Visual acuity is better for letters in rows than in columns

Abstract

The spatial resolving capacity of the visual system is usually measured with charts containing letters of different sizes displayed in horizontal rows or, less often, in vertical columns. Although previous studies suggest that acuity may not be equal for these two arrangements^1–7 it is implicitly assumed in clinical tests that orientation of an array of letters is of no consequence to the measurement. The work reported here shows that this assumption is incorrect. While undergoing an examination in an eye clinic, I noticed that letters of a given size seemed more difficult to identify when they were arranged in vertical compared with horizontal arrays. Subsequently, I tested a group of subjects using letters presented pseudo-randomly with different inter-letter spacings, in rows or columns. Subjects were required to identify letters from left to right or from top to bottom in each isolated row or column. A significantly higher number of errors was made for letters in columns than for those in rows. The effect does not depend on order or direction of letter presentation, and it is not found when acuity determinations are made using a non-letter test symbol. To explore the conjecture that the effect could be associated with previous experience, two additional groups of subjects were tested. First, bilingual Chinese, whose first language is read in both horizontal and vertical arrays, were tested using Chinese characters and English letters. Second, acuity determinations were made with young children who could identify letters but did not read. In each case, acuity was not significantly different for horizontal or vertical presentations, a finding consistent with the notion that learning may influence the development of visual resolution.