A compensatory role for declarative memory in neurodevelopmental disorders

Abstract

Most research on neurodevelopmental disorders has focused on their abnormalities. However, what remains intact may also be important. Increasing evidence suggests that declarative memory, a critical learning and memory system in the brain, remains largely functional in a number of neurodevelopmental disorders. Because declarative memory remains functional in these disorders, and because it can learn and retain numerous types of information, functions, and tasks, this system should be able to play compensatory roles for multiple types of impairments across the disorders. Here, we examine this hypothesis for specific language impairment, dyslexia, autism spectrum disorder, Tourette syndrome, and obsessive-compulsive disorder. We lay out specific predictions for the hypothesis and review existing behavioral, electrophysiological, and neuroimaging evidence. Overall, the evidence suggests that declarative memory indeed plays compensatory roles for a range of impairments across all five disorders. Finally, we discuss diagnostic, therapeutic and other implications.

Keywords: Aphasia; Attention deficit hyperactivity disorder (ADHD); Autism; Autism spectrum disorder (ASD); Basal ganglia; Cognitive behavioral therapy (CBT); Compensation; Declarative memory; Developmental coordination disorder (DCD); Developmental disorders; Dyslexia; Electrophysiology; Event-related potentials (ERPs); Explicit; Explicit memory; Functional magnetic resonance imaging (fMRI); Habit reversal training; Hippocampus; Language disorder; Medial temporal lobe (MTL); Neurodevelopmental disorders; Neuroimaging; Obsessive–compulsive disorder (OCD); Parkinson's disease (PD); Positron emission tomography (PET); Procedural memory; Sex differences; Specific language impairment (SLI); Specific learning disorder; Striatum; Tourette syndrome; Underdiagnosis.

Figure 1

Electrophysiological evidence from children and adults with specific language impairment (SLI). In an event-related potential (ERP) study (Fonteneau and van der Lely, 2008), syntactic anomalies elicited a left anterior negativity in typically developing children and adults. In contrast, in children and adults with SLI the anomalies elicited an N400, which has been linked to lexical/semantic processing and declarative memory (Kutas and Federmeier, 2011; Ullman, 2001). This suggests that, unlike typically developing individuals, individuals with SLI may rely on lexical/semantic processing and declarative memory to compensate for impairments at syntactic processing. Figure adapted from Fonteneau and van der Lely (2008).

Figure 2

Structural imaging evidence from children with dyslexia. In a structural magnetic resonance imaging (MRI) study (Krafnick et al., 2011), gray matter volume (GMV) was examined in children with dyslexia before and after a behavioral intervention. Following the intervention, there was a significant increase not only of reading abilities, but also of gray matter volumes in the left hippocampus/fusiform and the right hippocampus (both shown here), as well as the left precuneus and right cerebellum (not shown). Since larger hippocampal volumes have independently been shown to correlate with better declarative memory (Protopopescu et al., 2008; Schofield et al., 2009), the findings suggest that intensive behavioral intervention in dyslexia can lead to improved declarative memory as well as improved reading. Declarative memory performance was not directly assessed in this study. Figure adapted from Krafnick et al. (2011).

Figure 3

Functional imaging evidence from adults with obsessive-compulsive disorder (OCD). Typically developing adults and adults with OCD were given a procedural learning task (the serial reaction time task) during positron emission tomography scanning (Rauch et al., 1997). The two groups showed no behavioral differences on the task. However, their functional neuroanatomy differed. Whereas the typically developing group showed bilateral basal ganglia (and other) activation, the OCD group instead activated medial temporal lobe structures, including the hippocampus bilaterally. This suggests that OCD individuals may use declarative memory to fully compensate for at least some tasks that normally depend on procedural memory. L = left side; R = right side. Figure adapted from Rauch et al. (1997).