Mild hearing loss can cause apparent memory failures which increase with age and reduce with IQ

Abstract

Rabbitt (1968) tested the accuracy with which young adults with excellent hearing could repeat lists of numbers, words or continuous text which they heard over a system which maintained a constant ratio of white noise to speech signal irrespective of signal strength. Noise levels which allowed individual subjects correctly to repeat every word as they heard it (i.e. shadow without errors) nevertheless reduced their ability to remember what they had heard. This appeared to be because increased effort necessary to recognise words through low levels of noise prevented adequate rehearsal or elaborative encoding of material to be remembered. This finding that slight degradation of sensory input had secondary consequences on memory and comprehension of spoken material led to an interpretation of findings that 960 individuals aged from 50 to 82 years, in contrast to young adults, showed markedly better recall for word lists presented visually than for word lists presented auditorally, even when each word in each list was correctly read or repeated aloud. Audiometric screening of a sub—set of this population allowed 30 individuals with mild hearing losses (35 to 50 db) in each of the three age—decades 50 to 59, 60 to 69 and 70 to 79 years to be compared with matched controls who had pure—tone hearing losses of less than 35 db on memory for lists of 30 words presented visually or over a good—quality sound system. In both presentation modes lists were only scored if all words were correctly read or repeated. Individuals with slight hearing loss recalled visually presented words as well, but recalled auditorally presented words significantly less well, than their controls. For both groups recall accuracy reduced with age. For the control group age had equal effects on auditorally and visually presented material but in the hearing impaired group age significantly increased the advantage for visual over auditory presentation. Two subsequent experiments found (a) that when variance due to individual differences in unadjusted IQ test score and in information processing speed were taken into consideration chronological age, per se, had no further effect on the visual/auditory disparity; (b) that IQ test score and extent of hearing loss interacted in, respectively, reducing and increasing failures to make correct inferences from auditorally presented text. An information—processing model for these effects also predicts that a wide range of other behavioral difficulties in the elderly may often be misattributed to central changes when they are the consequence of mild, and easily remediable, peripheral sensory changes.

Key Words:

• age
• cognitive
• hearing—loss
• intelligence
• memory