Friday, August 24, 2007

From Helmholtz's Treatise on Physiological Optics

Today I share a long quote from Hermann von Helmholtz's Treatise on Physiological Optics. Helmholtz was one of the great intellectual figures of the 19th century, making seminal contributions in physiology and physics as well as psychology.

It might seem that nothing could be easier than to be conscious of one’s own sensations; and yet experience shows that for the discovery of subjective sensations some special talent is needed, such as PURKINJE manifested in the highest degree; or else it is the result of accident or of theoretical speculation. For instance, the phenomena of the blind spot were discovered by MARIOTTE from theoretical considerations. Similarly, in the domain of hearing, I discovered the existence of those combination tones which I have called summation tones.... It is only when subjective phenomena are so prominent as to interfere with the perception of things, that they attract everybody’s attention. Once the phenomena have been discovered, it is generally easier for others to perceive them also, provided the proper precautions are taken for observing them, and the attention is concentrated on them. In many cases, however – for example, in the phenomena of the blind spot, or in the separation of the overtones and combination tones from the fundamental tones of musical sounds, etc. – such an intense concentration of attention is required that, even with the help of convenient external appliances, many persons are unable to perform the experiments. Even the after-images of bright objects are not perceived by most persons at first except under particularly favorable external conditions. It takes much more practice to see the fainter kinds of after-images. A common experience, illustrative of this sort of thing, is for a person who has some ocular trouble that impairs his vision to become suddenly aware of the so-called mouches volantes in his visual field, although the causes of this phenomenon have been there in the vitreous humor all his life. Yet now he will be firmly persuaded that these corpuscles have developed as a result of his ocular ailment, although the truth simply is that, owing to his ailment, the patient has been paying more attention to visual phenomena. No doubt, also there are cases where one eye has gradually become blind, and yet the patient has continued to go about for an indefinite time without noticing it, until he happened one day to close the good eye without closing the other, and so noticed the blindness of that eye.

When a person’s attention is directed for the first time to the double images in binocular vision, he is usually greatly astonished to think that he had never noticed them before, especially when he reflect that the only objects he has ever seen single were those few that happened at the moment to be about as far from his eyes as the point of fixation. The great majority of objects, comprising all those that were farther or nearer than this point, were all seen double (Helmholtz 1856/1909/1962, vol. 3, p. 6-7, emphasis in original).

In a Cartesian (1641/1984) or Price-ian (1932) mood, it can seem almost impossible to doubt the correctness of your consequent judgments about your ongoing experience; but the leading figures of introspective psychology had quite the opposite opinion (as do I here and here). This was, no doubt, grounded in their experience of finding people disagreeing radically about their phenomenology, without any plausible physiological or behavioral or environmental differences underlying that disagreement; and of people changing their minds as their theories change, conforming too neatly to expectations, being swayed by the reports and opinions of their friends and advisors, and missing things that seem in retrospect to be obvious.

Consider Helmholtz’s own examples in this passage. The most familiar example to contemporary readers is the blind spot, which even in monocular vision can be very difficult to notice without aid. The musically or psychoacoustically trained will be familiar with combination tones and overtones, which are accompanying tones different in pitch from the fundamental tones produced by musical instruments. These tones surely add to our musical experience, but they can be very difficult to discern without training (see here for further explanation and a recreation the combination tone training exercises of Titchener 1901-1905). Whether Helmholtz is right about their being summation tones in particular (tones of the pitch characteristic of A+B, supposedly produced when sounds of frequency A and B occur together) remains unclear – buttressing his fundamental point. People sometimes notice bright afterimages – those that interfere with ordinary perception, especially, such as after having glanced at the sun – but rarely do they notice faint ones, which one might (with Helmholtz) think to be more or less a constant phenomenon of vision, or imperfections and floaters in the fluid that fills the eye, even when they’re looking for them; but is this imperfection in introspection, as Helmholtz supposes, or is our visual experience normally free of such perturbations?

Helmholtz’s final example is maybe the most striking: He suggests that most of the objects in the visual field, most of the time are seen double, but we fail to notice that. Reid (1764/1997) and Titchener (1910) and others make similar remarks (I discuss this also here and here). If you hold your finger near your nose and focus in the distance, the finger may seem to you to double. But is our visual experience of most objects like that? I can’t say it seems to me that way as I gaze about the room. But I haven’t had 10,000 trials of introspective training yet! Or maybe it’s Helmholtz and Reid and Titchener who are mistaken? But that only advances Helmholtz’s central point about the difficulty of the introspection. Or are we to suppose that Helmholtz and Reid saw most things double and the rest of us do not? – that everyone is right about his own experiences and wrong about everyone else’s? Besides the physiological and psychological implausibility of that (unless we see appropriate corresponding physiological or psychological differences), that supposition makes nonsense of people’s changing their minds....


Jim said...


William James, in "Psychology: A Briefer Course" also implied that a special talent (effort and attention) was needed to grasp subjective sensations.

James states "I have spoken as if our attention was wholly determined by neural conditions. I believe that the array of things as can be attended to it is so determined. No object can catch our attention except by neural machinery. But the amount of attention an oject receives after is has caught our mental eye is another question. It often takes effort to keep our mind upon it. We feel that we can make more or less of an effort as we choose. If this feeling is not deceptive...then of course effort contribues coequally with the cerebral conditions to the result.
Though it induces no new idea, it will deepen and prolong the stay in consciousness of innumerable ideas which else would fade more quickly away..It is often a matter of but a second more or less of attention at the outset, whether one system shall gain force to occupy the field and develop itself, and exclude the other, or be excluded itself by the other...the whole drama of the voluntary life hinges on the amount of attention, slightly more or slightly less, which rival motor ideas may receive...Effort may be an original force and not a mere effect and may be indeterminate in amount."

Eric Schwitzgebel said...

Thanks for the quote, Jim! That accurate introspection was a challenge, requiring the dedication of considerable attention, was pretty much the standard view at the time. James, actually, was one of the least insistent upon it among leading psychologists.

The quote here seems to apply not just to attention to experience -- amplifying and noticing aspects of the stream of experience via attention -- but "intellectual" attention more generally. One key question is the extent to which the act of attention interferes with and alters the experience to be observed. Comte was notoriously pessimistic about this, Titchener optimistic, and James (more reasonably) somewhere between.

Shady Grove Eye Vision Care said...

Most spots and floaters in the eye are harmless and merely annoying. Many will fade over time and become less bothersome. People sometimes are interested in surgery to remove floaters, but doctors are willing to perform such surgery only in rare instances.