Interest in motivation in schizophrenia has increased considerably in recent years (Gard et al., 2009; Kremen et al., 2016). Most studies have defined motivation as a core negative symptom in schizophrenia that is related to poor functional outcome (Fervaha et al., 2015; Foussias et al., 2015). One of the most recent conceptualizations delineates two possible sub-domains of negative symptoms in schizophrenia: diminished expression and motivational deficits (Blanchard and Cohen, 2006; Foussias and Remington, 2010; Messinger et al., 2011). Diminished expression involves blunted affect and poverty of speech, while motivational deficits refer to a decrease in goal-directed behavior and the associated internal processes, such as curiosity, interest, and drive, which prompt the individual to plan, initiate, and pursue activities (Andreasen, 1982; Nakagami et al., 2008). The latter domain typically involves avolition/apathy and anhedonia/asociality (Kelley et al., 1999; Kimhy et al., 2006), and it is usually referred to as apathy (Hartmann et al., 2015) or avolition (Foussias and Remington, 2010).

For many years, it was thought that patients with schizophrenia failed to pursue goal-directed activities because they did not find such activities enjoyable, and thus anhedonia was considered to be a key negative symptom of schizophrenia (Strauss, 2013; Strauss et al., 2014). However, recent findings question the presence of anhedonia in schizophrenia because it has been found that individuals with this illness report intact consummatory pleasure, comparable to that of healthy controls (Horan et al., 2006; Foussias and Remington, 2010; Strauss and Gold, 2012). Despite these preserved hedonic responses, however, it is clear that individuals with schizophrenia less frequently engage in motivated behaviors aimed at obtaining rewards and pleasurable outcomes (Myin-Germeys et al., 2000). In this context, a prominent line of research has indicated that motivational deficits can reflect disordered processing of rewards and value-based decision-making (Juckel et al., 2006; Strauss et al., 2013, 2014). It is also possible that the motivational deficits of patients with negative symptoms reflect low expectations about performing activities successfully (Bentall et al., 2010). Indeed, from the perspective of the cognitive model (Rector, 2004; Rector et al., 2005) it has been suggested that dysfunctional attitudes such as defeatist performance beliefs contribute to the development of negative symptoms and, consequently, to a lack of goal-directed behavior. Whatever the case, the study of motivation in schizophrenia is clearly important because it has been shown to have a significant influence on areas such as psychosocial functioning (Nakagami et al., 2010), quality of life (Buck and Lysaker, 2013), work outcome (Reddy et al., 2016), and cognitive function and treatment adherence (Campellone et al., 2016; Fiszdon et al., 2016).

Despite the long history of research on motivation in schizophrenia and the volume of literature available, there have been few reviews and bibliometric studies of this topic. The reviews which have been published have focused on the main measures used to explore apathy (Bortolon et al., 2017), the association between reward processing and motivational impairment (Strauss et al., 2014), or developments in the concept of apathy (Del-Monte, 2013). As for bibliometric studies, these have examined trends in the literature on antipsychotics (López-Muñoz et al., 2015), production of neurocognition literature in schizophrenia (Guilera et al., 2012), production of schizophrenia literature compared with the total medical literature (Theander and Wetterberg, 2010), and patient-centered medicine (Calton et al., 2009).

Given the relevance of motivational deficits to recovery and their association with negative functional outcomes, such as the inability to secure employment or to conduct normal activities of daily living (Cardenas et al., 2013), we believe it is important to examine, through an exhaustive search and bibliometric methods, the different terms used in studies related to motivation in schizophrenia. Bibliometric studies are useful tools for systematically assessing and analyzing research publications, and hence for evaluating the social and scientific relevance of a given discipline or field (Koskinen et al., 2008). Typically, a bibliometric study analyzes the evolution of scientific production though different indicators, including: quantity indicators, which measure the productivity of a particular researcher or research group; structural indicators, which measure connections between publications, authors, or research fields; and performance indicators, focused on the quality of a journal, researcher, or research group (Durieux and Gevenois, 2010). Accordingly, the aim of the present study was to identify current trends in, and the scope of, research on motivation in schizophrenia and to explore both how these trends have evolved over time and the relationship between them. To this end, we present descriptive information regarding the studies identified, including sample characteristics, terminology, and the instruments used to examine motivation. The value of a study of this kind is that it can illustrate the current research efforts being made in the field and help to achieve a better understanding of the main processes related to motivation. The qualitative information provided and the comprehensive picture obtained of the field of study make it a robust and effective complement to expert appraisals in this field.

Studies were compiled from the Web of Science platform by searching in the Medline and Web of Science TM Core Collection databases. These databases were chosen due to their broad and encompassing content regarding severe mental illness, including the characteristics, functioning, and recovery of patients. The study was conducted in accordance with the PRISMA Statement on preferred reporting items for systematic reviews and meta-analyses (Moher et al., 2010), as described below.

We have revised the main trends in the study on motivation in schizophrenia through exhaustive examination of the literature and bibliometric methods. Results showed a significant growth in the number of studies, especially in recent decades, with 20.49% of the studies included in this study being published in 2015. Since the late 1990s, the increase in the number of published studies on motivation in schizophrenia has followed exponential and logistic curves, reflecting the considerable and growing interest in this field (Price, 1963). However, if studies are analyzed according to the different motivation terms they use, then the upward trend in productivity does not apply equally to them all, with the exception of RR, which attracted particular interest in the scientific community between 2009 and 2016 (see Table 6), even though the term has been present for a period of 17 years of research.

The sheer scale of expansion challenges researchers' capacity to conduct a comprehensive analysis of the available motivation terms. Moreover, the concepts and definitions of motivation are complex, interdependent, and interdisciplinary (see Figure 3), yielding publications that may be grouped into three broad areas of interest, namely, neuroscience, psychology, and psychiatry. As evidenced by the results, research has focused particularly on the term “reward” as a means of understanding motivational impairments in schizophrenia (Barch and Dowd, 2010), providing important insights into the cognitive and neural mechanisms associated with these impairments (Strauss et al., 2014). This interest in neural mechanisms is clearly linked to the development of new brain techniques for studying RR, even though behavioral tasks continue to be the most widely used approach (see Table 7). In fact, the most used task to assess reward processing was the Effort Expenditure for Reward Task (EEfRT, 2009), a computerized effort-based decision-making task in which participants are given an opportunity on each trial to choose between two different task difficulty levels in order to obtain monetary rewards: a high effort option and a low effort option (Treadway et al., 2009, 2015).

Self-reports are the preferred instrument in the literature for assessing other associated terms in the study of motivation, such as IM, AP, SE, and AV. The widespread use of questionnaires may reflect the ease with which they can be attained and applied or may imply that there is greater confidence in psychometric measures than in neurophysiological or behavioral tools for assessing motivation. It should be noted, however, that some inconsistent results have emerged when using self-reports. For example, Barch et al. (2008) found that people with schizophrenia did not differ from participants without schizophrenia in two intrinsic motivation domains, as measured by the Motivational Trait Questionnaire (MTQ; Heggestad and Kanfer, 2000). However, Choi et al. (2012) found that patients with schizophrenia reported less IM than did healthy controls when rating their motivation using the Intrinsic Motivation Inventory (IMI) adapted for people with schizophrenia. These results illustrate how two instruments measuring the same construct are not properly adapted for people with mental disabilities like schizophrenia. In this respect, some authors have recently highlighted that only a small number of motivation self-reports are appropriate for patients with severe mental illness (Cooper et al., 2015). Despite this, there has been an increase in the use of different and new questionnaires and behavioral tasks to measure motivation. In addition, the set of terms used to refer to motivation has increased in recent years, so not only are more instruments required to measure different and specific motivational processes, but further efforts are needed to adapt these measures to people with mental disorders and their specific cognitive disabilities.

The least widely used concept in the study of motivation in schizophrenia is AV, the first study of which was published in 2010, the same year as the review by Foussias and Remington (2010), who in discussing two sub-domains of negative symptoms used the term “avolition” to refer to the motivational deficits observed in schizophrenia. However, this term has been slow to catch on, and AP seems to be preferred, even though AP and AV are often used as synonyms in the literature on schizophrenia (Bortolon et al., 2017).

Our evaluation of worldwide trends in research productivity in the field of motivation in schizophrenia over 61 years of study indicates that North America produces the most reports: the USA and Canada were the countries with the greatest volume of published research over the period studied, jointly accounting for more than 50% of the total output inspected. Among the other continents, Europe made a significant contribution, with the UK and Germany being the largest producers.

Given that institutional, national, and international research collaborations are directly related with the number of citations a publication receives, and that collaborations therefore play a key role in the main research agendas (Gazni and Thelwall, 2014; Hassan and Haddawy, 2015), the general index of collaboration in the study of motivation in schizophrenia (only 15.84% of the studies included) suggests that authors do not establish sufficient contact with their international colleagues. Of the two most productive continents referred to above, Europe generated more collaborative studies than North America. The country with the greatest research output was the USA, which recorded the largest number of collaborations with other countries in absolute terms (20); however, among the four most productive countries, the UK recorded the highest rate of collaboration (70.58% of published studies), followed by Germany (30.00%), Canada (28.57%), and the USA (24.69%). These data suggest that scientific production is centralized by countries and that professionals might not be benefiting from sharing and integrating procedures and knowledge generated by other groups. This pattern of production may be impeding further development of this research area.

Consistent with the literature (Qasim, 2016), we identified a large number of publications with a small number of citations per publication, and a small number of publications with a very high number of citations (32 studies with 16 or more citations). This illustrates the interest shown by the scientific community in the work of a group of authors (primarily from the USA and Germany) who are investigating the mechanisms and processes of reward dysfunction, a topic that accounted for 56.25% of the highly cited publications. The network derived from the co-citation analysis by author indicates that motivation in schizophrenia is a focus of research interest for authors working in three distinct but closely related areas, namely Neurocognitive function cluster, Reward processing cluster, and Psychiatry and Clinical approach cluster. It can be seen in Figure 5 that the co-citation relationship is stronger between researchers working in the first two of these areas; however, all three areas are involved in the enhancement of the functional outcomes in schizophrenia

The network resulting from the co-citation analysis by journal shows a strong tendency for co-citation between the journals Schizophrenia Research and Schizophrenia Bulletin (see Figure 3). This is consistent with the results for journal productivity, since these two journals accounted for 30.02% of the total literature production; Schizophrenia Research was the most productive journal and Schizophrenia Bulletin was among the top five. The network derived from the co-citation analysis by cited reference (see Figure 4) shows that the output of these two journals forms two clusters which we label “Cognition” and “Reward/Neuroimaging,” this being consistent with the term most widely used in studies of motivation in schizophrenia, namely RR. Note that the most oft-cited reference is Kay et al. (1987) a publication that provides the basis for current conceptualizations of negative symptoms in schizophrenia and methods for assessing them. Similar to previous findings (Qasim, 2016; Flis and van Eck, 2017), the analysis of the co-occurrence of terms showed the terms that best capture the literature in our revised topic. Our analysis revealed two main clusters of terms (see Figure 6). The dense area in red (Functional assessment cluster) encompasses terms related to performance, assessment, and functional outcomes. The cluster in green (Reward processing cluster) has the term “reward” at its center. This central position, coupled with the size of the corresponding node, indicates that it is this term which best reflects the co-occurrence of terms, and underlines that reward is the main dimension studied by the publications included in this study.

Research production is centralized by the fact that the majority of articles are restricted to a small number of journals and a small group of researchers. Although it is often considered that psychology, psychiatry, and neuroscience studies are well connected, our co-citation analysis identified opportunities for greater inter-disciplinary collaboration with biological and neuroimaging approaches, and between authors. As has been noted elsewhere, this is important, as a failure to consider work from other disciplines could lead to partial or incorrect conclusions (Stuckler et al., 2015).

Due to the heterogeneity in measurement methods, research designs, and ouitcome variables, it was not possible to perform a meta-analysis or to calculate pooled effect sizes. Further, we restricted our analysis to quantitative studies in order to facilitate the codification and generalization of data, although qualitative studies can also provide useful insights. Despite these limitations, the present study highlights the need to use standardized instruments and terminologies when designing studies on motivation in schizophrenia. Moreover, the field will also benefit from an increase in the level of collaboration between the various approaches within the psychological and psychiatry fields, and hence between authors from different countries.

All authors certify that they have participated sufficiently in the work to take public responsibility for the content, including participation in the concept, design, acquisition of data and writing of manuscript draft (AN-G, JG-B), codification of data, analysis and writing of the final manuscript (VC, AN-G), and revision of the manuscript (JG-B, VC).