Speech sounds are highly variable, yet listeners readily extract information from them and transform continuous acoustic signals into meaningful categories during language comprehension. A central question is whether perceptual encoding captures continuous acoustic detail in a one-to-one fashion or whether it is affected by categories. We addressed this in an event-related potential (ERP) experiment in which listeners categorized spoken words that varied along a continuous acoustic dimension (voice onset time; VOT) in an auditory oddball task. We found that VOT effects were present through a late stage of perceptual processing (N1 component, ca. 100 ms poststimulus) and were independent of categories. In addition, effects of within-category differences in VOT were present at a post-perceptual categorization stage (P3 component, ca. 450 ms poststimulus). Thus, at perceptual levels, acoustic information is encoded continuously, independent of phonological information. Further, at phonological levels, fine-grained acoustic differences are preserved along with category information.

A critical issue in perception is the manner in which top-down expectancies guide lower level perceptual processes. In speech, a common paradigm is to construct continua ranging between two phonetic endpoints and to determine how higher level lexical context influences the perceived boundary. We applied this approach to music, presenting participants with major/minor triad continua after brief musical contexts. Two experiments yielded results that differed from classic results in speech perception. In speech, context generally expands the category of the expected stimuli. We found the opposite in music: The major/minor boundary shifted toward the expected category, contracting it. Together, these experiments support the hypothesis that musical expectancy can feed back to affect lower-level perceptual processes. However, it may do so in a way that differs fundamentally from what has been seen in other domains.

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Background/RationaleIdentification of subtle cognitive impairments that may adversely affect quality of life in epilepsy is often difficult. The N2pc and lateralized readiness potential (LRP) event-related potential (ERP) paradigms are capable of detecting and quantifying covert, real-time neural processing with exquisite temporal sensitivity. We hypothesized that the N2pc and LRP are delayed in epilepsy.MethodsTen epilepsy subjects (4 mesial temporal, 1 extratemporal, 3 idiopathic generalized, and 2 new onset, each receiving one to three AEDs) and 20 controls underwent ERP recording from standard 10-20 electrode sites while performing a feature-conjunction visual-search task. Data analysis compared ERP waves from equivalent channels ipsilateral and contralateral to the target using Student's t-test, ANOVA, and jackknifing data resampling procedures.ResultsN2pc peak latency was significantly delayed in epilepsy subjects by approximately 35 msec (p < .016; data shown in the Table and Figure). LRP peak latency was also significantly delayed in epilepsy subjects. P1, N2, and P3 latencies did not differ between epilepsy and control subjects.ConclusionsThere is a significant delay in visual attentional processing in epilepsy subjects and additional impairment of movement preparation but not categorization. Although subtle and covert to bedside detection, these additive delays in the first half-second of neural processing could underlie subjectively impaired cognitive functioning in epilepsy patients and substantially impact their performance during demanding psychomotor tasks requiring rapid reaction time, such as driving. Future research will focus on determining the causes of these delays in cognitive processing, including epilepsy syndrome, brain lesions, and antiepileptic drugs.Delayed N2pc and LRP ERP Responses in Epilepsy