As noted above, Legge et al. (
2001) postulated that reading rate is limited by the “visual span,” operationally defined in terms of flanked letter identification. They mapped the identifiability of a triplet consisting of three random letters over the relevant part of the visual field. This “visual span function” describes proportion correct as a function of letter position in the visual field for text of a given size, spacing, and vertical eccentricity. They noted the similarity of their triplet test to Bouma's crowding test but suggested that the triplet performance might be limited by acuity, crowding, masking, or mislocalization (
Footnote 2). In fact,
Figures 6 and
8 rule out all the alternatives, showing that the triplet performance they measured is limited solely by crowding. Thus, the operationally defined “visual span” measures the number of uncrowded character positions in a line of text at a given spacing and vertical eccentricity (centered on the vertical midline). We call this the uncrowded span
u and write the conjecture as
where
r is the reading rate (characters per second) and
ρ is a proportionality constant with a value on the order of 10 Hz. (As a mnemonic, think of
ρ as the rate of glimpses and
u as the number of letters harvested per glimpse.) We define
r as characters per second, but we measure and report the traditional word/min. For English text, with an average of five letters plus a space per word, 1 word/min equals 0.1 character per second. For casual reading of a static page, typical values might be 280 word/min (i.e.,
r = 28 character/s), a
ρ of 4 Hz, and a span of 7. For central RSVP reading, participants striving to read as quickly as possible reach a rate of 910 word/min (i.e.,
r = 91 character/s) with a
ρ of 13 Hz and a span of 7. (We return to this comparison in the
Discussion section.) The uncrowded span
u depends solely on the spacing, the critical spacing constant
b, and the eccentricity. In writing
Equation 3, one anticipates that the proportionality constant
ρ will be independent of most experimental variables. Its variation among observers and with text difficulty was expected, but it is surprising that
ρ falls with increasing eccentricity.