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Contrast sensitivity: Psychophysical and evoked potential methods compared
CONTRAST EVOKED
SENSITIVITY: PSYCHOPHYSICAL AND POTENTIAL METHODS COMPARED MARIO W. CAUNOX
\viatton
Vision
JR
Laboratory. Human Engmeerin@ Diviston. Air Force Aerospace Laboratory. Wright-Patterson .41r Force Base. OH -IS??. U.S.A.
Medical
Abstract--Contrast sensitivity functions derived from stead! state evoked potential high correlatton with psychophysically derived threshold contrast sensitivity functions. holds as flicker rate. tvpe of flicker. optical correctton and vtsihllity conditions are substantial range. provided that VEP and ps~choph~si~~l data are recorded under the tal conditions
Campbell and MaKei (1970) studied the relationship between steady state visual evoked potentials (VEP) and threshold contrast sensitivity (the reciprocal of threshold contrast) for Rickering sine-wave gratings. They measured VEP amplitude over a range of contrasts and showed that under the conditions of their experiment. regressions fitted to plots of log contrast vs VEP amplitude intersected the contrast axis near the psychophysically measured threshold. The concept of a direct relationship between sinewave grating thresholds determined by psychophysical means and those determined from VEP data was further strengthened in subsequent papers by Campbell and Kulikowski (1972) and by Bodis-Wollner er nl. f 1972) where responses to a single spatial frequency were studied in some detail. Further studies demonstrating a relationship between contrast sensitivity functions (CSFs) derived from steady state VEP data and CSFs derived ps~chophysically have been conducted by Harris PI ul. (1976). Bodis-Wollner (1976. 1977). Pirchio (‘r ul. (1978) and by Fiorentini rt ul. (1980). On the other hand. Tyler et al. (1978, 1979) and Apkarian et of. (19811. using high contrast grattngs have consistently demonstrated functions of VEP amplitude vs spatial frequency that do not resemble contrast sensitivity funtions. These functions often show two sharply tuned peaks: one in the region between 0.51 c/deg and one in the region between 4 and 8 Ldeg. Apkarian et ul. (1981) claim that these peaks are similar in shape to tuning curves obtained from single cell recordings. In a similar vein Regan 11978) found that plots of VEP amplitude vs spatial frequency can be strongly affected by the flicker rate used to elicit the VEP. Regan claimed that low spatial frequencies produced VEPs by “local flicker” of large pattern regions while higher spatial frequencies elicited VEPs from pattern reversal. Thus, from Regan’s data. one would not expect plots of VEP vs spatial
Kewarch
thresholds shtr\\ This correlatton
changed
over a
same rxperimen-
frequency generated at some arbitrary flicker rate to resemble contrast sensitivity functions. While the discussion to this point has concentrated on steady state VEPs generated by sine-wave grating stimuli the reader is probably aware that most VEP research has been conducted using checkerboard patterns. perhaps because they produce stronger responses than sine-wave or square-wave gratings (Parker and Salzen. 1977). Steady state VEP responses to checkerboard patterns show a maximum VEP response for check sizes in the range of IO-15 min arc (Regan. 1977). This tuning is similar to the bandpass shape of the contrast sensitivity function. However, checkerboards are rich in spatial harmonic components and in this sense are more complex stimuli than sine-wave gratings. A great deal of current evidence. both psychophysical (Blakemore and Campbell. 1969: Graham and Nachmias. 1971: Graham er trl.. 1978) and neurophysiological (Maffei and Fioretini. 1972: Movshon L’?ui.. 1978: DeValois (‘r rti.. 1979) implies that these components may be processed by independent filters or channels. The physiological consequences of such processing would be more activity in the cortex and perhaps greater complexity of the VEP waveform in response to a checkerboard. The appropriateness of sine-wave gratings for VEP use was recognized by Parker and Salzen ( 1977) and by Jones and Keck (1978) who conducted studies of transient VEP responses using sine-wave grating stimuli. In fact. Jones and Keck commented that when sine-wave gratings were used. evoked response waveforms were remarkably consistent across subjects. Much of the interest in determining contrast sensitivity from VEP data has been generated by the increasing use of contrast sensitivity among psychophysicists for assessing certain aspects of visual performance. A relationship between individual threshold CSFs and the detection and identification
of complex objects has been established by Ginsburg. t 1978). Ginsburg cr trl. (1981) and by Ousle) c’r u/. (1981). References to clinical applications of CSFs can be found in review articles by DeVaiois and DeValois (19801 by Sekuier (1974) and in a recent paper by Ginsburg (19X1). Because of these relationships between contrast sensitivity and visual performance. a method of reliabiiy determining a CSF from evoked potential data would be valuable for clinical work. infant x4sion studies and as a luboratory research tool. As the brief literature review in this introduction has shown. there is considerable uncertainty in determining whether a VEP response at a particular spatial frequency will be related to the psychophysically derived contrast sensitivity at that spatial frequency. It is possible that part of the problem is due to the different contrasts and the variety of flicker rates that different researchers use. While the method of Campbell and Maffei (1970) appears to overcome some of these problems there are two difficulties with their method. First. Campbell and Maffei claim that their method is only good for spatial frequencies above 3 cjdeg. The second problem is the time required to obtain a CSF. Using the Campbell and MafTei method. Harris er al. (1976) determined VEP amplitudes at lOcontrast levels for each spatial frequency and extrapolated regression lines through the points to obtain a threshold. They estimated that approximately 10 min were required for data collection at each spatial frequency. This amounted to a minimum of 80min for data collection at the 8 spatial frequencies they used. Longer data blocks to achieve a higher signai to noise ratio at low contrasts and possible subject rest periods increased this time by an unspecified amount. Fiorentini et uI. (1980) show good agreement between threshold CSF shape and relative VEP amplitude recorded at a constant suprathreshoid contrast. This is potentially a more rapid method. than that of Campbell and Maffei (1970). but they did not investigate responses at spatial frequencies higher than 6c/deg. Possibly due to differences in flicker rate. their data and that of Tyler c’rtrl. (1978. 19791 do not agree, even though both were recorded at a fixed suprathreshold contrast. In order to avoid possible distortions in the VEPjCSF at high contrast due to saturation of VEP at some spatial frequencies. (Campbell and Maffei. 1970; Spskreijse.1973). I chose to maint;~~n COntraSt at the lowest possible level: the VEP threshold. To account for flicker effects, 1 chose to study VEP for flicker rates from 6.6 to 20 Hz thereby bracketing the flicker rates used by Campbell and Maffei (19701 and those used by Tyler pt ul. (1978). Findity. both counterphase and on-off (appearance-disappearance) flicker were studied. It was hoped that by systematically varying these parameters. some range of parameters could be found where VEP and psychophysical CSFs showed significant agreement.
Sine wave grating stimuli were displayed on a @cc Electronics monitor fP31 phosp~~or~ at a mean Iuminance of IOOcd ‘m’. A small fixatton spot .3mm m diameter was placed in the center of the screen. Subjects were instructed to view thz area of the screen near the fixation point. but were not required to fixate on it. Gratings were flickered either m counter-phase or in an on-off (appearance-disappcarancet mode. il\verage luminance remained constant for both flicker types. Flicker rates were 6.6. IO and 20 Hz. Countcrphase flicker at FHz produced pattern reversal twice per cycle at a rate of 2 FHz. On-off flicker at FHz produced a grating for one hatf period and a blank screen at the average luminance for the other haif period. Thus. the grating appeared once per cycle at a rate of FHz. Subjects sat a distance of 14Ocm from the screen which subtended an area 8.5 deg in width by 7 deg in height. They viewed the gratings binocularly with natural pupils against a dark surround. The monitor was calibrated with ~a Pritchard Spectra photometer and contrast was determined to be a linear function of peak-to-peak -_-axis voltage to a contrast of 0.7. Contrast is defined as (L,,, - L,r, V(L,,, -t L,,, 1 where L,,, is the luminance at a peak and Lmin is the luminance at a trough.
Evoked potentials were recorded between Beckman electrodes located at 0: and the left mastoid. The right matsoid was grounded. Impedance of electrode pairs was maintained at less than .4 KQ. Potentials were amplified by a Gnass PS a.c. preamp set at a gain of 10,000. Filter half amplitudes were at 1 Hz and i KHz. The ampti~er output was fed into a Nicotet 446 B Fast Fourier transform spectral analyzer. This analyzer contains an anti-aiiasing filter wzith an upper cutoff adjusted to the selected spectral range. The FFT analyzer computed the VEP spectrum from 0 to SOHz for ali recording sessions. All VEP records consisted of an average of 8 spectra, each computed from 8 set of data. Total data collection time for each record was approximately 1 tnin. A display on thf 446 B allowed viewing of the ~~c~umuiating spectral sum. All spectra were stored on magnetic tape for further analysis. A typical averaged VEP response. shown in Fig. 1% normally appeared as a very narrow peak rising above a broad band noise spectrum. Tbe minimal detectable response was limited by the average noise in the spectral region around the peak. A signal to noise ratio was defined as the spectra1 amptitude at the expected response frequency divided by the average amplitude across a 3 Hz wide spectral region centered on the stimulus frequency. A histogram of the ratio was determined from hundreds rrf spectral records obtained from all subjects for the conditions when 110
Contrast sensitivlt\
30
40
HZ Fig. I. Fast Fourier Transform ofa VEP waveform for counterphase Bicker at 10.125 Hz. The spectr~im shows two primary peaks, one at 20.25 Hz and one at 40.5 Hz. The broad peak a! 10 Hz is produced b) alpha activity and the peak a( 40.5 Hz is a harmonic of the pattern reversal rate. Decrbels are detincd US 10 log I l/r;3where Cir the r.m,s. voltage in a particular spectral band and I, is I V r.m.s. Spectral hands are 0. I-75 Hz wide.
stimulus was present. The histogram was approximatety Gaussian and showed that the signal to noise ratio defined above fell below 1.25 for approximately 95”,, of the trials. This ratio was chosen as a decision criterion for ali subsequent experiments. The ex~rjmenter adjusted contrast by means of a step attenuator. The contrast was changed in steps of 2 or 4 dB (2 dB when contrasts were less than 0,021 and VEP records were obtained at each step until threshold was bracketed by signal to noise ratios above and below the criterion level. Threshold was then defined as the contrast step (2 or 4 dBf below the lowest contrast at which the criterion signal to noise ratio had been exceeded. When the signal to noise ratio at the contrast step above threshold was between 1.3 and 1.3 the presence of a signal was always verified by determ~njng the signal to noise rario at the next highest contrast step. If this ratio was less than 1.25. the previous record was rejected as defining Ikreshold and the threshold search was resumed. Once the experimenter had acquired some experience. locating an individuals VEP threshold could he done rapidly. requiring about three data points ar each spatial frequency. Contrast sensitivity was defined as the reciprocal of the threshold contrasr.
Subjects were asked to adjust the contrast of the sttmulus using an analog attenuator until they could gust detect (11flicker or (3) the spatial structure of the grating. Subjects made five contrast settings at each spatial frequency and flicker rate used in the VEP
experiment. Psychophysical contrast sensitiGty was defined as the reciprocal of detection threshold contrast.
P’EP Dar0
Steady state VEP analysis by a Fast Fourier transform device enables the experimenter to observe the behavior of a number of harmonics in the spectrum. Counterphase flicker produced the more easily interpretable data in terms of harmonics. The threshold was always determined by tke spectral component at frequency 2 FHz. A companent at 4 FHr was of&n observed but did not relate to contrast in :I consistent manner across sessions. On-off flicker. on the other hand. usually showed a component at FHz, rhe Nicker frequency. as well as at 2 FHz. 3 FHz and higher karma&s. The relative strengths of the FHz and 2 FHz components varied from subject to subject. However both were repeatable across sessions and varied in a monotcmic manner with contrast. Therefore. it was decided that threshold would be determined by the component (FHz or 2 FWz) that reached the criterion level first. as contrast was increased. This decision was made before the data were compared to the psychophysical contrast sensitivity functions and it will be seen below that VEP/CSFs produced in this way
YO
6.6 HZ
10 HZ
20 HZ
SPATIAL
-Y
(CPOf
Fig. -1. Djstribution of thresholds determined by spectral components of on-off flicker at a frequency of FHz. The 2F component mediates threshold determination primarily at low spatial frequencies while for intermediate to high spatial frequencies the threshold is more often determined by the component at the flicker rate. F.
agree very well with psychophysical CSFs. A genera1 pattern for the relative strengths of the F and 2 FHt components in on-off fficker can be seen in Fig. 2. At a flicker rate of F = 6.6 Hz, thresholds at low spatial frequencies are generally determined by the 2 FHz com~nent while thresholds at higher spatial frequencies are determined by the FHz component. At the higher flicker rates the F and 2 FHz components are about equally effective for 0.5 c/deg gratings while the FHz component predominates for spatial frequencies of 1 cideg or higher. Threshold VEP,/CSFs for two subjects (Fig. 3) show the amount of variation to be expected in VEP data recorded over a period of 3 weeks. This figure shows the most and least variable of the six subjects tested with counterphase flicker at 6.6 Hz. Variability for
each subject was determined bx taktnp the ratio of the maximum contrast sensitivilj to the minimum colttrast sensitivity at each spatial frequent! and averag-
ing these values across spatial freyucnc!. The subjecr with the highest score was the most variable. Subjeci data of the type shown in Fig. 3 \-ir’rc averaged HCKW WSS~O~S HKI then across subjects to determine ;I mean CSF for each flicker rate and for each type’ of tlicker. Averages in all cases were calculated from the logarithms of [he contrast sensitl\,itics al each spst~l frcquency. The averaged VEPCSF &rt;~ for countt’rphase and on-off Bicker are shmvn in Fig. &I and h. The general shapes of the CW~L’~arc qurte diffcmx. The counterphase CSFs decrease in amplitude with flicker rate while on-off CSFs are nearly indcpcndmt flicker rate. Counterphasc CSF data for the two higher flicker rates are generally constant with spatial frequency to 4 cideg. decreasing above this spatial frcquency. The 6.6 Hz curve behaves somewhat differently and shows a peak at I or 3 c‘ deg. The on-off CSFs on the other hand. show a pronounced peak at 4 c!deg for all flicker rates.
Individual contrast sensitivities were determined from the average of the logarithms of 5 contrast sensitivity estimates. Individual subject standard deviations were about _+1Oqbof the mean. Mean contrast sensitivities for each condition were then computed from the iogarithms of the individual contrast sensitivities. Figure .5a,b shows flicker threshold CSFs averaged across subjects, for counterphase and on-off flicker. Figure SC,d shows pattern threshold CSFs for the two different types of flicker. Flicker threshold CSFs are similar in shape but not in magnitude for counterphase and on-off flicker while pattern threshold CSFs are quite different for the two types of flicker.
Pearson product
moment
correlation
coefficients
Fig. 3. VEP contrast sensitivity functions for subjects showing greatest and least variabilit) for counterphase flicker at 6.6 Hz.
Contrast
I
VEP CONTRAST
SENSITIVITY
FLICKER RATE
0 6.6HZ
91
sensitivity
.lOHZ
x
ZOHZ ON-OFF
COUNTER PHASE FLICKER
FLICKER
a
0.5 SPATIAL FREOUENCY
IN CPD
4.0
1.0
10.0
20.0
SPATIAL FREQUENCY IN CPO
Fig. 4. VEP threshold CSFs for counterphase and on-off flicker. The curves are averages over three repetitions from six subjects, Average magnitudes of the standard deviation are shown b! the symbols to the right of each panel.
used to compare mean VEP/CSFs to mean psychophysical CSFs in an attempt to determine whether the VEPKSF was more closely related to pattern or to flicker detection thresholds. For the purpose of correlation, each set of 3 CSFs seen in Figs 4 and 5 were considered as a set of 18contrast sensitivity values. Each value corresponds to a different pair of independent variables (spatial frequency and flicker rate). The VEP data in Fig. 4a and the psychophysical data in Fig. 5a and c were collected under conditions of counterphase flicker. The I8 contrast sensitivity
were
PSYCHOPHVSlCAL FLICKER RATE COUNTERPHASE z=I&
CONTRAST SENSTnVIIY .6.6
FLICKER
. 10 HZ
HZ
X 20 HZ
ON-OFF FLICKER
a
b
=
! I
FLICKER DETECTION
FLICKER DETECTION
d
PATTERN DETECTION
0.5
1.0
sical pattern
PATTERN DETECTION
4.0
10.0 21
0.5
1.0
values in Fig. 4a were correlated first with the 18 contrast sensitivity values in Fig. 5a to determine the correlation between the VEPCSF and psychophysical flicker CSF and then with the 18 values in Fig. 5c to determine the correlation between the VEPCSF and psychophysical pattern CSF. These are the upper two correlation coefficients in Table 1. Similar correlations for on-off flicker were obtained by correlating the VEP data in Fig. 4b with the psychophysical data in Fig. 5b and d. The on-off correlations are the lower two in Table 1. Counterphase flicker correlation coefficients show that the VEP CSF correlates equally well with psychophysical flicker or pattern CSFs. The on-off flicker VEP,CSF correlates highly with the psychophysical pattern CSF but not very well with the psychophysical flicker CSF. A test of the significance of the difference between correlation coefficients for flicker or pattern detection (Edwards. 1973) indicates that the ditference is significant to the 0.01 level for the on-off Ricker and not significant for the counterphase flicker. Thus. the on-off tlicker VEPCSFs are related primarily to psychophysical pattern threshold CSF data. While a relationship to psychophysical flicker threshold data cannot be ruled out for counterphase Bicker. a strong relationship to pattern threshold is an equally good hypothesis with the present data, One can conclude from these results that VEP threshold data averaged over a number of subjects are highly correlated with similarly averaged psychophy-
4.0
ID.0
20
threshold
Table
SPATIAL FREOUENCV IN CPD
FIN. 5. Payhoph!sical CSFs for counter phase and Htckrr. In (a. b) subjects adjusted contrast until they lust detect Hacker. In (c. d) subjects adjusted contrast they could just detect the bar pattern of the grating. age magmtudes of the standard deviation are shown symbols to the right of each panel.
on-off could unttl Averby the
data as a function
I. Correlation between psychophystcal CSF
Countcrphasc rlrcket-
VEP VEP
On-Of
VEP vs lhcker VEP cs pattern
llicka
vs flicker vs pattern
VEP
of spatial
and
0.957 0.965 0.65 1 0.941
92
M?\HKW
CANNON
JK
frequency and flicker rate for both cou~terphase and on-off flicker. However. it is obvious from Fig. 5 [c.d) that pattern threshold CSFs for counterphase flickering gratings are different from pattern threshold CSFs for on-off flicker. These data make it clear that success in relating VEP and psychophysical data require that both be recorded under the same experimental conditions. EXPERIMENT
2. COMPARISON
OF CHANGES
IN VEP AND PSYCHOPHYSICAL
CSFs
CAUSED BY CHANGES IN OFTICAL CORRECTION
AND VISABfLtTt
CONDITIONS
If evoked potential CSFs are directly related to psychophysi~l contrast sensitivity functions they should both show similar changes for a given change in the stimulus. Two experiments were designed to test this hypothesis. First, changes in the VEP/CSF were compared to changes in the psychophysi~l CSF for changes in optical correction of the subjects. Second. changes in the VEP/CSF were compared to changes in the psychophysical CSF for a reduction in the visibility of high spatial frequency components produced by placing a diffusing screen in front of the disptay. Results
Counterphase flickering gratings at a rate of 6.6 Hz (13.2 reversals per second) were used to study the effectscaused by changes in optical correction. Counterphase Ricker was chosen because of its simple response structure (primarily a response at 2F). Four subjects with corrected binocular Snellen acuity of 20/20 participated in the study. Binocular Snelien acuity without glasses ranged from 29130 to 2Of500. A VEP/CSF was obtained for each subject in both the corrected (glasses on) and uncorrected (glasses off) viewing conditions, The VEPKSF was an average of two measurements at each spatial frequency. Psychophysical contrast sensitivity functions were also determined from pattern detection thresholds of each subject in the corrected and uncorrected conditions. Individual threshold contrast sensitivity functions are shown for all four conditions in Fig. 6. While psychophysical and VEP contrast sensitivity functions do not have exactly the same shape or amplitude. ordering of the curves for the uncorrected condition is the same for both recording methods (Fig. 6c, d). CSF amplitudes decrease with decreasing acuity. However, if one computes the ratio of the CSF with glasses off to the CSF with glasses on for both VEP and psychophysical data and compares the two ratios. agreement is excellent. The four panels in Fig. 7 show these ratios for the individual subjects. Apparently, threshold changes due to changes in optical correction measured psychophys~a~ly are almost exactly reproduced by changes in VEP threshold.
SPAT1A.L FREOUENCY tN CPD
Fig. 6. Contrast sensitivity functions for liiur Subjectstrlth glaSSeSon (a.h) and glasses off (c.d\. Sneflen acuity of the subjects without glasses is indicated in panel td). VEP imd psychophysical CSFS show identical rank ordering as a function of fr%?tIenacuity for the glasses-off eon&ion Visibilit_reffects
The procedure followed in this experiment was almost identical to the procedure followed in the previous experiment except that instead of changing optical correction, a diffusing screen was placed in front of the monitor to selectively attenuate high spatial frequencies. Average luminance was also reduced b) about I@,; with the addition of the diffusing screen. but this condition prevailed for both VEP and psychophysical sessions. Four subjects with corrected binocular Sndlen acuity of 20,‘20 were used fur this experiment and gratings were counrerphase flickered at 6.6 Hz. Ratios of contrast sensitivity with diffusing screen to contrast sensitivity without diffusing screen were determined for both psy~bophysical and VEP methods. Individual data for both melhods are compared in Fig. 8. Agreement between VEP and psychophysical data is almost as good_ as the agreement shown in Fig. 7a-d. except for the subject in Fig. Xc. When the data for four subjects are averaged. the two average curves show excellent agreement (Fig. 9) with the photometrically measured MTF of the diffusing screen and with each other. Apparently. changes in display system MTF are reflected very accurately in both VEP and psychophysical methods when data from as few as four subjects are averaged.
The data in Figs 7 and 8 show that removal of optical correction or the addition of a diffusing screen can slightly increase the VEP/CSF at low spatial frequencies. This occurs where the ratios in these figures are greater than one. Regan and Richards ($973)
Contrast
0
“EP
’ PSYCHOPHYSICAL
RATIO
sensttivtt)
0 VEP
RATI
1 PSYCHOPHY5lCS -PHOTOMETRIC SCREEN
30)
kmy
::cmt
:
d SNELLEN ACUITY (20
500)
,,,I ._
.n n SPATIAL
4n
FREOUENCV
,n n
,
IN CPD
FIN. 7. Similarity between VEP and psychophystcal contrast sensitivity ratios for four subjects. The rattos were computed from the data of Fig. 6 by determining CS without glasses over CS with glasses at each spatial frequent!. .A ratto of one implies no change in CS due to removal of glasses. VEP and psychophysical ratios are in excellent agreement indicating. as expected. the greatest attenuation due to removal of glasses occurs at high spattal frequencles.
10
100
checkerboard in amplitude
pattern
amplitude
showed
small
checks
the expected
when the checks were blurred
checks. greater VEP
by
produced
in a
data
of Figs
7 and 8 indicate
ment (ratio greater wave
when
they
were
blurred.
than
gratings
are
and
Richards
Repan
contrasts.
used.
by a brightness
Three
VEP
task at higher
threshold
data
data
show
defocus
screen. In both cases
of sample means is significant
to the 0.025 level for two of the three subjects 0 YEP
’ PsvcHoPHV5Ic!
show
enhancement
indicating
(Fig.
a real but
hancement McCann They
small
showed
that
the
surround
surround
introduction increase
of glasses or introduction
SPATIAL
FREOUENCV
narrow had
sensitivity.
of a dinilsing
what is perceptually
surround
and
sur-
a nar-
raise contrast
sensi-
where attenuation
or diffusion
glasses
a
is extremety
due
small.
IN CPO
VEP and psychoph)slcal contrast scnsitlvity ratlos showins the degradation of the monitor MTF due to the presence of a diffusing screen. Both VEP and psychophyslcat threshold contrast Fensttic~~y functions were determtned with and without the diffusing screen in place. The curves represent the ratios of CS with diffusing screen over CS wlthout diffusing screen as a function of spatial frequent! and demonstrate good agreement between psychophystcal and VEP data. X
of
of
of
(19x0).
the edges of the dark
at 10~ spatial frequencies
to removal
Hall
contrast
and softens
illuminated
c). enfor
that previously
screen blurs
tlvity
explanation and
The removal
row
CSF
in the experiments
IO a display
could
who
Xa and
psychophysical
may be found
round. This may introduce
7.2
c. Fig.
c’t (II. (1978) and McCann
illuminated a dark
7b and
at 0.5 c deg. A possible
this enhancement
in
as well.
due to both
of diffusing
a test for the difference
the
which
seems to be present
at 0.5 c!deg
and the introduction
to
results.
sets of psychophysical
enhancement
enhance-
contrar)
matching
a slight enhancement
of the four
a slight
However.
but large
CSF
that
psychophysical
some of the psychophysical
The
IN CPO
I) may also occur when sine-
reduction
than 30 min arc. caused an increase in
FREOUENCY
Ftg. 9. Mean VEP and psychophysical CS ratms compared with photometrIcall) measured MTF of the dtffusin~ screen. The VEP and psychophysical ratio4 from FIN S were aleraged to compute the dashed cur\cs in this tigurc The solid curve represents the reduction of the monttor MTF due to the diffusing screen. Appparentlk both ps!chophgslcal and VEP data give an exccllcnt lit to the actual MTF degradatton.
were obtained VEP
that
I
I
“1’11’
SPATIAL
warned
DIFFUSING
m
: c --
SNELLEN
I
w
OF
SNELLEN
SNELLEN ACUITY (20
1.
MTF
Regan (197X) has expressed sponses to low spatial
concern
frequency
(I c!dep or lessl may have a significantly poral
tuning
than
high
spatial
psychophysical
temporal
pattern
same at all spatial
tuning
CSFs
frequencies:
VEP
re-
gratings
different
frequency
Data in Figs 4 and 5 show that under of this experiment.
that
sine wave
tem-
gratings.
the conditions
for both
VEP
are approximately sensitivity
and the
decreases
94
MARK W. CAS>ON JR
with flicker rate. Regan (1978). on the other hand. found that a counterphase flickered 1 c/deg grating had its largest reponse at a pattern reversal rate between 15 and 20Hz and showed a steep decline in amplitude above and below these rates. The fact that such tuning does not appear in the data of Fig. &a may be attributable to the different electrode configurations used in the two experiments. or to different conditions of luminance and contrast under which the experiments were conducted. It was noted previously (Fig. 2) that the 3F component of the VEP produced b) on-off flicker was usually the larger spectral component at the low spatial frequencies and low flicker rates. However. this effect is also present for a significant number of subjects at a flicker rate of 20 Hz: hence it must be associated more with low spatial frequencies than with flicker rate. Campbell and Maffei (1970) observed different suprathreshold behavior for VEPs recorded below Zcjdeg than for those recorded at 2c:deg and above. Perhaps the shift in frequency of maximum response shown in Fig. L ’ is another manifestation of this difference. Whatever the mechanism. the choice of the maximum response component to determine threshold gives a high correlation between VEP and psychophysical contrast sensitivity functions to spatial frequencies as low as 0.5 cideg. While the VEP/CSF data for counterphase flicker rates of 10 and 20 Hz in Fig. 3 are nominally low pass functions, it is of interest that both functions show a local minimum at about 2 c!deg. The location of this minimum is similar to that in some of the suprathreshold VEP data of Tyler er ai. (1978. 19791and of Apkarian et ul. (198 If who used flicker rates above 10Hz at a contrast of almost 0.8. Two of the six subjects in this experiment showed this type of behavior at VEP threshold. No such behavior appeared in the on-off flicker VEPKSFs. It is possible that the combination of counterphase flicker, high flicker rates and high contrast may hc required to produce the well defined multiple peaks observed by Tyler and his associates. While it has been established (Figs 7. 8) that changes in an indivudal CSF are measured with nearly equal accuracy using either VEP or psychophysical techniques the use of VEP to measure absolute contrast sensitivity for an individual requires more study. Means and standard deviations for VEP and psychophyslcal pattern detection data from IO subjects LIW~ counterphase flicker at h.h Hr. are sho\vn in Fig. IO. It IS evident that standard deviations for the VEP data are not significantly greater than those for psychophysical data. The mean curves have about the same shape but the psychophysical contrast sensitivtty is about a factor of 3 higher than the VEP CSF. at least when criterion VEPs are come puted as was done here. Analysis of the individual pattern detection CSFS for the 10 sub_$cts at each spatial frequency showed that rank correlation between VEP and psychophysi-
Ei
PSYCHOPHYSleS
f
VEP
i I
1.0 SPATIAL
I
8.0 FREQUENCY
,i*i_,,1
10 0
CPO
Fig. IQ VEP and ps~c~oph~sicai pattern contrast sensrtivity functions for counterphase flicker at 6.6 Hz. These data represent the means and standard deviations for lOsubjects. The standard deviation for the VEP data is
larger. on the average, than the standard deviation of the psychophysical data. but not excessively so, An Important result shown here is that the VEP data are consistent enough to show a clustering of CSF \&es around a mean such as we see in the psychophysical data.
cal CSF values was not significantty different from zero. Apparently, rank correlation between individual VEP and psy~~ophyai~~ CSFs is not to he expected when individual differences lie within the amplitude range indicated by the standard deviations in Fig. 10. However data from the four subjects in Fig. 6 show that when individual CSFs differ h> an amount greater than the standard deviations of Fig. 10. essentially perfect rank correlation between VEP and psychophysical CSFs result. The similarities between psychl~phys~c~l and VEP data shown in this paper bring OUI an imporrant point that can easily be overlooked. Objective VEP data for a population of observers agree very well with mean psychophysical data from the Same subjects obtained using the criterion dependent method of adjustment. but with present methods of stimulus presentation and analysis. the objective measure offers no advantage in accuracy over the p~ycl~ophysica~ method for measurement of CSFs. However, there are many cases where psychophysical methods either cannot be used or cannot answer specific questions about the state of neural pathways to the occipital cortex. fn such cases, the VEP threshold techniques may be useful for generating VEP/CSFs having many features m common with psychophysi~l CSFs. VEP/CSFs obtained by the method described in this paper appear to he free of many uncertainties present iti previous studies and to be interpretable in much the same way as psychophysical CSFs. It is hoped that this method may be significant step in developing a quant~t~ti~~e link between VEP and psychophysical datu.
Contrast Ack1loi~iedgemenrsc-lhls
work was supported in part by the AFAMRL Laboratory Director’s Fund. The author wishes to thank Brian Bennett. Penny Konys and Michael Riess for their techmcal assistance. Thanks also to Art Ginsburg. Wayne Martin and Alan Pantle for their constructive comments on the manuscript. Reprints of this article are identified by Air Force Aerospace Medical Research Laboratory as AFAMRL-TR-X1-137.
sensmvit~ patterns containmg two spatial frequencies: a comparison of single channel and multiple-channel models. I.isiorz Res. Il. 751-259. Graham N.. Robson J. G. and Nachmias J. (19781 Grating 1L\IOHRc.\. 18. summation in fovea and periphery x15-s25.
Harris L.. Atkinson J. and Braddtck 0. (1976) V’isual contrast sensitivity of a 6 month-old infant measured by the evoked potential. .\trric~c~ 264. 570 571 REFERESCES Harter M. R. and White C. T. (196X) Effects of contour sharpness and check st7e on visualI\ e\oked cortical Apkarian P. A.. Nakayama K. and Tyler C. W. (IYXli potentials, t’isiori Rrss. 8. 7Ol7l 1. Binocularity in the human visual evoked potential: facilttation. summation and suppression. ~~~~~~~)~~~~~,p~~.Jones R. and Keck M. J. 410781 Visual evoked response as a function of grating spattal frequent!. irti-c\f. (~~~~f~~l~~~. Clffi. ‘~~~l~~~~~i~~~~~~. 51, 3248. usiriii sci. 17. 657 659. Blakemore C. and Campbell F. W. (1969) On the exrstence Maffet L. and Fiorentini A. (1973) The visual cortex as a of neurons in the human visual system selectively sensispatial frequency analyzer. tisrort Rex 13. 135 1767. tive to the orientation and size of retinal images. J. P/r!McCann J. J. and Hall J. A. Jr (1980) Effects of averagesioi. 203, ‘37-260. luminance surrounds on the visibility of sine-wave gratBodis-Wollner 1. (1976) Vulnerability of spatial frequencv ings. J. opr. SW. .Irn. 70, 212 1719. channels in cerebral lesions. Narw 261.. 309-311.. _ McCann J. J.. Savoy R. L. and Hall J. A. Jr (19781 t’isibilit! Bodis-Wollner 1. (1977) Recoverv from cerebral blindness: of low-frequency sine-wave target: dependence on evoked potential and psychophysical measurements, number of cycles and surround parameters. liriott Rc\. Elrctroenceph. C/in. ~Vewophysiol. 42, 1I& I X4, 18,891-X94. Bodis-Wollner I.. Hendly D. D. and Kulikowski J. J. (IY72) Electrophysiological and psychophysrological responses Movshon J. A.. Thompson I. D. and Tolhurst D. J. (197X) Spatial and temporal contrast sensitivities of neurons in to modulation of a contrast grating pattern. Perccptinn 1, 341-349. areas 17 and lg of the cat‘s visual cortex. J. Pit~s~ol. 283. 101-120. Campbell F. W. and Maffei L. (1970) Electrophysiolo~ical Owsley C.. Sekuler R. and Boldt C. 11981 t Agmg and lowevidence for the existence of orientation and size deteccontrast vision: face perception. Ifwsr. ~~J~if~j~~~. ri.\ioir tors in the human visual system. J. Pht~of. 207. sci. 21. 362-365. 635-652. Parker D. M. and Saizen E. A. (1977) The spatial selectivity Campbell F. W. and Kuhkowski J. J. (1972) The visual of early and late waves within the human visual evoked evoked potential as a function of contrast of a grating response. Pwception 6. 85-95. pattern. J. Phvsiof. 222, 345-3.56. DeValois R. L. and DeValois K. K. (1980) Spatial vision. Pirchio M.. Spinelli D. Fiorentini A. and Maflei L. (1978) Am. Rer. Pswhol. 31. 309-341. Infant contrast sensitivity evaluated by evoked potenDeValois K. K.. DeValois R. L. and Yund W. E. (1979) tials. Brain 141. 179-184. Responses of striate cortex cells to grating and checkerRegan D. (1977) Steady-state evoked potentrals. J. opr. S’oc. board patterns. J. Physiol. 191, 483-505. .+lnr. 67, 1375-1489. Edwards A. L. (1973) Sratisical Methods. Third edn. Holt. Regan D. (1975) Assessment of visual acuity by evoked Rinehart & Winston. New York. potential: recording ambiguity caused by temporal Fiorentini A.. Pirchio M. and Spine& D. (1980) Scotopic dependence of spatial frequency selectivity. I c.GtttiRv.\. contrast sensnivity in infants evaluated by evoked poten18. 4% 443. tials. fntz~r. ~~~~~~i~~~. riarai Sci. 19, 950-Y% Regan D. and Richards W. I lY73) Brightness contrast :d Ginsburg A. P. 11978) Visual inrornlation processing based evoked potentntls. J. opt. Sot. itttt. 63, 606-61 t on spatial filters constrained by biological data. .-tit Sehuler R. (1974) Spatial vision. .4im. Ret-. P.~~~httf. 25, f-orce Aerc>.sptrce .$fcdictrl Rrsrtrrch L.~dwtnror~~ Rcpwt 712 _._, ‘3’ T‘R-7X-129. Sprkrcuse H.. Van Der Tweel L. H. and Zuidcma T. (19731 Ginsburg A. P. (1981) Spatial filtering and vision: impliContrast evoked responses in man. l’rsiolt Ri*\. 13, cations for normal and abnormal vision. In Proc. .sI,,u~. 1577 1601. .dp/Tl. P.sl~c,lroplt~.~ic~,s 10 Clinicrrl Pt~~h/rr~l,r. San Francisco. Tyler C. W.. Apkarian P. and Nakayama K. (lY7g) MulCA. Cambridge University Press. tiple spatial frequency tuning of electrical responses l‘rom Ginsburg A. P.. Evans D. W.. Sekuler R. and Harp S A. the I isual cortex. Espl Ertrin Res. 33, 535 550. (IYXI) Contrast sensitivity predicts pilots’ pcri”ormance m Tyler C. W.. Apakarian P.. Levi D. M. and Nakayama K. aircraft simulators. .-trfi. J. Opro,u. P/I,~\;~J/. Qr. 59. f 1979) Raptd assessment of visual function: an electronic 105 109. sweep rrchmque for the pattern VEP. frzcc\r. Ot~/rt/t& Graham N. and Nachmiss J. (1971) Detectton of gratmg 5htr;ri .Qi. IS. 703 7 13.
SENSITIVITY: PSYCHOPHYSICAL AND POTENTIAL METHODS COMPARED MARIO W. CAUNOX
\viatton
Vision
JR
Laboratory. Human Engmeerin@ Diviston. Air Force Aerospace Laboratory. Wright-Patterson .41r Force Base. OH -IS??. U.S.A.
Medical
Abstract--Contrast sensitivity functions derived from stead! state evoked potential high correlatton with psychophysically derived threshold contrast sensitivity functions. holds as flicker rate. tvpe of flicker. optical correctton and vtsihllity conditions are substantial range. provided that VEP and ps~choph~si~~l data are recorded under the tal conditions
Campbell and MaKei (1970) studied the relationship between steady state visual evoked potentials (VEP) and threshold contrast sensitivity (the reciprocal of threshold contrast) for Rickering sine-wave gratings. They measured VEP amplitude over a range of contrasts and showed that under the conditions of their experiment. regressions fitted to plots of log contrast vs VEP amplitude intersected the contrast axis near the psychophysically measured threshold. The concept of a direct relationship between sinewave grating thresholds determined by psychophysical means and those determined from VEP data was further strengthened in subsequent papers by Campbell and Kulikowski (1972) and by Bodis-Wollner er nl. f 1972) where responses to a single spatial frequency were studied in some detail. Further studies demonstrating a relationship between contrast sensitivity functions (CSFs) derived from steady state VEP data and CSFs derived ps~chophysically have been conducted by Harris PI ul. (1976). Bodis-Wollner (1976. 1977). Pirchio (‘r ul. (1978) and by Fiorentini rt ul. (1980). On the other hand. Tyler et al. (1978, 1979) and Apkarian et of. (19811. using high contrast grattngs have consistently demonstrated functions of VEP amplitude vs spatial frequency that do not resemble contrast sensitivity funtions. These functions often show two sharply tuned peaks: one in the region between 0.51 c/deg and one in the region between 4 and 8 Ldeg. Apkarian et ul. (1981) claim that these peaks are similar in shape to tuning curves obtained from single cell recordings. In a similar vein Regan 11978) found that plots of VEP amplitude vs spatial frequency can be strongly affected by the flicker rate used to elicit the VEP. Regan claimed that low spatial frequencies produced VEPs by “local flicker” of large pattern regions while higher spatial frequencies elicited VEPs from pattern reversal. Thus, from Regan’s data. one would not expect plots of VEP vs spatial
Kewarch
thresholds shtr\\ This correlatton
changed
over a
same rxperimen-
frequency generated at some arbitrary flicker rate to resemble contrast sensitivity functions. While the discussion to this point has concentrated on steady state VEPs generated by sine-wave grating stimuli the reader is probably aware that most VEP research has been conducted using checkerboard patterns. perhaps because they produce stronger responses than sine-wave or square-wave gratings (Parker and Salzen. 1977). Steady state VEP responses to checkerboard patterns show a maximum VEP response for check sizes in the range of IO-15 min arc (Regan. 1977). This tuning is similar to the bandpass shape of the contrast sensitivity function. However, checkerboards are rich in spatial harmonic components and in this sense are more complex stimuli than sine-wave gratings. A great deal of current evidence. both psychophysical (Blakemore and Campbell. 1969: Graham and Nachmias. 1971: Graham er trl.. 1978) and neurophysiological (Maffei and Fioretini. 1972: Movshon L’?ui.. 1978: DeValois (‘r rti.. 1979) implies that these components may be processed by independent filters or channels. The physiological consequences of such processing would be more activity in the cortex and perhaps greater complexity of the VEP waveform in response to a checkerboard. The appropriateness of sine-wave gratings for VEP use was recognized by Parker and Salzen ( 1977) and by Jones and Keck (1978) who conducted studies of transient VEP responses using sine-wave grating stimuli. In fact. Jones and Keck commented that when sine-wave gratings were used. evoked response waveforms were remarkably consistent across subjects. Much of the interest in determining contrast sensitivity from VEP data has been generated by the increasing use of contrast sensitivity among psychophysicists for assessing certain aspects of visual performance. A relationship between individual threshold CSFs and the detection and identification
of complex objects has been established by Ginsburg. t 1978). Ginsburg cr trl. (1981) and by Ousle) c’r u/. (1981). References to clinical applications of CSFs can be found in review articles by DeVaiois and DeValois (19801 by Sekuier (1974) and in a recent paper by Ginsburg (19X1). Because of these relationships between contrast sensitivity and visual performance. a method of reliabiiy determining a CSF from evoked potential data would be valuable for clinical work. infant x4sion studies and as a luboratory research tool. As the brief literature review in this introduction has shown. there is considerable uncertainty in determining whether a VEP response at a particular spatial frequency will be related to the psychophysically derived contrast sensitivity at that spatial frequency. It is possible that part of the problem is due to the different contrasts and the variety of flicker rates that different researchers use. While the method of Campbell and Maffei (1970) appears to overcome some of these problems there are two difficulties with their method. First. Campbell and Maffei claim that their method is only good for spatial frequencies above 3 cjdeg. The second problem is the time required to obtain a CSF. Using the Campbell and MafTei method. Harris er al. (1976) determined VEP amplitudes at lOcontrast levels for each spatial frequency and extrapolated regression lines through the points to obtain a threshold. They estimated that approximately 10 min were required for data collection at each spatial frequency. This amounted to a minimum of 80min for data collection at the 8 spatial frequencies they used. Longer data blocks to achieve a higher signai to noise ratio at low contrasts and possible subject rest periods increased this time by an unspecified amount. Fiorentini et uI. (1980) show good agreement between threshold CSF shape and relative VEP amplitude recorded at a constant suprathreshoid contrast. This is potentially a more rapid method. than that of Campbell and Maffei (1970). but they did not investigate responses at spatial frequencies higher than 6c/deg. Possibly due to differences in flicker rate. their data and that of Tyler c’rtrl. (1978. 19791 do not agree, even though both were recorded at a fixed suprathreshold contrast. In order to avoid possible distortions in the VEPjCSF at high contrast due to saturation of VEP at some spatial frequencies. (Campbell and Maffei. 1970; Spskreijse.1973). I chose to maint;~~n COntraSt at the lowest possible level: the VEP threshold. To account for flicker effects, 1 chose to study VEP for flicker rates from 6.6 to 20 Hz thereby bracketing the flicker rates used by Campbell and Maffei (19701 and those used by Tyler pt ul. (1978). Findity. both counterphase and on-off (appearance-disappearance) flicker were studied. It was hoped that by systematically varying these parameters. some range of parameters could be found where VEP and psychophysical CSFs showed significant agreement.
Sine wave grating stimuli were displayed on a @cc Electronics monitor fP31 phosp~~or~ at a mean Iuminance of IOOcd ‘m’. A small fixatton spot .3mm m diameter was placed in the center of the screen. Subjects were instructed to view thz area of the screen near the fixation point. but were not required to fixate on it. Gratings were flickered either m counter-phase or in an on-off (appearance-disappcarancet mode. il\verage luminance remained constant for both flicker types. Flicker rates were 6.6. IO and 20 Hz. Countcrphase flicker at FHz produced pattern reversal twice per cycle at a rate of 2 FHz. On-off flicker at FHz produced a grating for one hatf period and a blank screen at the average luminance for the other haif period. Thus. the grating appeared once per cycle at a rate of FHz. Subjects sat a distance of 14Ocm from the screen which subtended an area 8.5 deg in width by 7 deg in height. They viewed the gratings binocularly with natural pupils against a dark surround. The monitor was calibrated with ~a Pritchard Spectra photometer and contrast was determined to be a linear function of peak-to-peak -_-axis voltage to a contrast of 0.7. Contrast is defined as (L,,, - L,r, V(L,,, -t L,,, 1 where L,,, is the luminance at a peak and Lmin is the luminance at a trough.
Evoked potentials were recorded between Beckman electrodes located at 0: and the left mastoid. The right matsoid was grounded. Impedance of electrode pairs was maintained at less than .4 KQ. Potentials were amplified by a Gnass PS a.c. preamp set at a gain of 10,000. Filter half amplitudes were at 1 Hz and i KHz. The ampti~er output was fed into a Nicotet 446 B Fast Fourier transform spectral analyzer. This analyzer contains an anti-aiiasing filter wzith an upper cutoff adjusted to the selected spectral range. The FFT analyzer computed the VEP spectrum from 0 to SOHz for ali recording sessions. All VEP records consisted of an average of 8 spectra, each computed from 8 set of data. Total data collection time for each record was approximately 1 tnin. A display on thf 446 B allowed viewing of the ~~c~umuiating spectral sum. All spectra were stored on magnetic tape for further analysis. A typical averaged VEP response. shown in Fig. 1% normally appeared as a very narrow peak rising above a broad band noise spectrum. Tbe minimal detectable response was limited by the average noise in the spectral region around the peak. A signal to noise ratio was defined as the spectra1 amptitude at the expected response frequency divided by the average amplitude across a 3 Hz wide spectral region centered on the stimulus frequency. A histogram of the ratio was determined from hundreds rrf spectral records obtained from all subjects for the conditions when 110
Contrast sensitivlt\
30
40
HZ Fig. I. Fast Fourier Transform ofa VEP waveform for counterphase Bicker at 10.125 Hz. The spectr~im shows two primary peaks, one at 20.25 Hz and one at 40.5 Hz. The broad peak a! 10 Hz is produced b) alpha activity and the peak a( 40.5 Hz is a harmonic of the pattern reversal rate. Decrbels are detincd US 10 log I l/r;3where Cir the r.m,s. voltage in a particular spectral band and I, is I V r.m.s. Spectral hands are 0. I-75 Hz wide.
stimulus was present. The histogram was approximatety Gaussian and showed that the signal to noise ratio defined above fell below 1.25 for approximately 95”,, of the trials. This ratio was chosen as a decision criterion for ali subsequent experiments. The ex~rjmenter adjusted contrast by means of a step attenuator. The contrast was changed in steps of 2 or 4 dB (2 dB when contrasts were less than 0,021 and VEP records were obtained at each step until threshold was bracketed by signal to noise ratios above and below the criterion level. Threshold was then defined as the contrast step (2 or 4 dBf below the lowest contrast at which the criterion signal to noise ratio had been exceeded. When the signal to noise ratio at the contrast step above threshold was between 1.3 and 1.3 the presence of a signal was always verified by determ~njng the signal to noise rario at the next highest contrast step. If this ratio was less than 1.25. the previous record was rejected as defining Ikreshold and the threshold search was resumed. Once the experimenter had acquired some experience. locating an individuals VEP threshold could he done rapidly. requiring about three data points ar each spatial frequency. Contrast sensitivity was defined as the reciprocal of the threshold contrasr.
Subjects were asked to adjust the contrast of the sttmulus using an analog attenuator until they could gust detect (11flicker or (3) the spatial structure of the grating. Subjects made five contrast settings at each spatial frequency and flicker rate used in the VEP
experiment. Psychophysical contrast sensitiGty was defined as the reciprocal of detection threshold contrast.
P’EP Dar0
Steady state VEP analysis by a Fast Fourier transform device enables the experimenter to observe the behavior of a number of harmonics in the spectrum. Counterphase flicker produced the more easily interpretable data in terms of harmonics. The threshold was always determined by tke spectral component at frequency 2 FHz. A companent at 4 FHr was of&n observed but did not relate to contrast in :I consistent manner across sessions. On-off flicker. on the other hand. usually showed a component at FHz, rhe Nicker frequency. as well as at 2 FHz. 3 FHz and higher karma&s. The relative strengths of the FHz and 2 FHz components varied from subject to subject. However both were repeatable across sessions and varied in a monotcmic manner with contrast. Therefore. it was decided that threshold would be determined by the component (FHz or 2 FWz) that reached the criterion level first. as contrast was increased. This decision was made before the data were compared to the psychophysical contrast sensitivity functions and it will be seen below that VEP/CSFs produced in this way
YO
6.6 HZ
10 HZ
20 HZ
SPATIAL
-Y
(CPOf
Fig. -1. Djstribution of thresholds determined by spectral components of on-off flicker at a frequency of FHz. The 2F component mediates threshold determination primarily at low spatial frequencies while for intermediate to high spatial frequencies the threshold is more often determined by the component at the flicker rate. F.
agree very well with psychophysical CSFs. A genera1 pattern for the relative strengths of the F and 2 FHt components in on-off fficker can be seen in Fig. 2. At a flicker rate of F = 6.6 Hz, thresholds at low spatial frequencies are generally determined by the 2 FHz com~nent while thresholds at higher spatial frequencies are determined by the FHz component. At the higher flicker rates the F and 2 FHz components are about equally effective for 0.5 c/deg gratings while the FHz component predominates for spatial frequencies of 1 cideg or higher. Threshold VEP,/CSFs for two subjects (Fig. 3) show the amount of variation to be expected in VEP data recorded over a period of 3 weeks. This figure shows the most and least variable of the six subjects tested with counterphase flicker at 6.6 Hz. Variability for
each subject was determined bx taktnp the ratio of the maximum contrast sensitivilj to the minimum colttrast sensitivity at each spatial frequent! and averag-
ing these values across spatial freyucnc!. The subjecr with the highest score was the most variable. Subjeci data of the type shown in Fig. 3 \-ir’rc averaged HCKW WSS~O~S HKI then across subjects to determine ;I mean CSF for each flicker rate and for each type’ of tlicker. Averages in all cases were calculated from the logarithms of [he contrast sensitl\,itics al each spst~l frcquency. The averaged VEPCSF &rt;~ for countt’rphase and on-off Bicker are shmvn in Fig. &I and h. The general shapes of the CW~L’~arc qurte diffcmx. The counterphase CSFs decrease in amplitude with flicker rate while on-off CSFs are nearly indcpcndmt flicker rate. Counterphasc CSF data for the two higher flicker rates are generally constant with spatial frequency to 4 cideg. decreasing above this spatial frcquency. The 6.6 Hz curve behaves somewhat differently and shows a peak at I or 3 c‘ deg. The on-off CSFs on the other hand. show a pronounced peak at 4 c!deg for all flicker rates.
Individual contrast sensitivities were determined from the average of the logarithms of 5 contrast sensitivity estimates. Individual subject standard deviations were about _+1Oqbof the mean. Mean contrast sensitivities for each condition were then computed from the iogarithms of the individual contrast sensitivities. Figure .5a,b shows flicker threshold CSFs averaged across subjects, for counterphase and on-off flicker. Figure SC,d shows pattern threshold CSFs for the two different types of flicker. Flicker threshold CSFs are similar in shape but not in magnitude for counterphase and on-off flicker while pattern threshold CSFs are quite different for the two types of flicker.
Pearson product
moment
correlation
coefficients
Fig. 3. VEP contrast sensitivity functions for subjects showing greatest and least variabilit) for counterphase flicker at 6.6 Hz.
Contrast
I
VEP CONTRAST
SENSITIVITY
FLICKER RATE
0 6.6HZ
91
sensitivity
.lOHZ
x
ZOHZ ON-OFF
COUNTER PHASE FLICKER
FLICKER
a
0.5 SPATIAL FREOUENCY
IN CPD
4.0
1.0
10.0
20.0
SPATIAL FREQUENCY IN CPO
Fig. 4. VEP threshold CSFs for counterphase and on-off flicker. The curves are averages over three repetitions from six subjects, Average magnitudes of the standard deviation are shown b! the symbols to the right of each panel.
used to compare mean VEP/CSFs to mean psychophysical CSFs in an attempt to determine whether the VEPKSF was more closely related to pattern or to flicker detection thresholds. For the purpose of correlation, each set of 3 CSFs seen in Figs 4 and 5 were considered as a set of 18contrast sensitivity values. Each value corresponds to a different pair of independent variables (spatial frequency and flicker rate). The VEP data in Fig. 4a and the psychophysical data in Fig. 5a and c were collected under conditions of counterphase flicker. The I8 contrast sensitivity
were
PSYCHOPHVSlCAL FLICKER RATE COUNTERPHASE z=I&
CONTRAST SENSTnVIIY .6.6
FLICKER
. 10 HZ
HZ
X 20 HZ
ON-OFF FLICKER
a
b
=
! I
FLICKER DETECTION
FLICKER DETECTION
d
PATTERN DETECTION
0.5
1.0
sical pattern
PATTERN DETECTION
4.0
10.0 21
0.5
1.0
values in Fig. 4a were correlated first with the 18 contrast sensitivity values in Fig. 5a to determine the correlation between the VEPCSF and psychophysical flicker CSF and then with the 18 values in Fig. 5c to determine the correlation between the VEPCSF and psychophysical pattern CSF. These are the upper two correlation coefficients in Table 1. Similar correlations for on-off flicker were obtained by correlating the VEP data in Fig. 4b with the psychophysical data in Fig. 5b and d. The on-off correlations are the lower two in Table 1. Counterphase flicker correlation coefficients show that the VEP CSF correlates equally well with psychophysical flicker or pattern CSFs. The on-off flicker VEP,CSF correlates highly with the psychophysical pattern CSF but not very well with the psychophysical flicker CSF. A test of the significance of the difference between correlation coefficients for flicker or pattern detection (Edwards. 1973) indicates that the ditference is significant to the 0.01 level for the on-off Ricker and not significant for the counterphase flicker. Thus. the on-off tlicker VEPCSFs are related primarily to psychophysical pattern threshold CSF data. While a relationship to psychophysical flicker threshold data cannot be ruled out for counterphase Bicker. a strong relationship to pattern threshold is an equally good hypothesis with the present data, One can conclude from these results that VEP threshold data averaged over a number of subjects are highly correlated with similarly averaged psychophy-
4.0
ID.0
20
threshold
Table
SPATIAL FREOUENCV IN CPD
FIN. 5. Payhoph!sical CSFs for counter phase and Htckrr. In (a. b) subjects adjusted contrast until they lust detect Hacker. In (c. d) subjects adjusted contrast they could just detect the bar pattern of the grating. age magmtudes of the standard deviation are shown symbols to the right of each panel.
on-off could unttl Averby the
data as a function
I. Correlation between psychophystcal CSF
Countcrphasc rlrcket-
VEP VEP
On-Of
VEP vs lhcker VEP cs pattern
llicka
vs flicker vs pattern
VEP
of spatial
and
0.957 0.965 0.65 1 0.941
92
M?\HKW
CANNON
JK
frequency and flicker rate for both cou~terphase and on-off flicker. However. it is obvious from Fig. 5 [c.d) that pattern threshold CSFs for counterphase flickering gratings are different from pattern threshold CSFs for on-off flicker. These data make it clear that success in relating VEP and psychophysical data require that both be recorded under the same experimental conditions. EXPERIMENT
2. COMPARISON
OF CHANGES
IN VEP AND PSYCHOPHYSICAL
CSFs
CAUSED BY CHANGES IN OFTICAL CORRECTION
AND VISABfLtTt
CONDITIONS
If evoked potential CSFs are directly related to psychophysi~l contrast sensitivity functions they should both show similar changes for a given change in the stimulus. Two experiments were designed to test this hypothesis. First, changes in the VEP/CSF were compared to changes in the psychophysi~l CSF for changes in optical correction of the subjects. Second. changes in the VEP/CSF were compared to changes in the psychophysical CSF for a reduction in the visibility of high spatial frequency components produced by placing a diffusing screen in front of the disptay. Results
Counterphase flickering gratings at a rate of 6.6 Hz (13.2 reversals per second) were used to study the effectscaused by changes in optical correction. Counterphase Ricker was chosen because of its simple response structure (primarily a response at 2F). Four subjects with corrected binocular Snellen acuity of 20/20 participated in the study. Binocular Snelien acuity without glasses ranged from 29130 to 2Of500. A VEP/CSF was obtained for each subject in both the corrected (glasses on) and uncorrected (glasses off) viewing conditions, The VEPKSF was an average of two measurements at each spatial frequency. Psychophysical contrast sensitivity functions were also determined from pattern detection thresholds of each subject in the corrected and uncorrected conditions. Individual threshold contrast sensitivity functions are shown for all four conditions in Fig. 6. While psychophysical and VEP contrast sensitivity functions do not have exactly the same shape or amplitude. ordering of the curves for the uncorrected condition is the same for both recording methods (Fig. 6c, d). CSF amplitudes decrease with decreasing acuity. However, if one computes the ratio of the CSF with glasses off to the CSF with glasses on for both VEP and psychophysical data and compares the two ratios. agreement is excellent. The four panels in Fig. 7 show these ratios for the individual subjects. Apparently, threshold changes due to changes in optical correction measured psychophys~a~ly are almost exactly reproduced by changes in VEP threshold.
SPAT1A.L FREOUENCY tN CPD
Fig. 6. Contrast sensitivity functions for liiur Subjectstrlth glaSSeSon (a.h) and glasses off (c.d\. Sneflen acuity of the subjects without glasses is indicated in panel td). VEP imd psychophysical CSFS show identical rank ordering as a function of fr%?tIenacuity for the glasses-off eon&ion Visibilit_reffects
The procedure followed in this experiment was almost identical to the procedure followed in the previous experiment except that instead of changing optical correction, a diffusing screen was placed in front of the monitor to selectively attenuate high spatial frequencies. Average luminance was also reduced b) about I@,; with the addition of the diffusing screen. but this condition prevailed for both VEP and psychophysical sessions. Four subjects with corrected binocular Sndlen acuity of 20,‘20 were used fur this experiment and gratings were counrerphase flickered at 6.6 Hz. Ratios of contrast sensitivity with diffusing screen to contrast sensitivity without diffusing screen were determined for both psy~bophysical and VEP methods. Individual data for both melhods are compared in Fig. 8. Agreement between VEP and psychophysical data is almost as good_ as the agreement shown in Fig. 7a-d. except for the subject in Fig. Xc. When the data for four subjects are averaged. the two average curves show excellent agreement (Fig. 9) with the photometrically measured MTF of the diffusing screen and with each other. Apparently. changes in display system MTF are reflected very accurately in both VEP and psychophysical methods when data from as few as four subjects are averaged.
The data in Figs 7 and 8 show that removal of optical correction or the addition of a diffusing screen can slightly increase the VEP/CSF at low spatial frequencies. This occurs where the ratios in these figures are greater than one. Regan and Richards ($973)
Contrast
0
“EP
’ PSYCHOPHYSICAL
RATIO
sensttivtt)
0 VEP
RATI
1 PSYCHOPHY5lCS -PHOTOMETRIC SCREEN
30)
kmy
::cmt
:
d SNELLEN ACUITY (20
500)
,,,I ._
.n n SPATIAL
4n
FREOUENCV
,n n
,
IN CPD
FIN. 7. Similarity between VEP and psychophystcal contrast sensitivity ratios for four subjects. The rattos were computed from the data of Fig. 6 by determining CS without glasses over CS with glasses at each spatial frequent!. .A ratto of one implies no change in CS due to removal of glasses. VEP and psychophysical ratios are in excellent agreement indicating. as expected. the greatest attenuation due to removal of glasses occurs at high spattal frequencles.
10
100
checkerboard in amplitude
pattern
amplitude
showed
small
checks
the expected
when the checks were blurred
checks. greater VEP
by
produced
in a
data
of Figs
7 and 8 indicate
ment (ratio greater wave
when
they
were
blurred.
than
gratings
are
and
Richards
Repan
contrasts.
used.
by a brightness
Three
VEP
task at higher
threshold
data
data
show
defocus
screen. In both cases
of sample means is significant
to the 0.025 level for two of the three subjects 0 YEP
’ PsvcHoPHV5Ic!
show
enhancement
indicating
(Fig.
a real but
hancement McCann They
small
showed
that
the
surround
surround
introduction increase
of glasses or introduction
SPATIAL
FREOUENCV
narrow had
sensitivity.
of a dinilsing
what is perceptually
surround
and
sur-
a nar-
raise contrast
sensi-
where attenuation
or diffusion
glasses
a
is extremety
due
small.
IN CPO
VEP and psychoph)slcal contrast scnsitlvity ratlos showins the degradation of the monitor MTF due to the presence of a diffusing screen. Both VEP and psychophyslcat threshold contrast Fensttic~~y functions were determtned with and without the diffusing screen in place. The curves represent the ratios of CS with diffusing screen over CS wlthout diffusing screen as a function of spatial frequent! and demonstrate good agreement between psychophystcal and VEP data. X
of
of
of
(19x0).
the edges of the dark
at 10~ spatial frequencies
to removal
Hall
contrast
and softens
illuminated
c). enfor
that previously
screen blurs
tlvity
explanation and
The removal
row
CSF
in the experiments
IO a display
could
who
Xa and
psychophysical
may be found
round. This may introduce
7.2
c. Fig.
c’t (II. (1978) and McCann
illuminated a dark
7b and
at 0.5 c deg. A possible
this enhancement
in
as well.
due to both
of diffusing
a test for the difference
the
which
seems to be present
at 0.5 c!deg
and the introduction
to
results.
sets of psychophysical
enhancement
enhance-
contrar)
matching
a slight enhancement
of the four
a slight
However.
but large
CSF
that
psychophysical
some of the psychophysical
The
IN CPO
I) may also occur when sine-
reduction
than 30 min arc. caused an increase in
FREOUENCY
Ftg. 9. Mean VEP and psychophysical CS ratms compared with photometrIcall) measured MTF of the dtffusin~ screen. The VEP and psychophysical ratio4 from FIN S were aleraged to compute the dashed cur\cs in this tigurc The solid curve represents the reduction of the monttor MTF due to the diffusing screen. Appparentlk both ps!chophgslcal and VEP data give an exccllcnt lit to the actual MTF degradatton.
were obtained VEP
that
I
I
“1’11’
SPATIAL
warned
DIFFUSING
m
: c --
SNELLEN
I
w
OF
SNELLEN
SNELLEN ACUITY (20
1.
MTF
Regan (197X) has expressed sponses to low spatial
concern
frequency
(I c!dep or lessl may have a significantly poral
tuning
than
high
spatial
psychophysical
temporal
pattern
same at all spatial
tuning
CSFs
frequencies:
VEP
re-
gratings
different
frequency
Data in Figs 4 and 5 show that under of this experiment.
that
sine wave
tem-
gratings.
the conditions
for both
VEP
are approximately sensitivity
and the
decreases
94
MARK W. CAS>ON JR
with flicker rate. Regan (1978). on the other hand. found that a counterphase flickered 1 c/deg grating had its largest reponse at a pattern reversal rate between 15 and 20Hz and showed a steep decline in amplitude above and below these rates. The fact that such tuning does not appear in the data of Fig. &a may be attributable to the different electrode configurations used in the two experiments. or to different conditions of luminance and contrast under which the experiments were conducted. It was noted previously (Fig. 2) that the 3F component of the VEP produced b) on-off flicker was usually the larger spectral component at the low spatial frequencies and low flicker rates. However. this effect is also present for a significant number of subjects at a flicker rate of 20 Hz: hence it must be associated more with low spatial frequencies than with flicker rate. Campbell and Maffei (1970) observed different suprathreshold behavior for VEPs recorded below Zcjdeg than for those recorded at 2c:deg and above. Perhaps the shift in frequency of maximum response shown in Fig. L ’ is another manifestation of this difference. Whatever the mechanism. the choice of the maximum response component to determine threshold gives a high correlation between VEP and psychophysical contrast sensitivity functions to spatial frequencies as low as 0.5 cideg. While the VEP/CSF data for counterphase flicker rates of 10 and 20 Hz in Fig. 3 are nominally low pass functions, it is of interest that both functions show a local minimum at about 2 c!deg. The location of this minimum is similar to that in some of the suprathreshold VEP data of Tyler er ai. (1978. 19791and of Apkarian et ul. (198 If who used flicker rates above 10Hz at a contrast of almost 0.8. Two of the six subjects in this experiment showed this type of behavior at VEP threshold. No such behavior appeared in the on-off flicker VEPKSFs. It is possible that the combination of counterphase flicker, high flicker rates and high contrast may hc required to produce the well defined multiple peaks observed by Tyler and his associates. While it has been established (Figs 7. 8) that changes in an indivudal CSF are measured with nearly equal accuracy using either VEP or psychophysical techniques the use of VEP to measure absolute contrast sensitivity for an individual requires more study. Means and standard deviations for VEP and psychophyslcal pattern detection data from IO subjects LIW~ counterphase flicker at h.h Hr. are sho\vn in Fig. IO. It IS evident that standard deviations for the VEP data are not significantly greater than those for psychophysical data. The mean curves have about the same shape but the psychophysical contrast sensitivtty is about a factor of 3 higher than the VEP CSF. at least when criterion VEPs are come puted as was done here. Analysis of the individual pattern detection CSFS for the 10 sub_$cts at each spatial frequency showed that rank correlation between VEP and psychophysi-
Ei
PSYCHOPHYSleS
f
VEP
i I
1.0 SPATIAL
I
8.0 FREQUENCY
,i*i_,,1
10 0
CPO
Fig. IQ VEP and ps~c~oph~sicai pattern contrast sensrtivity functions for counterphase flicker at 6.6 Hz. These data represent the means and standard deviations for lOsubjects. The standard deviation for the VEP data is
larger. on the average, than the standard deviation of the psychophysical data. but not excessively so, An Important result shown here is that the VEP data are consistent enough to show a clustering of CSF \&es around a mean such as we see in the psychophysical data.
cal CSF values was not significantty different from zero. Apparently, rank correlation between individual VEP and psy~~ophyai~~ CSFs is not to he expected when individual differences lie within the amplitude range indicated by the standard deviations in Fig. 10. However data from the four subjects in Fig. 6 show that when individual CSFs differ h> an amount greater than the standard deviations of Fig. 10. essentially perfect rank correlation between VEP and psychophysical CSFs result. The similarities between psychl~phys~c~l and VEP data shown in this paper bring OUI an imporrant point that can easily be overlooked. Objective VEP data for a population of observers agree very well with mean psychophysical data from the Same subjects obtained using the criterion dependent method of adjustment. but with present methods of stimulus presentation and analysis. the objective measure offers no advantage in accuracy over the p~ycl~ophysica~ method for measurement of CSFs. However, there are many cases where psychophysical methods either cannot be used or cannot answer specific questions about the state of neural pathways to the occipital cortex. fn such cases, the VEP threshold techniques may be useful for generating VEP/CSFs having many features m common with psychophysi~l CSFs. VEP/CSFs obtained by the method described in this paper appear to he free of many uncertainties present iti previous studies and to be interpretable in much the same way as psychophysical CSFs. It is hoped that this method may be significant step in developing a quant~t~ti~~e link between VEP and psychophysical datu.
Contrast Ack1loi~iedgemenrsc-lhls
work was supported in part by the AFAMRL Laboratory Director’s Fund. The author wishes to thank Brian Bennett. Penny Konys and Michael Riess for their techmcal assistance. Thanks also to Art Ginsburg. Wayne Martin and Alan Pantle for their constructive comments on the manuscript. Reprints of this article are identified by Air Force Aerospace Medical Research Laboratory as AFAMRL-TR-X1-137.
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