Southern Illinois University School of Medicine, , Department of Pharmacology, Springfield, IL, USA.
Department of Biological Sciences, Purdue University, West Lafayette, IN, USA.
J Physiol. 2019 May;597(10):2767-2784. doi: 10.1113/JP277450. Epub 2019 Apr 21.
Temporal imprecision leads to deficits in the comprehension of signals in cluttered acoustic environments, and the elderly are shown to use cognitive resources to disambiguate these signals. To mimic ageing in young rats, we delivered sound signals that are temporally degraded, which led to temporally imprecise neural codes. Instead of adaptation to repeated stimuli, with degraded signals, there was a relative increase in firing rates, similar to that seen in aged rats. We interpret this increase with repetition as a repair mechanism for strengthening the internal representations of degraded signals by the higher-order structures.
To better understand speech in challenging environments, older adults increasingly use top-down cognitive and contextual resources. The medial geniculate body (MGB) integrates ascending inputs with descending predictions to dynamically gate auditory representations based on salience and context. A previous MGB single-unit study found an increased preference for predictable sinusoidal amplitude modulated (SAM) stimuli in aged rats relative to young rats. The results suggested that the age-degraded/jittered up-stream acoustic code may engender an increased preference for predictable/repeating acoustic signals, possibly reflecting increased use of top-down resources. In the present study, we recorded from units in young-adult MGB, comparing responses to standard SAM with those evoked by less salient SAM (degraded) stimuli. We hypothesized that degrading the SAM stimulus would simulate the degraded ascending acoustic code seen in the elderly, increasing the preference for predictable stimuli. Single units were recorded from clusters of advanceable tetrodes implanted above the MGB of young-adult awake rats. Less salient SAM significantly increased the preference for predictable stimuli, especially at higher modulation frequencies. Rather than adaptation, higher modulation frequencies elicited increased numbers of spikes with each successive trial/repeat of the less salient SAM. These findings are consistent with previous findings obtained in aged rats suggesting that less salient acoustic signals engage the additional use of top-down resources, as reflected by an increased preference for repeating stimuli that enhance the representation of complex environmental/communication sounds.
时间不精确导致在嘈杂的声学环境中对信号的理解能力下降，研究表明老年人会利用认知资源来消除这些信号的歧义。为了模拟年轻大鼠的衰老过程，我们传递了时间上退化的声音信号，这导致了时间上不精确的神经编码。与对重复刺激的适应不同，随着时间的推移，退化信号的发射率相对增加，这与老年大鼠相似。我们将这种随着重复的增加解释为通过高级结构增强退化信号的内部表示的修复机制。
为了更好地在具有挑战性的环境中理解言语，老年人越来越多地使用自上而下的认知和上下文资源。内侧膝状体（MGB）将上行输入与下行预测结合起来，根据显著性和上下文动态地对听觉表示进行门控。先前的 MGB 单细胞研究发现，与年轻大鼠相比，老年大鼠对可预测的正弦幅度调制（SAM）刺激表现出更高的偏好。结果表明，年龄退化/抖动的上游声码可能会导致对可预测/重复的声信号的偏好增加，这可能反映了对自上而下资源的更多使用。在本研究中，我们从年轻成年 MGB 中的单位记录，比较了标准 SAM 与不太显著的 SAM（退化）刺激引起的反应。我们假设，退化 SAM 刺激会模拟老年人中看到的退化上行声码，增加对可预测刺激的偏好。从植入 MGB 上方的可推进四极管簇中记录了单个单元。不太显著的 SAM 显著增加了对可预测刺激的偏好，尤其是在较高调制频率下。较高的调制频率引起的反应不是适应，而是随着每次连续试验/重复出现不太显著的 SAM，发射的尖峰数量增加。这些发现与先前在老年大鼠中获得的发现一致，表明不太显著的声信号需要更多地利用自上而下的资源，这反映在对增强复杂环境/通信声音表示的重复刺激的偏好增加上。
