Heritability in visual-geometric illusions: a family study

Transcription

1 Perception, 1979, volume 8, pages Heritability in visual-geometric illusions: a family study Stanley Coren Department of Psychology, University of British Columbia, Vancouver, British Columbia, V6T 1W5 Canada Clare Porac Department of Psychology, University of Victoria, Victoria, British Columbia, V8W 2Y2, Canada Received 20 November 1978, in revised form 12 February 1979 Abstract. The Miiller-Lyer and Ebbinghaus illusions were tested in 203 mother-father-offspring triads and 303 sibling pairs. Significant familial resemblances were found in all but the overestimated segment of the Ebbinghaus illusion. These results suggest that responses to visual-geometric illusions, mediated by optical or neural interactive mechanisms, show patterns of familial resemblance which may be based upon heritable factors. 1 Introduction For many years research has been conducted on individual differences in response to visual-geometric illusions. It is argued that knowledge of the factors that cause individuals to differ from, or to resemble, each other in their responses to an illusory stimulus lead to an understanding of the mechanisms which engender these perceptual phenomena. The two most commonly investigated dimensions of individual differences are age (see Coren and Girgus 1978a; Pick and Pick 1970 for reviews) and culture or environment (Berry 1971; Deregowski 1973; Segall et al 1966; Witkin 1967). The rationale for cross-cultural studies is somewhat interesting. When the differences observed between cultures are greater than those seen within a culture, it is argued that the observed differences are due to differential experience, or environment, which play a role in determining responses to geometric illusions. There is another technique, the family study, which uses a similar rationale in an attempt to isolate genetic components in a given trait. Here, one assesses whether the variability between families is greater than the observed variability within families. When such a condition is observed, it is argued that this may be due to shared genes among family members. Although the family-study technique has been widely employed in the investigation of spatial cognitive skills (Bock and Kolakowski 1973; Hartlage 1970; Stafford 1971; Yen 1975), very few studies have addressed themselves to familial resemblances in visual illusions (Smith 1953). This is unfortunate, since there are reasons why one might expect family members to show similar patterns of illusion susceptibility for some figures but not for others. For example, consider the Muller-Lyer and the Ebbinghaus configurations, which are shown in figure 1. There is evidence which suggests that these two illusions represent distinct classes of illusory distortion (Coren et al 1976). There is even evidence which suggests that the under- and overestimation components of each of these illusions may also be controlled by different factors (Coren and Porac 1978a; Coren et al 1978; Sekuler and Erlebacher 1971). One can predict a genetic component for the Miiller-Lyer illusion, while such a prediction does not easily emerge for the Ebbinghaus. Let us see how this comes about. The Muller-Lyer illusion is typical of many intersecting-line configurations in that a number of optical and neural mechanisms have been shown to play a role in its formation (Coren and Girgus 1978b). These include image blur caused by optical

2 304 S Coren, C Porac aberrations in the eye (Chiang 1968; Coren 1969; Coren et al 1978; Ward and Coren 1976), lateral inhibitory interactions on the retina (von Bekesy 1967; Coren 1970; Ganz 1966), and light scatter in the eye due to pigmentation differences (Coren and Porac 1978b; Pollack 1976). There is some evidence which suggests that such peripheral processes may account for as much as 65% of the observed illusion magnitude in the Muller-Lyer configuration (Girgus et al 1975). The importance of these mechanisms, in light of the paradigm which we are considering here, is that there is also evidence that several of these same factors contain heritable components which could promote a high degree of similarity between family members (Francois et al 1973; Sorsby 1970). The refractive ability of the eye, which of course affects the amount of image blur, has been shown to have definite genetic components (Francois 1961; Sorsby 1970; Sorsby et al 1962). In a similar manner, ocular pigmentation, which in turn affects light scatter within the eye, also has a heritable component (Bleehan 1975; Fialkow et al 1977; Matheny and Dolan 1975; Sorsby 1970). While no direct evidence for genetic factors in lateral inhibition exists, there is some evidence that neural responsiveness of the retina, and some brightness phenomena which may have lateral inhibitory components, do show evidence of heritability (Eysenck and Prell 1951; Francois 1961; Hill et al 1974; Schwartz et al 1974; Stanescu and Michiels 1976; Waardenburg et al 1961). Given the fact that several of the peripheral mechanisms which have been implicated in the formation of the Muller-Lyer illusion also show evidence of genetic transmission, one can extrapolate and suggest that there may also be heritability in the response to this intersecting-line illusion. For the Ebbinghaus illusion, the situation is somewhat more complex. The distortion observed in this configuration is most often explained by the operation of cognitive mechanisms (Coren 1971; Coren et al 1976; Coren and Miller 1974). In fact, an attempt to find direct evidence for the involvement of image blur in the Ebbinghaus illusion has produced negative results (Coren et al 1978). Other work indicates that light scatter due to light ocular pigmentation does not affect the magnitude of this illusion either (Coren and Porac 1978b). As for lateral inhibitory contribution to this effect, the situation is somewhat more ambiguous. Cooper and Weintraub (1970), Jaeger and Pollack (1977), and Motokawa (1970) have suggested such neural involvement. These theoretical formulations seem to predict the underestimation effects rather well, although their ability to predict the overestimation distortion is much poorer. Coren and Girgus (1978a) present an analysis which suggests that lateral inhibition models probably cannot adequately explain such overestimation effects. Thus it seems that if there are heritable components in the Ebbinghaus illusion they would most likely be manifest in the underestimated segment of the illusion where more peripheral factors, which have been shown to have heritable Figure 1. (a) The overestimated segment of the Muller-Lyer illusion; (b) the underestimated segment of the Muller-Lyer; (c) the overestimated segment of the Ebbinghaus illusion; (d) the underestimated segment of the Ebbinghaus.

3 Genetics of visual illusions 305 components, might operate. On the other hand, if higher-level spatial abilities are also heritable, as some evidence suggests (i.e., Bock and Kolakowski 1973), we might also find resemblances among relatives for both segments of the Ebbinghaus illusion. In order to determine whether individual differences in illusion magnitude are partially explicable in terms of genetic factors affecting illusion-forming mechanisms, the following family study was conducted. 2 Method 2.1 Stimuli. The Miiller-Lyer and Ebbinghaus illusions shown in figure 1 served as stimuli. The over- and underestimation portions of each configuration were presented separately. In each Muller-Lyer segment, the length of the horizontal shaft was 8 cm, the length of the wings was 2 cm, and the wings were oriented at 45 from the horizontal extent. For both portions of the Ebbinghaus illusion, the diameter of the central circle was 14 mm. Each center circle was surrounded by four illusion-inducing circles. For the underestimated half of the illusion, the large inducing circles had a diameter of 23 mm, while for the overestimated portion the small inducing circles measured 5 mm in diameter. Separation between the central and inducing circles was 3 mrn. In both configurations, the lines were 1 mm wide and were drawn in black (4% reflectance) on a white (85% reflectance) background. 2.2 Subjects The sample consisted of 203 triads of mother, father, and one offspring. In addition, 303 sibling pairs were used. In order to minimize environmental contributions to between-family differences, the sample was restricted to a reasonably homogeneous socioeconomic level. The subjects were families of the faculty and the professional staff of the University of British Columbia. All were Caucasian, and had been born in Western Europe or North America. Biological relatedness was established by means of self-report. 2.3 Procedure To obtain measures of illusion magnitude, each family member made separate judgments of the over- and the underestimated portions of the two illusions. When judging the length of the horizontal shaft of the Muller-Lyer illusion, each observer selected a line which appeared to be of the same length as the test extent. This line was selected from a graded series of line lengths which ranged in length from 6 1 cm to 10-1 cm in 2 mm steps. In an analogous fashion, each observer judged the size of the central circles in the Ebbinghaus illusion by selecting a comparison circle which appeared to be equal in size from a graded series of circles. This series consisted of circles which ranged in diameter from 11 mm to 16 mm in 0-5 mm steps. This method of illusion measurement has been shown to be as reliable as direct adjustment procedures (Coren and Girgus 1972). 3 Results and discussion When we are dealing with a graded character which is caused by the action of several genes, the degree of resemblance expected between relatives can be represented by a simple mathematical expression,.subject to certain assumptions. These assumptions are: (i) the trait is completely under genetic determination; (ii) there is intermediate or graded inheritance, rather than dominance and recessiveness; (iii) there is random mating; (iv) the effect of the genes is perfectly additive and not modified by other genes. If these criteria are met, the expected correlation between relatives becomes the geometric mean of the regression coefficient of the score measured in relative A regressed upon the value obtained from relative B and the regression of B upon A (Falconer 1964).

4 306 S Coren, C Porac Table 1 shows the correlation coefficients expected for various familial relationships, based upon the proportion of genes in common, if we are dealing with a genetically transmitted trait. Violations of these assumptions will affect the actual correlations obtained in predictable ways as we shall discuss later. To obtain measures of familial resemblance Pearson product-moment correlations between family members were computed. In order to control for possible biases due to family size, each family was represented only once in each correlation. Thus, where there are two or more offspring, the parent-offspring correlation is based upon the parent and one randomly selected child. In addition, the correlation between the midparent value (the mean of the observed trait in the two parents) and one offspring was also computed. Although intraclass correlations are often used to assess the degree of resemblance between family members, the Pearson product-moment correlation coefficient is equivalent to this statistic when the number of pairs is large (Loehlin et al 1975; Wright 1969). In addition, the product-moment coefficient allows one to control for some variables known to be correlated with illusion magnitude through partial-correlation techniques. For instance a nongenetic factor which influences illusion response [hence violates assumption (i) above] is age. It is well-known that both the Miiller-Lyer and the Ebbinghaus distortions show age-related changes in illusion magnitude (Coren and Girgus 1976, 1978b; Coren and Porac 1978a; Piaget 1969; Pollack 1964). The existence of such age trends could artificially inflate some of the familial correlations since siblings tend to be contemporaneous, and measurements taken from older parents will tend to be associated with measurements taken from older offspring. The present analyses were carried out by using partial correlations which removed the effect of the age of each family member. The results of this analysis are shown in table 2, which presents a breakdown of the obtained correlations between specific familial pairs as a function of illusion configuration. When interpreting a set of family correlations, such as these, the obtained values may differ widely from the theoretical values in table 1 owing to violations of any of the assumptions outlined above. For example, any degree of dominance and recessiveness will tend to lower the correlations obtained. Conversely, since family members live or have lived together, some correlations (particularly those between siblings) may be increased by the effect of shared environment. This could be due to physiological factors arising as a result of common nutritional history, for example, or of common experiences which may affect cognitive style. Because of this, the most useful procedure is to consider the obtained pattern of correlations. Within this context, the strongest evidence for a genetic component in any trait would be significant parentoffspring correlations and significant sibling correlations, all of which are of approximately the same magnitude. Inspection of table 2 reveals that the underestimated segment of the Muller-Lyer illusion (figure lb) shows the expected pattern (with the exception of the father-son correlation, which is not significant). The fact that two out of the three sibling Table 1. Expected correlations between family members. Relationship Theoretical proportion of Theoretical correlation genes in common coefficient Midparent-child Parent-child Siblings Spouses (1 x0-5) 1/2 =0*7 (0-5 x0-5) 1/2 = 0-5 (0-5 x0-5) 1/2 = 0-5 (0x0/ /2 = 0-0 a Mean of the two parents.

5 Genetics of visual illusions 307 Table 2. Correlations of illusion magnitude respondents removed). Relationship Midparent - offspring Mother-daughter Mother-son Father-daughter Father-son Daughter -daughter Son-son Son-daughter Mother-father *p<0-05; **p<0-01 I. Pairs Miiller-Lyer illusion underestimated 0-198** 0-252** 0-201* 0-199* ** 0-361** 0-192* among family members (with the effect of age of both overestimated 0-160** 0-194* 0-254** * Ebbinghaus illu: sion underestimated 0-116* ** 0-222* overestimated correlations are larger than the parent-offspring correlations may also suggest some environmental contribution. For the other illusions the situation is quite different. The overestimation illusion in the Ebbinghaus configuration shows no evidence for familial similarities at all. The overestimated segment of the Muller-Lyer and the underestimated segment of the Ebbinghaus illusions are more ambiguous. For the overestimated portion of the Miiller-Lyer, both of the mother-offspring correlations are significant, while only the daughter-daughter correlation shows a significant relationship among siblings. While this could suggest some sex-linked genetic transmission, the evidence for heritability here is not as strong as that for the underestimated segment of the illusion. The underestimated segment of the Ebbinghaus illusion shows only two significant familial relationships. These are the cross-sex correlations of mother-son and father-daughter. Although similar correlational patterns have been taken as suggesting possible genetic transmission in some complex spatial tasks (Bock and Kolakowski 1973; Yen 1975), in the absence of significant sibling correlations such a conclusion seems to be unwarranted for these data. There is an alternate way of assessing whether these data are consistent with any notion of genetic transmission of illusion magnitude. This involves the midparent value (which is the mean of the two parential values). Quantitative geneticists often use the regression of offspring on the midparent to indicate the upper limit on any possible heritable components within a phenotype (Falconer 1964; McClearn and DeFries 1973). This value can be directly related to the correlation coefficient as indicated in table 1. When one looks at the midparent-offspring correlations, significant values are found for both segments of the Muller-Lyer, and a somewhat weaker, but significant, correlation is found for the underestimated segment of the Ebbinghaus illusion. It thus becomes clear that parents and offspring do resemble one another in terms of their responses to the Muller-Lyer illusion and also in terms of their responses to the underestimation distortion in the Ebbinghaus illusion. The results of this experiment are theoretically important, since considerations of the mechanisms involved in the formation of illusory percepts suggest that we might explain some individual differences in illusory response on the basis of familial membership. Although further work must be done to establish the reasons for these farhilial similarities, the present results suggest the possibility of genetic transmission of some factors associated with the formation of visual-geometric illusions.

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