A Temporal Sampling Basis for Visual Processing in Developmental Dyslexia

Abstract

Knowledge of oscillatory entrainment and its fundamental role in cognitive and behavioral processing has increasingly been applied to research in the field of reading and developmental dyslexia. Growing evidence indicates that oscillatory entrainment to theta frequency spoken language in the auditory domain, along with cross-frequency theta-gamma coupling, support phonological processing (i.e., cognitive encoding of linguistic knowledge gathered from speech) which is required for reading. This theory is called the temporal sampling framework (TSF) and can extend to developmental dyslexia, such that inadequate temporal sampling of speech-sounds in people with dyslexia results in poor theta oscillatory entrainment in the auditory domain, and thus a phonological processing deficit which hinders reading ability. We suggest that inadequate theta oscillations in the visual domain might account for the many magno-dorsal processing, oculomotor control and visual deficits seen in developmental dyslexia. We propose two possible models of a magno-dorsal visual correlate to the auditory TSF: (1) A direct correlate that involves "bottom-up" magnocellular oscillatory entrainment of the visual domain that occurs when magnocellular populations phase lock to theta frequency fixations during reading and (2) an inverse correlate whereby attending to text triggers "top-down" low gamma signals from higher-order visual processing areas, thereby organizing magnocellular populations to synchronize to a theta frequency to drive the temporal control of oculomotor movements and capturing of letter images at a higher frequency.

The Mediation Role of Dynamic Multisensory Processing Using Molecular Genetic Data in Dyslexia

“…The current findings support our previous results, implicating RAP and visual motion processing as the most solid EPs of DD [ 51 ], and provide further support for the role of deficits in the processing of transient and dynamic auditory and visual stimuli in the etiology of DD [ 27 , 28 , 36 , 37 , 42 , 47 , 48 , 79 , 80 , 81 , 82 , 83 , 84 , 85 , 86 , 87 , 88 ]. These findings further support the dominant, albeit controversial account [ 82 , 83 ], of the M-D theory of DD [ 47 ].…”

“…According to the general M-D theory [ 35 , 47 , 80 , 83 , 84 , 86 ], DD is due to a multimodal sensory impairment in the processing of transient and dynamic stimuli [ 36 , 83 , 85 , 86 ], which might arise from a deficit in neural pathways involved in the fast transmission and processing of sensory information [ 82 , 84 , 89 ]. Successful sequencing depends on the accurate timing of auditory and visual sensory inputs, which leads to hearing accurately the changes in the amplitude and/or frequency of the sounds and to rapidly recognizing and sequencing written letters [ 79 ]. Therefore, accurate timing facilitates the formation of precise memory representations of the order of sounds (phonological processing) and letters in a word (orthographic processing).…”

“…This ability depends upon deploying attention accurately and in the correct sequence [ 33 , 36 , 89 ]. Such sequential allocation of attention depends upon the properties of “transient” systems in the brain, which is mediated by networks of “magnocellular” neurons whose size enables them to react rapidly to temporal transients [ 36 , 50 , 79 , 90 , 91 , 92 , 93 ]. Segregated magno- and parvo-cellular processing routes are well documented in the visual system from the lateral geniculate nucleus up to the level of the primary visual cortex [ 94 , 95 , 96 ].…”

“…Additionally, auditory analogies to magno- and parvo-cellular auditory processing streams have been suggested at the cortical level [ 97 , 98 , 99 , 100 , 101 ]. These multi-sensory deficits in dynamic processing of transient stimuli [ 79 ] could be linked with typical impairments in integrating visual symbols with their corresponding speech sounds. Although there is a debate about causal relationships between multisensory dynamic processing and print-to-speech sound integration, as well as their neural bases, these processes all require precise and rapid timing mechanisms across distributed brain networks in which perceptual neural noise exclusion is fundamental [ 79 , 102 , 103 , 104 , 105 , 106 ].…”

An Evolutionary Perspective of Dyslexia, Stress, and Brain Network Homeostasis

“…The visuospatial processes with relevance to dyslexia involve the magnocellular-dorsal stream (MDS). Multiple studies have found evidence linking dyslexia to the MDS, visuospatial orienting, and the right posterior parietal cortex (e.g., Facoetti et al, 2001 ; Franceschini et al, 2018 ; Fu et al, 2018 ; Vidyasagar, 2019 ; Archer et al, 2020 ). The MDS or “where” stream relays retinal signals, via the lateral geniculate nucleus and along secondary visual pathways, to the right hemisphere dorsal attention network (DAN), which leads in top-down feedback control of visuospatial processing ( Underleider and Mishkin, 1982 ; Grefkes et al, 2010 ).…”