Neuronal conditioned responses in the dorsal hippocampus to combined acoustic stimulation and extracellular polarization

1.	Responses of dorsal hippocampal neurons were investigated during combinations of an acoustic stimulus with a direct current (anode), stimulating the cell through the recording microelectrode. Of the 45 cells, 22% formed conditioned responses of activation or inhibitory types which corresponded with the character of the reinforcing agent. Cells not responding to the stimuli did not form temporary connections. Approximately half of the conditionally responding neurons formed extinction and differentiation to an unreinforced stimulus.
2.	During periodic presentation of combinations of acoustic and electrical stimulation 30% of the 25 cells formed conditioned responses to time. Significant trace reproduction of conditioned responses was observed during 10 to 12 omissions of the stimuli. Cells forming ordinary conditioned responses could produce conditioned responses to time also. They were formed more rapidly.
3.	Prolonged periodic presentation of the acoustic stimulus leads to rhythm binding of the previous stimulation. These responses differ from conditioned responses to time in their short duration, the absence of spontaneous recovery of the responses, and their occurrence within a definite time interval.
4.	It is suggested that the characteristics discovered during the formation of simple temporary connections and observed in various model situations in neurons belonging to different brain structures are common to many conditioned-reflex acts.