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, June 2003, Pages
Early binocular vision in human infants: limitations on the generality of the Superposition Hypothesis, ,
The Ohio State University, College of Optometry, 320 W 10th Avenue, Columbus, OH , USAThe Superposition Hypothesis states that the binocular vision of newborn infants blends together the monocular visual responses of the two eyes, even when the visual stimulus evokes binocular rivalry in adults. According to the Superposition Hypothesis, this blending is replaced by binocular rivalry after the emergence of stereopsis [Neonate Cognition: Beyond the Blooming, Buzzing Confusion, Erlbaum, Hillsdale, NJ, 1985, p. 37; Early Visual Development Normal and Abnormal, Oxford University Press, New York, 1993, p. 201]. The main evidence for the Superposition Hypothesis is a preferential looking experiment [Vision Res., 26 (3) (], in which 8&12-week-old infants fixated a rivalrous (for adults) dichoptic plaid, in preference over a fusible grating of parallel lines. This report describes our attempt to repeat that important experiment. Infant stereopsis emerged at 8.6 weeks under our conditions, but infants did not preferentially fixate the dichoptic plaid at any age between age 5 and 16 weeks. Control experiments showed that our result was not due to technical differences between their experiment and ours (red/green vs. polarizing glasses, the use of a fixation point, or the infant observation apparatus). Therefore, blending of the visual responses to rivalrous (for adults) stimuli is not a general feature of the pre-stereoptic infant visual system.1. IntroductionSeveral groups of investigators have studied binocular vision in infants (; ; ; ; ), yet surprisingly little is known about why stereopsis emerges when it does. One seminal hypothesis on the topic (; ) suggests that the binocular vision of newborn infants is qualitatively unlike that of adults. Whereas adult vision exhibits binocular rivalry when one eye is presented with vertical stripes and the other with horizontal stripes in the same position in the visual field, the hypothesis of
states that neonatal vision blends together the two eyes& responses to those same stimuli. The resulting blended percept should be similar to what would occur if the dichoptically presented stimuli were superimposed and presented simultaneously to one or both eyes. Their hypothesis (henceforth, the &Superposition Hypothesis&) states that the developmental milestone that underlies the emergence of stereopsis is the sorting out of these visual responses into distinct right-eye and left-eye signals in the primary visual cortex. These signals can then serve as the basis of stereopsis at a higher brain site, presumably including the infero-temporal and medial-temporal cortex (). The Superposition Hypothesis is important because it suggests a behaviorally observable correlate of the postnatal maturation of visual cortex. For example, one might consider using a clinical test based on the Superposition Hypothesis to find out whether a particular infant is still in his or her critical period before planning treatment for amblyopia.1.1. Psychophysical evidenceThe Superposition Hypothesis was tested by the following simple preferential looking experiment (). If a normal adult views a dichoptic stimulus consisting of a vertical grating presented to the right eye and a horizontal grating presented to the left, a rivalrous percept results (). Shimojo et al. studied infant preferential looking using rivalrous dichoptic grating stimuli over the period from 2 months, at which age infants generally do not have stereopsis, to age 6 months, when most infants do have stereopsis. When older infants viewed a dichoptic, rivalrous (for adults) stimulus on one side of the stimulus display, and a binocularly fusible (for adults) horizontal grating seen by both eyes on the other side, the older infants preferred to look at the fusible single grating ( shows our versions of these stimuli). This was not surprising. What was surprising was that when infants younger than about age 14 weeks viewed the same two gratings, they preferred to look at the dichoptic pattern, which was rivalrous to adults. The Superposition Hypothesis explains this unexpected preference by positing that the dichoptic gratings (which are rivalrous to adults) look like a perceptually blended stationary plaid pattern similar to an old-fashioned checked tablecloth. It is well-known (e.g., ) that young infants prefer to look at more complex stimuli over simpler ones, and the Superposition Hypothesis says that infants prefer to look at the dichoptic pattern because it is more complex. A subsequent study () showed a correlation between the age of first stereopsis and the age at which infants started to prefer to fixate the fusible pattern over the dichoptic pattern, which is rivalrous to the adult observer.Fig. 1. Examples of stimuli. (A,B) Stimulus type 1; (C,D) stimulus type 2; (E,F) stimulus type 3. (G) A Julesz-style stereogram used in Experiment III. Stimuli were viewed through red/green stereo glasses, so red stimulus regions were seen by the right eye, green regions were seen by the left eye, and yellow regions were seen by both eyes. has suggested that physiological summation of the signals arising in the two eyes is not required by the psychophysical results of Shimojo et al. Experiments with human adults indicate that binocular fusion occurs when the hemi-stereograms are high in spatial frequency and low in contrast (), especially when the duty cycle is large (thin stripes against a background) (). If infants were to show perceptual fusion of dichoptic stimuli during the pre-stereoptic period, it might be because even stimuli at low spatial frequency and 100% contrast are only a few times detection threshold at that age. This argument suggests that the two concepts that make up the Superposition Hypothesis, perceptual fusion inferred from psychophysical experiments and the hypothesized physiological summation of the signals originating in the right and left eyes, should be considered separately.1.2. Binocular fusion in infancyAside from the studies of Shimojo and his colleagues (; ) and , there is little clear evidence that binocular fusion precedes the emergence of stereopsis.
compared the emergence of binocular fusion of random dot correlograms and stereopsis of random dot stereograms using psychophysical and visually evoked potential (VEP) techniques. The proportion of infants showing both types of binocular interaction were entirely similar when measured using each technique (compare Figs. 3 and 4 of Birch and Petrig to each other and to , Figs. 2&4). These two studies knit together nicely the results of previous studies in which each of the four types of data were collected separately (VEP, fusion: (; ; ); VEP, stereopsis: (); psychophysics, stereopsis: (; ); psychophysics, fusion: (; )). Recall that the Superposition Hypothesis predicts that nonselective combination, in which the incoming visual signals are simply added together point-by-point throughout the region of binocular overlap, should be the native state of infant binocular vision. Stereopsis should emerge well after birth, after the incoming monocular signals are sorted out. The close agreement between the time courses of maturation of binocular fusion (indicated by VEP response to, or a behavioral preference for, stimuli modulated in their correlation between the right and left-eye stimuli) and stereopsis (indicated by VEP response to, or a behavioral preference for, stimuli modulated in their amount of horizontal binocular disparity) is not easily reconciled with the Superposition Hypothesis.
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No articles found.Impact oscillations with multiple modes between a pantograph and an overhead rigid conductor line in a railway current collection
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1Department of Mechanical Engineering, Fukui University of Technology, Japan
2Department of Mechanical Engineering, Keio University, Japan
Kiyotaka Yamashita, Department of Mechanical Engineering, Fukui University of Technology, 3-6-1 Gakuen, Fukuishi, Fukui 910-0028,
Japan Email: yamashita{at}fukui-ut.ac.jp
A theoretical and experimental investigation was conducted into impact oscillations between a pantograph and an overhead rigid
conductor line in a railway current system. The contact problem was modeled as impact oscillations of an intermediate spring-supported
beam excited by an oscillating plate. First, the impact oscillations of a beam with a sinusoidal vibrating plate were theoretically
examined. The impact relations are derived considering the effects of multiple vibration modes. Using some of the concepts
behind periodic oscillations, qualitatively distinct impact oscillations of the beam with multiple modes are clarified. The
interactions between the lowest two modes during the impacts are also discussed. Moreover, we conducted an experiment using
a similar model based on the analytical model. Qualitative agreement was demonstrated between the experimental and theoretical
results. Some experimental results reveal specific impact oscillations which correspond to the theoretical results considering
the interactions between the lowest two vibration modes.
Received April 2, 2010.
Accepted July 10, 2010.
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