Imágenes de páginas
PDF
EPUB
[ocr errors]

meant, and what is the drift of the experiments in point. And in considering the centripetal aspect of nervous processes, we may with advantage appropriate to the three-level scheme the three terms. impression, sensation, perception-using 'impression' as the lowestlevel term, sensation' as the middle-level term, 'perception' as the highest-level term; and taking impression to denote an effect that does not reach consciousness, sensation (Empfindung) to denote a felt impression, perception (Wahrnehmung) to denote a sensation in relation with its felt circumstances, i.e. the group of associated sensations that complete the mental picture or representation of the moment (Vorstellung), forming what may be designated as a field of attention.

The reaction time is the interval between the application of a stimulus and the responsive signal indicating that the stimulus has been felt.' This interval is conveniently measured by arranging an electro-magnet to mark on a revolving cylinder (1) the moment when a tactile, auditory, or visual stimulus is applied, and (2) the signalling movement by which the person experimented upon indicates that he has felt the stimulus. The reaction time. varies in different subjects, with different modes of stimulation, and with different degrees of attention and of health, between ten and twenty hundredths of a second.

Average values of the reaction time are:

[graphic][subsumed][merged small][subsumed][subsumed][merged small]

The total reaction time is composed of (1) the time occupied in conduction up sensory and down motor nerves and (2) the time occupied in the central elaboration, during which entering impression gives rise to outgoing impulse. Thus, in the simplest

case, where the skin of the hand is stimulated by an induction shock, and the signal given by the same or by the opposite hand, the interval (say '15") between stimulus and signal is made up of the time of conduction along sensory nerve (say '02'') + the time of conduction along motor nerve and the muscular latency (say 03") + the time of cerebral elaboration, i.e. perception of sensation and formation of volition (say '10"). It is the last of these three factors that varies most-with idiosyncrasy, attention, health, &c. and it is partly on this account, partly because the sensibility differs at different parts of the skin, that measurements of the reaction time with cutaneous stimulation near and far from the head give no admissible data for estimating rate of conduction along sensory nerves. The reaction time is shorter when attention is concentrated upon the intended movement than when it is concentrated upon the expected stimulus; this signifies that the formation of volition involves more labour than the perception of sensation, the preparatory influence of attention being more effective if bestowed upon the outgoing than upon the incoming event.

[ocr errors]

With regard to individuality, it is not the case that persons reputed quick and wide-awake' have a shorter reaction time than persons of an apparently sleepy and phlegmatic temperament. Quick' people often have a long reaction time; slow' people often have a short reaction time; the relation, although common, is, however, not constant enough to constitute the rule. Experiments can easily be devised so as to yield an approximate estimate of the shortest time required to discriminate between two sensations (discrimination time), and of the shortest time required to determine an act of volition (volition time). For instance, the hand of a person, with bandaged eyes, on whom the simple reaction time to touch has been determined to be, say, 15", is stimulated on the little finger or on the thumb, with the understanding that only one of these stimuli is to be signalled; the reaction time is now found to be, say, 17", from which the conclusion is drawn that 02" was the discrimination time, i.e. that required to distinguish between the two different sensations. Or the experiment is conducted with two signals, on the understanding that one signal is to be used when the little finger is touched, and the other when the thumb is touched; the reaction time under these conditions being found to be 20", is considered to be the sum of 15", the simple reaction time,+02", the discrimination time, + '03", the volition time. The experiments may be still further complicated

NN

visual area of the two sides overlap in such a way that the central spots of both retinæ are projected together near the middle line, while each occipital lobe, right and left, represents, as stated by Munk, the left and right retina. considers that the

B

a

a

b

FIG. 288.-OCCIPITO-RETINAL CONNECTIONS, AC-
CORDING TO MUNK, FROM EXPERIMENTS ON
DOGS AND MONKEYS.

Each occipital lobe, A, B, C, is connected with a and c of the opposite retina, and with b of the same side.

Ferrier angular

gyrus is more particularly related to the area of distinct vision,' i.e. the macula lutea of the opposite side.

Of the many ingenious schemata that have been proposed to illustrate the connection between retina and cortex, that of Munk gives the clearest and most probable view. It exhibits, as the main consequence of lesion of the occipital cortex on one side, paralysis of the same sides of the two retinæ, most pronounced in the opposite eye, and is in harmony with our knowledge of optic-tract degeneration; section of one optic nerve gives degeneration in the opposite optic tract towards A and C, and in the tract of the same

side towards B', not reaching higher than the anterior corpus quadrigeminum and pulvinar (v. Gudden). Subsidiary features of the view held by Munk with regard to the cortical representation of the yellow spot, &c., are, however, very problematical. He considers that in each occipital area three regions may be distinguished—an internal region, A, connected with the nasal half of the opposite retina; a middle region, c, connected with the yellow spot of the opposite retina; and an external region, B, connected with the temporal half of the retina on the same side. But any determination of the field of vision on animals is obviously a matter of extreme uncertainty; and on man there are no clinico-pathological records enabling us to map out any retino-cortical correspondence; our knowledge is limited to the fact that cortical lesions in cases of cortical hemiopia have most

frequently been found in the vicinity of the cuneate lobule, viz. in an occipital situation.

[ocr errors]

Epilepsy. The minute study of epileptic spasms has contributed largely to our knowledge of cerebral mechanism. Clinically, the form of epilepsy which most nearly concerns the physiologist is that known as Jacksonian, or cortical epilepsy, which has a closely similar counterpart in the epileptoid attacks produced on animals by experimental stimulation of the cortex. A typical epileptic fit of cortical origin is divisible into the definite stages (1) of the warning sensation, or aura; (2) the fit proper, or period of excessive muscular discharge; and (3) a post-epileptic stage, characterised by muscular debility, which may amount to actual paralysis, and which is referable to an exhaustion of discharged' cortex. The entire phenomenon is comparable to a protracted series of explosions, arising in an over-irritable or definitely irritated spot of the cortex as its focus of origin. The character of the warning and the mode of commencement of a fit give a clue to the actual seat of irritation; the order of invasion. and the character of the convulsion are the outward sign of the directions along which the original cortical disturbance has spread. The warning sensation may consist in a peculiar sensation in the muscles of the limbs in which the convulsion is about to commence, or it may take all kinds of subjective forms, from a simple flash of light, or a sound, smell, or taste, to an elaborate representation of scenery or of music. Such muscular, visual, or auditory auræ, if well characterised, are to be taken as symptoms pointing to the area within which irritability is overstepping the boundary-line into actual irritation. The fit proper -clinically divided into a short tonic period, during which muscular contraction is at its height, and a longer clonic period, during which muscular contraction is resolving itself—is significant of excessive discharge from the motor area of the cortex. Physiologically, the feature of greatest interest is the order in which the limbs become affected at the very commencement of the fit. It may commence in any one limb; ordinarily it does so by twitchings of the highly-specialised extremities most used in voluntary movement, viz. the thumb and fingers, and thereafter involves the forearm and arm, the opposite upper extremity, the face, and finally the lower extremities. But the order of invasion is by no means invariable. The part first invaded does, however, furnish valuable hints towards determining the precise seat for operative interference in cases where there may be reason

able hope of cure by removal of irritation: a fit beginning in the left great toe points to a discharging lesion of the upper part of the right Rolandic area; a fit beginning in the right hand points to a discharging lesion about the middle of that area.

х

FIG. 289.-EPILEPTOID DIFFUSION.

The motor cortex of the left hemisphere is removed. (1) Stimulation of the motor cortex on the opposite side has provoked an epileptoid attack; diffusion must have taken place in the bulbospinal grey matter. (2) Stimulation of the exposed white matter from the motor cortex of the left hemisphere has reached

the right hemisphere through the corpus callosum, and has provoked an epileptoid attack, which has arisen in the right motor cortex, and must have diffused in the bulbo-spinal grey matter. (3) No epileptoid attack could be produced by stimulation of white matter after removal of the motor cortex on both sides.

The variability of the order of invasion deserves attention; we are naturally led to inquire whether the diffusion occurs in the cortex, or in subordinate bulbo-spinal centres. The answer to this question is, that the diffusion probably occurs in the cortex and in subordinate centres. Or there may be no diffusion, the epileptoid spasms remaining limited to a single limb, or to one side of the body, as a monospasm or as a hemispasm.

The phenomena of experimental epilepsy confirm this interpretation, and fix it in certain particulars. An epileptoid attack remaining locally restricted, or taking place as the prelude to a general convulsion of the body, is apt to occur whenever experimental stimulation of the Rolandic area is at all prolonged or strong; the attack occurs or begins in the limb or part functionally connected with the particular cortical area excited; and if it become generalised the order of invasion is, as in clinical cases, a variable one. Its more usual but by no means regular order is supposing the spasm to begin in the right upper extremity-as follows: right upper extremity + right lower extremity + left upper and lower extremities, i.e. the invasion is longitudinal before it is transverse. This order favours the view that the diffusion has occurred in the cortex of one side rather than in the spinal cord, where, as we have seen, diffusion of reflex stimuli is transverse before it is longitudinal.

The great part played by the cortex in the extension of an epileptoid storm is, moreover, demonstrated in the experiments of Munk and of Heidenhain. Munk arrested the partial epilepsy which he had excited in a single limb by rapidly excising the

« AnteriorContinuar »