Reaching behavior in eighteen- to thirty-two-week-old infants was studied as a function of ... changes from 8 ⢠5 degrees in the newborn to 5 ⢠0 degrees in the adult, .... the dark, especially when wearing the convergence-decreasing prisms.
Perception, 1977, volume 6, pages 139-144
Binocular convergence as a determinant of reaching behavior in infancy Claes von Hofsten Department of Psychology, University of Uppsala, Box 227, S-751 04 Uppsala, Sweden Received 1 June 1976
Abstract. Reaching behavior in eighteen- to thirty-two-week-old infants was studied as a function of binocular convergence. The infant looked at the object to be reached for through prism arrangements which changed convergence only. The reaches obtained were nearly always directed at the virtual object defined by convergence. Corrections of the reaches, if any, were made rather late and often not before the hand arrived at the place of the virtual object. 1 Introduction By the age of eighteen to twenty weeks infants reach for and grasp nearby objects which they are fixating visually (White et al 1964; Trevarthen and Ingersoll 1970; Bower 1974). The reach is described as very accurate. This investigation concerns the type of visual information that determines the reach. Reaching behavior can be functionally analyzed into a ballistic part, triggered visually, and a corrective part, guided by visual feedback. The ballistic part gets the hand to the vicinity of the object being reached for, where corrections can be made to secure a smooth grasp. Visually triggered reaching requires absolute distance information. The subject must know where in space the object is situated relative to himself before the reach starts. Visually guided reaching utilizes relative depth information. The position of the hand relative to the object must be seen for the correction of the reach. Bower (1974) has extensively discussed relative and absolute depth information from a developmental point of view. There are powerful mechanisms of relative depth perception sensitive to binocular parallax, movement parallax, and optical expansion and contraction. These types of visual information are invariant over growth, and it has been shown that they can specify the relative positions of objects precisely in very young infants. Thus there is reason to believe that the organism is preadapted to be able to use such information. The problem of absolute distance perception is much more complicated. If the function of absolute distance perception is to enable the organism to act efficiently on objects in the near space, then the spatial variables specifying absolute distance have to be translated into a form appropriate for the control of spatial motor movements. The motor system, however, grows more than does any sense organ. A functional perceptual-motor coordination must therefore be subject to much adaptation over growth. Prewiring of such a system seems less appropriate. The puzzling fact, however, is that neonate reaching is described as purely ballistic and rather well aimed. The hit rate is about 40% (Bower 1974). Therefore it seems that some type of absolute distance perception may, after all, be present in the newborn infant. Could binocular convergence give critical information that determines ballistic reaching in infancy? Although convergence has been subject to much controversy in the past, recent evidence points to this possibility. It has been shown with adults that estimated absolute distance varies with convergence in a very predictable manner (von Hofsten 1976). It has also been shown that pointing at nearby targets with the unseen hand can be determined by this source of information (Foley and Held 1972).
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The first important question regarding the role of convergence in infant ballistic reaching is whether infants converge on objects at all. Slater and Finlay (1975b), taking into account the displacement of the fovea relative to the visual axis, were recently able to demonstrate that the newborn baby does converge on objects displayed in the visual field. The displacement of the fovea relative to the visual axis changes from 8 • 5 degrees in the newborn to 5 • 0 degrees in the adult, so that, as the whole eye grows, the distance between the fovea and the visual axis is approximately constant (Slater and Finlay 1975a). It has further been observed that congenitally blind infants converge their eyes on their unseen hands as they move them to and fro (Bower 1976, personal communication). This interesting observation suggests an innate coordination between convergence and reaching behavior, which might be of significance for the perception of absolute distances. The precision of such a system, however, depends on its ability to adapt to changes in the relation between convergence and arm length. But binocular convergence is not the only possible source of absolute distance information. Movement parallax combined with information about head and eye movements specifies absolute distance in space. This visual-kinesthetic information has been extensively discussed by Johansson (1973), who also showed that adults are able to use such information in the perception of absolute distance. There is no doubt that movement parallax information about absolute distance might prove to be the most efficient one in a structured visual environment. If only one object is available in the visual field, however, this source of information should be less efficient for two reasons. First, in such a situation there are no relative motions on the retina, and it is well-known that the eye is less sensitive to absolute retinal motion. Second, the motion components specifying the motion of the object and the motion of the subject can not be abstracted with only one motion element available (Johansson 1971). These two motion components are therefore by necessity confounded. Binocular convergence does not suffer from this shortcoming. This source of absolute distance information should be as effective with one as with several objects present in the visual field. Consequently, the optimal situation for demonstrating the effectiveness of binocular convergence as a determinant of reaching behavior would be to let the subjects reach for a single object suspended in the air in front of them. This was done in the present experiment in a situation where convergence was varied but accommodation and movement parallax information were held constant. The reaches were performed both in light and in total darkness where only the object to reach for was visible. The dark condition was introduced in order to eliminate movement parallax information more efficiently and to prevent corrections of the reaches. 2 Method The binocular convergence was manipulated with pairs of Fresnel displacing prisms fitted to planolenses mounted on National Health spectacle frames designed for sixmonth-old infants. The Fresnel prisms consisted of a series of wafer-thin prisms lying in adjacent strips on a thin platform of plastic. There was a very slight acuity decrement compared to conventional glass prisms. Accommodation was not altered by the prisms. Two pairs of spectacles were used. The first pair was mounted with 4-diopter prisms displacing the visual field away from the sagittal plane for each eye. This arrangement decreases binocular convergence by 4-58 deg, and makes an object appear further away. The second pair was mounted with 10-diopter prisms displacing the visual field toward the sagittal plane for each eye. This arrangement increases binocular convergence by 11 • 46 deg, and makes the object appear closer.
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A small bell, painted white, was used as the object to be reached for. The baby was lying on its back on a couch beneath the bell as shown in figure 1. A closedcircuit television camera, which could be used both in ordinary light and in infrared light, was placed about 150 cm behind the infant's head. The infrared light was provided by a specially constructed light source. This light was totally invisible for the human eye. The infants reaching activity was recorded on a Sony videorecorder.
Figure 1. The experimental situation. 2.1 Procedure The subject was placed lying on its back on a couch beneath the small bell as shown in figure 1. The distance from the eyes to the bell was between 11 and 13 cm. The larger distance was used for the larger babies. Four conditions were used. Each baby was tested with both pairs of spectacles in the dark and in the light. In the dark condition the only thing visible in the room was the bell, which was illuminated by a small spotlight placed behind the baby's head. The experiment started either with the convergence-increasing spectacles or with the convergence-decreasing spectacles. The spectacles were changed between each condition in order to prevent any adaptation to the prisms during the experiment. The first and the last condition were both either in the dark or in the light. The baby was allowed to make a maximum of three reaches in each condition. If this was not obtained within three minutes the next condition was introduced. 2.2 Subjects Ten infants ranging in age from eighteen weeks to thirty-two weeks served as subjects. 3 Results The infants performed a total of forty-two reaches in the experiment. The distribution of the reaches on the different conditions for each infant is shown in table 1. There was a large variation in the number of reaches performed by the infants in the experiment. Only two reached in all four conditions. Most infants became bored with the experiment after one or two conditions and just stopped reaching or protested loudly. As shown in table 1, the infants were more passive in the dark, especially when wearing the convergence-decreasing prisms. The object was obviously not attractive enough in this condition. The trajectories of the reaches were transcribed by hand from the video tape. The midpoint between the infant's thumb and index finger was used as a measure of the position of his hand. The tape was wound forward by hand and the position of the infant's hand was marked after every 100 ms interval. The position of the infant's head and eyes and the position of the real object were also marked on the plotting paper for each reach. As the distance from the infant's eyes to the real object was known, the position of the virtual object could then be calculated. The interocular distance was thereby assumed to be 49 mm (Davenport 1940).
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Table 1. The distribution of reaches in the different conditions of the experiment. (D is dark condition; L is light condition; N is convergence-increasing prisms, which make the object look nearer, F is convergence-decreasing prisms, which make the object look further away.) Subject
Sa Mo Mt Ta Mi Jo Le Re Ch Ry
Number of reaches FD
FL
ND NL
0 2 0 0 0 1 0 0 0 0
0 3 0 3 3 3 3 2 0 2
0 3 1 0 0 3 1 0 0 0
1 3 0 3 0 3 0 0 2 0
Age (weeks)
18 22 22 22 23 26 31 31 32 32
Convergenceincreasing prisms
Convergencedecreasing prisms
O
Sa, 18 weeks
Mi, 23 weeks
Mo, 22 weeks
' Mo, 22 weeks
o U
Jo, 26 weeks
Jo, 26 weeks
Figure 2. Virtual object reaches in the light, transcribed from videorecords. The time interval between each dot plotted is 100 ms. The real object is indicated by a circle with a solid outline, and the virtual object is indicated by a circle with a broken outline.
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The results are summarized in table 2 and examples of reaches are shown in figure 2. As can be seen from the table, thirty-four out of forty-two reaches were aimed at the virtual object. A large proportion of these, fifteen reaches in the light and seven reaches in the dark, were not corrected before getting to the virtual object. In some cases grasping movements at the place of the virtual object were also observed. With the convergence-increasing prisms, only one reach was not aimed at the virtual object. In this reach the hand went up beyond the real object, grasped in the air and returned. The number of reaches not directed at the virtual object was larger with the convergence-decreasing prisms. It should be noted, however, that only in five out of these eight reaches did the hand grasp the real object in the end and that only in three cases did the hand grasp the real object without first hitting it with some other part of the hand. There were nine symmetrical two-hand reaches in the experiment. Table 2. Frequencies of different kinds of reaches in the experiment (D, L, N, and F are as in table 1). Others
Virtual object reaches not corrected FL FD NL ND
5 0 1 6
corrected after getting corrected before getting to the virtual object to the virtual object 5 0 4 1
5 0 7 0
4 3 0 1
4 Discussion In the present experiment purely ballistic reaching was successfully distinguished from visually guided reaching. This held true for both the dark and the light condition. Visually guided corrections, if any, were introduced rather late and the corrections were often time-consuming. The result shows that binocular convergence plays an important role in the control of reaching behaviour in infancy. Although both accommodation and movement parallax specified the correct position of the object in the present situation, a majority of the reaches were clearly aimed at the position of the virtual object as specified by convergence. This is not to say that ballistic reaching behavior in infancy is determined by convergence alone. In a more structured visual environment, movement parallax is likely to be a very efficient source of depth information. The result also shows that the ballistic part of the reach is well aimed. This is in accordance with Bower and Wishart (1972), who showed that infants of the same age group would accurately reach for objects in a situation where the lights went out before the reach started. A precisely tuned perceptual-motor system for the control of ballistic arm movements is very functional indeed. It enables the infant to get the hand to the vicinity of the object being reached for before any corrections are needed. This means increased efficiency and reduced attentional load of the task. The objections raised against such a perceptual-motor system concern the rapid changes due to growth in the shape and position of the eyes and the size of the limbs and body (McDonnel 1975). A system for absolute distance perception that cannot accommodate such changes is, of course, useless. In the case of convergence, however, a closer analysis reveals that the different changes due to growth compensate one another. The growth of the arm is compensated for by the increase in the interocular distance and the decrease in the angular displacement of the foveas relative to the visual axes. Figure 3 shows how the convergence at arms length changes with age. Data on the growth of the arm were obtained from Davenport (1944) and on
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the growth of the interocular distance from Davenport (1940). The angular displacement of the fovea relative to the visual axis was calculated from Larsen (1971) and from Slater arid Finlay (1975a). As can be seen from figure 3, the convergence at arms length is approximately constant and nearly zero during the first year of life.
Age (years)
Figure 3. The binocular convergence at arms length for different age groups. Acknowledgements. Data for the present study were collected in Dr T G R Bower's laboratory at the University of Edinburgh. The author gratefully acknowledges the generous assistance and advice of Dr Bower, Jane Dunkeld, and Jennifer Wishart. The investigation was made possible by grants to the author from the Swedish Council for Social Science Research and by grants to Dr. Bower from the British Medical Research Council (MRC-grant G969-559C). References Bower T G R, 1974 Development in Infancy (San Francisco: Freeman) Bower T G R , Wishart J G, 1972 "The effects of motor skill on object permanence" Cognition 1 165-172 Davenport C B, 1940 "Post-natal development of the head" Proceedings of the American Philosophical Society'83 1-216 Davenport C B, 1944 "Post-natal development of the human extremities" Proceedings of the Americal Philosophical Society 88 375-456 Foley J M, Held R, 1972 "Visually directed pointing as a function of target distance, direction and available cues" Perception and Psychophysics 12 263-268 Hofsten C von, 1976 "The role of convergence in visual space perception" Vision Research 16 193-198 Johansson G, 1971 "Visual motion perception. A model for visual motion and space perception from changing proximal stimulation" Report 98, Department of Psychology, University of Uppsala, Uppsala, Sweden Johansson G, 1973 "Monocular movement parallax and near-space perception" Perception 2 135-146 Larsen J S, 1971 "The sagittal growth of the eye III. Ultrasonic measurement of the posterior segment (axial length of the vitreous) from birth to puberty" Acta Ophtalmologica 49 441 -453 McDonnel P, 1975 "The development of visually guided reaching" Perception and Psychophysics 18 181-185 Slater A M, Finlay J M, 1975a "The corneal-reflection technique and the visual preference method: sources of error" Journal of Experimental Child Psychology 20 240-247 Slater A M, Finlay J M, 1975b "Binocular fixation in the newborn baby" Journal of Experimental Child Psychology 20 248-273 Trevarthen C B, Ingersoll S M, 1970 "Coordination of eye and hand in human infants" in California Institute of Technology: Biology Annual Report page 182 White B L, Castle P, Held R, 1964 "Observations on the development of visually-directed reaching" Child Development 35 349-364
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©1977 a Pion publication printed in Great Britain
