Learning to avoid dangerous signals while preserving normal behavioral responses to safe stimuli is essential for everyday behavior and survival. Like other forms of learning, fear learning has a high level of inter-subject variability. Following an identical fear conditioning protocol, different subjects exhibit a range of fear specificity. Under high specificity, subjects specialize fear to only the paired (dangerous) stimulus, whereas under low specificity, subjects generalize fear to other (safe) sensory stimuli. Pathological fear generalization underlies emotional disorders, such as post-traumatic stress disorder. Despite decades of work, the neuronal basis that determines fear specificity level remains unknown. We identified the neuronal code that underlies variability in fear specificity. We performed longitudinal imaging of activity of neuronal ensembles in the auditory cortex of mice prior to and after the mice were subjected to differential fear conditioning. The neuronal code in the auditory cortex prior to learning predicted the level of specificity following fear learning across subjects. After fear learning, population neuronal responses were reorganized: the responses to the safe stimulus decreased, whereas the responses to the dangerous stimulus remained the same, rather than decreasing as in pseudo-conditioned subjects. The magnitude of these changes, however, did not correlate with learning specificity, suggesting that they did not reflect the fear memory. Together, our results identify a new, temporally restricted, function for cortical activity in associative learning. These results reconcile seemingly conflicting previous findings and provide for a neuronal code for determining individual patterns in learning.
Results
Experimental setupWhereas AC is involved in auditory fear conditioning, its role in the specificity of fear learning remains controversial. To establish the relationship between sound-evoked activity in the primary auditory cortex and differential fear conditioning (DFC), we recorded simultaneous neural activity from hundreds of neurons. We tracked the same neurons before and after DFC, using two-photon imaging of a virally expressed fluorescent calcium probe (GCaMP6 (Chen et al., 2013), Fig. 1). Longitudinal imaging of neuronal activity in large ensembles of neurons in AC before and after conditioning allowed us to compare the representation of the CS stimuli before and after learning ( Fig. 2A).We conditioned mice by exposure to a sequence of 10 repeats of two tones, one of which coterminated with a foot-shock (CS+, 15 kHz), and one which did not (CS-, 11.4kHz). Pseudo-conditioned mice were presented with the same stimuli (CS, 11.4 kHz and 15 kHz), but the foot-shock occurred during periods of silence between the stimuli (Fig. 2B). We measured recall of the fear memory by presenting the same stimuli to the mouse in a different context and measuring the % of time the mouse froze (Fig. 2C). Learning specificity was defined as a difference between freezing to CS+ and CS-during me...