Auditory-evoked response in the clinically normal dog: middle latency components.

The middle latency components (6 to 60 ms) of the auditory-evoked response were recorded in clinically normal mature dogs (n = 15 mixed breed). To record the middle latency response (MLR), a series of suprathreshold auditory clicks was presented monaurally to nonanesthetized and anesthetized dogs at a stimulus rate of 5/s. To determine effects of stimulus intensity on wave amplitude and latency in nonanesthetized dogs, clicks were presented at 90-, 70-, and 50-dB hearing level. Filtered electroencephalic activity during the first 100 ms after each click was recorded and averaged digitally for 1,024 consecutive stimuli. The response pattern in the nonanesthetized dog was a repeatable polyphasic configuration characterized by 3 to 4 major waves. Peaks of the waves were labeled, using a method developed in human beings. Mean latency ranges for the positive (P) or negative (N) peaks that comprised the waves across intensity levels were 7.43 to 7.77 ms (No), 11.07 to 11.57 ms (Po), 13.62 to 14.20 ms (Na), 20.04 to 20.68 (Pa), 39.08 to 39.37 ms (Nb), and 54.25 to 55.21 ms (Pb). The only variations in waveform morphologic features were the additional wave formed by peaks Nx (24.80 to 25.94 ms) and Px (30.0 to 30.62 ms), which occurred in 44% of the records, and the missing wave formed by No and Po in 6.6% of the records. Varying the stimulus intensity had no effect (P less than 0.05) on wave amplitude or latency.(ABSTRACT TRUNCATED AT 250 WORDS)