Auditory Cortical Responses to the Interactive Effects of Interaural Intensity Disparities and Frequency

J. R. Mendelson
2000 Cerebral Cortex  
Under natural conditions, stimuli reaching the two ears contain multiple acoustic components. Rarely does a stimulus containing only one component (e.g. pure tone burst) exist outside the realm of the laboratory. For example, in sound localization the simultaneous presence of multiple cues (spectral content, level, phase, etc.) serves to increase the number of available cues and provide the listener with more information, thereby helping to reduce errors in locating the sound source. The
more » ... source. The present study was designed to explore the relationship between two acoustic parameters: stimulus frequency and interaural intensity disparities (IIDs). By varying both stimulus frequency and IIDs for each cell, we hoped to gain insight into how multiple cues are processed. To this end, we examined the responses of neurons in cat primary auditory cortex (AI) to determine if their sensitivity to IIDs changed as a function of stimulus frequency. IIDs ranging from +30 to -30 dB were presented at different frequencies (frequency was always the same in the two ears). We found that approximately half of the units examined exhibited responses to IIDs that varied as a function of stimulus frequency (i.e. displayed some form of IID × Freq dependency). The remaining units displayed IID responses that were not clearly related to stimulus frequency. Materials and Methods Recording Preparation Nine adult cats with otoscopically clean ears were used in this study. The surgical procedures employed have been approved by the Canadian Council for Animal Care (CCAC) and comply with the stipulations regarding the care and use of experimental animals set out by the American Physiological Association. Initially, animals were tranquilized with an i.m. injection of ketamine hydrochloride (10 mg/kg) and acetylpromazine maleate (0.1 mg/kg) to allow for venous cannulation. Animals were then given sodium pentobarbital (30 mg/kg i.v.) followed by i.m. injections of atropine (1 mg) to reduce salivation, and dexamethasone sodium phosphate (0.14 mg/kg) to help prevent brain edema. A surgical level of anesthesia was maintained throughout the experiment with a constant i.v. infusion of sodium pentobarbital (2 mg/kg/h) in lactated R inger's solution. Body temperature was maintained at 37.5°C. The EEG and EKG were recorded continuously throughout the experiment to monitor any changes in the state of anesthesia. Anesthesia was considered adequate if the EEG was characterized by relatively large amplitude, low frequency activity coupled with intermittent spindles, and the EKG indicated a heart rate of ∼170 beats/min. Both pinnae were surgically ref lected and the external meatuses exposed to allow for insertion of sound delivery tubes. The temporal muscle was transected on the left side and a craniotomy was performed over the middle of the ectosylvian gyrus. All wound margins and pressure points were generously infiltrated with a long-acting local anesthetic (bupivicaine hydrochloride 2.5%). Stimulation and Recording The animals were located in an electrically shielded sound-attenuating chamber (IAC). Glass-coated platinum-iridium microelectrodes (impedance 0.7-1.3 MΩ) were used to record extracellular single-unit responses. The microelectrode was aimed orthogonal to the surface of the primary
doi:10.1093/cercor/10.1.32 pmid:10639393 fatcat:ldlnuenaqngt3boiuxwtinctam