Limbic and sensory connections of the inferior parietal lobule (area PG) in the rhesus monkey: A study with a new method for horseradish peroxidase histochemistry

The caudal part of the inferior parietal lobule (area PG) was injected with horseradish peroxidase (HRP) in 6 hemispheres of 5 rhesus monkeys. The retrograde transport of HRP resulted in the labeling of neurons in diverse cortical and subcortical areas. In cortex, labeled neurons were noted in prefrontal cortex (areas 8, 45, 46), in the banks of the intraparietal and superior temporal sulci, in medial parietal cortex, in cingulate cortex, in the retrosplenial area, in area TF and the caudal portions of the parahippocampal region. Subcortical sites with labeled neurons included the nucleus basalis of the substantia innominata, the claustrum, the pulvinar and intralaminar thalamic nuclei, the pretectal area, the nucleus locus coeruleus and the raphe nuclei. Although many of the labeled neurons were seen in layers IIIc and V, each cortical area had an individual laminar pattern of labeled neurons. In these experiments, a benzidine dihydrochloride (BDHC) method was used which yields a blue reaction-product at sites containing HRP. BDHC affords superior visibility of labeled neurons, and a significant improvement in sensitivity when compared to a diaminobenzidine procedure in matching series of sections. Additional sections were also stained with a method which allows the simultaneous demonstration of HRP (blue) and acetylcholinesterase (reddish-brown). These revealed that virtually all substantia innominata (nucleus basalis) neurons which project to area PG are also rich in the enzyme acetylcholinesterase. These afferents of PG may be classified into 'sensory association', 'limbic' and 'reticular' categories. It is argued that this arrangement of afferent input may afford a convergence of limbic and sensory information in area PG and that this may subserve a significant function in the process of sensory attention.