Engram for the complex signals in the mouse brain: distinct neuronal ensembles for compound conditioning stimulus and its components

Abstract

Natural learning involves multiple sensory modalities receiving complex stimuli. Elemental learning theories suggest separate encoding and association of each component in a compound signal. Configural theories predict the formation of a representation of the entire complex signal, which is associated with the second event. We developed a mouse model of fear conditioning to a compound tone-light cue or its separate components to test these alternative theories. First, we investigated the memory dynamics of compound cues and their individual components and discovered that they mature at varying times following conditioning. We demonstrated that the memory of the components matures at different intervals after training: memory of the auditory stimulus and the auditory component of CCS is demonstrated behaviorally right after training, whereas memory of the light stimulus and the light component of CCS matures within three days. The memory of CCS, its components, and discrete conditioned stimuli persists for an extended period, up to one month. A similar dissociation was observed in extinction experiments, revealing that the extinction of memory for one CCS component did not affect the memory of the other component when the extinction procedure began a day after training. In addition, when the extinction procedure began seven days after training, while the memory was fully mature, the extinction of one component of contextual conditional stimuli led to the extinction of the other component. Next, c-Fos imaging was conducted to examine cellular activity across multiple brain regions, including the frontal, prelimbic, cingulate, retrosplenial, parietal, primary and secondary visual, primary and secondary auditory cortices, as well as the hippocampus and amygdala. This examination was performed following conditioning using either the entire compound cue or its individual components. We discovered different cortical activation patterns between compound-cue and single-cue conditioning. Conditioning to the compound cue activated prelimbic and frontal associative cortices, whereas single cues did not. Third, we demonstrated that retrieval of memory only through the entire compound cue, and not through single cues, activated the parietal cortex, primary visual cortex, mediolateral secondary visual cortices, and hippocampal CA1. Fourth, through in vivo two-photon imaging, we examined retrieval-induced c-Fos expression in the parietal cortex of fos-EGFP transgenic mice and identified at least three distinct neuronal populations with differential response specificity to the compound signal and its components. Taken together, our data suggest that intricate signals have the potential to establish both integral and elemental neuronal representations. These representations can be used separately in behavior and have different long-term memory dynamics.