Tuesday, June 19, 2012

The summer of grant writing...

... is good for $$ (well, at least in theory), bad for blog posts.  Here's what I've been up to:

Consider the following scenario:  two children are identified in school as having a specific reading impairment.  This is a common scenario in the United States, where between 17.5 and 36% of elementary aged children meet the broad diagnostic criteria for dyslexia defined in the DSM-IV (APA, 1994; Shaywitz, Shaywitz, Fletcher, & Escobar, 1990; Perie, Grigg, & Donahue, 2005), which specifies, essentially, that children can be diagnosed with dyslexia so long as they have difficulty reading in the absence of other apparent learning disabilities.  One child may have difficulty reading as a result of perceptual impairment that prevents effective processing of orthography, while the other child may have a delayed or otherwise incomplete phonological awareness, preventing the critical link between orthography and semantics that is often posited to be necessary in learning to read (see Harm & Seidenberg, 2004, for review).  Effective treatments for these two children would therefore be different.

In order to discover the separate loci of impairment in these two children, under a common mode of current practice, each would need to complete a battery of behavioral assessments, designed to assess ability in different sub-domains of reading—such as the perceptual and phonological impairments in fact present.  Even supposing that a satisfactorily nuanced cognitive assessment is provided, production demands may obscure the diagnostic relevance of any particular behavioral test:  it is well documented in the behavioral literature that production demands can substantially alter the estimated reading ability of a child, unrelated to that child’s actual expertise in a particular domain (see Stanovich, Cunningham, & Cramer, 1994, for a particularly striking example of this phenomenon in the case of phonological awareness.)

What if instead there were a measure that could inform perceptual, phonological, and semantic processing as they unfold in real time, in a single record from a single test?  A measure that could be collected in the absence of overt responses, in order to measure process-pure underlying expertise, free from contamination by response demands?  Such a measure exists!  The Event-Related Potential (ERP) technique provides millisecond level temporal resolution, is decomposable into well-studied, functionally specific components, and can be collected in the absence of an overt task.  Recent data in the ERP literature underscore ERPs as a tool that can probe language ability in the absence of overt responses; Parise & Csibra (2012) have demonstrated semantic priming effects in 9 month old infants through analysis of the N400 ERP component—illustrating receptive vocabulary ability in children that certainly cannot respond verbally to behavioral assessments.  This study reiterates in infants what is already known in even the adult bilingualism literature, where productive competency in overt tasks has been shown to be dissociated from receptive competency as revealed by implicit tasks in combination with ERPs (Tokowicz & MacWhinney, 2005).

Not only can ERPs reveal process-pure receptive abilities, they can do so for multiple sub-components of reading in a single record—even in children with specific reading impairment.  Any ERP recorded in response to an orthographic stimulus will display components that represent automatic processing at the perceptual, phonological, and semantic levels of representation, components known to change with development and in specific reading impairment.  Specifically, the visual N1, which peaks between ~100-150 ms post stimulus onset, is known to represent processing of the onset of a visual stimulus (e.g, Luck, Heinze, Mangun, & Hillyard, 1990), and has been shown to be modulated differentially in children with and without a diagnosis of dyslexia (Araujo, Bramao, Faisca, Petersson, & Reis, 2012)—consistent with subtypes of dyslexia that result from perceptual impairment.  Moving forward in time, the N250 component has been hypothesized to reflect the grouping of orthographic or phonological features into more complex representations (see Grainger & Holcomb, 2009, for review), and has a delayed peak in children with reading disorders—consistent with subtypes of dyslexia that result from phonological impairment.  Finally, the N400 component is a known marker of attempted lexical-semantic access (see Federmeier & Laszlo, 2009, for review), and has been shown to still be developing towards its adult functionality in our target demographic (e.g., Coch et al., 2002).

The ERP technique provides for measurement of process-pure ability in perceptual analysis, phonological recoding, and semantic access in a single record.  Of course, ERPs are more expensive and invasive than traditional behavioral measurements, but the advantages that they provide warrant exploration of their use in clinical assessment.  This research program will achieve such an exploration.   

No comments:

Post a Comment