Interneurons were not included in further analyses. The position of the LED pair on the rat’s headstage was tracked by an overhead camera. Epochs in which the animal ran less than 2.5 cm/s or more than 100 cm/s (tracking artifacts) were removed from the data set. The remaining position data were smoothed using a 21-sample boxcar window filter (400 ms, ten samples on each side). The head direction of the rat was monitored for each tracking sample by plotting of the relative positions of the two LEDs onto the horizontal

plane. A directional tuning function was then generated for each cell by plotting selleck kinase inhibitor of the firing rate as a function of the rat’s directional heading. Only cells with more than 80 spikes and an average rate of more than 0.2 Hz were included in the analyses.

Maps for spike frequency and time were smoothed prior to statistical analysis and graphical presentation with a 1D Gaussian LDK378 cost kernel with a SD of 6°. The directional tuning of each cell was expressed by the length of the mean vector of the circular firing-rate distribution. Head direction cells were defined as cells with mean vector lengths above the chance level, estimated for each age group by a shuffling procedure. For each of the 400 permutations of the shuffling procedure, the entire sequence of spikes fired by the cell was time-shifted along the animal’s path by a random interval between 20 s and the total trial length minus 20 s, with the end of the trial wrapped onto the beginning. A polar firing-rate map was then constructed, and the mean vector length was determined. A distribution of mean vector lengths was then generated for the entire set of permutations from all cells in the sample, and the 95th percentile was determined. Head direction

cells were identified as cells in which the mean vector length exceeded the 95th percentile of the shuffled distribution. The stability of direction-tuned cells was evaluated by correlation of either the spikes in each half of a trial (within-trial stability) or the spikes of two consecutive trials (between-trial stability). Cells were only included in analyses if the rat had moved its head through Liothyronine Sodium all four directional quadrants. Tetrodes were not moved after the last recording day. The rat received an overdose of pentobarbital and was perfused with an intracardial injection of 0.9% saline followed by 4% formaldehyde. The brain was stored in 4% formaldehyde for at least 48 hr. After this, the brain was quickly frozen and cut in 30 μm sections. The slices were mounted on glass and stained with cresyl violet. The final position of the tip of each tetrode was identified on digital pictures of the brain sections. Experiments were performed in accordance with the Norwegian Animal Welfare Act and the European Convention for the Protection of Vertebrate Animals Used for Experimental and Other Scientific Purposes.