Influence of missed catch trials on the visual field in normal subjects.

A total of 20 healthy individuals purposely missed an increasing number of individual catch trial questions (false positive or negative errors or fixation losses) when tested on the Humphrey Field Analyzer to determine the effect on the normal visual field. As determined by Statpac, the global indices and probability maps became significantly altered from those for the control fields at a prevalence of 20% for false negatives and 33% for fixation losses and false positives. However, the perimeter's recorded prevalence of missed catch trials showed a wide distribution from the percentage purposely missed. A high prevalence of missed catch trials was also indicated by a greater than normal mean defect and number of questions asked. This study suggests that although the number of missed catch trials are often recorded inaccurately, they help to identify unreliable normal visual fields, as do the mean defect and the number of questions asked.