COLOUR CODING OF PATTERN RESPONSES IN lMAN INVESTIGATED BY EVOKED POTENTIAL FEEDBACK AND DIRECT PLOT TECHNIQUES D. REGAN Department of Communication. University of Keele, Staffs. ST5 5BG. England

Abstract-Visual sensitivity can be directly measured by arranging that evoked potential (EP) amplitude controls stimulus intensity (method of evoked potential feedback). A second technique is to directly plot graphs of EP amplitude vs the required stimulus parameter: this method gives more precise estimates of the shapes of EP plots than the conventional procedure of measuring individual points. The practical details. advantages and drawbacks of these two rapid techniques are discussed. Pattern EPs are mediated via colour channels, whereas EPs to rapidly Hickering lights are mediated by pooled colour signals. The red pattern channel’s peak sensitivity falls at a wavelength no shorter than 550-610 nm compared with the 555 nm peak for flicker sensitivity. An objective analogue of the psychophysical increment threshold procedure is made possible by means of EP feedback. Stimulus pattern intensity is approximately proportional to adapting intensity down to a pattern intensity of 5-10 td. below which EP amplitude attenuates rapidly. ISTRODCCTION

METHODS

Judgements of perceptual threshold are not only subjecttve but also labile. Furthermore. threshold psychophysics does not deal with everyday vision where sensations are well above threshold. But suprathreshold psychophysics, which does deal with everyday sensation levels. must handle judgements of sensory magnitude. On this point it is not yet clear to what extent such judgements really do represent sensory magnitude (Stevens, 196I ; Warren and Warren, I963 ; Treisman. 196-t). In this situation an objective means of studying human visual perception would be clearly welcome, and it is easy to understand the current interest in evoked potentials (EPs). Evoked potential amplitude can provide an objective correlate of visual threshold, for pattern perception (Campbell and Maffei. 1970; Regan. 1971b: Spekreijse, Tweel and Zuidma, 1973). However. a major strength of EP methods is that they are not restricted to sensory threshold levels. Used with an eye to its pitfalls. EP recording can complement superthreshold psychophysics by providing objective measures of the neurophysiological responses to visual stimuli when visual sensations are well above threshold. One major hindrance to a more effective use of EP recording is the slowness of the commonly-employed averaging method. Long experimental procedures often introduce their own errors. since the quantity being measured changes with time. This article presents two rapid methods for objectively studying colour and pattern vision. and discusses how the results might extend the findings of psychophysical experiments. especially at everyday levels of sensation.

Visual

st~dator

The field of view. illustrated in Fig. I(a) (upper right), was a 2’ x 1’ pattern of bright and dark checks (almost 100 per cent contrast) whose wavelength was either 676 or 544 nm. The retinal illumination from the bright squares was IX td. Superposed on the pattern was an unpatterned circular desensitizing light patch. 6: dia. and generally of a different colour to the pattern. The two dark bars illustrated in Fig. l(a) assisted fixation and accommodation. The light choppers, Ci and CZ in Fig. 1. were used in heterochromatic flicker photometry. The pattern’s intensity was controlled in steps by Wratten neutral density filters. while the desensitizing beam’s intensity was controlled continuously by a 2 log unit servo-driven neutral density wedge (W) used in combination with neutral densitv filters. Lenses LZ and L4 were placed so that they were seen-in Maxwellian view. Both lenses were located 17 cm from the eye so that planes passing through LZ and L-t appeared in sharp focus to an eye viewing through spectacle lens L5 and achromatizing lens A. The checkerboard transparency T was placed close to lens LZ. Light intensities were calibrated on an energy- basis with a Hilger Swartz vacuum thermopile. A photomultiplier was used for increased sensitivity when calibrating neutral density filters and wedge. Light choppers Cl and C2 were used for subjective calibration of luminance by heterochromatic flicker photometry. The retinal ihuminatton of the pattern was measured as follows: The chequerboard transparency T was removed, leaving the interference filter and the 2’ x 2’ stop in place. A milk glass diffuser and a 2’ x I’ stop were then placed in front of L4 so that the subject saw a single 2’ x 1’ stimulus field, and the two light patches were matched by heterochromatic flicker photometry. Next. the exit pupil P was removed. and the luminance of the white patch was measured using an SE1 photometer. A straight-

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Fig. I(a). Visual stimulator. Insert upper right shows stimulus field. This was a 3 x 2. pattern oiY min checks on u hich an unpatterned. 6 dia patch of desensitizin: light has superposed. The lower beam illuminated the pattern. and the upper beam gave the desensitizin g light. Key: A-500 W Xenon arc lamp: B%beamsplitter: M-mirror: Ll to Li-lenses: W--N.D. wedge: F-interference filters (IOnm halfwidth) and Wratten K.D. filters; T-checkerboard transparency; V-vibrator: A-achromatizing lens; P-

forward calculation then gave the retlnal illumination pattern u hen viewed through the e\it pupal.

1.5 mm dia exit pupil.

of the

Noise \vas mainlq due to spontaneous EEC activity and took the form of a.c. added to the d.c. signal. This could be attenuated bq increasing the time constants filters FI. F2 and F3. These filters were of the three-stage passive RC t.“pe. The time constant of each filter could be independentI! set to an> one of seben values separated b> factors of 2. between 0.07 see and 4d set per stage. A valuable advantage of this system o\cr some commcrcial devices that use the phase-locked principle is that the system described above measures amplitude for one selected harmonic component of the signal: the Fourier anal>ser is not intluenced by signal harmonics other than that for which it is tuned (here F Hz). Furthermore. and in practice this is an important point. if filters FI and F3 have small time constants. then the amphtude calibr;ltion is little altered by abrupt changes of signal phase during recording (see Results below). ErorCrd porrjlritrl f&dh&. The feedback mode of analysis is described in Fig. I(b). 4s mentioned aboLe. the output of F3 m Fig. l(b) \vas proportional to the running average of the evoked potential’s amplitude. This was fed to a summing amplifier S where it was subtracted from an adjustable d.c. criterion level CL. The direrence activated a servo drive circuit D that powered an B.C. servo motor SM. The motor drove wedge W so as to increase adapting light intensity when EP amplitude was more than the criterion level. and to decrease desensitizing light intensity when EP amplitude

of

Fowtr~ d~ui/~~~i.~of r/w EEC. Conisntionsl Ag-i\gCI electrodes were placed ( I I on the inion: 13) 9 cm anterior to the mion along the midline. and (3) on the left mastoid. Bipolar rxordings Lvere made between electrodes I and 1: electrode ? \ias grounded. .I squarewaveoffrequency F 1 Hz from a signal generator (not shoan) drove the vibrator V (Fig. la) so as to oscillate the checkerboard transparent) through one check width. thus gi\ino F pattern reversals per second. The signal generator also t\d FHz sine and cosine signals to analogue multipliers Xl I Fig. I b). The product of the EEG and sine (2xFt) passed through filter Ft and the product of the EEG and cosine (3zFt) passed through filter FZ before reaching a de\ics R. u hose output was proportional to the square root oi the sum of the squares of its inputs. This signal then passed through a filter F3. The running average of ecoked potential amplitude was proportional to the d.c. voltage at the output of filter FZ (Regan. 1965. 1966a. 1966b. 1973). This running average was fed both to a recording potentiometer P and averager A. The signal processing so far described can be regarded in its net result as equivalent to narrow bandwidth frequency filtering. with the centre frequencylocatedeuactlyon the signal frequencylhere F Hz).

(b)

Fig. I(b). Apparatus in evoked potential feedback mode. Key: A-EEG amplifier: M-multiplier (Burr Brown. model -iO97,3): FI. F2. F3-low pass filters; R-device for computing (.Y’ + 1.‘); where x and ~‘are its inputs (Teledyne Philbrick. model 4352); A-averager: P-recording potentiometer; ssumming amplifier: D-servomotor drive unit: SM--50 Hz a.c. servomotor: W-neutral density wedge. Fig. I(c). Apparatus

in direct

plotting mode (averaging of graphs). Key: p&positional feedback signal).

Pt-multi

turn

potentiometer:

Colour coding of pattern responses in man was less than the criterion level. The response speed of the wedge (see Results) was controlled by (a) altering the gear ratio between the servomotor and wedge, and (b) altering the amount of velocity feedback to the servomotor. Direcr plot mode. For the direct plot mode of operation the apparatus of Fig. l(b) was modified as shown in Fig. I(c). The inputs to the servomotor were now as shown in Fig. l(c). The required waveform (here a 0.05 Hz ramp) were fed from a signal generator to a multiplier where it modulated the amplitude of a 50 Hz sinewave. Only in this form would the ramp waveform be accepted by the 50 Hz servomotor SM.. Positional servo control was arranged as follows. A multi-turn potentiometer was driven from the wedge by !ears. When the potentiometer was energized by 50 Hz a.c., Its 50 Hz output voltage was proportional to the linear position of the wedge. This signal (p~posit~onal feed back), indicating actual wedge position was fed to a summing amplifier S where it was subtracted from the 50 Hz rampmodulated waveform that represented the required wedge position. The difference between these 50 Hz signals drove the servomotor until the wedge reached the required position. The amplitude ofa 50 Hz velocity feedback signal (not shown) was adjusted to give an acceptable compromise between response speed and overshoot of the wedge. An a.c. servocontrol svstem was chosen in order to minimize drift of wedge posiiion in this application. RESULTS R~~~z~~i~rg awagr

misdial of response arnp~~t~~e

Previousdescriptions(Regan. 1965. 1966a. t966b) of the Fourier analysis (i.e. synchronous detection) method of recording evoked potentials outlined a major advantage of the method. This is that EP amplitude or phase is available in the form of a running average. Figure 2 shows the moment-to-moment running average of EP amplitude as a function of time. The onset of visual stimulation was at zero on the time axis. The subject was then presented with a 2’ x 2’ pattern of red (676 nm) bright and dim (100 per cent contrast) checks, each of 9 min subtense. Bright and dim checks

Fig. 2. Running average of evoked potential amplitude. The light was switched on at zero on the time axis. The trace was recorded by potentiometer P in Fig. I(b). Response time roughly 5 set (IO-90 per cent). All filters (F 1. FZ and F3 in Fig. l(b) were three-stage passive RC low-pass filters of time constant 1.2 sec. each stage.

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exchanged places 7 times+ec. ‘amplitudes of the 7 Hz component of the evoked potential were plotted as ordinates in Fig. 3, confirming previous reports that moment-to-moment values of the running average remain substantially constant with time (Regan, 1965. I966a. 1966b. 1972a Fig. 5.12). It should be noted that. in any given situation. before Fourier analysis can be accepted as a valid approach it must be shown experimentally that the running averages of EP amplitude and phase are substantially constant. When these conditions hold. the EPs have been labelled “stead)-state EPs” (Regan. 1965, 1966a). The plot of Fig. 2 would be completely flat if (a) EP amplitude remained perfectly steady with time, and (b) if there were no noise. The short-term fluctuations seen in Fig. 2 seem to be mainly caused by variations in EP amplitude rather than by spontaneous EEG activity (i.e. noise) (Regan 1972a. Fig. 5.12). In practice it was found that these short-term fluctuationscould be minimized by carefully controlling both fixation and accommodation; for steady-state EPs, in contrast to transient EPs. “psychological” variables such as attention seemed unimportant. The time constants used to record Fig. 2 were similar to those used in Fig. 5. Therefore the fluctuations in Fig. _1 give an impression of the fluctuations that must be handled by the EP feedback method (and by the direct plot method). It is easy to see how the running average display of Fig. 2 would immediately show the experimenter what EP effect he had produced when he abruptly changed some stimulus parameter. For example, if he suddenly placed a trial lens in front of the subject’s e?e. an immediate increase or decrease of response amphtude in Fig. I would tell him at once whether the lens sharpened or degraded the retinal image (Regan, 1973a). More generally. the display shows the effect of changing parameter B (here, image sharpness) upon the EP elicited by varying a different parameter A (here. spatial contrast). Other examples of this procedure are the effect of abruptly changing the wavelength of chromatic adaptation (parameter B) upon the EP elicited by Hickering intensity (parameter A) (Regan. 1968) and the effect of abrupt!y.changing flicker phase (parameter B) upon the EP ehclted by flickering intensity (parameter A) (Regan, 1965, 1966a). The running-average facility is the basis both of the direct graphical plotting method and of the EP feedback technique described below. Cbite out graphs rather fhan imficidunl poinrs The effect of a stimulus parameter upon EP amplitude was obtained directly in the form of a graphical plot. In general. parameter B was co~zri~~~o~~s~~ varied while continuously recording the EPs elicited by oscillating parameter A. For example. the EPs in Fig. 3 were elicited by a red checkerboard pattern. The bright and dark squares repetitively exchanged places 6

D. REGAN

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t 6 97 10 2 x-:

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Fig. 3. Averaging of graphs: direct plotting mode. This shows how a graph is made smoother by averaging several rapidly-recorded samples of the whole graph. The running average of evoked potential amplitudes (ordinates) was continuously recorded while log stimulus intensity was changed with a ramp waveform (abscissae). The upper four traces are individual samples of the whole graph. The middle trace is the average of the upper four samples and shows some smoothing. The lower graph shows two traces. each of which is the average of 16 samples. The noise trace is the average of I6 samples made with the stimulus occluded. All titters (Fi. FZ and F3 in Fig. lb) were 3 stage passive RC low-pass filters, each stage of @3 set time constant.

times+. At the same time the intensity of a superposed yellow adapting light was continuously changed (log intensity was varied through I.9 log units by a ramp waveform of period 25 set). Figure 3 shows how the amplitude of the red pattern EP was affected. The upper part of Fig. 3 shows four separate recordings. It can be seen that each individual trace give an approximation to the required curve (an important control. discussed below, is that similar plots were obtained for increasing and for decreasing intensities). However, each trace each shows irregular noisy fluctuations. Smoothing the EP graph by aueraging versus smootk-

ing by controi ofjiter time constants. Figure 3 shows how the recording was smoothed by averaging. The single trace in the middle is the average of the four

separate recordings shown at the top of the figure. The lower part of Fig. 3 shows that further smoothing could be obtained by averaging 16 rather than four individual records. Two traces are shown to illustrate variability. The trace labelled “noise” was recorded in the same way as the two other traces except that the pattern was occluded. Of course averaging must be used with discretion. for when too many traces are averaged the experiment is unduly prolonged, variability increases, and the procedure becomes self-defeating. Alternatively. the individual EP piots of Fig, 3 (upper) could be smoothed by increasing the time constants offilters Fl, F3 and F3 in Fig. 1. Figure 4(a) and (b) ifiustrate this smoothing effect. The stimulus was similar to that of Fig. 3 except that log intensity of the adapting light was varied with a triangular waveform (dotted line). Each trace in Fig. 4 is the average of I6 separate records. In Fig. 4(a) the filter time constants were short, and the traces showed marked noise fluctuations. These fluctuations were much reduced when the filter time constants were increased by a factor of 2 (Fig. 4(b)). Increasing the filters’ time constants had, however, the following disadvantages. (a) The EP plot was more delayed, so that the correspondence between instantaneous EP amplitude and instantaneous stimulus intensity was degraded. For example. the point of minimLlm EP amplitude in Fig. -I(b) can be seen to be delayed behind the point of maximum adapting intcnsity (dotted line); this delay was much less in Fig. 4(a) where the filter time constants were shorter. (b) The shape of the plot was distorted. (c) Detailed features of the plot were lost. These disadvantages were countered by increasing the time of the plot, for example by doubling the length of the abscissa in Fig. 4(b) to 100 sec. Experimentally. however, this counter was to some extent self-defeating, for prolonging the trace seemed to increase the moment-to-moment variability of the EP. I found that a practical compromise between filtering and averaging could be reached empirically along the following lines. (I) By ensuring that the physiological stimulus was well controlled so that moment-tomoment EP variations were minimal (e.g. by maintaining good fixation and accommodation). (2) By arranging that all EP analysis frequencies were distant from the subject’s alpha frequency. (3) Since reducing sweep time was experimentally found to reduce fluctuations of EP amplitude, the shortest sweep was chosen that was compatible with (4) Filter time constants that gave acceptably smoothed individual records without unacceptable distortion of their shapes. (5) Averaging a&v traces usually gave useful smoothing (e.g. averaging four traces approximately halved the amplitude of ffuctuations). Averaging too many traces was, however, self-defeating. The balance between the filter time constants of Fl and F1 and of F3 was found to be important. If the (similar) time constant of Fl and F2 was small, SO that smoothing was mainly due to filter F3. then the noise

Colour coding of pattern responses in man

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Fig. 1. Averaging of graphs: distortions caused by EP and by apparatus. Fig. 4(a). Shows that hysteresis and dynamic nonlinearities are negligible in directly-plotted evoked potential graphs. The running average of evoked potential amplitudes (ordinates) was continuously recorded while log stimulus intensity was changed with a triangular waveform (shown dotted). The EP traces are approximately similar for increasing and decreasing intensities, and minimum EP amplitude approximately coincides with minimum stimulus intensity. Each trace is the average of 16 samples. The middle trace shows the speed of response to switching a 6 Hz sinewave calibration signal on and off (onset shown by lowermost trace). All filters (Fl, F2 and F3 in Fig. lb were three stage passive RC low-pass filters. each stage of 0.15 sec. time constant. ‘01 98l-

s6 a.

W5Cl 24-

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level was high (i.e. the amplitude of the output of F3 was high when the stimulus was occluded). When smoothing was mainly carried out by filters Fl and F2. then the output of F3 was much smaller when the stimulus was occluded; on the other hand, with this latter low-noise arrangement, an abrupt change in EP phase caused a temporary error in the measurement of EP amplitude. In other words variations in EP phase were seen as variations of EP amplitude. Therefore, this low-noise arrangement could only be used when preliminary experiments showed that abrupt changes of EP phase were not likely to occur. Fenrurrs ofme EP plot. Figure 3 (lower) shows that EP amplitude was little affected by adding adapting light until. at higher adapting intenstties, EP amplitude started to attenuate roughly proportionally to log intensity; EP amplitude fell to noise level at the approximate point on the intensity axis where the pattern just disappeared. This is in line with reports of previous experiments in which contrast was varied at constant intensity (Campbell and Maffei, 1970; Regan. 1971b. 1973b; Spekreijse, Tweel and Zuidma. 19i3). These two parts of the curve merged at a well-defined bend or “knee”. This knee has been used as a marker to enable EP studies to be carried out at a known suprathreshold level (Regan. 1971b. 1974). Is the EP plot the same for increasing and for decreasing intensity’! This question cannot be answered when filter time constants delay and distort the plot as in Fig. 4(b). According!y. filter time constants were set to low values in Ftg. -l(a) as illustrated by the calibration trace in the centre of the figure. This calibration shows the system’s speed of response when a 6 Hz calibration signal was abruptly switched on and off. In the upper part of Fig. 4(a) the EP traces seem to be approximately similar whether intensity was increasing (left half of Fig. -I) or decreasing (right half of Fig. 4). Much of the residual delay is probably due to the filters and not to the EP. In other words, hysteresis is negligible for the EP. The force of this point is that if hysteresis were appreciable, then the EP feedback method would not have been feasible. Rwming average of response semitirify: tial feedback

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Fig. 4(b). Shows how increasing the time constants of the Fourier analyser’sfilters (Fl, F2, F3 in Fig. l(b)) smooths the plot. but also causes distortion. Filter time constants in Fig. 4(a)were doubled in Fig. 4(b). Compared with the EP traces of Fig. 4(a). the trace in Fig. 4(b) is smoother. but the minimum is delayed, and the plot is asymmetric. Each trace is the average of 16 samples.

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evoked poterr-

Relarice spectral semiticiryfor pattern EPs. Figure 5 shows records made with the apparatus arranged to write out the running average of EP semiricity rather than EP amplitude. Some care was needed in selecting a criterion level for EP amplitude that was not too high. If this were set too near the knee (Fig. 3 lower half), a fluctuation might take the ND wedge to a position where EP amplitude was attenuated by increasing intensity so that feedback became ineffective [one way of reducing this problem was to use patterns of lower (e.g. 30 per cent) contrast]. At the other extreme, the EP feedback method could be easily used with criterion levels so low that the stimulus pattern was only 0.2-0.3 log units above subjective threshold.

As described above. a disc of monochromatic dewxsirizing light (wavelength ;.I was superposed on the red checkerboard pattern. The checkerboard eticited an EP urhose running average controlled the adapting Light‘s intensitv so as to maintain EP amptirude constant at a criterion Ievzl of 7 microvolts (chosen arbitrarily 1. Throughout this experiment the red pattern was not changed in any \~a-: only the superposed desensitizing light was altered. The traces marked EP in Fig 5 are running averages of EP amp~i&Liderecorded during this condition of EP feedbxk. The trace marked ‘Ct;edpein the Lft side of the figure shuu-s the movements over a period of SOsees ofthe SD wedge in the desensitizing beam whose position was controlled by the EP. The wedge can be seen to hunt up and down over roughI! x if:! log units. it was found to be important to ensure that the servo-driven wedge‘s speed of response did not greatly exceed the response speed of the Fourier analyser (set by filters Fl. F2 and F3). other\visz hunting became excessive. Response time in the EP feedback condition of Fig. 5 was approximately 17 sec. The wavelength of the adapting light was then changed from +low (590 nml to blue (477 nmf while leaving the red ehtckerboard unchanged. At once EP 590 nm

437nm

Fig. 5. Evoked potential feedback. EP amplitude controls stimulus intensity so as to maintain EP amplitude constant (here at 7 !iV). Traces marked “wcdpe” record stimulus intensity in log units (ordinates) vs time (abscissa). while traces marked “EP” record the running average of EP amplitude in microt;olts(ordinatesf versus time (abscissa. The desensitizing light is yellow (595 nm) in the left half of the Fig., and is changed to blue 1137nm) in the right half. fn order to maintain EP amplitude constant. EP feedback then produces an immediate 1.7 log unit increase of desensitizing light intensity b> displacing the wedge position. 411 filters fF1, F1 and F3 in Fig. I(b)) xen 3 staee passive RC low-pass filters. each stage of timeconstant 1.2 sec. The practical response time of the whole EP feedback system was about 17 52C.( 10-90 per CtYlT).

rb)

Fig. 6. Spectral sensirivit? of red pattern EPs (a) and incremental plots (bt determined b) EP feedback method. Fig. 61a1. Filled circles sho\v sensitivity of red (676 nm) pattern EPs to superposed desensitizing hpht of~rinble WBYCLength iabscissae). Dotted line is relative luminous efficiency curve mwsured by heterochromatic Hickrr photometry. Fig. 6(b). Abscissa are checkerboard pattern’s intensity. Ordinates plot the UI~ in which the intensity of the adapting light was changed in order to maintain EP amplitude

constant. Filled circles-both pattern and adapting light of woveletqth

676 nm. Open triangles-pattern adapting light 619 nm.

543 nm,

feedback drove the wedge through rough@ I.7 log units so as to maintain EP amplitude at 7itV by increasing the intensity of the blue desensitizing light. Figzrz 5 {right side) shows that the xsedge position then stabilized. hunting about this new position. Thus. if a red checkerboard pattern was to elicit an EP ofconstant criterion levei 7 )tV. then n blue 337 nm adapting light had to be I.7 log units more intense than a yellow 590 nm adapting light. In other ivords. visual sensitivity as measured by the red pattern EP was 1.7 log units less to blue light than to yellow light. This experiment was then repeated for a range of adapting wavelengths. In this way. visual spectral sensitivity \sas measured using the amplitude ofred pattern EPs as a probe. The filled circles of Fi_e.6(a) show the result. Although these measurements wre rather imprecise. visual sensitivity for pattern EPs seemed to peak at roughiy %3@6iO nm. These points did not fail on the dashed curve of Fig. 6(a). This curve shows visual sensitivity measured bt heterochromatic flicker photometry and agrees \vlth the standard CIE curve in its peak wavelength of approGmately 555 nm. Srrprtrrhws/~~Miwt-‘~w~tr I~J~‘~~s~~~c~~Ic’/IIs. The appatatus was set to maintain EP amplitude at 3 &IVby EP feedback. Both pattern and adapting light were green (S-Mnm). Throughout this etwriment the wavelengths of both the red pattern and the superposed desensitizing light were kept constant. However. both their intensities changed. With the pattern’s bright squares of brightness 5900 troIandr EP amplitude was allowed to control the intensity of the adapting lighr. Both wedge position and EP amplitude werr: continuously recorded as in Fig. 5.

Colour coding of pattern responses in mdn A 03 ND tilter was then placed in front of the pattern. The wedge was allowed to re-stabilize. and its position recorded for I min or so. A further 0.3 ND filter was placed in front of the pattern and the new wedge position recorded again. This was repeated until the pattern’s intensity had been reduced by a total of 3 log units. An ascending series of readings were then taken. These measurements are shown as filled circles in Fig. 6(b). At higher intensities the points fell on a straight line whose slope was roughly unity (note that both intensity axes are logarithmic). When the pattern became dimmer than roughly -I to IO td. EP amplitude attenuated rapidly. The experiment was repeated with a green (544 nm) pattern and red (619 nm) adaptation (open triangles in Fig. 6(b). Again, this gave a straight line plot whose slope also approximated to unity. and again EP amplitude attenuated rapidly at low pattern intensities. However the green-red plot was displaced from the green-green curve by an amount corresponding to the greater sensitivity of green pattern EPs to green light than to red light (Regan. 1974). DISCUSSlO\

In many, perhaps most. EP experiments the end point is a graph that describes a functional relationship

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between an EP feature and some stimulus parameter. Although it is common practice to firstly measure individual points and then plot such a graph. these individual points are an unnecessary intermediate stage. When stimulation is repetitive the running average facility enables EP graphs to be plotted directly, so bypassing. the stage of measuring individual points.’ Averaging graphs rather than points has the advantages of speed and precision. The speed increase is often large, though difficult to estimate quantitatively; compared with averaging points it probably lies between I5 times (Fig. 3 above) and 100 times (Regan, 1973a). Increased precision can be understood as follows. Take the experiment of Fig. 3 as an esample. Imagine that the sl~clpr of the lowermost EP trace in Fig. 3 remains constant with time. except that the whole curve moves up and down from moment to moment. This notion is illustrated in Fig. 7(a) where five curves show the hypothetical relationships between EP amplitude and stimulus intensity at five points in time. This argument is restricted to the type of EP variability shown in Fig. 7(a). so that each of the ftte lines can also be taken to represent a directly-recorded plot using the method of Fig. 3. The filled circles in Fig. 7(a) show EP amplitudes obtained in the usual way by five separate successive av-erages. In Fig. 7(b) the dotted line joins these five points. while the full line is the average of the five direct plots of Fig. 7(a) obtained by the method of Fig. 3. It is clear from Fig. 7 that the shape of the graph is estimated much more reliably by the method of averaging vvhole graphs than by the conventional method of separately averaging individual points. The simplified and extreme hypothetical situation of Fig. 7(a) is considered in order to clarify the argument. However, in both the present experiments and previous work (e.g. Regan. 1973a. Fig. 3c) the esperimental situations

‘The transient response of a linear system to infrequently-repeated stimuli can be imagined to overlap to an increasing extent as the stimulus frequency is slowly increased until. in the steady state regime. no response cycle can be attributed to any particular stimulus cycle. To be classified as a steady-state EP, the running averages of EP phase and amplitude must remain substantially constant with time (Regan. 1965. 1966a): it is necessary to demonstrate this empirically in any new stimulus situation. In a linear system the overlapping of successive responses would lead only to linear superposition so that the steady(a) state response could be calculated from the transient response ii1 r/w 3ame straiylrrfbrwatd wuyfbr an_v liwar sprm. In other words, for any linear system. transient and steadvstate responses give the same information. The situation *is quite different for nonlinear systems. For nonlinear systems there is no general way of calculating the steady-state response from the transient response. In order to do this. the nature of the particular nonlinearity must be quantitatively known. Conversely, a comparison of the empirically-measured transient and steady-state responses can reveal something about the particular nonlinear system being studied. 0 -1 -2 0 -1 -2 In other words steady-state and transient responses can give !NTENSlTY some complementary information about nonlinear systems. The force of this pomt is that. as is well known. EPs and Fig. 7. Averaging whole graphs can determine their shapes stimulus parameters are related in ways that show marked more precisely than averaging points. nonlinearities (e.g. Tweel and Lunel. 1965; Spekreijse, 1966; Fig. 7(a). The five lines illustrate the hypothetical relation Regan. 1972a).Hence these remarks apply to EPs and there is experimental evidence that transient and steady-state EPs between EP amplitude and stimulus intensity at different indeed give complementary information about the visual moments. The filled circles represent values ofEP amplitude obtained by averaging. system. For further discussion about the practical distinction Fi,g. 7(b). The dashed line joins the five separately-deterbetween transient and steady-state EPs see Regan. 1972a. mrned points. The continuous line is the average of the five pp. 236-237. separately-determined graphs in Fig. 7(a).

D. RECAV

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studied did approximate sufficiently closely to this hypothetical situation that the direct recording of graphs did in practice give a useful improvement in the precision of estimating the shapes of EP graphs. It is some years since Grey Walter’s (1966) first dramatic demonstrations that electrical brain activity can directly control the occurrence of visual stimuli. There is. however, one feature of the evoked potential feedback method I describe in this article that does not seem to have been reported before. This feature is that EP amplitude controls some stimuius feature in a graded or continuous manner. iMany experimental questions call for measurements of response sensitivity rather than response amplitude. In practice, sensitivity can be calculated from a series of amplitude measurements. but this is a clumsy procedure. Kesponse feedback enabtes sensitivity to be measured directly. For example. Padmos and van Norren (1972) used a “vector voltmeter” to establish an ERG feedback loop and thusenable the dark adaptation curve to be plotted directly. Signal to noise levels are generally much less favourable for EPs than for ERGS. However, as reported above. evoked potential feedback is a practical possibility with human subjects by means of the Fourier analysis method. In this way. EPs can be used in direct measurements of sensitivity. It is interesting to note that this procedure is not feasible in all animals: in goldfish. for example. the tectal evoked potential (though not the ERG) shows such gross nonlinearities and hysteresis when stimulus intensity is COUr~~~~~oz~s~~ changed, that EP feedback would be out of the question (Regan. Spekreijse. Schellert and van den Berg. 1971). .4lthough the two EP methods are presented here in the contest of pattern stimuli, it is clear that in principle they could be used to study other visual parameters or even nonvisual (e.g. auditory) responses. Had the criterion level been set a little nearer to the threshold of pattern perception. then the incremental EP method of Fig. 6(b) would have constituted an EP equivalent of Stiles two-colour increment threshold technique (Stiles, 1946). However, the method of Fig. 6(b) is more akin to an objective suprathreshold version of the Stiles technique. Stiles showed that abrupt changes in the slope of the increment threshold plot demonstrated the presence of more than one colour mechanism and. conversely, how a straight Iine plot shoued that only one mechanism was active. Figure 6(b) shows that the equivalent EP method is limited by the abbreviated plot that results from the rapid attenuation of EP amplitude at low pattern intensities. It has previously been reported that EPs can be elicited by either a checkerboard or grating pattern of alternate

equiluminant

red and green checks or bars;

EPs are elicited each time the red and green checks or bars exchange places (Regan and Sperling. 1971; Regan. 1972b) or each time the pattern appears from a previously unpatterned field (Regan and Spekreijse, 1973). The EPs are specific to pattern: they are clear even when ocular chromatic aberrations are cancelled

(Regan. 1973b). The existence of such chromatic contrast EPs means that visual signals from red-sensitiv-e and green-sensitive cones are still segregated when they reach pattern-~nsitive neurons. It seems likely that the spectral sensitivity curve of Fig. 6(a) is dominated by the red channel since (a) the pattern stimulus is far into the red (676 nm). and (b) the spectral sensitivity curve of Fig. 6(a) peaks at roughly j80610 nm; simple additive intrusion from the green channel would displace the peak towards green wavelengths, whereas the wavelength of the 580-6lOnm peak is towards the long-wavetength extreme of cstimates for the peak sensitivities of the red pigment. red receptor or the rcY(red) mechanism (Pitt. 1944: Stiles 1949. 1953. 1959: Thomson and Wright. 1953: Baker and Rushton, 1965; Rushton. 1965: WeaIe, 1939: Wald, 1964; Brown and Wald. 196-l: Xfarks, Dobelle and Mac~ichol, 1964; Vos and Walraven. 19701. Acknowledgements---I am grateful to Dr. %‘. S. Stiles for reading this manuscript. I thank Robert F. Cartwright for invaluable technical assistance. I thank Mr. H. Wardell. Mr. R. Mortal1 and workshop staff for constructing equipment, I thank Hazel Henry for help in preparing this MS. I am grateful to the Medical Research Council for supporting this work.

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