It is clear that lead exposure in animals and humans may result in impairment in visual and auditory function. An inability to perceive or process sensory information would obviously interfere with performance on any particular test. More importantly, however, deficits in higher-order sensory processing would make it difficult for the child to learn and respond appropriately to the environment. In fact, it is difficult to define the point at which deficits in sensory processing may be defined as "cognitive" deficits. Whereas there have been studies of first-order sensory function in both animals and children, higher-order sensory processing has been studied in very few instances in either children or animals.
Elevated auditory thresholds in the range of speech frequencies were observed as a function of increasing blood lead concentrations in children in the Second National Health and Nutritional Survey (NHANES) II . A study in Poland documented higher auditory thresholds across a range of frequencies as a function of blood lead levels, including blood lead concentrations below 10 |g/dl . Monkeys exposed to lead from birth, with blood lead concentrations of 30 |g/dl at the time of testing, exhibited impaired detection of pure tones at 13 years of age, with the pattern of impairment being somewhat idiosyncratic . A number of elec-trophysiological studies in animals demonstrated impairment at various parts of the auditory pathway in monkeys and other animals related to lead exposure [99-103], although electrophysiological changes have not been universally observed .
Pure-tone thresholds provide only basic, first-level information concerning auditory function. For example, an individual may have normal pure-tone detection and still have difficulty distinguishing speech. Speech generally comprises small but rapid changes in frequency and amplitude. Frequency and amplitude difference thresholds (i.e., the threshold for detection of a change in frequency or amplitude) have been assessed in various animal species, including monkeys and guinea pigs, and may be more sensitive to disruption by toxic exposure than pure tone detection thresholds . A test that is used clinically to evaluate auditory (language) processing is the determination of the infant's ability to distinguish "ba" and "da" and, at a later age, "bi" and "di." Monkeys can also discriminate human speech sounds. In a study with rhesus monkeys, there was evidence that developmental lead exposure impaired the ability of young monkeys to discriminate speech sounds ("da" and "pa") based on an electrophysiological procedure . Lead-exposed children are impaired on the Seashore Rhythm Test [7, 77], which requires the subject to discriminate whether pairs of tone sequences are the same or different. This is a simplified discrimination compared with analysis of speech sounds. Lead-exposed children also had a decreased ability to identify words when frequencies were filtered out (i.e., when information was missing ). Increased lead body burdens were associated with impaired language processing on difficult but not easy tasks , with impairment of the development of word recognition , and with impaired auditory comprehension . The degree to which the deficits in language development are the result of deficits in higher-order auditory processing is unknown.
Lead also produces deficits in visual function. Altmann et al.  reported changes in visual evoked potentials in a cohort of over 3800 4-year-old children with average blood lead concentrations of 42 | g/dl, but no differences in visual acuity or spatial contrast sensitivity functions. In a study in infant monkeys with very high lead levels (300 to 500 | g/dl), one infant appeared to develop temporary blindness . Monkeys exposed to lead during the first year of life with blood lead concentrations during exposure of 85 | g/dl, but not those with 60 | g/dl, had impaired scotopic (very low luminance) spatial contrast sensitivity at 3 years of age . Both spatial and temporal (motion) contrast sensitivity were examined in monkeys with lifetime exposure to lead, with steady-state blood lead concentrations of 25 to 35 |g/dl . Lead-exposed monkeys exhibited deficits in temporal vision at low and middle frequencies under low-luminance conditions, with no other impairments. Lead-induced changes in electrophysiologic responses have also been observed in monkeys exposed developmentally to lead, particularly under low-luminance conditions [100, 115, 116]. Consistent with these findings, rod function was found to be preferentially affected by lead in rats .
Only the most basic level of visual function has been assessed in either children or animals with respect to potential effects of lead exposure. Half of the primate brain is devoted to visual processing, yet neither experimental nor epidemiological researchers have explored the effects of lead exposure in higher-order visual processing.
Lead impairs primary sensory function in both animal models and children. The only study higher-order function in animals (discrimination of speech sounds) identified impairment produced by lead. There are also hints from studies in children of sensory system impairment. All of the "cognitive" endpoints assessed in children require higher-order processing in the visual or auditory systems, including tests of memory, attention, executive function, learning, or any other categorization of performance. Yet there has been virtually no exploration of higher-order sensory processing in either children or animal models; therefore, the degree to which deficits identified as cognitive are actually sensory is unknown. This failure is of considerable practical relevance, since intervention strategies may be different depending on the actual nature of the deficit.
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