From Sensory Perception to Lexical-Semantic Processing: An ERP Study in Non-Verbal Children with Autism.

This study examines electrocortical activity associated with visual and auditory sensory perception and lexical-semantic processing in nonverbal (NV) or minimally-verbal (MV) children with Autism Spectrum Disorder (ASD). Currently, there is no agreement on whether these children comprehend incoming linguistic information and whether their perception is comparable to that of typically developing children. Event-related potentials (ERPs) of 10 NV/MV children with ASD and 10 neurotypical children were recorded during a picture-word matching paradigm. Atypical ERP responses were evident at all levels of processing in children with ASD. Basic perceptual processing was delayed in both visual and auditory domains but overall was similar in amplitude to typically-developing children. However, significant differences between groups were found at the lexical-semantic level, suggesting more atypical higher-order processes. The results suggest that although basic perception is relatively preserved in NV/MV children with ASD, higher levels of processing, including lexical- semantic functions, are impaired. The use of passive ERP paradigms that do not require active participant response shows significant potential for assessment of non-compliant populations such as NV/MV children with ASD.

Effects of Presentation Rate and Attention on Auditory Discrimination: A Comparison of Long-Latency Auditory Evoked Potentials in School-Aged Children and Adults.

Decoding human speech requires both perception and integration of brief, successive auditory stimuli that enter the central nervous system as well as the allocation of attention to language-relevant signals. This study assesses the role of attention on processing rapid transient stimuli in adults and children. Cortical responses (EEG/ERPs), specifically mismatch negativity (MMN) responses, to paired tones (standard 100-100 Hz; deviant 100-300 Hz) separated by a 300, 70 or 10 ms silent gap (ISI) were recorded under Ignore and Attend conditions in 21 adults and 23 children (6-11 years old). In adults, an attention-related enhancement was found for all rate conditions and laterality effects (L>R) were observed. In children, 2 auditory discrimination-related peaks were identified from the difference wave (deviant-standard): an early peak (eMMN) at about 100-300 ms indexing sensory processing, and a later peak (LDN), at about 400-600 ms, thought to reflect reorientation to the deviant stimuli or "second-look" processing. Results revealed differing patterns of activation and attention modulation for the eMMN in children as compared to the MMN in adults: The eMMN had a more frontal topography as compared to adults and attention played a significantly greater role in childrens' rate processing. The pattern of findings for the LDN was consistent with hypothesized mechanisms related to further processing of complex stimuli. The differences between eMMN and LDN observed here support the premise that separate cognitive processes and mechanisms underlie these ERP peaks. These findings are the first to show that the eMMN and LDN differ under different temporal and attentional conditions, and that a more complete understanding of children's responses to rapid successive auditory stimulation requires an examination of both peaks.

Plasticity in developing brain: active auditory exposure impacts prelinguistic acoustic mapping.

A major task across infancy is the creation and tuning of the acoustic maps that allow efficient native language processing. This process crucially depends on ongoing neural plasticity and keen sensitivity to environmental cues. Development of sensory mapping has been widely studied in animal models, demonstrating that cortical representations of the sensory environment are continuously modified by experience. One critical period for optimizing human language mapping is early in the first year; however, the neural processes involved and the influence of passive compared with active experience are as yet incompletely understood. Here we demonstrate that, while both active and passive acoustic experience from 4 to 7 months of age, using temporally modulated nonspeech stimuli, impacts acoustic mapping, active experience confers a significant advantage. Using event-related potentials (ERPs), we show that active experience increases perceptual vigilance/attention to environmental acoustic stimuli (e.g., larger and faster P2 peaks) when compared with passive experience or maturation alone. Faster latencies are also seen for the change discrimination peak (N2*) that has been shown to be a robust infant predictor of later language through age 4 years. Sharpening is evident for both trained and untrained stimuli over and above that seen for maturation alone. Effects were also seen on ERP morphology for the active experience group with development of more complex waveforms more often seen in typically developing 12- to 24-month-old children. The promise of selectively "fine-tuning" acoustic mapping as it emerges has far-reaching implications for the amelioration and/or prevention of developmental language disorders.

Oscillatory support for rapid frequency change processing in infants.

Rapid auditory processing and auditory change detection abilities are crucial aspects of speech and language development, particularly in the first year of life. Animal models and adult studies suggest that oscillatory synchrony, and in particular low-frequency oscillations play key roles in this process. We hypothesize that infant perception of rapid pitch and timing changes is mediated, at least in part, by oscillatory mechanisms. Using event-related potentials (ERPs), source localization and time-frequency analysis of event-related oscillations (EROs), we examined the neural substrates of rapid auditory processing in 4-month-olds. During a standard oddball paradigm, infants listened to tone pairs with invariant standard (STD, 800-800 Hz) and variant deviant (DEV, 800-1200 Hz) pitch. STD and DEV tone pairs were first presented in a block with a short inter-stimulus interval (ISI) (Rapid Rate: 70 ms ISI), followed by a block of stimuli with a longer ISI (Control Rate: 300 ms ISI). Results showed greater ERP peak amplitude in response to the DEV tone in both conditions and later and larger peaks during Rapid Rate presentation, compared to the Control condition. Sources of neural activity, localized to right and left auditory regions, showed larger and faster activation in the right hemisphere for both rate conditions. Time-frequency analysis of the source activity revealed clusters of theta band enhancement to the DEV tone in right auditory cortex for both conditions. Left auditory activity was enhanced only during Rapid Rate presentation. These data suggest that local low-frequency oscillatory synchrony underlies rapid processing and can robustly index auditory perception in young infants. Furthermore, left hemisphere recruitment during rapid frequency change discrimination suggests a difference in the spectral and temporal resolution of right and left hemispheres at a very young age.