Infant temporal contrast sensitivity at low temporal frequencies

00424989192 $5.00 + 0.00 Copyright c‘ 1992 Pergamon Press Ltd

b’is;an Res. Vol. 32. No. 6, pp. 1157-1162, 1992 Pmted tn Great Britain. All rights reserved

Research Note Infant Temporal Contrast Frequencies DAVIDA Y. TELLER,*? MONIKA R. MAHAL*

DELWIN

Sensitivity at Low Temporal

T. LINDSEY,*

CORINNE

M. MAR,* ANNEMARIE

SUCCOP,*

Received 15 April 1991: in revised form 23 October 1991

The data on infant temporal contrast sensitivity functions (TCSFs) are scarce and contradictory. Earlier studies suggest that critical flicker frequency (CFF) is adultlike at 2-3 months postnatal (Regal, D. M., 1981 Yifion Research, 21, 54%555), while contrast sensitivity at low temporal fr~uencies remains poor. If both of these firings are true, then infant TCSFs are much fiatter than those of adults. In the present study, we have re-investigated 2-month-olds’ contrast thresholds at low temporal frequencies. To match the conditions of Regal’s CFF study, test fields were embedded in a luminance-matched surround. As in previous studies, low contrast sensitivities were found. Models of infants’ flat TCSFs are discussed. Infant vision Infant temporal vision Temporal contrast sensitivity

INTRODUCTION The properties of adult temporal contrast sensitivity functions (TCSFs) are well established (de Lange, 1958; Watson, 1986). Classical data from Kelly (1971) are shown in Fig. 1. Different spatial parameters yield TCSFs of different shapes, particularly in that the TCSF is typically band-pass for uniform fields and low-pass for counterphase-modulated gratings. In both cases, contrast thresholds are 1% or less at maximum sensitivity. Above 10 Hz, sensitivity falls off dramatically, and the high-frequency cut-off (at which 100% contrast is required for threshold) occurs at about 50 Hz. Thus, contrast thresholds increase by at least two orders of magnitude in the temporal frequency range between 10 and 50 Hz. The development of temporal resolution in infants has only rarely been studied. In particular, few behavioral studies of infant TCSFs have been carried out, and the currently available results are puzzling. In an early study, Regal (1979, 1981) used square-wave flicker to investigate critical flicker frequency (CFF) in infants. Small (3.6”) uniform test fields were used. Regal found CFFs of 41, 50 and 52 Hz in 4-, 8- and IZweek-old infants respectively. Thus, under Regal’s conditions, the high*Department ofPsychology,NI-25, University of Washington, Seattle, WA 98195, U.S.A. *Departmentof Physiology/Biophysics, NI-25, University of Washington, Seattle. WA 98195, U.S.A. $The CFFs of 3-month-old infants have been reinvestigated recently, with isochromatic red and blue stimuli and an ON: OFF duty cycle of 1: 5.6.CFFS of approx. 22 Hz were reported (Mercer & Adams, 1989).

frequency cut-off of young infants is apparently similar to that of adults by 2-3 months postnata1.S Similar high values of infants’ CFFs have been reported in electroretinographic studies (Heck & ZetterstriSm, 1958; Horsten & Winkelman, 1962, 1965; cf. Hartmann & Banks, 1992). On the other hand, there are three studies suggesting that infants’ sensitivity to low- and mid-frequency sinusoidal flicker is much lower than that of adults. Counterphase-modulated grating targets were used in all three cases. In the earliest study, Atkinson, Braddick and French (1979) report a spatial CSF from a single ‘I-weekold infant tested with large fields of IO Hz counterphase flicker. The infant’s best performance was a contrast threshold of about 15% for 0.2 and 0.5 c/deg targets. More recently, Hartmann and Banks (1984, 1992) tested 6- and 12-week-old infants with large, 0.1 c/deg fields modulated at 1, 5 and 20 Hz. 6- and 12-week-olds’ best contrast thresholds at 5 Hz, were 30 and 15% respectively. And in the most recent study, Swanson and Birch (1990) tested 4- to 8-month-old infants with small patches of 0.35 and 1 c/deg gratings. 4-month-olds’ best contrast thresholds were about 20% at low temporal frequencies (2 and 4 Hz), and near 100% at higher temporal frequencies (8 and 17 Hz). Even at 8 months, the infants’ best contrast thresholds barely reached 10%. In sum, the presently avaitable data suggest that infants’ high-temporal-frequency cut-offs are similar to those of adults (Regal, 1981), while their sensitivities at lower temporal frequencies are one to two orders of magnitude below those of adults. If all of these reports are true, infant TCSFs are much flatter than those of adults, and thereby provide an interesting challenge to

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of the development of spatiotemporal vision (see below). There are two important diff&nces in skull parameters between Regal% CFF study and the three studies TCSFs. The first arises from different methods of creating time-varying stimuli. Regal produced squarewave flicker by inte~pting a t~n~ten beam with a rotating disk, and his “blank” field was similarly a tungsten beam,flickered at 75 Hz. All other investigators have used sinewave Bicker, gmxbaxi on video display systems, and their “blank” &Us have also been produced on video displays, presumabty containing some residual flicker at the frame rate of the video system wed (67 Hz in Swanson CQE Birch, 1990, unspecified in Atkinson er aC., 1979; and in Hartmann t Banks, 1984, 1992). The second diaerence has to do with surround conditions. Regal used a p~~arly simple display: a small i%ckering field embedded to the left or right of center in a large surround tield, with the Bickering target, “blank” and surround field all closely matched in time-average luminance. The videogenerated targets used by other investigators have formed by the edges necessarily contained salient of the video displays. In addition, Swan~n and Biroh deli~~l~ introduced outline “winduws” ~~~ the locations of target and blank on the two sides of their display. For a variety of purposes, we recently undertook a brief study of infants’ temporal contrast sensitivity in the low temporal frequency range. We began by u&g 13” fields of sinusoidal fhoker presented on a video display system. As did earlier i~v~ti~to~, we found remarkablylow tcxnporalcontrast sensitivities. mod&

of

However, we noted that the infants were drawn to the edges of the video screen more strongly than to the stimuli, and stared indiscriminately toward either side of the display on many trials. It seemed possible that use of target fields embedded in a iumi~an~-match~ surround, like those used by Regal (1981), might yield higher performance levels (cf. Lythgoe & Tansley, 1929 de Lange, 1958; Kelly, 1959). Accordingly, we constructed a surround screen which allowed presentation of small, modulated stimuli in a matched surround. Infants were tested with both sine- and square-wave modulation with this display. This dispiay, however, had its own perceptual cornplexity. When adult observers viewed the display, they noted that the smah “blank” stimuli created by the video screen had a highly visible temporal flicker, presumably caused by the adult’s resolution of a residual flicker at the 67 Hz frame rate of the display. This phenomenon was particularly noticeable during eye movements, which spread the individual video frames out across the retina, and provided the impassion of a series of Gelds spread out in space. The “video blank” field also exhibited perceptual color changes, particularly in conjunction with eye movements. It thus seemed possible that these phenomena, if seen by the infants, could attract the infant’s gaze to the blank field and defeat the purpose of the experiment. We therefore devised two control conditions to examine this possible artifact (cf. Regal, 1979). In brief, none of the stimulus variations we tried resulted in high contrast sensitivity to low temporal frequency modulation in infants. METEZODS Subjects and observers fnfants were recruited from the Infant Studies Subject Pool at the University of W~~n~ton. Informed consent was obtained fram parents. Individual infants were tested for l-6 1-hr sessions, between the 49th and 65th postnatal day (median = 56 days). Within the experimental condition being tested as many stimulus conditions as possible were run within each subject; thus, the results are a mixture of within- and ~tw~n-su~eet compa~sons. A total of St g-week-old infants were tested; of these, 33 completed at least one data set and are included in the final results. The infants were tested by 2 observers, h4M and AS. In the earlier phases of the experiment in which sinewave modulation was tested, MM’s average percent correct was consistently higher than ASS, AS’s_performante improved ~ad~lly over time, and her later data, for square-wave modulation, approach those of MM. These differences are d~urne~t~ in Fig. 2. Apparatus Stimuli were generated on a high rm~luti~n RGB monitor (Barco CDCT 6451) driven by au Apple M~.cn. m&ocomputer with an %-bitcolor card. The computer graphics syst~ was capable uf ~uemting 256 discrete levels of white light intensity. The non~int~rla~ frame

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surround screen and above the infant’s head, prevented the adult observer from seeing the stimuli. To search for potential artifacts that could be created by the residual frame-rate temporal variations of the “blank” video field, a sliding shield (the “occluder”), matched in reiIectance to the surround screen, was mounted directly behind the surround screen. Under some conditions of the experiment (see below), the occluder was used to block the infant’s view of one side or the other of the video screen, thus providing an “occluded blank” condition with no residual flicker.

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PERCENT CONTRAST FIGURE 2. Representative infant psychometric functions, (A, Bf Sine-wave modulation. Temporal frequencies 1 (openk~k~), 2 (solid S~U~PZS), 4 (solid triangles) or 7.5 (solid circles) Hz. (A) ObserverMM; (B) AS. (6, D) Square-wave modulation. Temporal frequencies as in (A) and (B). (C) Observer MM; (D) AS. (E) A single infant tested with both sine- (solid symbols) and square-wave (Open symbols) modulation. Temporal frequencies 2 (squares), 4 (triangles), 7.5 (circles) Hz. Observer MM. (F) A single infant tested at 2 HZ with the video blank (solid squares) and occluded blank (open squares) ~nditions. Observer MM.

rate of the monitor was 66.6 Hz. The time average luminance of the video screen was 1.4 log ft-L (1.9 log cd/m*). On each trial of the experiment, one side of the screen was modulated at a rate of 1,2,4 or 7.5 Hz, while the other side remained at the time average luminance value (except for the residual 66.6 Hz flicker from the frame-rate of the display), Both sine-wave and (nominal) square-wave flicker were used. To eliminate the edges of the video screen, a white cardboard screen (the “surround” screen) was mounted 6 cm in front of the video screen. Two 6.6” square holes cut in the surround screen formed the stimuli. The surround screen was front-illu~nat~ by the fluorescent room illumination. The CIE chromaticity coordinates of the video monitor were (0.328, 0.317) in early experiments, and (0.446, 0.392) in later experiments. In each instance, theater gels were used to cover the room lights as needed, to provide color and luminance matches (for adults) between the surround and the stimuli. A video camera was mounted behind the surround screen, in front of the video screen. Via a small mirror and a peephole in the surround screen, the video camera provided the observer with a view of the infant’s face. An infrared source was mounted on the front of the surround screen, 48 cm below the peephole. A hot mirror mounted on the screen 15 cm below the peephole, at a slight angle from vertical, reflected the i.r. source toward the infant’s eyes to create a prominent cornea1 regection of the i.r. source. A large shield, located 30 cm from the

Procedures for calibration and lin~~tion of the video phosphor output have been described previously (Lindsey & Teller, 1989). The chromaticity coordinates of the video screen were calibrated with a Minolta TV color analyzer II. The actual contrast of a nominal 100% contrast modulation was calibrated as follows. Square-wave modulations of 1,2,4,7-S and 15 Hz were used. At each temporal frequency, 100 samples were taken within a 15 msec window (spanning slightly more than 1 frame of the video signal) at the center of the on and off phases of the square-wave modulation. These values were averaged to estimate the maximum and minimum luminances. True contrasts for all nominal contrasts were estimated as proportions of these values; the true contrast values are plotted in Fig. 2. At 15 Hz the maximum achievable contrast was only 14%; for this reason, the maximum flicker rate used was 7.5 Hz. Procedures

The forced-choice preferential looking (FPL) technique was used (Teller, 1979). The adult observer held the infant 3Ocm in front of the surround screen, and watched the infant’s looking pattern on the viewing monitor. The prominent cornea1 reflections of the i.r. source assisted the observer in sensing the infant’s direction of gaze. On each trial, the observer used the infant’s looking pattern to judge the location of the flickering stimulus. Trial-by-trial feedback was provided. A maxims of four temporal frequencies, 1, 2,4 and 7.5 Hz, was used in testing each infant. Bach temporal frequency was tested at three contrast levels. The order of testing of temporal frequencies was counterbalanced among infants. A minimum of 20 trials per point (60 trials per psychometric function) was completed for each temporal frequency before the next temporal frequency was begun. Four conditions were run. In the first condition (16 infants), sinusoidal modulation was used. In the second condition (9 infants), (nominal) square-wave modulation was used. In these cases, as many temporal frequencies as possible were tested within each individual infant. Three additional infants were also tested with both square- and sine-wave modulation at one or more temporal frequencies. In the third (“occluded blank”) condition (3 infants), the occluder was used to occlude the blank side of the

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video screen on half of the trials; i.e. on these

trials

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stimuli consisted of a modulated field on one side and a blank occluder on the other. This condition was used to see whether the perceptual artifacts associated with the “video blank” might be attracting the infant’s attention, and so reducing the infant’s tendency to look at the modulated stimulus field. In the fourth condition (2 subjects), the “video blank” was tested directly against the “occluded blank” on some trials, to see whether or not the perceptual artifacts associated with the “video blank” for adults were sufficient to attract the infant’s visual attention. On interleaved trials, 1 or 2 Hz stimuli were presented, paired against the occluded blank stimulus, in order to maintain the infant’s attention and show that abovechance performance could be maintained by that particular infant. Since the highest possible performance was being sought in this experiment, it was run by the more sensitive of our 2 observers (MM), with 45 to 55 trials per point. Data analysis Contrast threshold (75% correct) were estimated from the psychometric functions by linear interpolation. Points plotted below the abscissas in Fig. 3 represent cases in which performance for all contrasts fell below 75% correct. RESULTS Representative psychometric functions for individual infants are shown in Fig. 2. Figure 2(A) shows an infant tested with sine-wave modulation by observer MM at 1, 2 and 4 Hz. This figure is typical of our best infant/observer performance. Performance exceeded 75% correct at 50% contrast at 1 and 2 Hz, and at 100% contrast at 4 Hz. Figure 2(B) shows a second infant, tested with sine-wave m~ulation at 1, 2, 4 and 7.5 Hz by observer AS. This figure is typical of our worst level of infant/observer performance; in fact, no individual data point exceeds 75% correct for any temporal frequency. Figure 2(C) and (D) show two infants tested with and are both selected to square-wave modulation, demonstrate typical infant/observer performance levels. In both cases, performance exceeds 75% only at very high contrasts at the three lowest temporal frequencies, while at 7.5 Hz neither infant/observer’s performance reached threshold. Figure 2(E) shows a single infant tested with both square- and sine-wave modulation. This particular infant was among the most sensitive we tested, particularly at 7.5 Hz. Performance is highly similar with both sineand square-wave modulation. Figure 2(F) shows the results of varying the form of the blank stimulus between the “video blank” and the “occluded blank”. Performance is slightly worse, rather than better, with the occluded blank, thus demonstrating that ehmmation of the video artifacts visible to adults did not improve the performance of the infants. The

FREQUENCY (Hz) FIGURE 3. infant temporal CSFs. Each symbol represents a threshold from a single infant psychometric function; dotted lines show group medians. Symbols below the abscissa represent inbts who responded at <75% correct at 100% contrast; numbers of infants in this category are given in parentheses. (A) MM, sine-wave modulation; 03) AS, sine-wavemodulation; (C) MM, square-wave modulation; @) AS, square-wave modulation.

same result was shown by all three infants tested in this condition. A similar result was reported briefly by Regal (1979). Finally, when confronted with the “video blank” paired with the occluded blank, the infants were not attracted to looking at the “video blank”; performance remained at chance for both infants tested under these conditions (data not shown). Threshold values from all completed psychometric functions, for both sine- and square-wave modulation, are shown separately for the two observers in Fig. 3. For sine-wave modulation [Fig. 3(A, B)], there is a clear (although small) difference between observers. AS’s performance remained b&w 75% correct, even at 100% contrast, on over half of her data sets, while MM was always able to produce a~ve-threshed performance at the highest contrast levels. For square-wave modulation [Fig. 3(C, D)J, MM’s performance remained very much the same, while AS’s performance improved somewhat, and the interobserver difference is consequently somewhat smaller. Since sine-wave modulation was tested first, we are inclined to attribute AS’s improved performance with square-wave modulation to practice, rather than to the difference between square- and- sine-wave modulation, but further data would be required to estabhsh this point ~~tiveIy. Even for the more sensitive of our two observers, MM, for both sine- and square-wave modulation, median threshoids were near or above 50% contrast at all temporal frequencies. ‘Thus; the major outcome of this study is to oonfirm the low sensitivities Of infants to low-frequency temporal modulation reported by earher investigators.

RESEARCH DISCUSSION Contrary to our initial expectations, infants’ performance remained low throughout our entire series of experiments. Despite our best efforts, 2-month-old infants exhibited no inclination or ability to attend visually to low-contrast temporally modulated stimuli in the l-7.5 HZ frequency range. Neither a relatively high luminance, nor matched surround fields, nor squarewave modulation, not the “occluded blank” induced the infants to increase their performance levels. Our data are thus consistent with the results of Atkinson et al. (1979). Hartmann and Banks (1984, 1992), and Swanson and Birch (1990), and fail to resolve the apparent inconsistency between those data and the high CFFs measured behaviorally (Regal, 1981) and with ERGS (Heck & Zetterstrijm, 1958; Horsten & Winkelman, 1962, 1965). The apparently high CFFs and flattened TCSFs of infant temporal vision provide an interesting contrast to the case of infant spatial vision. In the case of spatial CSFs, there is general agreement between infants’ diminished contrast sensitivity at low spatial frequencies and their diminished cut-off frequency, or grating acuity (Atkinson, Braddick & Moar, 1977; Banks & Salapatek, 1978; Dobson & Teller, 1978). In consequence, the mid-to-high frequency range of infant spatial CSFs can be modeled reasonably well by assuming developmental decreases of both absolute sensitivity and “spatial scale”; i.e. with various combinations of downward and leftward shifts of the adult function or spatial mechanisms derived from models of the adult function (Wilson, 1988; cf. Banks & Bennett, 1988). If infants’ TCSFs are as flat as they appear to be, this modeling approach will not work in the temporal domain. The infants’ high CFF prevents any combination of downward and leftward shifts of adult TCSFs from fitting the infant data if the cut-off frequency is included in the curve-fitting exercise. Moreover, temporalfrequency mechanisms derived from the data of adult subjects (e.g. Mandler & Makous, 1984) have sensitivity maxima corresponding to threshold contrasts of < 5%, and vary in sensitivity by at least a factor of 20 from maximum to cut-off. Hence, no combination of downward or leftward shifts of any single such mechanism, or set of such mechanisms, can be used to model the flat TCSFs demonstrated by infants. We are left with two basic alternatives. The first is to assume that there is still something artifactural or discountable about the available data on infant temporal resolution-either the demonstrated CFF is artifactually high or the demonstrated sensitivities at low temporal frequencies are artifactually low-and that data collected in the future will reveal infant TCSFs that resemble adult TCSFs more closely in shape. This assumption preserves the possibility of modeling infant

*It wouldof COU~SZ

be interesting to replicate the early ERG measurements of infant CFFs (Heck & Zetterstrom, 1958; Horsten & Winkelman,1962, 1965). and extend such measurements to include the whole temporal frequency range.

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TCSFs by means of downward and leftward shifts of adult TCSFs or temporal mechanisms derived from them; i.e. by a combination of losses of sensitivity and decreases in temporal scale, the latter accomplished by increases in the duration of the temporal impulse response function (Swanson & Birch, 1990). The second alternative is to accept the possibility that infant TCSFs are, in fact, much flatter than those of adults, and to seek a model to account for these flat functions. This perspective immediately draws one’s attention to the fact that infants seem to respond to virtually the whole range of temporal frequencies to which adults respond, but only when high contrasts are presented. This perspective also suggests an analogy to a phenomenon associated with adult supra-threshold temporal contrast matching (Bowker. 1983; Georgeson, 1987): apparent contrast grows more rapidly with stimulus contrast for temporal frequencies displaced from the peak than for frequencies near the peak of the thresholdlevel TCSF. As a result, TCSFs produced by contrast matching become increasingly flatter at higher suprathreshold levels. In fact, at levels well above the absolute detection threshold, stimuli matched in physical contrast also match approximately in apparent contrast, producing at least a rough “contrast constancy” for adult tempora1 vision. If infants’ temporal sensitivity were governed by processes like those that produce the supra-threshold constant-contrast contours of adults, very flat infant TCSFs would result. This outcome could occur if infants’ TCSFs were to develop “from the bottom up”, with infant detection thresholds corresponding to the constant contrast contours produced by standard stimuli of increasingly lower physical contrast as age increases. In such a case, the low-pass or band-pass TCSF typical of adult vision would emerge only as the infant’s absolute sensitivity approached the detection thresholds of adult subjects. The flat supra-threshold contrast matching functions of adults have been modeled by hypothesizing that temporal impulse response functions decrease in duration and become less biphasic as contrast increases above threshold levels (Georgeson, 1987). We have applied Georgeson’s model to the infant case (Lindsey, unpublished calculations). We assumed that infants are 100 times less sensitive than adults to low frequency flicker. and reduced the time scale of the infant impulseresponse function to approx. one-third that of the adult function assumed by Georgeson. Predictions of adult and infant temporal contrast sensitivity were obtained by Fourier transformation of the infant and adult impulse-response functions. The resulting amplitude spectra predict an infant TCSF that shows low contrast sensitivity at low temporal frequencies, is flat out to about 25 Hz, and cuts off at about 50 Hz, thus providing good agreement with all of the infant temporal data available to data.* It is interesting to note that this model differs fundamentally from that of Swanson and Birch (1990). Since

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we accept the possibility that infant and adult CFFs are similar, and infant TCSFs are flatter than those of adults, our model must produce an increase in infant temporal band~dth, and therefore must assume a decrease in the duration of the temporal impulseresponse function of infants with respect to that of adults. Our model thus involves an increase rather than a decrease of temporal scale in infancy, i.e. a rightward rather than a leftward shift of the adult function along the temporal frequency axis, to fit the infant data. Although this modeling option is appealing, it is important to note that spa&r/ supra-threshold contrastmatching contours also flatten at higher supra-threshold levels in adults (Georgeson & Sullivan, 1975; Bowker, 1983), and may do so in infants (Stephens & Banks, 1985). Yet as discussed above, the infant’s thresholdlevel spatial CSF apparantly retains a sharp highfrequency fall-off, and is reasonably modeled by downward and leftward shifts of the adult function. Thus, if the present model is correct, the developmental aspects of spatial and temporal vision in infants are governed by changes of scale in opposite directions. Additional empirical and theoretical work will be needed to provide a unified description of the development of spatiotemporal vision.

REFERENCES Atkinson, J., Braddick, 0. & French. J. (1979). Contrast sensitivity of the human neonate measured by the visual evoked potential. Investigative i?phthaimoiogy and Visual Science, 18, X0-21 3. Atkinson, J., Braddick, 0. & Moar, K. (1977). Development of contrast sensitivity over the first 3 months of life of the human infant. V&ion Research, f 7, 1037-1044. Banks, M. S. & Bennett, P. J. (1988). Optical and photoreceptor imrnaturities limit the spatial and chromatic vision of human neonates, Journal of the Optical Society of America A, 52059-2079. Banks, M. S. & !hlaptek, P. (1978). Acuity and contrast sensitivity in I-, 2-, and 3.month-old human infants. Zn~e~figar~~ Ophtkaimalogy and Visual Science, 17, 361-365. Bowker, D. 0. (1983). Suprathreshold spatiotemporal response characteristics of the human visual system, Journal of the Opficat satiety of America, 73, 436-440. Dobson, M. V. &Teller, D. Y. (1978). Visual acuity in human infants: A review and comparison of behavioral and electrophysiological studies. Vision Research, 18, 1469-1483. Georgeson, M. A. (1987). Temporal properties of spatial contrast vision. Vision Research, 27, 765-780. Georgeson, M. A. & Sullivan, G. D. (1975). Contrast constancy: Debhtrring in human vision by spatial frequency channels. Journal of Phystbiagy, Lambn,

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NOTE Hartmann, E. E. % Banks, M. S. (1984). Development of temporal contrast sensitivity. international Conference on Infancy Studies, New York, U.S.A. Hartmann, E. E. % Banks, M. S. (1992). Temporal contrast sensitivity in human infants Vision Research, 32. t 163-f t68. He-& J. & Zetterstriim, B. (1958). Analyse des photopischen flimmerelektroretinogramms bei neugeborenen. International Journal af Ophthalmology, 135, 204-210.

Horsten, G. P. M. t Winkehnan, J. E. (1942). Electrical activity of the retina in reiation to histological differentiation in infants born prematurely and at f&l-term. Vision Research, 2, 269-276. Horsten, G. P. M. & Winkelman, J. E. (1965). The electrical activity of the eye in the first few days of life. Acfn Physiologica et Phar~cologi~a

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Kelly, D. H. (1959). Effects of sharp edges in a flickering field. Journal of the Optical Society of America, 49. 730-7’32.

Kelly, D. H. (1971). Theory of flicker and transient responses, II. Counterphase gratings. Journal of the Optical Society of America, 61, 632-640.

de Lange, H. (1958). Research into the dynamic nature of the human fovea-cortex systems with intermittent and modulated light I. Attentuation characteristics with white and colored light. Joumfd of the Optical Society of America, 48, 777.-784.

Lindsey, D. T. & Teller, D. Y. (1989). lnguence of variations in edge blur on minimally distinct border judgments: A theoretical and empirical investigation. Journal of the Optical Society of America A. 61. 446-458.

Lythgoe, R. & Tam&y, K. (1929). The relation of the critical frequency of flicker to the adaptation of the eye. Proceedings of the Ro.yal Society of London B, 10.X 60-92.

Mandler, M. 8. & Makous, W. (1984). A three-channel model of temporal frequency perception. Vi&n Research, 24, 18%1-1887. Mercer, M. E. & Adams, R. I. (1989). Wavelength effects on critical flicker frequency in humaninfants.Perceptual and Motor Skills, 68, 1083-1087. Regal, D. M. (1979). Development of critical flicker frequency in human infants. Ph.D. thesis, University of Washington, U.S.A. Regal, D. M. (1981). Development of critical flicker frequency in human infants. Vision Research, 21. 549-555. Stephens, B. R. & Banks, M. S. (1985). The development of basic mechanisms of pattern vision. IL Contrast constancy. Journal of Experimentat Child Psychofogy, 45, 528-547.

Swanson, W. H. & Birch, E. E. (1990). Infant spatiotemporal vision: Dependence of spatial contrast sensitivity on temporal frequency. Vision Research, 30, 1033-1048.

Teher, D. Y. (1979). The forced-choice preferential looking procedure: A psychophysical technique for use with human infants. In@tt Behavior and Deselopment,

2, 135-153.

Watson, A. B. (1986). Temporal sensitivity. In Boff, K., Kaufman, L. & Thomas, J. P. (Ed@, ~~d~ok of eruption and hums performance. Vomme i: Sensory processes and perception. New York. Wiley/Interscience: New York. Wilson, H. R. (1988). Development of spatiotemporal mechanisms in infant vision. Vision Research. 28, 611-628. Acknowledgement-This

research was supported EY 02920 and BY 04470 to DYT.

by NIH grants