Hippocampal representation of touch and sound guided behavior

Understanding the mechanisms by which sensory experiences are stored is a longstanding challenge for neuroscience. Previous work has described how the activity of neurons in the sensory cortex allows rats to discriminate the physical features of an object contacted with their whiskers. But to date there is no evidence about how neurons represent the behavioral significance of tactile stimuli, or how tactile events are encoded in memory. To investigate these issues, we recorded single-unit firing and local field potentials from the CA1 region of hippocampus while rats performed a tactile task. On each trial, the rat touched a plate with its whiskers and, after identifying the texture of the plate, turned to the left or right to obtain its reward. Two textures were associated with each reward location. Over one-third of the sampled neurons encoded the identity of the texture: their firing differed for the two stimuli associated with the same reward location. Over 80 percent of the sampled neurons encoded the behavioral significance of the contacted texture: their firing differed according to the reward location with which it was associated. Texture and reward location signals were present continuously, from the moment of stimulus contact through the entire period of reward collection. The local field potential power spectrum varied across the different phases of behavior, showing that signals of single-units were present within a sequence of different hippocampal states. The influence of location was examined by training rats to perform the same task in different positions within the room. The responses of neurons to a given stimulus in different locations were independent. This was not the case for reward location signals: neurons that carried a signal in one location were more likely to carry a signal in the other location. In summary, during a touch-guided behavior, neurons of the CA1 region represent both non spatial (texture identity) and spatial (reward location) events. Additional experiments were carried out, on another set of rats, to generalize some of the above findings from the tactile to the auditory modality. On each trial, the rat leaned into the gap and heard one of four sounds which were distributed along a vowel continuum from "A" to "I". After identifying the sound, the rat turned to the left or right to obtain its reward. Two sounds were associated with each reward location, and the experiment was repeated on 2 platforms. As in the tactile task, more than 80 percent of neurons represented reward location and more than 25 percent of neurons represented the identity of the sound (the vowel). The role of context on the stimulus and reward location signals was the same as in the tactile experiments. Representations of sounds were independent across 2 platforms but the representations of reward location were not: neurons that carried a signal in one location were more likely to carry a signal in the other location. These responses were absent during passive listening to the sounds.