WHY LOW CONTRAST LETTERS ARE DIFFICULT TO RECOGNIZE

Title WHY LOW CONTRAST LETTERS ARE DIFFICULT TO RECOGNIZE
Author, Co-Author Lei Liu, Hillary Gauthier
Topic
Year
2004
Day
Program Number
Poster 6
Room
Affiliation
Lighthouse International
Abstract PURPOSE: Low contrast letter charts are widely used in optometry practice and research. However, little is known about how low contrast limits correct recognition of letter-like stimuli.

METHODS: Twelve characters, which were morphs of a block-E letter, were created. They all had the 3 horizontal bars of the E, and differed in the way the 4 end gaps were sealed. For example, a square "5" was created by sealing the upper left and the lower right end gaps, and a square "8" was created by sealing all 4 end gaps. The stroke width was 10’. Each character was 50'x50' in size. Contrast thresholds for detecting and recognizing these characters were measured for 100, 200 and 1500 msec stimulus durations. Five normal subjects (23-49 yrs) participated in the study. Informed consents were obtained.

RESULTS: Contrast threshold for detecting characters increased with decreasing duration. At the 3 durations tested, contrast thresholds for recognition were 3 to 3.5 times of those for detection. An analysis of character confusions ruled out feature inhibition as the cause of observed errors, because when an error occurred, the subject was just as likely to "gain" a non-existing gap seal as he/she was to "lose" an existing one. The causes of errors were different for long and short durations. For stimulus duration of 1500 msec, the stimulus, though very visible, did not appear stable. It changed appearance 2 to 3 times during the 1500 msec, each time appearing as a legitimate character. Selecting one of these characters as the response could result in error. Such instability diminished when contrast increased. When duration was short, there seemed to be not enough time to pick up all the information about a low-contrast stimulus. The low contrast might have restricted the attention window to such a degree that the subject could inspect only part of the stimulus when shifting attention or initiating a saccade was impossible.

CONCLUSIONS: Perceptual instability and restricted attention window may contribute to the failure to recognize low-contrast suprathreshold letters, though they may operate on different time scales.
Affiliation of Co-Authors Lighthouse International
Outline