Biao Tian

Hierarchical Organization of the Human Auditory Cortex Revealed by Functional Magnetic Resonance Imaging

The concept of hierarchical processingthat the sensory world is broken down into basic features later integrated into more complex stimulus preferencesoriginated from investigations of the visual cortex. Recent studies of the auditory cortex in nonhuman primates revealed a comparable architecture, in which core areas, receiving direct input from the thalamus, in turn, provide input to a surrounding belt. Here functional magnetic resonance imaging (fMRI) shows that the human auditory cortex displays a similar hierarchical organization: pure tones (PTs) activate primarily the core, whereas belt areas prefer complex sounds, such as narrow-band noise bursts.

Modality-specific frontal and parietal areas for auditory and visual spatial localization in humans

Although the importance of the posterior parietal and prefrontal regions in spatial localization of visual stimuli is well established, their role in auditory space perception is less clear. Using positron emission tomography (PET) during auditory and visual spatial localization in the same subjects, modality-specific areas were identified in the superior parietal lobule, middle temporal and lateral prefrontal cortices. These findings suggest that, similar to the visual system, the hierarchical organization of the auditory system extends beyond the temporal lobe to include areas in the posterior parietal and prefrontal regions specialized in auditory spatial processing. Our results may explain the dissociation of visual and auditory spatial localization deficits following lesions involving these regions.

Serial and parallel processing in rhesus monkey auditory cortex

Auditory cortex on the exposed supratemporal plane in four anesthetized rhesus monkeys was mapped electrophysiologically with both pure‐tone (PT) and broad‐band complex sounds. The mapping confirmed the existence of at least three tonotopic areas. Primary auditory cortex, AI, was then aspirated, and the remainder of the cortex on the supratemporal plane was remapped. PT‐responses in the caudomedial area, CM, were abolished in all animals but one, in which they were restricted to the high‐frequency range. Some CM sites were still responsive to complex stimuli. In contrast to the effects on CM, no significant changes were detectable in the rostral area, R.

A PET study of human auditory spatial processing.

To learn more about human auditory spatial processing, we used positron emission tomography (PET) to measure regional cerebral blood flow in human volunteers engaged in sound localization tasks. Spectral and binaural cues of localized sound were reproduced by a sound system and delivered via headphones. During localization tasks, subjects activated inferior parietal lobules (IPL) bilaterally. In a second experiment, matched in design to the first, subjects made non-spatial auditory discriminations based on frequency, activating the IPL bilaterally with left hemispheric predominance. A between-study comparison revealed that the right IPL was significantly more activated during the sound localization task compared with the feature discrimination task, suggesting a preferential role for the right IPL in auditory spatial processing.

Reply to "What', 'where' and 'how' in auditory cortex'

ventral pathway is responsible for identifying the speaker. Similarly, the dorsal pathway processes the melody of an instrumental piece, while the ventral pathway recognizes the instrument by its timbre. This ‘What/How’ model of functional segregation requires experimental confirmation. In particular, it will be important to understand how such functional segregation interacts with hemispheric lateralization, a feature of the auditory cortex that characterizes the human brain.

Analogues of simple and complex cells in rhesus monkey auditory cortex

Receptive fields (RFs) of neurons in primary visual cortex have traditionally been subdivided into two major classes: “simple” and “complex” cells. Simple cells were originally defined by the existence of segregated subregions within their RF that respond to either the on- or offset of a light bar and by spatial summation within each of these regions, whereas complex cells had ON and OFF regions that were coextensive in space [Hubel DH, et al. (1962) J Physiol 160:106–154]. Although other definitions based on the linearity of response modulation have been proposed later [Movshon JA, et al. (1978) J Physiol 283:53–77; Skottun BC, et al. (1991) Vision Res 31(7-8):1079–1086], the segregation of ON and OFF subregions has remained an important criterion for the distinction between simple and complex cells. Here we report that response profiles of neurons in primary auditory cortex of monkeys show a similar distinction: one group of cells has segregated ON and OFF subregions in frequency space; and another group shows ON and OFF responses within largely overlapping response profiles. This observation is intriguing for two reasons: (i) spectrotemporal dissociation in the auditory domain provides a basic neural mechanism for the segregation of sounds, a fundamental prerequisite for auditory figure-ground discrimination; and (ii) the existence of similar types of RF organization in visual and auditory cortex would support the existence of a common canonical processing algorithm within cortical columns.

Processing of “what” and “where” in auditory association cortex

Abstract The auditory system serves a dual function of identifying sound objects and localizing sounds in space. Where in the cerebral cortex are these dual functions of hearing to be found? Neurons in primary auditory cortex (A1) respond primarily to tones of a single frequency. However, several additional areas have been identified recently both physiologically and anatomically in auditory association cortex surrounding A1. More complex response properties are encountered in these areas of the auditory belt suggesting functional specialization. In addition, the anatomical projections of the different belt areas target more remote regions in parietal and prefrontal cortex that are known to subserve specific functions. Parietal cortex, for instance, participates in spatial analysis; different areas of prefrontal cortex are involved in the processing of space and object information. In humans, functional neuroimaging has made specialized auditory processing streams especially evident by lighting up cortical areas that are jointly activated during specific tasks. This article summarizes recent evidence from both human and nonhuman primates supporting the existence of specialized processing streams in auditory association cortex.