In the Lanthony D-15 test, a subject is shown is a color arrangement consisting of 15 color caps that form a color circle covering the visual spectrum. The subject is asked to select the cap that is closest in hue to a reference cap. The patient places caps in the tray in an orderly transition of hue. The caps are designed so that a person with normal color vision or a mild color-vision deficit will arrange the caps in a perfect color circle. The test does not indicate the degree of color deficiency other than to separate those with normal color vision and mild congenital color blindness from patients with moderate to severe color deficits. It can distinguish significant defects, particularly if a person is screened prior to exposure to a toxicant.
Audiometry measures the ability to discriminate pure tones (500, 1000, 2000, 4000, 6000, and 8000 Hz) presented at 5-dB intervals through a headset. The test takes about 10 minutes to complete and requires concentration on the part of the subjects to distinguish between tones just above and just below the threshold of detection. In the early stages, exposure selectively affects the ability to detect high-frequency sounds (4000, 6000, and 8000 Hz). After continuing exposure, speech frequency sounds (500–2000 Hz) may also be affected.
Audiometry can distinguish between sensorineural hearing loss and hearing loss due to middle ear infections or central nervous system lesions by incorporating additional tests, including measurement of bone conduction, audio evoked potentials, and immittance. Testing of the latter includes acoustic-reflex testing (based on reflex tightening of the tympanic membrane by the stapedius muscle after presentation of an auditory signal and measured by changes in tympanic membrane impedance) and evaluation of decay in the acoustic reflex (associated with central nervous system lesions).
The acoustic reflex threshold is established by using an ascending and descending 5-dB increment-bracketing procedure to determine the minimal intensity required for person to note a change in middle ear compliance. An abnormal reflex decay occurs when a stimulus is present at 10 dB above the reflex threshold, and the amplitude of the reflex decreases to less than half its original value in 10 seconds or less. The main objective in performing immittance measurements is to obtain information on the site of lesions by investigating acoustic reflex findings.