(g) Universal hyperæsthesia, paræsthesia, and anæsthesia may be observed. The last condition, in the insane, is very apt to give rise to delusions of non-identity and death of the body.
THE SENSORY DISTURBANCES OF THE SPECIAL SENSES are well worth separate consideration.
(a) Optic Apparatus.—Hyperæsthesia of the retina shows itself directly in photophobia, and indirectly (reflexly) by lachrymation and involuntary closure of the eyelids. Paræsthesiæ of the optic nerve and retina show themselves as flashes or projections of white or colored light in the visual field. These may be irregularly or generally distributed in the field, or appear as hemiopic (vide infra), or sector-like forms. Phenomena of this order may be experimentally produced by pressure on the eyeball or by the application of electricity over or near the eye. Photopsiæ of most varied sorts, as flashes, colored scotomata, or quasi forms may immediately precede epileptic seizures or attacks of migraine, constituting an optic aura. In some cases this assumes a definite picture form, when it partakes of the character of an hallucination. Anæsthesia of the optic nerve and retina varies infinitely in degree, from slight blurring or veiling of vision (amblyopia) to complete blindness (amaurosis). Another result is sluggishness or complete immobility of the iris under the action of light. As regards distribution, optic anæsthesia may affect the visual fields uniformly and generally, or it may assume definite geometric forms, or may appear in irregular patches (scotomata).
The definite geometric defects are classed under the general head of hemianopsia, by which term is meant that one horizontal or vertical half of the visual field is obscured. (1) Horizontal hemianopsia is not bounded by a very sharp or straight boundary-line, and is almost always due to intraocular disease (retinal lesions, embolism of one large branch of the retinal artery, injuries, etc.). (2) Vertical hemianopsia is usually marked by a sharply-defined vertical limit in the visual field, passing through the point of fixation, or a little to one side of it more usually, leaving central vision very acute. (α) Temporal hemianopsia, in which the temporal halves of the visual fields are dark, represents anæsthesia of the nasal halves of the retinæ, and is usually caused by a lesion of the chiasm of the optic nerve, so placed at its frontal or caudal edge as to injure the fasciculi cruciati. This variety is usually bilateral, but a lesion might be so situated as to affect only one fasciculus cruciatis. (β) Nasal hemianopsia, in which the inner (nasal) halves of the visual fields are dark, represents anæsthesia of the temporal halves of the retinæ, and is caused by a lesion injuring one fasciculus lateralis or both fasciculi. In the former case the nasal hemianopsia would be unilateral; in the second case, bilateral or symmetrical, (γ) Lateral or homonymous hemianopsia is that condition in which physiologically similar halves of the visual field are darkened; for example, the temporal half-field of the left eye and the nasal half-field of the right. This represents anæsthesia of the nasal half of the left retina and of the temporal half of the right. The patient can only see, with one or both eyes, the right half of any object held directly in front of him. In such cases the lesion is always caudad of the chiasm, and may consist in interruption of the right optic tract, of disease of the primary optic centres (corpus geniculatum laterale and lobus opticus) on the right side, of the caudo-lateral part of the right thalamus, of the caudal extension of the internal capsule or optic fasciculus within the right occipital lobe, of the right superior parietal lobule or gyrus angularis penetrating deep enough to interrupt the optic fasciculus; or, finally, the lesion may injure the visual centre itself—viz. the cortex of the right cuneus and fifth temporal gyrus (of Ecker). Hemianopsia of any type may be incomplete or only sector-like—i.e. involving only a quadrant or less of one visual field or of both fields. (Vide article on Localization).
Perception of color may be reduced, confused, or abolished in the retina, either a diffused general way, throughout the field of vision, or following the laws of hemiopic distribution. In cases of hysteria, achromatopsia is not rarely met with, affecting the eye corresponding to the side on which the skin is analgesic or where paralysis exists. Hysterical achromatopsia may be transferred from one eye to another by the application of metals, by hypnotic manipulations, etc.
Hemianopsic phenomena may be functional and transient, as witnessed just before attacks of migraine or sick headache.
Attempts recently made, from purely theoretical considerations, to locate centres in the occipital cortex for perception of light, form, and color separately, are wholly unjustified or at least premature.
Loss of reflex pupillary movements is a symptom of much importance. It occurs chiefly under these conditions: (1) with paralysis of the iris due to lesion of the third cerebral nerve; (2) with amaurosis or anæsthesia of the retina; (3) with posterior spinal sclerosis. The last condition is distinguished from the others by the fact that while the reflex iritic movements are lost, the quasi-voluntary movement of accommodation efforts is preserved. This condition is known as the Argyll-Robertson pupil.
Diplopia, or double vision, is due to paresis or paralysis of one or more of the ocular muscles, and as such is to be classed under motor symptoms.
Megalopsia (apparent enlargement of objects) and micropsia (apparent reduction in size of objects) are sometimes due to disorder of the accommodation apparatus within the eye, and to local diseases causing displacement of the rods and cones of the retina; but they are often, no doubt, fanciful (in neurasthenia and hysteria). The same remarks apply to monocular diplopia.