The precise nature of the sensations is not yet fully understood. It is pretty clear that the “minimal stimuli” here employed do not give rise to purely tactile sensations of low intensity. This seems to be established by the fact brought out by Dr. Louis Robinson that the parts of the skin having the most acute tactile sensibility, the tips of the fingers and the tip of the tongue, are “scarcely at all sensitive {52} to titillation”.[30] It has been pointed out by Wundt that the sensations in this case, as in that of some other skin sensations, tend to spread themselves out, other and even distant parts of the surface being engaged by means of the mechanism of reflex sensation.[31] This in itself suggests that the sensations of tickling are more allied to organic than to purely tactile sensations. It is supposed that the light stimuli set up in the skin certain organic changes, more particularly modifications of the circulation of blood in the small vessels.[32]
It is well known that not all parts of the skin are equally susceptible of the effect of tickling. Certain areas, for example, the sole of the foot and the armpit, are commonly said to be “ticklish places”. In the answers to questions sent out by Dr. Stanley Hall we find the order, as determined by most frequent naming of the part, to be as follows: the sole of the foot, the armpit, the neck and part under the chin, the ribs, and so forth. The inquiries brought out the fact that there are considerable differences of experience here, some saying that they were ticklish in all parts, others only in one. The method adopted in this inquiry clearly affords no accurate measurement of comparative sensibility.[33] {53}
A more scientific attempt to measure this was made by Dr. Louis Robinson, who carried out a large number of experiments on children from two to four years of age with the definite purpose of testing the degree of responsiveness by way of laughter. According to his results the order of decreasing sensibility is as follows: (1) the region in front of the neck; (2) the ribs; (3) axillae; (4) bend of elbow; (5) junction of ribs and abdominal muscles; (6) flanks; (7) region of the hip joint; (8) upper anterior part of the thigh.[34]
A glance at these statements shows that the determination of the scale of ticklish sensibility over the surface is not yet completed. Dr. L. Robinson, by the way, mentions neither the sole, a highly ticklish spot in the popular creed, nor the palm, which, as we shall see, is decidedly a ticklish region.[35] It is highly desirable that more precise experimental inquiries should be directed to these local variations of ticklishness, and that, after the seats of the higher degrees of the sensibility have been ascertained, the question should be considered whether these are marked off by any definite peculiarities of structure.
It is probable that the sensations included under the head of ticklishness are not all of the same quality. It seems safe to say that in all cases the sensation is complex to this extent, that it is composed of a tactile and an organic factor. But we may see that the complexity is often greater than this. An obvious instance is the addition {54} of a peculiarly irritating effect when the orifice of the ear or nostril is tickled, an effect due to the action of the stimulus on the hairs, which are specially abundant here.[36] Some surfaces, too, which are free from hair, appear to be endowed with a special modification of the ticklish sensibility. In my own case, at any rate, light touches on the sole, have, as long as I can remember, excited sensations which seem to have almost a character of their own. A further complication probably occurs when the tickling grows rougher and approaches to a digging of the fingers into the soft parts of the armpits; for here the nerve-endings lying deeper are pretty certainly stimulated.
Lastly, it is important to add that prolongation of the tickling seems to introduce changes in the intensity, if not also in the quality of the sensations. Hence it would appear that the sensations falling under the head of ticklishness, though they have certain common characteristics, may vary considerably.
Since we are here concerned with these sensations as provocatives of laughter, it behoves us to look rather closely at their feeling-tones. As largely organic sensations they may be expected to have a strongly marked element of the agreeable or disagreeable; and this is what we find. I, at least, cannot conceive of myself as having the proper sensational experience of tickling, and yet being wholly indifferent.
When, however, we ask what is the precise feeling-tone of one of these sensations, we find no simple answer forthcoming. Some psychologists view them as having, in general, an unpleasant character.[37] On the other hand, children are {55} certainly fond of being tickled, ask for it, and make a pastime of it. This at once suggests that we have here to do with a complexity of feeling-tone, as, indeed, our study of the sensations would lead us to suppose.
It is, I think, a plausible supposition that no sensation coming under the head of tickling is merely agreeable or disagreeable. It seems always to be of a mixed feeling-tone: some sensational elements being pleasant, others unpleasant, though analysis may be unable to attribute with exactness their respective tones to the several elements.
Adopting this hypothesis, we should expect that the differences in the composition of the sensations already dealt with would lead to the result that, whereas some are preponderantly agreeable, others are rather disagreeable. And this, I believe, accords with the results of observation. The tickling sensations excited by stimulating the hairy orifices of the ear and the nostril are said by Dr. Louis Robinson to be “distinctly distasteful”. The sensations produced by tickling the sole of the foot are commonly held, at least by older children and adults, to be disagreeable in all degrees of their intensity. This certainly accords with my own self-observation. The lightest touch, say from a shampooer’s hand, is to me distinctly “nasty,” with an uncanny nastiness which I cannot hope to describe.