Fig. 1: Experimental design and histology of recorded neurons.

A Schematic representation of an experimental setup for extracellular recording of auditory-evoked responses in a rat brain. A schematic lateral view where the hippocampus is shown in violet. Stimuli were sequences of 75 ms pure tones played by a speaker coupled to one of the ears. Red arrows represent a possible pathway for auditory information during the experimental session. B Coronal hippocampal section, 4.5 mm caudal from bregma, with electrolytic lesions in Cornus ammonis 1 (CA1) and dentate gyrus (DG). Scale bar, 500 µm. Electrophysiological recordings were made using either multichannel or tungsten electrodes. C A classical oddball sequence (top) consists of a number of repetitions of a standard tone (STD), with a deviant tone (DEV) at a different sound frequency occurring with low probability. In the current study, only the last STD tone before a DEV tone was used for analysis. To account for the different responses to both sound frequencies, another oddball sequence was played with the STD and DEV roles inverted (second panel). We also used cascade (third and fourth panels) and many-standard sequences (bottom) as non-repetition controls. D Mismatch responses recorded following this procedure (DEV, deviant; STD, standard; CTR, control) can be decomposed in 3 indices (iPE, index of prediction error; iRS, index of repetition suppression; iMM, index of neuronal mismatch). E Coronal section of the hippocampus, indicating the location of CA1 and DG regions. The red line indicates the approximate location of the multichannel probe. F Enlarged view of the DG with the electrode probe overlaid, showing the approximate position of recording sites across different sublayers (gl: granule cell layer; Hilus: hilus (polymorphic layer); CA3: cornu ammonis, area 3; ml: molecular layer). G LFP traces from all recorded channels in one animal (n = 30 channels in 1 rat), aligned to deviant stimulus onset (0 ms), which include the standard tone before the deviant (left vertical blue line), the deviant (vertical red line) and the standard tone after the deviant (right vertical blue line). Channels where units exhibited significant CSI are traced in red. H Time-amplitude plot of LFP activity across channels (n = 30 channels in 1 rat), revealing prominent oscillatory activity in the hilus and granule cell layer (in the infrapyramidal blade). Red and blue lines above the plot indicate deviant and standard tone presentations, respectively. The underlying data for this Figure can be found in Supplementary Data 1.