Progressive loss in episodic memory is a risk factor for cognitive decline and dementia and a core criterion for Alzheimer’s disease (AD; Albert et al., 2011; McKhann et al., 2011). According to Tulving (2002), episodic memory refers to the memory of past and personal experiences that are spatially and temporally anchored (see also Renoult and Rugg, 2020 for a theoretical review). It allows us to re-experience past events with vivid details but also to project ourselves into future events (Schacter et al., 2007). This capacity to experience the passage of time and to navigate in time, also known as mental time travel, contributes to a sense of identity and self-continuity (Conway, 2005). Consequently, besides experiencing difficulties in recollecting past events, the decrease in episodic memory abilities in AD leads to other disorders. It is currently well-documented that people with AD may experience disruptions in the sense of self (El Haj et al., 2019), anxiety regarding future events (Kloszewska, 1998), and deficits in time conceptualization (e.g., interval judgment and recall of past events; Requena-Komuro et al., 2020).

Concerning the latter, several studies have shown how time conceptualization is characteristically affected in AD, using various tasks focusing on different time intervals. For instance, some of these studies assessed the ability to make temporal judgements about short intervals (i.e., seconds) and highlighted that AD participants show time distortions (i.e., overestimation or underestimation of time intervals), compared to healthy, aged participants (e.g., Carrasco et al., 2000; Papagano et al., 2004; El Haj et al., 2013). Other tasks focused on longer intervals (i.e., days, months, or years) and showed that people with AD experience difficulties navigating between different periods of life (i.e., difficulties apprehending the present or imagining the future and interferences of the past in the present; Shiromaru-Sugimoto et al., 2018). The Autobiographical Interview (AI; Levine et al., 2002) has been widely used to assess the ability to travel mentally in time through discourse production. In this task, AD participants must recall and recount personal events from different periods (their youth, their middle age, and their recent past), and imagine future events (Addis and Tippett, 2004; Addis et al., 2009; Irish et al., 2011, 2012a,b). Based on participant narratives, two categories of discursive details are generally extracted: internal and external detail. Details that are directly linked to the event are classified as internal details. They convey information about the time, place, or emotions that are related to the event and rely heavily on episodic memory processes. In contrast, external details provide semantic information that is not directly linked to the event (e.g., general semantic knowledge, metacognitive statements, etc.) and involve semantic memory processes specifically. These studies showed that AD participants have specific difficulties reliving past events and using past experiences to project themselves in the future, and that those difficulties are related to episodic memory impairment. Indeed, compared to control participants, they produce less internal details for each period, suggesting an impairment in mental time travel, while no such difference is observed for external details.

The combination of internal and external details in a narrative builds the temporal framework of the reported event, but other language markers such as verbal inflection and temporal adverbs can also be used to express the temporality of the event in a more precise way and testify to mental time travel. Language through time reference (i.e., the linguistic expression of time) is indeed an important way to express concepts of past, present, and future that are related to every human experience (Benveniste, 1974; Harner, 1982; Zhang and Hudson, 2018). From a linguistic perspective, time reference concerns three main categories: tense and grammatical aspect (which are grammatical categories), and lexical aspect (a lexical category). These categories are essential to organize the localization of events on the timeline (Grisot, 2018), indicating when and how the events took place. While tense refers to when the event took place (i.e., before, at the same time, or after the time of speech, referring, respectively, to a past, present, or future event; Reichenbach, 1947), aspect indicates how the event took place. More precisely, grammatical aspect corresponds to the degree of completion of an event, which can be finished (perfective aspect) or ongoing (imperfective aspect; Comrie, 1976). In many languages, in particular tensed languages like French or English, tense is principally expressed by verb tenses through inflectional morphology (e.g., simple past, simple present, future, etc.) and temporal adverbials (e.g., yesterday, now, in 5 years, etc.). Grammatical aspect is also expressed by inflectional morphemes bound to verbs (e.g., walked_perfective vs. was walking_imperfective) and perfective or imperfective adverbials (e.g., last Monday vs. every Monday; Vetters, 1996; Grisot, 2018). Another linguistic device to convey aspect is lexical aspect, which refers to the temporal profile of the internal course of an event (Vetters, 1996; Grisot, 2018). It is related to verb semantics and to their argument structure, independent of marking for tense and grammatical aspect. For instance, following classification of Vendler (1957, 1967), verbs can be categorized as an activity, an achievement, an accomplishment, or a state depending on the use of the verb (e.g., “run” is an activity but “run a mile” is an accomplishment, depending on whether the final boundary of the situation—its end—is expressed or not; see Grisot, 2018 for more details about the differences between the four classes of lexical aspect in English and in French). Analyzing the use of morphological marking of tense and grammatical aspect on inflected verbs, classes of verbs (i.e., lexical aspect), and temporal adverbials in language production and comprehension could then be an effective way of assessing time processing abilities.

Interestingly, many studies focused on the processing of functional categories expressed by verbal morphology (such as tense, grammatical aspect, lexical aspect, and agreement) in AD. Using for instance sentence-completion tasks (Fyndanis et al., 2013, 2018; Manouilidou et al., 2020) or connected speech (e.g., Kavé and Levy, 2003; Sajjadi et al., 2012; see Auclair-Ouellet, 2015 for a systematic review) these studies aimed to specify the extent of morphological impairment in AD. Results revealed that all functional categories are impaired in AD participants, but not all to the same extent. Indeed, grammatical aspect is more impaired than tense and agreement (Fyndanis et al., 2013), and tense is more impaired than agreement (Fyndanis et al., 2013, 2018).

Psycholinguistic models of inflectional morphology generally explain deficits in terms of rule-processing impairments, phonological-encoding impairments, lexical-retrieval impairments, or reductions in cognitive resources (e.g., Ullman et al., 1997; Pinker, 1998; Faroqi-Shah and Thompson, 2004, 2007; Bastiaanse et al., 2011). However, studies of patients with post-stroke aphasia and primary progressive aphasia (PPA) suggest that those difficulties may be related to semantics, and more specifically to time conceptualization (e.g., Bates and Wulfeck, 1989; Auclair-Ouellet et al., 2016; Mack et al., 2021). Because it is known that semantic difficulties in AD become more severe with the progression of the disease, the question of whether time reference impairments may emerge as a result of time conceptualization impairments can be asked.

The relationship between time conceptualization and morphosyntactic expression of time is best understood through models of connected speech production. According to some models, building a temporal framework is essential to tell a coherent and complete story, but also to communicate efficiently. Telling a story requires the ability to recall an event and to create a mental representation of the situation described to communicate it verbally (Zwaan, 2008). Several dimensions are needed to create a complete event representation, including time (Zwaan et al., 1995a,b; Zwaan and Radvansky, 1998). When telling a story, the storyteller has to construct a temporal framework to ensure the listener understands the story. Because the temporal course of the story is not always continuous, as time shifts are quite common in narratives (Zwaan, 1996, 2008), the storyteller needs to go back and forth in time mentally to construct an event representation. Furthermore, to make sure that the listener will be correctly situated within the temporal framework and will understand the story, the storyteller also needs to give appropriate linguistic cues, such as verb tenses and temporal adverbials. As mentioned above, grammatical aspect and tense are more impaired than agreement in AD. Differences in the degree of impairment for different functional categories shed light on their underlying processes, as tense and grammatical aspect demand the integration of conceptual information in addition to grammatical information (Fyndanis et al., 2012). Likewise, the subjective and speaker-related dimensions involved in grammatical aspect processing (Fyndanis et al., 2012), which are absent for tense and agreement, may compromise temporality processing in AD. Since morphosyntactic marking of temporality relies on subjective dimensions associated with semantics and conceptual processes, it may be impaired in AD. These considerations raise the question of the extent to which mental representations of an event are integrated in memory (Zwaan, 2008) and then translated into language. In this perspective, time reference impairments may be related to time conceptualization impairments in AD.

In sum, time conceptualization impairments may manifest themselves as difficulties in mental time travel and in expressing temporality using lexical-semantic and morphosyntactic devices. However, the relationship between these two impairments in Mild Cognitive Impairment (MCI) and AD remains unclear. The goal of the present study is to perform a systematic review of the literature on the relationship between mental time travel impairments and time reference impairments in MCI and AD.

This systematic review is based on the guidelines proposed by the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA; Page et al., 2021a). Using the Population-Intervention-Comparison-Outcomes (PICO) tool, the following research question was formulated: what influence does the ability of Alzheimer’s disease patients to conceptualize time have on their production and comprehension of time reference (tense and grammatical aspect via verbs and adverbs)?

This research question guided the choice of electronic databases, search strategy, study selection, and data extraction.

The initial literature search in the three electronic databases yielded 3,209 records. The updated literature search performed in the same electronic databases resulted in 44 new records for Medline, 78 new records for Scopus, and five new records for PsychInfo, for a total of 3,336 records. Duplicates and records marked as ineligible by automation tools were removed, and 1,129 records remained. Screening of titles and abstracts resulted in 25 records. Two articles met the inclusion criteria after full-text reading. One additional study, which also met the inclusion criteria but was not identified by databases, was manually included. A flow diagram illustrating the identification and selection of the studies is provided in Figure 1.

The goal of this systematic review was to explore the potential link between the abilities in time conceptualization (e.g., mental time travel, episodic memory, and interval judgments), and in time reference (e.g., verbal inflection and temporal adverbials) in MCI and AD. Only three studies, met all selection criteria and were included in the review (Ahmed et al., 2013; Irish et al., 2016; Seixas-Lima et al., 2020). Ahmed et al. (2013) assessed time conceptualization in AD with subtests of the CAMCOG (Roth et al., 1986) focused on orientation, and remote and recent memory while Irish et al. (2016) and Seixas-Lima et al. (2020) used a modified version of the AI and analyzed the number of internal vs. external details. Concerning time reference measures, Ahmed et al. (2013) analyzed the proportion of verbs with inflection through connected speech with a picture description task, Irish et al. (2016) analyzed the number of present tense and past tense verbs produced by participants in their narratives, and Seixas-Lima et al. (2020) reported the proportion of past and present tense inflection. Even though both categories of measures of interest for our review were present in these three studies, only Irish et al. (2016) and Seixas-Lima et al. (2020) investigated the potential link between them. Interestingly, while Irish et al. (2016) found that AD participants produced fewer inflected verbs during the production of internal details compared to control participants, Seixas-Lima et al. (2020) found no difference in verbal inflection between aMCI and control participants. One reason for these different results in verbal inflection could be the nature of the different cognitive profiles in AD and aMCI participants. Indeed, in the study of Seixas-Lima et al. (2020), aMCI participants produced more external details (i.e., semantic information) compared to control participants, which may indicate that they still have difficulties to access episodic information and then to travel mentally in time. Although these difficulties are present, they may not yet be visible in verbal inflection (i.e., time reference), but may become so with the progression of the disease.

The results of this systematic review are quite surprising. Indeed, people with AD present well-documented time conceptualization difficulties as well as time reference difficulties, whose functional origins are widely debated (e.g., Fyndanis et al., 2013, 2018). Yet, very little work made connections between these two facets of time processing. We believe the likely reasons behind this gap in the literature are two-fold: firstly, the existence of separate lines of inquiries and research fields, and second, the emphasis on the dissociations between regular and irregular verbs in studies of morphology.

Each of our actions is temporally anchored and represented on the mental timeline. While autobiographical memory and mental time travel enable us to relive past events or to imagine future ones, language is an important way of conveying experiences and observing mental time travel processes in others. The number of internal and external details in narrations and time reference (verbal inflection and temporal adverbs use) are two language indicators of mental time travel which can both be impaired in AD. Even if mental time travel and time reference difficulties could be related in AD, the literature published so far is not conclusive. Considering the significant overlaps in the understanding of temporality as it is experienced in mental time travel and expressed in language, studies investigating the role of time conceptualization in time reference in people with AD are warranted.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

ES, NA-O, and MF contributed to conception and design of the study. ES organized the database. ES and MS analyzed the data. ES wrote the first draft of the manuscript. MS, CG, and NA-O wrote sections of the manuscript. All authors contributed to the article and approved the submitted version.

This work was supported by the Swiss National Science Foundation (grant number 10001F_197862 attributed to MF).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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