Effects of video display character size, clarity, and color on P-300 latency.

The P-300 is a positive polarity electroencephalographic (EEG) signal occurring about 300 ms after a surprising, novel, or unique stimulus. The amplitude of the P-300 is dependent on the degree of attention and importance the subject has assigned to the stimulus and on its relative novelty. P-300 latency is dependent on the time it takes the subject to mentally process the stimulus and determine if the information it carries is novel. In Experiment I, the latencies of P-300's for 40 and 80 column size video display characters were compared. The effect of dioptric blur (mean add power +2.44 D) on latency was also determined. It was found that 40 column size characters could be interpreted more quickly than 80 column size characters and that blur increased the processing time equally for both character sizes. In Experiment II, P-300 latencies for green and amber display screens were compared. Display screen color did not significantly affect latency, neither did the addition of lens powers ranging from a mean of -1.15 D to a mean of +1.15 D over the subjects' habitual lenses. However, it was found that latencies significantly increased when a mean add of +2.29 D for the green display or a mean add of +2.57 D for the amber display were used. (Chromatic aberration accounted for the difference in lens power for the green vs. amber display.) These results demonstrate that character size and clarity are significant factors in determining the time required to mentally process a stimulus. They also demonstrate that there is no clear advantage to using an amber vs. a green screen when speed of mental processing (as measured by P-300 latency) is used as a criterion.