In this study, the THINC-it tool was used as a quick, easy-to-use, and practical screening tool to assess MDD adults’ daily cognitive functions. Previous studies have shown that the corresponding clinical factors affect patients’ cognitive abilities [21]. Thus, a transversal naturalistic study was conducted to highlight different cognitive profiles of patients with specific clinical symptoms of depression.

Objective and subjective cognitive impairment exists in MDD

A previous study found that cognitive symptoms were pervasive and that MDD patients had impaired cognitive domains such as reduced executive functioning, attention, memory, learning, psychomotor speed, and verbal processing [4, 22, 23]. In the THINC-it tool assessment, a difference between objectiveand subjective cognitive impairment was found in depression, and subjective cognitive impairment was significantly correlated with social dysfunction and anxiety [24, 25]. Previous studies also found that subjective cognitive impairments improve more quickly when depression symptoms are relieved early [26]. Compared to the healthy controls, the MDD patients had significantly different THINC-it total scores and Spotter, Codebreaker, Trails, and PDQ-5-D scores, but no differences were found in the Symbol Check in our study. The results of the present study were consistent with these previous studies and showed that MDD patients had impairments in most cognitive domains, including attention/vigilance, processing of speed, psychomotor speed, executive function, and subjective cognition. However, we did not find evidence of impaired working memory. The observed differences might partly be explained by the use of Symbol Check scores to assess dual tasks. Most of the study subjects, including healthy controls, were easily distracted by multivariate tasking, resulting in lower scores that were indistinguishable.

Age and age at onset should be considered predictors of objective cognitive impairment

We found that age and age at onset were associated mainly with impairment in cognitive domains such as attention/alertness, processing speed, working memory, and executive function. Previous literature reported that the aggravation of cognitive impairments was associated with increasing age [27], however, the difference remained in the domain of specific cognitive impairment. Rapp et al. [28] showed that depressed patients in both the early-onset and late-onset groups had cognitive impairment, but the late-onset patients demonstrated a wider range of cognitive impairment, with more severe symptoms. From the biological point of view [29], one probable explanation might be the association with increased incidence of vascular disease and the presence of more severe cognitive impairments in elderly patients. Additionally, the results of this study support the difference between objective and subjective cognitions. We chose the THINC-it as a cognitive screening tool due to its wide applicability and found that age and age at onset should be considered predictors of objective but not subjective cognitions, which was consistent with the findings of Srisurapanontet et al. [30]. Neurobiologically, these findings seem rational because objective cognition is mainly determined by brain functions, which are superior to those of younger individuals in terms of physiological functions. However, a previous study indicated that age and severity of depression might predict differences between objective and subjective cognitions [31]. Thus, these observations should still be confirmed in further studies.

The severity of depressive symptoms and sleep problems were associated with impaired working memory, information processing speed, and executive functioning

In this study, the presence of anxiety symptoms did not appear to be associated with subjective or objective cognitive impairments, but we found that an increase in depression severity was associated with impaired working memory, processing speed, and executive function but not with subjective cognitive impairment, which was concordant with the findings of Cha et al. [20]. However, some scholars held contrasting views and reported a correlation between the severity of depression and subjective cognitive impairment but not with objective cognitive impairment [32,33,34] and even suggested that the severity of depression was not affected by the performance of the whole cognitive field [35]. On the one hand, the obvious limitations of these works were the small sample size and limited neuropsychological tests, while on the other hand, patients enrolled were generally young, had full-time employment, believed cognitive impairment was equivalent to IQ damage, and subjectively refused to acknowledge the decline in work and social function caused by illness. Patients’ stigma is influenced by their cultural background. As a result of the introversion of traditional Chinese culture, Chinese people are used to suppressing their emotions and desires, often asking people to self-absorptionor and self-demand, but not asking others or trying to change the external environment. However, Western culture have an extroverted nature and are more open to change the environment. Consequently, whether different cultural backgrounds differentially affect subjective and objective cognitive impairment in depression still needs further research. In addition, there is consensus that sleep disorders impair cognitive functions despite the unclear underlying mechanisms. In the present study, we found that sleep problems were an important risk factor for cognitive impairment but did not distinguish between subjective and objective cognitive functions. Some previous research supported this conclusion [36, 37]. Our study used a cross-sectional design and collected only a few data relevant to cognitive impairment. In a recent study [20], it was reported that both sleep problems and depression severity independently predicted subjective cognitive impairment, therefore indicating that these should be viewed with caution and need further confirmation.

Education as a protective factor in the objectivecognitive domains of processing speed

Our results indicated that education was a protective factor in the objective cognitive domains of processing speed. The previous conclusion indicated that the degree of education represented some level of intelligence and could partially counteract cognitive impairment [38]. In addition, there is evidence showing that the estimated education and intelligence level are protective factors against potential cognitive dysfunction or may even compensate for cognitive competence when the damage has already occurred [39]. Thus, activities such as increasing reading hours, playing logical games, or learning new skills designed to increase patients’ cognitive reserve might be beneficial. In contrast, there is still a need for further validation regarding the use of the THINC-it in patients with low levels of education.

Limitations

The current study also had some shortcomings. First, only patients from the outpatient department were recruited, hence, the current findings may not be generalized to all MDD Chinese patients. Second, some factors potentially influencing the results, such as occupation and medication, were not taken into account. Third, as this was a cross-sectional study, the longitudinal relationship between cognition, depressive symptoms, and other functional outcomes need further investigation.