The perception of suprathreshold sinusoidal flicker measured by light- and dark-phase matching

RESEARCH NOTE

THE PERCEPTION OF SUPRATHRESHOLD SINUSOIDAL FLICKER MEASURED BY LIGHT- AND DARK-PHASE MATCHING SVEINMAGNUSSES and ROALD A. BJ~RKLUND institute of Psychology. University of Oslo. Box 1094 Blinder, Oslo 3. Norway (Received 20 April

In contrast to the vast literature on human flicker sensitivity (Kelly, 1972) comparatively few studies have dealt with the perception of suprathreshold sinusoidal flicker; in particular it is not known whether the lowfrequency attenuation of the psychophysical response observed at threshold persists at suprathreshoid levels of perception. Early flicker matching experiments by Veringa (1958) were confined to frequencies above 6 Hz. and in a more recent study by Regan and Beverley (1973), investigating the relationship between flicker amplitude and the subjective flicker magnitude, only one frequency was tested. Varju (1964) measured the subjective brightness of sinusoidal flicker and his results indicate a slight decrease in brightness towards lower frequencies. Marks (1970) had his subjects estimate the apparent depth of modulation of trapezoidal flicker (a waveform approximating a pure sine-wave) and found evidence for low-frequency attenuation at low stimulus amplitudes but not at higher amplitudes: however, frequencies below 3 Hz were not included. In the present experiments the perception of suprathreshold sinusoidal flicker was measured by light- and dark-phase matching (Magnussen and Glad, 1975). a procedure analogous to that used by Bryngdahi (1966) and Fiortntini and Maffei (1973) in studies of suprathreshold sinusoidal gratings. The stimulus arrangement (Fig. 1) and experimental procedure were similar in principle to those described by Magnussen and Glad (1975). Briefly, the subject adjusted a comparison flash of variable sign (increment or decrement) and magnitude to match the apparent brightness of either the maxima or the minima of a sinusoidal flicker stimulus: both test and comparison fields were embedded in a larger steady surround, the luminance of which was equal to the time-average luminance of the flickering test stimulus. The display was viewed monocularly through a 2.5 mm artificial pupil at a distance of approximately 6Ocm, taking the usual precautions to avoid head movements. Field luminance was lOcd/m*. A black mark midway between test and comparison fields assisted steady fixation. Test and comparison stimuli were generated on the face of two cathode-ray tubes (CRT). The light froin each CRT was reflected into the eye by a full-reflecting prism and masked off by a white cardboard screen (surrounding field) containing two sharply cut circular windows. The cardboard screen was illuminated by V.I.

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a separate source, its colour matched to the P31 phos-

phor of the CRT’s by a specially prepared filter. Temporal modulation of the test field was achieved by generating a homogeneous raster on a Tektronix T922 CRT and modulating its luminance in time. Stimulus waveform, as calibrated with the aid of a photo-cell and a Tektronix T534 storage oscilloscope,

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Fig. 1. A to-scale representation of the display (scale in visual angle). Insets diagram the luminance-time charactcristics of the test and comparison fields. The test field was symmetrically about the modulated sinusoidally, luminance of the steady surround. with varying frequency and amplitude. In the comparison field 0.4~~ luminance increments (+) or decrements (-1 of variable magnitude were presented with 1 set intervals: the luminance of the comparison field during these null-periods was equal to the surround luminance (IO cd/m’). It was the subject’s task to adjust the magnitude of the comparison flash to match the extreme values of the temporal brightness distribution of the sinusoidal flicker stimulus.

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Fig. 2. Flicker sensitivity functions for the two subjects obtained with rhc method of adjustment. Each point is the mean of five settings. Smooth curves Wed by eye.

showed no distortion within the frequency and amplitude ranges used in the present experiments. To provide the comparison field characteristics. a lowfrequency spatial square-wave grating was generated on the other CRT. The grating was presented for 0.4s~~ spaced by 1 see blanks (a lOcd!m’ homogeneous raster). Increment and decrement comparison flashes. as required for “light”- and “dark”-phase matches, respectively, were obtained by positioning the bright. respectively dark, part of the grating behind the mask. Flash amplitude was thus controlled by the spatial contrast of the grating, and could be adjusted on a ten-turn potentiometer operated by the observer. Suprathreshold frequency-response curves were collected for flicker amplitudes between 0.1 and 0.65. Modulation amplitude, m. is defined by (L,,, - L,,,).’ U-,A, +L,,,) where L,, and L,,,;, are the luminance maxima and minima of the flicker cycle. In a series of measurements m was fixed and five light-phase and five dark-phase matches were made for selected frcquencies between 0.5 Hz and the CFF for that particular value of m. Before each series of measurements test and comparison fields were blanked and the observer adapted for a suitable time to the field luminance. The whole experimental program was run twice: in half of the sessions frequencies were tested in an ascending order, in the ocher half in a decending order of presentation. The results from the different sessions were combined. In addition to the suprathreshold measurements, flicker thresholds were determined using the method of adjustment. Figure 2 shows flicker sensitivity functions for two subjects (the authors). The results compare with previous measurements for small-field center-surround patterns at comparable Iuminances (Kelly, 1969). peak sensitivity occuring around 5 Hz with a highfrequency cut-off around 30Hz and a moderate attenuation of sensitivity at low frequencies. When the light- and dark-phase matches were plotted on a finear luminance scale against flicker fre-

quency almost perfectly symmetric pairs of curves emerged. To compare with the flicker threshold data the light- and dark-phase matches were therefore computed according to the formula (B,,, - &,I is,,, A- &,,,,) where B,,, and B,,, denote the maximum and minimum brightness of the flicker cycle as specified in terms of matching luminance. This is the temporal analogue of the subjective contrast index adopted by Bryngdahl (1966) in the spatial domain. and it provides an estimate of rricztir~ chanyes in subjrcficr r~~~~or~7~con~rasr [the term ‘-subjective temporat contrast” is preferred to the alternative “subjective modulation amplitude’. since the method only considers the extreme points in rhs temporal brightness variations). Figure 3 shows results for four stimulus amplitudes. tn being 0.1 [open circles), 0.3 (squares). 0.5 !triangles) and 0.65 (i;llrd circlesr. The smooth curves through the data points are drawn to a high-frequency cut-off predicted from the individual flicker sensitivity functions (Fig. 1). which gives an excellent fit to the high-frequency data, particularly for RAB. The results indicate frequen~~~e~ndent changes in the perception of Bicker above threshold that parallel the changes in flicker threshold. As under threshold conditions the curves exhibit a more or less clearly defined peak around 5 Hz: however. the amount of decrease in subjective temporal contrast below 5 Hz appears to vary with stimulus amplitude. being almost absent for m = 0.65. By extrapolation these results suggest the low-frequency fall-off to be most pronounced at threshold. and a gradual transition to high suprathreshold levels. This cannot easily be seen from the foregoing graphs: therefore Fig. 4 was prepared. In this plot the threshold and suprathreshold data base been individuaIly normalized by dividing for each curve the peak-frequency value with ths values obtained at each of the other frequencies, and the results are plotted on the ordinate as log -‘relative change“ (signs neglected). In Fig. 4 the results for m = 0.5 have been omitted for sake of graphical clarity, the threshold data are represented by open squares and dashed curves, otherwise symbols as in Fig. 3. The results confirm that the low-frequency branch climbs steadily from threshold onwards: all curves join around 5 Hz and share a common high-frequency fate. It should perhaps be mentioned that the comparison of Fig. 4 may not be completely justified since the suprathreshoid data represent the changes in the perception of a frequency-varying stimulus of fixed ~plitude whereas the threshold data represent the Ricker amplitude required to attain a fixed perceptual criterion (just flickering), but the general picture should be correct. It is currently fashionable to analyse visual spatiotemporal interactions in terms of sustained and transient neural channels (see MacLeod, 1978). In relation to the present results it is interesting to note the psychophysical (Keesy. 1972: Kulikowski and Tolhurst, 1973) and neurophysiologizal (Ikeda and Wright. 1975) demonstrations that transient channels show a marked preference for medium temporal frequencies whereas sustained channels respond equally well to low and medium temporal frequencies. It might be speculated that the gradual disappearance of the Iowfrequency inhibjtion of the psychophysical response

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Flicker frequency, Hz Fig. 3. Subjective temporal contrast of suprathreshold sinusoidal flicker as a function of flicker frequency. Each point represents the value computed on basis of ten light-phase and ten dark-phase matches. Curve parameter is stimulus amplitude. m being 0.1 (Ok 0.3 (5). 0.5 (A) and 0.65 (e). Smooth curves are fitted by eye under the restriction that the high-frequency branch is drawn to a cut-off predicted from the individual flicker sensitivity functions. Resufrs for both subjects.

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Fig. 4. ‘Pu’ormalizedthreshold and suprathreshold frequency-response relationships. A replot of the flicker threshold data of Fig. 2 (Cl) is shown together with replotted suprathreshold data from Fig. 3 representing modulation amplitudes 0.1 (0). 0.3 (W) and 0.65 (0). Smooth curves fitted by eye.

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at supratbreshotd stimulus amplitudes suggested bjFig. 1 could be due to an increasing dominance of the sustained mechanism over the transient mechanism at frequencies below 5 Hz as the stimu!us amplitude increases above the Bicker threshold. presumably set by the sensitivity of the transient mechanism. Acitnosfedgummr-We thank 3. Breitmeyer for reading an cariier draft of the manuscript. REFERESCES Bryngdahl 0. (19661 Characteristics of the v-isual system: psychophysical measurements of the response to spatial sine-wave stimuli in the photopic region. 1. opr. Sot. Am. 56, 81 l-821. Fiorentini A. and b
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Kelly D. H. (19691 Flickering patterns and lateral mhibition. 1. opr. Sot. Am. 59, t361-1370. Kelly D. H. (19721 Flicker. In Handbook o~Srn.sor,r Phrsioing.~, Vol. VII 4 [edited by Jameson D. and Hurvich L. M.). Springer-Verlag. Berlin. Kulikowski J. J. and Tolhurst D. J. (IY73f Psychophysicai evidence for sustained and transient detectors in human vision. j. Ph@uf., Lonri. 232, 149-162. %iacLeod D. I. A. (1978) Visual sensitivity. A. Rer. Ps.rchoI. 29, 613645. Ivtagnussen S. and Glad 4. (1975) Brightness and darkness enhancement during flicker: perceptual correlates of neuronal Et- and D-systems in human vision. Erpl Brain R&s. 22, 399-413. %