Ensemble recordings in awake rats: achieving behavioral regularity during multimodal stimulus processing and discriminative learning.

To meet an increasing need to examine the neurophysiological underpinnings of behavior in rats, we developed a behavioral system for studying sensory processing, attention and discrimination learning in rats while recording firing patterns of neurons in one or more brain areas of interest. Because neuronal activity is sensitive to variations in behavior which may confound the identification of neural correlates, a specific aim of the study was to allow rats to sample sensory stimuli under conditions of strong behavioral regularity. Our behavioral system allows multimodal stimulus presentation and is coupled to modules for delivering reinforcement, simultaneous monitoring of behavior and recording of ensembles of well isolated single neurons. Using training protocols for simple and compound discrimination, we validated the behavioral system with a group of 4 rats. Within these tasks, a majority of medial prefrontal neurons showed significant firing-rate changes correlated to one or more trial events that could not be explained from significant variation in head position. Thus, ensemble recordings can be combined with discriminative learning tasks under conditions of strong behavioral regularity.

A split microdrive for simultaneous multi-electrode recordings from two brain areas in awake small animals.

Complex cognitive operations such as memory formation and decision-making are thought to be mediated not by single, isolated brain structures but by multiple, connected brain areas. To facilitate studies on the neural communication between connected brain structures, we developed a multi-electrode microdrive for chronically recording ensembles of neurons in two different brain areas simultaneously. The "split drive" contains 14 independently movable microdrivers that were designed to hold tetrodes and to permit day-to-day adjustment of dorsoventral position in the brain. The limited weight of the drive allowed rats to adjust well to the headstage after recovering from surgery and permitted stable recording sessions across at least several weeks. In addition to describing the design and assembly of the split drive, we also discuss some important individual parts of microdrives used for tetrode recordings in general. Furthermore, the split drive was applied to two widely separated and connected brain structures, the hippocampus and ventral striatum. From these two areas, stable ensemble recordings were conducted in rats performing a reward-searching task on a triangular track, yielding group sizes of about 15 and 25 units in the dorsal hippocampus and ventral striatum, respectively.