c:: Frequency response characteristics for sinusoidal movement in the fovea and periphery* ==> 0.' SOO O.S 2.0

Transcription

1 Freqency response characteristics for sinsoidal movement in the fovea and periphery* C. WILLIAM TYLER and JEAN TORRES Northeastern University, Boston, Massachsetts 211 Threshold sensitivity was measred for sinsoidal movement of bright 1-deg lines against a dark backgrond as a fnction of oscillation freqency and retinal location. Sensitivity was greatest in the fovea and at a freqency of 1-2 Hz. Peripheral sensitivity was more narrowly tned than foveal sensitivity. The presence of a stationary reference line affected mainly the foveal sensitivity. The reslts are interpreted as evidence for both position- and velocity-sensitive mechanisms in the movement detection system. The techniqes of analysis of linear systems were first applied to the stdy of visal fnction by de Lange (192, 198a, b) in his experiments on threshold flicker. The systems analysis approach, often involving the se of sinsoidal stimli, has fond wide acceptance among vision researchers. By measring threshold amplitdes, sensitivity can be plotted as a fnction of freqency of the sinsoid, on log-log coordinates. This type of plot is called a freqency response fnction 1 (FRF). Its se greatly facilitates nderstanding of the processes nderlying the system response. FRFs for sine-wave flicker have been obtained as a fnction of variables sch as retinal adaptation level (Kelly, 1961), field size (van der Tweel, 196), chromatic stimlation (de Lange, 198a, b; Kelly, 1962), and spatial pattern (Kelly, 1969a;Levinson, 1964). Keesey (197) has stdied the effects of target size, lminance, srrond, and retinal-image movement on sensitivity to sinsoidal flicker. Kelly (1969a) has derived the temporal properties of a lateral interaction mechanism. The systems analysis approach has also been sccessflly applied to aspects of temporal visal fnction other than flicker. Schickman ( 197 ), for example, has sed low-freqency sinsoidally modlated light as a masking stimls for brief light flashes. Wavelength-modlated light has been sed in a systems analysis approach to color vision (Regan & Tyler, 1971a, b). In the field of movement perception, investigations of both apparent movement for sqare-wave modlated displacement (Tyler, 1972a) and stereoscopic depth movement for sinsoidal line displacement (Tyler, 1971) have revealed important i..formation concerning the respecti.!~ *Spported by the Fondations Fnd for Research in Psychiatry. Grant processing systems by the se of freqency response analysis. Clemmesen (1944) has sed sinsoidally modlated displacement ụ. IIlI - c:>!!! >.... ==> :: 1 c:: 2. ~ - c:: E > SOO E 1 2.'.2. O.S in his work on the perception of moving targets in the fovea and periphery. However, complete FRFs for movement sensitivity have not been reported in any of the literatre srveyed. The experiments described in this paper serve the twofold prpose of measring the FRF of the hman observer to sinsoidally moving targets in the fovea and periphery, and of dedcing the natre of the visal system in determining the form of the FRF. Althogh few investieators have worked with sinsoidal motion, data are available on the variation in sensitivity to linear and rotary motion as a fnction of retinal position. McColgin (1961) fond that rate sensitivity for these two types of motion varied in a similar manner, the threshold velocity increasing linearly with eccentricity at a somewhat greater rate along the vertical than Fffrfl1iency 1Hz.) Fig. 1. Threshold sensitivity in seconds of arc to sinsoidal movement with a stationary reference, plotted as the reciprocal of threshold amplitde, as a fnction of oscillation freqency. Retinal eccentricity is the parameter of the for crves. Eccentricities sed were,, 1, and 2 deg, reading downwards on the graph. Ss were J.T. (filled circles) and C.W.T. (open circles). vertical bars represent one standard deviation on each side ofthe mean. An indication is given of the slope of sensitivity above the highest freqency for visible movement. These lines are the shallowest slope possible, since movement was invisible at the next freqency. Note decreasing sensitivity and change in form of crves with increasing eccentricity. The obliqe straight line represents slope of nity approximated by peripheral crves at low freqencies Copyright 1972, Peychonemie Society, Inc., Astin, Texas Perception & Psychophysics, 1972, Vol. 12 (2B)

2 -: :e -.; c: QJ en 1 2 1: QJ E ~ o 1 J:T l-r--, , -----, r--,--~--, I I I I (ti)./.2 I Fig. 2. Threshold sensitivity in seconds of arc to sinsoidal movement with reference (filled circles) and withot reference (open circles). S J.T.'s pper crves show deg eccentricity, lower crves 2 deg eccentricity. Bars represent one standard deviation on each side of mean. The dashed line shows a slope of nity. Note differential effects of removal of reference at and 2 deg eccentricity. along the horizontal meridian. However, he does not give data that are detailed enogh to permit comparison between foveal and 2-deg peripheral sensitivities, as will be done in the present experiment. EXPERIMENT I Apparats and Method A fast-phosphor dal-beam oscilloscope was sed for presentation of two vertical lines, at 1-fL lminance against a dark srrond. The freqency response of the phosphor was 2 Hz ± 3 db. A very dim backgrond glow from the CRT was perceptible. One line was held steady, while the second line cold be sinsoidally displaced in the horizontal direction by means of a variable freqency fnction generator. Any effect of position ces from the CRT glow was certainly negligible in relation to the bright reference line. At a fixation distance of 3 m, both lines were 1 deg high and approximately 1 min wide. They were viewed binoclarly with natral ppil, either in central vision or with fixation on a bright vertical bar target located at, 2 freqency (Hz.) 2 1, or 2 deg sbtense from the center of the stimls. Dark adaptation was not necessary since the stimli were of photopic lminance. Thresholds were measred by the method of adjstment, with an eqal nmber of ascending and descending measrements being taken at each of a range of freqencies from.1 to 3 Hz. The S adjsted a linear potentiometer to determine the jst-stationary threshold. The E altered the gain of the oscilloscope so that the potentiometer setting was always near the optimal position. For foveal viewing, the reference line was placed 1 min away from the target, while for peripheral viewing it was located at a distance of 3 min. Pilot experiments on the effects of anglar distance between reference line and target indicate that the distance of the reference line has very little effect on the movement sensitivity within wide limits. For example, varying line separation in foveal viewing from 1 to 3 min affected the thresholds by only a factor of 1. at both. and Hz. De to difficlty with the apparats, measrements were taken first for the freqencies of. Hz and higher for S J.T. At a later date, additional readings were taken for the lower freqencies, between.1 and 1. Hz, the later data being calibrated with the earlier measrements to obtain the final crve. A total of eight measrements were taken for the freqencies below 1. Hz. Data were also obtained for a second S (C.W.T.) in the fovea and 2 deg in the periphery, two measrements being taken at each freqency. Both Ss wore fll refractive correction dring experiments. Reslts and Discssion The reslts of Experiment I are shown in Fig. 1 for J.T. (fll lines) and C.W.T. (dashed lines). The form of the FRF is similar for the two Ss, both in the fovea and at 2 deg in the periphery. The most notable reslt is that sensitivity to sinsoidal movement is highest in the fovea at all freqencies. Sensitivity decreases as distance of the retinal image from the fovea increases. Earlier research by Clemmesen (1944) shows that at 2 or 3 Hz oscillation freqency, sensitivity in the fovea is greater than at deg eccentricity by a factor of 3. A similar difference is evident in Fig. 1. The apparent peaks in sensitivity in the peripheral crves between.7 and 7 Hz are probably not repeatable. They are not consistent from one crve to the next and do not reach significance at p =.1 on a two-tailed t test. Clemmesen (1944) does not report any threshold measrements for freqencies other than 2. and 3. Hz, bt he states that between 1.1 and. Hz, sensitivity was not affected by freqency. The present data for all retinal positions are consistent with these findings, as the differences across the range of 1 to Hz are within one standard deviation of the sensitivity at 1. Hz. (A single exception to this statement is shown by the -deg measrement at 2 Hz, bt two exceptions wold be expected by chance alone, since a difference of one standard deviation represents a confidence level of p =.1, or one exception in every seven cases.) Between.2 and Hz, the decrease in movement sensitivity with increasing retinal angle from the fovea is approximately niform across freqencies for S J.T. (fll lines). Ths, the largest change, abot. log nit, occrs between and deg of retinal angle. The differences from to 1 deg and from 1 to 2 deg are similar at abot.3 log nit each. The data of S C.W.T. (dashed lines) are in accord with these differences in showing a total decrease in sensitivity of close to 1. log nit between and Perception & Psychophysics, 1972, Vol. 12 (2B) 233

3 2 deg, which is close to the total decrease of 1.1 log nits for S J.T. At freqencies greater than abot Hz, it is no longer the case that sensitivity is independent of freqency or that the decrease in sensitivity with retinal angle is niform acrols freqencies. The foveal sensitivity remains relatively high, while peripheral sensitivity shows a relative decrease as freqency increases. Ths, by 1 Hz, senaitivity at 2 deg is redced by 1.31g nits for C.W.T. and by nits for J.T. The peripheral sensitivity is ths more sharply tned than foveal sensitivity in the high-freqency region. For both Ss, the peripheral data below 1 Hz conform to a slope of nity (shown as the straight line in Fig. 1) within the limits of experimental error. The foveal sensitivity appears to level off somewhat at very low freqencies and is ths more broadly tned than peripheral sensitivity at both high and low freqencies. This observation sggested that foveal and peripheral low-freqency movement detection might involve different types of processing systems. This possibility was investigated in a second experiment in which the effect of the removal of the reference line in the movement freqency response was examined. EXPERIMENT II Apparats and Method The conditions were the same as for Experiment I, except that the stimls was presented withot the reference line. One ascending and one descending series of measrements were taken, for the fovea and 2-deg peripheral fixation only, with the same Sa Ȧrgments are presented below that the reslts do not appear to be contaminated by eye movements. As an empirical check, a sbsidiary experiment was designed. The problem is to provide a fixation stimls that does not simltaneosly act as a position reference. Since eye tracking ability is negligible above 3 Hz (St. Cyr & Fender, 1969), fixation on a line oscillating at higher freqencies shold maintain the eye in a constant position. However, if the oscillations are of large amplitde, the "fixation" stimls will be moving across the retina, which mst redce its efficacy as a position reference. Therefore, a movement FRF was measred nder conditions similar to those of the previos experiments bt with the reference line oscillating at 3. Hz with an amplitde of 1 min. Reslts Figres 2 and 3 show the effects of removing the reference line on - e,.!i -....c.,. 1: E ~ Cl""--r r----r----r----r--,---y----,.1 Freqency (Hz.) 1 :lo Fig. 3. As for Fig. 2, bt with S C.W.T. Data with reference is replotted from Fig.l. movement sensitivity. For C.W.T., the data with reference line are replotted from Fig. 1. The data with reference for J.T. consist of new measrements which are similar to those in Fig. 1 within experimental error. For both Ss, removal of the reference line has a marked effect on foveal movement sensitivity for freqencies below 1 Hz, while it has a mch smaller effect at. 2 deg eccentricity. For J.T. (Fig. 2), there is virtally no change in peripheral sensitivity after removal of the reference line. For C.W.T. (Fig. 3), there appears to be a small redction in sensitivity, approximately.3 log nits. The form of the foveal crves withot a reference line resembles that of the peripheral crves, both with and withot a reference line. The difference in sensitivity between foveal and peripheral crves withot reference may therefore be described for both Ss as a redction in sensitivity of abot.8 log nit (a factor of 6.3), together with a shift of the sensitivity crve along the freqency axis of abot an octave (a factor of 2). Role of Eye Movements The role of tracking eye movements in the prodction of the present data is not regarded as significant, for the following reasons. Where the reference line was present, the S was instrcted to base the jdgments on relative displacement and not to track the target. With regard to Experiment II, the data reviewed by St. Cyr and Fender (1969) show that the gain of tracking eye movements is negligible above abot 3 Hz. Had tracking eye movements played a significant role, removal of the reference line shold have had no effect on foveal sensitivity between 3 and 1 Hz, since eye movement tracking is not possible on this freqency range. Similarly, the effect of tracking eye movements in the freqency region below. Hz can be rled ot, since the stdies of sinsoidal movement tracking reviewed by St. Cyr and Fender show that the oclomotor gain and phase shift remain essentially constant below. Hz, and hence can have no effect on the freqency dependence of the movement sensitivity. In peripheral observations, there is no possibility that eye movements occrred, since a foveal fixation target was present. Althogh these considerations rle ot the effects of tracking eye 234 Perception & Psychophysics, 1972, Vol. 12 (2B)

4 , / "./ "'--<,.. 2 / e- /. en SO -~ ~ > 1 ;: I:.. II: U en 2. E > \ t.ooo.----" /~"'-.<\ \ /~.) 1.---' Fig. 4. Threshold sensitivity to sinsoidal movement with stationary reference (filled cirlces, replotted from Fig. 1) and with reference oscillating at 3. Hz (open circles). S C.W.T. Note similarity of effect to Fig. 3. movements, it is possible that the "noise" introdced by involntary movements might tend to redce the sensitivity to retinal movement. This sggestion is spported by the data of Fender and Nye (1961), which show that the spectral density of involntary eye movements dring fixation remains approximately constant at min of arc between.1 and 1 Hz, then falls steadily to abot 1 min of arc at 1 Hz. Similarly, the difference between reference and no-reference conditions in the present experiment is redced steadily between 1 and 1 Hz. However, the effect of removing the reference does not remain constant between.1 and 1 Hz bt increases by abot. log nit for both Sa. This observation indicates that the interference effect of involntary eye movements cold not be inflencing the threshold in this low-freqency region, since the interference shold remain constant between.1 and 1 Hz. If the no-reference FRF is not inflenced by eye movements, how cold fixation have been maintained in the absence of a reference line? We sggest that the backgrond glow of 2. Freqency 1Hz.' 1 2 the CRT cold have served this fnction. It is possible that this diffse light cold act as a fixation target withot providing position ces. Certainly it did not provide position ces; otherwise, there wold not be a large difference between the reference and no-reference FRFs. The sbsidiary experiment provides frther evidence that eye movements do not accont for the effect of removal of the reference line. Movement sensitivity (open circles in Fig.4) was measred for S C.W.T. with the reference oscillating at 3. Hz, sch as to provide an eye fixation ce bt little retinal position information. Stationary reference sensitivity (fll circles in Fig. 4) is shown for comparison. It has a similar form to that previosly measred (Fig. 1). With C.W.T., the FRF with the oscillating reference shows characteristics similar to the no-reference condition. Sensitivity in the high-freqency region is only slightly affected by oscillation of the reference, whereas sensitivity at low freqencies is redced by abot 1 log nit. At very low freqencies, the oscillating reference crve (Fig. 4) falls less steeply than the no-reference crve (Fig. 3). This difference may well be attribtable to the fact that the oscillating reference sensitivity is leveling off to abot the same amplitde as the reference oscillations (1 min). This sggests that oscillating the reference only redces the position information when the oscillations are mch larger than the threshold amplitde of the test line. DISCUSSION Ifthe above reasoning is correct, the effects of eye movements are not important in prodcing the difference between reference and no-reference conditions below 1 Hz. Conseqently, the fact that removal or oscillation of the reference line affects foveal sensitivity below 1 Hz bt not above sggests that the foveal movement sensitivity is mediated by a system consisting of at least two components. One component operates at the lower freqencies when there is a reference line, and the other component operates at the higher freqencies regardless of the presence of a reference line and is less sensitive than the fimt component below 1 Hz. The second high-freqency component also characterizes the peripheral system, which appears to be insensitive to the presence or absence of a reference line. Leibowitz (19) has previosly sggested that the movement was detected by a position detector at long exposre drations and a velocity detector at short drations, bt he did not consider the effects of eye movements. The form of the sensitivity to sinsoidal movement can be sed to determine the natre of two components. Consider first the high freqencies where sensitivity is limited very steeply and does not discriminate between the components. Here the target appears as a flickering bar of variable width. The sensitivity is therefore probably determined in a similar fashion to flicker sensitivity. lves (1922), Veringa (1961), and Kelly (1969a, b, 1971) have sggested t h a t high-freqency flicker sensitivity is limited by a retinal diffsion process that wold probably operate in a similar manner with the high-ireqency movement sensitivity even thogh, for the latter, the phase of the flicker varies continosly across the width of the perceived bar. On the other hand, the low-freqency sensivitities do discriminate between the two components. Ifsensitivity in the foveal reference condition were determined by the retinal positions of the lines alone, it wold be expected to be freqency independent, i.e., a Perception & Psychophysics, 1972, Vol. 12 (2B) 23

5 horizontal line on the graph. The data in Figs. 1-4 show that, indeed, sensitivity is relatively naffected by freqency p to 2 Hz in this condition. Frthermore, removal of the reference greatly redces sensitivity. For these reasons, movement sensitivity in the foveal reference condition is attribted to a position detection mechanism. Conversely, movement sensitivity in all three peripheral locations (Fig. 1) and in the no-reference conditions (Figs. 2 and 3) show strong freqency dependence. If movement sensitivity were determined by the maximm velocity occrring in the stimls, sensitivity wold increase in direct proportion to freqency (fll line in Fig. 1 and dotted lines in Figs. 2 and 3). The data conform to this fnction between.1 and 1 Hz, and hence spport the hypothesis that the peripheral and no-reference sensitivity is determined by a velocity detection mechanism. Ths, the sinsoidal movement sensitivity data spport Leibowitz's (19) speclation that movement processing involves position detection in the fovea when a reference is available, and velocity detection in peripheral observation and when no reference is present. On the basis of the straight line asymptotes to the low-freqency data, the velocity threshold in the fovea in the absence of a reference is calclated as abot 3.3 min of arc/sec for J.T. and 3. min of arc/sec for C.W.T. These vales are slightly lower than classical measres of velocity sensitivity with no stationary reference stimli (Abert, 1886). Similarly, velocity threshold with a reference line for S J.T. was abot 2., 4., and 1. min of arc/sec at, 1, and 2 deg eccentricity, respectively. Removal of the reference line hardly affected 2-deg sensitivity. For C.W.T., velocity threshold at 2 deg eccentricity was abot 7. min of arc/sec with a reference and abot 12. min of arc/sec withot. An interesting aspect of these data is that velocity threshold withot a reference line is only redced by abot. log nit from the fovea to 2 deg eccentricity in both Ss, compared with twice this redction in peak displacement sensitivity at 2-3 Hz oscillation. This redction is also mch less than the.9-log-nit redction in photopic grating acity from the fovea to 2 deg eccentricity fond by Kerr (1971). Ths, the retinal periphery can be said to be specialized for velocity detection in that redction in peripheral sensitivity for velocity is less by a factor of 2 than redction in peripheral acity relative to the fovea. The present data accord with those of Abert (1886) in that nder no conditions is the periphery more sensitive than the fovea, either to displacement or velocity. Finally, it shold be noted that, at maximm sensitivity, movement is detectable for only 8-1 sec of arc displacement. This is abot 1% of the width of the bar, which is at the limit of optical resoltion. Vernier acity has abot the same vale for similar stimls conditions (Tyler, 1972b), so that the temporal and spatial resoltion of differences in lminance distribtions are eqivalent. It may be assmed that the line-spread fnction of a point of light has a normal distribtion to a first approximation (Fry, 19). A shift of 1% in the mean width of a normal distribtion prodces a redistribtion of approximately 8% of the lminance of the bar from one side to the other, or a maximm contrast difference of 1%. This is higher than the contrast threshold of abot 1 % obtained by Hecht and Mintz (1939). The difference may be de in part to the temporal.integration that mst take place for the movement to be observed. In any case, this discssion provides a conceptal framework in which discrimination of displacement of sch_fine magnitde can take place. REFERENCES AUBERT. H. Die Bewegngsempfindng. Archiv r die gesamte Phvsfologie, , CLEMMESEN. V. Central and indirect vision of the light-adapted eye. Acta Physiologica Scandinavica, 194, 9(Sppl. 27) de LANGE. H. Experiments on flicker and some calclations on an electrical analoge of the foveal systems. Phvsica, de LANGE. H. Research into the dynamic natre of the hman fovea-cortex systems with intermittent and modlated light: I. Attenation characteristics with white and colored light. Jornal of the Optical Society of America, 198a, de LANGE, H. Research into the dynamic natre of the hman fovea-cortex systems with intermittent and modlated light: II. Phase shift in brightness and delay in color perception. Jornal of the Optical Society of America, 198b, FENDER, D. H., & NYE, P. W. An investigation of the mechanisms of eye movement control. Kybemetik FRY. G. A. Blr of the retinal image. ColmbUS, Ohio: Ohio State University Press. 19. HECHT. S. & MINTZ. E. U. The visibility of single lines at varios illminations and the retinal basis of visal resoltion. Jornal of General PhYsiology , IVES. H. E. A theory of intermittent vision. Jornal of the OpticalSociety of America , KEESEY. U. T. Variables determining flicker sensitivity in small fields. Jornal of the Optical Society of America KELLY. D. H. Visal responses to time-dependent stimli: I. Amplitde sensitivity measrements. Jornal of the Optical Society of America KELLY. D. H. Visal responses to time-dependent stimli: IV. Effects of chromatic adaptation. Jornal of the Optical Society of America, 1962, KELLY. D. H. Flickering patterns and lateral inhibition. Jornal of the Optical Society of America. 1969a. 9, KELLY. D. H. Diffsion model of linear flicker responses. Jornal of the Optical Society of America, 1969b, 9, KELLY. D. H. Theory of flicker and transient responses: I. Uniform fields. Jornal of the Optical Society of America. 1971, 61, KERR. J. L. Visal resoltion in the periphery. Perception & Psychophysics , ; LEIBOWITZ. H. Effect of reference lines on the discrimination of movement. Jornal of the Optical Society of America , LEVINSON. J. Non-linear and spatial effects in the perception of flicker. Docmenta Ophtbalrnotogica, McCOLGIN, F. H. Movement thresholds in peripheral vision. Jornal of the Optical Society of America REGAN. D & TYLER. C. W. Temporal smmation and its limit for wavelength changes: An analog of Bloch's law for color vision. Jornal of the Optical Society of America. 1971a REGAN, D. &; TYLER. C. W. Some dynamic featres of color vision. Vision Research. 1971b. 11, SCHICKMAN. G. M. Visal masking by low freqency sinsoidally modlated light. Jornal of the Optical Society of America. 197, ST. CYR. G. J.. & FENDER, D. H. Non-linearities of the hman oclomotor system: Gain. Vision Research TYLER. C. W. Stereoscopic depth movement: Two eyes less sensitive than one. Science. 1971, TYLER. C. W. Temporal characteristics of apparent movement: Phi movement vs, Omega movement. Qarterly Jornal of Experimental Psychology in press. TYLER. C. W. Periodic vernier acity. Paper presented to the Association for Research in Vision and Opthalmology. April. 1972b. van der TWEEL, L. H. Cortical responses to modlated light in the hman sbject. Acta Physiologica Scandinavica. 196, VERINGA. F. Enige natrl<:indige aspecten van het zien van gemodleerd licht. Thesis. University of Amsterdam, (Accepted for pblication April 23, 1972.) NOTE 1. The term "freqency response fnction" (FRF) will be sed in preference to the common "modlation transfer fnction" (MTF). MTF has the implication that the fnction is mathematically transformable to predict other types of response fnction. whereas FRF describes oniy the type of data that have been obtained. 236 Perception ~ Psychophysics, 1972, Vol. 12 (2B)