Spike-rate intensity functions of cat cortical neurons studied with combined tone-noise stimuli

In the auditory cortex of anesthetized cats, single neurons were studied for their sensitivity to tones presented against backgrounds of continuous wide-spectrum noise. Tone pulses and noise stimuli were mixed acoustically and presented using calibrated, sealed stimulating systems. Data collected were spike-rate intensity functions for tones delivered alone and in the presence of noise. In most neurons, noise of any given intensity induced tone sensitivity shifts that were greatest for frequencies to which the neurons were most sensitive. When the sensitivity loss was in excess of about 15 dB, continuous noise usually caused a steepening of the slope of the tone intensity function. These data suggest that the excitatory response area of a cortical neuron is shaped by multiple, incompletely overlapping inputs of varying sensitivities. In the presence of a continuous noise mask, the disparate thresholds of these inputs may be brought into closer register, resulting in a steepened rate intensity function. These observations may be germane to the neural genesis of the "recruitment" seen in the loudness judgments of normal listeners for masked tones.