Abstract: Speech fluency and coherence are two important indicators for assessing age-related changes in speech production, representing the global and local processing of speech planning in older adults respectively. With increasing age, older adults often exhibit off-topic and declines in fluency. Previous studies have relied primarily on controlled laboratory experiments that predominantly focused on the vocabulary and sentence-level analysis, which may not fully capture the complexity of real-life speech situations. In addition, these studies have typically examined age-related changes in fluency and coherence independently without considering the interaction, leading to inconsistent results.
The term “natural language” refers to the spontaneous speech in real-life contexts. In the context of natural communication, the environment is characterized by complexity and variability, which may affect the quality of speech production in elderly individuals due to changes in their language abilities and cognitive functions. However, it is often overlooked that older adults engage in a trade-off between verbal fluency and coherence within natural situations, influenced by external factors and their own cognitive abilities. They assess their cognitive abilities, increase fluency by relying on familiar topics, and compromise global discourse coherence; alternatively, they may slow down the pace of conversation to allow for more time to plan and sustain social interaction. By employing these adaptive strategies, older adults demonstrate a discernible trade-off between fluency and coherence.
The trade-off and adaptive changes of the elderly individuals in discourse communication can be explained by the pragmatic change hypothesis at the social level and the inhibition deficit hypothesis at the cognitive level. According to the pragmatic change hypothesis, older adults exhibit changes in speech production due to their increased focus on conversational processes and the opportunities for interaction, aiming to achieve specific communication goals. Therefore, older adults must make trade-offs to maintain the quality of information exchange, sacrificing some fluency for a tightly focused discourse or deviating from the topic to ensure fluency in speech, which is closely related to individual communicative motivations and task demands. Individual cognitive function, particularly inhibitory control function, plays a crucial role in speech production. Inhibitory control skills help maintain conversation topics, update the content of speech, and facilitate transformation. The inhibition deficit hypothesis suggests that the older adults struggle with suppressing task-irrelevant thoughts and content, making them more prone to introducing unrelated topics during conversations. This leads to more irrelevant information entering working memory and consequently affecting coherent speech expression. At the same time, older adults may also reduce fluency by slowing their speaking rate to allow sufficient time to inhibit irrelevant information interference.
Previous studies have shown a significant overlap between the brain regions involved in the speech fluency task and those involved in the coherent propositional speech task. Specifically, bilateral frontal lobes, particularly the dorsolateral prefrontal cortex, inferior frontal gyrus, and angular gyrus, have been identified as shared neural substrates underlying the trade-off between speech fluency and coherence. Additionally, several distinct regions play crucial roles in either verbal fluency or coherence processing independently, such as the superior temporal gyrus and middle temporal gyrus involved in fluency processing, and the right dorsolateral prefrontal cortex involved in coherence processing.
There is a lack of systematic research on the dynamic pattern of fluent and coherent speech in older adults. In the future, it is necessary to go beyond the single evaluation standard of speech aging and use natural language processing technology to analyze the speech in interactive situations with brain imaging studies. The comprehensive exploration should include various factors influencing speech aging and investigate how older adults can effectively balance their limited cognitive resources to achieve optimal adaptation based on their cognitive abilities. These discussions will contribute to the understanding of the mechanisms underlying adaptive changes in speech aging and facilitate targeted improvement of speech production in older adults.

Key words: natural speech production, aging, speech fluency, speech coherence, trade-off

摘要： 言语流畅性与连贯性多被用于单独评价老年人的言语产生质量。伴随年龄增长老年人在自然言语产生中常常无法兼顾二者,或因社交动机提高流畅性而损失连贯性,或在任务需求下提高连贯性导致流畅性下降,表现为对流畅性和连贯性的权衡。语用改变理论和抑制损伤理论可以解释老年人权衡不同言语产生指标的动机和认知机制,双侧背外侧前额叶、额下回和角回作为言语流畅性和连贯性共享的神经基础支持两者的权衡,颞上回、颞中回与右背外侧前额叶则分别在流畅性和连贯性中发挥重要作用。未来研究应突破对言语产生老化的单一评价标准,区分不同老化阶段中言语产生的差异,进一步探索老年人言语加工的动态变化模式。

关键词: 自然言语产生, 老化, 言语流畅性, 言语连贯性, 权衡