Oscillatory “temporal sampling” and developmental dyslexia: toward an over-arching theoretical framework

Human cognitive systems such as language represent the sensory world as unitary. For example, the “speech signal” is perceived as a single auditory stimulus, and different visual features and textures in the visual field are perceived holistically as unitary objects. Yet sensory neuroscience demonstrates the diversity of encoding in the neural systems supporting sensory perception. Different aspects of sensory information are processed in parallel, often at different timescales and by different populations of neurons. Further, research into the function of neuronal oscillations (e.g., Buzsáki and Draguhn, 2004) reveals a key role in processing sensory information. In this special issue, we investigate the developmental implications of one neuroscientific theory based on oscillations that is relevant to reading education and developmental dyslexia, the “temporal sampling” framework (hereafter TSF, Goswami, 2011). The TSF identifies specific oscillatory mechanisms that may be impaired in dyslexia. We expect that deeper understanding of neural mechanisms of information-processing will eventually enable deeper understanding of developmental disorders of learning, such as dyslexia. Developmental dyslexia is a disorder in the acquisition of successful reading and spelling skills that is found in∼7% of children across cultures. The ability to read and write—the achievement of literacy—is one of the most complex and sophisticated cognitive skills developed by the brain. Literacy skills develop as a result of direct teaching, usually in childhood, and become fluent during years of practice. By adulthood, most brains have read millions of words. This makes it difficult to disentangle cause from effect when studying sensory and neural factors. Dyslexia is usually only diagnosed after 2–3 years of schooling, when the brain has already had considerable experience of reading and reading tuition. Dyslexia is diagnosed when children who are receiving adequate teaching and who have no overt sensory or neurological problems fail to develop fast, efficient and age-appropriate reading and spelling. The TSF was proposed as the neural basis for the extraction of phonological information from speech via auditory oscillatory encoding (Goswami, in press). Based on work by Poeppel, Greenberg, Giraud, Ghitza and many others (Giraud and Poeppel, 2012, for a recent summary), the TSF linked “sampling” of the speech stream by auditory cortical networks operating at different timescales or oscillatory frequencies (delta, theta, gamma) to the emergence of phonological (linguistic) coding of speech by children. Poeppel and others argued that cortical oscillations enabled the representation of different temporal rates of amplitude modulation in the complex speech signal. These temporal frequency bands yield complementary “windows” of information relating to cognitive linguistic units such as syllables (theta rate) and phonemes (gamma rate, see Poeppel, 2003). Accordingly, the TSF proposed that atypical oscillatory sampling at one or more temporal rates in children with dyslexia could cause phonological difficulties in specifying linguistic units such as syllables. Phonological difficulties in dyslexia are related to difficulties in the accurate perception of amplitude “rise time” (related to modulation rate). The TSF proposed that atypical oscillatory entrainment at syllable-relevant rates of amplitude modulation (delta [∼ stressed syllable rate], theta [∼ syllable rate]) could be one neural cause of the “phonological deficit” found in children and adults with dyslexia across languages and orthographies. This theory is about early developmental mechanisms, nevertheless impaired oscillatory sampling in auditory cortex could, over developmental time, lead to atypical functioning of the leftlateralized “reading network” identified in many fMRI studies of older children (Richlan, 2012, for a recent overview; Clark et al., 2014, for a relevant longitudinal study). This should be true across languages. Indeed, rise time perception is impaired in English, French, Spanish, Hungarian, Finnish, Chinese, and Dutch dyslexic children (see Goswami and Leong, 2013, for an overview). The “phonological deficit” in dyslexia is found in all of these languages, and manifests as difficulties in oral tasks such as recognizing prosodic stress, counting syllables, and counting or deleting phonemes (the smallest phonological units in a language; Ziegler and Goswami, 2005, for a cross-language review). These and other phonological tasks are considered by some of the auditory-based contributions to the special issue (Lehongre et al., 2013; Power et al., 2013; White-Schwoch and Kraus, 2013; Sela, 2014 this issue). The aim of this special issue, however, was to simultaneously invite colleagues who work on visual sensory processing