Cite as:
Scharff L F V, Ahumada A J, 2005, "Understanding text polarity effects" Perception 34 ECVP Abstract Supplement

Understanding text polarity effects

L F V Scharff, A J Ahumada

Scharff and Ahumada (2002 Journal of Vision 2 653 - 666) measured paragraph readability and letter identification for light text and dark text. For all tasks and conditions, responses to light text were slower and less accurate. Repetition of the letter-identification task on a single well-calibrated monitor has demonstrated that the result is not an artifact of apparatus. One potential explanation of the polarity effect is that it results from sensitivity and resolution differences between the ON and OFF pathways that differentiate in the retina. Another possibility is that the polarity effect is the result of increased experience with dark text on light backgrounds. To distinguish between these alternatives, we tried to separate the polarity of the contrast signal from the polarity of the letter by using a pedestal only slightly larger than the letters. The positive letters were placed on a negative pedestal so that the letter was at zero contrast with respect to the large background, but had positive contrast with respect to the local surround. Similarly, negative contrast letters were placed on a positive pedestal. If the physiological hypothesis is correct, the polarity of the pedestal, rather than the polarity of the letters, should control the performance. We presented randomised blocks of all combinations of three contrast levels and two polarities, both with and without a pedestal for the same 12 letters we used earlier. The task was to identify the presented letter as quickly as possible by typing it on a standard keyboard. The results without the pedestal replicated our earlier results. With pedestals, the polarity difference reversed with respect to text polarity. Negative pedestals (positive letters) led to faster and more accurate responses, thus supporting the physiological hypothesis.

[Supported by the HMP Project of NASA's Airspace Systems Program.]

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