OCTOBER 1975

VOLUME 65, NUMBER 10

JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

Sinusoidal flicker characteristics of primate cones in response to heterochromatic stimuli* Robert M. Boyntont and William S. Baron: Centerfor Yisual Science, University of Rochester, Rochester, New York 14627 (Received 23 November 1974; revision received 7 June 1975) Electrophysiological recordings of primate photoreceptors have been obtained and frequency response characteristics of the red, green, and blue cones have been determined and compared to previous psychophysical findings. Using cynomolgus monkeys, we recorded the foveal local electroretinogram, which is dominated by the late receptor potential, and obtained criterion-response threshold data for sinusoidally flickered test stimuli against complementary chromatic adapting backgrounds. Our results support the hypotheses that (a) the shapes of the MTFs of the red and green cone systems are identical and are determined solely by the photoreceptors at high frequencies, and (b) the blue cones have an MTF with a lower corner frequency than the red- and green-cone systems. Index Headings: Vision; Color; Electroretinogram; Modulation transfer.

The purpose of this study is to investigate electrophysiologically the temporal-response properties of the three types of primate cones, by monitoring late receptor potentials. Sinusoidal stimuli are used, under conditions of selective chromatic adaptation.

to be descriptive of cone functioning. At higher frequencies, where the lateral networks cannot follow, the responses of the cones themselves become limiting for either counterphase bars or homogeneous fields.

The motivation for the study lies in three previous reports. Two of these were concerned with the response of photopic "mechanisms" to flickering light, as deduced from psychophysical experiments where conditions of complementary chromatic. adaptation were used in an effort to isolate them. Green' and Kelly2 each did this, using steady chromatic-adapting fields upon which superimposed test fields of complementary colors were flickered sinusoidally. The purpose of their experiments was to determine modulation thresholds of flicker perception, whose reciprocals were plotted as a function of temporal frequency to yield a temporal modulation transfer function (MTF).

is that of Boynton and Whitten,

The two studies differed mainly with respect to the spatial parameters of the test field. Kelly's test pattern occupied his entire 8° adapting area and, in some of his experiments, it consisted of counterphase gratings. Green modulated only a homogeneous central 20 portion of his 120 adapting field. These procedural differences affect mainly the low-frequency portions of their functions. This difference does not concern us, however, because the. emphasis in the present paper is mainly on high-frequency behavior; for purposes of comparison, the data of Kelly have been arbitrarily selected. Both Kelly and Green found that, when spatially ho-

mogeneous fields were flickered, the MTF exhibited a maximum sensitivity at intermediate frequencies. For high frequencies, Kelly proposed that the cones limit the response, and that sensitivity is further reduced at low frequencies by the effects of lateral neural inhibition. Kelly's argument is that the lateral inhibitory processes have MTFs of their own, these having a lower cutoff frequency than those of cones. 3 If true, it follows that, at low frequencies where the lateral inhibitory networks can follow the flickering input, they should reduce the over-all sensitivity of the system below that otherwise characteristic of individual cones. The use of counterphase gratings reduces the low-frequency inhibition and reveals MTFs of a simple low-bandpass variety, believed

The third study that provides background for this paper 4

who found electrophys-

iological evidence that in cynomolgus macaque monkeys (known to be visually similar to humans) the temporal response of blue (B) cones was different from that of red (R) and green (G) cones. Although sinusoidal stimuli were not used in that study, responses obtained from blue cones to square-wave stimuli were very sluggish, implying a MTF with much greater attenuation at high frequencies than would be characteristic of the red and green cones. Kelly found little difference in the shape of his R and G functions.

But if Kelly's R, G, and B

MTF curves are equated at low frequencies, the highfrequency attenuation for the B mechanism is greater, and the B curve lies below those of the other two types of cone mechanisms at intermediate frequencies. The trichromatic vision of the human or the macaque monkey requires that the eye must possess, in addition to B cones, two different kinds of cones with peak sensitivities at the longer wavelengths: the R cones with a peak spectral sensitivity at about 575 nm, and the G cones with a peak sensitivity at about 535 nm. Although the relative numbers of cones of these two types in the normal retina may differ from one individual to another, 5 no evidence suggests that their temporal responsiveness differs. Neither Kelly nor Green found any difference of the shapes of the temporal MTF for R and G mechanisms. Additional evidence for the temporal similarity between red and green cones also appeared in three further studies: (a) the work of Rushton and his colleagues, 5 which revealed no difference between the bleaching kinetics of the red and green absorbers, (b) a psychophysical study by Estdvez and Spekreijse,

7

and (c) a dark-adaptation

study by v. Norren and Padmos.i The data we have to report are consistent with this failure to find temporal differences between red and green cones. Distinctions must nevertheless be drawn between B cones and the other two types. For this purpose, following v. Norren and Padmos, we adopt the expression "R, G" cones to specify the combined activity of red and green cones. 191

Copyright © 1975 by the Optical Society of America

R. M. BOYNTON

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The proportion of R to G contribution must vary in our experiments, but without a detectable effect upon the

temporal responses. METHODS

The details of our experimental procedure have been fully documented elsewhere.

9

"-1 We have found it possi-

ble to record late receptor potentials from the eyes of cynomolgus macaques, by insertion of a single microelectrode that usually permits recovery of the animal and reuse of the eye. The preparation is ordinarily stable and permits experiments to be undertaken that require several hours to complete. The visual characteristics of cynomolgus monkeys have been shown to be very

similar to those of humans." the results

of our experiments

Therefore, we suggest that can be generalized

to hu-

man receptor responses. The potential that we record is a local ERG (LERG),

which originates from many thousands of cells in the neighborhood of the electrode

tip, and is therefore

the

sum of extracellular potentials from all three types of cones. Rods and other retinal elements also contribute, unless precautions are taken to eliminate their potentials from the record. The animal is maintained on sodium pentobarbitol, and lies in a supine position with its head supported by an

atraumatic head holder. The pupils are dilated with 3 Cyclogyl, and a 0. 8 cm retrobulbar

injection of 2%

Xylocaine is used to help eliminate residual eye movements. In addition, mechanical stability of the eye is obtained by threading a stainless steel ring under the conjunctiva near the limbus, and then suturing this ring to an external ring attached to the head holder. A minor incision is made in the lateral canthus, and the bulbar conjunctiva is cleared from the sclera about 4 mm temporal from the limbus. At this location, a hypodermic needle is inserted, which acts as a guide for the microelectrode. The microelectrode is directed to the fovea centralis under ophthalmoscopic observation. The entry angle is adjusted by use of an angularly calibrated stereotaxic apparatus whose center of rotation coincides with the point of entry through the sclera. In previous experiments with yellow light, we have shown that the foveal local ERG that we record is dominated by the cone late receptor potential (LRP); for this reason it is legitimate to regard the records from our experiments as LRPs. We achieve this cone domination of the LERG by a combination of procedures.

Record-

ings are always made from the rod-free center of the fovea, where the components of the inner nuclear layer to which the cones connect (and which trigger the unwanted b-wave component of the ERG) are displaced lat-

erally and are oriented less radially than in the rest of the retina. Rods are unable to follow a sinusoidally flickering light at the higher frequencies used. Light adaptation also suppresses rods and eliminates the c wave. Our control experiments have mainly involved the use of sodium aspartate,

which when injected into the 3

eye causes vasoconstriction of the retinal vesselS1 as well as synaptic blocking. 14 We used this type of con-

Vol. 65

AND W. S. BARON

trol under some of the conditions of the experiments reported in this paper, with results that have been partially described elsewhere." In previous experiments with yellow light, the fundamental frequency of the response was always the same as that of the stimulus; at low amplitudes, the response was sinusoidal. In the heterochromatic experiments of this paper, second-order harmonics (relative to the stimulus frequency) were sometimes observed in the response. In some cases, the responses were perfectly sinusoidal, with alternate cycles of the same height, but at twice the stimulus frequency. However, in most cases the amplitudes of alternate waves differed; the wave form was therefore complex. Efforts to correlate the occurrence of second-order harmonics with the experimental conditions that elicited them have so far proved unsuccessful, mainly because the phenomenon is not generally replicable. The complex wave forms that result tend to complicate the measurements. Our procedure has been simply to measure the over-all peakto-trough amplitude. Linear interpolation, between zero and responses

of 60 p.V and less,

was used in order to

determine thresholds at a criterion response of 10 TV. When the responses were sinusoidal, and of the same frequency as the stimulus, interpolation provided an accurate assessment of the thresholds, "1but this procedure results in a somewhat erroneous threshold for nonsinusoidal (and therefore nonlinear) responses. A measure of this error is the range of thresholds obtained for a given heterochromatic condition if the magnitudes of the responses used for the interpolation were varied. This was done for 5 separate sets of conditions, for sample sizes of 3, 3, 5, 6, and 8 responses.

The mean

of the 5 standard deviations, each expressed as a proportion of its mean, was 0.443, or ±0.16 on the scale of logarithms used in some of the figures of this paper. All experiments were conducted with 100 circular fields, centered upon the fovea, using filtered chromatic light supplied by red, green, and blue gelatin filters (Kodak Wratten 24, 61, and 47) in the test field, super-

imposed upon complementary cyan, purple, and yellow adapting fields (Wratten 44A, 35, and 16). In some ex-

periments, field.

monochromatic stimuli were used in the test

SPECTRALSENSITIVITYON A YELLOWBACKGROUND

This experiment consists of determining the spectral sensitivity

of the receptors

at 3 and 10 Hz with the test

stimuli presented against a yellow background of 10 000 td that was selected to enhance the relative contribution of B cones to the total measured response. Spectral sensitivity was evaluated by determining the relative radiance required, at each wavelength, to elicit a 10 piVcriterion response. The experiment was performed on three animals. Responses to each of these were obtained in pairs at each of nine wavelengths at 30 nm intervals

from 420 to

640 nm. Mean data are shown in Fig. 1. Here it is seen that, as frequency is increased from 3 to 10 Hz, there is a reduction of sensitivity at short wavelengths,

Oct. 1975

FLICKER CHARACTERISTICS

\b

0 0

CD1 LU

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Instrument limitations did not permit us to hold the adapting level constant; instead we used a method that Kelly16 originally employed in this pioneering psychophysical studies of heterochromatic flicker. Instead of adding a constant mean illuminance of the test stimulus, we always added it at 100%modulation, and its threshold illuminance was the dependent variable. Under conditions where sensitivity is high, the amount of added test light that is required for threshold is small; therefore it does not disturb the state of adaptation. Under such conditions, an n-fold increase of threshold illuminance has the same significance as an n-fold increase of modulation threshold at a constant test illuminance. However, as the test-stimulus frequency is increased and sensitivity to flicker is thereby progressively reduced, the mean threshold level of the superimposed flickering

2

CD, I-

OF PRIMATE CONES

test stimulus-which is also increasing-begins to alter the state of adaptation of the eye; in the limit, it be comes the primary determinant of the adaptation level. To distinguish our curves from true MTFs, we will refer to them as "amplitude-frequency" curves, taking this nomenclature 420

460

500

540

580

620

660

WAVELENGTH (nm)

FIG. 1. Spectral sensitivity, evaluated as the reciprocal of the relative energy required to elicit a 10 ,uVcriterion response to flickering

stimuli of 3 Hz (circles)

and 10 Hz (squares).

Data

are averaged values from three experimental sessions.

and an increase at long ones. The short-wave maximum of Fig. 1 is at 450 nm for both frequencies, and seems therefore to be dominated by the activity of B cones,

whose peak sensitivity is very near to this wavelength.4 " 5 The reduction of short-wave sensitivity with increasing frequency, as calculated at 450 nm for the average of the four experiments, is consistent with the statement that, for the adapting conditions used, the MTF for B cones decreases by about 0. 2, on the scale of logarithms, between 3 and 10 Hz. Our results are consistent with those of Kelly, in showing that the B cones have a differ-

ent MTF from that of the long-wave cones, one which falls off sooner and more rapidly as a function of increasing frequency. MODULATION TRANSFER FUNCTION vs AMPLITUDEFREQUENCY CURVES

In the psychophysical

experiments

of Kelly? and Green,'

the adapting fields consisted of intense backgrounds of one color, upon which a constant mean illuminance of the test stimulus was superimposed. Without changing the mean illuminance of the test stimulus, threshold modulationwas determined as a function of test frequency. In order not to disturb materially the state of adaptation established by the more intense steady adapting light, the amount of added test light was kept small. The permissible amount of added test light is arbitrary within rather wide limits, a fact that was used to advantage by Green and Kelly in their efforts to deduce the absolute sensitivities of their mechanisms.

from Kelly.

16

Although the use of the

amplitude-frequency procedure complicates the interpretation of our experimental data, it also has advantages relative to what could have been deduced from the use of constant adaptation levels and variable modulation. The advantage results from the linear behavior of receptor responses at high frequencies, observed in the present experiments as well as in our earlier work on homochromatic flicker. (Kelly was the first to point this out, in the context of his psychophysical experiments.) At the limiting frequency of flicker detection, threshold is determined solely by the absolute amplitude of the test stimulus. The. significance of this, where receptors are concerned, is that the signals generated by them at these high frequencies are determined exclusively by the absolute difference of the rates of photon absorption associated with the peaks and valleys of the cyclic stimulus, independent of any steady absorption rate upon which the fluctuation might be superimposed. Critical flicker frequency therefore becomes limited by the maximum variation of amplitude that can be physically produced, which necessarily is that produced by a stimulus of 100%modulation.

MAIN EXPERIMENT: CONDITIONS TESTED

The experiments to be reported in the remaining experimental sections of this paper consist of the determination of 13 amplitude-frequency curves, most of them at eight frequencies. The results to be shown are based upon averaged data from two normal subjects, obtained without the use of retinal clamping, drugs, or other procedures that would alter the normal retinal physiology. See the Methods section for specification of the red, green, blue, purple, yellow, and cyan filters used. Frequencies 60 Hz.

tested were 3, 5, 10, 20, 30, 40, 50, and

Blue test stimuli were presented upon yellow backgrounds of 0, 10 000, 40 000, 200 000, and 800 000 td.

R. M. BOYNTON

1094 I

Vol.

65

seem to be consistent with the hypothesis that the heavy line drawn through selected points in Fig. 2 represents the shape of the MTF of the B cones. This interpretation is not immediately obvious and requires considerable explanation.

I

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AND W. S. BARON

Coincident points. There are three pairs of coincident points (indicated with arrows) where the thresholds do not differ, despite substantial differences of the adapting illuminances. In all three of these cases, the threshold is found to be independent of adaptation level over at least a fourfold range of adapting illuminance. How can this be? The answer relates to the linear behavior at high frequencies that has already been discussed. If amplitude sensitivity is independent of adaptation level along the high-frequency asymptote of the amplitudefrequency curve for B cones, as well as for R, G cones previously tested, then the existence of the three pairs

10 -

-J

100 -

of coincident points in Fig. 2 may be interpreted

-J

on the

hypothesis that they lie on a high-frequency asymptote that is characteristic

through them-part that asymptote. 1000 I-

10000 I

X

3

5

l

i

l

10

I

I

I

l

|

.

20

40

80

(Hz) FREQUENCY

FIG. 2. Amplitude-frequency curves for blue test stimuli, of frequency shown on abscissa, against steady yellow backgrounds of the indicated retinal illuminances: o, zero, a, 10000 td; A, 40000 td; O, 200000 td; V, 800000 td.

The heavy curve

drawn through the family of curves is believed to represent the temporal modulation transfer

function of the B cones.

The

filled symbols represent results treated in Table I, believed to

Bleaching and the R, G cones. With the exception of most of the points lying on the zero-background curve, it seems likely that the points to the right of the heavy line in Fig. 2 indicate sensitivity dominated by the activity of the R, G cones (despite the use of a blue test stimulus). To understand this, recall from the first experiment of this paper evidence presented to show that, as frequency is increased, the relative contribution of B cones is diminished relative to that of R, G cones. Furthermore, if the heavy curve actually does represent the high-frequency asymptote of the B cones, any points to the right of it logically cannot be due to the activity of such cones,

because the observed sensitivity is higher than what B cones can mediate. If the foregoing argument is correct, why then do not the high-frequency limbs of the curves for 10000, 40000, that would and 200 000 td reach a common asymptote-one The answer, we of a R, G mechanism? be characteristic

be due to R, G cones.

Red test stimuli were presented on cyan backgrounds of 0, 2500, 10 000, and 155 000 td.

Green test stimuli were presented on purple backgrounds of 0, 1000, 4000, and 57 500 td.

We had hoped initially that the use of these conditions of complementary test and adapting stimuli, which Wald17 and Kelly3 reported to have isolated the three types of cones, would have given us an opportunity to

observe in turn the temporal-response each type of cone, uncontaminated

properties of

by other mechanisms.

The results, which turned out otherwise, are neverthe-

less revealing. MTF OF THE B CONES

Figure 2 shows the results

If so, a line drawn

of B cones.

of the heavy line of Fig. 2-describes

of experiments

propose, is related to the phenomenon of photopigment bleaching. "1 For the most part, Kelly's phychophysical experiments were done at adapting illuminances that bleached an insignificant fraction of photopigment. His data show that his generalization about the high-frequency asymptote fails slightly at his highest adapting level of 9300 td. Assuming a half -bleach constant of 20 000 td, this adapting stimulus would bleach 32% of the pigment. At his next-lowest

adapting level of 850 td, only 4% of

the pigment would be bleached, which is insignificant; at all of his other levels, bleaching would even be less. Because bleaching serves only to reduce the probability of absorption of incident photons, its adaptive effect should be independent of the frequency of the test stimulus, resulting in a displacement of the entire amplitudesensitivity curve downward relative to what would occur in the absence of bleaching.

in which

flickering blue test stimuli were superimposed upon steady yellow backgrounds at the five specified levels of retinal illuminance. The results, which are complex,

The top of the B-cone curve.

The heavy curve of Fig.

2 also passes through two additional noncoincident points at 3 and 5 Hz (plotted as squares).

At the top of Fig. 3

Oct. 1975

FLICKER CHARACTERISTICS

OF PRIMATE CONES

1095

nates. This procedure permits at least a rough extrapolation to a zero-background level, and suggests that the difference between the result obtained for 10 000 td -

of yellow adaptation and what would be obtained with no

LU,

background is negligible. This in turn suggests that the receptors that contribute to the response, being virtually unaffected by the 10 000 td yellow background, are the B cones. This justifies drawing the curve of Fig. 2 through the point at the upper left for 3 Hz and, by the same kind of argument, through the point at 5 Hz as well.

0

2.0

LU

-o

(-J I

/

0

//

/

/

Rod intrusion with zero background. It is clear from the uppermost (zero background) curve of Fig. 2 that, when the background is physically reduced to zero, nothing of the sort predicted from the foregoing argument actually occurs; instead, sensitivity increases more than ten times. This effect, which seems to be almost

iL0 0-

erw I1.0

0--__-

surely due to rods, would correspond

in Fig. 3 to a

precipitous drop in the tvi curve, beginning at some 4

(unknown) illuminance below 10000 td. Although we have

5

LOG ADAPTING INTENSITY

6 (LOG TD)

not done the experiment, it is also likely that a detailed examination of such a branch of the tvi curve would reveal that it has scotopic properties. Phase shifts. If a response were coming exclusively from rods, the extracellular potential picked up by a microelectrode positioned in the fovea centralis would be 1800 out of phase with the rod activity, because the central fovea, having no rods, would be an inactive region of the retina. (See Tomita1 8 for an excellent discussion of this point.) Because the responses at 3 Hz, both for the zero-background curve and the 10 000 td yellow-background curve, were sinusoidal in form, it was possible to measure their phase relationships. Assuming that both rods and cones are able to follow a 3 Hz stimulus with little or no phase lag, there could be a 1800 phase shift between these two 3 Hz responses. A phase shift close to this was found, which provides further evidence that the 3 Hz response obtained with the 10 000 td adapting background is from cones, and that the response obtained with a zero background is from rods.

30

-J

to

I

Cf)

LUJ

I- 20 LU

LUt

-J

LU

10

0

50000 ADAPTING

INTENSITY

100 000 (TD)

FIG. 3. Top. Replot of the 3 Hz data obtained with yellow adaptation from Fig. 2 in the form of a tvi curve (og threshold

vs log illuminnnce). Bottom. Lower three points from the upper plot, expressed in linear coordinates, and extrapolated to a zero adapting level.

As the frequency of the stimulus is increased, the zero background curve of Fig. 2 becomes progressively closer to the 10000 td curve, and at 60 Hz is coincident with it. This final pair of coincident points seems ascribable to R, G cones, the rod contribution having been eliminated for one or both of the following reasons: (a) at this high frequency, more than 1000 td are required for threshold, corresponding to an effective mean adapting level of more than 500 td of blue light, probably not quite enough to saturate the rods, but certainly sufficient to reduce their sensitivity substantially; (b) it is unlikely that the rods, even if unadapted, would be able to follow a 60 Hz input as well as the R, G cones.

is shown the tvi curve (log threshold vs log adapting illuminance) represented by the four points at 3 Hz on yellow backgrounds in Fig. 2. The curve appears very nearly to have reached a zero slope at an adapting level of 10 000 td. This is even more apparent at the bottom of Fig. 3, where the lowest three points of the curve have been replotted on less familiar arithmetic coordi-

The bottom of the B cone curve. Evidence will be presented later in support of the hypothesis that the 12 filled data points for high frequency in Fig. 2, despite the use of a blue test stimulus, are due to the activity of R, G cones. The inverted triangle for 40 Hz at the bottom, through which the B-cone curve has been drawn, represents a threshold value that probably cannot be

R. M. BOYNTON TABLE I. Summary of data for conditions under which the threshold appears to have been determined by the activity of R, G cones. The first two columns specify stimulus conditions. The third column gives the value of A, by which thresholds presumably have been increased (sensitivity decreased) by the Bleaching should also reduce bleaching of R, G cone pigments.

the effectiveness of the steady-state backgrounds: the effective background (trolands times fraction of unbleached pigment) is given in the fourth column.

The last four columns show, for

the four frequencies indicated, 10 ,uVthreshold adjusted for the effects of bleaching. The experimentally determined thresholds can be calculated by adding A to each of the values shown. The value (1. 90) in the 40 Hz column has not been included in

determining the means and standard deviations given at the bottom of the table.

The points given in Figs, 2, 8, and 9 as

filled symbols correspond to the conditions for which threshold values are reported in this table. These points should all fall

AND W. S. BARON

Vol. 65

curves should provide a good estimate of the temporal MTF of the R, G cones.

(a) The use of 100% modulation

ensures, provided that other mechanisms that have higher sensitivites do not intrude, that all measurements are appropriate

of the MTF

asymptote

to the high-frequency

of the mediating mechanism. This means that the increase of adaptation levels caused by increasing testthreshold illuminances at high frequencies should not affect the shape of the curve. (b) As mentioned in the foregoing, existing evidence contra-indicates rod intrusion.

(c) The MTF of the B cones at high frequencies

lies below that of the R, G; thus there is no problem of secondary branching of the functions at high frequencies, as occurred inthe experiments that involved blue test (d) The use of red vs

stimuli upon yellow backgrounds.

along the high-frequency asymptote of the R, G cones. The extent to which they do so is revealed by the standard deviations shown.

green test stimuli does not, of course, result in unique activation of R vs G cones, respectively. If the MTFs of these classes of cones differed, each result would

Effective background

represent a weighted average of the two kinds of activity, the components of which would be difficult to deduce.

Background Condition

G/P

30

40

50

60

0

2.02

2.17

2.68

3.16

2500 10000

0.05 0.18

2200 6670

1.86

2.20 2.28

2.56 2.63

2.92 3.04

155 000

0.94

17710

1000 4 000

0.02 0.08

950 3330

1.90

2.24 2.48

2.65 2.73

2.93 3.08

57 500

0.58

14840

2.94

3.26

2.52

3.13

2.69 2.93 2.87

2.96 3.24

0

R, G (av)

R/C

at frequency (Hz) indicated

(td)

A

(td)

10 PVthresholds(logtd)-A

0

3.20

0

0

0

B/Y

10000 40000 200000

0.17 0.48 1.04

6 670 13330 18180

(800000)

1.61

19510

Mean n

1.99 1.97

2.22 2.40 2.26

tivity

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I I

I

I

(1.90)

1.95

2.26

2.72

3.09

0.059

0.099

0.132

0.055

ascribed to the activity of R, G cones; it is therefore attributed to B cones. The argument, in its quantitative form, demands that the temporal behavior of the R, G cones be considered first; this will not be done until the next section. The quantitative argument will indicate (see Table I) that this point is 0. 36 higher on the logarithm scale than it probably should be (more than 3. 5 standard deviations)

But the fact that the two zero-background curves do not differ at all means that there is no difference between the MTFs of the R and G cones; therefore the weights that enter into the average are immaterial. The result-

in order for it to be due to the ac-

10 H

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100 H