On the Relationship between Theory of Mind and Syntax in Clinical and Non-Clinical Populations: State of the Art and Implications for Research
Elena Fontana1, Mauro Adenzato1, 2, *, Jacopo S. Penso1, Ivan Enrici2, 3, Rita B. Ardito4
1 Department of Psychology, University of Turin, Turin, Italy
2 Neuroscience Institute of Turin, Turin, Italy
3 Department of Philosophy and Educational Sciences, University of Turin, Turin, Italy
4 Department of Neuroscience, University of Turin, Turin, Italy
Over the years, different explanations have been given on the relationship between syntax and Theory of Mind, i.e., the ability to attribute mental states to others and predict, describe, and explain behavior based on such mental states. In the present study, we focus on the relationship between false-belief understanding as a crucial aspect of Theory of Mind, and on the ability to master the syntax of complementation, i.e., the ability to produce and comprehend sentences in a recursive way.
Our purpose is to test two main hypotheses on the relationship between false-belief understanding and the ability to master the syntax of complementation: the dependence and the independence of false-belief understanding on syntactic complementation.
We analyze studies on children with typical development, deaf children with deaf signing or hearing parents, children with specific language impairment, children diagnosed with autism spectrum disorder, longitudinal and training studies, and studies on adults with neuropsychological disorders.
Strengths and weaknesses of the two hypotheses are discussed and limitations of the current state of knowledge are presented. A lifespan approach taking into account both the emergence and maintenance of false-belief understanding and using both implicit and explicit false-belief tasks is proposed to face the issue discussed.
Keywords: Aphasia, Atypical development, Syntactic complementation, False-belief understanding, Syntax, Theory of Mind.
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* Address correspondence to this author at the Department of Psychology, University of Turin, via Po, 14-10123 Turin, Italy; Tel: +39.011.670.30.39; E-mail: firstname.lastname@example.org
Theory of Mind (ToM) is the ability to attribute mental states to others and predict, describe, and explain behavior based on such mental states [1Baron-Cohen S. Mindblindness: An Essay on autism and Theory of Mind 1995.-4Adenzato M, Brambilla M, Manenti R, et al. Gender differences in cognitive Theory of Mind revealed by transcranial direct current stimulation on medial prefrontal cortex. Sci Rep 2017; 7: 41219.[http://dx.doi.org/10.1038/srep41219] [PMID: 28117378] ]. ToM is thought to be at the core of any successful social interactions and communication process [5Cavallo M, Enrici I, Adenzato M. The comprehension of social situations in a small group of patients with frontotemporal dementia and Alzheimer’s disease. Acta Neuropsychol 2011; 9: 167-76., 6Poletti M, Enrici I, Adenzato M. Cognitive and affective Theory of Mind in neurodegenerative diseases: neuropsychological, neuroanatomical and neurochemical levels. Neurosci Biobehav Rev 2012; 36(9): 2147-64.[http://dx.doi.org/10.1016/j.neubiorev.2012.07.004] [PMID: 22819986] ], and one of its key components is the False Belief (FB) understanding, i.e., the understanding of another person’s wrong belief in relation to one’s own knowledge of the real state of affairs [7Wimmer H, Perner J. Beliefs about beliefs: representation and constraining function of wrong beliefs in young children’s understanding of deception. Cognition 1983; 13(1): 103-28.[http://dx.doi.org/10.1016/0010-0277(83)90004-5] [PMID: 6681741] ]. Accordingly, ToM is generally tested through classic FB tasks: these tests provide evidence that children can or cannot distinguish between the thoughts and feelings they themselves currently have with those that can be possibly held by others [8Douglas SK. False Belief Task. In: Goldstein S, Naglieri JA, editors. Encyclopedia of Child Behavior and Development. Boston, MA: Springer; 2011. pp 637-39.] (but see [9Scott RM, Baillargeon R. Early false-belief understanding. Trends Cogn Sci (Regul Ed) 2017; 21(4): 237-49.[http://dx.doi.org/10.1016/j.tics.2017.01.012] [PMID: 28259555] , 10Wellman HM, Cross D, Watson J. Meta-analysis of theory-of-mind development: The truth about false belief. Child Dev 2001; 72(3): 655-84.[http://dx.doi.org/10.1111/1467-8624.00304] [PMID: 11405571] ] for critical reviews on FB tasks). Although we know much about ToM and, in particular, about FB understanding, it is still not entirely clear which factors are involved in its development and maintenance. It is very complex to study social skills as separate from other abilities as the language, considering also that they develop around the same life-period [11Adenzato M, Garbarini F. The As If in cognitive science, neuroscience and anthropology: A journey among robots, blacksmiths, and neurons. Theory Psychol 2006; 16: 747-59.[http://dx.doi.org/10.1177/0959354306070515] -13Enrici I, Adenzato M, Ardito RB, et al. Emotion processing in Parkinson’s disease: A three-level study on recognition, representation, and regulation. PLoS One 2015; 10(6): e0131470.[http://dx.doi.org/10.1371/journal.pone.0131470] [PMID: 26110271] ]. Over the years, different explanations on ToM development have been given, but one of the most interesting is the relationship between FB understanding, and syntactic complementation, i.e., the embedding of one propositional argument under another proposition, as is needed for the expression of propositional attitudes such as beliefs and states of knowledge [14De Villiers JG, De Villiers PA. Linguistic determinism and the understanding of false beliefs. In: Mitchell P, Riggs KJ, editors. Children’s reasoning and the mind. Hove, UK: Psychology Press; 2000. pp. 191-228.]. Syntax is a mechanism that allows humans to understand and construct an infinite number of sentences from a finite number of elements and complementation is the scaffolding of this mechanism [15Chomsky N. Syntactic structures 1957., 16Vicari G, Adenzato M. Is recursion language-specific? Evidence of recursive mechanisms in the structure of intentional action. Conscious Cogn 2014; 26: 169-88.[http://dx.doi.org/10.1016/j.concog.2014.03.010] [PMID: 24762973] ].
Here we discuss the state of the art of the field and propose an interpretative structure presenting two main hypotheses on the relationship between FB understanding, as a key ability of ToM, and complementation, as a crucial aspect of syntax. To this end, studies on both children with typical or atypical development, longitudinal and training studies, and studies on adults with neuropsychological disorders are presented and discussed. The different samples mentioned above allow us also to refer to different dimensions, such as the development, maintenance and deficit of one or both the abilities taken in to account (FB and/or syntactic complementation)
2. THE TWO INTERPRETATIVE HYPOTHESES: THEORY OF MIND DEPENDENCE OR INDEPENDENCE UPON SYNTAX
A debated idea is whether we need syntax for the development of the ability to understand FB tasks. According to the literature, there are two main interpretative hypotheses on the relationship between ToM and syntax, i.e., that ToM depends on syntax or that ToM and syntax are independent.
As pointed out by the hypothesis of ToM development dependence upon syntax, FB understanding owes its development to a previously developed linguistic base (e.g., [14De Villiers JG, De Villiers PA. Linguistic determinism and the understanding of false beliefs. In: Mitchell P, Riggs KJ, editors. Children’s reasoning and the mind. Hove, UK: Psychology Press; 2000. pp. 191-228., 17San Juan V, Astington JW. Bridging the gap between implicit and explicit understanding: How language development promotes the processing and representation of false belief. Br J Dev Psychol 2012; 30(Pt 1): 105-22.[http://dx.doi.org/10.1111/j.2044-835X.2011.02051.x] [PMID: 22429036] ]). The proponents of this hypothesis argue that syntax and, more specifically, mastery in complementation, support the crucial processes for FB understanding. According to the linguistic determinism theory proposed by Jill and Peter De Villiers, the acquisition of the ability to master the syntax of complementation in childhood provides the representative structure necessary for the understanding of FBs by converting them into propositional attitudes [14De Villiers JG, De Villiers PA. Linguistic determinism and the understanding of false beliefs. In: Mitchell P, Riggs KJ, editors. Children’s reasoning and the mind. Hove, UK: Psychology Press; 2000. pp. 191-228., 18De Villiers JG, Hobbs K, Hollebrandse B. Recursive complements and propositional attitudes. In: Roeper T, Speas M, editors. Recursion: Complexity in cognition. Dordrecht: Springer; 2014. pp. 221-242.]. When we talk about mental states (e.g., “I think that Sally is afraid”), we use a rather complex construction, which consists of the main clause (“I think that...”) and subordinate clause embedded into the first (“Sally is afraid”). An important stage in development occurs when children understand that the sentential complements, which depend on the verb think, may be false after careful comparison with the world. In this syntactical structure, the content can, therefore, have an independent truth value: sentential complements promote an explicit representation of an embedded subordinate clause with a true-or-false truth value [19De Villiers JG. Can language acquisition give children a point of view? In: Astington JW, Baird JA, editors. Why language matters for Theory of Mind. Oxford: Oxford University Press; 2005. pp. 186-219.[http://dx.doi.org/10.1093/acprof:oso/9780195159912.003.0010] , 20Barak L, Fazly A, Stevenson S. Gradual acquisition of mental state meaning: A computational investigation. In: Bello P, Guarini M, McShane M, Scassellati B, editors. Proceedings of the 36th Annual Conference of the Cognitive Science Society. Austin, TX: Cognitive Science Society; 2014. pp. 1-6.]. From this viewpoint, syntactic complementation is a necessary precursor to the acquisition of FB understanding, offering the means to debate about mental states and reality.
On the other hand, the hypothesis of ToM development as independent of syntax, syntax and ToM are related but separate skills. As the hypothesis of dependence previously described proposed syntactic complementation as a key component for FB understanding, we expect that a lack of this ability leads to the collapse of the FB component of ToM, also when ToM is already acquired. The proponents of this hypothesis argue that mastery in complementation is not a scaffolding ability for FB understanding, even if these two skills develop in the same life-period. To support the hypothesis of independence, some researchers conducted studies on adult participants with syntactic deficits to explore this issue and found contrasting results. Case studies on aphasic patients, who pass ToM tasks despite their severe linguistic deficit, have contributed to a different way to conceptualize the relationship between ToM and syntax. Siegal and Varley [21Siegal M, Varley R. Aphasia, language, and theory of mind. Soc Neurosci 2006; 1(3-4): 167-74.[http://dx.doi.org/10.1080/17470910600985597] [PMID: 18633785] ] proposed that grammar is marginally important for ToM, although it is an important source for the mediation of conversation. Evidence on patients with aphasia indicates that explicit knowledge of grammar is not necessary to scaffold ToM reasoning.
The hypothesis of independence was further supported by studies on ToM reasoning in adult patients with right hemisphere damage suggesting a double dissociation between grammar on one hand, and ToM and pragmatics, on the other [22Martin I, McDonald S. Weak coherence, no theory of mind, or executive dysfunction? Solving the puzzle of pragmatic language disorders. Brain Lang 2003; 85(3): 451-66.[http://dx.doi.org/10.1016/S0093-934X(03)00070-1] [PMID: 12744957] ].
The proponents of this hypothesis do not totally deny the utility of syntactic functions in the comprehension of others’ mental states. Complementation could facilitate ToM when the system fails but it is not essential when everything is operating optimally. According to Siegal et al. [23Siegal M, Varley R, Want SC. Mind over grammar: reasoning in aphasia and development. Trends Cogn Sci (Regul Ed) 2001; 5(7): 296-301.[http://dx.doi.org/10.1016/S1364-6613(00)01667-3] [PMID: 11425618] ], syntax plays a critical role in reducing mistakes in the transmission of knowledge but its presence alone cannot guarantee understanding social relationships and communication.
3. THE TWO INTERPRETATIVE HYPOTHESES: STUDIES FOR DIFFERENT SAMPLES
The studies in the domain under consideration have been conducted at different stages of development (children, adolescents, and adults) and in samples presenting both typical and atypical development.
3.1. Longitudinal and Training Studies in Typical Development
To understand the relationship between FB understanding and syntactic complementation, various longitudinal and training studies have been conducted. Longitudinal studies analyzed the development of an ability across the lifespan. Astington and Jenkins [24Astington JW, Jenkins JM. A longitudinal study of the relation between language and theory-of-mind development. Dev Psychol 1999; 35(5): 1311-20.[http://dx.doi.org/10.1037/0012-16220.127.116.111] [PMID: 10493656] ] assessed ToM and language development in 3-year-old children over a period of 7 months at approximately 3.5-month intervals and their results seem to support the hypothesis that the syntactic subcomponent of language is involved in promoting the development of FB understanding. In a subsequent study on typically developing 3-5-year-old children tested in four sessions over the course of 1 year, De Villiers and Pyers [25De Villiers JG, Pyers JE. Complements to cognition: A longitudinal study of the relationship between complex syntax and false-belief-understanding. Cogn Dev 2002; 17: 1037-60.[http://dx.doi.org/10.1016/S0885-2014(02)00073-4] ] found that the mastery of syntax for sentential complements was the best predictor of success in FB tasks. The authors conclude that complementation is necessary to enable children to represent the correct syntactic form of others’ belief states in their own mind.
Training studies attempted to verify the effect of experience. Hale and Tager-Flusberg [26Hale CM, Tager-Flusberg H. The influence of language on theory of mind: A training study. Dev Sci 2003; 6(3): 346-59.[http://dx.doi.org/10.1111/1467-7687.00289] [PMID: 16467908] ] pre-tested children (ages 36–58 months) with FB and sentential complement tasks to ensure that they had not yet acquired FB understanding or sentential complements, and with relative clause tasks to assess their ability to include the relative clause information in their speech. The children were divided into three different groups and assigned to three different training types: the first was set up on the development of FBs, the second on sentential complements, and the third on relative clauses. The training was scheduled in two training sessions within one week of each other. After the second training session, all children were post-tested with tasks similar to those from the pre-test session. The results supported the hypothesis of linguistic influence on the development of ToM: the training on complementation and the acquisition of sentential complements as specific linguistic constructions led to improved performance on FB tasks, in contrast to relative clause training that did not yield improved FB understanding (see also [27Lohmann H, Tomasello M. The role of language in the development of false belief understanding: A training study. Child Dev 2003; 74(4): 1130-44.[http://dx.doi.org/10.1111/1467-8624.00597] [PMID: 12938709] ]). Mo et al. [28Mo S, Su Y, Sabbagh MA, Jiaming X. Sentential complements and false belief understanding in Chinese Mandarin-speaking preschoolers: A training study. Cogn Dev 2014; 29: 50-61.[http://dx.doi.org/10.1016/j.cogdev.2013.11.001] ], in a study involving Mandarin-speaking children (ages 40–55 months), showed that training on sentential complements depending on communication verbs (e.g., say) significantly improved FB understanding. The children were divided into four training groups and each training was scheduled in two 25-minute-training sessions within two weeks of each other and 7-day-interval between sessions. Interestingly, they also found that FB training only (i.e., without sentential complement training) improved FB performance but not sentential complement understanding, suggesting that explicit FB understanding can emerge even when children have little competence with sentential complement constructions.
These studies give a controversial support to the hypotheses: longitudinal studies on typical development seem to support the hypothesis of dependence, but training studies highlight that also FB training and not only complementation improve FB understanding.
3.2. Studies of People with Hearing Loss (Deaf and Hard-of-Hearing)
In deaf children born in families with hearing parents, both language ability [29Peterson CC, Wellman HM, Liu D. Steps in theory-of-mind development for children with deafness or autism. Child Dev 2005; 76(2): 502-17.[http://dx.doi.org/10.1111/j.1467-8624.2005.00859.x] [PMID: 15784096] , 30Wellman HM, Peterson CC. Deafness, thought bubbles, and theory-of-mind development. Dev Psychol 2013; 49(12): 2357-67.[http://dx.doi.org/10.1037/a0032419] [PMID: 23544856] ] and FB understanding [31Meristo M, Falkman KW, Hjelmquist E, Tedoldi M, Surian L, Siegal M. Language access and theory of mind reasoning: Evidence from deaf children in bilingual and oralist environments. Dev Psychol 2007; 43(5): 1156-69.[http://dx.doi.org/10.1037/0012-1618.104.22.1686] [PMID: 17723042] , 32Schick B, de Villiers P, de Villiers J, Hoffmeister R. Language and theory of mind: A study of deaf children. Child Dev 2007; 78(2): 376-96.[http://dx.doi.org/10.1111/j.1467-8624.2007.01004.x] [PMID: 17381779] ] are acquired later than in children with typical development. On the contrary, studies on deaf children with signing parents show that children who learn sign language at an age comparable to those with typical development can fluently master syntax and display ToM skills comparable to age-matched hearing peers [33Lederberg AR, Schick B, Spencer PE. Language and literacy development of deaf and hard-of-hearing children: Successes and challenges. Dev Psychol 2013; 49(1): 15-30.[http://dx.doi.org/10.1037/a0029558] [PMID: 22845829] ]. De Villiers and De Villiers [34de Villiers PA, de Villiers JG. Deception dissociates from false belief reasoning in deaf children: implications for the implicit versus explicit theory of mind distinction. Br J Dev Psychol 2012; 30(Pt 1): 188-209.[http://dx.doi.org/10.1111/j.2044-835X.2011.02072.x] [PMID: 22429041] ] found that FB understanding in orally taught deaf children is independent from general vocabulary skills but is dependent upon the ability to manage false sentential complements.
Interesting findings come from hearing-impaired children with cochlear implants. Remmel and Peters [35Remmel E, Peters K. Theory of mind and language in children with cochlear implants. J Deaf Stud Deaf Educ 2009; 14(2): 218-36.[http://dx.doi.org/10.1093/deafed/enn036] [PMID: 18796736] ] pointed out that in both linguistic-syntactic and FB tasks, these children performed comparably to deaf children born into signing families, with both groups showing better results in verbal than non-verbal ToM tasks. These findings do not guarantee the predictive ability to master the complementation but merely attest to the importance of previous conversations about mental states. Holding conversations using meta-representative skills increases the development of ToM reasoning, and vice versa, an environment lacking interpersonal and communication possibilities often leads to difficulties in ToM reasoning in deaf children ([31Meristo M, Falkman KW, Hjelmquist E, Tedoldi M, Surian L, Siegal M. Language access and theory of mind reasoning: Evidence from deaf children in bilingual and oralist environments. Dev Psychol 2007; 43(5): 1156-69.[http://dx.doi.org/10.1037/0012-1622.214.171.1246] [PMID: 17723042] ]; on the importance of conversation, see also [36Harris PL, de Rosnay M, Pons F. Language and children’s understanding of mental states. Curr Dir Psychol Sci 2005; 14(2): 69-73.[http://dx.doi.org/10.1111/j.0963-7214.2005.00337.x] ]).
3.3. Studies of People with Autism Spectrum Disorders
Children with Autism Spectrum Disorders (ASD) face a delay in the acquisition of syntax and significant difficulties in FB tasks [37Baron-Cohen S, Leslie AM, Frith U. Does the autistic child have a “theory of mind”? Cognition 1985; 21(1): 37-46.[http://dx.doi.org/10.1016/0010-0277(85)90022-8] [PMID: 2934210] -40Durrleman S, Burnel M, Thommen E, Foudon N, Sonié S, Reboul A, et al. The language cognition interface in ASD: Complement sentences and false belief reasoning. Res Autism Spectr Disord 2016; 21: 109-20.[http://dx.doi.org/10.1016/j.rasd.2015.10.003] ]. Tager-Flusberg and Sullivan [41Tager-Flusberg H, Sullivan K. Predicting and explaining behavior: A comparison of autistic, mentally retarded and normal children. J Child Psychol Psychiatry 1994; 35(6): 1059-75.[http://dx.doi.org/10.1111/j.1469-7610.1994.tb01809.x] [PMID: 7995844] ] reported that language ability predicted FB performance with an accuracy of 90% in children with ASD and that syntactic comprehension was the best predictor. Tager-Flusberg [38Tager-Flusberg H. Language and understanding minds: Connections in autism. In: Baron-Cohen S, Tager-Flusberg H, Cohen DJ, editors. Understanding other minds: Perspectives from autism and developmental cognitive neuroscience. Second edition. Oxford: Oxford University Press; 2000. pp. 1-45.] noted that children with ASD, who fail in ToM reasoning tasks, could not deduce the sentential complementation in phrases depending on communication verbs. The syntax of complementation seems to be significant for the development of the representational mind especially in ASD. Similarly, Durrleman et al. [40Durrleman S, Burnel M, Thommen E, Foudon N, Sonié S, Reboul A, et al. The language cognition interface in ASD: Complement sentences and false belief reasoning. Res Autism Spectr Disord 2016; 21: 109-20.[http://dx.doi.org/10.1016/j.rasd.2015.10.003] ] set up an experimental study on children with ASD (ages 6.9 to 14.4 years) and found that non-verbal ToM was significantly correlated with the understanding of complement clauses. More specifically, by using regression analyses, they highlighted that morphosyntax played an important role to determine ToM success (for similar results, see also [42Stephanie D, Julie F. Exploring links between language and cognition in autism spectrum disorders: Complement sentences, false belief, and executive functioning. J Commun Disord 2015; 54: 15-31.[http://dx.doi.org/10.1016/j.jcomdis.2014.12.001] [PMID: 25637130] ]).
Lind and Bowler [43Lind SE, Bowler DM. Language and theory of mind in autism spectrum disorder: The relationship between complement syntax and false belief task performance. J Autism Dev Disord 2009; 39(6): 929-37.[http://dx.doi.org/10.1007/s10803-009-0702-y] [PMID: 19205856] ] noted that the syntax of complementation and performance in FB tasks were significantly related in children with ASD matched in age and verbal ability with the comparison group, but neither the bivariate nor the partial correlations between the two abilities differed between the groups. Whereas, in this study, correlational analyses have been carried out, based on these results, it is not possible to identify the direction of causality, but it is possible to observe a simple correlation between ToM competence and syntax of complementation. Other studies adopted a similar position: syntax could facilitate ToM reasoning without implying that this ability is necessary for ToM development (e.g., [44Colle L, Baron-Cohen S, Hill J. Do children with autism have a theory of mind? A non-verbal test of autism vs. specific language impairment. J Autism Dev Disord 2007; 37(4): 716-23.[http://dx.doi.org/10.1007/s10803-006-0198-7] [PMID: 16977496] ]).
The studies on People with ASD seem to support mainly the hypothesis of dependence but, as it is pointed out by some studies, the results could be interpreted as the influence of reasoning rather than the cause of the development of the FB understanding
3.4. Studies of Children with Specific Language Impairment
Children with Specific Language Impairment (SLI) experience significant syntactic difficulties. Thus, several studies investigated children with SLI to better understand the relationship between syntax and ToM development [45Miller C. False belief and sentence complement performance in children with specific language impairment. Int J Lang Commun Disord 2004; 39(2): 191-213.[http://dx.doi.org/10.1080/13682820310001616994] [PMID: 15204451] -48Andrés-Roqueta C, Adriàn JE, Clemente RA, Katsos N. Which are the best predictors of theory of mind delay in children with specific language impairment? Int J Lang Commun Disord 2013; 48(6): 726-37.[http://dx.doi.org/10.1111/1460-6984.12045] [PMID: 24165368] ].
Farrar et al. [47Farrar MJ, Johnson B, Tompkins V, Easters M, Zilisi-Medus A, Benigno JP. Language and theory of mind in preschool children with specific language impairment. J Commun Disord 2009; 42(6): 428-41.[http://dx.doi.org/10.1016/j.jcomdis.2009.07.001] [PMID: 19647837] ] observed that in preschool children with SLI, general grammar and vocabulary contributed to ToM reasoning, whereas sentential complementation abilities did not contribute independently. These findings not only emphasize the generic syntactic ability for overcoming FB tasks but they also display that the ability in syntactic complementation should be less relevant than general language abilities.
Miller [49Miller CA. False belief understanding in children with specific language impairment. J Commun Disord 2001; 34(1-2): 73-86.[http://dx.doi.org/10.1016/S0021-9924(00)00042-3] [PMID: 11322571] ] pointed out that the mastery of embedded sentential complementation was related to ToM but there was no evidence of causality. Miller compared three groups of children: children with SLI, age-matched children with typical development, and younger typically developed children with linguistic abilities similar to those of SLI children. The experiment consisted of four conditions applied to FB tasks. Children with SLI had linguistic abilities that are below age expectations but they showed intact other cognitive skills including ToM abilities (see also [50Davies C, Andrés-Roqueta C, Norbury CF. Referring expressions and structural language abilities in children with specific language impairment: A pragmatic tolerance account. J Exp Child Psychol 2016; 144: 98-113.[http://dx.doi.org/10.1016/j.jecp.2015.11.011] [PMID: 26716575] ]). This study showed that children with SLI performed like younger children in the linguistically demanding “think” and “pretend” conditions, but they were more similar to their age-matched peers in the “look” and “show” conditions. These results emphasize that FB understanding and complementation are not causally related, even if they are required for the linguistic demands of some ToM tasks (see also [46Miller CA. Developmental relationships between language and theory of mind. Am J Speech Lang Pathol 2006; 15(2): 142-54.[http://dx.doi.org/10.1044/1058-0360(2006/014)] [PMID: 16782686] , 51Farrant BM, Maybery MT, Fletcher J. Language, cognitive flexibility, and explicit false belief understanding: Longitudinal analysis in typical development and specific language impairment. Child Dev 2012; 83(1): 223-35.[http://dx.doi.org/10.1111/j.1467-8624.2011.01681.x] [PMID: 22188484] ]).
The main studies on children with SLI maintain a more cautious position in the interpretation of the results: the data presented do not allow one to lean for one or the other hypothesis, even if they detect some kind of influence between the two components in analysis.
3.5. Studies of Patients with Aphasia
In order to analyses the relationship between ToM and syntax, here we discuss three case reports of patients who became aphasic following damage to the left hemisphere.
Varley and Siegal [52Varley R, Siegal M. Evidence for cognition without grammar from causal reasoning and ‘theory of mind’ in an agrammatic aphasic patient. Curr Biol 2000; 10(12): 723-6.[http://dx.doi.org/10.1016/S0960-9822(00)00538-8] [PMID: 10873809] ] were the first to systematically investigate the relation between grammar and ToM by studying patients with aphasia. They reported the case of S.A., a patient with severe agrammatic aphasia but with preserved ToM reasoning. S.A. had a large left hemisphere lesion that affected his linguistic abilities, except for sentence comprehension mediated by a more general preserved pragmatic competence. His spontaneous writing consisted of article-noun, adjective-noun, and quantifier-noun combinations without grammatical structure. Despite these impairments, preserved ToM understanding tested with FB tasks and executive functioning was observed. This dissociation was also noted in two other cases: M.R., reported by Varley et al. [53Varley R, Siegal M, Want SC. Severe impairment in grammar does not preclude theory of mind. Neurocase 2001; 7(6): 489-93.[http://dx.doi.org/10.1093/neucas/7.6.489] [PMID: 11788740] ], revealed impaired performance in executive functioning and linguistic tasks but retained ToM reasoning in a picture-based task. P.H., a patient investigated by Apperly et al. [54Apperly IA, Samson D, Carroll N, Hussain S, Humphreys G. Intact first- and second-order false belief reasoning in a patient with severely impaired grammar. Soc Neurosci 2006; 1(3-4): 334-48.[http://dx.doi.org/10.1080/17470910601038693] [PMID: 18633798] ], showed severe agrammatical aphasia, similar to S.A. and M.R., with specific impairment in embedded complement sentences and relative clauses. Despite these difficulties, ToM reasoning tested on non-verbal first-order and second-order FB tasks was above chance. Similar findings were reported by Ramachandra and Mikajlo [55Ramachandra V, Mikajlo B. Affective Theory of Mind may be unimpaired in people with aphasia. Procedia Soc Behav Sci 2013; 94: 164-5.[http://dx.doi.org/10.1016/j.sbspro.2013.09.080] ] (see also [56Willems RM, Benn Y, Hagoort P, Toni I, Varley R. Communicating without a functioning language system: Implications for the role of language in mentalizing. Neuropsychologia 2011; 49(11): 3130-5.[http://dx.doi.org/10.1016/j.neuropsychologia.2011.07.023] [PMID: 21810434] ]; for a brief overview see [21Siegal M, Varley R. Aphasia, language, and theory of mind. Soc Neurosci 2006; 1(3-4): 167-74.[http://dx.doi.org/10.1080/17470910600985597] [PMID: 18633785] ]). In sum, case studies of patients with aphasia are presented by researchers advocating the independence of ToM from grammatical abilities, proposing that ToM reasoning is possible even when specific grammatical constructions are impaired.
DISCUSSION AND CONCLUSION
The role of language in ToM is well recognized, but the relative contribution of different language aspects remains debated [57Milligan K, Astington JW, Dack LA. Language and theory of mind: Meta-analysis of the relation between language ability and false-belief understanding. Child Dev 2007; 78(2): 622-46.[http://dx.doi.org/10.1111/j.1467-8624.2007.01018.x] [PMID: 17381794] -60Tettamanti M, Vaghi MM, Bara BG, Cappa SF, Enrici I, Adenzato M. Effective connectivity gateways to the Theory of Mind network in processing communicative intention. Neuroimage 2017; 155: 169-76.[http://dx.doi.org/10.1016/j.neuroimage.2017.04.050] [PMID: 28438665] ]. In the present work, we have focused on the relationship between FB understanding and syntactic complementation with a controversial emerging picture.
First, the linguistic determinism theory has had a strong influence but has not been without criticisms. Studies in line with this theory mainly come from longitudinal and training studies and from studies with deaf children born into hearing families. However, contrasting results were observed as well. For example, Lohmann and Tomasello [27Lohmann H, Tomasello M. The role of language in the development of false belief understanding: A training study. Child Dev 2003; 74(4): 1130-44.[http://dx.doi.org/10.1111/1467-8624.00597] [PMID: 12938709] ] found in their training study that sentential complement syntax is sufficient for FB understanding in experimental settings, but it might not be a necessary condition in FB comprehension in ecological conditions. Studies on children speaking non-English languages did not support the effectiveness of training on ToM performance. In a study on Swedish preschoolers, Clausen-Bruun [61Clausen-Brunn M. Does training production of tensed complements accelerate the development of Theory of Mind? Lund University Libraries; 2013. Available from
http://lup.lub.lu.se/student-papers/record/4139686] found no evidence supporting the role of active production of tensed complements (i.e., full propositions embedded within sentences) in ToM development. A study by Perner et al. [62Perner J, Sprung M, Zauner P, Haider H. Want that is understood well before say that, think that, and false belief: a test of de Villiers’s linguistic determinism on German-speaking children. Child Dev 2003; 74(1): 179-88.[http://dx.doi.org/10.1111/1467-8624.t01-1-00529] [PMID: 12625444] ] reported that German children understand desires earlier than beliefs, although both verb types take compliments with a similar structure. Cheung [63Cheung H. False belief and language comprehension in Cantonese-speaking children. J Exp Child Psychol 2006; 95(2): 79-98.[http://dx.doi.org/10.1016/j.jecp.2006.05.002] [PMID: 16782125] ] and Tardif et al. [64Tardif T, So CW, Kaciroti N. Language and false belief: evidence for general, not specific, effects in cantonese-speaking preschoolers. Dev Psychol 2007; 43(2): 318-40.[http://dx.doi.org/10.1037/0012-16126.96.36.1998] [PMID: 17352542] ] found that in Cantonese-speaking children, general language comprehension, rather than specific language structure per se, accounted for FB understanding. Tompkins [65Tompkins V. Mothers’ cognitive state talk during shared book reading and children’s false belief understanding. Cogn Dev 2015; 36: 40-51.[http://dx.doi.org/10.1016/j.cogdev.2015.08.004] ] proposed that an important aspect for the prediction of children’s FB understanding could be the mother’s use of contrastives (i.e., the complement syntax that directly contrasted thought and reality). Comparing mothers’ cognitive state talk assessed during shared book reading with 3- to 5-year-olds in relation to children’s FB understanding six months later, the authors found that, in opposition to the mother’s use of contrastives, nor the total use of cognitive state vocabulary neither the complement syntax using the verb “think” predicted children’s FB understanding. In their interesting review, Farrar and colleagues [66Farrar M, Benigno JP, Tompkins V, Gage NA. Are there different pathways to explicit false belief understanding? General language and complementation in typical and atypical children. Cogn Dev 2017; 43: 49-66.[http://dx.doi.org/10.1016/j.cogdev.2017.02.005] ] analyze different studies and suggest that it seems that syntactic complementation has an influence only on atypical development and not also in typically developing children, where general language seems to have a more relevant effect.
Second, to understand why studies investigating the relationship between syntax and ToM yielded mixed results, it is important to pay attention to the nature of the experimental tasks. Many ToM tasks are not cognitively equivalent and might draw on divergent mental abilities, which is a first limitation of the current literature. As emphasized by Apperly et al. [67Apperly IA, Samson D, Humphreys GW. Domain-specificity and theory of mind: evaluating neuropsychological evidence. Trends Cogn Sci (Regul Ed) 2005; 9(12): 572-7.[http://dx.doi.org/10.1016/j.tics.2005.10.004] [PMID: 16271505] ], there is no definitive evidence for the domain specificity of ToM, especially belief understanding assessed with FB tasks. FB tasks are not well suited to study the relationship between ToM and syntactic complementation, because just passing these tasks requires abilities other than ToM, such as attention and memory [68Bloom P, German TP. Two reasons to abandon the false belief task as a test of theory of mind. Cognition 2000; 77(1): B25-31.[http://dx.doi.org/10.1016/S0010-0277(00)00096-2] [PMID: 10980256] ]. Furthermore, it is important to keep in mind that FB tasks normally used in literature vary for the kind of FB understanding assessed, explicit or implicit [69Low J, Perner J. Implicit and explicit theory of mind: state of the art. Br J Dev Psychol 2012; 30(Pt 1): 1-13.[http://dx.doi.org/10.1111/j.2044-835X.2011.02074.x] [PMID: 22429029] ]. In the former kind of assessment the participant is asked to predict a behavior explicitly by means of a verbal answer consciously controlled, while in the latter kind of assessment procedural non-verbal behaviors such as anticipatory looking or eye gaze are used as an indirect measure of FB understanding. The difference between implicit and explicit ToM was firstly addressed by Clements and Perner [70Clements WA, Perner J. Implicit understanding of belief. Cogn Dev 1994; 9: 377-95.[http://dx.doi.org/10.1016/0885-2014(94)90012-4] ] who analyzed eye movements and distinguished a period of implicit understanding of FB preceding the onset of explicit understanding. In their experiment, children ages 2 years 11 months were able to look at the correct solution in a FB scenario although they verbally answered incorrectly. Subsequently, Baillargeon [9Scott RM, Baillargeon R. Early false-belief understanding. Trends Cogn Sci (Regul Ed) 2017; 21(4): 237-49.[http://dx.doi.org/10.1016/j.tics.2017.01.012] [PMID: 28259555] , 71Onishi KH, Baillargeon R. Do 15-month-old infants understand false beliefs? Science 2005; 308(5719): 255-8.[http://dx.doi.org/10.1126/science.1107621] [PMID: 15821091] , 72Baillargeon R, Scott RM, He Z. False-belief understanding in infants. Trends Cogn Sci (Regul Ed) 2010; 14(3): 110-8.[http://dx.doi.org/10.1016/j.tics.2009.12.006] [PMID: 20106714] ], using a violation-of-expectation method, reported that 15-month-old infants already show some implicit understanding of FB when tested with an entirely non-verbal task (although explanations of these results not invoking FB understanding have been proposed, see [73Perner J, Ruffman T. Psychology. Infants’ insight into the mind: how deep? Science 2005; 308(5719): 214-6.[http://dx.doi.org/10.1126/science.1111656] [PMID: 15821079] , 74Ruffman T, Taumoepeau M, Perkins C. Statistical learning as a basis for social understanding in children. Br J Dev Psychol 2012; 30(Pt 1): 87-104.[http://dx.doi.org/10.1111/j.2044-835X.2011.02045.x] [PMID: 22429035] ]).
Explicit and implicit FB understanding may dissociate during development and in atypical populations [75Frith CD, Frith U. Implicit and explicit processes in social cognition. Neuron 2008; 60(3): 503-10.[http://dx.doi.org/10.1016/j.neuron.2008.10.032] [PMID: 18995826] ] but are closely related in typical adults [76Kanske P, Böckler A, Trautwein F-M, Singer T. Dissecting the social brain: Introducing the EmpaToM to reveal distinct neural networks and brain-behavior relations for empathy and Theory of Mind. Neuroimage 2015; 122: 6-19.[http://dx.doi.org/10.1016/j.neuroimage.2015.07.082] [PMID: 26254589] ]. Thus, future research should carefully consider the kind of tasks used to assess ToM abilities and use a mix of tasks covering both implicit and explicit processes (even considering that children understand diverse beliefs before FBs, as well as different mental states before belief, with a progression from desire to diverse belief to FB [77Wellman HM, Liu D. Scaling of theory-of-mind tasks. Child Dev 2004; 75(2): 523-41.[http://dx.doi.org/10.1111/j.1467-8624.2004.00691.x] [PMID: 15056204] ]).
Third, the findings by Low [78Low J. Preschoolers’ implicit and explicit false-belief understanding: relations with complex syntactical mastery. Child Dev 2010; 81(2): 597-615.[http://dx.doi.org/10.1111/j.1467-8624.2009.01418.x] [PMID: 20438463] ] in 3- and 4-year-olds indicated that while in explicit FB tasks a relationship between ability for syntactic sentential complements and FB performance is detectable, this is not the case when implicit FB is assessed in this population. On the other hand, the findings by Newton and De Villiers [79Newton AM, de Villiers JG. Thinking while talking: adults fail nonverbal false-belief reasoning. Psychol Sci 2007; 18(7): 574-9.[http://dx.doi.org/10.1111/j.1467-9280.2007.01942.x] [PMID: 17614864] ] show that in a dual-task setting in adults (ages 18–35), verbal shadowing disrupts the ability to monitor a character’s belief in a non-verbal FB task. In contrast, Forgeot d’Arc and Ramus [80Forgeot d’Arc B, Ramus F. Belief attribution despite verbal interference. Q J Exp Psychol (Hove) 2011; 64(5): 975-90.[http://dx.doi.org/10.1080/17470218.2010.524413] [PMID: 21161856] ] presented a completely non-verbal paradigm and observed an independence between FB understanding and verbal reasoning in adulthood. The results show that, actually, verbal shadowing decreased the overall performance and not only FB understanding. Similarly, Dungan and Saxe [81Dungan J, Saxe R. Matched false-belief performance during verbal and nonverbal interference. Cogn Sci 2012; 36(6): 1148-56.[http://dx.doi.org/10.1111/j.1551-6709.2012.01248.x] [PMID: 22486780] ] found that success in FB tasks in adulthood is equal during verbal and non-verbal interference; these results show that verbal shadowing has no a specific effect, suggesting that syntax is not necessary in FB understanding in adulthood. These results are important as they highlight the second limitation of the current literature, a limitation concerning the comparison of results coming from populations, at least in principle, not comparable.
Fourth, another important question concerns the distinction between the impact of syntax on the development of ToM understanding and the impact of syntax on ToM understanding when ToM is already acquired. This distinction led to a different interpretative structure in the discussion of the state of the art of this field, in particular for what concerns the kind of dependence between syntax and ToM. In fact, the hypothesis of ToM development dependence upon syntax could be split in two sub-hypotheses: a temporary dependence, i.e., complementation is necessary for the emergence of FB understanding but this dependence disappears later on, and a permanent dependence, i.e., that complementation is necessary for the emergence as well as for the maintenance of FB understanding. As previously discussed, findings on case studies of patients with aphasia are discussed by researchers advocating the independence of ToM from grammatical abilities; nevertheless, it is important to note that these findings falsify only the permanent dependence hypothesis, but they can not allow to exclude a temporary dependence between complementation and FB understanding.
One important limitation of the present brief overview is the lack of a careful analysis concerning the influence of culture on the development of both FB understanding and syntax: the experimental data come from many different cultures and languages, and culture has an impact on both dimensions of analysis (see for example [82Gut A, Mirski R. In Search of a Theory: The interpretative challenge of empirical findings on cultural variance in mindreading. studies in logic. Grammar and Rhetoric 2016; 48(1): 201-30.]). Consequently, the nature of the link between syntax and FB understanding remains unresolved even for reasons related to culture that makes the comparison between studies thorny.
In conclusion, we tried to identify salient features of both the dependence and independence hypothesis, but the cases used by the different authors are not fully comparable. Discussing the relationship between syntactic complementation and FB understanding at developmental stages is one aspect, which differs when both functions have already been acquired [83Im-Bolter N, Agostino A, Owens-Jaffray K. Theory of mind in middle childhood and early adolescence: Different from before? J Exp Child Psychol 2016; 149: 98-115.[http://dx.doi.org/10.1016/j.jecp.2015.12.006] [PMID: 26827097] ]. Taking into account the difference between the role played by syntax in ToM development and in ToM understanding when ToM is already acquired is crucial in facing this issue. A lifespan approach taking into account both the emergence and maintenance of FB understanding and using both implicit and explicit FB tasks would be beneficial in the future. Adopting this comprehensive approach would not only have scientific value for our knowledge of the relationship between syntax and ToM but would have significant clinical and educational implications for individuals with ToM difficulties.
CONSENT FOR PUBLICATION
CONFLICT OF INTEREST
The research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Mauro Adenzato, Rita B. Ardito and Ivan Enrici were supported by University of Turin grants (“Ricerca scientifica finanziata dall’Università”). This study was supported by Ministero dell'Istruzione, dell'Università e della Ricerca – MIUR project “Dipartimenti di Eccellenza 2018 – 2022” to Dept. of Neuroscience “Rita Levi Montalcini”. All authors listed have made substantial intellectual contribution to the work and approved it for publication. Elena Fontana wrote the first version of the manuscript.
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