Cortical Representation of Species-Specific Vocalizations in Guinea Pig

We investigated the representation of four typical guinea pig vocalizations in the auditory cortex (AI) in anesthetized guinea pigs with the aim to compare cortical data to the data already published for identical calls in subcortical structures - the inferior colliculus (IC) and medial geniculate body (MGB). Like the subcortical neurons also cortical neurons typically responded to many calls with a time-locked response to one or more temporal elements of the calls. The neuronal response patterns in the AI correlated well with the sound temporal envelope of chirp (an isolated short phrase), but correlated less well in the case of chutter and whistle (longer calls) or purr (a call with a fast repetition rate of phrases). Neuronal rate vs. characteristic frequency profiles provided only a coarse representation of the calls’ frequency spectra. A comparison between the activity in the AI and those of subcortical structures showed a different transformation of the neuronal response patterns from the IC to the AI for individual calls: i) while the temporal representation of chirp remained unchanged, the representations of whistle and chutter were transformed at the thalamic level and the response to purr at the cortical level; ii) for the wideband calls (whistle, chirp) the rate representation of the call spectra was preserved in the AI and MGB at the level present in the IC, while in the case of low-frequency calls (chutter, purr), the representation was less precise in the AI and MGB than in the IC; iii) the difference in the response strength to natural and time-reversed whistle was found to be smaller in the AI than in the IC or MGB.本研究以麻醉豚鼠为实验对象，探究四种典型豚鼠发声在其听觉皮层（Auditory Cortex，简称AI）中的表征情况，旨在将皮层数据与已发表的、针对皮层下结构——下丘（Inferior Colliculus，简称IC）和内侧膝状体（Medial Geniculate Body，简称MGB）中相同发声的研究数据进行对比。与皮层下神经元相似，皮层神经元通常也会对多种发声产生响应，其响应可与发声的一个或多个时域成分实现时间锁定。AI中的神经元响应模式与啭鸣（chirp，一种孤立的短音节）的声音时域包络相关性良好，但针对啁啾声（chutter，较长发声）、口哨声（whistle，较长发声）或呼噜声（purr，短语重复速率较快的发声），其相关性则相对较弱。神经元放电速率-特征频率分布图仅能粗略表征发声的频谱特征。将AI的神经元活动与皮层下结构的神经元活动进行对比后发现，针对单个发声，神经元响应模式从IC到AI的转化过程存在差异：其一，啭鸣的时域表征未发生改变，口哨声与啁啾声的表征在丘脑水平发生转化，而呼噜声的响应则在皮层水平发生转化；其二，对于宽带发声（口哨声、啭鸣），其频谱的放电速率表征在AI和MGB中得以保留，且与IC中的水平一致；而对于低频发声（啁啾声、呼噜声），AI和MGB中的表征精度不及IC；其三，相较于IC或MGB，AI中自然口哨声与时反口哨声的响应强度差异更小。