Directional Hearing, Temporal Order and Auditory Pattern in Peripheral and Brain Stem Lesions

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Abstract
Tests have been applied to the following subjects: (1) 25 normal individuals; (2) the same individuals during temporary impairment of the brain stem by barbiturate (3 mg/kg); (3) 32 patients affected by acute nucleo-reticular vestibular syndrome; (4) 7 patients affected by unilateral Menière's disease, and (5) 1 patient affected by acoustic neuroma with well preserved hearing Directional hearing was tested by changing the DLi and DLt two pure tones (400 and 600 Hz) presented through earphones or two fixed loudspeakers placed at 30 from the azimuth and with balanced intensity: temporal order and auditory patterns were tested with the same frequencies by changing the DLt or the order of presentation of the stimuli at fixed intensity On the basis of the results obtained in the normal subjects, a distinction can be made between the patients affected by brain stem impairment and those affected by Menière's disease. In these patients it was very difficult to obtain reliable discriminations due to the failure of the affected cochlea, especially during an acute phase of the disease, to perform adequate time, intensity and frequency analysis In brain stem patients, spontaneous or pharmacological, the directional hearing judgments were impaired whereas temporal order and auditory pattern discrimination were normal. It is suggested, therefore, that these various central-hearing tests do not investigate analogous performances, but on the contrary different ones. This difference can be attributed to different structures, or to different performance along the same pathways but also to an identical structure of different sensitivity to the noxious agents