Brosch, Michael and Christoph E. Schreiner. Time course of different auditory streams (Bregman 1990). Numerous psyforward masking tuning curves in cat primary auditory cortex. chophysical studies have demonstrated that the temporal J. Neurophysiol. 77: 923–943, 1997. Nonsimultaneous two-tone stimulus context affects the perceptual quality of individual interactions were studied in the primary auditory cortex of anestheauditory events. Detection thresholds of individual sounds tized cats. Poststimulatory effects of pure tone bursts (masker) on can be elevated (Lüscher and Zwislocki 1947), and the subthe evoked activity of a fixed tone burst (probe) were investigated. The temporal interval from masker onset to probe onset (stimulus jective pitch and loudness (Stevens and Davis 1938) of audionset asynchrony), masker frequency, and intensity were paramet- tory events can be altered by preceding and succeeding rically varied. For all of the 53 single units and 58 multiple-unit sounds. Although the perceptual consequences of the tempo-clusters, the neural activity of the probe signal was either inhibited, ral stimulus context have been studied intensively, little infacilitated, and/or delayed by a limited set of masker stimuli. The formation is available on the central neural mechanisms and stimulus range from which forward inhibition of the probe was neural structures underlying the processing of time-varying induced typically was centered at and had approximately the size stimuli.

The studies presented here show evidence that representations of discrete phonological categories are available to human auditory cortex, using an adapted oddball paradigm. Brain activity was recorded using a 37-channel biomagnetometer while 8 subjects listened passively to synthetic speech sounds. In an experiment contrasting stimuli from an acoustic /dæ/-/tæ / continuum, a magnetic mismatch field (MMF) was elicited in a sequence of stimuli in which phonological category labels occurred in a many-to-one ratio, but no acoustic many-to-one ratio was present. In order to isolate the contribution of phonological category labeling to the MMF responses, the acoustic parameter of voice onset time, which distinguished standard and deviant stimuli, was also varied within the standard and deviant categories. No MMF was elicited in a control experiment in which the acoustic distribution of stimuli was identical to the first experiment, but the many-to-one distribution of phonological categories was removed. The design of these studies make it possible to demonstrate the all-or-nothing property of phonological category membership in mismatch responses. This approach contrasts with a number of previous studies of phonetic perception using the mismatch paradigm which have demonstrated the graded property of enhanced

Mapping from acoustic signals to lexical representations is a complex process mediated by a number of different levels of representation. This paper reviews properties of the phonetic and phonological levels, and hypotheses about how category structure is represented at each of these levels, and evaluates these in light of relevant electrophysiological studies of phonetics and phonology. The paper examines evidence for two alternative views of how infant phonetic representations develop into adult representations, a structure-changing view and a structure-adding view, and suggests that each may be better suited to different kinds of phonetic categories. Electrophysiological results are beginning to provide information about phonological representations, but less is known about how the more abstract representations at this level could be coded in the brain.

Abstract. In spite of a large number of studies at different levels of the auditory system, there is still no satisfactory physiological account for the perception of speech. We argue here that many of the problems in creating such an account are common to other processing tasks of the auditory system, such as the extraction of pitch and sound localization. These difficulties are related to the global aspects of these percepts, which depend on integrating information over large bandwidth and on temporal context. We describe a possible framework for a solution- the Reverse Hierarchy Theory, recently formulated to account for difficulties of similar character in the relationships between visual psychophysics and the physiology of the visual system. We show that the concepts of Reverse Hierarchy Theory can be easily mapped into the auditory system, and that it generates natural explanations to some perplexing features of auditory perception of global structures such as pitch, space and speech. We argue that in the context of Reverse Hierarchy Theory, primary auditory cortex seems to occupy a pivotal role in that it generates auditory objects over which higher-order processing tasks can be performed.