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Perea, M. & Gomez, P. (2012) Subtle increases in interletter spacing facilitate the encoding of words during normal reading.PLoS ONE 7(10): e47568. doi:10.1371/journal.pone.0047568
— They only measured eye movements. No comprehension accuracy. Disappointing. However, it’s data does point towards possibly refuting the significance one of our oldest adages: “Any man who would letterspace blackletter would steal sheep.” ~Goudy, F. (1936).
Bayer, M. Sommer, S. and Schacht, A. (2012). Font size matters—Emotion and attention in cortical responses to written words. PLoS ONE, 7(5): e36042. 1–6. doi:10.1371/journal.pone.0036042