Magnocellular-dorsal pathway and sub-lexical route in developmental dyslexia

Abstract

Although developmental dyslexia (DD) is frequently associate with a phonological deficit, the underlying neurobiological cause remains undetermined. Recently, a new model, called "temporal sampling framework" (TSF), provided an innovative prospect in the DD study. TSF suggests that deficits in syllabic perception at a specific temporal frequencies are the critical basis for the poor reading performance in DD. This approach was presented as a possible neurobiological substrate of the phonological deficit of DD but the TSF can also easily be applied to the visual modality deficits. The deficit in the magnocellular-dorsal (M-D) pathway - often found in individuals with DD - fits well with a temporal oscillatory deficit specifically related to this visual pathway. This study investigated the visual M-D and parvocellular-ventral (P-V) pathways in dyslexic and in chronological age and IQ-matched normally reading children by measuring temporal (frequency doubling illusion) and static stimuli sensitivity, respectively. A specific deficit in M-D temporal oscillation was found. Importantly, the M-D deficit was selectively shown in poor phonological decoders. M-D deficit appears to be frequent because 75% of poor pseudo-word readers were at least 1 SD below the mean of the controls. Finally, a replication study by using a new group of poor phonological decoders and reading level controls suggested a crucial role of M-D deficit in DD. These results showed that a M-D deficit might impair the sub-lexical mechanisms that are critical for reading development. The possible link between these findings and TSF is discussed.

Keywords: dorsal stream; phonological decoding; reading acquisition; reading disability; transient system; visual disorder.

FIGURE 1

The difference between the chronological age- and IQ-matched (CA) controls and dyslexic children in the FD task measuring the M-D pathway functionality (i.e., M threshold) in the Experiment 1. ^*p < 0.05.

FIGURE 2

The difference between chronological-age and IQ matched (CA) controls and the subtypes of children with DD (i.e., poor phonological decoders, hereafter PPD and non-impaired phonological decoders, hereafter NPD) in the FD task (Experiment 1). ^*p < 0.05.

FIGURE 3

The scatter-plot shows that at individual level the 75% of the PPDs (the red triangle) are at least 1 SD under the mean of the CA controls (the green circle) in the FD task (Experiment 1).

FIGURE 4

No difference was found between age and IQ matched (CA) controls and the children with DD in the HPR task measuring the P-V pathway functionality (i.e., P threshold) in the Experiment 1.

FIGURE 5

The difference between the new sample of PPDs and reading-level and IQ-matched (RL) controls in the FD task (Experiment 2). ^*p < 0.05.

FIGURE 6

(A) The scatter-plot of the individual data showing the difference between the children without DD (i.e., CA and RL controls) and the subtypes of children with DD (NPD and PPD) in the FD task measuring the M-D pathway functionality (Experiment 1 and 2). The findings did not change running the ANCOVA without the outliers. Error bars represent one standard deviation of the mean. (B) The plot of the individual data showing the difference between the children without (i.e., CA controls) and with DD in the HPR task (Experiment 1). Error bars represent one standard deviation of the mean.