Early lead exposure affects auditory temporal processing in chicks

The effects of low-level lead on hearing were investigated in newborn chicks. Hearing was measured using an unconditioned response—a delay in subjects’ ongoing peeping when they detect a new sound. Lead acetate was introduced into the air space of the egg at either 12 or 14 days of incubation in doses that varied between 36 and 88 mg kg⁻¹. Subjects were then tested at 0 and 4 days after hatching. A test of absolute thresholds showed no effect of the lead. A test of backward masking, however, showed a significant deficit in temporal processing. In backward masking a short tone pip is followed by a brief silent period and then by a longer and louder noise. Ability to hear the brief signal despite the subsequent masker is a test of rapid hearing. Lead-exposed chicks were deficient in this task, especially at 4 days of age and with a brief silent period between signal and masker. Human children with reading disabilities show a similar deficit. It is thus possible that variations of this test can elucidate basic developmental deficits that are also involved in human perceptual disorders and dyslexia. Copyright © 1999 John Wiley & Sons, Ltd.