Scientists find out why aphasia patients lose the ability to talk about the past and future

(Press-News.org) An international team of researchers, including scientists from the HSE Centre for Language and Brain, has identified the causes of impairments in expressing grammatical tense in people with aphasia. They discovered that individuals with speech disorders struggle with both forming the concept of time and selecting the correct verb tense. However, which of these processes proves more challenging depends on the speaker's language. The findings have been published in the journal Aphasiology.

Aphasia is a severe speech disorder, often resulting from a stroke, in which individuals lose the ability to speak coherently. In particular, this can manifest as incorrect use of verb tenses, making it difficult for patients to talk about past or future events, significantly complicating everyday communication.

To investigate the origins of these difficulties, researchers from universities in Russia, Greece, Italy, the US, and Norway conducted an experiment. They hypothesised that tense expression impairments could stem from two distinct processes: encoding and retrieval. During encoding, a speaker forms the concept of time (for example, whether an action occurred in the past, present, or future). During retrieval, they select the correct verb form to express that concept. To understand the impact of each process, the scientists carried out experiments with aphasia patients speaking four different languages: Greek, Russian, Italian, and English. These languages were chosen because they structure verb tenses differently, allowing the researchers to examine how language-specific features influence encoding and retrieval of tense in aphasia patients.

To aid in diagnosis, the researchers designed two sentence-completion tasks. The first task asked participants to fill in blanks in sentences, requiring both processes—encoding and retrieval. They had to complete the sentence according to the model, considering the change in the tense form of the verb. For example: ‘Yesterday, the gardener watered the flowers. Tomorrow, the gardener will... the flowers.’ The second tasks expected participants to complete sentences without changing the verb tense. They were given the phrase ‘to water the plants’ and heard the example sentence ‘The gardener is currently collecting mushrooms.’ Then they were then prompted to begin a sentence with ‘The gardener is currently...’ and complete it with the phrase ‘watering the plants’ in the correct form, resulting in ‘is watering the plants.’

By comparing the results from these tasks, the researchers could determine whether the primary difficulties arose during encoding or retrieval.

The study revealed that most participants experienced impairments in both encoding and retrieval, but the severity of these issues varied depending on the language and the individual. For instance, Russian- and English-speaking participants struggled more with the retrieval task, while Greek- and Italian-speaking participants faced challenges primarily during encoding. Interestingly, difficulties in expressing time were selective. Some patients had trouble referencing the past, while others struggled with the future. 

Olga Buivolova

‘These findings are crucial for understanding how aphasia patients lose the ability to express time differently, depending on the characteristics of their language,’ explained Olga Buivolova, Research Fellow at the HSE Centre for Language and Brain and one of the study’s authors. ‘We can now better evaluate which aspects of time pose the greatest challenges for patients and begin developing more tailored therapeutic approaches.’

As researchers note, the main conclusions of the study may also have practical implications for neurorehabilitation. Firstly, this experimental method can help identify the underlying causes of difficulties with using verb tenses. This means that speech therapists and neuropsychologists will be able to work more thoroughly and effectively with patients on speech recovery.

Secondly, the study helps to understand how differences between languages can affect the symptoms of aphasia. This is important for developing standardised tests and methods that consider the specifics of a speaker's native language, ultimately leading to more accurate and comprehensive diagnosis of patients with aphasia.

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