the visual system is specialized to extract features from complex natural scenes that have a unique statistical structure ( simoncelli and olshausen , 2001 ; felsen et  al . , 2005a ) , including edges and contours that change in space and time across the field of view .
although neurons in the primary visual cortex ( v1 ) respond best to local image features that fall within their receptive fields ( rfs ) , their responses are strongly modulated by stimuli placed in the surrounding regions of visual space ( blakemore and tobin , 1972 ; nelson and frost , 1978 ; allman et  al . , 1985 ; gilbert and wiesel , 1990 ) .
typically , stimulating the surround suppresses responses to stimuli in the rf ( jones et  al .
, 2003 ; ozeki et  al . , 2009 ; adesnik et  al . , 2012 ) , and this suppression is more pronounced when using natural surround images than when using their phase - scrambled versions devoid of complex structure ( guo et  al . , 2005 ) .
visual circuits are thus particularly sensitive to integrating salient image components across natural scenes , which may contribute to contour integration and  pop - out  phenomena at the perceptual level ( knierim and van essen , 1992 ) .
concomitantly , surround modulation by natural images alters the firing rate distribution of individual neurons , whereby their responses become more selective and sparse ( vinje and gallant , 2000 , 2002 ; haider et  al . ,
sparse codes are considered efficient , because they are able to transfer more information with fewer spikes ( olshausen and field , 2004 ) .
taken together , surround modulation contributes to contextual processing of sensory information and increases the efficiency of neural representations for natural scenes ( sachdev et  al . , 2012 ) .
how do neural circuits become specialized to integrate and efficiently represent information from complex natural scenes , which contain image features that extend beyond the rf of any individual neuron ?
neurons in v1 are feature selective already at eye opening ( hubel and wiesel , 1963 ; blakemore and van sluyters , 1975 ; chapman and stryker , 1993 ; krug et  al . , 2001 ;
, 2001 ; rochefort et  al . , 2011 ; ko et  al . , 2013
however , little is known about the development of surround modulation and its dependence on early sensory experience , and how this impacts the ability to encode complex natural scenes .
surround modulation is mediated by excitatory and inhibitory interactions at different stages of the mature visual pathway , including the retina ( olveczky et  al .
2006 ) and visual cortex ( stettler et  al . , 2002 ; angelucci and bressloff , 2006 ; girardin and martin , 2009 ; ozeki et  al . , 2009 ; haider et  al . , 2010 ;
, 2012 ; nienborg et  al . , 2013 ; vaiceliunaite et  al . ,
2013 ) . since both excitatory and inhibitory circuits refine after eye opening ( frgnac and imbert , 1984 ; katagiri et  al . , 2007 ;
, 2011 ; ko et  al . , 2013 ) and are  susceptible to changes in visual experience ( ruthazer and stryker , 1996 ; zufferey et  al . , 1999 ;
2004 ) , the effectiveness of surround modulation may be expected to change during postnatal development .
the extent to which this may improve the processing of full field natural scenes is , however , unknown . in this study , we show that circuits mediating surround modulation require sensory experience to become preferentially sensitive to natural stimulus statistics across the rf and its surround . in mature mouse v1 , neuronal firing to natural movies presented to the rf became more selective when costimulating the surround with natural movies than with phase - scrambled movies lacking the higher - order statistical regularities of natural scenes .
in contrast , this preferential sensitivity of center - surround interactions for natural scenes was absent in immature , visually naive v1 after eye opening and in mature animals that were reared without visual input .
mechanistically , the surround - induced increase of response selectivity was mediated by transient membrane potential hyperpolarization that coincided with moments of greatest depolarization during rf stimulation .
these transient hyperpolarizing events were most effective in limiting spiking during full - field natural movie stimulation in adult v1 , consistent with the increased effectiveness of the natural surround stimuli in improving response selectivity .
therefore , normal visual experience is required for the refinement of neuronal circuits that contribute to the selective coding of natural scenes by spatially integrating information from the entire field of view .
to study the effectiveness of surround modulation during postnatal development , we carried out in  vivo whole - cell recordings from individual neurons in cortical layer 2/3 of monocular v1 in immature mice with limited visual experience ( 15  days after eye opening , p14p19 , n  = 18 from 7 mice ) and in visually mature mice with at least 18  days of normal visual experience ( p32p40 , n  = 21 from 10 mice ) . to determine the exact rf size of each recorded neuron
, we alternated the presentation of a naturalistic movie within apertures of increasing size ( isoluminant gray surround ) and the corresponding surround ( annulus ) regions ( figure  1a ; see experimental procedures ) . in
both mature and immature v1 , neuronal firing was stimulus size dependent ( figure  1b ) .
responses first increased and then decreased with increasing aperture size , while response rate decreased for the corresponding surround stimuli ( figure  1b , see figure legend for details ) . the rf size  defined by the aperture diameter at which neurons exhibited a maximal response without a significant response to the corresponding annulus stimulus  was similar for the two age groups ( figure  1e ; mean   sem , mature , 29.9   10 ; immature , 35.3   18 , p  = 0.26 , t test ) . while responses decreased significantly during full - field stimulation with natural movies ( rf  + natural surround ; figures 1c and 1d ) compared to stimulation of the rf alone ( p  < 0.01 for both mature and immature mice , paired t test ) , they were suppressed more in mature v1 ( figure  1f , mature , 71.9%   3.6% ; immature , 35.3%   15.6% , p  = 0.019 , t test ) .
these results show that neurons in immature v1 exhibit surround suppression within a few days after eye opening , but that the suppressive effect of the surround becomes stronger with age . to determine how the developmental strengthening of surround suppression influences the selectivity of neuronal responses
, we computed a measure that captures the distribution of spiking across stimulus repetitions ( here referred to as  selectivity ,  also known as  lifetime sparseness  ; vinje and gallant , 2002 ; lehky et  al . , 2005 ; tolhurst et  al . , 2009 ;
response selectivity increased significantly during natural surround stimulation compared to stimulation of the rf alone ( figure  1 g ; p  < 0.01 , paired t test ) , and this increase was significantly greater in mature animals ( mature , 7.5%   1.1% ; immature , 3.0%   1.1% , p  = 0.008 , t test ) . a reduced spike rate and increased selectivity only add to the efficiency of a neuronal representation if the information about the stimulus is adequately maintained ( laughlin , 2001 ; vinje and gallant , 2002 ) .
hence , the amount of information per spike should increase to compensate for fewer evoked spikes . in both age groups , costimulating the surround significantly increased the information per spike ( see experimental procedures ) relative to the stimulation confined to the rf ( figure  1h , p  < 0.01 , paired t test ) .
this increase tended to be higher in mature than in immature v1 ( mature , 41.9%   6.3% ; immature , 26.2%   8.2% , p  = 0.2 , t test ) , but the effect did not reach significance .
very similar results were obtained in a separate data set using juxtacellular single - cell recordings ( figure  s1 available online ) , indicating that any alterations of the intracellular milieu caused by the whole - cell recording technique did not influence the results .
these age - dependent effects of the surround on firing rate suppression were not influenced by any differences in rf size or absolute firing rate between of neurons recorded in the two age groups ( figure  s2 ) .
taken together , these data indicate that visual circuits are capable of spatial integration already at eye opening , but that surround modulation becomes more effective at suppressing firing and increasing response selectivity to natural scenes with age . in adult monkey v1 ,
the effectiveness of surround modulation depends on the higher - order structure of natural scenes ( e.g. , extended contours ) , because responses to natural images in the rf are suppressed less when randomizing the phase of natural images in the surround ( guo et  al . , 2005 ) .
we therefore tested whether neurons in mature mouse v1 also exhibit the dependency of rf - surround interactions on the statistical properties of surround stimuli .
we compared how responses to the natural movie presented in the rf were altered by costimulating the surround either with the same natural movie ( rf  + natural surround ) or with the phase - randomized version of the same movie ( rf  + phase - randomized surround , figure  2a ) .
note that the phase randomization only removes the higher - order structure in natural images without altering their contrast or spatial frequency composition ( see experimental procedures ) .
accordingly , full - screen presentations of natural and phase - randomized stimuli evoked similar activity levels in both age groups ( figure  s3 ) . for the following analysis
, we only included neurons whose responses were significantly suppressed in at least one  of the surround conditions ( mature , 21/21 cells ; immature , 13/18 cells ; see experimental procedures ) .
in mature v1 , costimulation of the surround with both natural and phase - randomized stimuli reduced firing rates significantly ( figure  2b ; p  = 9   10 , one - way anova ) , increased response selectivity ( figure  2c ; rf  + natural surround , 7.5%   1.1% , p  < 0.001 ; rf 
+ phase - randomized surround , 3.7%   0.9% , p  < 0.001 ; t  test ) and mutual information per spike ( figure  2d ; rf  + natural surround , 41.8%   7.4% , p  < 0.001 ; rf  + phase - randomized surround , 20.6%   6.2% , p  <
importantly , however , stimulating the surround with natural movies decreased firing rates significantly more than phase - randomized surround movies ( figure  2b ; p  < 0.001 , paired t test ) .
this led to significantly greater increases in both selectivity and mutual information per spike during natural compared to phase - randomized surround stimulation ( figures 2c and 2d ; p  < 0.001 and p  = 0.005 , respectively , paired t  test ) .
thus , neurons in mature v1 are sensitive to the higher - order regularities of natural stimuli extending beyond the rf boundary , which makes their responses more selective and informationally efficient .
we next determined whether the increased sensitivity of v1 neurons for natural surround stimuli is already apparent within a few days after eye opening .
in immature mice , the costimulation of the rf with either natural or phase - randomized surround stimuli generated significant spike rate suppression ( figures 2e and 2f , p  = 0.0007 , one - way anova ) , increased response selectivity ( figure  2 g , natural surround , 4.7%   1.3% , p  < 0.001 ; phase - randomized surround , 4.3%   1.8% , p  < 0.001 ; t test ) , and information transmitted per spike ( figure  2h , natural surround , 43.2%   7.8% , p  < 0.001 ; phase - randomized surround , 40.7%   12.8% ,
however , neither the amount of response suppression nor the increase in response selectivity and information per spike was significantly different between the two types of surround stimuli ( figures 2f2h ; p  = 0.17 , p  = 0.72 and p  = 0.67 , respectively ; paired t test ) .
thus , in contrast to experienced animals , neurons in immature v1 did not differentiate between naturalistic and phase - scrambled stimuli in the surround , suggesting that early circuits mediating surround modulation are not yet preferentially sensitive for higher - order structure of natural scenes extending beyond the rf .
we next investigated whether the age - dependent increase in the sensitivity of center - surround interactions for natural scenes can be explained by differences in subthreshold membrane potential dynamics during different stimulus conditions ( figures 3a and 3f ) .
previous reports indicate that surround stimulation leads to more hyperpolarized membrane potential ( vm ) relative to rf stimulation alone ( haider et  al .
, 2010 , 2013 ) , which is partly attributed to increased inhibition in the cortical network ( haider et  al . , 2010 ; adesnik et  al . , 2012 ;
however , when averaged over the entire stimulation period , we found that costimulation of the surround with either natural or phase - scrambled movies slightly depolarized the median absolute vm in immature and mature mice ( figures 3b and 3h ; p  = 0.017 and p  < 0.0001 , respectively ; friedman s test ) .
because it is unclear how such small average differences in vm could contribute to changes in the spiking response selectivity , we focused our analysis on how vm temporal dynamics are altered by surround stimulation .
we quantified moment - to - moment differences in vm between rf and full - field stimulation for each neuron ( vm  = vmrf+surround  vmrf ; see experimental procedures ) .
both natural and phase - randomized surround stimuli induced hyperpolarizing ( negative vm ) and depolarizing ( positive vm ) vm changes relative to rf stimulation alone ( figures 3c and 3 g ) .
plotting the median vm of each cell against its average change in firing rate revealed that vm was strongly correlated with the firing rate suppression during full - field stimulation in mature , but not in immature mice ( figures 3d and 3i ; see figure legend for details ) .
moreover , the distribution of vm was shifted to more negative values during natural than phase - randomized surround stimulation in mature v1 ( figure  3d , p  = 0.027 , wilcoxon rank sum test ) , but not in immature v1 ( figure  3i , p  = 0.6 , wilcoxon rank sum test ) .
how could relatively small differences in vm between natural and phase - randomized surround stimulation lead to pronounced differences in firing rate suppression incurred by these surround stimuli in mature v1 ? to address this question , we determined the dependency of vm on the particular membrane potential value ( relative to spike threshold ) elicited by the rf stimulus at each time point during movie presentation ( vmrf ) .
strikingly , in both age groups , vm exhibited a negative linear dependency on membrane depolarization during rf stimulation : neurons were relatively most hyperpolarized during rf  + surround stimulation ( negative vm ) specifically at times when vmrf was closest to spiking threshold ( figures 3e and 3j ) . which mechanisms underlie the pronounced surround - induced relative hyperpolarization when the vm is most depolarized during rf stimulation ?
surround stimulation has been shown to increase synaptic inhibition ( haider et  al . , 2010 , 2013 ;
we therefore tested the influence of chloride ( cl)-mediated conductances on the inverse relationship between vm and vmrf .
we performed whole - cell recordings using an elevated cl concentration in the intracellular solution ( [ cl]i , see experimental procedures ) to modify the reversal potential of gabaa - mediated conductances ( figure  4a ) .
compared to data recorded with the normal cl - concentration ( figure  3e ) , recordings with elevated [ cl]i revealed a rightward shift of vm values in both natural and phase - randomized surround conditions at all values of vmrf ( figure  4b ) .
these data suggest that an increased cl conductance during surround stimulation at least in part contributes to the negative relationship of vm and vmrf .
the cl conductance may also account for the depolarizing effect of surround costimulation ( positive vm values , figure  4b ) at very hyperpolarized levels of vmrf , if these fall below the reversal potential for gabaa - mediated conductances .
given that the increased cl conductance is most likely mediated by gabaa receptors , we explored the likely sources of gabaergic inputs by targeting parvalbumin ( pv ) and somatostatin ( som ) inhibitory interneurons with whole - cell recordings ( figures 4c and 4d ; see experimental procedures ) .
we found that firing rates of pv and som neurons were on average only slightly but significantly reduced by costimulation of the surround relative to stimulation of the rf alone , irrespective of the surround stimulus type ( figure  4e).the relative firing rate decrease was smaller in both pv and som cells compared to putative pyramidal ( pyr ) cells during either surround stimulus condition ( figure  4f , rf  + natural surround , pyr - pv p  = 8   10 ; pyr - som p  = 8   10 ; rf  + phase - scrambled surround , pyr - pv p  = 0.01 ; pyr - som p  = 0.01 , mann - whitney u test ) .
since pv and som cells maintained relatively high firing rates during rf  + surround stimulation , both interneuron classes can be expected to contribute to the increased inhibition of pyr cells during surround stimulation .
importantly , neither pv nor som cells were preferentially selective for the natural and phase - randomized surround stimuli ( figure  4f , pv p  = 0.12 and som p  = 0.14 , for rf  + natural versus rf  + phase - randomized surround , paired t test ) .
this is consistent with the observation that the rightward shift of the vm and vmrf relationship after elevating [ cl]i was not associated with a change in the slope of the relationship ( figure  4b ) , suggesting that surround stimuli of different statistics cause no major difference in the average increase of cl conductance .
these data further imply an additional involvement of other , most likely excitatory conductances , suggesting that surround suppression is rooted in the modulation of temporally balanced excitation and inhibition ( ozeki et  al . , 2009 ) .
our results thus far suggest that the increased response suppression and selectivity of putative pyramidal neurons during rf  + natural surround stimulation in mature mice could not be explained by a net difference in the amount of inhibition during the two surround conditions . instead
, our results raise the possibility that the timing of inhibition may be important for this selective suppression , because the difference in vm between natural and phase - randomized surround stimulation was largest at most depolarized vmrf ( i.e. , closest to spike threshold ) in mature mice ( figure  5c ; see also figure  3e ) . to examine the timing of surround - induced hyperpolarization in more detail
, we determined the temporal progression of vm before the occurrence of a spike during rf stimulation ( see experimental procedures ) . at times preceding action potential firing events during rf stimulation ( corresponding to instances when the vm is most depolarized , figure  5c ) , natural surround stimuli hyperpolarized the vm more than phase - randomized surround stimuli ( figures 5a , 5c , and s4d ) .
this difference in the relative hyperpolarization between natural and phase - randomized surround ( vm difference ) was significantly larger in mature mice compared to immature mice ( figures 5a5c and s4h ) , both when vm was binned relative to vmrf ( p  = 0.006 , t test ) and relative to rf spike time ( p  = 0.0004 , t test ) .
these findings are consistent with the greatest spike rate suppression during natural surround stimulation in mature v1 ( figures 2b and 3d ) , and suggest that suppression is caused by time - locked vm hyperpolarization that curtails spike generation at moments of largest vm depolarization . accordingly , natural surround stimulation significantly reduced the likelihood that large - amplitude , depolarizing synaptic events ( > 3  mv change within 5  ms , see experimental procedures ) triggered a spike in mature v1 ( figure  5d ; rf versus rf  + natural surround , p  = 1   10 ; rf  + natural surround versus rf  + phase - randomized surround , p  = 0.01 ; kruskal - wallis test and post hoc mann - whitney u test ) , but not in immature v1 ( figure  5e ; p  = 0.19 , kruskal - wallis test ) , even though the number of large - amplitude events did not differ between the stimulus conditions ( figures 5f and 5 g ; p  = 0.34 and p  = 0.59 for mature and immature mice , respectively ; kruskal - wallis test ) .
interestingly , even in instances of action potential firing during surround stimulation , the vm during rf  + natural surround stimulation was more hyperpolarized prior to spike generation compared to rf  + phase - randomized surround stimulation in mature mice ( figure  s4 ) , suggesting that the relative magnitude of excitation and inhibition governs spike generation during full - field stimulation . taken together ,
natural surround stimuli most effectively recruit precisely timed hyperpolarization to increase the selectivity of spiking to stimuli in the rf .
the results thus far suggest that there is an age - dependent increase in sensitivity of visual circuits for features in natural movies extending beyond the rf , which confers greater response selectivity to neurons in v1 . to determine whether this increased sensitivity for the statistical structure of full - field natural scenes depends on visual experience during development , we carried out recordings in mature mice that were reared in the dark until p32p40 and therefore never experienced normal visual input
. the estimated rf size did not differ significantly between the dark - reared , immature , and normal mature mice ( p  = 0.65 , one - way anova ; figure  s5 ) . similar to immature and mature animals ,
costimulation of the surround suppressed neuronal responses and increased their selectivity in dark - reared mice ( figures 6a and 6b ; firing rate change rf  + natural surround , 60.7%   7.9% , p  < 0.001 ; rf  + phase - randomized surround , 52.3%   10.3% , p  < 0.001 , n  = 15 ; t test ) , indicating that the capacity of visual circuits for surround modulation was maintained and not disrupted by rearing animals devoid of visual experience .
importantly , however , we observed no significant differences between the effects of the natural and phase - randomized surrounds on responses to stimuli in the rf ( figure  6b ) in terms of firing rate ( rf  + natural versus rf  + phase - randomized surround , p  = 0.33 , paired t test ) , response selectivity ( p  = 0.23 , paired t test ) , or information transmitted per spike ( p  = 0.88 , paired t test ) . differences in the level of spike suppression were not related to differences in absolute firing rates in any age group ( figure  s6 ) .
the indifference of dark - reared v1 neurons to the statistical properties of surround stimuli was also reflected at the level of subthreshold membrane potential dynamics ( note that the cellular sensitivity for spiking to membrane potential changes was comparable to the other age groups ; figure  s5 ; azouz and gray 2003 ) .
the median vm in dark - reared mice was not significantly altered by costimulation of rf and surround ( figures 6c and 6d , n  = 19 ; p  = 0.33 ; friedman s test ) .
similar to normally reared , mature mice , there was a strong dependence of vm ( figure  6e ) on the level of vm depolarization during stimulation of the rf alone ( figures 6 g and 6h ) .
however , the distribution of vm was not different between natural and phase - randomized surround stimulation conditions ( figure  6f , p  = 0.21 , wilcoxon rank sum test ) , and vm at most depolarized vm during rf stimulation was not significantly different when costimulating the surround with natural and phase - randomized stimuli ( figures 6 g and 6h ) in dark - reared v1 , similar to the observations in immature v1 ( figures 3f3j ) .
accordingly , while the likelihood of spiking during large - amplitude depolarizing events ( which were unaltered in frequency of occurrence across conditions ; figure  6j ; p  = 0.82 , kruskal - wallis test ) was reduced , it was not significantly different between the two surround conditions ( figure  6i ; p  = 0.18 , kruskal - wallis test across all conditions ) . these findings are consistent with a similar level of firing rate suppression by phase - randomized and natural surround stimuli ( figure  6b ) in these visually inexperienced but mature animals .
thus , the emergence of neuronal sensitivity for image features extending beyond the rf boundaries requires visual experience after eye opening .
our findings provide evidence for a progressive developmental refinement of visual processing to the global statistics of the natural environment , as hypothesized previously ( olshausen and field , 1996 ; berkes et  al . , 2011 ; sachdev et  al . ,
v1 we observed developmental improvements in coding efficiency for natural scenes after eye opening ( increased response selectivity and mutual information rate ) , which was brought about by an increased neuronal sensitivity for natural scene statistics in the rf surround , but not for surround stimuli lacking the statistical regularities of natural scenes .
this emergence of efficient processing of natural stimuli was dependent on sensory experience , because it was absent in animals reared without visual input . in cat and monkey v1 , costimulation of rf and
its surround with naturalistic stimuli leads to more sparse and efficient responses than during stimulation of the rf alone ( vinje and gallant , 2000 ; haider et  al . ,
similarly , we found that in mature mouse v1 , the full - field naturalistic movie was most effective for reducing spike rate and increasing selectivity and information per spike , consistent with the idea that neural codes are constrained by the same factors across mammalian species ( i.e. , energy consumption and information transmitted ) .
our findings reveal the existence of circuit mechanisms for improving coding efficiency beyond that provided by the filter characteristics of the rf alone ( olshausen and field , 1996 ; david et  al . , 2004 ; felsen et  al . , 2005b ) , which depend on the specific structure of natural scenes spanning the rf and its surround . while phase sensitivity of the surround in general has been suggested before ( guo et  al . , 2005 ;
, 2007 ; xu et  al . , 2005 ) , we show that the sensitivity to the spatiotemporal stimulus correlations across rf and surround is a plausible mechanism for improving neuronal selectivity . at the population level in mouse v1 ,
recent experiments indicate on the one hand that surround suppression is orientation tuned ( self et  al . , 2014 ) and on the other hand that the representations of natural stimuli are sparser than those of phase - scrambled stimuli ( froudarakis et  al . , 2014 )
our data not only suggest a circuit mechanism for this increased coding efficiency of natural scenes but also reveal its developmental dependency .
importantly , while surround suppression was apparent albeit weaker already in the first days after eye opening , the surround - induced increase in response selectivity and information per spike were unspecific to the statistical properties of the surround stimuli in these visually inexperienced mice .
the circuit mechanisms for increasing response selectivity are therefore present but not yet sensitive to detect the higher - order stimulus correlations of natural scenes in the immature visual pathway .
moreover , neurons in dark - reared , mature v1 were also indifferent to the statistics of surround stimuli . visual experience , therefore , may be required to promote the refinement of neuronal circuits to detect congruent information across the field of view , which leads to improved response selectivity of mature v1 neurons for features embedded in full - field natural scenes .
we note , however , that this refinement may not only depend on visual experience , as dark - rearing may also delay the development of visual circuits ( fagiolini et  al .
, 1994 ; iwai et  al . , 2003 ; espinosa and stryker , 2012 ) .
cortical inhibition likely plays a role in surround - induced response suppression in v1 ( haider et  al .
our results extend this idea by revealing how costimulation of the rf surround affects membrane potential dynamics to suppress neuronal firing ; while the average membrane potential was altered little by surround stimulation , the principal effect of the surround was to counteract membrane depolarization generated by stimulation of the rf alone . specifically , we observed an experience - dependent increase of relative membrane hyperpolarization by natural surround stimuli at times of large depolarizing events during rf stimulation .
this hyperpolarization was partly mediated by an increased cl conductance , most likely through gabaa receptors .
yet the average firing rates of pv and som interneurons , although slightly reduced by surround stimulation , were not different between natural compared to phase - randomized surround stimulation in mature v1 .
hence , the preferential sensitivity for natural scene statistics in the surround was not mediated by a relative increase of inhibitory tone .
rather , we identified transient increases in membrane hyperpolarization during natural relative to phase - randomized surround stimulation , particularly at times that coincided with moments of greatest depolarization during rf stimulation .
these temporal differences in the magnitude of hyperpolarization resulted in increased spike suppression , and thereby increased the response selectivity for features in full - field natural scenes in mature v1 , but not in the immature or visually deprived v1 .
therefore , our results suggest that sensory experience during maturation exerts a prominent influence on the recruitment of inhibition  particularly with respect to its timing relative to potential firing events  to generate more selective coding of visual features embedded in natural scenes .
our results are broadly consistent with observations in cat v1 , where there is a transient increase of inhibition during surround suppression with drifting grating stimuli ( ozeki et  al . , 2009 ) , which ultimately results in an overall reduction of both excitatory and inhibitory conductances when the circuit reaches a balanced state .
our results , however , underscore the importance of transient hyperpolarization prior to spiking events as a mechanism for effective surround suppression during ongoing stimulation with natural movies .
a probable explanation for this difference is that the statistical properties of grating stimuli are much narrower than that of the naturalistic stimuli used in our study .
the continuous variation of spectrotemporal content of naturalistic movies may prevent cortical circuits ever reaching a stable state of balanced excitation and inhibition that is observed when using narrowband grating stimuli .
these results suggest a possible functional role for the elaboration of both excitatory and inhibitory intracortical circuits , which are susceptible to changes in sensory experience in the period after eye opening ( ruthazer and stryker , 1996 ; zufferey et  al . , 1999 ;
, 2001 ; chattopadhyaya et  al . , 2004 ; katagiri et  al . , 2007 ; ko et  al . , 2013 )
we propose that circuit connectivity is shaped by exposure to the statistical structure of the natural environment ( e.g. , extended contours or edges ) after the onset of vision , which increases the effectiveness of surround modulation when viewing naturalistic stimuli to which animals are typically exposed .
our data suggest that visual experience optimizes spiking output by refining the timing and magnitude of inhibition recruited by the surround . in conclusion ,
our results support the idea that visual circuits mature in an experience - dependent manner to become sensitive to the statistical structure of natural stimuli extending beyond the boundaries of the rf . while the basic rf properties are established by the time of eye opening ( hubel and wiesel , 1963 ; blakemore and van sluyters 1975 ; chapman and stryker , 1993 ; krug et  al . , 2001 ; white et  al . , 2001 ;
, 2011 ; ko et  al . , 2013 ) , efficient representations of natural stimulus features  in terms of selectivity , information transfer , and energy consumption ( barlow , 1961 ; simoncelli and olshausen , 2001 ; laughlin , 2001)are not inherent to sensory circuits but require visual experience to develop .
all experimental procedures were licensed and performed in accordance with institutional and national animal welfare guidelines .
data were obtained from c57bl/6 mice aged postnatal day ( p ) 1419 ( immature age group , n  = 7 ) or p32p40 ( mature age group n  = 10 ; dark - reared age group n  = 8) . for dark rearing ,
mice were initially anaesthetized with a mixture of fentanyl ( 0.05  mg / ml ) , midazolam ( 5.0  mg / kg ) , and medetomidin ( 0.5  mg / kg ) .
anesthesia was maintained with a low concentration of isoflurane ( typically 0.5% mixed with o2 ) delivered by a small nose cone .
the position and size of a neuron s rf were determined in similar way as described before ( jones et  al . , 2001 , 2002 ) .
first , rf center position was mapped with pseudorandomized sparse noise stimulus sequence ( white and black flashing patches on an isoluminant gray background ) .
then , the rf radius was estimated by determining a circular area of half - maximal spike responses to the same pseudorandomized sparse noise stimulus .
next , we presented the naturalistic movie within an aperture centered on the rf , surrounded by an isoluminant gray ( see supplemental experimental procedures ) .
these apertured naturalistic movies were interleaved with a naturalistic movie shown in the surrounding annulus of the same size ( i.e. , center not stimulated ) .
we explicitly used the naturalistic movie in this procedure ( rather than a grating stimulus ) to achieve best estimates , because rf radius and surround effects are dependent on contrast , which is constantly fluctuating in movies , but not in grating stimuli .
the radius of the aperture and the surrounding annulus were systematically varied ( typically 0.4 to 2 times the originally estimated rf radius in 5 steps ) .
the aperture size that elicited the strongest response ( firing rate ) , but no significant response to the annulus stimulus of the corresponding size , was defined as the rf size ( jones et  al . , 2001 ;
ozeki et  al . , 2009 ) ( see figure  1b ) . mean and distribution of rf sizes obtained in this way were very similar for the three experimental groups ( figure  s5 ) .
next , the naturalistic movie was presented in one of the following ways : in the rf condition , the naturalistic movie was presented within a rf - sized aperture , masking all portions of the movie outside the calculated rf with an isoluminant gray screen . to ensure a smooth transition to the surround ,
linear alpha - blending ( 0.3/ ) was applied at the border of the rf and gray surround . in the natural surround condition ,
the naturalistic movie was shown full - field . in the phase - randomized surround condition ,
the natural movie was shown in the central aperture , while the phase - randomized movie covered all the surrounding portions of the screen . to determine the influence of the surround alone ,
after each stimulus presentation , a constant gray screen was shown for 1,000  ms .
each condition was typically presented 11 times , and the first repetition was discarded from the analysis to eliminate onset - related effects .
cells were included for further analysis if during at least one movie frame if any of the two rf  + surround conditions elicited a significant response modulation ( p  < 0.01 , randomized two - sided t test ) .
there were no significant differences in the cortical recording depth between the age groups ( range , 85430  m beneath cortical surface ; 212   15 , 207   20 , and 198   17  m , mean   sem , for mature , immature , and dark - reared mice , respectively ; p  = 0.86 ; one - way anova ) .
in short , pipettes were advanced into the cortex at 40 angle with a high positive pressure until the electrode tip was at the depth of approximately 100  m ( corresponding to superficial layer 2/3 ) .
the resistance of the pipettes was typically 68 m , which were filled with a solution containing 110  mm potassium gluconate , 4  mm nacl , 40  mm hepes , 2  mm atp - mg , and 0.3  mm gtp - nacl ( adjusted to ph 7.2 with koh , 290 mosm ) . for experiments with elevated cl reversal potential , 5  mm of potassium gluconate was replaced by 5  mm kcl in the internal solution .
the membrane potential was filtered at 50  hz ( humbug ) and digitized at 10 khz ( national instruments , usa ) .
pv and som cells were targeted for whole - cell recordings in different transgenic mouse lines ( pv - gfp mice , meyer et  al . , 2002 ; gin mice , oliva et  al . , 2000
, 2010 ; hofer et  al . , 2011 ) , using either 30  m alexa fluor 594 or alexa fluor 488 ( life technologies , uk ) in the internal solution .
the targeted cells and patch pipettes were visualized using a custom - built two - photon microscope in the green and red channels with excitation at 880 and 930  nm , respectively .
all analysis was performed with built - in or custom - made functions in matlab ( mathsworks , usa ) .
selectivity index ( si ) and mutual information ( mi ) were calculated as described before ( haider et  al .
, 2010 ; borst and theunissen , 1999 ) and are explained in detail in the supplemental experimental procedures .
moment - to - moment differences in vm ( vm ) between rf and rf  + surround conditions for each neuron were calculated in frame - wide bins ( 33  ms , figures 3d and 3i ) or 1  ms bins ( figures 3e and 3j ) from spike - removed traces ( spikes removed at spike threshold , see below ) .
the mean vm during either surround stimulation for each frame was plotted either against the mean vm relative to spiking threshold ( 5  mv binning ) or against the relative time before firing a spike ( 500  ms to 1  ms in 50  ms bins ) during the rf stimulation .
the membrane potential preceding a spike was first identified , and the membrane potential value at which the second derivative of the membrane potential was maximal was defined as threshold .
analysis of depolarizing events was carried out by quantifying the number and size of transient positive membrane deflections .
events were detected with a moving window ( bin width 5  ms ) with an amplitude threshold of 3  mv .
an individual event was regarded to have triggered a spike if the peak amplitude of the event was followed by an action potential .
statistical significance for repeated measurements of the same cell with different stimuli was assessed using the paired student s t test and anova for reaped measurements ( parametric data ) or wilcoxon sign - rank test and friedman s test ( nonparametric data ) .
m.p . and t.d.m .- f . conceived of the experiments and wrote the paper . e.s . , y.h . , and m.p .
 summaryat eye opening , neurons in primary visual cortex ( v1 ) are selective for stimulus features , but circuits continue to refine in an experience - dependent manner for some weeks thereafter . how these changes contribute to the coding of visual features embedded in complex natural scenes remains unknown . 
 here we show that normal visual experience after eye opening is required for v1 neurons to develop a sensitivity for  the statistical structure of natural stimuli extending beyond the boundaries of their receptive fields ( rfs ) , which leads to improvements in coding efficiency for full - field natural scenes ( increased selectivity and information rate ) . 
 these improvements are mediated by an experience - dependent increase in the effectiveness of natural surround stimuli to hyperpolarize the membrane potential specifically during rf - stimulus epochs triggering action potentials . 
 we suggest that neural circuits underlying surround modulation are shaped by the statistical structure of visual input , which leads to more selective coding of features in natural scenes . 
Introduction
Results
Discussion
Experimental Procedures
Author Contributions