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The meaning of non-monotonic psychometric functions in the assessment of infant preferential looking acuity. A reply to Bankset al. (1982) and Telleret al. (1982)
C’ivwn Rrs. Vol. 13. No. 9. pp. 917-920. 1983 Primed in Great Britain. All rights reserwd
Copyright
0042-6989 83 53.00+0.00 C 1983 Pergamon Press Lrd
LETTER TO THE EDITOR THE MEANING OF NON-MONOTONIC PSYCHOMETRIC FUNCTIONS IN THE ASSESSMENT OF INFANT PREFERENTIAL LOOKING ACUITY. A REPLY TO BANKS et al. (1982) AND TELLER et al. (1982) (Receiced 21 May 1982; in revisedform
The rapid assessment of infant visual acuity has been a topic of considerable interest in recent years. One of the most successful methods has been preferential looking (PL). The PL technique takes advantage of the fact that, given a choice between a high contrast, low spatial frequency grating and a blank field, most infants will preferentially fixate the grating. We have published evidence demonstrating that some visible gratings are not preferentially fixated but are, in fact, avoided (Held et al., 1979). We showed infant psychometric functions that were non-monotonic. As spatial frequency increased, preferential looking fell from near lOOo/, preference for the grating to significantly less than 50% before rising to levels not significantly different from 50% at high spatial frequencies. Since any significant deviation from 50% indicates an ability to see the stimulus and since these “dips” had not been previously reported in the PL literature, we suggested, on the basis of published data, that PL acuity might have been underestimated when only those points significantly above 50% were considered. We have also reported on a fast PL method that gains in reliability when these “dips” occur (Gwiazda ef al., 1980). Recently, two articles have appeared in this journal reporting failures to find “dips” below 50% in PL experiments (Teller et al., 1982: Banks et al., 1982). Both papers also question the usefulness of the fast method proposed in Gwiazda ef al. (1980). The papers raise interesting and important issues pertaining to the measurement of PL acuity. In this letter, we will address three questions raised by these articles: (I) why don’t Banks et ul. and Teller er al. find dips below 50% in their PL data? (2) In light of these results, what is the significance of the dip? (3) How do the shapes of the psychometric function and the constraints on infant testing limit the choice of psychophysical methods? Teller ef ul. (1982) report that they do not find negative preference dips in PL data from Allen (1979). In our original paper (Held ef al., 1979) we argued that their version of PL would not produce 917
27 September 1982)
dips. Teller et al. (1982) agreed. In our version, infants are seated in a dark room facing two bright stimuli. One is a grating. At low frequencies, infants prefer the grating over the other stimulus which is a bright but blank region. If the grating is aversive, however, the infant looks at the other bright field and preferential looking for the grating drops below 50%. In the Teller version, the grating stimulus is presented on either one side or the other of a large gray field. The entire field is of the same average luminance as the grating. There is no second bright stimulus, as such. Thus, if the grating is aversive, the infant will look about at random since there is no second choice stimulus. No dip below SOY/,would be expected and none is seen. Teller et al. argue that they get preference above 50% where we get preference below 50% (see their Fig. 4). They suggest that their observers, given feedback about the actual position of the stimulus, would learn to interpret the infant’s aversion as a clue to the side containing the stimulus. We do not give the observer feedback. Their suggestion is certainly possible though Banks et al. argue that it is unlikely that an observer could change criteria rapidly enough as the stimulus changed from one spatial frequency to another. As a cause for the dip, Teller et al. suggest that “neural non-linearities” might produce a dimming of the gratings that are fixated less than 5096 of the time. Rather than an aversion for these gratings, they suggest that the infant’s behavior may reflect a simple preference for the brighter stimulus, the blank field. Banks et al. note that their method should have revealed any such effect. As it did not, they consider the explanation to be implausible. Another possible explanation for their failure to find negative values in the range of our negative values is the shallowness of the psychometric functions cited by Teller er al. Our psychometric functions are characteristically much steeper. Of the 16 infants whose functions are shown in Fig. I of Held ef al. (1979), I5 drop from better than 80’~ preference for the grating to near or below a 50%
prcfxencr frrquznc)
level o\er t. range of one ott2ie
of spatial
The ps:lchometnc funcuon, used b> Teller t’i al are dmu\n from -\llen (1979) and tend to be shallow (see Fig. 4 in Trllcr ). The luminance is lower i 10.6cd/m’ vs 33cdjm’). The stimulus color IS dltferent (Banks: black and green CRT vs Held. black and white slides}. The upper range of spatial frequencies dltl‘ers (Banks: up to 4c,‘deg vs Held: up to 18c;deg). The age range of the infants barely overlaps (Banks: 1-3 months vs Held: 3-12 months). The spatial configuration is different (Banks: left and right fields adjacent vs Held: fields separated by 25 ‘). Finally. they gave the observer feedback on each trial. We do not. As Banks er al. point out. It is not clear which variables are important here. However, whatever the critical differences may be, the results of those differences are very clear. In addition to the absence of any negative dips In the Banks ef (I/. data, there are gross differences between their psychometric
functIoni Jnd our> 4~ nated aho\*. iii: i‘un, ‘ii!>X are usually steeper The II! p~>chc~~stx I‘un;.,,*:~, shown b> Bank\ e’r (11 L~rz her\ sh;l!lo.:\ Oni‘. I .( 10 Infants rber shoL% L, prciersnoe thcri equ,~‘. ilk exceeds SO”., for an! Liequenc) For 3 2t’ thz m:.tn;x no points on the function are blgniticantl> ;t;1o\i chance levels. It is clear that the infants in the B.tnks study hate no aversion for any of the srzmuh However. It IS also clear that the) hate ltttls prsference for any of the stimuli either. Bank5 or (11.t 1982~ state that they do not obtatn high preferences for ant of their stimuh because their Infants are boung {j--i5 weeks) and they do not use ION spatial frequenclzs. However. they do use frequenclrs d.s lots as 1 c deg and infants as old as I5 weeks. We obtain near lO(Y; preference for gratings of I S cldeg 1n infants of 12-14 weeks (see Infants D.S., K S. and D.H in Held et cl/.. 1979). Our psychometric functions dip only to 35-40”;;. We ran 48 trials per spatial frequent) m order to demonstrate the reality OF these small deviations. Banks (af used stimuli that provoke only mild preferences and (b) had only 25 to 35 trials per frequency. Consequently, it would be less likely that the Banks’ data would show significant dips. even if the infant found some stimuli mildly acersive. The failure of Banks et al. and Teller cv N/. to tind dips in their versions of PL does not constitute a failure to replicate our original results. The methods are too different. However, their results do show thrtt the dip does not appear in al1 PL acuity experiments Their analyses of our results show the e&et to be reliable in our data. Thus, ue are faced with a dilemma. What is the meaning of a negative preference that can be reliably produced using one PL paradigm but not others7 The existence of negattve preference reminds us that. in using PL. we are studying infant vision through the filter of infant behavior and that infants do not always behave as we expect. This fact is of particular importance as we extend the use of PL to the study of other \~sual functions In recent studies of binocular vision. for instance. &ants seemed to look away from strongly rivalrous stimuh (Held t)r nf.. 1980: Bi-rch cr rrl. 1983). Of course, we would not expect to find negative prererences
in iill two-chorce
pnr~d~grns
Fill-
example. aversise behavtor uould be unlikeI> In operant techniques where either the child or the experimenter is reinforced for locating the “correct” stimutus (e.g. Birch et cd., 1980; Mayer and Dobson. 1982). The dip reminds us that a priori assum&o-ns about infant preferences may not he correct. Doing PL experiments under such assumptions can and has led to errors. Given that our methods and those of Teller et (11 and Banks er a/. seem to yield psychometric functions of diEerent shape. It might be tempt& IO pronounce one version better. This seems to us to be a futile exercise. All of these vanatlons have been carefully
done by competent lnkestrgators.
,411 of the
Letter to the Editor
PL methods seem to yield qualitatively similar results. Our version of PL does seem to produce steeper psychometric functions and works until the infant is 12 months of age. Other methods produce shallower functions and stop working earlier. However. our method tends to produce non-monotonic functions. These are difficult to treat theoretically and preclude the use of standard curve-fitting procedures to estimate acuity. Other methods produce monotonic data that can be treated in the conventional manner. Any one of these methods, executed with appropriate care, seems to produce useful data. In assessing infant acuity three factors are important: accuracy. speed, and simplicity. This is particularly true in a clinical setting. As a result, there has been considerable interest in developing fast and simple methods for reliably determining PL acuity. We have proposed one such method, a modified. descending staircase (Gwiazda ef ul.. 1980). It can be implemented using standard carousel slide projectors. This allows the use of bright, highcontrast patterns. Further, it is both cheaper and more portable than methods relying on CRT displays and/or computer selection of stimuli. Both Teller ef nl. and Banks et nl. raise questions about this method. The method works best with nonmonotonic psychometric functions such as those obtained in our lab. It works tolerably well with monotonic functions but is less precise. This assertion has recently been supported by an independent computer simulation of the method {Nachmias, 1982) with the important proviso that each session must start with a spatial frequency that the infant sees and prefers to fixate. Banks et al. compare our method to a standard two-down, one-up staircase. Their data show that the standard method works somewhat better for monotonic functions but that both methods are about equally reliabfe for nonmonotonic functions. The two-down, one-up method is more difficult to implement with slide projectors. In our method, the projectors either go forward or backward one step on each trial. Moving back two slide positions is awkward and produces a cue as to the correctness of the observer’s response. As noted above. we do not normally give feedback to the observer. Further, since standard staircase designs assume a monotonic psychometric function, it is not clear that it is appropriate to use them when the underlying function is likely to be non-monotonic. However, Banks e! af. are probably correct in asserting that our method is not the method of choice if the underlying function is likely to be monotonic. This leads to the unhappy conclusion that the method of constant stimuli, requiring many more trials, should be used unless one is sure about the shape of the underlying psychometric function. Teller er al. raise an additional problem. They correctly note that our stimuli are not sequentially
919
jnde~nd~t~ This is an inevitable result of the use of slide projectors for stimulus presentation. Given a fixed set of slides and any version of an up-down staircase. sequential dependencies are unavoidable. For example, if the infant prefers the stimulus on slide !V and avoids the stimulus on N + 1, then the method will alternate between those two slides producing a non-random distribution. Of course, this will be a problem for any staircase method using a fixed order of slides in conjunction with reversals of direction of the slide tray. The problem is mitigated in our method because we do not give feedback to the observer abour the correctness of his or her responses. Thus the observer does not know if the slides have been advanced or retreated and, as a result, the observer would have a difficult time forming hypotheses about the future location of the stimuli based on anything other than the infant’s behavior. The problem would be very serious if a fixed order of slides was used in conjunction with a method such as that reported by Teller er al. Since they give feedback to the observer, given a fixed order of slides and a staircase method, observers would be able to predict the correct side with 100% assurance on some trials. This would be particufarly true if the target could be on one side for only two or three trials in a row. In the absence of feedback, the effects of non-random order are probably not large and are outweighed by the advantages of using a simple apparatus. In summary, our version of PL reliably produces steep, non-monotonic psychometric functions in infants up to I year of age. The methods of Banks er ui. and Teller ef ui. reliably produce shallower but monotonic functions in infants up to six months of age. All methods so far used document the increase in PL acuity with age and all can be used to give an estimate of the PL acuity of an infant. The choice of psychophysical methodology is constrained by knowledge or lack of knowledge about the shape of the underlying psychometric function. If we know that the underlying psychometric is monotonic, standard staircase methods work well. ff we know there is a dip, the method proposed in Gwiazda et ai. (1980) is appropriate. If we know nothing, the method of constant stimuli is probably the safest course. Finally, ttie existence of reliable negative preferences in our data demonstrates that PL, though exceedingly useful, must be used with care. We are inferring the properties of infant vision by observing infant behavior. We should not fall into the trap of presuming to know the nature of infant preferences for a set of stimuli before we have examined those preferences. AcX-noltl~~~emenI-This research was supported by a grant from the National Eye institute (No. EY-01191). ~e~urf~enf
of‘ Psvchoiogy Massachuselts hiiiure of Teclmologj Camhriffge .\I.4 @I39 U.S.A.
JERESIYM. WOLFE JA%TGWIAZDA RICHARDHELD
Letter to the Edltor
910 REFEREXCES
J (1979) Visual acuity development III human tnfdnts up to 6 months of age Ph.D dlssrrtatlon. L’nib of Washmgton. Banks Xl. S.. Stephens B. R and Dannemliler J. L. ( 1982) A failure to observe negatt\e preference II-Iinfant acult) testing. pensionRes. 22. 1025-1032. Birch E. E.. Naegele J. Bauer J. A. and Held R. (1980) Visual acuity of toddlers tested by operant and preferenteal looking techniques. Incest. Ophrhai. risual SCL. Suppi.. xl, 210. Birch E E.. Gwiazda J. and Held R. (1983) The development of vetgence does not account for the onset of stereopsls. Perception. To be published. Gwiazda J.. Wolfe J. ,M.. Brill S., Mohindra I. and Held R. (I 980) Quick assessment of preferential looking acuity in infants. Am. J. Optom. Physiol. Opt. 57, 420-427. Held R., Birch E. and Gwiazda J. (1980) Stereoacuity of
Allen
Held R.. Gwlazda J Brtll S Mohmdrn I. and Uo;te J (1979) Infant visual acutt) IS underestimated because near threshold gratmgs are not preferenuall> fixated Lform Res 19, 1377-1379 Mayer D. L. and Dobson V. (1982) VISUSIacult) dcrelopment m Infants and young children ds assessed bq operant preferential lookmg. I’ision Res 22. I ISI-I t53. Nachmias J (1981) Startmg point bias of a recent psychophysical method. Am J. Oprom. Phrsiol Opr 19. 845-837.
Teller D. Y.. Mayer D. L., Makous W. L and Allen J. L. (1982) Do preferential lookrng techniques underesttmare Infant visual acuity? Cbion Res. 22, 101-1024. Teller D. Y., Morse R., Borton R. and Regal D. (1974) Visual acuitv for vertical and diagonal grarmgs m human infants. Vi&on Res. 14, 1433-1439.
Copyright
0042-6989 83 53.00+0.00 C 1983 Pergamon Press Lrd
LETTER TO THE EDITOR THE MEANING OF NON-MONOTONIC PSYCHOMETRIC FUNCTIONS IN THE ASSESSMENT OF INFANT PREFERENTIAL LOOKING ACUITY. A REPLY TO BANKS et al. (1982) AND TELLER et al. (1982) (Receiced 21 May 1982; in revisedform
The rapid assessment of infant visual acuity has been a topic of considerable interest in recent years. One of the most successful methods has been preferential looking (PL). The PL technique takes advantage of the fact that, given a choice between a high contrast, low spatial frequency grating and a blank field, most infants will preferentially fixate the grating. We have published evidence demonstrating that some visible gratings are not preferentially fixated but are, in fact, avoided (Held et al., 1979). We showed infant psychometric functions that were non-monotonic. As spatial frequency increased, preferential looking fell from near lOOo/, preference for the grating to significantly less than 50% before rising to levels not significantly different from 50% at high spatial frequencies. Since any significant deviation from 50% indicates an ability to see the stimulus and since these “dips” had not been previously reported in the PL literature, we suggested, on the basis of published data, that PL acuity might have been underestimated when only those points significantly above 50% were considered. We have also reported on a fast PL method that gains in reliability when these “dips” occur (Gwiazda ef al., 1980). Recently, two articles have appeared in this journal reporting failures to find “dips” below 50% in PL experiments (Teller et al., 1982: Banks et al., 1982). Both papers also question the usefulness of the fast method proposed in Gwiazda ef al. (1980). The papers raise interesting and important issues pertaining to the measurement of PL acuity. In this letter, we will address three questions raised by these articles: (I) why don’t Banks et ul. and Teller er al. find dips below 50% in their PL data? (2) In light of these results, what is the significance of the dip? (3) How do the shapes of the psychometric function and the constraints on infant testing limit the choice of psychophysical methods? Teller ef ul. (1982) report that they do not find negative preference dips in PL data from Allen (1979). In our original paper (Held ef al., 1979) we argued that their version of PL would not produce 917
27 September 1982)
dips. Teller et al. (1982) agreed. In our version, infants are seated in a dark room facing two bright stimuli. One is a grating. At low frequencies, infants prefer the grating over the other stimulus which is a bright but blank region. If the grating is aversive, however, the infant looks at the other bright field and preferential looking for the grating drops below 50%. In the Teller version, the grating stimulus is presented on either one side or the other of a large gray field. The entire field is of the same average luminance as the grating. There is no second bright stimulus, as such. Thus, if the grating is aversive, the infant will look about at random since there is no second choice stimulus. No dip below SOY/,would be expected and none is seen. Teller et al. argue that they get preference above 50% where we get preference below 50% (see their Fig. 4). They suggest that their observers, given feedback about the actual position of the stimulus, would learn to interpret the infant’s aversion as a clue to the side containing the stimulus. We do not give the observer feedback. Their suggestion is certainly possible though Banks et al. argue that it is unlikely that an observer could change criteria rapidly enough as the stimulus changed from one spatial frequency to another. As a cause for the dip, Teller et al. suggest that “neural non-linearities” might produce a dimming of the gratings that are fixated less than 5096 of the time. Rather than an aversion for these gratings, they suggest that the infant’s behavior may reflect a simple preference for the brighter stimulus, the blank field. Banks et al. note that their method should have revealed any such effect. As it did not, they consider the explanation to be implausible. Another possible explanation for their failure to find negative values in the range of our negative values is the shallowness of the psychometric functions cited by Teller er al. Our psychometric functions are characteristically much steeper. Of the 16 infants whose functions are shown in Fig. I of Held ef al. (1979), I5 drop from better than 80’~ preference for the grating to near or below a 50%
prcfxencr frrquznc)
level o\er t. range of one ott2ie
of spatial
The ps:lchometnc funcuon, used b> Teller t’i al are dmu\n from -\llen (1979) and tend to be shallow (see Fig. 4 in Trllcr ). The luminance is lower i 10.6cd/m’ vs 33cdjm’). The stimulus color IS dltferent (Banks: black and green CRT vs Held. black and white slides}. The upper range of spatial frequencies dltl‘ers (Banks: up to 4c,‘deg vs Held: up to 18c;deg). The age range of the infants barely overlaps (Banks: 1-3 months vs Held: 3-12 months). The spatial configuration is different (Banks: left and right fields adjacent vs Held: fields separated by 25 ‘). Finally. they gave the observer feedback on each trial. We do not. As Banks er al. point out. It is not clear which variables are important here. However, whatever the critical differences may be, the results of those differences are very clear. In addition to the absence of any negative dips In the Banks ef (I/. data, there are gross differences between their psychometric
functIoni Jnd our> 4~ nated aho\*. iii: i‘un, ‘ii!>X are usually steeper The II! p~>chc~~stx I‘un;.,,*:~, shown b> Bank\ e’r (11 L~rz her\ sh;l!lo.:\ Oni‘. I .( 10 Infants rber shoL% L, prciersnoe thcri equ,~‘. ilk exceeds SO”., for an! Liequenc) For 3 2t’ thz m:.tn;x no points on the function are blgniticantl> ;t;1o\i chance levels. It is clear that the infants in the B.tnks study hate no aversion for any of the srzmuh However. It IS also clear that the) hate ltttls prsference for any of the stimuli either. Bank5 or (11.t 1982~ state that they do not obtatn high preferences for ant of their stimuh because their Infants are boung {j--i5 weeks) and they do not use ION spatial frequenclzs. However. they do use frequenclrs d.s lots as 1 c deg and infants as old as I5 weeks. We obtain near lO(Y; preference for gratings of I S cldeg 1n infants of 12-14 weeks (see Infants D.S., K S. and D.H in Held et cl/.. 1979). Our psychometric functions dip only to 35-40”;;. We ran 48 trials per spatial frequent) m order to demonstrate the reality OF these small deviations. Banks (af used stimuli that provoke only mild preferences and (b) had only 25 to 35 trials per frequency. Consequently, it would be less likely that the Banks’ data would show significant dips. even if the infant found some stimuli mildly acersive. The failure of Banks et al. and Teller cv N/. to tind dips in their versions of PL does not constitute a failure to replicate our original results. The methods are too different. However, their results do show thrtt the dip does not appear in al1 PL acuity experiments Their analyses of our results show the e&et to be reliable in our data. Thus, ue are faced with a dilemma. What is the meaning of a negative preference that can be reliably produced using one PL paradigm but not others7 The existence of negattve preference reminds us that. in using PL. we are studying infant vision through the filter of infant behavior and that infants do not always behave as we expect. This fact is of particular importance as we extend the use of PL to the study of other \~sual functions In recent studies of binocular vision. for instance. &ants seemed to look away from strongly rivalrous stimuh (Held t)r nf.. 1980: Bi-rch cr rrl. 1983). Of course, we would not expect to find negative prererences
in iill two-chorce
pnr~d~grns
Fill-
example. aversise behavtor uould be unlikeI> In operant techniques where either the child or the experimenter is reinforced for locating the “correct” stimutus (e.g. Birch et cd., 1980; Mayer and Dobson. 1982). The dip reminds us that a priori assum&o-ns about infant preferences may not he correct. Doing PL experiments under such assumptions can and has led to errors. Given that our methods and those of Teller et (11 and Banks er a/. seem to yield psychometric functions of diEerent shape. It might be tempt& IO pronounce one version better. This seems to us to be a futile exercise. All of these vanatlons have been carefully
done by competent lnkestrgators.
,411 of the
Letter to the Editor
PL methods seem to yield qualitatively similar results. Our version of PL does seem to produce steeper psychometric functions and works until the infant is 12 months of age. Other methods produce shallower functions and stop working earlier. However. our method tends to produce non-monotonic functions. These are difficult to treat theoretically and preclude the use of standard curve-fitting procedures to estimate acuity. Other methods produce monotonic data that can be treated in the conventional manner. Any one of these methods, executed with appropriate care, seems to produce useful data. In assessing infant acuity three factors are important: accuracy. speed, and simplicity. This is particularly true in a clinical setting. As a result, there has been considerable interest in developing fast and simple methods for reliably determining PL acuity. We have proposed one such method, a modified. descending staircase (Gwiazda ef ul.. 1980). It can be implemented using standard carousel slide projectors. This allows the use of bright, highcontrast patterns. Further, it is both cheaper and more portable than methods relying on CRT displays and/or computer selection of stimuli. Both Teller ef nl. and Banks et nl. raise questions about this method. The method works best with nonmonotonic psychometric functions such as those obtained in our lab. It works tolerably well with monotonic functions but is less precise. This assertion has recently been supported by an independent computer simulation of the method {Nachmias, 1982) with the important proviso that each session must start with a spatial frequency that the infant sees and prefers to fixate. Banks et al. compare our method to a standard two-down, one-up staircase. Their data show that the standard method works somewhat better for monotonic functions but that both methods are about equally reliabfe for nonmonotonic functions. The two-down, one-up method is more difficult to implement with slide projectors. In our method, the projectors either go forward or backward one step on each trial. Moving back two slide positions is awkward and produces a cue as to the correctness of the observer’s response. As noted above. we do not normally give feedback to the observer. Further, since standard staircase designs assume a monotonic psychometric function, it is not clear that it is appropriate to use them when the underlying function is likely to be non-monotonic. However, Banks e! af. are probably correct in asserting that our method is not the method of choice if the underlying function is likely to be monotonic. This leads to the unhappy conclusion that the method of constant stimuli, requiring many more trials, should be used unless one is sure about the shape of the underlying psychometric function. Teller er al. raise an additional problem. They correctly note that our stimuli are not sequentially
919
jnde~nd~t~ This is an inevitable result of the use of slide projectors for stimulus presentation. Given a fixed set of slides and any version of an up-down staircase. sequential dependencies are unavoidable. For example, if the infant prefers the stimulus on slide !V and avoids the stimulus on N + 1, then the method will alternate between those two slides producing a non-random distribution. Of course, this will be a problem for any staircase method using a fixed order of slides in conjunction with reversals of direction of the slide tray. The problem is mitigated in our method because we do not give feedback to the observer abour the correctness of his or her responses. Thus the observer does not know if the slides have been advanced or retreated and, as a result, the observer would have a difficult time forming hypotheses about the future location of the stimuli based on anything other than the infant’s behavior. The problem would be very serious if a fixed order of slides was used in conjunction with a method such as that reported by Teller er al. Since they give feedback to the observer, given a fixed order of slides and a staircase method, observers would be able to predict the correct side with 100% assurance on some trials. This would be particufarly true if the target could be on one side for only two or three trials in a row. In the absence of feedback, the effects of non-random order are probably not large and are outweighed by the advantages of using a simple apparatus. In summary, our version of PL reliably produces steep, non-monotonic psychometric functions in infants up to I year of age. The methods of Banks er ui. and Teller ef ui. reliably produce shallower but monotonic functions in infants up to six months of age. All methods so far used document the increase in PL acuity with age and all can be used to give an estimate of the PL acuity of an infant. The choice of psychophysical methodology is constrained by knowledge or lack of knowledge about the shape of the underlying psychometric function. If we know that the underlying psychometric is monotonic, standard staircase methods work well. ff we know there is a dip, the method proposed in Gwiazda et ai. (1980) is appropriate. If we know nothing, the method of constant stimuli is probably the safest course. Finally, ttie existence of reliable negative preferences in our data demonstrates that PL, though exceedingly useful, must be used with care. We are inferring the properties of infant vision by observing infant behavior. We should not fall into the trap of presuming to know the nature of infant preferences for a set of stimuli before we have examined those preferences. AcX-noltl~~~emenI-This research was supported by a grant from the National Eye institute (No. EY-01191). ~e~urf~enf
of‘ Psvchoiogy Massachuselts hiiiure of Teclmologj Camhriffge .\I.4 @I39 U.S.A.
JERESIYM. WOLFE JA%TGWIAZDA RICHARDHELD
Letter to the Edltor
910 REFEREXCES
J (1979) Visual acuity development III human tnfdnts up to 6 months of age Ph.D dlssrrtatlon. L’nib of Washmgton. Banks Xl. S.. Stephens B. R and Dannemliler J. L. ( 1982) A failure to observe negatt\e preference II-Iinfant acult) testing. pensionRes. 22. 1025-1032. Birch E. E.. Naegele J. Bauer J. A. and Held R. (1980) Visual acuity of toddlers tested by operant and preferenteal looking techniques. Incest. Ophrhai. risual SCL. Suppi.. xl, 210. Birch E E.. Gwiazda J. and Held R. (1983) The development of vetgence does not account for the onset of stereopsls. Perception. To be published. Gwiazda J.. Wolfe J. ,M.. Brill S., Mohindra I. and Held R. (I 980) Quick assessment of preferential looking acuity in infants. Am. J. Optom. Physiol. Opt. 57, 420-427. Held R., Birch E. and Gwiazda J. (1980) Stereoacuity of
Allen
Held R.. Gwlazda J Brtll S Mohmdrn I. and Uo;te J (1979) Infant visual acutt) IS underestimated because near threshold gratmgs are not preferenuall> fixated Lform Res 19, 1377-1379 Mayer D. L. and Dobson V. (1982) VISUSIacult) dcrelopment m Infants and young children ds assessed bq operant preferential lookmg. I’ision Res 22. I ISI-I t53. Nachmias J (1981) Startmg point bias of a recent psychophysical method. Am J. Oprom. Phrsiol Opr 19. 845-837.
Teller D. Y.. Mayer D. L., Makous W. L and Allen J. L. (1982) Do preferential lookrng techniques underesttmare Infant visual acuity? Cbion Res. 22, 101-1024. Teller D. Y., Morse R., Borton R. and Regal D. (1974) Visual acuitv for vertical and diagonal grarmgs m human infants. Vi&on Res. 14, 1433-1439.