integrating sensory information and producing the appropriate motor output are the basic functions of the nervous system , and the neural networks underlying these two functions are tightly linked .
the basal ganglia are involved in various functions , including motor learning , planning , and execution , as well as in decision making and reward ( haber , 2008 ; middleton and strick , 2000 ; schultz et  al . ,
the input layer of the basal ganglia , striatum , receives glutamatergic inputs from multiple cortical areas , including sensory , motor , and prefrontal cortices ( alloway et  al . , 2009 ;
these projections are characterized by a high degree of divergence and convergence ( flaherty and graybiel , 1991 ) , enabling striatal neurons to integrate inputs from different cortical areas and modalities ( chudler et  al . , 1995 ;
nagy et  al . , 2005 , 2006 ; wilson et  al . , 1983 ) .
corticostriatal projections originate from both hemispheres and are mediated by different subtypes of pyramidal neurons ( carman et  al . , 1965 ;
kress et  al . , 2013 ; knzle , 1975 ; lei et  al . , 2004 ; wall et  al . ,
2013 ) , suggesting that striatal neurons may receive bilateral sensory input with different synaptic properties .
striatal neurons respond to sensory input from different modalities such as tactile , auditory , and visual input ( brown et  al . , 1996 ;
nagy et  al . , 2005 ; schulz et  al . , 2009 ; wilson et  al . , 1983 ) .
due to the high convergence in the corticostriatal pathway and the loose topographical correspondence ( kincaid et  al . , 1998 )
, individual striatal neurons may be involved in tactile - visual sensory integration ; however , such synaptic integration at the single neuron level has not yet been shown in striatum .
striatal projection neurons ( msns ) are divided into two main subpopulations according to their projection via the direct or indirect pathway ( alexander and crutcher , 1990 ; smith et  al . ,
1998 ) , with direct pathway neurons facilitating motor activity and indirect pathway neurons inhibiting it ( albin et  al . , 1989 ; kravitz et  al . ,
both subpopulations receive cortical as well as thalamic inputs ( doig et  al . , 2010 ) ; however , it has been debated whether contralateral and ipsilateral corticostriatal projections are selective or biased in targeting direct and indirect pathway msns , respectively ( kress et  al . , 2013 ; lei et  al . , 2004
corticostriatal synapses formed onto direct and indirect pathway neurons and interneurons have different properties ( calabresi et  al .
, 1996 ; fino and venance , 2011 ; kreitzer and malenka , 2008 ; surmeier et  al . , 2007 ) , suggesting that striatal neurons of different types may also respond differently to sensory input in  vivo .
msns recorded in  vivo are characterized by low discharge frequencies ( adler et  al . , 2012 ; berke et  al . , 2004 ;
wilson , 1993 ) , suggesting that a large fraction of their synaptic inputs are subthreshold and do not often contribute to action potential discharge .
we therefore used whole - cell striatal recordings in order to study synaptic responses to tactile and visual stimuli .
we show that neurons throughout dorsal striatum respond to bilateral whisker stimulation in a type - dependent manner and that neurons in dorsomedial striatum perform multisensory integration .
whole - cell patch - clamp recordings were obtained from neurons in dorsolateral striatum ( n  = 59 neurons ) , dorsomedial striatum ( n  = 50 neurons ) , and layer v of s1 barrel field ( n  = 20 neurons ) . of all recorded neurons ( n  = 129 ) , 45 were stained and morphologically reconstructed ( see experimental procedures ) , three of which were cortical pyramidal neurons and 42 were striatal neurons .
striatal neurons were located between 1,854 and 2,613  m below the pia , and the average recording time for all neurons was  48   20  min ( minimum  = 9  min , maximum  = 100  min ; n  = 129 ) .
the striatal regions targeted for recordings were selected according to the existence of corticostriatal projections from primary somatosensory and visual cortices ( s1 and v1 , respectively ) following anterograde tracing ( see experimental procedures ; figures 1a , 1b , 3a , and 3b ) . as described in previous studies ( alloway et  al . , 1999 , 2006 ;
2001 ) , projections from s1 ( barrel field ) were found throughout the dorsal striatum , with higher concentration in dorsolateral striatum ( figures 1b ; figure  s1 available online ) .
projections from visual cortex , on the other hand , were located dorsomedially in proximity to the lateral ventricle ( figure  3b ) , in agreement with previous reports in other species ( donoghue and herkenham , 1986 ; faull et  al . , 1986 ; norita et  al . , 1991 ; serizawa et  al . , 1994 ) .
all recorded cortical and striatal neurons exhibited slow wave oscillations ( figures 2a , 3c , s4 , s6 , and s7 ) with bimodal distribution of the membrane potential ( figures s4 and s7 ) , as previously described ( wilson and kawaguchi , 1996 ) .
striatal neurons , however , had longer up state durations than cortical neurons and rarely discharged action potentials , unlike cortical neurons , which discharged several aps during up states ( figure  s4 ) .
the input resistance of recorded neurons was extracted using step current injections and was calculated separately for up and down states ( figures s4f , s4h , and s4i ) . unlike cortical neurons , striatal neurons had higher input resistance at up states and at more depolarized membrane potentials ( figure  s4f ) , suggesting that inward rectification in msns ( kita et  al . ,
1984 ; nisenbaum and wilson , 1995 ) is the dominant factor determining their input resistance as recorded at the soma , even in the presence of the synaptic barrages occurring during the up states . in order to study bilateral sensory integration , we delivered brief air puffs to the whisker pads on both sides
( see experimental procedures ) and recorded subthreshold responses to ipsilateral , contralateral , and bilateral stimulation in dorsolateral striatal neurons .
using the same stimulation protocol , we compared the whisker - evoked responses in striatal msns to cortical regular spiking neurons ( putative pyramidal cells ) in layer 5 of the barrel cortex ( figures 1 and s2 ; table 1 ) .
all recorded neurons responded to both ipsilateral and contralateral whisker stimulation ( cortical n  = 17 ; striatal msns n  = 20 ) .
sensory responses were classified according to those occurring during  up  or  down  states , including cases in which sensory stimulation triggered state transitions ( reig and sanchez - vives , 2007 ) ( figure  1 and s2 , respectively ) .
response onset delays were significantly longer for ipsilateral than contralateral whisker stimulation in both cortical and striatal neurons ( bf ipsilateral : 25.11   6.49  ms , contralateral : 13.32   3.61  ms , p  < 0.001 , n  = 17 ; dorsolateral striatum ipsilateral : 28.45   6.94  ms , contralateral : 19.78   3.42  ms , p  < 0.001 , n  = 20 ) ( figure  1f ) .
corresponding differences were observed also for response peak latencies ( bf ipsilateral : 71.13   21.98  ms , contralateral : 37.75   8.52  ms , p  < 0.001 , n  = 17 ; striatum ipsilateral : 66.88   26.07  ms , contralateral : 52.62   15.82  ms , p  < 0.01 , n  = 20 ) ( figure  1 g ) .
onset and peak latencies in response to contralateral and bilateral stimulation were significantly shorter in cortical neurons ( figures 1f and 1 g ) .
response amplitudes for both cortical and striatal neurons were always larger when occurring during down states than during up states ( figures 1h and s2f , respectively ) .
contralateral whisker stimulation evoked larger amplitudes than ipsilateral stimulation in both cortical and striatal neurons ( cortex during down states , ipsilateral : 9.11   3.41  mv , contralateral : 14.07   5.67  mv , p  < 0.001 ; striatum during down states , ipsilateral : 10.27   3.27  mv , contralateral : 15.65   6.90  mv , p  < 0.01 ) ( figure  1h ) .
however , when stimuli were delivered simultaneously to both sides , a significant increase in amplitude was observed in striatal responses but not in cortical ones ( cortex bilateral : 15.72   7.09 mv ; striatum bilateral : 20.37   6.52 mv , p  < 0.05 ) ( figure  1h ) .
the increase in amplitude in the striatal msns is also reflected in the slopes of responses to bilateral stimulation ( figure  1i ) , both suggesting a higher sensitivity to bilateral input in striatal msns than that observed in s1 layer 5 pyramidal cells .
this difference could be explained by the higher temporal separation between ipsilateral and contralateral responses in cortical neurons compared to msns ( response peak latency : cortex  = 33.38   21.46  ms ; striatum  = 14.26   24.96  ms , p  < 0.05 , data not shown ) .
sensory and electrically evoked excitation of striatal neurons is accompanied by inhibition ( pidoux et  al . , 2011 ) , originating from striatal interneurons and msn collaterals ( kos and tepper , 1999 ; tunstall et  al . , 2002 ) .
in order to decompose the excitatory and inhibitory response components , we used a low - chloride intracellular solution ( see experimental procedures ) enabling us to hold recorded neurons at the reversal potential for excitation ( 5  mv ) or gabaa inhibition ( 70  mv , figures 2a2d ) . in all recorded msns , excitation preceded inhibition for ipsilateral , contralateral , and bilateral whisker stimulation ( onset delays : ipsilateral excitation 24.63   3.76  ms , inhibition 40.6   9.7ms , p  <
0.01 ; contralateral excitation 19.16   2.8  ms , inhibition 28.98   1.93 ms , p  < 0.001 ; bilateral excitation 19.71   3.27  ms , inhibition 25.61   3  ms , p  < 0.01 , n  = 8) ( figure  2c ) . 
regardless of the stimulus condition ( ipsilateral , contralateral , or bilateral whisker stimulation ) , larger excitatory responses were matched by larger inhibitory ones ( figure  2d ) .
spontaneous activity was also mediated by mixed excitatory and inhibitory synaptic barrages during up states , as recorded at the respective reversal potentials ( figures 2a and s4 ) .
we next wanted to assess the stimulus intensity dependence of the different response conditions . to that end
, we altered the pressure generating the air puff , resulting in different deflection intensities .
the air puff duration remained constant ( 15  ms ) , and the air pressure was changed in a range between 0 and 30  psi ( 0 , 2 , 5 , 10 , 20 , and 30 psi ) ( figure  2e ) .
for all three stimulus conditions , onset and peak delays were shortened , and amplitudes and slopes increased when air puff pressure was increased from 2 to 20 psi ( figures 2e2i ) .
interestingly , response durations , measured as the width at 75% amplitude , were inversely related to the amplitudes and stimulus strength ( figures 2j2l ) , possibly due to curtailing of responses by the increased inhibitory component ( figure  2d ) .
the relationship between ipsilateral and contralateral responses was maintained for different stimulus intensities , with contralateral ( and bilateral ) stimulation evoking earlier onset and peak response latencies ( figures 2f and 2 g ) , with larger amplitudes and rise slopes ( figures 2h and 2i ) . in order to study responses of striatal neurons to visual stimulus
, we obtained whole - cell recordings from neurons in dorsomedial striatum ( figure  3 ) .
the recording area was selected according to the presence of axonal projections from cortical v1 ( figures 3a and 3b ,
n  = 5 ) , which were clustered in dorsomedial portions of the striatum , near the lateral ventricle ( figure  3b ) .
visual stimuli were presented to the contralateral eye as brief light flashes from a white led ( see experimental procedures ) during whole - cell recordings in striatum and lfp recordings from v1 .
as described above , responses were sorted offline to those occurring during up or down states .
the onset delay for contralateral visual responses during down states was 98.43   19.05  ms ( ranging between 56.7 and 141.0  ms , n  = 25 ) , almost five times longer than the onset response to whisker stimulation ( 19.78   3.42  ms , figure  1f ) and similar to previously reported visual responses ( schulz et  al . , 2009 , 2011 ) .
such delays are expected when comparing to the long and variable delays ( 50130  ms ) described for visual responses in mouse visual cortex ( niell and stryker , 2008 ; takagaki et  al . , 2008 ) . in order to verify the occurrence of cortical visual responses
, we obtained extracellular recordings ( lfp ) in v1 simultaneously with the striatal whole - cell recordings ( n  = 16 , figures 3c and 3d ) . onset and peak
latencies were slightly earlier , although not significantly different between cortical and striatal visual responses ( figures 3e and 3f ) , which may be due to the large response variability .
the amplitude of visual responses was 13.28   5.38  mv and slopes 0.14   0.15  mv / ms ( data not shown , n  = 25 ) .
visual responses were evoked in 25 of dorsomedial neurons ( 66% ) and in none of the dorsolateral ones ; however , all neurons in both regions responded to bilateral whisker stimulation ( figure  4 ) .
figure  4c depicts the position of anatomically reconstructed neurons , in which both tactile and visual stimulation were presented ( n  = 30 ) .
all neurons that responded to visual stimuli were located medially ( figure  4c , orange points ) , close to the area receiving axonal projections from v1 ( figure  3b ) .
the differences in onset and peak delays between whisker and visual sensory responses suggested that the strongest responses in striatal neurons would not occur when stimuli are presented simultaneously , but rather when visual stimulation precedes tactile stimulation . to test this prediction
, we presented the whisker stimuli at different time intervals with relation to the visual stimuli ( figure  4d ) .
indeed , in all cases ( n  = 7 ) maximal response amplitudes were evoked when whisker deflections followed the visual stimuli synchronized to the respective response onsets ( relative interval 102.11   20.98  ms , figure  4e ) .
multisensory responses did not summate linearly , with maximal response amplitudes smaller and earlier than those predicted by the linear sum of unimodal responses ( independent whisker 13.73   3.15  mv , independent visual 13.23   4.66  mv , synchronized onset 18.19   4.54  mv , n  = 7 ) ( figures 4d and 4e ) .
this sublinear summation is expected due to the proportional inhibitory component of sensory responses ( figures 2a2d ) but may also be mediated by activation of common cortical association areas ( olcese et  al . , 2013 ) .
as shown above neurons in both dorsolateral and dorsomedial striatum responded to whisker stimulation ; however , there were notable differences in responses recorded in both striatal regions .
dorsolateral msns had stronger and faster responses ( figure  4f ) , as reflected in response amplitudes ( contralateral whisker , dorsolateral striatum : 15.6   6.9  mv , dorsomedial 8.4   3.67  mv ; bilateral dorsolateral 20.37   6.52  mv , dorsomedial 10.29   4.42  mv ) ( figure  4h ) and slopes ( ipsi - dorsolateral 0.36   0.24  mv / ms , dorsomedial 0.16   0.08  mv / ms ; contralateral dorsolateral 0.63   0.46  mv / ms , dorsomedial 0.17   0.09  mv / ms ; bilateral dorsolateral 1.08   0.63  mv / ms , dorsomedial 0.28   0.18  mv / ms , dorsolateral n  = 20 , dorsomedial n  = 24 ) ( figure  4i ) . there were no differences in onset delays ( ipsi - dorsolateral 28.45   6.94  ms , dorsomedial 31.8   8.35  ms , p  = 0.16 ; contra - dorsolateral 19.78   5.09  ms , dorsomedial 18.24   6.68  ms , p  = 0.41 ; bilateral dorsolateral 20.49   5.19  ms , dorsomedial 18.39   5.71  ms , p  = 0.23 , data not shown ) ; however , there were significant differences for peak delays in all conditions , reflecting the slower response rising slopes ( figure  4 g ) .
these differences in sensory responses correlate to the differences between dorsolateral and dorsomedial striatum in the density of axonal projections from primary somatosensory cortex ( figures 1b and s1 ) . in summary , neurons located in the dorsomedial striatum integrate tactile and  visual sensory inputs , with maximal responses when the respective response onsets are aligned .
dorsolateral msns do not respond to visual stimulation but have larger and faster responses to whisker stimulation than dorsomedial ones .
the large majority of striatal neurons are msns , projecting via the striatonigral ( direct ) and striatopallidal ( indirect ) pathways .
since these projection pathways are believed to have different roles in basal ganglia function , it is important to understand whether and how they differ in their integration of sensory input . to that end
, whole - cell recorded and electrophysiological identified msns were subsequently immunostained with d1 antibody ( see experimental procedures ; figures 5a , s5 , and s6 ) in order to classify them as direct or indirect pathway msns .
the staining allowed us to classify msns into d1-expressing and putative d2-expressing msns , which we refer to below as d2 msns . using a drd2 bac transgenic mouse we verified that the d1 antibody did not stain d2-egfp msns ( figure  s5 ) , suggesting that recorded msns that were d1 negative are indeed d2 msns .
msn subtype identification was obtained for 28 msns : 15 d1 positive and 13 d2 neurons ( figures 5a and s6 ) . while similar in most of their electrophysiological properties , d2 msns had higher input resistance than d1 msns , as measured by injection of a depolarizing and hyperpolarizing current step during down states ( in mohm , depolarized d1 112.85   25.76 and d2 158.81   28.93 , p  < 0.001 ; hyperpolarized d1 88.13   27.7 and d2 107.81   39.01 , p  < 0.05 ) ( figure  5c ) .
similar results were reported for d1 and d2 msns in recent slice studies ( gertler et  al .
the differences in onset delays between contralateral and ipsilateral responses were significantly larger in d1 msns ( d1 ipsi : 31.18   6.49  ms , contra : 17.07   7.97  ms , p  <
0.001 ; d2 ipsi : 25.16   3.68  ms , contra : 20.38   5.35  ms , p  <
this difference was caused by the response onset for ipsilateral stimulation that was longer in d1 than d2  msns ( d1 : 31.18   6.49  ms , d2 25.16   3.68  ms , p  <
peak responses to contralateral and ipsilateral stimulation also had different latencies in d1 msns ( ipsi 89.85   44.44  ms , contra 63.08   24.87  ms , p  < 0.01 ) but were similar in d2 msns ( ipsi 70.33   23.47  ms , contra 63.71   23.52  ms , p  = 0.192 ) ( figures 5d , 5f and 5 g ) .
response amplitudes were overall larger in d1 msns than d2 msns , significantly so for contralateral whisker stimulation ( d1 msns 15.14   5.47  mv , d2 msns 10   4.93  mv , p  < 0.05 ) ( figure  5h ) .
the two msn subpopulations also had differences in the response slopes , where d1 msns  had faster slopes for contralateral versus ipsilateral stimuli  ( p  < 0.05 ) but not d2 msns ( p  =  0.13 , see table 2 ; figure  5i ) .
both  d1 and d2 msns had the fastest slopes for bilateral  stimulation ( see table 2 ; figure  5i ) .
differences between  d1 and  d2 msns in their responses to whisker stimulation persisted  also when occurring during up states ( figure  s6 ) ; however , no differences were observed in spontaneous up states  amplitudes or durations ( vm down states  d1  72.28    5.44  mv , d2  72.66   3.02  mv ; ap threshold d1 
46.24   3.31  mv , d2  45.71   3.73  mv ; up state amplitude d1 11.41   3.16  mv , d2 12.24   4.6  mv ; up state duration d1 0.62   0.18 s , d2 0.61   0.26 s ; ap frequency d1 0.16   0.31  hz , d2 0.10   0.13  hz ) ( figure  s6 ) .
we also recorded from identified msns in dorsomedial striatum that responded to  whisker and visual stimulation ( five d1-msns and three d2-msns ) , thus showing that both msn types integrate multimodal  sensory inputs ( figures 5 , 5j , and 5k ) . the  small sample size of type - identified visually responding msns prevents us from drawing conclusions regarding their respective integration properties . in summary ,
msns belonging to the direct and indirect pathways exhibited significant differences in their bilateral integration of tactile sensory inputs .
direct pathway msns ( d1 ) responded earlier ( onset delay ) , stronger ( amplitude ) , and faster ( slope ) to contralateral compared to ipsilateral whisker stimulation . in contrast , in d2 msns contralateral and ipsilateral stimulation resulted in more similar response properties , with ipsilateral responses being earlier ( onset ) and faster ( slope ) .
these results suggest that d1 and d2 msns have different roles in their sensory integration , d1 msns tuned to detect differences between ipsilateral and contralateral whiskers than d2 msns , which act as integrators of bilateral inputs .
striatal interneurons form a small albeit diverse minority in the  striatal microcircuitry ; therefore , our method of  blind  whole - cell patch - clamping resulted in a rather small number of interneurons . out of 109
recorded striatal neurons , four were classified as interneurons , of which two were fast spiking ( figures s7a  s7d ) and two others were cholinergic interneurons ( figures s7e  s7h ) .
neurons were initially classified according to their recorded electrical properties and following morphological staining , according to the aspiny dendrites and the large soma size ( in the case of the cholinergic interneurons ) .
fs interneurons displayed narrow action potentials , relatively depolarized rest potential , high discharge rate of action potentials during up states , and no apparent inward rectification .
cholinergic interneurons were characterized by their voltage sag response to current step injections , depolarized membrane potential , and spontaneous discharge activity .
they also displayed spontaneous slow wave activity , although the amplitude range was not as wide as in fs interneurons or msns yet was sufficient to phase - lock spontaneous discharge to the cortical oscillations as recorded simultaneously in s1 and v1 ( figure  s7 ) .
as in all other recorded neurons , interneurons responded to either whisker stimulation with stronger and earlier responses to the contralateral whisker deflection ( figures s7c , s7 g , and s7i ) , and in one interneuron where visual responses were tested , the recorded fs responded to visual input ( figure  s7c ) .
in this study we used whole - cell patch - clamp recordings to study the integration of bilateral and multimodal sensory information in striatal neurons .
we show that individual striatal neurons integrate bilateral as well as multisensory inputs , that both spontaneous and sensory evoked inputs are comprised of overlapping excitatory and inhibitory synaptic input , and that msns of the direct and indirect pathways differ in the way they integrate bilateral sensory input .
all neurons recorded in dorsal striatum , including projection neurons and interneurons , responded to bilateral whisker stimulation in a type - dependent manner , and a population of dorsomedial neurons also responded to visual input .
recordings were obtained under anesthesia , enabling activation of sensory pathways while avoiding interference with motor related inputs . in future studies
it would be of interest to study these sensorimotor interactions in the unanesthetized , behaving animal .
all neurons recorded in the dorsal striatum responded to stimulation of whiskers of both sides .
the responses differed in latency , slope , and amplitude , with contralateral whisker stimulation inducing larger and earlier responses as seen also in the recordings from cortical neurons .
there are , however , notable differences between the striatal and cortical responses , suggesting different integration properties . onset delays for contralateral and bilateral responses in dorsolateral msns were 6 to 7  ms longer than the cortical ones ( figure  1f ) .
this result , together with the anatomical tracing data , supports the idea that the responses in striatal neurons under our experimental conditions are generated primarily by cortical inputs without engaging a thalamic shortcut ( mowery et  al . , 2011 ) .
bilateral responses were relatively larger in striatal neurons than in cortical ones ( figure  1h ) , showing that striatal msns act as integrators of bilateral sensory input to a higher degree than cortical neurons .
this result can be explained by the time differences between ipsilateral and contralateral onset latencies , which were shorter in striatal neurons , in particular d2 msns ( figure  5 g , inset ) .
the sensory responses we observed were almost entirely subthreshold , compared to a larger fraction of suprathreshold responses to contralateral whisker stimulation recently reported in rats ( pidoux et  al . , 2011 ) .
the discrepancy in the measured contralateral responses may be explained by differences in the  air puff duration and pressure settings ( see experimental procedures ) , striatal coordinates ( medial - lateral or rostro - caudal axes ) , and species ( rat and mouse ) , but it may also be attributed to differences between intracellular sharp and whole - cell recordings ( staley et  al . , 1992 ) .
neurons in both dorsolateral and dorsomedial striatum responded to bilateral whisker stimulation ; however , these responses differed in several aspects .
response amplitudes and slopes were larger in dorsolateral striatum ( figures 4f4i ) ; however , onset latencies were similar between the two regions , suggesting that both receive monosynaptic inputs from s1 .
these results together with the anatomical results ( figure  1b ) suggest that the primary target receiving whisker information is indeed the dorsolateral striatum . a larger striatal area , however , receives the sensory input generating an attenuated and slower response , which , in the case of the dorsomedial striatum , also receives sensory information from a different modality ( figures 4a4e ) .
these differences in sensory input to dorsolateral and dorsomedial striatal regions may underlie the differences in their discharge pattern as recorded during task performance ( thorn et  al . , 2010 ) .
sensory - evoked responses in striatal neurons in all tested cases were composed of excitatory and inhibitory components ( figure  2 ) .
inhibition followed excitation by a few milliseconds , suggesting that it was mediated by intrastriatal gabaergic neurons driven by the same excitatory input .
although the inhibitory component was smaller in amplitude than the excitatory input , it could be strong enough to shape striatal output by preventing or delaying msn discharge ( kos and tepper , 1999 ) . in  vivo studies in neocortex
have shown that visual and tactile sensory input induces temporally complex inhibitory inputs mediated by gabaergic interneurons ( haider et  al . , 2013 ; monier et  al . , 2003
is mediated by gabaergic interneurons , in the striatum at least part of the inhibitory component may arise from msn collaterals ( tunstall et  al . , 2002 ) , in addition to that from gabaergic interneurons .
it is not known which interneurons provide the observed inhibition onto msns ; however , likely candidates are parvalbumin - expressing fs interneurons ( gerfen et  al . , 1985 ) .
, 2010 ; kos and tepper , 1999 ; planert et  al . , 2010 ) and are the first neurons to be activated by cortical input , even before neighboring msns ( mallet et  al . , 2005 ) .
a similar form of feedforward inhibition from fs interneurons onto projection neurons exists in the thalamocortical pathway , where an early activation of fs interneurons by thalamic synaptic input provides rapid disynaptic inhibition of excitatory neurons ( cruikshank et  al . , 2007 ) .
inhibition was also present during ongoing activity , in particular during up states ( figures 2a and s4 ) , as reported also in cortical up states ( haider et  al .
this form of inhibition is likely to originate from striatal neurons that are active during up states ; however , the identity of these neurons is not clear . in our recordings , only a small fraction of msns discharged spontaneous action potentials , whereas the small sample of recorded fs interneurons were more spontaneously active ( figure  s7 ) .
striatal inhibition may also arise from external sources such as neurons in globus pallidus , which increase their discharge rate during striatal up states ( goldberg et  al . , 2003 ) .
a particularly robust pallidostriatal inhibitory pathway is mediated by the recently described arkypallidal neurons ( mallet et  al . , 2012 ) .
the source of inhibition during both  spontaneous activity and sensory - evoked responses should be addressed in future studies using cell - type - specific manipulations .
visual responses were seen in neurons recorded in the dorsomedial striatum in a relatively large yet restricted striatal region that also received axonal projections from visual cortex , thus suggesting that at least part of the response was mediated by excitatory projections from visual cortex ( figure  3 ) .
visually evoked responses were recently described in neurons from rat dorsal striatum , mediated mainly by subcortical inputs following disinhibition of superior colliculus ( schulz et  al .
recorded neurons were located more laterally than in the present study , which together with the different species and anesthesia may explain the different visual responsiveness . as in the current study
, responses were almost entirely subthreshold and had similar latencies ( schulz et  al . ,
2009 ) , suggesting that the visual responses in our recordings may originate from multiple afferent pathways , both cortical and subcortical .
multisensory responses have been described in different basal ganglia nuclei including the striatum using intracellular , extracellular , and optical recordings ( chudler et  al . , 1995 ; cui et  al . , 2013 ;
hikosaka et  al . , 1989 ; nagy et  al . , 2005 , 2006 ; wilson et  al . , 1983 ) .
those studies show that a fraction of striatal neurons changed their discharge rate when presented with sensory input of different modalities , mainly somatosensory and auditory .
our findings support these studies and describe the synaptic input underlying such cross modal interactions .
the striatal neurons we recorded in dorsal striatum showed clear preference for whisker stimulation , while only a fraction of them responded to both tactile and visual input , depending on their topographic location ( figure  4c ) .
responses to whisker stimulation were significantly earlier than those to visual stimulation ( figure  4 ) . in the neocortex
, whisker responses also show shorter latencies ( manns et  al . , 2004 )
the longer latencies in visual responses are in part attributed to retinal processing and may functionally be compensated via retinal motion prediction ( berry et  al . , 1999 ) .
another possible explanation may lie in the nature of  these stimuli , with tactile stimulation originating from nearby objects , whereas visual input would originate from more distal objects , before touching the whiskers .
these differences in processing time between visual and tactile stimuli suggest that introducing a time lag between tactile and visual stimuli would result in increased responsiveness in the striatum , as observed in cortical multisensory areas ( olcese et  al . , 2013 ) .
in agreement with the cortical studies , we showed that maximal response amplitude occurred when visual and whisker inputs simultaneously impinged onto postsynaptic msns . in the current study we only studied visual and tactile sensory integration ; however , the striatum also integrates other sensory modalities such as auditory ( bordi and ledoux , 1992 ) and olfactory ( mcdonald , 1991 ; novejarque et  al . ,
we obtained recordings under anesthesia , enabling the selective activation of sensory pathways , without  contamination  by motor related interactions .
further research addressing multisensory information should also consider other sensory modalities as well as the effects of anesthesia , brain - state , and motor activity on sensory integration ( haider et  al .
we recorded from several neuronal subtypes and observed differences in their spontaneous activity and response to sensory stimuli .
differences in input resistance between direct and indirect pathway msns have been observed in slices ( gertler et  al . , 2008 ;
we found similar results in  vivo , showing higher input resistance in d2 msns , which supports the higher excitability and activity rate of this subpopulation ( cui et  al .
bilateral cortical input has been reported for both direct and indirect pathway msns ; however , there is a debate regarding the bias in target preference for the different cell types ( kress et  al . , 2013 ; lei et  al . , 2004 ; reiner et  al . , 2010 ; wall et  al . ,
the difference we observed in the bilateral integration between d1 and d2 msns may reflect differences between the afferent corticostriatal pathways to the two populations . in this case , our results would support the findings describing ipsilateral as well as contralateral corticostriatal projection onto both d1 and d2 msns and a stronger ipsilateral corticostriatal projection to d1 than d2 msns ( kress et  al . ,
another explanation may lay in the intrinsic properties of msns subpopulations , in particular the increased excitability of d2 msns and their calcium - mediated dendritic depolarization , which were shown to be different from d1 msns ( day et  al . , 2008 ) .
although our data set is too small to enable a quantitative characterization , a few observations are important to note .
all interneurons displayed the slow wave oscillations as was the case for msns and cortical pyramidal cells .
as seen in figures s4 and s7 , interneurons differed in the oscillation amplitudes and shape , suggesting different connectivity patterns conveying the afferent inputs to these neurons .
all interneurons responded to bilateral whisker stimulation , and in a single fs interneuron tested for visual stimulation , such responses were indeed observed , showing that fs interneurons as well as msns perform multisensory integration .
further studies should elucidate the functional role of the different interneuron types in sensory integration using the large and growing arsenal of molecular tools ( fenno et  al . , 2011 ) .
all experiments were performed according to the guidelines of the stockholm municipal committee for animal experiments .
adult c57bl6 mice of both sexes between 2 and 6  months of age were used to perform the experiments ( n  = 92 ) .
anesthesia was induced by intraperitoneal injection of ketamine ( 75  mg / kg ) and medetomidine ( 1  mg / kg ) diluted in 0.9% nacl .
temperature was maintained between 36c37.5c using a feedback - controlled heating pad ( fhc inc . ) .
craniotomies were made at five sites for patch - clamp and extracellular recordings : ap 0  mm from bregma , l 2.5  mm ( dorsomedial striatum ) ; ap 0  mm from bregma , l 3.75  mm ( dorsolateral striatum ) ; ap 1.5 mm , l 3.25 mm ( s1 ) ; ap 1.5  mm , l 2.0  mm ( m1 ) ; ap  3.5  mm , l 2.5  mm ( v1 ) ( following paxinos and franklin , 2001 ) .
whole - cell recordings were obtained from dorsolateral striatum between 1,8542,613  m deep and in layer v of cortical barrel field between 617863  m from the pia , in a perpendicular penetration angle .
signals were amplified using multiclamp 700b amplifier ( molecular devices ) and digitized at 20 khz with a ced acquisition board and spike 2 software ( cambridge electronic design ) .
patch pipettes were pulled with a flaming / brown micropipette puller p-87 ( sutter instruments ) and had an initial resistance of 512 m. extracellular recordings were obtained using tungsten electrodes with impedances of 1 to 2 m. the electrodes were placed in infragranular layers in somatosensory ( bf ) , motor ( m1 ) , and visual ( v1 ) cortex with an angle between 15 and 25. recordings were amplified using a differential ac amplifier model 1700 ( a - m systems ) and digitized at 20 khz with ced and spike-2 simultaneously with the whole - cell recording .
whisker stimulation was obtained by brief air puffs delivered by a picospritzer unit ( picospritzer iii , parker hannifin ) via 1  mm diameter plastic tubes placed at 20  mm in front of the whiskers of both sides .
air puffs ( 15  ms duration ) were given at least 40 times for each stimulus condition ( ipsilateral , contralateral , or bilateral stimulation ) in a random order , with 5  s of interstimulus interval .
visual stimulation was delivered by a white led positioned 50  mm from the contralateral eye .
stimulus duration was 10  ms and was delivered with interstimulus intervals of at least 5 s. the eye was covered with vaseline in order to prevent drying , as previously described ( holtmaat et  al . , 2009 ) .
visual responses were confirmed by monitoring the activation of the contralateral visual cortex using extracellular recordings ( figure  3 ) .
tracer injections were made using glass pipettes ( borosilicate , od  = 1.5  mm , i d  = 1.18  mm ) with a tip diameter of 510  m .
a total of 150250 nl of bda 10% ( 10,000 mw lysine - fixable biotin dextran amine , molecular probes ) was dissolved in 0.9% nacl and fast green ( to aid visualization of the injected tracer ) .
a single injection was done for each cortical area and animal using the coordinates described above , as taken from paxinos and franklin ( 2001 ) .
three to six days following injections , animals were transcardially perfused with a solution containing 4% formalin and 14% saturated picric acid dissolved in 0.1  m phosphate buffer ( pb ) ( ph 7.4 ) .
coronal slices ( 20  m thick ) of both hemispheres containing the entire striatum ( from ap 1.7  mm to ap 2.3  mm , following paxinos and franklin , 2001 ) were obtained using a cryostat and collected on gelatin - coated slides .
sections were incubated over night with cy3-conjugated streptavidin ( jackson immunoresearch laboratories ) and neurotrace 500/525 green fluorescent nissl stain ( invitrogen ) diluted ( 1:1,000 ) in 1% bsa ,
0.3% triton x-100 in 0.1  m pb . at the end of each electrophysiological experiment the mouse was perfused and the brain was placed in fixative solution for 1 to 2  hr in order to stain the neurobiotin - filled neurons ( same procedures and solution described above ) .
sections ( 1012  m thick ) mounted on gelatin - coated slides were incubated overnight with cy2-conjugated streptavidin ( jackson immunoresearch laboratories ) diluted ( 1:500 ) in 1% bsa , 0.3%
triton x-100 in 0.1  m pb . in between all experimental procedures , slices were washed with 0.01  m pbs .
the shortest recording duration for a stained neuron was 24  min , and the average was 55.44   17.87  min ( n  = 45 ) .
reconstructed striatal neurons were immunolabeled for the detection of d1 dopamine receptors , where we found msns that clearly expressed d1 or not ( d1  n  = 15 ; d2  n  = 13 ) .
primary and secondary antibodies were diluted in 1% bsa , 0.3% triton x-100 in 0.01  m pbs .
we used fluorescent and confocal microscopy to recognize the msn receptor expression ( d1 ) . in order to control for the efficacy of the d1 receptor expression described above , we stained slices from d2 egfp mice , showing that d2 expressing neurons were not stained by the antibody ( figure  s5 ) .
 summarythe basal ganglia are involved in sensorimotor functions and action selection , both of which require the integration of sensory information . in order to determine how such sensory inputs are integrated , we obtained whole - cell recordings in mouse dorsal striatum during presentation of tactile and visual stimuli . 
 all recorded neurons responded to bilateral whisker stimulation , and a subpopulation also responded to  visual stimulation . 
 neurons responding to both visual and tactile stimuli were located in dorsomedial striatum , whereas those responding only to whisker deflections were located dorsolaterally . 
 responses were mediated by overlapping excitation and inhibition , with excitation onset preceding that of inhibition by several milliseconds . 
 responses differed according to the type of neuron , with direct pathway msns having larger responses and longer latencies between ipsilateral and contralateral responses than indirect pathway msns . 
 our results suggest that striatum acts as a sensory  hub  with specialized functional roles for the different neuron types . 
Introduction
Results
Discussion
Experimental Procedures