LETTERS

Light adaptation in cone vision involves switchingbetween receptor and post-receptor sitesFelice A. Dunn1, Martin J. Lankheet4 & Fred Rieke2,3

We see over an enormous range of mean light levels, greater thanthe range of output signals retinal neurons can produce. Even high-lights and shadows within a single visual scene can differ 10,000-fold in intensity—exceeding the range of distinct neural signals by afactor of 100. The effectiveness of daylight vision under theseconditions relies on at least two retinal mechanisms that adjustsensitivity in the 200 ms intervals between saccades1. One mech-anism is in the cone photoreceptors (receptor adaptation)2–5 andthe other is at a previously unknown location within the retinalcircuitry that benefits from convergence of signals from multiplecones (post-receptor adaptation)6,7. Here we find that post-receptoradaptation occurs as signals are relayed from cone bipolar cellsto ganglion cells. Furthermore, we find that the two adaptivemechanisms are essentially mutually exclusive: as light levelsincrease the main site of adaptation switches from the circuitryto the cones. These findings help explain how human cone visionencodes everyday scenes, and, more generally, how sensory systemshandle the challenges posed by a diverse physical environment.

Adaptation enables weak sensory inputs to be amplified and pro-tected from noise while preventing strong inputs from saturating neuralresponses. Past physiological work has shown that ganglion cells6,7, theoutput cells of the retina, adapt at lower light levels than cones2–5 and

one of the downstream targets of the cones, horizontal cells8–10. Suchstudies provide evidence for adaptation in the retinal circuitry and inthe cone photoreceptors. However, differences in recording conditionshinder direct comparison of cone and ganglion cell adaptation; thus, theproperties of receptor and post-receptor adaptation remain unclear. Inaddition, past studies have not separated cone- and rod-mediated sig-nals, and rods or rod circuitry could contribute to the identified post-receptor adaptation (see Supplementary Information). Here we trackcone-mediated signals from the input to the output cells of the primateretina to identify where adaptation occurs and to compare directly theproperties of adaptation at different sites.

We studied the midget (parvocellular-projecting) and parasol(magnocellular-projecting) circuits that form two key parallel read-outs of the cone signals (Fig. 1a). Figure 1 characterizes adaptation ofcone photoreceptors and cone-mediated currents of ON (depolariz-ing to light increments) ganglion cells measured in directly compar-able conditions (cones in slices, ganglion cells in a flat mount; seeMethods). The Methods and Supplementary Information describeisolation of cone-mediated signals (Supplementary Figs 1 and 2).

Both receptor and post-receptor adaptation were apparent. Figure 1c,d shows responses to brief flashes in the absence and presence of adapt-ing backgrounds with intensities near typical interior lighting (Fig. 1c;

1Program in Neurobiology and Behavior, 2Howard Hughes Medical Institute and 3Department of Physiology and Biophysics, University of Washington, Seattle, Washington 98195,USA. 4Functional Neurobiology and Helmholtz Institute, Utrecht University, 3584 CH Utrecht, The Netherlands.

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Figure 1 | Midget and parasol ganglion cellsadapt at lower backgrounds than L cones.a, Schematic of primate midget and parasolpathways with fluorescent images of cones in sliceand ganglion cells in flat mount. b, Backgrounddependence of the response amplitude(mean 6 s.e.m.). Amplitudes (Amp) weremeasured between 2–8 Hz and normalized by theamplitude in darkness (Ampdark). Cone data werefitted from equation (1) (Methods) with a 5 100,b 5 1.3 and c 5 0.00029. c, d, Current responsesto flashes in darkness and on backgroundsproducing ,1,000 and ,10,000 P* s21.e, Average cone current responses (in pA)normalized by the flash strength (in P*) indarkness (red) and on backgrounds producing1,100 (blue) and 7,600 (green) P* s21, responsesof ON midget ganglion cells on backgroundsproducing 0.5 (red), 690 (blue) and 7,000 (green)P* s21, and responses of ON parasol ganglion cellson backgrounds producing 0.5 (red), 870 (blue)and 6,800 (green) P* s21. Grey traces showresponses at intermediate backgrounds.

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1,000 absorbed photons per cone per second; P* s21) and indirect sun-light (Fig. 1d; 10,000 P* s21). Figure 1e collects average flash responsesfrom multiple cells of each type at each background tested. Dim back-grounds (less than 1,000 P* s21) produced a small increase in the coneflash responses (compare red and blue traces), but substantially reducedthe amplitude and duration of the responses of both midget and parasolganglion cells. Thus post-receptor adaptation dominated at low back-grounds. Bright backgrounds (greater than 10,000 P* s21) reduced theamplitude and duration of the responses of cones and ganglion cells(compare blue and green traces). The cone responses continued toadapt without circuit feedback (see Supplementary Fig. 4). Previousprimate cone recordings did not exhibit substantial changes in kineticswith background light4,5; the background-dependent kinetic changes wefind suggest that primate cones are similar to cones of other species11,12,rather than the anomaly (see Supplementary Information).

Ganglion cell responses were reduced by backgrounds 10–100times dimmer than those that reduced cone responses, consistent

with past work6,7. Figure 1b summarizes the background dependenceof response amplitude in cones and ganglion cells. Amplitudes wereaveraged for response components between 2 and 8 Hz, becauseadaptation is strongest at these temporal frequencies6,7. Midget gan-glion cells recorded in flat mount and slice preparations exhibitedsimilar adaptation, indicating that slicing did not substantially com-promise the underlying mechanisms. Parasol ganglion cell responseswere reduced more and at lower backgrounds than midget ganglioncell responses, demonstrating that post-receptor adaptation at lowlight levels was produced independently in the two pathways.

To identify the location of post-receptor adaptation, we recordedfrom the ON midget and DB4 or DB5 cone bipolar cells that conveycone signals to midget and parasol ganglion cells (Fig. 2a)13. To test foradaptation of the synapse between cones and cone bipolar cells, we heldthe cone bipolar voltage fixed and measured the light-evoked inputcurrents. As with the cones, modest backgrounds (Fig. 2b, blue tracesin Fig. 2d) produced little or no adaptation in the cone bipolar currents

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Figure 2 | Post-receptor adaptation occurs insignal transfer from cone bipolar cells toganglion cells. a, Fluorescent images of a midgetcone bipolar cell (left panel) and a diffuse conebipolar cell (right panel) in slice. OPL, outerplexiform layer; INL, inner nuclear layer; IPL,inner plexiform layer. b, c, Current responses toflashes in darkness and on backgroundsproducing ,1,000 and ,10,000 P* s21. d, e, Leftpanels, average current (d) and voltage(e) responses of midget cone bipolar cells indarkness (red) and on backgrounds producing1,200 (870 for voltage analysis) P* s21 (blue) and9,600 (9,900) P* s21 (green). Right panels,average current (d) or voltage (e) responses ofdiffuse cone bipolar cells in darkness (red) and onbackgrounds producing 1,000 (1,000 for voltageanalysis) P*s21 (blue) and 8,200 (8,600) P* s21

(green). Grey traces show responses atintermediate backgrounds. f, Backgrounddependence of response amplitude(mean 6 s.e.m.). Amplitudes (Amp) weremeasured between 2–8 Hz and normalized by theamplitude in darkness (Ampdark). Cone fit andganglion cells are from Fig. 1b.

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whereas bright backgrounds (Fig. 2c, green traces in Fig. 2d) producedsubstantial adaptation. Figure 2f compares the background dependenceof the amplitude of the cone bipolar current responses with that of thecones and ganglion cells; adaptation in both the midget and diffuse conebipolar currents tracked that of the cones and showed no evidence forthe post-receptor adaptation apparent in the ganglion cell responses.

To test for adaptation in the conversion of cone bipolar synapticcurrents to voltages, we characterized the effect of backgrounds onthe bipolar voltage responses. The background dependence of thecone bipolar voltages (Fig. 2e, f) resembled that of their synapticinput currents and of the cone signals. The absence of post-receptoradaptation in the output voltages of midget and diffuse cone bipolarcells, and its presence in the midget and parasol ganglion cell synapticinputs, localizes post-receptor adaptation to signal transfer betweencone bipolar cell and ganglion cell in both pathways. This locationhas been implicated in other forms of adaptation14–16.

How do receptor and post-receptor adaptation together controlsensitivity? The relative contributions of receptor and post-receptoradaptation were clearer when individual temporal frequencies of theresponses were examined in the frequency domain. The left column ofFig. 3 compares amplitude spectra for cones, diffuse cone bipolar cellsand parasol ganglion cells in darkness and in the presence of a back-ground producing 1,000 P* s21—spanning the range of backgroundswhere post-receptor adaptation dominates and the cones adapt little,

if at all. The right panels compare amplitude spectra for backgroundsproducing 1,000 and 10,000 P* s21—a range throughout which conescontribute to adaptation. The insets show time-domain flash res-ponses. The amplitude spectra allowed us to determine how muchadaptation in the cone bipolar cells and ganglion cells was inheritedfrom the cones themselves and how much was produced by post-receptor adaptation. We applied the ratio of cone amplitude spectraat two backgrounds (for example, ratio of the red to blue points inFig. 3a, left panel) to the amplitude spectrum of the downstream cell atthe lower background (for example, blue points in Fig. 3b, left panel).This procedure generated a predicted amplitude spectrum (forexample, blue dotted line in Fig. 3b, left panel) for the downstreamcell, assuming adaptation was entirely inherited from that in the cones.

At low backgrounds, the diffuse cone bipolar voltages matchedthe cone prediction well, consistent with little or no effect of post-receptor adaptation, whereas the parasol ganglion cell responsesdiffered substantially from the cone prediction because of post-receptor adaptation (Fig. 3c, left panel). The contributions of coneand downstream adaptation were quite different when the conesbegan to adapt (Fig. 3, right panel). The change in diffuse conebipolar cell and parasol ganglion cell spectra between 1,000 and10,000 P* s21 matched the predictions based on cone adaptation(dashed green lines in Fig. 3b, c, right panel), indicating that coneadaptation dominated adaptation of the bipolar and ganglion cellresponses produced by the change in background. Thus a single sitedominated adaptation at all backgrounds: post-receptor adaptationat low backgrounds and receptor adaptation at high backgrounds.

How do the properties of receptor and post-receptor adaptationcompare? Figure 3 shows clear differences in how the two forms ofadaptation affected ganglion cell responses. Post-receptor adaptationpreferentially attenuated low frequencies (Fig. 3c, left), consistentwith previous studies6,7,17, whereas receptor adaptation attenuatedall temporal frequencies (Fig. 3c, right; Supplementary Fig. 3; see alsoref. 5). Thus the dominant form of adaptation depends on both lightintensity and temporal frequency; these dependencies are likely tocontribute to inconsistencies in previous descriptions of adaptationbased on behaviour (reviewed by ref. 18).

Having distinct operating ranges requires coordination to ceasepost-receptor adaptation when receptor adaptation becomes active.The dominance of cone adaptation at high backgrounds cannot beexplained simply by cone adaptation eliminating the signal pro-ducing post-receptor adaptation, because cones continue to producedistinguishable maintained currents at backgrounds well above thosewhere post-receptor adaptation saturates (Fig. 4a; see also figure 7 inref. 4). Instead, at high backgrounds post-receptor adaptation mustbecome insensitive to changes in the mean cone signal, explaining theswitch from post-receptor to receptor adaptation.

What rules determine the backgrounds at which receptor andpost-receptor adaptation operate? Cone responses to backgroundswhere post-receptor adaptation operates are obscured by noise(Fig. 4b, left; ref. 5), particularly when viewed for the ,100 ms periodcharacterizing the integration time for adaptation10,19 (see alsoref. 20). This high level of noise would cause the gain of a mechanismbased on the response of a single cone to fluctuate, making adapta-tion ineffective or even detrimental. Indeed, cone adaptation beginsonly for backgrounds that produce noticeable changes in the coneresponse (Fig. 4b, right panel). Thus the onset of adaptation seems toreflect a balance of the benefits of matching a cell’s limited responserange to the range of input signals encountered21 and the drawbacksassociated with basing such gain controls on noisy signals.

Convergence in the retinal circuitry can improve signal-to-noise,providing the adaptive mechanism with a more reliable signal.Diffuse cone bipolar cells receive input from ,10 cones, but the noisein their flash responses at low backgrounds is similar to that ofthe cones (Fig. 4c; ratio of cone bipolar to cone signal-to-noise0.67 6 0.06; mean 6 s.e.m.; see Methods). This is consistent withthe lack of post-receptor adaptation in the cone bipolar signals.

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Figure 3 | The primary site of adaptation switches from circuitry to conesas light levels increase. a, b, c, Amplitude spectra of flash responses for Lcones (a), diffuse cone bipolar cells (b) and parasol ganglion cells(c). Amplitudes (Amp) at each temporal frequency were normalized by themaximum amplitude spectra in the dark (Ampdark max.). Left panels,responses in darkness (red) and in the presence of background producing,1,000 P* s21 (blue); right panels, responses in the presence of backgroundsproducing ,1,000 (blue) and ,10,000 P* s21 (green). Insets show time-domain responses. Adaptation predicted from the cone responses (dottedlines in b and c) was calculated by multiplying the postsynaptic amplitudespectra at the lower of the two backgrounds by the appropriate ratio of thecone amplitude spectra (r1, r2). Data are mean 6 s.e.m.

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The convergence necessary for post-receptor adaptation could beprovided by an amacrine cell or by voltage-independent synapticintegration within the ganglion cell. For example, peripheral parasolganglion cells receive input from hundreds of cones, and their flashresponses achieve a higher signal-to-noise ratio at low backgrounds(Fig. 4d; ratio of parasol to cone signal-to-noise, 6.0 6 0.3). Thus anadaptive mechanism based on converging cone inputs can operatemore reliably at low backgrounds than one based on cone or conebipolar signals. This reasoning suggests that post-receptor adaptationin central midget ganglion cells, which receive input from singlecones, could differ considerably from that in the periphery.

As we make saccades to explore a visual scene, retinal neuronsencounter a wide range of light intensities. Receptor and post-receptoradaptation permit the amplification required to see objects in shadowswhile avoiding saturation from the sky. The combination of these adapt-ive mechanisms allows the visual system to encode details in a scene withgreater fidelity than a standard camera at a single exposure setting. Thestrategy the retina employs—shifting the dominant site of adaptation tomatch the reliability of the input signals—demonstrates an elegant prin-ciple for accurate information processing in sensory perception.

METHODS SUMMARYPrimate (Macaca fascicularis, Macaca nemestrina and Papio anubis) retinas were ob-

tained through the Tissue Distribution Program of the National Primate Research

Center at the University of Washington and dark-adapted for 1 h before isolation

from the pigment epithelium. Isolated flat-mounted pieces of retina or retinal slices

were perfused with bicarbonate-based Ames medium (Sigma) equilibrated with 5%

CO2/95% O2 at 34–37 uC while we recorded the current or voltage responses ofindividual cone photoreceptors, cone bipolar cells and ganglion cells. Cells were

targeted for recording under infrared differential interference contrast microscopy.

Cells were identified at the end of each recording from fluorescence images.Spatially uniform light stimuli were delivered from three light-emitting diodes

(LEDs) with peak outputs at 470, 513 and 640 nm. An accurate view of adaptation

of cone-mediated signals required separation of rod- and cone-mediated res-

ponses at the lowest backgrounds possible. Cone-mediated signals were probed

with brief 640 nm flashes that preferentially stimulated long-wavelength-sensitive

cone photoreceptors in the absence and presence of constant background light

produced by simultaneous activation of all three LEDs. Contributions from

the rod photoreceptors were minimized by pharmacologically blocking rod-

mediated inputs to cone bipolar cells or by suppressing rod-mediated signals withshort-wavelength-adapting light in cone bipolar and ganglion cells. Responses to

brief flashes on varying background lights were averaged across cells and analysed

in the time and frequency domains.

Full Methods and any associated references are available in the online version ofthe paper at www.nature.com/nature.

Received 27 June; accepted 6 August 2007.Published online 12 September 2007.

1. Yarbus, A. L. Eye Movements and Vision (Plenum, New York, 1967).2. Boynton, R. M. & Whitten, D. N. Visual adaptation in monkey cones: recordings of

late receptor potentials. Science 170, 1423–1426 (1970).3. Valeton, J. M. & van Norren, D. Light adaptation of primate cones: an analysis

based on extracellular data. Vision Res. 23, 1539–1547 (1983).4. Schnapf, J. L., Nunn, B. J., Meister, M. & Baylor, D. A. Visual transduction in cones

of the monkey Macaca fascicularis. J. Physiol. (Lond.) 427, 681–713 (1990).5. Schneeweis, D. M. & Schnapf, J. L. The photovoltage of macaque cone

photoreceptors: adaptation, noise, and kinetics. J. Neurosci. 19, 1203–1216 (1999).6. Lee, B. B., Pokorny, J., Smith, V. C., Martin, P. R. & Valberg, A. Luminance and

chromatic modulation sensitivity of macaque ganglion cells and humanobservers. J. Opt. Soc. Am. A 7, 2223–2236 (1990).

7. Purpura, K., Tranchina, D., Kaplan, E. & Shapley, R. M. Light adaptation in theprimate retina: analysis of changes in gain and dynamics of monkey retinalganglion cells. Vis. Neurosci. 4, 75–93 (1990).

8. Lee, B. B., Dacey, D. M., Smith, V. C. & Pokorny, J. Horizontal cells reveal conetype-specific adaptation in primate retina. Proc. Natl Acad. Sci. USA 96,14611–14616 (1999).

9. Smith, V. C., Pokorny, J., Lee, B. B. & Dacey, D. M. Primate horizontal celldynamics: an analysis of sensitivity regulation in the outer retina. J. Neurophysiol.85, 545–558 (2001).

10. Lee, B. B., Dacey, D. M., Smith, V. C. & Pokorny, J. Dynamics of sensitivity regulationin primate outer retina: the horizontal cell network. J. Vis. 3, 513–526 (2003).

11. Baylor, D. A. & Hodgkin, A. L. Changes in time scale and sensitivity in turtlephotoreceptors. J. Physiol. (Lond.) 242, 729–758 (1974).

12. Matthews, H. R., Fain, G. L., Murphy, R. L. & Lamb, T. D. Light adaptation in conephotoreceptors of the salamander: a role for cytoplasmic calcium. J. Physiol.(Lond.) 420, 447–469 (1990).

13. Boycott, B. B. & Wassle, H. Morphological classification of bipolar cells of theprimate retina. Eur. J. Neurosci. 3, 1069–1088 (1991).

14. Baccus, S. A. & Meister, M. Fast and slow contrast adaptation in retinal circuitry.Neuron 36, 909–919 (2002).

15. Hosoya, T., Baccus, S. A. & Meister, M. Dynamic predictive coding by the retina.Nature 436, 71–77 (2005).

16. Manookin, M. B. & Demb, J. B. Presynaptic mechanism for slow contrastadaptation in mammalian retinal ganglion cells. Neuron 50, 453–464 (2006).

17. Tranchina, D., Gordon, J. & Shapley, R. M. Retinal light adaptation—evidence for afeedback mechanism. Nature 310, 314–316 (1984).

18. Hood, D. C. Lower-level visual processing and models of light adaptation. Annu.Rev. Psychol. 49, 503–535 (1998).

19. Yeh, T., Lee, B. B. & Kremers, J. The time course of adaptation in macaque retinalganglion cells. Vision Res. 36, 913–931 (1996).

20. Fairhall, A. L., Lewen, G. D., Bialek, W. & de Ruyter Van Steveninck, R. R. Efficiencyand ambiguity in an adaptive neural code. Nature 412, 787–792 (2001).

21. Laughlin, S. A simple coding procedure enhances a neuron’s information capacity.Z. Naturforsch. C 36, 910–912 (1981).

Supplementary Information is linked to the online version of the paper atwww.nature.com/nature.

Acknowledgements We thank D. Dacey, O. Packer, B. Peterson and T. Haun forproviding primate tissue; P. Newman and M. Wixey for technical assistance;J. Cafaro, E. J. Chichilnisky, T. Doan, W. Dunn, G. Horwitz, R. Kiani, G. Murphy,F. Soo, B. Wark for comments on the manuscript; and B. Lundstrom for commentson figures. This work was supported by the National Institutes of Health (F.R.),Achievement Rewards for College Scientists Foundation (F.A.D.) and the HowardHughes Medical Institute (F.R., and F.A.D. with a predoctoral fellowship).

Author Contributions F.A.D., M.J.L. and F.R. participated in all aspects of this work.

Author Information Reprints and permissions information is available atwww.nature.com/reprints. The authors declare no competing financial interests.Correspondence and requests for materials should be addressed to F.R.(rieke@u.washington.edu).

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Figure 4 | Signal-to-noise of the adapting signal improves with convergenceand increasing light level. a, Left panel, current responses of an L cone tosteps doubling in intensity from 4,800 P* s21; right panel, normalized steadystate response amplitudes for 12 L cones fitted with equation (2) (Methods;I1/2 5 45,000 6 7,000 P* s

21, n 5 0.7 6 0.05). Current responses of an L cone(b), a diffuse cone bipolar cell (c) and a parasol ganglion cell (d) to stepsproducing 200 P* s21 (left panels) and 2,000 P* s21 (right panels).

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METHODSTissue. Slice and flat mount preparations of primate retina followed proceduresdescribed previously22. All recordings were from peripheral retina (.30 u eccent-ricity, based on ganglion cell dendritic field sizes) isolated from the pigment

epithelium.

Recordings and cell types. Cone photoreceptors were recorded in slices pre-pared as described previously23,24. Currents (holding potential 260 mV) weremeasured using perforated or whole-cell voltage clamp with an internal solution

containing (in mM): 125 potassium-aspartate, 10 KCl, 10 HEPES, 5 NMG-

HEDTA, 1 MgCl2, 0.5 CaCl2, 4 Mg-ATP, 0.5 Tris-GTP, 0.1 Alexa-488 orAlexa-750 and 0.05 mg ml21 amphotericin-B for perforated-patch recordings;

pH was adjusted to 7.2 with NMG-OH and osmolarity was ,280 mOsm. Long(L)-wavelength-sensitive cones were distinguished from middle (M)- and short

(S)-wavelength-sensitive cones on the basis of their higher sensitivity to 640 nm

light.

Cone bipolar responses were measured in slices by whole-cell voltage- or

current-clamp recordings with the same internal solution used for the cones.

Each recorded cell was identified as a diffuse or midget cone bipolar from fluor-

escence images acquired after the experiment (Fig. 2a)13. Ganglion cells were

recorded in a flat mount preparation using an internal solution containing (in

mM): 105 CsCH3SO3, 10 TEA-Cl, 20 HEPES, 10 Cs2-EGTA, 2 QX-314, 5 Mg-

ATP, 0.5 Tris-GTP and 0.1 Alexa-488 or Alexa-750; pH was adjusted to 7.2 with

Cs-OH and osmolarity was ,280 mOsm. Midget and parasol cells were targetedon the basis of soma size and sustained versus transient responses to light steps.

Identification was confirmed with fluorescence images acquired after each record-

ing (Fig. 1a). We restricted our study to ON cone bipolar and ON ganglion cells.

Junction potentials (approximately 210 mV) have not been corrected.Light stimuli and isolation of cone-mediated responses. All stimuli uniformlyilluminated a 630mm diameter circular area centred on the recorded cell.Calibrated photon fluxes (photonsmm22 s21) were converted to absorbedphotons per cone per second (P* s21) on the basis of the L cone spectral sens-

itivity, measured LED spectrum, and a collecting area of 0.37mm2 (ref. 4).Adapting backgrounds generated by simultaneous activation of all three LEDs

produced near equal photon absorption rates in L and M cones, and eightfold

lower rates in S cones. Responses with and without backgrounds were probed

with 10-ms flashes from the 640 nm LED. Flash strengths were chosen to elicit

responses with amplitudes , 25% of the maximal response. In a subset of experi-ments we used multiple flash strengths to check for linearity; none of the results

reported depended systematically on flash strength. Flashes from the 640 nm

LED stimulated L cones 5 times more effectively than M cones, whereas the

adapting background equally stimulated L and M cones. The combination of

flash and background minimized the possibility that M cones could compensate

for the L cones in downstream retinal cells that receive inputs from both cone

types. M and L cones adapted similarly (data not shown).

We probed cellular responses using long-wavelength light to isolate cone

inputs with minimal rod contamination (Supplementary Fig. 1). Light fromthe 640 nm LED was absorbed 560 times more effectively by L cones than rods.

In the cones themselves this preferential activation was sufficient to eliminate

measurable contamination from electrically-coupled rods (Supplementary Fig.

1a)25. However, cone bipolar cell and ganglion cell responses to 640-nm wave-

length flashes delivered in the dark still had a substantial rod-mediated com-

ponent. For ganglion cells, rod signals were suppressed by a constant background

from the 470 nm LED producing ,3 (for parasol cells) and up to 120 (for midgetcells) absorbed photons per rod per second (Supplementary Fig. 1b, c, bottom);

this background reduced the rod responses by a factor of at least 10 without

noticeably affecting cone responses (Supplementary Fig. 1b, c, top). For ON conebipolar cells, signals mediated by the rod bipolar pathway were suppressed by

blocking rod bipolar cell input to AII amacrine cells with the AMPA-receptor

antagonist DNQX (10 mM) or NBQX (5mM). Similar results were obtainedwhen rod signals were suppressed by 470 nm light (Supplementary Fig. 2).

Data processing. Data were filtered at a bandwidth of 0–60 Hz in Figs 1c, d, and2b, c, and at a bandwidth of 0–100 Hz in Figs 1e and 2d, e. The background

dependence of the response amplitude was taken as the average of the amplitude

spectra across 2–8 Hz in Figs 1b and 2f, and across 18–22 Hz in Supplementary

Fig. 3.

Fits. The dependence of response amplitude on background (Figs 1b and 2f)were fitted empirically:

fit~aj jzbIBaj jzIB

� �cIBz1ð Þ{1 ð1Þ

The first term accounts for the increase in response amplitude with back-

ground seen in some cells: a, determines the intensity at which the increase

begins and b the amount of increase. The second term describes the decreased

amplitude with background, with c setting when the decrease begins.

In Fig. 4a, the background dependence of steady state response amplitudenormalized by the maximum current, R/Rmax, was fitted with the Hill equation:

R

Rmax~

1

1z(I1=2=IB)n ð2Þ

where I1/2 is the half maximal background and n is the Hill exponent describing

the relationship between response amplitude and background intensity, IB.

Signal-to-noise. Signal-to-noise was calculated as the ratio between the flashresponse amplitude scaled by the number of isomerizations (for example, red

traces in Fig. 1e and 2d) and the standard deviation of current in darkness.

Image processing. Output levels, brightness and contrast of background and cellimages in Figs 1a and 2a were adjusted independently to enhance the cell profile

(Adobe Photoshop). Background image of retinal layers in Fig. 2a was obtained

from extracellular sulphurhodamine. Each panel is a projection of a stack of

confocal images.

22. Dunn, F. A., Doan, T., Sampath, A. P. & Rieke, F. Controlling the gain of rod-mediated signals in the mammalian retina. J. Neurosci. 26, 3959–3970 (2006).

23. Field, G. D. & Rieke, F. Nonlinear signal transfer from mouse rods to bipolar cellsand implications for visual sensitivity. Neuron 34, 773–785 (2002).

24. Armstrong-Gold, C. E. & Rieke, F. Bandpass filtering at the rod to second-order cellsynapse in salamander (Ambystoma tigrinum) retina. J. Neurosci. 23, 3796–3806(2003).

25. Schneeweis, D. M. & Schnapf, J. L. Photovoltage of rods and cones in the macaqueretina. Science 268, 1053–1056 (1995).

doi:10.1038/nature06150

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TitleAuthorsAbstractMethods SummaryReferencesMethodsTissueRecordings and cell typesLight stimuli and isolation of cone-mediated responsesData processingFitsSignal-to-noiseImage processing

Methods ReferencesFigure 1 Midget and parasol ganglion cells adapt at lower backgrounds than L cones.Figure 2 Post-receptor adaptation occurs in signal transfer from cone bipolar cells to ganglion cells.Figure 3 The primary site of adaptation switches from circuitry to cones as light levels increase.Figure 4 Signal-to-noise of the adapting signal improves with convergence and increasing light level.