Fig. 6.     Comparative distribution of basal forebrain cells projecting to the prefrontal cortex (PFC), the primary somatosensory cortex (S1) and the auditory cortex (AUD). These maps were generated from 12 sections from three brains using the retrograde tracer Fluoro-Gold deposited in these cortical areas. Only the outlines and the corpus callosum are shown. Lateral view, left is rostral, right is caudal. White dots label the presence of cholinergic neurons, yellow down triangle symbolizes retrograde non-cholinergic cells and the red up triangle marks the location of cholinergic projection neurons. Note that the center of gravity of S1 projection neurons are slightly more rostral than that for auditory projection neurons.