Introduction

Dementia is an umbrella term for progressive neurocognitive disorders that impair memory, thinking, language, reasoning, and other cognitive abilities. The progressiveness of cognition decline will further lead to impaired daily functioning and independence and decreased quality of life (QoL). Apart from the negative impact on people living with dementia’s individuality and life, dementia takes a toll on caregivers by placing care burdens on them. The World Health Organization (WHO) classifies it as a public health priority, considering its global prevalence and profound impact on people living with dementia and caregivers [1]. Globally, it affects over 55 million people and witnesses 10 million new cases each year, with the figure estimated to hit 139 million by 2050 as populations age [2]. Notably, while age-specific dementia incidence has declined in some Western countries due to improvements in healthcare and lifestyle, the absolute number of people living with dementia continues to rise [3]. Among the modifiable risk factors identified by the Lancet Commission, social isolation in later life is a significant contributor to dementia risk [3]. This highlights the critical importance of fostering social connectedness as both a preventive and therapeutic strategy.

Existing interventions for people living with dementia can be divided into pharmacological and nonpharmacological ones. Pharmacological treatment arguably demonstrates marginal efficacy (e.g., 1.5-ADAS-Cog-point improvement in cognitive function with donepezil) [4], significant adverse effects such as somnolence, strokes, falls, and pneumonia [5,6], which are strong predictors of poor QoL [7,8]. The minimal benefits and the variety of potential adverse effects highlight the urgent need for nonpharmacological interventions in dementia treatment [9].

Within the broad spectrum of nonpharmacological approaches, psychosocial interventions play a pivotal role. These interventions shift the focus of treatment from neurochemical pathways to psychosocial, social, and behavioral strategies to enhance well-being, preserve function, and reduce distress. Such interventions are characterized by structured routines (individualized or group-based), where emotional regulation, person-centered care, and people living with dementia’s engagement are prioritized [10]. The effects of psychosocial interventions have been proven with evidence. Specifically, sustained effects have been found in cognition [3,11,12], emotion and psychological well-being [13,14], behavior symptom management [10,15], social engagement and quality of life [11,12,16].

According to Cohen-Mansfield et al., people living with dementia’s engagement in constructive and meaningful activities underpins the possibility of these interventions taking effect [17]. They investigated the mechanism of engagement and proposed that engagement is affected by environmental attributes, person attributes, and stimulus attributes, as well as the interactions between these factors [17]. In this sense, interaction facilitates engagement, emphasizing the dynamics between these factors. Similarly, Blumer discussed the notion of interaction and suggested that it is a dynamic process where individuals interpret and assign meaning to each other’s actions through symbols (e.g., language, gestures) [18]. The reciprocal process of interaction is measurable by capturing eye movements, facial expressions, verbal feedback, and physical movements [17]. In dementia care, person-centered interaction is prioritized, which upholds the personhood of people living with dementia and promotes empathy, respect, and emotional connection [19]. Integrating the above interpretations of interaction, interventions can be categorized by person-environment interaction, person-caregiver interaction, person-stimulus interaction, and group-based interaction.

Person-environment interactions, including sensory stimulations, modify the physical or sensory environment to reduce agitation. Whear et al. [20] reviewed that spending time in the garden reduces levels of agitation among care home residents, and Van Maanen et al. [21] found that bright light therapy is effective for sleeping problems. Similarly, the more dynamic type—the interaction between people living with dementia and caregivers elicits holistic benefits. For example, individual reminiscence therapy, which fosters trust and emotional connection between people living with dementia and the caregiver, reported benefits in mood, well-being, and cognitive functions [22]. Stimulus-driven interactions feature a variety of triggers, including music, art, social robots, and animals, during which people living with dementia are expected to make meaningful responses to triggers. Such interactions can be sensory (e.g., animals and social robots) and cognitive (e.g., music), contributing to improved behavioral and psychologic symptoms [23], decreased stress and anxiety [24,25], and improved quality of life [25]. The last type of intervention, group-based interaction (e.g., cognitive stimulation therapy), provides a platform where people living with dementia can communicate with fellow participants or caregivers. Group discussions and synchronized movements in these interventions promote social cohesion and shared experiences in structured settings (Spector et al., 2003), improve social skills, reduce loneliness [12], and stimulate collaborative cognition [10].

The communicative feature of these interactions is beneficial to dementia treatment. Communication problems facing people living with dementia manifested as expressive language deficits (e.g., anomia) [26], receptive language challenges (e.g., impaired comprehension) [27], impaired pragmatic and social communication ability [28], repetitive speech [29], and nonverbal communication declines [30]. The variety of psychosocial therapies provides multimodal interactions for people living with dementia to maintain or improve their communication ability. For example, person-environment and stimulus-driven interactions elicit nonverbal communication by changing physical environments (e.g., light) or providing tangible objects. Moreover, group-based interactions can inspire social communication and language comprehension. Involving in interactions with the environment, stimulus, peers, and caregivers, people living with dementia are encouraged to express themselves verbally and nonverbally, as well as understand peers and caregivers.

Apart from the benefits in communication problems, interactions bear the potential to improve dementia care outcomes. Meaningful interactions can promote people living with dementia’s psychological well-being [31,32] as well as physical well-being [33,34]. Specifically, to evaluate the effects of social interaction in a cognitive training program, they were randomly assigned to a high social interaction (HSI) group and a low social interaction (LSI) group while receiving cognitive training [34]. Their study suggests that social interaction (i.e., open-floor communication with experimenters) is positively related to people living with dementia’s cognitive outcomes and quality of life (QoL). However, the settings of interactions investigated were in general dementia care, focusing on the attitudes and communications of nursing staff. Moreover, existing research on the efficacy of interactions was primarily based on qualitative analysis (i.e., interviews of nursing staff), lacking statistical reasoning of the relation between interactions and outcomes. Therefore, there is a dearth of studies on interactions taking into account the mutual performances of caregivers and care recipients (i.e., people living with dementia) in therapeutic settings and conducting quantitative analysis on the relation between interactions and treatment outcomes.

Previous studies measuring interactions in dementia care focus on communications between people living with dementia, caregivers, and healthcare professionals [35–37]. Specifically, Williams et al. [37] developed an observation scale tackling the communication problems between people living with dementia and their caregivers. According to their work, the Verbal and Nonverbal Interaction Scale-CR (VNVIS-CR) takes into account caregivers’ verbal and nonverbal communicative behaviors and categorizing these behaviors into sociable and unsociable types. It provides a holistic guide to measuring caregivers’ behaviors but oversimplifies people living with dementia’s communicative performance, as verbal outputs are measured in terms of frequencies without further classification. Comparably, Mabire et al. [36] paid more attention to people living with dementia’s interaction behaviors and developed a multimodal scale to observe how they communicate with residents as well as care staff from visual, verbal, and facial perspectives. Meanwhile, interaction measures in structured psychosocial activities vary across different settings. Therapies including robots and animals as stimuli have spotlighted both people living with dementia’s verbal and nonverbal performance [38–40] while interventions concerning music [41], communication [42], and group-based activities [43] paid more attention to their verbal outputs. The majority of interaction measures have been examined quantitatively [38–42], where frequencies and durations of their performance were recorded. Apart from quantitative studies, Orfanos et al. [44] probed into interactions in group-based cognitive stimulation therapy qualitatively, which revealed participants’ perception of interaction.

Given the diversity of psychosocial interventions, interaction measures have taken different focuses to align with different treatment goals. A systematic synthesis of existing interaction measures can facilitate a comprehensive understanding of interactions in dementia care and the efficacy of various approaches. To have a better understanding of interactions in a wide range of therapies, this study aims to give a systematic review of the existing interaction measures applied in empirical studies and tackle the following questions:

1. What are the nature and characteristics of the studies in which interaction with people living with dementia was measured (e.g., type of intervention, characteristics of participants)?
2. Which aspects of interactions are measured (e.g., verbal or nonverbal)?
3. How were interactions measured in different therapeutic settings (e.g., parameters; qualitative or quantitative in nature; linguistics or pragmatic; coding systems; framework/analytic methods used)?
4. Did these studies try to investigate the relation between measures and outcomes? If yes, further investigation will be carried out to describe the relationship between measures and outcomes.

Methods

This paper will examine the existing research on psychosocial interventions for people living with dementia where participants’ interactions in corresponding therapeutic settings were measured. A comprehensive review will be conducted to include quantitative studies and qualitative studies with empirical evidence. The review follows the Preferred Reporting Items for Systematic Reviews and Meta‐analyses (PRISMA) guidelines (S1 File) [45]. The study protocol has been registered at the Campbell Systematic Reviews (cl2.20250120).

Supporting information