scholarly journals Auditory Cortex Phase Locking to Amplitude-Modulated Cochlear Implant Pulse Trains

2008 ◽  
Vol 100 (1) ◽  
pp. 76-91 ◽  
Author(s):  
John C. Middlebrooks
Keyword(s):  
Auditory Cortex ◽  
Cochlear Implant ◽  
Amplitude Modulation ◽  
Cortical Neurons ◽  
Modulation Depth ◽  
Phase Locking ◽  
Modulation Frequency ◽  
Electrical Pulse ◽  
Electrode Arrays ◽  
Pulse Trains

Cochlear implant speech processors transmit temporal features of sound as amplitude modulation of constant-rate electrical pulse trains. This study evaluated the central representation of amplitude modulation in the form of phase-locked firing of neurons in the auditory cortex. Anesthetized pigmented guinea pigs were implanted with cochlear electrode arrays. Stimuli were 254 pulse/s (pps) trains of biphasic electrical pulses, sinusoidally modulated with frequencies of 10–64 Hz and modulation depths of −40 to −5 dB re 100% (i.e., 1–56.2% modulation). Single- and multiunit activity was recorded from multi-site silicon-substrate probes. The maximum frequency for significant phase locking (limiting modulation frequency) was ≥60 Hz for 42% of recording sites, whereas phase locking to pulses of unmodulated pulse trains rarely exceeded 30 pps. The strength of phase locking to frequencies ≥40 Hz often varied nonmonotonically with modulation depth, commonly peaking at modulation depths around −15 to −10 dB. Cortical phase locking coded modulation frequency reliably, whereas a putative rate code for frequency was confounded by rate changes with modulation depth. Group delay computed from the slope of mean phase versus modulation frequency tended to increase with decreasing limiting modulation frequency. Neurons in cortical extragranular layers had lower limiting modulation frequencies than did neurons in thalamic afferent layers. Those observations suggest that the low-pass characteristic of cortical phase locking results from intracortical filtering mechanisms. The results show that cortical neurons can phase lock to modulated electrical pulse trains across the range of modulation frequencies and depths presented by cochlear implant speech processors.

scholarly journals Amplitude modulation encoding in auditory cortex: Comparisons between the primary and middle lateral belt regions

2020 ◽  
Author(s):  
Jeffrey S. Johnson ◽  
Mamiko Niwa ◽  
Kevin N. O’Connor ◽  
Mitchell L. Sutter
Keyword(s):  
Auditory Cortex ◽  
Firing Rate ◽  
Amplitude Modulation ◽  
Single Unit ◽  
Modulation Depth ◽  
Phase Locking ◽  
Primary Auditory Cortex ◽  
Auditory Stimuli ◽  
Phase Lock ◽  
Basic Properties

ABSTRACTIn macaques, the middle lateral auditory cortex (ML) is a belt region adjacent to primary auditory cortex (A1) and believed to be at a hierarchically higher level. Although ML single-unit responses have been studied for several auditory stimuli, the ability of ML cells to encode amplitude modulation (AM) – an ability which has been widely studied in A1 – has not yet been characterized. Here we compare the responses of A1 and ML neurons to amplitude modulated (AM) noise in awake macaques. While several of the basic properties of A1 and ML responses to AM noise are similar, we found several key differences. ML neurons do not phase lock as strongly, are less likely to phase lock, and are more likely to respond in a non-synchronized fashion than A1 cells, consistent with a temporal-to-rate transformation as information ascends the auditory hierarchy. ML neurons tend to have lower temporally (phase-locking) based best modulation frequencies than A1. At the level of ML, neurons that decrease firing rate with increasing modulation depth become more common than in A1. In both A1 and ML we find a prevalent class of neurons with excitatory rate responses at lower modulation frequencies and suppressed rate responses relative to the unmodulated carrier at middle modulation frequencies.

scholarly journals Ability of primary auditory cortical neurons to detect amplitude modulation with rate and temporal codes: neurometric analysis

2012 ◽  
Vol 107 (12) ◽  
pp. 3325-3341 ◽  
Author(s):  
Jeffrey S. Johnson ◽  
Pingbo Yin ◽  
Kevin N. O'Connor ◽  
Mitchell L. Sutter
Keyword(s):  
Amplitude Modulation ◽  
Cortical Neurons ◽  
Modulation Depth ◽  
Modulation Frequency ◽  
Primary Auditory Cortex ◽  
Detection Methods ◽  
Lower Envelope ◽  
Detection Thresholds ◽  
Modulation Detection ◽  
Temporal Measures

Amplitude modulation (AM) is a common feature of natural sounds, and its detection is biologically important. Even though most sounds are not fully modulated, the majority of physiological studies have focused on fully modulated (100% modulation depth) sounds. We presented AM noise at a range of modulation depths to awake macaque monkeys while recording from neurons in primary auditory cortex (A1). The ability of neurons to detect partial AM with rate and temporal codes was assessed with signal detection methods. On average, single-cell synchrony was as or more sensitive than spike count in modulation detection. Cells are less sensitive to modulation depth if tested away from their best modulation frequency, particularly for temporal measures. Mean neural modulation detection thresholds in A1 are not as sensitive as behavioral thresholds, but with phase locking the most sensitive neurons are more sensitive, suggesting that for temporal measures the lower-envelope principle cannot account for thresholds. Three methods of preanalysis pooling of spike trains (multiunit, similar to convergence from a cortical column; within cell, similar to convergence of cells with matched response properties; across cell, similar to indiscriminate convergence of cells) all result in an increase in neural sensitivity to modulation depth for both temporal and rate codes. For the across-cell method, pooling of a few dozen cells can result in detection thresholds that approximate those of the behaving animal. With synchrony measures, indiscriminate pooling results in sensitive detection of modulation frequencies between 20 and 60 Hz, suggesting that differences in AM response phase are minor in A1.

scholarly journals Differential contributions of synaptic and intrinsic inhibitory currents to speech segmentation via flexible phase-locking in neural oscillators

2020 ◽  
Author(s):  
Benjamin R. Pittman-Polletta ◽  
Yangyang Wang ◽  
David A. Stanley ◽  
Charles E. Schroeder ◽  
Miles A. Whittington ◽  
...  
Keyword(s):  
Auditory Cortex ◽  
Speech Processing ◽  
Cortical Neurons ◽  
Phase Locking ◽  
Outward Current ◽  
Speech Segmentation ◽  
Oscillatory Activity ◽  
Acoustic Features ◽  
Neural Oscillators ◽  
Speech Stream

AbstractCurrent hypotheses suggest that speech segmentation – the initial division and grouping of the speech stream into candidate phrases, syllables, and phonemes for further linguistic processing – is executed by a hierarchy of oscillators in auditory cortex. Theta (~3-12 Hz) rhythms play a key role by phase-locking to recurring acoustic features marking syllable boundaries. Reliable synchronization to quasi-rhythmic inputs, whose variable frequency can dip below cortical theta frequencies (down to ~1 Hz), requires “flexible” theta oscillators whose underlying neuronal mechanisms remain unknown. Using biophysical computational models, we found that the flexibility of phase-locking in neural oscillators depended on the types of hyperpolarizing currents that paced them. Simulated cortical theta oscillators flexibly phase-locked to slow inputs when these inputs caused both (i) spiking and (ii) the subsequent buildup of outward current sufficient to delay further spiking until the next input. The greatest flexibility in phase-locking arose from a synergistic interaction between intrinsic currents that was not replicated by synaptic currents at similar timescales. Our results suggest that synaptic and intrinsic inhibition contribute to frequency-restricted and - flexible phase-locking in neural oscillators, respectively. Their differential deployment may enable neural oscillators to play diverse roles, from reliable internal clocking to adaptive segmentation of quasi-regular sensory inputs like speech.Author summaryOscillatory activity in auditory cortex is believed to play an important role in auditory and speech processing. One suggested function of these rhythms is to divide the speech stream into candidate phonemes, syllables, words, and phrases, to be matched with learned linguistic templates. This requires brain rhythms to flexibly phase-lock to regular acoustic features of the speech stream. How neuronal circuits implement this task remains unknown. In this study, we explored the contribution of inhibitory currents to flexible phase-locking in neuronal theta oscillators, believed to perform initial syllabic segmentation. We found that a combination of specific intrinsic inhibitory currents at multiple timescales, present in a large class of cortical neurons, enabled exceptionally flexible phase-locking, suggesting that the cells exhibiting these currents are a key component in the brain’s auditory and speech processing architecture.

scholarly journals Cochlear-Implant High Pulse Rate and Narrow Electrode Configuration Impair Transmission of Temporal Information to the Auditory Cortex

2008 ◽  
Vol 100 (1) ◽  
pp. 92-107 ◽  
Author(s):  
John C. Middlebrooks
Keyword(s):  
Animal Model ◽  
Auditory Cortex ◽  
Cochlear Implant ◽  
Pulse Rate ◽  
Temporal Information ◽  
Electrode Configuration ◽  
Bipolar Electrode ◽  
Pulse Trains ◽  
Speech Reception ◽  
High Pulse

In the most commonly used cochlear prosthesis systems, temporal features of sound are signaled by amplitude modulation of constant-rate pulse trains. Several convincing arguments predict that speech reception should be optimized by use of pulse rates ≳2,000 pulses per second (pps) and by use of intracochlear electrode configurations that produce restricted current spread (e.g., bipolar rather than monopolar configurations). Neither of those predictions has been borne out in consistent improvements in speech reception. Neurons in the auditory cortex of anesthetized guinea pigs phase lock to the envelope of sine-modulated electric pulse trains presented through a cochlear implant. The present study used that animal model to quantify the effects of carrier pulse rate, electrode configuration, current level, and modulator wave shape on transmission of temporal information from a cochlear implant to the auditory cortex. Modulation sensitivity was computed using a signal-detection analysis of cortical phase-locking vector strengths. Increasing carrier pulse rate in 1-octave steps from 254 to 4,069 pps resulted in systematic decreases in sensitivity. Comparison of sine- versus square-wave modulator waveforms demonstrated that some, but not all, of the loss of modulation sensitivity at high pulse rates was a result of the decreasing size of pulse-to-pulse current steps at the higher rates. Use of a narrow bipolar electrode configuration, compared with the monopolar configuration, produced a marked decrease in modulation sensitivity. Results from this animal model suggest explanations for the failure of high pulse rates and/or bipolar electrode configurations to produce hoped-for improvements in speech reception.

scholarly journals Temporal Pitch Perception in Cochlear-Implant Users: Channel Independence in Apical Cochlear Regions

2021 ◽  
Vol 25 ◽  
pp. 233121652110206
Author(s):  
Andreas Griessner ◽  
Reinhold Schatzer ◽  
Viktor Steixner ◽  
Gunesh P. Rajan ◽  
Clemens Zierhofer ◽  
...  
Keyword(s):  
Cochlear Implant ◽  
Lateral Wall ◽  
Apical Region ◽  
Electrode Arrays ◽  
Short Delay ◽  
Pulse Trains ◽  
Temporal Cues ◽  
Single Sided Deafness ◽  
Temporal Rate ◽  
Bilateral Deafness

Two-electrode stimuli presented on adjacent mid-array contacts in cochlear-implant users elicit pitch percepts that are not consistent with a summation of the two temporal patterns. This indicates that low-rate temporal rate codes can be applied with considerable independence on adjacent mid-array electrodes. At issue in this study was whether a similar independence of temporal pitch cues can also be observed for more apical sites of stimulation, where temporal cues have been shown to be more reliable than place cues, in contrast to middle and basal sites. In cochlear-implant recipients with single-sided deafness implanted with long lateral-wall electrode arrays, pitch percepts were assessed by matching the pitch of dual-electrode stimuli with pure tones presented to the contralateral normal-hearing ear. The results were supported with an additional pitch-ranking experiment, in a different subject population with bilateral deafness. Unmodulated pulse trains with 100, 200, and 400 pulses per second were presented on three pairs of adjacent electrodes. Pulses were separated by the minimal interchannel delay (1.7 µs) in a short-delay configuration and by half the pulse period in a long-delay configuration. The hypothesis was that subjects would perceive a pitch corresponding to the doubled temporal pattern for the long-delay stimuli due to the summation of excitation patterns from adjacent apical electrodes, if those electrodes were to activate largely overlapping neural populations. However, we found that the mean matched acoustic pitch of the long-delay pulses was not significantly different from that of the short-delay pulses. These findings suggest that also in the apical region in long-array cochlear-implant recipients, temporal cues can be transmitted largely independently on adjacent electrodes.

scholarly journals Human subjective response to steering wheel vibration caused by diesel engine idle

2005 ◽  
Vol 219 (4) ◽  
pp. 499-510 ◽  
Author(s):  
M Ajovalasit ◽  
J Giacomin
Keyword(s):  
Diesel Engine ◽  
Amplitude Modulation ◽  
Modulation Depth ◽  
Paired Comparison ◽  
Modulation Frequency ◽  
Steering Wheel ◽  
Subjective Response ◽  
Passenger Cars ◽  
Direct Evaluation ◽  
Wheel Vibration

This study investigated the human subjective response to steering wheel vibration of the type caused by a four-cylinder diesel engine idle in passenger cars. Vibrotactile perception was assessed using sinusoidal amplitude-modulated vibratory stimuli of constant energy level (r.m.s. acceleration, 0.41 m/s2) having a carrier frequency of 26 Hz (i.e. engine firing frequency) and modulation frequency of 6.5 Hz (half-order engine harmonic). Evaluations of seven levels of modulation depth parameter m(0.0, 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0) were performed in order to define the growth function of human perceived disturbance as a function of amplitude modulation depth. Two semantic descriptors were used (unpleasantness and roughness) and two test methods (the Thurstone paired-comparison method and the Borg CR-10 direct evaluation scale) for a total of four tests. Each test was performed using an independent group of 25 individuals. The results suggest that there is a critical value of modulation depth m = 0.2 below which human subjects do not perceive differences in amplitude modulation and above which the stimulus-response relationship increases monotonically with a power function. The Stevens power exponents suggest that the perceived unpleasantness is non-linearly dependent on modulation depth m with an exponent greater than 1 and that the perceived roughness is dependent with an exponent close to unity.

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