It was previously mentioned that cortical damage in both animals and humans leads to deficits in discriminating pitch patterns, particularly when the elements in the pattern are rearrangements of individual elements. This type of finding is indicative of the important role played by auditory cortical regions in processing complex sound patterns. In animals, these patterns are especially important for functions such as responding to conspecific vocalizations, but they are also relevant for interpreting many sounds from the environment.
Many aspects of pattern detection appear to occur automatically, as indexed by electrical and magnetic changes measured from STG. Auditory cortical areas appear to be highly sensitive to changes or deviations from a background, even when subjects are not listening to the stimulus but are engaged in another task. The cortex responds to simple deviations in frequency or intensity but also to subtle changes in more complex patterns, indicating that the auditory system is specialized to some extent for ongoing pattern analysis and detection of novelty. Such a mechanism inplies an important survival value since changes in the auditory environment could signal danger or an event needing attention.
In humans, the perception of speech is among the most complex types of patterns that must be processed and will be discussed later. Another example of pattern perception that is related to speech, but is independent of it, pertains to the processing of voices. Human voices contain characteristic acoustic features that are unlikely to exist in other environmental sounds. Recently, it was proposed on the basis of fMRI data that regions in the upper bank of the superior temporal sulcus may be specialized for processing auditory information related to the perception of voices. These parabelt regions would be well situated to extract higher order stimulus features according to the hierarchical nature of the inputs to these areas.
Another important example of complex auditory pattern perception concerns music. Music, which appears to be both unique to and ubiquitous in the human species, offers a particularly rich source of information concerning human auditory cortical processing. Cases of specific loss of musical functions following brain damage have been described dating back to the 19th century. Recently, detailed studies of patients with bilateral damage to auditory cortices have revealed that certain specific aspects of musical function, notably involving discrimination of pitch patterns, may be selectively abolished while leaving other auditory functions, including speech, intact. These findings lead to the conclusion that aspects of music may have a distinct neural substrate, independent of other processing domains.
Studies of patients with unilateral damage to the anterior STG have generally shown that discrimina tion of melodic patterns is most affected by damage to the right STG, although usually milder disturbance may also be seen after left STG lesions. These findings are in accord with the data reviewed previously indicating that aspects of low-level pitch processing are affected by right HG damage. Thus, whereas more fundamental aspects of auditory processing are dependent on the core regions, damage outside of the core area results in impairments in higher order processes. Deficits in processing tonal patterns are also described following damage to right auditory cortical areas when the stimuli consist of complex spectral elements, such as musical chords, or when the discrimination entails differences in the distribution of spectral energy, such as when changes in harmonic structure affect the timbre of musical instruments. Functional imaging data also suggest that core and belt areas of the right STG are involved in tonal processes. Imaging studies have found neural activity changes, often bilaterally but usually greater on the right, in these areas in tasks requiring analysis of pitch or of pitch patterns, including melodies and chords (Fig. 4).
These findings extend to other types of tasks as well, including situations in which tones must be retained over brief time intervals. In this type of task, working memory mechanisms are involved. Working memory is important for all types of complex auditory processing since sounds necessarily unfold over time, and a mechanism must therefore exist for holding auditory information on-line so that relationships between elements can be appreciated. In the case of melodies, working memory for pitch involves belt areas of the right auditory cortex, as shown both by lesion studies and by functional imaging studies in normal listeners. The latter studies have also shown that pitch judgments depend on interactions between auditory regions and frontal lobe cortices since frontal cortical regions become more active when tones must be retained over time intervals filled with distractor tones. Together, these findings support the idea ofa hierarchy of processing, with basic aspects of pitch analysis being carried out in the core areas in HG, and more complex processes important for pattern analysis dependent on cortical areas in the belt region anterior and possibly posterior, to HG. The perceptual analyses carried out within these regions in turn must interact with frontal lobe regions, presumably via the connectivity described previously, for situations in which working memory is important.
Another example of a complex function is auditory imagery, or the ability to imagine sounds in the absence
Figure 4 Positron emission tomography scans showing neural activity in superior temporal regions in response to auditory stimulation. Each image shows a horizontal slice through the temporal cortex at the position indicated by the dotted line in the inset. Changes in cerebral blood flow (arrows) are superimposed on structural magnetic resonance images. The left image shows increased activity in the left planum temporale in response to hearing speech sounds compared to a control condition of acoustically matched noise bursts. The right image shows bilateral activity in belt areas of the superior temporal gyrus, with stronger activity on the right side, in response to hearing tonal melodies.
of real auditory stimulation. This phenomenon has been studied in the context of musical imagery as well as imagery for speech. Several studies using functional imaging, as well as behavioral lesion techniques, have shown that auditory cortical areas, especially those in the right STG, are involved in the subjective experience of imagining a melodic pattern. Thus, there is an overlap between the cortical areas recruited for perceiving a sound and imagining it. An example of a similar phenomenon has been described for silent lip reading, which results in cortical activation of HG and surrounding areas, particularly on the left. This functional overlap between cortical zones involved in perception and imagery has also been described in the visual modality. These findings suggest that sensory information may be re-evoked by mechanisms that engage the sensory cortices that are initially involved in processing perceptual information from the environment.
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