The effects of entorhinal cortex lesions on type 1 and type 2 theta.

The effects of bilateral entorhinal cortex lesions on type 1 and type 2 theta generation in the guinea pig were studied in the chronic preparation. EEG recordings from dentate generator zones demonstrated a decrease in the relationship between ongoing type 1 motor movements and theta generation. Before lesioning theta activity accompanied type 1 behaviors (i.e., walking and head movements) in 100% of the samples, while subsequent to lesioning theta activity was present in only 58% of the samples. In addition, prior to lesioning, type 2 theta was present in 100% of the samples taken during immobile sensory processing, e.g., tactile stimulation, while after lesioning type 2 theta was present in only 34% of the samples. Type 2 theta is selectively sensitive to disruption by atropine sulfate. In the present experiment atropine sulfate eliminated all theta activity after entorhinal lesions. Consequently as no type 1 theta appeared to be present it was argued that the entorhinal cortex is a critical route for type 1 theta. However, as the behavioral and sensory correlates of both type 1 and type 2 theta were disrupted it was suggested that the entorhinal cortex is an integral part of both systems.