Hippocampal rhythmic slow activity (RSA; theta): a critical analysis of selected studies and discussion of possible species-differences.

The literature concerning the correlates of hippocampal RSA (theta) has seen a wealth of hypotheses generated from seemingly contradictory data. Two possible reasons for this are examined here. (1) An analysis of the maximum published RSA amplitudes in over 70 papers shows that there is enormous variation in how effective various research groups have been in tapping the hippocampal RSA generator zones. It is suggested that this variation is a major source of 'contradictory data'. The enormous variability is probably due to the fact that the laminar structure of the hippocampus, and the location of two seemingly independent 180 degrees out-of-phase RSA generators, results in very disparate signals being recorded by electrodes of different configurations. Electrodes which are not optimally placed result in records which may provide misinformation as to whether or not the hippocampus is in the RSA 'mode'. The results of studies with less than adequate records must therefore be viewed with great caution. (2) An explanation often evoked to account for much of the controversy is that of species differences. This idea is examined and it is suggested that there are probably not major species differences in that all of the species appropriately examined thus far have neural systems capable of producing both an atropine-sensitive and an atropine-resistant form of RSA. All species (with the exception of primates) also show relations of RSA to ongoing motor behavior. However, there are definitely species differences in the neural mechanisms underlying the production of atropine-sensitive, immobility-related RSA. Although all species appear to be capable of producing immobility-related RSA some do so only rarely (e.g. rats), while others do so frequently, particularly in response to sensory stimulation (e.g. rabbits, cats, guinea pigs). Therefore, the answer to the question as to whether there are species differences in the occurrence of RSA may be yes, or not, depending upon how specifically the question is posed.