32According to predictive coding theory, perception emerges through the interplay of neural circuits 33 that generate top-down predictions about environmental statistical regularities and those that 34 generate bottom-up error signals to sensory deviations. Prediction error signals are hierarchically 35 organized from subcortical structures to the auditory cortex. Beyond the auditory cortex, the 36 prefrontal cortices integrate error signals to update prediction models. Here, we recorded neuronal 37 activity in the medial prefrontal cortex of the anesthetized rat while presenting oddball and control 38 stimulus sequences, designed to separate prediction errors from repetition suppression effects of 39 mismatch responses. Robust mismatch signals were mostly due to prediction errors. The encoding of 40 a regularity representation and the repetition suppression effect over the course of repeated stimuli 41 were fast. Medial prefrontal cells encode stronger prediction errors than lower levels in the auditory 42 hierarchy. These neurons may, therefore, represent the neuronal basis of a fundamental mechanism 43 of hierarchical inference. 44 45 46 Keywords: auditory processing, predictive coding, prediction error, mismatch negativity, medial 47 prefrontal cortex, anaesthesia, neuronal activity 48 49 3 50 51 100 unpredictable deviations from the auditory background. These cells may, therefore, represent the 101 neuronal basis of predictive activity in FCs. 102 103 104
Results
105Context-dependent responses across fields in mPFC. 106 To seek experimental evidence for predictive coding signals in the mPFC, we recorded neuronal 107 activity across all fields in 33 urethane-anesthetized rats. We recorded 83 multiunits (AGM [medial 108 agranular cortex]: 25; ACC [anterior cingulate cortex]: 20; PL [prelimbic cortex]: 20; IL [infralimbic 109 cortex]: 18) and tested 384 tones (AGM: 13; ACC: 90; PL: 81; IL: 81) as part of oddball paradigms 110 and suitable control sequences, namely, cascade and many-standards conditions (Fig. 1). Figure 2 111 illustrates 5 examples of electrolytic lesions in three Nissl-stained sections at different recording sites 112 across the mPFC. Regardless of their anatomical location in all mPFC fields, we found sound-driven 113 neuronal activity to pure tones (0.6-42.5 kHz; 25-70 dB SPL). We assessed significantly increased 114 responses to sound by comparing baseline-corrected spike counts after stimulus presentation against 115 a simulated null peristimulus time histograms (PSTH) with a firing rate equal to the baseline 116 (detailed in Methods). We tested the neuronal response to each pure tone as either being deviant, 117 standard, or part of a control sequence and only included those frequency tones that demonstrated a 118 significant response to any condition. Thus, 86.2% (331/384) of deviant stimuli evoked a significant 119 neuronal discharge, while 26.6% (102/384) of standard stimuli, 26.6% (102/384) of many-standards 120 stimuli and 32.3% (124/384) of cascade stimuli evoked a firing rate ...