be accounted for in terms of cochlear evolution. The observed changes in cochlear function are discussed in relation to outer-, middle-, and inner-ear modifications. A basis for the limits of the critical period for audiogenic seizure ‘priming’ is hypothesized.