Speech perception depends on processing of key features in the sound stream and modulations of the sound envelope. These two processing capacities may be reflected in MMN (permanent feature detector) and N1 (transient stimulus detector) components respectively (Naatanen 1990). Developmental disruption of these processing capacities may result in distinct language disorders.
In a first study of dyslexic adults, a behavioural impairment in pitch discrimination was correlated with a MMN deficit, correlated with phonological skills. In contrast, N1 to tones with short rise times was not impaired, suggesting impaired feature but intact envelope processing in this group.
A second study suggested that processing of both sound features and envelope contribute to behavioural deficits in SLI. Speech MMN magnitude was reduced in SLI and correlated with performance deficits in non-word repetition and listening span. Half of the SLI children also displayed impairments in discriminating speech sounds, correlated with the amplitude of N1 potentials.
A third study examined ERP correlates of sensitivity to sound rise-time, an important cue for speech envelope processing which accounts for a large proportion of variance in children's phonological and reading skills (Goswami et al. 2002). Preliminary data in adults and children suggest that N1 and MMN are also sensitive to sound rise time and might distinguish dyslexics from age-matched controls.
In summary, N1 and MMN are sensitive neural markers for distinct aspects of speech sound processing correlated to phonological skills, and can assist in elucidating the developmental trajectories of their dysfunction in language disorders.