The responses of single units of the inferior colliculus of the cat to acoustic stimulation.

Abstract

Discharges of single units of the inferior colliculus in response to acoustic stimulation have been studied in anaesthetized and decerebrate cats by means of extracellularly placed electrodes. A tonal stimulus may cause a unit to discharge either with a short burst of spikes or with a slowly adapting train of spikes. For each unit there is a ‘characteristic frequency’ at which the threshold is lowest, with rising thresholds for frequencies on either side. In response to tonal stimuli presented to one ear, some units (classed as Group A units) may respond over a relatively wide range of the audible spectrum, while other units (classed as Group B units) are only activated by tonal frequencies within a narrow band of the spectrum. Some Group B units, however, may be activated by two separate bands of frequencies at different parts of the spectrum; this occurs even though the unit can only be activated by stimuli delivered to one ear. An increase in the intensity of the stimulus causes an increase in the rate of firing of the unit, and also a broadening in the range of tonal frequencies which can activate the unit. This takes place to a greater extent for Group A units than for Group B units. Masking of the unitary responses to clicks by different tonal backgrounds was examined. A click response may be abolished by a particular tonal frequency: on either side of this frequency the characteristics of the responses (i. e. the mean latency and number of spikes per discharge) are altered. This may take place over a large or small range of the audible spectrum. Tonal stimuli delivered to either ear may activate the same unit. In such cases, the unit may have two ‘characteristic frequencies’, but these lie close to one another. The range of tonal frequencies which activates the unit, however, differs for the two sides. Click stimulation alone presented to either ear may activate the same unit. When click stimuli are delivered in succession to the two ears, there is revealed an absolutely unresponsive period during which the second stimulus cannot evoke a detectable response. The duration of this unresponsive period is dependent upon which ear is stimulated first, and is not the same for the two sides. For most units, the absolutely unresponsive period was longer when the contralateral ear was stimulated first. With progressive increase in the interval between the two stimuli there is a relatively unresponsive period during which the characteristics of the responses are altered, until finally a constant response to the second stimulus is obtained. If the two stimuli are separated by short intervals, summation of the two responses may take place. When the sound source is moved in a semicircular horizontal plane in front of the head, and click stimuli are delivered at every 15°, an asymmetrical distribution with respect to the latency is revealed. For those units which are activated by click stimuli delivered to either ear, a progressive and linear change in the mean latency of the response occurs as the source is moved from one ear to the opposite ear. When the sound source is placed at the midline, however, there is a deviation from the linear relationship and the mean latency is significantly decreased. This decrease does not occur for responses from units activated by stimuli presented to one ear alone. The significance of these findings is discussed, particularly in relation to central mechanisms for the localization of sound in space.