Behavioral flexibility requires the prefrontal cortex and striatum, but it is unclear if these structures play similar or distinct roles in adapting to novel circumstances. Here, we investigate neuronal ensembles in the medial frontal cortex (MFC) and the dorsomedial striatum (DMS) during one form of behavioral flexibility: learning a new temporal interval. We studied corticostriatal neuronal activity as rodents trained to respond after a 12-second fixed interval (FI12) learned to respond at a shorter 3-second fixed interval (FI3). On FI12 trials, we found that a key form of temporal processing—time-related ramping activity—decreased in the MFC but did not change in the DMS as animals learned to respond at a shorter interval. However, while MFC and DMS ramping was stable with successive days of two-interval performance, temporal decoding by DMS ensembles improved on FI3 trials. Finally, when comparing FI12 vs. FI3 trials we found that more DMS neurons than MFC neurons exhibited differential interval-related activity early in two-interval performance. These data suggest that the MFC and DMS play distinct roles during temporal learning and provide insight into corticostriatal circuits.

1Behavioral flexibility requires the prefrontal cortex and striatum. Here, we investigate neuronal 2 ensembles in the medial frontal cortex (MFC) and the dorsomedial striatum (DMS) during one 3 form of behavioral flexibility: learning a new temporal interval. We studied corticostriatal 4 neuronal activity as rodents trained to respond after a 12-second fixed interval (FI12) learned to 5 respond at a shorter 3-second fixed interval (FI3). On FI12 trials, we discovered time-related 6 ramping was reduced in the MFC but not in the DMS in two-interval vs. one-interval sessions. 7 We also found that more DMS neurons than MFC neurons exhibited differential interval-related 8 activity on the first day of two-interval performance. Finally, MFC and DMS ramping was 9 similar with successive days of two-interval performance but DMS temporal decoding increased 10 on FI3 trials. These data suggest that the MFC and DMS play distinct roles during temporal 11 learning and provide insight into corticostriatal circuits. 13 Behavioral flexibility requires learning to adapt to uncertainty. Two forebrain structures 14 critical for flexibility are the prefrontal cortex and striatum (Fuster, 2008; Kehagia et al., 2010). 15 Prefrontal cortical neurons densely innervate the striatum (Gabbott et al., 2005; Wall et al., 2013) 16 and disruptions of either structure profoundly impact the learning of new goals, rules, and 17 strategies (Hart et al., 2018; Ragozzino, 2007). Dysfunctional corticostriatal circuits and 18 connectivity are implicated in a range of psychiatric and neurological disorders (Deutch, 1993; 19 Shepherd, 2013). However, the relative roles of prefrontal and striatal networks during 20 behavioral flexibility are unclear. 21One task that provides an ideal window into behavioral flexibility is interval timing, 22 which requires participants to estimate an interval of several seconds via a motor response. 23Across species, interval timing requires the prefrontal cortex and striatum (Coull et al., 2011; Merchant and de 25 Lafuente, 2014). Work from our group and others has shown that both prefrontal and striatal 26 neurons encode temporal information via 'time-related ramping' activity-or monotonic changes 27 in firing rate over a temporal interval (Bakhurin et al., 2017; Donnelly et al., 2015; Emmons et 28 al., 2017; Kim et al., 2018; Narayanan, 2016; Wang et al., 2018). Our past work suggested that 29 ramping activity in neurons of the medial frontal cortex (MFC) and the dorsomedial striatum 30 (DMS) is very similar, with ~40% of neurons in each area exhibiting such activity (Emmons et 31 al., 2017). We have also found that MFC inactivation attenuates DMS ramping (Emmons et al., 32 2019, 2017) and that MFC stimulation is sufficient to increase DMS ramping (Emmons et al., 33 2019). These data suggest that DMS ramping is closely linked to MFC ramping and suggest the 34 hypothesis that MFC and DMS ensembles respond similarly as animals learn new temporal 35 intervals. By contrast, recordings from prima...