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Visual evoked potentials provide a quantitative measure of the visual system. The function measured includes that of the optic nerve, through the optic chiasm and tract, to the lateral geniculate bodies, and the geniculocalcarine projection to the visual cortex. The use of small-sized stimuli tests the foveal region, emanating primarily from the central fifteen degrees. The most reliable information relates to lesions of the optic nerve, and is derived by individual testing of each eye. An important application of visual evoked responses is as a screen for multiple sclerosis lesions. Abnormalities have also been shown in other conditions such as glaucoma, parkinsonism, and cortical blindness. It can also be used to measure visual acuity in infants. Since the response relies on a visual image reaching the retina, it is important to screen initially for any significant decrease in visual acuity. If possible, this should be corrected. If not, flash stimuli may yield some information, although this may or may not be as sensitive a test.

Pursuit has quite different dynamic characteristics when compared with optokinetic movements, and neuronal pathways seem to be mostly separate. Anatomical pathways are separate from those that transmit signals for compensatory movements. Examples of pursuit cells in the middle superior temporal area with clear eye and head velocity signals are described in this chapter. The primate cortical visual areas named MT and MST have received considerable attention since they appear to represent the upper stages of a tightly linked and hierarchically organized pathway for the analysis of visual motion. In light of these studies, the chapter re-examines the question of how cortically extracted visual motion information is utilized for control of voluntary pursuit of moving visual targets. The results indicate that information carried by a combination of inputs, including at least visualmotion, eye movement, and head movement, converges in a specific subregion of MST to produce neurones capable of encoding the motion of objects in extrapersonal space. The output of these neurones provides a representation of stimulus motion that could be used for a variety or perceptual and motor processes, including the control of smooth-pursuit eye movements.

Functional loss of vision or visual fields can present some of the most difficult diagnostic challenges, but even more difficult to diagnose are patients who have, in addition to functional visual loss, an organic disease. No matter how functional the symptoms appear, the clinician has to go that extra step to find any true pathology. This is why different field techniques may be more appropriate than the more sophisticated techniques. A tangent screen, for instance, may be better to control the patient’s response than a Humphrey Visual Field Analyzer. Remember the functional patients get sick, too. Patients with functional field loss can be placed into one of three general groups: neurasthenics, hysterics, or malingerers. The neurasthenic patient usually has many complaints, not limited to the visual system or to one set of visual symptoms such as field loss. The complaints, as well as the degree of field or visual defect, frequently vary from one examination to another, as well as during an examination as fatigue increases. The spiral field is often found in neurasthenic patients. The patient with hysteria, on the other hand, usually has a single ocular complaint. This chapter discusses the hysterical loss of acuity or fields. The usual field defect is a severe tubular type of contraction. The spiral field is sometimes seen in these patients. The malingerer may be the most difficult patient with functional loss, particularly if he has previously been examined by another physician and has acquired more experience with field testing than he had when first examined. The spiral field is not limited to functional loss; it can also be seen as a fatigue phenomenon in sick patients when the field testing is unduly prolonged. Its essential feature is that the field becomes progressively smaller as a specific isopter is tested for a second and third time with the same size of test object. shows all the points that were tested with a 3-mm white test object.

This chapter covers the processing and application of electrical signals from the body, in particularly the electroencephalogram (EEG), the electrocardiogram (ECG), and the electromyogram (EMG). The EEG and ECG will be considered in their monitoring capacity. The EMG will be discussed along with simulation and neuromuscular blockade and monitoring. The electrocardiogram (ECG) is a surface reflection of the propagation of electrical depolarisation and repolarisation over the various contractile chambers of the heart. Depolarisation is the trigger for releasing the stored contractile energy in the cardiac muscle. Each chamber also produces electrical action and polarising recovery potentials associated with the mechanical contribution of the recovery. The ECG can be divided into two major components: one associated with the propagation of excitation and recovery of the atria; the other with these events occurring in the ventricles. Excitation of the atria gives rise to the P wave, after which the atrial contractions propel blood into the ventricles. An atrial recovery wave exists, but it is rarely seen, as it is obscured by ventricular excitation, which is signalled by the QRS wave. During the later part of the QRS wave, ventricular contraction commences. Recovery of the ventricles is preceded by the T wave. The ECG labels, i.e. PQRST, are shown in Figure 18.1. To localise the direction of excitation and recovery of the heart chambers (and also to estimate the extent of cardiac injury), a variety of electrode arrangements can be used. The electrodes (which are normally disposable silver–silver chloride as described in Chapter 5) are positioned on easily located anatomical landmarks such as the right arm (RA), the left arm (LA), and the left leg (LL), with the right leg usually providing the reference or common. The standard (1, II, III), augmented (aVR, aVL, aVF) and precordial (V) leads are routinely recorded by electrocardiographers. It is possible to locate the direction of excitation and recovery by considering that the direction of the event (excitation or recovery) is at right angles to the isoelectric lead (i.e. the lead with equal forces in the positive and negative). This can be demonstrated by forming an equilateral triangle (Einthoven’s triangle) such as in Figure 18.2(a).