the flash electroretinogram ( erg ) provides an excellent noninvasive means to assess retinal function .
the largest feature of this response is the b - wave , which follows the initial negative a - wave .
the b - wave corresponds to the granit 's   pii component and is believed to be generated by the activity of on - bipolar cells .
however , a number of studies have suggested that the pii component may arise from several sources , thus complicating the interpretation of b - wave loss in studies of retinal disease . potential contributors to the pii include rod and cone bipolar cell responses , with the later showing faster rise and decay kinetics . on- and off - responses can also be visualized in erg responses from primates , rabbits , cats , and guinea pigs [ 47 ] .
the responses of on- and off - bipolar cells may have distinct temporal signatures in the b - wave as on - bipolar cells have been shown to be more sustained compared with their off - counterparts in turtle retina .
more recent studies suggest that bipolar cells appear to segregate into those producing transient and sustained responses [ 9 , 10 ] .
have attributed sustained and transient off - bipolar cell responses to differences in ionotropic glutamate receptors subtypes : kainate and ampa receptors on sustained and transient off - bipolar cells , respectively .
the temporal characteristics of bipolar cell responses are also thought to be modified by inhibitory processes [ 1113 ] .
a number of studies have shown that inhibition of glycine and gaba receptors can modify bipolar cell output [ 14 , 15 ] .
for example , inhibition of gabac receptors produces more sustained on bipolar cell responses [ 1618 ] as well as smaller and slower erg b - wave [ 17 , 19 , 20 ] .
more recently , herrmann and coworkers   showed that rod bipolar cell and the dark - adapted b - wave dynamic range in mice are extended by gabac - mediated sustained chloride current . whether a similar effect is seen in the rat erg has yet to be defined .
first , the putative pii was isolated by removing photoreceptoral and associated glial and epithelial responses .
second , response amplitudes are considered as a function of stimulus luminance systematically across a range of fixed times . finally , by applying this analytical approach to data collected following pharmacological manipulation , it is possible to show that the rat erg b - wave contains distinct fast and slow components .
all animal experimental procedures were conducted with approval from our institutional animal experimentation ethics committee ( a04001 ) and in accordance with the  australian code of practice for the care and use of animals for scientific purposes  .
adult long evans rats between 10 and 12 weeks of age ( 180270  g , monash animal services , clayton , vic , aust ) were maintained at 22c in a 3070 lux environment with a 12-hour light / dark cycle ( on at 8 am ) .
animals were dark adapted overnight ( > 12 hours ) and prepared under dim red light ( max ~ 650  nm ) .
anesthesia was induced with intramuscular injections of 60  mg / kg ketamine ( ketamil 100  mg / ml , troy laboratories , smithfield , australia ) and 5  mg / kg xylazine ( xylazil 100  mg / ml , troy laboratories ) .
corneal anesthesia was achieved by instillation of one drop of proxymetacaine ( opthetic 0.5% , allergan , frenchs forest , nsw , australia ) .
mydriasis was induced with one drop of 0.5% tropicamide ( mydriacyl , alcon laboratories , frenchs forest , nsw , australia ) , giving complete pupil dilation ( 4  mm ) within 15 minutes that was maintained for the duration of experiments .
body temperature was maintained ( 37  0.5c ) by placing the rat over a circulating - water heat pad ( mgw lauda , lauda - knigshoffen , germany ) and covering the animal with an insulating blanket .
the animal and heat pad were mounted on a custom - built platform with the rat 's neck and abdomen lightly secured to the stage with velcro to minimize movement during recordings .  
the active electrode was placed at the center of the cornea , and the reference was a circular scleral electrode placed around the equator . a stainless steel needle ( f - e2 - 30 , grass - telefactor , west warwick , ri , usa ) inserted subcutaneously into the tail served as the ground .
following electrode placement 1.0% carboxymethylcellulose sodium ( celluvisc , allergan , irvine , ca , usa ) was applied to aid electrical conductivity .
the signals were band - pass filtered ( 0.3  hz1  khz ) over a 2560 ( 640  ms ) epoch with a 4  khz sampling rate . a 50  hz notch filter was applied post hoc to eliminate line noise .  
stimulus duration of 256  ms was used to better visualize potential slow and fast components .
white flashes ( 5  watt leds , luxeon , philips lumileds lighting co. , ca , usa ) were delivered via a ganzfeld integrating sphere ( total diameter of 36  cm and 13  cm diameter aperture ; photometric solutions international , huntingdale , vic , australia ) .
stimulus energy was calibrated using an il-1700 research radiometer ( international light , newbury port , ma , usa ) to return values in scotopic cdm .
erg responses were obtained for flash luminances from 2.58 to 2.82  log  cdm ( 2.58 , 2.26 , 1.94 , 1.54 , 0.95 , 0.83 , 0.62 , 0.28 , 0.04 , 0.36 , 0.76 , 0.97 , 1.27 , 1.68 , 2.18 , 2.53 , and 2.82  log  cdm ) .
the interstimulus interval was increased progressively from 45 to 240  s , to allow for complete recovery of b - wave amplitudes as established in pilot trials .
as previously described [ 23 , 24 ] , pharmacological agents were injected into the vitreal chamber via a 30-gauge needle attached with polyethylene tubing ( inner diameter = 0.38  mm ) to a hamilton syringe ( sge syringes , ringwood , australia ) .
the needle was introduced through the pars plana , approximately 1  mm posterior to the superior limbus and at an angle of 45 degrees to avoid the lens .
contact with the lens resulted in opacification , and data from such animals were excluded ( < 10% of animals ) .
aliquots of 25  l were used with the upper limit chosen to minimize intraocular pressure changes that might compromise retinal function .
pharmacological agents , 2-amino-4-phosphonobutyric acid ( apb , 400  m ) , 6-cyano-7-nitroquinoxaline-2,3-dione ( cnqx , 250  m ) , and ( 1,2,5,6-tetrahydropyridine-4-yl)methylphosphonic acid ( tpmpa , 100  m ) were obtained from sigma - aldrich ( sigma - aldrich pty . ltd . , castle hill , nsw , australia ) .
apb and tpmpa were diluted using distilled h2o , whereas cnqx was mixed in the minimal volume of dimethyl sulphoxide ( dmso ) that gave dilution .
the concentrations stated represent the estimated final vitreal concentrations assuming complete dilution and a rat vitreal volume of 50  l .   a total of thirteen animals received intravitreal injection of the pharmacological agents .
four animals had one eye treated with cnqx , four animals received a combination of apb and cnqx , and the remaining five had one eye treated with tpmpa with the contralateral untreated eye serving as a control in both cases .
in addition to the 12 control eyes , an additional ten animals were included as controls in order to increase the size of the normal pool ( n = 22 ) .
following drug injection , single erg waveforms were collected at five - minute intervals ( stimulus of 0.15  log  cdm ) to track the retinal response for 40 minutes , ensuring a stable drug effect before the erg protocol began .  
average ( n = 4 ) apb / cnqx treated signals were subtracted from individual control waveforms at each energy level to remove receptoral , retinal pigment epithelial and glia contributions . following this subtraction ,
the ascending limb of the b - wave was described by taking amplitudes at fixed times in 10  ms steps .
amplitudes following stimulus offset were collected at fixed times ( every 10  ms ) after the waveform had been rezeroed to the amplitude at stimulus offset ( 256  ms ) .
this step was necessary to remove the influence of the variable decay of the b - wave following its peak . a hyperbolic function ( 1 )
the reasoning here is that if the response at a particular fixed time during a light step is mediated by a single underlying generator , then a single hyperbolic function with an exponent of 1 should adequately describe the intensity - response relationship [ 27 , 28 ] .
the presence of a deviation from a single hyperbolic function implies contributions from multiple mechanisms .
the function is given by maximal amplitude ( vmax , v ) , semisaturation constant ( k , log  cdm ) , and slope ( n ) .
consider 
 ( 1)v(i)=vmax(inin+kn ) . to determine whether data should be fit with a single or multiple hyperbolic function ,
an f - statistic is employed , which provides an unbiased method to assess whether using multiple mechanisms ( and additional variables ) provides a statistically superior fit ( p < 0.05 ) to the data set ( e.g. , see figure 4 ) .
this approach considers the null hypothesis that a complex model is not a better descriptor of the data than a single hyperbolic function , given the change in the merit function ( sum - of - squares ) and the degrees of freedom .
statistical analysis was performed using sigmastat ( v3.11 , systat software inc . , 2004 , il , usa ) .
two - way repeated measures ( rm-)anova was conducted to consider interactions between treatment and stimulus energy .
a holm - sidak post hoc test was applied following two - way rm - anova to consider the source of significant differences ( p < 0.01 ) .
the holm - sidak protects against increased type 1 errors associated with multiple comparisons but is considered more powerful than the bonferroni correction . throughout this study ,

 figure 1 shows representative control and drug - treated waveforms in response to 256  ms flashes .
figure 1(a ) shows that at dimmer luminances , the erg waveform is dominated by a corneal positive b - wave .
the response remains sustained for 100  ms following light offset before it shows relaxation back to baseline .
the relaxation overshoots the initial baseline to be corneal negative at 150  ms after flash offset . at higher luminances , both
the b - wave develops an early transient peak ( 100  ms ) followed by a slower tail and finally amplitude relaxation , after stimulus offset .   following application of apb / cnqx ( thin grey line ) ,
the b - wave is completely abolished , leaving a corneal negative waveform that grows in amplitude with higher light levels .
figure 1(b ) shows the putative pii following subtraction of apb / cnqx - treated signals to remove presumed contributions from photoreceptors , rpe , and glial cells ( thin grey line of figure 1(a ) ) .
this approach shows that the putative pii is sustained at low light levels and becomes more transient at brighter levels .  
figures 1(c)to 1(f ) consider the response following stimulus offset after signals are rezeroed to the amplitude at 256  ms ( stimulus offset ) .
the amplitude and timing characteristics of the post - offset relaxation have several phases . at low light levels , from 2.58 to 0.83  log  cdm , increasing luminance results in a progressively larger ( more negative ) relaxation ( figure 1(c ) , direction of grey arrow ) . for stimuli between 0.83 and 0.97  log  cdm ,
however , the relaxation becomes progressively smaller ( figure 1(d ) , grey arrow ) , but only after a latency of 100  ms after stimulus offset . between 0.97 and 2.17  log  cdm ,
the relaxation at stimulus offset again becomes more negative , but only after a latency of 40  ms ( figure 1(e ) ) . for brighter light levels of 2.17 to 2.82  log  cdm , the relaxation amplitude tends to become smaller again ( figure 1(f ) ) .  

figure 2(a ) overlays cnqx ( black line ) and control ( thin grey line ) pii waveforms and shows that application of cnqx reduces the early transient peak of the b - wave . in addition , the relaxation after stimulus offset appears delayed with cnqx .
figure 2(b ) compares control ( grey area represents the standard error of the mean ) and cnqx - treated amplitudes at a fixed time of 50  ms ( a50 ) .
cnqx intensity - response functions show a plateau between 1.271.68  log  cdm and a secondary plateau at higher luminances , suggesting that at least two mechanisms contribute to the isolated pii under these conditions .
0.035 ; treatment x luminance interaction , f1,17 = 59.8 , p < 0.001 ) for stimulus luminances greater than 0.04  log  cdm . 

figure 2(c ) shows that at a400 ( 144  ms after offset ) , the control intensity response - function shows an initial growth in amplitude that makes a plateau at approximately 0.95  log  cdm .
this is followed by a second phase of amplitude growth ( between 1 and 2  log  cdm ) and finally a reduction in magnitude at the highest luminous energies ( > 2  log  cdm ) .
cnqx application resulted in a second phase of amplitude growth that was approximately 1 log unit more sensitive compared with the control data .  

figure 3(a ) shows representative pii waveforms following inhibition of gabac receptors ( black line ) compared with controls ( thin grey line ) .
the offset relaxation was less apparent following tpmpa treatment , consistent with smaller onset amplitudes .
analysis of the intensity - response characteristics at a50 ( during light step , figure 3(b ) ) and a400 ( after light step , figure 3(c ) ) shows that tpmpa resulted in changes that are qualitatively similar to cnqx treatment ( see figure 2 ) .
specifically , at a400 , tpmpa resulted in reduced amplitudes as well as a more sensitive second phase of amplitude increase .  
figures 4(a ) and 4(b ) show average amplitudes at selected fixed times optimized with a hyperbolic function ( representative fixed times of a90 and a220 ) . at these fixed times , a simple hyperbolic function ( thick line ) fails to fully describe the data , suggesting the presence of more than one component .
the same data are better modeled by the summation ( thin solid line ) of two hyperbolic functions ( dashed and grey lines ) , both having their exponents constrained to one .
an f - test showed that a significantly better fit to the data could be achieved with the two hyperbolic functions ( p < 0.05 ) .  
figures 4(c ) and 4(d ) show optimized vmax and k50 plotted as a function of fixed time for the two components , which have been designated as m(1 ) and m(2 ) .
the parameters show a systematic change across time . in figure 4(d ) , it was apparent that the two hyperbolic functions show distinct sensitivities ( k50 ) .
if at each time we designate those hyperbolic functions that show larger k50 as m(1 ) and those with smaller k50 as m(2 ) , then the corresponding vmax values for m(1 ) and m(2 ) are shown in figure 4(c ) .
figure 4(d ) shows that the less sensitive transient m(1 ) component underlies the earliest part of the isolated pii . beyond a fixed time of 80 
ms , the more sensitive but sustained m(2 ) comes to dominate the rat pii .
interestingly , at later fixed times ( > 170  ms ) , another feature appears to grow in amplitude to contribute ~20% to the pii .
this secondary amplitude growth , designated as m(3 ) , may reflect the presence of an additional slower component or the relaxation of m(1 ) to a baseline during a light step .
the k50 for m(1 ) ( figure 4(d ) ) shows a transition at 170  ms , to become less sensitive ( arrows ) consistent with the possibility of intrusion from another component .
its intensity - response function reveals a low sensitivity , suggesting the possibility that it might arise from the cone pathway .
the waveform designated m(2 ) shows an increase in sensitivity to plateau at later times , consistent with a single component . 

figure 5(a ) applies the same modeling approach to amplitudes at various fixed times following stimulus offset .
the example shown here is for the fixed time of a400 , which shows an initial growth in amplitude , followed by a reduction in magnitude between 0.62 and 1.27  log  cdm and finally a second phase of amplitude growth .
this intensity - response profile can not be modelled using one or even two hyperbolic functions .
indeed , it can only be adequately modeled with three functions ( figure 5(a ) ) , the two negative components designated m(4 ) and m(5 ) , with the addition of a positive component designated m(6 ) .  
the merit function ( log sum of square ( ss ) error ) for the two- and three - component models was compared using an f - ratio .
figure 5(b ) shows that beyond a fixed time of 340  ms , the addition of a third positive hyperbolic function provides a statistically smaller ss compared with a two - function model .
the vmax parameters for the three components increase with longer fixed times as shown in figure 5(c ) .
figure 5(d ) shows that the k50 parameters for the three components are distinct from each other and remain relatively constant across the range of fixed times after a light step .  

figure 6 shows the effect that cnqx and tpmpa have on the parameters of the putative m(1 ) and m(2 ) components . for vmax , m(1 )
was reduced by cnqx treatment ( ~59% ) at early fixed times ( < 150  ms ) , whereas m(3 ) appeared to be unaffected .
tpmpa produced a smaller attenuation of m(1 ) ( ~44% , figure 8(a ) ) but produced a small enhancement of m(3 ) .
in contrast , figure 6(b ) shows that both cnqx and tpmpa produced similar effects on the sustained component ( m(2 ) ) . 

figure 6(c ) shows that application of cnqx removed the transition in k50 between the m(1 ) to m(3 ) components .
likewise , tpmpa application also removed this transition ; however , k50 was larger for early times with tpmpa .
this pattern of k50 change following cnqx or tpmpa suggests that m(1 ) may be a common process whose sensitivity is differentially modified at early ( more sensitive , < 150  ms ) and later fixed times ( less sensitive , > 180  ms ) .
figure 6(d ) shows that cnqx and tpmpa had little effect on the k50 of m(2 ) .   as was the case for the control data , those recorded following cnqx and tpmpa application
were evaluated to consider if three components are evident following a light step . in both cnqx- and tpmpa - treated data
, the log merit function shows that there was not a significant reduction in error to warrant the use of the more complex three hyperbolic function models ; thus , only two are needed to explain the trends in the drug - treated eyes .
figures 7(a ) and 7(b ) show vmax after a light step derived from control and drug - treated data for m(4 ) and m(5 ) .
eyes treated with cnqx or tpma returned smaller amplitudes compared with controls . the reductions in amplitudes after a light step may simply reflect drug - induced reductions in amplitudes during the light step .
the k50 is shown for each function in figure 7(d ) . a t - test shows for m(4 )
that the cnqx k50 is larger than that of controls ( control : 2.55    0.04 , cnqx : 2.12    0.05 ; p < 0.001 ) , whereas tpmpa was similar . for m(5 ) , both cnqx and tpmpa treatments produced k50 's that were smaller than controls ( control : 0.78    0.02 , cnqx : 0.39    0.04 , tpmpa : 0.47    0.05 ; p < 0.001 ) .
we find that the isolated pii ( control waveform  apb / cnqx sensitive waveforms ) in the pigmented rat electroretinogram has complex luminance response characteristics during and after a 256  ms light step .
our approach of modeling the intensity - response function across a range of fixed times revealed that during a light step , the rat pii contains transient and sustained responses , which relax to baseline after the light step .
in addition , there was a slow corneal positive response that follows stimulus offset .  
consistent with previous studies of rodent b - waves [ 28 , 30 ] , we also find that the isolated pii amplitude ( in our case for a fixed time of 50  ms , a50 ) intensity - response function returned a slope of 0.99 ( 0.89 , 1.09 ) ( 2.5% , 97.5% ) .
thus , a single hyperbolic function can describe the earliest portions of the rat pii leading edge .
however , a single hyperbolic function can not fully describe the intensity - response data for later fixed times after stimulus onset .
such complex behavior has previously been attributed to interactions between negative piii and positive pii components [ 3133 ] . however , even after subtracting corneal negative waveforms isolated using apb / cnqx , the pii intensity - response function retains its complex profile , suggesting that there are multiple components underlying the response . to model the amplitude , intensity - response function extracted at fixed times after stimulus onset required two hyperbolic functions ( figure 4 ) , which segregate into processes that have high and low sensitivities to light ( figure 4(d ) ) and
are sustained ( m(2 ) ) and transient ( m(1 ) ) ( figure 4(c ) ) , respectively .
this is consistent with previous studies suggesting that the b - wave contains sustained and transient responses ( also termed slow and fast pii ) [ 1 , 9 , 34 , 35 ] . here
, we show that these sustained and transient components do not arise from interactions with corneal negative responses .
application of cnqx to block ionotropic glutamate receptors does not completely remove either m(1 ) or m(2 ) , indicating that the transient and sustained responses do not arise simply from on- and off - bipolar .  
in addition to these sustained and transient components , there appears to be a third longer latency ( 140190  ms ) component ( designated as m(3 ) ) .
this component has low amplitude ( ~ 20% , figure 4(c ) ) and is less sensitive than m(1 ) for times longer than ~140  ms ( figure 4(d ) ) .
treatment with either cnqx or tpmpa removes the transition of k50 from a more sensitive m(1 ) to a less sensitive m(3 ) , producing a k50 profile that resembles a single mechanism ( figure 6(c ) ) .
this outcome suggests that m(3 ) is likely to be a continuation of m(1 ) during a light step .
lateral elements in the ipl , in particular amacrine cells , can modulate the on - bipolar cell response and the erg b - wave [ 11 , 17 , 19 , 36 , 37 ] .
inhibitory input has been shown to confer transient and sustained characteristics to bipolar cell neurotransmitter release [ 11 , 38 ] .
recordings from single cells suggest that such inhibition occurs via gabaa and gabac receptors onto rod and cone on - bipolar cells in rats .
show that higher proportions of gabac to gabaa receptors - mediated inputs produce more prolonged inhibition , whereas a relative higher proportion of gabaa to gabac results in more transient inhibition .
this is consistent with the finding that inhibition of gabac receptors using tpmpa makes bipolar cell responses in mouse , rat , and amphibian retina more sustained [ 16 , 39 ] with a smaller response range .
a less hyperpolarized resting membrane potential arising from loss of a tonic gaba - mediated current [ 21 , 41 ] would account for smaller and more sustained b - wave .
provide compelling evidence that dopamine - mediated gaba release possibly from horizontal cells , acting on both gabaa and gabac receptors , modulates the dynamic range of rod bipolar cells via a sustained cl current .
activation of gabac receptors increases presynaptic cl influx , hyperpolarizing bipolar cell and thereby inhibiting ca entry via l - type ca - channels [ 11 , 21 , 38 ] .
studies have shown that l - type ca channels may be differentially modified as a function of time following light onset [ 38 , 42 ] , implying that the on - bipolar cell responses may be both enhanced and inhibited at different times . in rat retina , a17 amacrine cells are known to make reciprocal synapses onto rod bipolar cells [ 11 , 43 ] . it has been suggested , based on data from tiger salamander retina , that a glycinergic amacrine cell initially inhibits the a17 amacrine , thereby delaying its action at gabac receptors on on - bipolar cell axon terminals . in this way
, amacrine cell inhibition of bipolar cells will occur only after the glycinergic amacrine cell inhibition onto the a17 amacrine cell is removed ( estimated to be ~150  ms , ) .
this mechanism provides an explanation for our finding that gabac - mediated inhibition produces a more transient pii component , by differentially modifying the pii at early and later times .
specifically , our tpmpa data shows that there is amplitude enhancement and increased light sensitivity from 70 to 160  ms after onset of the light step , as well as a decrease in light sensitivity for times later than 180  ms ( figure 6(c ) ) .
moreover , that cnqx results in the same qualitative outcome is consistent with amacrine cell involvement , as ampa / ka inhibition will affect both glycinergic and gabaergic amacrine cells .
.   suggest that dopamine released from amacrine cells might also modulate the tonic gaba currents at the level of rod bipolar cell dendrites .
thus , modification of bipolar cell responses might occur at both outer and inner plexiform layers .
it is possible that transient and sustained components described here can reflect both cone and rod bipolar cell responses .
the difference in response range of m(2 ) and m(1 ) might suggest rod and cone on - bipolar cell contributions to these components , respectively .
previous work by nixon and colleagues   has shown that the rod b - wave is ~1.5 log units more sensitive than the cone b - wave for short duration flashes .
this difference is comparable to that found at early fixed times for m(1 ) and m(2 ) seen here ( < 120  ms , ~1.5 log units ) .
however , interpreting m(1 ) and m(2 ) to be cone and rod mediated needs to guarded as responses from single rat rod bipolar cells to a step of light have a faster onset   than the m(2 ) components isolated in this study .
the response following the offset of a light step could not be modeled with two components and required a third hyperbolic function ( figure 7 ) . at very dim intensities
however , with brighter intensities , two additional components become apparent , which are opposite in polarity ( m(5 ) negative and m(6 ) positive ) . these offset components
indeed , the k50 values for m(4 ) and m(5 ) correlate well with those for m(3 ) and m(2 ) ( compare figure 7(d)to 6(c ) and figure 7(d)to 6(d ) , respectively , consistent with this proposal .
this possibility is also supported by the findings that cnqx ( reduction at a5050.7  1.5% and a400  56.1  4.1% , p = 0.19 ) and tpmpa ( reduction at a50  41.9  2.5% and a400  41.1  5.0% , p = 0.40 ) induce comparable reductions in amplitude for both onset ( a50 ) and negative offset components ( a400 ) . a corneal positive component ( figure 7(c ) )
apparent following stimulus offset ( here designated m(6 ) ) was only present in control eyes .
naarendorp and williams   have shown that positive off - component arising after a light step in rat was abolished after inhibition of ionotropic glutamate receptors , indicating an origin from hyperpolarizing bipolar cells .
our finding that the m(6 ) was removed by cnqx , which inhibits ionotropic receptors , correlates well with this interpretation .
hence , this component may also be modulated by disruption to inner retinal feedback as it is known that cnqx also modifies amacrine cell function .
putative components m(1 ) to m(6 ) are plotted in figure 8 to yield the composite pii waveform in response to a 256  ms light step . based on our analysis ,
we believe that the isolated scotopic pii in rats contains at least three separate components . during a light step , there is an initial transient - on response , which is made more transient by gabac - mediated inhibitory processes .
the majority of the pii for times greater than ~100  ms reflects a sustained - on component , likely arising from rod bipolar cells .
after a light step , transient - on and sustained - on components relax to baseline .
in addition , there is a positive sustained - off component that is likely to arise from the off - pathway , but it may also be modulated by gabac receptors .
thus , assessing the intensity - response functions at early and later times during a light step provides a means by which transient and sustained contributions to the rat pii erg can be identified .
 the most dominant feature of the electroretinogram , the b - wave , is thought to reflect on - bipolar cell responses . 
 however , a number of studies suggest that the b - wave is made up of several components . 
 we consider the composition of the rat b - wave by subtracting corneal negative components obtained using intravitreal application of pharmacological agents to remove postreceptoral responses . by analyzing the intensity - response characteristic of the pii across a range of fixed times during and after a light step , we find that the rat isolated pii has 2 components . 
 the first has fast rise and decay characteristics with a low sensitivity to light . 
 gabac - mediated inhibitory pathways enhance this transient - on component to manifest increased and deceased sensitivity to light at shorter ( < 160  ms ) and longer times , respectively . 
 the second component has slower temporal characteristics but is more sensitive to light . 
 gabac - mediated inhibition enhances this sustained - on component but has little effect on its sensitivity to light . 
 after stimulus offset , both transient and sustained components return to baseline , and a long latency sustained positive component becomes apparent . 
 the light sensitivities of transient - on and sustained - off components are consistent with activity arising from cone on- and off - bipolar cells , whereas the sustained - on component is likely to arise from rod bipolar cells . 
1. Introduction
2. Material and Methods
3. Results
4. Discussion
5. Summary