Methodological issues in the assessment of achromatic retinal thresholds with the B.C.U.P.

Abstract

Two sources of error - subject and instrumental, which are inherent in the data collected, can affect the validity and reliability of achromatic retinal thresholds in neurophthalmological research. These errors depend on the numbers of measurements, with increases in trials or subjects increasing the degree of confidence and accuracy. Errors from instrumental calibration and pupil diameter estimations are independent of wavelength but those from filter band widths are wavelength dependent - least for yellow, more for red and most for blue filters. At best an accuracy of between 0.1 and 0.2 log quanta can be expected. Abbreviated (eight filter) testing is valid for the mid-range of the visible spectrum, but variance increases for blue stimuli. Smoothing the data does not infringe on its integrity and this can be used to indicate the probable outline of the extended testing curve. When background luminance is high, difficulties in differentiating between achromatic and chromatic stimuli seem to be a problem of attention fluctuating between colour and flicker among younger subjects, whereas in older subjects this cognitive confusion is due to losses in sensitivity for blue and/or decreased flicker fusion rate.

Key Words:

• achromatic retinal thresholds
• sources of error
• validity and reliability of measures
• cognitive confusion
• attentional fluctuations