The calorictest is more useful in distinguishing malfunction of the labyrinth on both sides. By bending the neck backward about 60 degrees, the two horizontal canals become essentially vertical. When warm water is introduced into the left ear, the specific gravity of the endolymph decreases due to heating, causing the endolymph in the outer portion of the loop of the left semicircular canal to rise. This sets up a convection flow of endolymph, deflecting the kinocilia of the left ampullary crest hair cells toward the utricle, as if the head had rotated to the left. The discharge of the afferent fibers supplying this canal increases, resulting in nystagmus with the fast phase toward the left. This nystagmus creates a sensation that the environment is spinning to the right, and the person tends to fall to the right. The opposite effects are produced if cold water is placed in the ear. A mnemonic expression that can help remember the direction of the nystagmus in the calorictest is COWS ("cold opposite, warm same"). In other words, cold water in one ear results in a fast phase of nystagmus toward the opposite side, and warm water causes a fast phase toward the same side.

Activity in the superior colliculus is related to computation of the direction and amplitude of the saccade. The deep layers of the superior colliculus contain a topographic motor map of saccade locations. Information from the superior colliculus is forwarded to distinct sites for control of horizontal and vertical saccades. The horizontal gaze center consists of neurons in the paramedian pontine reticular formation, near the abducens nucleus. The vertical gaze center is located in the reticular formation of the midbrain, specifically the rostral interstitial nucleus of the medial longitudinal fasciculus and the interstitial nucleus of Cajal. The circuitry and operation of the horizontal gaze center are better understood than those of the vertical gaze center. Neurons in the horizontal gaze center show activity patterns responsible for horizontal saccades.

The neural circuitry by which saccades are generated is depicted in Fig. 9.28A. The activity of certain types of neurons found in the gaze center responsible for horizontal saccades is shown in Fig. 9.28B. Each horizontal gaze center has excitatory burst neurons that project to motor neurons in the ipsilateral abducens nucleus and to the internuclear neurons that excite medial rectus motor neurons in the contralateral oculomotor nucleus. It also has inhibitory burst neurons that inhibit the contralateral abducens. These burst neurons are capable of extremely high bursts of spikes. Moreover, the gaze center has neurons showing tonic activity and burst-tonic activity.

Normally, both inhibitory and excitatory burst neurons are inhibited by omnipause neurons located in the nucleus of the dorsal raphe. When a saccade is to be made, activity from the frontal eye fields or the superior colliculus, or both, inhibits the omnipause cells and excites the burst cells on the contralateral side. The resulting high-frequency bursts in the excitatory burst neurons provide a powerful drive to motor neurons of the ipsilateral lateral rectus and contralateral medial rectus muscles. At the same time, inhibitory burst neurons enable relaxation of the antagonists. The initial bursts of these neurons allow strong contraction of the appropriate extraocular muscles, overcoming the viscosity of the extraocular muscle and enabling rapid movement to occur.
Thecalorictestismoreusefulbecauseitcandistinguishbetweenmalfunctionofthelabyrinthsonthetwosides.Theneckisbentbackwardabout60degreessothatthetwohorizontalcanalsareessentiallyvertical.Ifwarmwaterisintroducedintotheleftear,theendolymphintheouterportionoftheloopoftheleftsemicircularcanaltendstoriseasthespecificgravityoftheendolymphdecreasesbecauseofheating.Thissetsupaconvectionflowofendolymph,andasaresult,thekinociliaoftheleftampullarycresthaircellsaredeflectedtowardtheutricle,asiftheheadhadrotatedtotheleft;thedischargeoftheafferentfibersthatsupplythiscanalincreases;andnystagmusoccurswiththefastphasetowardtheleft.Thenystagmusproducesasensationthattheenvironmentisspinningtotheright,andthepersontendstofalltotheright.Theoppositeeffectsareproducedifcoldwaterisplacedintheear.AmnemonicexpressionthatcanhelpinrememberingthedirectionofthenystagmusinthecalorictestisCOWS (“coldopposite,warmsame”).Inotherwords,coldwaterinoneearresultsinafastphaseofnystagmustowardtheoppositeside,andwarmwatercausesafastphasetowardthesameside.
posterior parietal areas). Activity in the superior colliculus is related to computation of the direction and amplitude of the saccade. Indeed, the deep layers of the superior colliculus contain a topographic motor map of saccade locations. From the superior colliculus, information is forwarded to distinct sites for control of horizontal and vertical saccades, referred to as the horizontal and vertical gaze centers, respectively. The horizontal gaze center consists of neurons in the paramedian pontine reticular formation, in the vicinity of the abducens nucleus (
Fig. 9.28A ). The vertical gaze center is located in the reticular formation of the midbrain: specifically, the rostral interstitial nucleus of the medial longitudinal fasciculus and the interstitial nucleus of Cajal. Because the circuitry and operation of the horizontal gaze center are better understood than those of the vertical gaze center, it is discussed here in detail. However, cells showing analogous activity patterns have been described in the vertical gaze center.
Fig. 9.28A is an overview of the neural circuitry by which saccades are generated, and
Fig. 9.28B shows the activity of certain types of neurons found in the gaze center that are responsible for horizontal saccades. Each horizontal gaze center has excitatory burst neurons that project to motor neurons in the ipsilateral abducens nucleus and to the internuclear neurons (which excites medial rectus motor
CHAPTER 9 Organization of Motor Function neurons in the contralateral oculomotor nucleus). It also has inhibitory burst neurons that inhibit the contralateral abducens. These burst neurons are capable of extremely high bursts of spikes (up to 1000 Hz). Moreover, the gaze center has neurons showing tonic activity and burst-tonic activity.
Normally, both inhibitory and excitatory burst neurons are inhibited by omnipause neurons located in the nucleus of the dorsal raphe. When a saccade is to be made, activity from the frontal eye fields or the superior colliculus, or both, leads to inhibition of the omnipause cells and excitation of the burst cells on the contralateral side. The resulting high-frequency bursts in the excitatory burst neurons provide a powerful drive to motor neurons of the ipsilateral lateral rectus and contralateral medial rectus muscles (see
Fig. 9.28A ); at the same time, inhibitory burst neurons enable relaxation of the antagonists. The initial bursts of these neurons allow strong contraction of the appropriate extraocular muscles, which overcomes the viscosity of the extraocular muscle and enables rapid movement to occur.