Spatial Localization

The ability to localize sounds in space is a function that appears to depend on the integrity of the primary auditory cortex. The presence of cortical neurons sensitive to interaural differences would be consistent with this functional role. Experimental studies in animals provide the most consistent data. In many species, unilateral destruction of A1 leads to a disturbance in localizing sound sources in the contralateral field. This deficit can be quite specific; for example, if the damage is to a restricted frequency band in the cortex, the animal will be unable to localize within that frequency band but will be unimpaired at other frequencies. Discrimination of sound sources at different locations is sometimes still possible even after complete destruction of A1, however, if the behavioral response entails a simple discrimination rather than responding to a true position in space. Thus, these findings support the concept that auditory cortex is necessary to construct a representation of auditory space rather than merely computing interaural acoustic differences, which is accomplished at earlier levels of the auditory system.

Data on human auditory spatial localization are not as clear as those from controlled animal studies. Patients with unilateral or bilateral hemispheric lesions have often been described who show deficits in localizing sound sources in space, but the precise nature of the damage and type of deficit are not well correlated. Some studies report greater localization problems in the hemifield contralateral to the lesion, but other findings indicate that deficits may exist in both auditory hemifields after damage to the right cerebral hemisphere, which is known to play an important role in spatial processing generally. However, it is important to distinguish the effects of large hemispheric lesions that affect parietal and frontal lobes as well as auditory cortex from more restricted damage to the superior temporal region. The former are likely to result in generalized spatial processing impairments and hemispatial neglect, whereas the latter result in selective auditory localization problems.

Neuroimaging findings indicate that perception of moving sounds or registering the visual position of auditory targets, engages parietal cortex, in keeping with the known role of these areas in spatial processes. Another important finding is that patients with complete removal of one hemisphere early in life show only very mild disturbances of auditory localization, in contrast with the severe effect of much more restricted lesions of auditory cortex occurring in adulthood. Therefore, there may be considerable plasticity in the functional role of auditory cortices—a finding not suspected from the animal literature, perhaps because most such studies are done only in the acute phase and in adult animals, thus not allowing for plasticity to manifest itself.

Anxiety and Depression 101

Anxiety and Depression 101

Everything you ever wanted to know about. We have been discussing depression and anxiety and how different information that is out on the market only seems to target one particular cure for these two common conditions that seem to walk hand in hand.

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