Photomicrographs illustrating coronal sections through primary visual cortex (V1) of the macaque monkey brain. (A) Nissl-stained section showing the lamination pattern (cortical layers I to VI) of neurons in this region. (B) Staining of V1 with plasma from one child with autism (age 6 years). Note that in this panel and Figure 2E, the highest numbers of labeled neurons are located in the superficial layers (I to III). (C) Section through area V1 demonstrating in situ hybridization with a probe to glutamic acid decarboxylase 67 (GAD67). The brown reaction product illustrates neurons that are GABAergic. (D) Very similar representation of GABAergic neurons identified immunohistochemically with a monoclonal antibody to γ-aminobutyric acid (GABA). (E) Staining very similar to that shown in Figure 2B of plasma from another child with autism (age 5 years). (F) Section through area V1 that was reacted with plasma from a typically developing child (age 5 years). While there is light, nonspecific background staining that resembles the distribution of Nissl-stained cell bodies, there is no specific labeling of GABAergic neurons. Calibration bar, 100 μm.