Spike discharges from single fibers in the auditory nerve of anesthetized cats were recorded with microelectrodes. Rates of discharge were measured as functions of the frequencies and levels of either single tones or two tones presented simultaneously. We found that the presence of a second tone diminishes the responses to the first tone if appropriate stimulus parameters are chosen. All fibers tested showed this two-tone inhibition. Response areas and inhibitory areas were defined from isorate contours. The general characteristics of the inhibitory areas are found to be similar for a population of over 300 fibers. than 300 fibers that were examined systematically with respect to two-tone inhibition. I. METHODS Most of our experimental techniques and data-processing methods have been described in detail elsewhere [-Kiang, Watanabe, Thomas, and Clark (1965); Kiang, Sachs, and Peake (1967)•. A total of 40 cats, anesthetized with intraperitoneal injections of Dial urethane (75 mg/kg), was used. The activity of single fibers was recorded with 3M KCl-filled micropipets inserted into the auditory nerve under visual observation. During the recording periods, the animals were placed in a double-walled chamber constructed for sound isolation and electrical shielding. Stimulus control and electrode manipulation were performed by the experimenter from outside the chamber. Acoustic stimuli were delivered by a Brtiel & Kjeer 1-in. condenser earphone into a closed system sealed into the central half of the cut external ear canal. A -}-in. condenser microphone was used to measure sound-pressure levels at the tympanic membrane. In this paper, stimulus levels are specified in two ways. In some cases, stimulus levels are expressed in terms of voltage input to the condenser earphone. In most cases, stimulus levels are expressed in terms of peak-to-peak stapes displacement, in order to correct for the transfer characteristics of our acoustic system and the middle ear. [-See Kiang, Sachs, and Peake (1967) for a discussion of these corrections.• An automatic frequency-sweeping technique was used to obtain iso-rate contours for single fibers [-Pfeiffer and Crist (1966)•. The details of this technique are de-

The activity of single auditory-nerve fibers was examined with and without electric stimulation of the crossed olivocochlear bundle (COCB). In the presence of tones at the characteristic frequencies (CF) of the fibers, the effect of COCB stimulation is, in general, to reduce the spike-discharge rate. The time characteristics of the effect were studied for several parameters of the electric shocks, acoustic stimuli, and fiber characteristics. Some effects of COCB stimulation on “spontaneous” and acoustically generated activity are discussed with reference to cochlear mechanisms.

Single unit activity in the posteroventral cochlear nucleus (PVCN) was recorded for a variety of stimulus conditions. The units were classified according to their response characteristics. The locations of units were plotted onto a three-dimensional block model of the cochlear nucleus. Certain types of units that responded best to the onsets of stimuli were located predominantly in the octopus cell region of the PVCN. The remainder of the PVCN, which contains a rather heterogeneous collection of small and multipolar cells, was found to contain several types of units with the dominant type being "chopper" units.