274 THE EAR of a peripheral lesion (otolith organ); or non-fatiguable, associated with a lesion involving central vestibular connections. Rotation Tests. The principle of rotation tests has been described, but these tests now have a very limited place, e.g. where there is a contra-indication to douching the ears, and in children. A more delicate turning test employing minimal stimuli (cupulometry) has been introduced. A subthreshold accelera- tion is given to the turning-chair until a certain angular velocity is attained and this is maintained for a certain time until every reaction has ceased. The chair is then suddenly stopped. This is repeated at different velocities and the duration of after-sensation and after-nystagmus are recorded on a logarithmic scale or 'cupulogram*. Rotation tests at present play a relatively subsidiary role in vestibular investigation. Caloric Test. The main advantage of the caloric test is that each ear can be tested separately. Syringeing the ear with cold or hot water induces convection currents within the semicircular canals and therefore stimulates them with resulting vertigo and nystagmus. In the caloric test this is done under controlled conditions. The patient lies with the head 30° above the horizontal (Fig. 152). This brings the horizontal semicircular canals into a vertical plane. Cold syringeing cools the most superficial part of the semicircular canal and gives rise to a current hi the endolymph away from the ampulla, e.g. cold syringeing of the left ear will produce a current away from the ampulla of the left semicircular canal and induce nystagmus with a slow phase towards the left and a quick phase to the right. There will be a sensation of rotation to the left and if the stimulus is great enough nausea and vomiting. Douching with warm water produces exactly the opposite effect. The tests are carried out on each ear with water at 30 and 44 °C. The ear is freely irrigated for 40 seconds. During testing the eyes are fixed in the straight ahead position upon an object on the ceiling (Fig. 152). The nystagmus elicited is thus of second degree. The measure of response is the number of seconds between the commencement of the stimulus and the cessation of nystagmus. The results are recorded as shown in Fig. 153. The two continuous lines represent a 3-minute period subdivided into intervals of minutes and 20 and 10 seconds. The interrupted lines denote response durations. Normal variations occur in response duration, the average being 1J-2J minutes, the hot reactions being slightly shorter than the cold. Where there is depression of function on one side, the response to hot and cold douching on that side will be depressed— canal paresis (Fig. 153, CT). In some cases nystagmus in one direction, e.g. nystagmus to the left, may be more readily induced and this is known as directional preponderance of nystagmus (Fig. 153, B). Directional preponder- ance of nystagmus may be evidence of labyrinthine imbalance. In many cases there is a combination of canal paresis and directional preponderance. Etectronystagmography. Eye movements may be recorded by utilizing the standing electrical potential between the cornea and retina. This can be picked up by electrodes placed close to the eyes and the difference in signal between the two eyes, when amplified, gives an accurate record of eye deviation. This method may be used for detecting more accurately the presence of spontaneous or positional nystagmus and may also replace direct observation of nystagmus during caloric and rotation tests. Eye movements can be recorded with the eyes closed, making the detection of nystagmus more sensitive (Fig. 154).