Neuroelectrical correlates of categorical perception for place of articulation in normal and lead-treated rhesus monkeys

Categorical perception of place of articulation contrasts was evaluated in rhesus monkeys. The monkeys had been chronically exposed to subclinical levels of lead, either from conception to birth, or for approximately 6 months beginning at birth, or were never exposed to lead. Auditory evoked responses were recorded from scalp electrodes placed over left and right hemispheres during stimulus presentation. The brain responses recorded from the right hemisphere of the normal control group of monkeys discriminated between the categories of [dae] and [gae]. Categorical discriminations were also noted for monkeys exposed to lead either prenatally or postnatally. These discriminations, in contrast, were found over only the left hemisphere. In additions, postnatal exposure resulted in categorical discrimination associated with slower latency components, suggesting a less mature pattern than that obtained for prenatally exposed monkeys. Finally, the brain responses of the animals in the normal control and postnatal exposure conditions evidenced reliable within-category, as well as between-category, discriminations. These results suggest that the neurocortical mechanisms associated with categorical perception of place information may differ between human and nonhuman primates and that early exposure to lead alters these processes.