Functional Organization of Primary Auditory Cortex Maps of Stimulus Features

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The primary auditory field (AI) has been identified anatomically and studied physiologically in a wide range of mammalian species, including humans. It is reciprocally and topographically tied to MGBv. Within AI, neurons are typically sharply tuned for frequency. Like other sensory cortices, AI exhibits a columnar representation of the peripheral sensory epithelium: Neurons occupying a vertical column tend to have the same or very similar CF. A tone just above threshold would activate a relatively small population of neurons in a cortical band having length, depth, and width (a cortical ''sheet''). AI is thus said to be organized into isofrequency bands or sheets. The organization of the AI tonotopic map varies from one subject to the next, possibly suggesting that environmental factors may be involved in the formation and shaping of functional maps. It is now known that the AI tonotopic map exhibits plasticity because it undergoes dramatic change following a cochlear lesion. One can only

Figure 6 (A and B) Lateral and dorsal views, respectively, of the human brain showing the approximate extent and locations of known auditory fields on the lateral and superior surfaces of the superior temporal gyrus. (C) Diagram of the organization of auditory cortical fields on the same temporal lobe areas of the rhesus monkey (adapted with permission from Hackett, T. A., Stepniewski, I. and Kaas, J. H., J. Comp. Neurol. 394, 475-495, 1998).

Figure 6 (A and B) Lateral and dorsal views, respectively, of the human brain showing the approximate extent and locations of known auditory fields on the lateral and superior surfaces of the superior temporal gyrus. (C) Diagram of the organization of auditory cortical fields on the same temporal lobe areas of the rhesus monkey (adapted with permission from Hackett, T. A., Stepniewski, I. and Kaas, J. H., J. Comp. Neurol. 394, 475-495, 1998).

wonder if, for example, the improvement in hearing performance over time exhibited by subjects with a cochlear prosthesis may, to some degree, be attributed to auditory cortical plasticity.

In addition to a tonotopic representation within AI, studies of experimental animals have revealed spatial maps related to other functional properties associated with pure tones, ripple spectra noise, broadband transients, frequency sweeps, and binaural stimuli. The picture that emerges from this work is one of primary auditory cortex being made up of overlaid topographic representations ofnumerous independent stimulus features. To complicate matters even further, these representations, which are based on neuronal firing rate and spike timing, depend on stimulus intensity. Thus, individual stimulus features per se are probably not coded at the cortex simply by some fixed place within the primary auditory field. Instead, they may be coded by spatiotemporal activation patterns created by neuronal assemblies within cortex that change in dynamic ways to reflect the many and changing acoustic features that make up complex natural sounds, including speech.

Several auditory cortical maps represent fundamental acoustic features of a stimulus (e.g., spectrum and noise bandwidth), whereas others represent derived properties (e.g., binaural interactions) or perceptual qualities (e.g., pitch). The binaural organizational patterns distributed across AI are examples of computational maps representing the direction of a sound source in acoustic space. Because there is no extraction of spatial direction by the cochlea, such maps can only result from neural interactions taking place in the lower auditory brain stem, where spike trains arriving over the two monaural channels converge. Similarly, a cortical map of pitch may be laid out orthogonal to the tonotopic map. Pitch is the psychological attribute associate with fundamental frequency even in the absence of spectral energy at the pitch frequency. Hence, its representation on cortex would provide evidence for a higher order cortical representation of a percept rather than simply a sensation tied directly to the physical attributes of a complex stimulus.

AI cortical neurons may encode the frequency of temporal modulation of the sound envelope in the phase-locked activity of cortical neuronal assemblies. Indeed, the phase-locked thalamocortical input may provide the upper frequency limit of pitch encoding (approximatley 400-600 Hz) based on temporal mechanisms. Studies of cortical coding of species-specific vocalization and human speech in monkey have revealed no simple correlation between a single cortical neuron's response properties and a particular utterance. Thus, auditory cortical neurons may be specialized to respond more on the basis of the presence or absence of certain acoustic components embedded in a vocalization rather than on a unique vocalization per se. fMRI studies in humans have also revealed that activation of the core auditory area differed little from the surrounding cortex when speech and nonspeech sounds were presented to normal-listening subjects, again indicating that these fields are more involved in detecting the acoustic rather than linguistic parameters of speech. Indeed, the tonotopic constraints imposed on AI and the highly individualistic responses of its neurons to complex sounds suggest that specific mechanisms for identification of individual phonemes, for example, would not reside in this field and, moreover, that such specificity to speech or other species-specific communication sounds is more likely accomplished by ensembles of cortical neurons rather than by single cells. On the other hand, fields on the ventral aspect of the temporal lobe and on the temporal-parietal boundaries of the left cerebral hemisphere show greater activation to speech than to nonspeech sound.

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Listening to the Binaural Beat

Listening to the Binaural Beat

When you were a kid were you fascinated by those dog whistles that you could blow, not hear but all the dogs in the vicinity would come running? The high pitch was something that only they could here, and though it seemed the dogs didn't seem to arrive in droves as they did in the movies, it was enough for perhaps your pet dog to prick up his ears before sliding back into sleep.

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