Physiology

The vast majority of information concerning neurophysiology of the auditory cortex derives from animal studies. To what extent these findings apply directly to humans is not known; moreover, human auditory cortical neurons may exhibit properties that are not found in other species, considering humans' specific ability to process complex sounds such as speech and music. Nevertheless, it is likely that much of the basic organization of the auditory cortex is at least similar to a first approximation between monkeys and humans.

A. Tonotopy

Tonotopic organization refers to the systematic topographical arrangement of neurons as a function of their response to tones of different frequencies. Tonotopy is a feature of the cortex but is also found throughout the auditory neuraxis, from the most peripheral level, at the basilar membrane of the cochlea as well as throughout most brain stem nuclei, mid-brain, and thalamic levels. In primary auditory cortex, A1, single-unit recordings indicate that the majority of neurons demonstrate frequency selectivity, which is typically measured by establishing frequency threshold tuning curves for different frequencies of stimulation. The frequency for which a given neuron responds at the lowest intensity level is referred to as the characteristic frequency or best frequency. These best frequencies have been shown to be organized along a linear dimension of A1. In the human auditory cortex, it has been well established with electrophysiological and functional imaging techniques that high frequencies are represented in the most medial portion of HG, with lower frequencies represented more laterally along the gyrus. In monkeys, the more anterior field

R is also known to exhibit tonotopy, but its frequency representation is reversed with respect to A1 so that the boundary between the two corresponds to low frequencies. This arrangement is similar to that of visual cortical regions that are adjacent to one another, which show similar reversals in the retinotopic organization of visual input.

B. Complex Response Properties

Although neurons in A1 typically show clear frequency selectivity, many of these neurons display more complex properties. Even neurons with the same best frequency may display different tuning functions. For example, some have very wide receptive fields, meaning that they respond well to tones over a wide frequency range, whereas others are more narrowly tuned. Still others have multiple best frequencies so that they respond strongly to two separate frequencies but not necessarily to frequencies in between. These features are thought to arise from interactions with convergent excitatory and inhibitory inputs from various afferent sources, as described previously. Thus, unlike auditory nerve fibers, whose tuning curves primarily reflect the mechanical tuning properties of the basilar membrane, cortical neurons display properties that are the result of neural interactions and thus represent higher order processing.

Beyond the core regions, the degree of tonotopic organization appears to break down. Some of the cortical fields adjacent to A1 have some tonotopic organization, but in belt and parabelt areas this organization is either absent or only weakly present. Neurons in these areas tend to have complex response properties and are generally not well characterized by simple frequency tuning curves. In the lateral belt areas, neurons tend to discharge with higher rates to narrowband noise bursts than to pure tones, in contrast with core area neurons whose discharge rates are typically highest for pure tones. In addition, the lateral belt neurons tend to have preferred bandwidths; that is, they respond best to sounds that cover a particular frequency range and less well to sounds that cover greater or lesser ranges. These properties suggest that neural integration over specific frequency ranges is occurring, most likely as a result of converging inputs from neurons in the core areas.

In several animal species, it has been shown that many auditory neurons respond not only to frequency information but also to temporal properties of the stimulus. There are neurons that respond primarily with excitation to the onset of the stimulus but are insensitive to its offset or vice versa. Other neurons have more sustained responses throughout the period of stimulaion, and still others respond with inhibition or with a combination of these features. Some neurons have been described in unanesthetized monkeys with very complex receptive field properties that take into account both spectral and temporal information. For example, there may be excitation to a particular frequency region during some time period, whereas another frequency region elicits inhibition for a different time period.

There are also neurons sensitive to frequency modulation. These changes in frequency that can be either monotonic (e.g., frequency glides, where the frequency changes in a consistent direction) or periodic (when the frequency moves up and down at regular time intervals, such as in vibrato). Also, neural responses that follow amplitude modulation, or changes to the amplitude envelope of a sound, have been described. These changes may also be of a periodic nature (e.g., in a tremolo), or responses may occur to abrupt changes in envelope, such as occur when an object strikes another object. These abrupt changes are also found in human consonant speech sounds. Because of their dynamic response properties, it is possible that these types of neurons may play a role in processing complex sounds such as vocalizations, which contain time-varying energy distributed in distinct spectral bands (formants). It is highly likely that similar neural responses occur in humans for processing of speech sounds.

In human studies, the distinction between primary and extraprimary fields, as determined from anatomical data such as that reviewed previously, is generally supported by physiological data. For example, the core region located in HG is seen to respond with the shortest latencies to clicks in depth-electrode recordings, whereas surrounding areas have longer latencies, consistent with the idea that information is passed from one region to the next in a hierarchical arrangement. Also, functional imaging shows that the core areas respond well to stimuli such as noise bursts, whereas the surrounding regions generally respond to stimuli with more complex properties. Also relevant are cortical stimulation studies in which the cortex is stimulated while patients undergo neurosurgery. These observations have shown that stimulation of HG generally leads to nonspecific percepts, such as buzzing or hissing. In contrast, stimulation of anterior and lateral STG regions sometimes results in complex auditory hallucinations. In some cases, subjects report hearing music, speech, or voices quite vividly. These observations are consistent with the idea of a hierarchical arrangement between primary and extraprim-ary areas.

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