Paul C. Nelson

A phenomenological model of peripheral and central neural responses to amplitude-modulated tones

A phenomenological model with time-varying excitation and inhibition was developed to study possible neural mechanisms underlying changes in the representation of temporal envelopes along the auditory pathway. A modified version of an existing auditory-nerve model [Zhang et al., J. Acoust. Soc. Am. 109, 648-670 (2001)] was used to provide inputs to higher hypothetical processing centers. Model responses were compared directly to published physiological data at three levels: the auditory nerve, ventral cochlear nucleus, and inferior colliculus. Trends and absolute values of both average firing rate and synchrony to the modulation period were accurately predicted at each level for a wide range of stimulus modulation depths and modulation frequencies. The diversity of central physiological responses was accounted for with realistic variations of model parameters. Specifically, enhanced synchrony in the cochlear nucleus and rate-tuning to modulation frequency in the inferior colliculus were predicted by choosing appropriate relative strengths and time courses of excitatory and inhibitory inputs to postsynaptic model cells. The proposed model is fundamentally different than others that have been used to explain the representation of envelopes in the mammalian midbrain, and it provides a computational tool for testing hypothesized relationships between physiology and psychophysics.

Wide-Dynamic-Range Forward Suppression in Marmoset Inferior Colliculus Neurons Is Generated Centrally and Accounts for Perceptual Masking

An organisms ability to detect and discriminate sensory inputs depends on the recent stimulus history. For example, perceptual detection thresholds for a brief tone can be elevated by as much as 50 dB when following a masking stimulus. Previous work suggests that such forward masking is not a direct result of peripheral neural adaptation; the central pathway apparently modifies the representation in a way that further attenuates the inputs response to short probe signals. Here, we show that much of this transformation is complete by the level of the inferior colliculus (IC). Single-neuron extracellular responses were recorded in the central nucleus of the awake marmoset IC. The threshold for a 20 ms probe tone presented at best frequency was determined for various masker-probe delays, over a range of masker sound pressure levels (SPLs) and frequencies. The most striking aspect of the data was the increased potency of forward maskers as their SPL was increased, despite the fact that the excitatory response to the masker was often saturating or nonmonotonic over the same range of levels. This led to probe thresholds at high masker levels that were almost always higher than those observed in the auditory nerve. Probe threshold shifts were not usually caused by a persistent excitatory response to the masker; instead we propose a wide-dynamic-range inhibitory mechanism locked to sound offset as an explanation for several key aspects of the data. These findings further delineate the role of subcortical auditory processing in the generation of a context-dependent representation of ongoing acoustic scenes.

Neural correlates of context-dependent perceptual enhancement in the inferior colliculus

In certain situations, preceding auditory stimulation can actually result in heightened sensitivity to subsequent sounds. Many of these phenomena appear to be generated in the brain as reflections of central computations. One example is the robust perceptual enhancement (or “pop out”) of a probe signal within a broadband sound whose onset time is delayed relative to the remainder of a mixture of tones. Here we show that the neural representation of such stimuli undergoes a dramatic transformation as the pathway is ascended, from an implicit and distributed peripheral code to explicitly facilitated single-neuron responses at the level of the inferior colliculus (IC) of two awake and passively listening female marmoset monkeys (Callithrix jacchus). Many key features of the IC responses directly parallel psychophysical measures of enhancement, including the dependence on the width of a spectral notch surrounding the probe, the overall level of the complex, and the duration of the preceding sound (referred to as the conditioner). Neural detection thresholds for the probe with and without the conditioner were also in qualitative agreement with analogous psychoacoustic measures. Response characteristics during the conditioners were predictive of the enhancement or suppression of the ensuing probe response: buildup responses were associated with enhancement, whereas adapting conditioner responses were more likely to result in suppression. These data can be primarily explained by a phenomenological computational model using dynamic (adapting) inhibition as a necessary ingredient in the generation of neural enhancement.

Enhancement in the Marmoset Inferior Colliculus: Neural Correlates of Perceptual “Pop-Out”

Although new information about external stimuli cannot be generated centrally, it is clear that the auditory system can selectively suppress or enhance different features of the peripheral response to acoustic stimulation. One example is the robust perceptual “pop out” of a single component within a broadband sound whose onset time is delayed relative to the remainder of the complex. Single auditory nerve fibers do not exhibit enhanced responses using such stimuli (J Acoust Soc Am 97:1786-1799, 1995); the percept is presumably derived from the amplification in the central auditory system of some set of features within the population peripheral response. The goals of this study were (1) to determine whether this neural integration occurs at or below the inferior colliculus (IC), and (2) to compare the effects of specific stimuli and parameter variations between physiological and psychophysical experiments. Single-unit activity was recorded in the IC of the awake marmoset in response to stimuli loosely modeled after previous behavioral and physiological studies of the enhancement effect. A 100-ms best-frequency (BF) tone was presented within a wideband sound with a spectral notch centered on BF. In many IC neurons, responses were significantly larger to this stimulus when a 500-ms preceding signal consisting of the band-reject complex was presented than when silence was presented prior to the probe.

Comparison of level discrimination, increment detection, and comodulation masking release in the audio- and envelope-frequency domains.

In general, the temporal structure of stimuli must be considered to account for certain observations made in detection and masking experiments in the audio-frequency domain. Two such phenomena are (1) a heightened sensitivity to amplitude increments with a temporal fringe compared to gated level discrimination performance and (2) lower tone-in-noise detection thresholds using a modulated masker compared to those using an unmodulated masker. In the current study, translations of these two experiments were carried out to test the hypothesis that analogous cues might be used in the envelope-frequency domain. Pure-tone carrier amplitude-modulation (AM) depth-discrimination thresholds were found to be similar using both traditional gated stimuli and using a temporally modulated fringe for a fixed standard depth (ms = 0.25) and a range of AM frequencies (4-64 Hz). In a second experiment, masked sinusoidal AM detection thresholds were compared in conditions with and without slow and regular fluctuations imposed on the instantaneous masker AM depth. Release from masking was obtained only for very slow masker fluctuations (less than 2 Hz). A physiologically motivated model that effectively acts as a first-order envelope change detector accounted for several, but not all, of the key aspects of the data.

Comparison of intensity discrimination, increment detection, and comodulation masking release in the envelope and audio‐frequency domains

In the audio‐frequency domain, the envelope apparently plays an important role in detection of intensity increments and in comodulation masking release (CMR). The current study addressed the question whether the second‐order envelope (‘‘venelope’’) contributes similarly for comparable experiments in the envelope‐frequency domain. One set of experiments examined the relationship between gated intensity discrimination and continuous‐carrier increment detection. In contrast to the asymmetry observed in the audio‐frequency domain (listeners are more sensitive to increments), AM‐depth discrimination thresholds were found to be the same in conditions with a continuous (modulated) carrier and with traditional gated stimuli for AM frequencies ranging from 4–64 Hz. The second set of experiments compared the amount of CMR in a tone‐in‐noise detection task when slow, regular fluctuations were imposed on the masking waveform in both domains. A significant release from masking of a 32‐Hz signal in the modulation frequ...