Department of Otolaryngology - Head and Neck Surgery, Queen's Medical Centre, Nottingham, NG7 2UH, UK.
J Physiol. 2013 Jul 1;591(13):3401-19. doi: 10.1113/jphysiol.2013.253062. Epub 2013 Apr 29.
Amplitude modulation (AM) is a pervasive feature of natural sounds. Neural detection and processing of modulation cues is behaviourally important across species. Although most ecologically relevant sounds are not fully modulated, physiological studies have usually concentrated on fully modulated (100% modulation depth) signals. Psychoacoustic experiments mainly operate at low modulation depths, around detection threshold (∼5% AM). We presented sinusoidal amplitude-modulated tones, systematically varying modulation depth between zero and 100%, at a range of modulation frequencies, to anaesthetised guinea-pigs while recording spikes from neurons in the ventral cochlear nucleus (VCN). The cochlear nucleus is the site of the first synapse in the central auditory system. At this locus significant signal processing occurs with respect to representation of AM signals. Spike trains were analysed in terms of the vector strength of spike synchrony to the amplitude envelope. Neurons showed either low-pass or band-pass temporal modulation transfer functions, with the proportion of band-pass responses increasing with increasing sound level. The proportion of units showing a band-pass response varies with unit type: sustained chopper (CS) > transient chopper (CT) > primary-like (PL). Spike synchrony increased with increasing modulation depth. At the lowest modulation depth (6%), significant spike synchrony was only observed near to the unit's best modulation frequency for all unit types tested. Modulation tuning therefore became sharper with decreasing modulation depth. AM detection threshold was calculated for each individual unit as a function of modulation frequency. Chopper units have significantly better AM detection thresholds than do primary-like units. AM detection threshold is significantly worse at 40 dB vs. 10 dB above pure-tone spike rate threshold. Mean modulation detection thresholds for sounds 10 dB above pure-tone spike rate threshold at best modulation frequency are (95% CI) 11.6% (10.0-13.1) for PL units, 9.8% (8.2-11.5) for CT units, and 10.8% (8.4-13.2) for CS units. The most sensitive guinea-pig VCN single unit AM detection thresholds are similar to human psychophysical performance (∼3% AM), while the mean neurometric thresholds approach whole animal behavioural performance (∼10% AM).
调幅（AM）是自然声音的普遍特征。在不同物种中，神经对调制线索的检测和处理在行为上是重要的。尽管大多数生态相关的声音不是完全调制的，但生理研究通常集中在完全调制（100%调制深度）的信号上。心理声学实验主要在低调制深度下运行，约为检测阈值（约 5% AM）。我们向麻醉豚鼠呈现了正弦幅度调制的音调，在一系列调制频率下，调制深度从零到 100%系统变化，同时记录下耳蜗腹核（VCN）神经元的尖峰。耳蜗核是中枢听觉系统中第一个突触的部位。在这个位置，对 AM 信号的表示进行了显著的信号处理。尖峰序列根据对幅度包络的尖峰同步的向量强度进行分析。神经元表现出低通或带通时间调制传递函数，随着声音水平的增加，带通响应的比例增加。表现出带通响应的单元比例随单元类型而变化：持续斩波器（CS）>瞬态斩波器（CT）>初级样（PL）。尖峰同步随调制深度的增加而增加。在最低调制深度（6%）下，对于所有测试的单元类型，仅在接近单元最佳调制频率的地方观察到显著的尖峰同步。因此，调制调谐随着调制深度的降低而变得更加尖锐。为每个单独的单元计算了作为调制频率函数的 AM 检测阈值。斩波器单元的 AM 检测阈值明显优于初级样单元。在 40dB 时 AM 检测阈值比纯音尖峰率阈值高 10dB 时明显更差。在最佳调制频率下，高于纯音尖峰率阈值 10dB 的声音的平均调制检测阈值（95%置信区间）分别为 PL 单元 11.6%（10.0-13.1），CT 单元 9.8%（8.2-11.5）和 CS 单元 10.8%（8.4-13.2）。豚鼠 VCN 单单元中最敏感的 AM 检测阈值与人类心理物理性能（约 3% AM）相似，而神经测量阈值接近整个动物行为性能（约 10% AM）。
