The Role of Auditory Attentional Processing and Attentional Behaviour in Accounting for Deficits in Cognitive Abilities of Children Exposed to Environmental Lead

Abstract

The effects of lead poisoning have been known for centuries and the deleterious effects of chronic, low to moderate levels of lead exposure on children's cognitive functioning is now well-established (Needleman & Gatsonis, 1990; Schwartz, 1995). Similarly, low and moderate levels of lead exposure early in life have been frequently associated with behavioural disturbances later in life in children and animals. However, few studies have systematically investigated the link between lead-induced deficits in cognitive functioning and lead-induced behavioural disturbances. This project describes a series of studies investigating the effects of lead on auditory processing and attentional behaviour, and their ability to account for the deleterious effects of lead on IQ in children participating in the Port Pirie Cohort study. Capillary blood samples were collected from the children at various ages from birth to 2 years, and annually thereafter until the age of 7. A measure of lifetime PbB was calculated for each child using the geometric mean of all the blood lead samples. This measure of lifetime PbB was used as a continuous explanatory variable in each of the studies described below. A variety of sociodemographic, health, anthropometric, birth and developmental covariates and potential confounders were recorded prospectively and concurrently. Using Needleman et al.'s (1996) analysis protocol, 8 of these covariates were included in all final regression models. Study One: A total of 387 children from Port Pirie, Australia (mean age of 7.7 years) participated in a study of children’s simple auditory processing. Children completed a series of monaural listening tasks across 4 ear/hand conditions. It was hypothesised that auditory reaction time and auditory processing accuracy would be related the lifetime PbB. Analysis revealed no significant associations between lifetime PbB and the reaction time to target words and a small significant association (3.6%) between lifetime PbB and accuracy of target word detection in just one of the four ear/hand conditions. The strongest associations were found between PbB and false responses to distractor words (10.1%, after adjusting for confounders). It was speculated that this pattern of results could be explained via lead effects on other processes such as impairments to impulse control or attentional processes. Study Two: Study Two investigated the effect of lifetime PbB on children’s complex auditory processing. In this study participants completed dichotic listening tasks across 4 different ear/hand conditions. After controlling for potential confounders, lifetime PbB accounted for 6.2% of the variance in the accuracy of target word detection. A weak direct association (3.0% of the adjusted variance) was also found between lifetime PbB and target word reaction time. No significant associations were found between lifetime PbB and the percentage of false responses or the speed of the false responses. Again the pattern of results was suggestive of a deficit in attentional processes such as freedom from distraction and impulse control. Study Three: This study hypothesised that lead-induced impairments to attentional behaviour - similar to those observed in Studies One and Two - would be observed in other contexts. The mothers of 492 cohort children, and teachers of 454 of the children participated in this study. The mean age of children for whom ratings were collected was 7 years and 4 months. Two versions of the Conner’s Behavior Rating Scale were used to rate the children’s behaviour: The Parent Rating Scale and the Teacher Rating Scale. Lifetime PbB accounted for a significant amount of the variance in Conner’s Parent Behavior Rating Scale scores. It accounted for 5.9% and 7.3% of the unadjusted variance in the Immature/Inattentive, and Hyperactive/Impulsive scores respectively. After adjusting for covariates, each of these associations remained significant. The associations between lifetime PbB and girls' behaviour scores were much higher (11.2%) than the PbB associations with boys' behaviour scores (4.6%). The unadjusted associations between lifetime PbB and Teacher Rating Scores, although much lower, were also significant and accounted for between 1.2% and 2.4% of the variance in the behaviour scores. However, after controlling for potential covariates these associations were no longer significant. These results indicated that increases in lifetime PbB were associated with higher Hyperactivity/Impulsivity and Immature/Inattention and scores on the Conner's Parent Rating Scale, but not the Conner's Teachers Rating Scale. Study Four: In this study it was hypothesised that a significant association would exist between children’s auditory processing performance and parent and teacher behavioural ratings of inattention and impulsivity. Correlational analysis was conducted on the data from Studies One, Two, and Three, to ascertain if the behavioural patterns as reported by the children's parents and teachers, were associated with the children's performance on the auditory processing tasks. Correlational analysis revealed highly significant correlation coefficients in the direction predicted between auditory processing scores and the Inattention and Hyperactivity/Impulsive scores of the Conner's Parent and Teacher Rating Scales. Study Five: The purpose of this study was to examine the extent to which auditory processing and attentional behaviour scores could account for the lead-induced deficits in children's global cognitive abilities. Combining IQ data with data from Studies One, Two, and Three, a hierarchical analysis was conducted with lifetime PbB as the continuous explanatory variable. The analysis entered potential covariates at step 1, behaviour ratings at step 2, auditory processing efficiency in step 3, auditory processing speed in step 4, and lifetime PbB at step 5. The results indicated that when parent and teacher ratings of behaviour and auditory processing efficiency were included in the model, lifetime PbB no longer accounted for a significant proportion of Verbal, Performance or Full-Scale IQ scores. On the other hand, further analyses demonstrated that the significant associations between lifetime PbB and parental behaviour ratings could not be accounted for by including IQ scores as mediating variables in the analytical model. Conclusion:The results of this project provide evidence for the deleterious effect of lead exposure on children's auditory processing performance and parent ratings of behaviour. Further, including these variables in an analysis of the effects of lifetime PbB on IQ served to markedly attenuate direct effect of lead on IQ to the level of non-significance. Taken together the data implicates disturbances to attentional processes (such as impulsivity and inattention) as possible mediators of lead-induced deficits in IQ.