Matching words to mental images is primarily the function of Wernicke’s area, while combining images according to imposed rules is the function of the lateral prefrontal cortex (LPFC) [1
]. The latter function of synthesizing a novel mental image from objects stored in memory is called mental synthesis
]. Mental synthesis is neurologically different from other key components of imagery, such as simple memory recall and dreaming. Unlike dreaming, which is spontaneous and not controlled by the LPFC [4
], mental synthesis is controlled by and completely dependent on an intact LPFC [6
]. Unlike simple memory recall, which involves the recall of a single object encoded at some point in the past, mental synthesis involves the combination of two or more objects stored in memory [2
Mental synthesis is defined narrowly in order to separate it from other components of executive function, such as attention, impulse control, and working memory. Mental synthesis is not a synonym of problem-solving, as complex problems can often be solved via amodal completion [12
], spontaneous insight [14
], integration of modifiers [2
], and other mechanisms, that either do not require the LPFC or do not involve the combination of objects stored in memory.
Mental synthesis is highly developed in neurotypical individuals well before the age of six [15
], but it is known to be a common challenge for children with Autism Spectrum Disorder (ASD). As a consequence, ASD symptoms often include a phenomenon called stimulus overselectivity
, whereby an individual cannot mentally combine disparate objects from memory into a novel image [16
]. For example, s/he will have difficulty accomplishing a seemingly trivial task, such as an instruction to “pick up a red crayon that is under the table”, which requires the individual to combine three different features, i.e., the object itself (crayon
), its color (red
), and its location (under the table
). The LPFC must then mentally integrate all of these into a new mental image, a red crayon under the table
, in order to take the correct action. When asked to pick up a red crayon under the table
, a child with ASD who is unable to mentally synthesize the crayon with its color and location may attend to the word “crayon” and ignore both its location and the fact that it should also be red, therefore picking up any available crayon. The impaired mental synthesis affects virtually every area of an individual’s verbal, cognitive, and social functioning, including lack of comprehension of flexible syntax and spatial prepositions [19
Furthermore, unlike vocabulary acquisition, which can be spread throughout one’s lifetime, there is only a short critical period for the development of mental synthesis capacity, since acquisition of neural networks essential for the LPFC ability to combine new images diminishes greatly after early childhood [1
]. As a result, thirty to forty percent of individuals diagnosed with ASD experience lifelong impairment in the ability to understand flexible syntax and spatial prepositions [20
]. These individuals, commonly referred to as having low-functioning ASD, typically exhibit full-scale IQ (intelligence quotient) below 70 [21
] and usually perform below the score of 85 in non-verbal IQ tests (see Boucher et al. [22
]; in fact, mental synthesis ability and the associated understanding of flexible syntax and spatial prepositions may be the most salient differentiator between high-functioning and low-functioning ASD).
The ASD community is very aware of this early critical period and there is wide consensus that intense early intervention should be administered to children as soon as they are diagnosed with ASD [23
]. The goals of speech language pathologists (SLPs) and Applied Behavior Analysis (ABA) therapists happen to be built around the construct of mental synthesis, and therefore it happens to be highly targeted in these treatments. SLPs commonly refer to mental synthesis developing techniques as “combining adjectives, location/orientation, color, and size with nouns”, “following directions with increasing complexity”, and “building the multiple features/clauses in the sentence” [24
]. In ABA jargon, these techniques are known as “visual-visual and auditory-visual conditional discrimination” [25
], “development of multi-cue responsivity” [17
], and “reduction of stimulus overselectivity” [18
]. Despite the widespread recognition of the importance of early development of mental synthesis abilities, there is a lack of awareness of mental synthesis definition, its underlying neurology, and of psychometric tests that could measure a child’s progress in acquiring mental synthesis. Most language assessment tests rely heavily on a child’s vocabulary and are therefore an inadequate gauge of mental synthesis acquisition.
In this article, we present the first study of a caregiver-completed evaluation tool designed specifically to measure the acquisition of mental synthesis. This new instrument, called the Mental Synthesis Evaluation Checklist (MSEC), has been designed to complement the Autism Treatment Evaluation Checklist (ATEC) [29
]. We hope that the combined ATEC/MSEC score will provide a better measure of a child’s general improvement and that the MSEC will be able to assess the function of the developing LPFC (i.e., both mental synthesis and syntactic language comprehension).
The MSEC was developed in two broad phases: (1) instrument development and (2) psychometric evaluation. The methods and results of each of these phases are described separately below.
In typical children, Wernicke’s area develops concurrently with the lateral prefrontal cortex (LPFC), but in children with ASD, development of one cortical area can significantly outpace the other. Commonly, the development of Wernicke’s area is significantly faster than the development of the LPFC and, as a result, understanding of words significantly outpaces acquisition of mental synthesis and its dependent functions such as understanding of flexible syntax and spatial prepositions [19
]. In these children, whose Wernicke’s area and LPFC are developing asynchronously, we ought to measure their functions separately. It is not uncommon to observe the following developmental steps in children who acquire language with a significant delay: they start to understand some individual words and phrases (Wernicke’s area), then develop understanding of more complex syntactic language (LPFC), and only after that they begin to verbally express themselves, first with individual words and then with complete sentences (Broca’s area). The existing tests and evaluations adequately assess the former (receptive vocabulary acquisition) as well as the latter (expressive language development), but, critically, miss assessing the middle step, which heralds the LPFC function of mental synthesis and the corresponding understanding of syntactic language. Therefore, there is a substantial gap in the ability of these tests to faithfully measure a child’s developmental progress. To fill this gap, the Mental Synthesis Evaluation Checklist (MSEC) was developed. Its design closely adhered to the recommendations of the US Food and Drug Administration patient-reported-outcomes development guide [61
]. MSEC consists of 20 items that capture four aspects of mental synthesis acquisition from the perspective of caregivers of children with ASD: (1) Sentence structure: understanding noun-adjective combinations, spatial prepositions, and flexible syntax; (2) Narrative comprehension: understanding of stories and explanations; (3) Creative manifestations: drawing and make-believe activities; and (4) Simple arithmetic: number manipulations with increasing complexity.
Information collected from the literature, key opinion leaders, and caregivers helped to inform a scientifically grounded and relevant measure of mental synthesis in children with ASD. Although a rigorous qualitative approach was given to elicit and to collect caregivers’ input throughout the initial steps of instrument development, some limitations in sample selection should be noted. We only interviewed a sample size of caregivers who volunteered to share their experience. This may have caused some selection bias and affected generalizability of the new instrument. Future studies should further validate its use with parents of various subcategories of ASD children. Despite this limitation, the new instrument demonstrated good psychometric qualities. The factor analysis confirmed unidimensionality of the test items, implying that all the items measure a single concept. The MSEC test exhibited good internal consistency and adequate test–retest reliability for patients with stable ASD level after follow-up periods of 3 and 9 months. The construct validity of the MSEC was confirmed by its positive correlation with the ATEC Communication subscale. The MSEC scores were significantly different for children with different ASD severity levels, confirming the construct validity of the new instrument. The MSEC may be conveniently used as a compliment to existing measures of ASD (e.g., ATEC).
Although the current empirical evaluation demonstrated a strong evidence of good psychometric properties, such as validity and internal consistency, there are some noteworthy limitations. The study population has been selected from registered users of MITA, which may have introduced some bias toward technologically advanced caregivers. Test–retest reliability was good although not excellent. This is likely because the time gap between test administrations was too long, as evaluations were administered with a periodicity of three months. Future studies should be conducted to recheck and improve the test–retest reliability of MSEC.
The MSEC questionnaire described in this manuscript is copyright-free and can be used by researchers as a complimentary subscale for the Autism Treatment Evaluation Checklist. We hope that the addition of MSEC to existing evaluations will make the combined assessment less punctured and more sensitive to small steps in a child’s development. As MSEC does not rely on productive language, it may be an especially useful tool for assessing the development of nonverbal and minimally verbal children.