LOW FUSION CONVERGENCE AS A FACTOR IN READING DISABILITY T H O M A S HARRISON E A M E S

WEST SOMERVILLE, MASSACHUSETTS

This article presents a study of the amplitude of fusion convergence for letters of three different sizes, ranging from 8 mm. gothic to primer size. A group of 88 reading-disability cases is compared statistically with a control group of 52 cases which were taken at random from grades II through IV in a public school, the range in age of the two groups being approximately equal. The physiological basis of fusion is considered very briefly and the method of measuring the amplitude of fusion convergence by the Wells method is outlined.

According to Verhoeff1, three images are involved in binocular single vision: two monocular images, one arising from either eye, and a third or binocu­lar image resulting from fusion of the other two. In order that this may occur, it is necessary that the light images formed in either eye should fall on cor­responding retinal areas, which are sup­posed to have a point-to-point represen­tation on the cerebral cortex in the cal-carine area. Each fovea is believed to have connections with both hemi­spheres, affording a double representa­tion2. If the monocular images fall on noncorresponding points, it is evident that fusion of the two will be imperfect or absent, depending on the separation of the stimulated areas.

This is particularly important in reading-disability cases, for imperfect fusion of letter forms may produce both letterlike and unfamiliar combinations of characters, which tend to confuse the child and to retard his progress in learn­ing to read. This was described in an

earlier study3 which demonstrated the importance of exophoria in cases of ■ difficult reading.

The present study was made for the purpose of determining whether or not fusional dysfunction is an important factor in reading disability. The mea­sure of fusion chosen for observation was the amplitude of fusion converg­ence, which was measured by means of the phorometer-stereoscope and the familiar divided-word charts (ON NE) . Prism power, base out, was slowly and evenly introduced before each eye until the N's fused and the letters appeared to form the word ONE. This amount (A) was noted, and then higher prism power was introduced gradually until the word ONE started to jumble or un­til a portion of it suddenly disappeared. This point (B) was noted, and the measurement of the amplitude of fusion convergence was regarded as B minus A4.

It was thought that fusion might vary according to the size of the test

Table

Size of Type.

8 mm.

5 mm.

Primer

Unselected (control) Group.

Median 20.333 Qi 16.333 Q3 24.400

Median 14.200 Qi 10.300 Q3 18.000

Median 8.545 Qi 6.333 Q3 10.714

Reading-Disability

Group.

17.750 10.250 25.000

11.000 6.333

17.000

5.250 0.733 8.250

Difference in Medians.

2.583

3.200

3.295

Above figures in prism diopters of fusion convergence as measured by the Wells Method with the phorometer-stereoscope.

709

710 THOMAS HARRISON EAMES

letters used, and that children who rated normally on the ordinary Wells I Series (ON NE) might fail rather sig­nificantly when faced with the smaller type such as is used in school. Accord­ingly a new set of divided-word charts was prepared in three sizes; the first being of plain gothic type, 8 mm. in height; the second, 5 mm. in height; and the third, of the size and style of type used in primers. The separation of the centers of the N's was 6 cm. on each of the three test cards. Other separa­tions were prepared in each size for ex­perimentation in stereoscopic develop­ment of fusion convergence should this be indicated.

A group of eighty-eight reading dis­ability cases was compared with an un-selected or control group of fifty-two cases taken at random from grades II, III , and IV in a public school5, the age range of both groups being approxi­mately equal. The median amplitude of fusion convergence was found to dimin­ish with the size of the test letters in both groups, but the reading disability group presented a lower median for each of the three letter sizes, and the difference between the medians of the two groups increased as the size of the letters decreased. More than three fourths of the reading-disability cases fell below the level of the control-group

median in the distribution of test results for the size letters used in ordinary readers and primers used in the first four grades.

This indicates that the amplitude of fusion convergence may be expected to be lower among non readers, particu­larly for small letters, and that low fusion amplitude is a factor in reading disability. Fusion should be investi­gated in all cases of poor reading and when found inadequate, treatment should be instituted.

Summary 1. The median amplitude of fusion

convergence is lower among nonread-ers than among unselected cases.

2. The reading-disability g r o u p shows a greater percentage of cases below the level of the median of the unselected group in all three sizes of type used in the study.

3. The smaller the type the lower the amplitude of fusion convergence in both groups, but the percentage of reading-disability cases falling below the level of the median of the unselected group, increases as the size of the letters di­minish.

4. The amplitude of fusion converg­ence is an important factor in reading disability.

31 Chester Street.

References 1 Verhoeff, F. H. Binocular perspective. Amer. Jour. Physiol. Optics, v. 6, no. 3. 2 Troland, L. T. Visual phenomena and their stimulus correlations. Found. Exp.

Psych., 1929, p. 211. 3 Eames, T. H. A comparison of the ocular characteristics of unselected and reading

disability groups. Jour. Educ. Res., 1932 (March), v. 25, p. 211. *The sterescopic development of the fusion faculty. American Optical Co., 1928, pp. 31-32. 5 Unselected cases were obtained through the cooperation of Superintendent Frank A.

Scott, Miss Eleanor Hayes, and Mr. Rush of the Belmont, Massachusetts, School De­partment.