Approved For Release 2003/04/18 : CIA-RDP96-00791R000200040003-0 ‘ 316 KELLY AND SAVOIE Subj. HP CONTRAST Adjustments —- Ist Run —— 2nd Run e<—~ 3rd Run 0.5 1 2 5 SPATIAL FREQUENCY (cpd) 10 Fig. 5. Contrast sensitivity vs spatial frequency for S H.P. Same conditions as Figs. 3 and 4. Dashed lines show data from three FCS runs, averaged to obtain filled-circle points. times greater than the adjustment sensitivity. The most important result shown in Fig.5 is the nature of the variability among FCS runs and among adjustments runs, Although the FCS data are about as variable as the adjustment data, the adjustment variability is mainly systematic, while the variability among the FCS runs seems random. (Note that the FCS curves from individual sessions cross and recross each other and the mean curve several times.) The data were subjected to a chi-square test of the hypothesis that mean rank contrast sensitivity is independent of session number. This hypothesis can be rejected for the adjustments data (p < .001), but cannot be rejected for the FCS data (p = .1). Now, if the FCS method eliminates systematic intra-S variability from one run to the next, and if this intersession variability is caused mainly by changes in S’s criterion, then we would expect this method to minimize the variability among Ss as well. The data shown in Fig. 6 tend to confirm this expectation. Here the mean FCS curve for S HP. is repeated, together with similar data for two other young, emmetropic Ss. Note the close similarity among all three contrast sensitivity curves, particularly at frequencies below 2 cpd. These data (and. others not reported here) all tend to confirm the presence of a low-frequency inhibiting effect with a relatively steep slope (about 2 in log-log coordinates). DISCUSSION Instructing 8 to try to detect any perturbation of the uniform field may be important in making the CPYRGHT ‘ adjustment curves so similar in shape to the FCS curves. One should not assume that our results would be obtained if S were instructed to detect the presence of a grating, or to report its orientation, count its fringes. ete. The use of other subjective criteria can doubtless change the shape of the sine wave threshold curve. When minimum threshold values are not required, and occasional criterion differences among and within Ss can be tolerated, the method of adjustments is obviously preferable, because it yields equally smooth curves in about a 10th of the time required by the FCS method.! However, the FCS method eliminates criterion differences and provides information about S's performance (not just his judgments). Summarizing our results with five Ss. ranging from naive to quite experienced: (1) Both methods yield the same curve shape. but the FCS method gives significantly greater sensitivity than that obtained by the method of adjustments: the increase may be as great as a factor of 5 (depending on S’s adjustment criterion). (2) Presumably because it is independent of threshold criteria, the FCS method does not show systematic changes of sensitivity from one run to the next (as the method of adjustments sometimes does.) (3) The variability of the FCS data is mainly random and can therefore be made quite small by taking enough data. (4) When this is done, individual differences (among young, emmetropic Ss) tend to disappear. These results are essentially what would be expected. based on the differences between the two psychophysical methods. We conclude that, if the S is appropriately instructed, the shape of his sine wave grating sensitivity curve is not affected by using the method of adjustments. Moreover, it seems likely that this negative result would be maintained if the present study were extended to other criterion-dependent Forced-Choice Staircase CONTRAST —O— Subj. HP —~ Subj. EB —o- Suhj. RS ee Ld 0.5 5 SPATIAL FREQUENCY (cpd} Fig. 6. Contrast sensitivity vs spatial frequency for three young, emmetropic Ss, obtained by the FCS technique. Approved For Release 2003/04/18 : CIA-RDP96-00791R000200040003-0
CPYRGHT SINE-WAVE CONTRAST SENSITIVITY 317 SELECT NEW EXECUTIVE STAIRCASE Ton ee ree INTERFACE CONTROL CONTRAST FAUING RESPONSE. REINFORCEMENT STAIRCASE CONTROL LAST 3 PRESENTATIONS oa'.e? INHIBIT FURTHER SELECTION RESPONSE HISTORY was PRESENT RESPONSE RIGHT? WAS LAST RESPONSE RIGHT? WAS RESPONSE BEFORE THAT RIGHT? 3d “WRA” SEQUENCE ? SET INCREMENT TO NEXT WAS ! SMALLER \ ALUE CONTRAST CHANGEE LAST TIME ? MARK CONTRAST NOT CHANGED CONTRAST CHANGE DECREASE COATRAST BY INCRENEST INCREASE CONTRAST | BY INCAENENT Fig. 7. Flow-chart representation of the FCS computer program. psychophysical methods in general. Thus, our results various bookkeeping functions. and calculates and prints tend to support the sine wave threshold data obtained by these methods. APPENDIX The Forced-Choice Staircase (FCS) Computer Program - The operation of the FCS computer program can be divided into five functional categories: executive. interface control. staircase control, response history, and contrast change (see Fig. 7). The executive section of the program initializes the experiment, selects (from a random table) one of the 12 stimulus patterns for presentation at each trial, performs Approved For Release 2003/04/18 out the average data at the end of an experimental session. The staircase control portion of the program operates on a parameter table for the staircase currently selected; it keeps track of the previous three responses. the previous eight contrasts. the present contrast, and contrast increment size (and other parameters tor internal use). These historical data are needed for each staircase to determine its subsequent contrasts. The interface control portion of the program controls the stimulator interface, which operates the entire sequence of each trial: stimulus generation, contrast fading, marker tones, and reinforcement. When S makes a response, the computer reads data from the interface. indicating whether the response was right or wrong (Or : CIA-RDP96-00791R000200040003-0
Approved For Release 2003/04/18 : CIA-RDP96-00791R000200040003-0 ~° * CPYRGHT . 318 KELLY AND SAVOIE whether the “pause” switch was set). The response history section is the heart of the FCS program: Fig. 7 shows a flow chart of this section and its relation to the other parts of the program. Each staircase follows the algorithm according to its own history, independent of the other staircases. The next contrast to be presented (for the selected staircase) is calculated on the basis of the present contrast, the present response, the previous two responses, the contrast increment, and whether the contrast was changed after the previous response. The contrast algorithm is as follows: (1) When S makes a wrong (W) response, the contrast of that pattern is increased for its next presentation. After he makes two successive right (R) responses to the same pattern, its contrast is decreased (this sets the average end point of each staircase at about 75% correct). The new contrast is obtained by multiplying (or dividing) the preceding contrast by a fixed ratio (i.e., a constant log increment). (2) Each time the sequence W,R,R occurs, the size of this increment is decreased. (This is most likely to occur when the contrast increases from below to above the 75% correct level.) The successive increment sizes ate: 8X, 2X, 1.3X. (3) After the sequence W,R,R has occurred three times with a given pattern, the staircase terminates with three more trials using the smallest increment, and that pattern is not presented again. The session ends when all staircases have terminated. The last eight contrasts from each staircase are averaged to obtain the threshold. The minimum number of trials before termination but after the second W,R,R sequence is eight. Therefore, all contrasts being averaged were necessarily presented during the period when the smallest contrast increment was in effect. REFERENCES Békésy, G. v. A new audiometer. Acta Oto-laryngologica, 1947, 35, 411-422. . Blackwell. H. R. Contrast thresholds of the human eye. Journal of the Optical Society of America, 1946, 36, 624-643. , Campbell. F. W., & Green, D. G. Optical and retinal factors affecting visual resolution. Journal of Physiology, 1965, 181,° §76-593. Comsweet, T. N, The staircase method -in psychophysics. American Journal of Psychology, 1963, 75, 483-491. Davidson, M. A perturbation analysis of spatial brightness interaction in flashed visual fields. Doctoral thesis, University. of California, Berkeley, 1966. Heinemann, E. G. The relation of apparent brightness to the threshold for differences in luminance. Journal of Experimental Psychology, 1961, 61, 389-399. . Kelly, D. H. Frequency doubling in visual responses. Journal of the Optical Society of America, 1966, 56, 1628-1633. Kelly, D. H. Effects of sharp edges on the visibility of sinusoidal gratings. Journal of the Optical Society of America, 1970, 60, 98-103. Kelly, D. H. Transient effects on the visibility of low spatial frequencies. Journal of the Optical Society of America, 1971, 61, 1576 (A). Kelly, D. H. Adaptation effects on spatio-temporal sine-wave thresholds. Vision Research, 1972, 12, 89-101. Kelly, D. H. Lateral inhibition in human colour mechanisms. Journal of Physiology, 1973, 228, 55-72. Kohayakawa, Y. Contrast-difference thresholds with sinusoidal gratings. Journal of the Optical Society of America, 1972, 62, 584-587. Rose, R. M., Teller, D. Y., & Rendleman, P. Statistical properties of staircase estimates. Perception & Psychophysics, 1970, 8. 199-204. Van Meeteren, A. Spatial sinewave response of the visual system (a critical literature survey). Soesterberg: Institute for Perception RVO-TNO, 1966. Westheimer, G. Modulation thresholds for sinusoidal light distributions on the retina. Journal of Physiology, 1960, 152, 67-74, NOTE 1. Because the FCS sessions are long and tiresome, a number of preliminary experiments were conducted to determine the most suitable compromise between speed and end-point stability. We found that the variability shown in Fig. 5 could not be decreased significantly without prohibitive increases in mn length. The number and sizes of our increments were governed by the following constraints (see Cornsweet, 1963): (a) For maximum efficiency, the smallest increment must be fixed at _ about the same size as the contrast-difference threshold for sinusoidal gratings; at a spatial frequency of 2.1 cycles/deg, Kohayakawa (1972) obtained jnds of 30% to 60% with low-contrast gratings. (Note that this is much larger than the jnd of luminance.) (b) All staircases must start at a common baseline; for this purpose, we chose 100% contrast. (To start at equal distances from threshold would require an a priori assumption of the curve shape we are measuring, which could bias the result.) (c) If the contrast is to approach threshold quickly, the initial increment must be much larger than specified in (a). The increment should decrease as the contrast approaches threshold, but not so rapidly as to run a high risk of being “trapped” in a small increment far from threshold. (d) Subject to constraint (c), the total number of increments should be minimized. (Received for publication January 16, 1973; accepted March 15. 1973; revision received March 29, 1973.) Approved For Release 2003/04/18 : CIA-RDP96-00791R000200040003-0