head direction cells are specialized neurons that fire only when an animal faces a certain range of directions in the horizontal plane , independent of the location and speed of the animal [ 2 , 3 ] . these neurons , which exist in a variety of brain regions , are already almost fully developed at the time when animals begin exploring the outside world , at the age of postnatal day 1618 ( p16p18 ) , a few days after the eyes open at p14p15 [ 8 , 9 ] . the present study was designed to determine whether head direction tuning is present at even earlier ages , before the eyelids open and at a time when rat pups still spend nearly all of their time in the nest . if relative firing directions are maintained from one experimental trial to another , before the appearance of vision , it would point to strong innate components in the mechanism for directional tuning in the brain . a total of 163 cells were sampled from 14 rat pups while the pups moved around twice for 10  min in a circular or square recording box . eighty - six of these cells were recorded during the last 34  days before eye opening ; 77 cells were recorded 12 days after eye opening . pre - eye - opening data were obtained on p11 in one rat , p12 in three rats , p13 in six rats , p14 in eight rats , and p15 in one rat ; post - eye - opening data were collected on p14 in one rat , p15 in eight rats , and p16 in eight rats . the tetrodes were placed in presubiculum in seven rats , in parasubiculum in four rats , at the border between pre- and parasubiculum in two rats , and in medial entorhinal cortex ( mec ) in one rat ( figure  1 ; figure  s1 available online ) . the tetrodes were distributed across deep and superficial layers of pre- and parasubiculum and deep layers of mec . the pups moved freely across the recording arena and covered the entire range of  head directions . median running speeds increased from 7.6  0.1  cm / s before eye opening to 9.4  0.2  cm / s after eye opening ( means across animals  sem ; t(102 )  = 6.9 , p  < 0.001 ) . mean coverage of the recording box increased from 85.7%  0.8% to 91.5%  0.8% ( t(102 )  = 5.0 , p  < 0.001 ) . head - direction - tuned cells were present from the first day when cells could be identified in the target area ( p11 and upward ; figures 1 and 2a ) . to compare directional tuning before and after eye opening , we computed , for each cell , the length of the mean vector for the distribution of firing rates across the 360 of possible head directions . cells were classified as head direction cells if their mean vector was longer than the 95 percentile of a distribution of mean vector lengths for shuffled firing rates ( figure  2b ) . before eye opening , after eye opening , 54 out of 77 cells ( 70.1% ) were classified as head direction cells . the percentage of head direction cells was significantly larger than expected by chance for both age groups ( large sample binominal test with an expected p0 of 0.05 ; pre - eye opening : z  = 27.1 , p  <  0.001 ; post - eye opening : z  = 26.3 , p  < 0.001 ) ( figure  2c ) . the percentage of head direction cells did not increase from pre - eye opening to post - eye opening ( z  = 0.2 , p  = 0.84 ) , but their  directional tuning improved ( mean vector length  sem before eye opening : 0.47   0.02 ; after eye opening : 0.61  0.03 ; t(111 )  = 3.8 , p  < 0.001 ) ( figure  2d ) . the firing rates of the head direction cells did not change significantly from pre - eye - opening to post - eye - opening trials ( 0.54  0.06 and 0.68  0.09 , respectively ; t(111 )  = 1.3 , p  = 0.20 ) , but they were lower than in adult animals . eye opening was accompanied by significant increases in the directional stability of the cells that passed the selection criterion ( figures 2e and 2f ) . before eye opening , directional correlations between the first and the second half of the trial were low , with a mean value of 0.24  0.05 ( t(48 )  = 5.2 , p  <  0.001 ) . after eye opening , on p15p16 , the within - trial correlation increased to 0.60  0.05 , whereas the between - trial correlation increased to 0.53  0.04 . both increases were significant ( within trial : t(93 )  = 5.4 , p  < 0.001 , between trial : t(104 )  = 10.5 , p the increase in stability most likely contributed to the increase in mean vector length in the time - averaged data . after eye opening , directional preferences also rotated along with external cues when the cue card was moved along the wall of the cylinder ( figure  s2 ) . during these trials , the cylinder was enclosed by curtains and rotated with the animal out of sight . when the cue card was placed back in the original position , the cells rotated back to the original position . we asked whether the directional tuning of different cells remained coherent across trials , i.e. , if their relative firing directions were maintained , in the presence of the instability in absolute firing directions prior to eye opening . mean absolute firing directions were calculated for each cell on consecutive trials in which two or more cells passed the 95 percentile criterion for mean vector length . two or more head direction cells were recorded simultaneously in nine pairs of recording trials ( 22 cells , 20 cell pairs ) . the difference between mean firing direction on the first and the second trial was calculated for each cell . the average change in preferred firing direction was 113.1  8.4 ( mean  sem ; figures 3a and 3b ) . the change in mean firing directions for individual cells was then compared to the change in relative firing direction for each cell pair recorded on the same two trials , i.e. , the change across trials in the angular difference between the mean vectors of each cell pair . in contrast to the absolute changes , the drift in pairwise differences was small ( figure  3a ) , with a mean value of no more than 20.6  3.6 , clearly lower than the mean for changes in individual cells ( comparison of frequency distributions :   = 37.2 , degrees of freedom  = 9 , p  < 0.001 ; figure  3b versus figure  3c ) . the maintained differences in firing direction were also apparent in circular correlations of the angular distribution of firing rate between simultaneously recorded cell pairs . circular correlations between cell pairs were calculated for each of the two recording trials with at least three simultaneously recorded cells . the directional correlation of a cell pair was highly correlated between trial 1 and trial 2 ( data set with ten cell pairs : pearson product - moment correlation , r  = 0.95 ; data set with three cell pairs : r  = 0.79 ) . thus , even though the head direction cells displayed low stability before eye opening , the ensemble of head direction cells drifted in a coherent manner . these findings show that head direction cells are widely present in parahippocampal areas well before rat pups open their eyes . the directional tuning of these cells is unstable , however , in that peak firing directions drift over the course of minutes in individual trials and change completely between discrete trials . despite this instability , simultaneously recorded cells maintain relative firing directions , suggesting that a directional map is already present , although anchoring to an external reference frame has not been established . the fact that cells exhibit directional firing before eye opening is consistent with data from adult animals showing that head direction cells maintain directional tuning in complete darkness even though the preferred tuning direction drifts over extended time intervals . recordings from adult animals further demonstrate that head direction cells use external visual landmarks to determine firing direction . rotation of a visual cue card , for example , leads to a corresponding rotation of firing direction on the subsequent trial . the present findings extend these observations by showing ( 1 ) that head direction cells develop independently of both vision and outbound navigational experience in young rat pups and  ( 2 ) that young pups are able to compute instantaneous direction based on integration of angular movement alone . furthermore , when visual input becomes available at p14p15 , this  information is used to calibrate firing direction almost instantly , suggesting that anchoring of directional preferences to the external world can proceed with minimal learning . the relative independence of vision points to alternative sources of sensory input , such as vestibular information , as more important for the process of updating firing in head direction cells . in agreement with this possibility , lesions of the vestibular labyrinth or inactivation of the vestibular hair cells disrupt head direction signals in the anterior dorsal thalamic nucleus and the dorsal presubiculum in adult rats [ 15 , 16 ] . in rat pups , the main features of the vestibular system are in place at an early stage of development . when rat pups are placed on their backs on a surface , for example , they try to right themselves shortly after birth , indicating an early sense of body position . the observation that directional signals emerge before eye opening is consistent with a role for vestibular and other nonvisual modalities in the formation of the head direction signal . finally , the coherent drift of head direction cells in rat pups is  reminiscent of the maintenance of directional relationships among cell pairs in adult animals [ 14 , 18 ] . the coherence of the population activity has implications for the developmental mechanism of head direction tuning . properties of the head direction system have most often been explained by a ring - shaped attractor neural network [ 1921 ] , in which cells have strong intrinsic connections that are set up such that only one part of the network is active at any given time . in the presence of sensory inputs , activity in the network shifts along the  connectivity ring , in correspondence with movement of the head , and different sets of cells are activated accordingly . internal coherence would be expected in such a network , even in the absence of external sensory signals , and therefore these data support such a model . a total of six male and eight female juvenile rats were used for the experiments . post - eye - opening data from three of the rats were included in a previous study . the pups lived with their mother and siblings in transparent plexiglas cages in a temperature- and humidity - controlled vivarium less than 30  m from the recording arena . the animals were kept on a 12  hr light/12  hr dark cycle and had free access to food and water throughout the experimental period . pregnant mothers were checked multiple times per day between 8 a.m. and 8 p.m. p0 was defined as the first day a new litter was observed . recordings were obtained from ten rats before their eyes opened at p14p15 . when a slit between the eye lids was observed on one or both sides , each animal was tested over a period of 26  days between p11 and p16 . the rats were anesthetized in an induction chamber with 5% isoflurane and 2000  ml / min room air . after induction of anesthesia , the rat was secured in a stereotactic frame , the air flow was reduced to 1,2001,600  ml / min , and isoflurane was gradually reduced to 0.5%1.0% . the rat received a subcutaneous injection of bupivacaine ( marcaine ) near the incision site and carprofen ( rimadyl ) as a general analgesic . each rat was implanted with a miniature microdrive with two tetrodes aimed at pre- or parasubiculum . the tetrodes were made of 17  m platinum - iridium wire cut flat to the same level . the tetrodes were platinum plated to reduce impedances to approximately 200 k at 1 khz . coordinates for the tetrode tips were 3.33.5  mm lateral from the midline , 1.51.8  mm in front of the transverse sinus , and 2.02.5  mm ventral to the dura . depth of anesthesia was monitored using tail and pinch reflexes and by observation of the animal s breathing . shortly after surgery , the pup was placed back with its mother and siblings . rats were extensively handled to ensure that pups with implants were accepted upon return to the cage . the rat pup rested on a flower pot covered with a towel while the signal was checked . the pup was connected to an eight - channel light - weight counterbalanced cable connecting the implant to a computer through an ac - coupled unity - gain operational amplifier . the recorded signal was band - pass filtered between 0.8 and 6.7 khz and amplified 6,000 to 14,000 times . recorded spikes were stored at 48 khz with a 32 bit time stamp . a camera in the ceiling tracked the  positions of two light - emitting diodes ( leds ) placed on the head stage . the diodes were positioned 3.5  cm apart and aligned transversely to the animal s body axis . when the signal exceeded approximately four times the noise ratio , the rat pup was placed in a small cylinder ( 50  cm diameter , 50  cm height ) and was allowed to explore freely for two consecutive trials of 10  min each . the rat rested in the flower pot , on a pedestal , between the trials ( 515  min ) . two rats were run in a 50  cm  50  cm square enclosure ( 50  cm height ) for a similar duration . the walls of the arenas were covered with black adhesive plastic with a prominent white cue card ( 25  cm  50  cm ) placed centrally on one side . the oldest rats ( p15p16 ) were given chocolate or vanilla biscuit crumbs to enhance motivation . after the recording session , the tetrodes were generally moved further , and new cell clusters were obtained . the pups were warmed by handling before and after recording to prevent temperature loss . in a subset of animals in the post - eye - opening group , an additional trial was recorded in which the cue card in the recording arena was shifted 90 clockwise . in this trial , the recording arena was enclosed by black curtains so that no distal cues were visible . cell identification was done manually using a graphical cluster cutting tool , with 2d projections of the multidimensional parameter space consisting of waveform amplitudes . 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 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 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 95 percentile was determined . head direction cells were identified as cells in which the mean vector length exceeded the 95 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 all four directional quadrants . the rat received an overdose of pentobarbital and was perfused with an intracardial injection of 0.9% saline followed by 4% formaldehyde . after this , the brain was quickly frozen and cut in 30  m sections . 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 . , r.f.l . , and i.u.k . collected data , t.l.b . and r.f.l . analyzed data , and t.l.b .
summarymammalian navigation is thought to depend on an internal map of space consisting of functionally specialized cells in the hippocampus and the surrounding parahippocampal cortices [ 17 ] . 
 basic properties of this map are present when rat pups explore the world outside of their nest for the first time , around postnatal day 1618 ( p16p18 ) [ 810 ] . 
 one of the first functions to be expressed in navigating animals is the directional tuning of the head direction cells [ 8 , 9 ] . to determine whether head direction tuning is expressed at even earlier ages , before the start of exploration , and to establish whether vision is necessary for the development of directional tuning , we recorded neural activity in pre- and parasubiculum , or medial entorhinal cortex , from p11 onward , 34  days before the eyelids unseal . 
 head direction cells were present from the first day of recording . 
 firing rates were lower than in adults , and preferred firing directions were less stable , drifting within trials and changing completely between trials . yet the cells drifted coherently , i.e. , relative firing directions were maintained from one trial to the next . 
 directional tuning stabilized shortly after eye opening . 
 the data point to a hardwired attractor network for representation of head direction in which directional tuning develops before vision and visual input serves primarily to anchor firing direction to the external world .