Fiber groups in the dorsal psalterium of the guinea pig

The dorsal psalterium (the dorsal hippocampal commissure) of the guinea pig was electrophysiologically studied with a view to possible commissural fiber groups and their properties. Three different groups of commissural fibers were identified, which had conduction velocities (mean +/- SE) of 7.47 +/- 0.14, 2.47 +/- 0.08, and 0.96 +/- 0.04 m/s, respectively, and absolute refractory periods (mean +/- SE) of 0.92 +/- 0.04, 1.26 +/- 0.03, and 2.05 +/- 0.09 ms, respectively. The three groups of fibers did not travel in separate bundles within the dorsal psalterium and were arranged in parallel fashion. The fibers with the fastest mean conduction rate were found responsible for the excitatory synaptic effects evoked in the entorhinal area by dorsal psalterium stimulation, demonstrated by previous studies.