Pain is an interesting case, and I'll discuss it in more detail later.

Both types of pain signals end up in the primary somatosensory cortex -- at least for . However, the suffering com"suffering

Although in principle, the modality of sensation is determined by four factors (proximal stimulus, receptor organ, sensory tract, and sensory projection area), in the final analysis the projection area is the most important. If the auditory nerve were connected to the visual cortex, we would see sounds instead of hearing them (we know this because direct electrical stimulation of the visual cortex produces visual sensations). Regardless of where the sensory impulse comes from, what matters is where it goes.   The Doctrine of Specific Nerve Energies is correct, but the nerve energies in question have more to do with the sensory projection areas than they do with the sensory receptors or the sensory tracts.

Some Problematic Modalities

With respect to the four characteristic features of a sensory modality -- proximal stimulus, receptor organ, sensory tract, and projection area -- some modalities -- especially vision and audition -- are clearly organized and well understood.  The other modalities are apparently more complicated; certainly they are more poorly understood.

For example, the chemical senses interact. (1) a head cold clogs the nasal passages, preventing the person from smelling properly; but food is also tasteless. (2) Cigarette smoke clogs the taste pores, but perfume is also odorless.

The skin senses are hopelessly complex. At least six different nerve endings are found in the skin, but there is no isomorphism (1:1 relation) between the type of receptor, and the corresponding sensory experience when that receptor is stimulated electrically. To make things worse, neural impulses from all six receptor types are transmitted along the same afferent tracts to the spinal cord and up to the brain, and all project to the same somatosensory cortex in the parietal lobe. So the puzzle is: how are the skin senses kept separate? There is a prize for the person who comes up with the best answer, which you can collect in Stockholm.

Pain

Consider, as well, the experience of pain -- a common example offered in philosophical discussions of qualia.

Sensory Substitution

There's Something It's Like

In the present context, however, the most important point is that "there's something it's like" to see something, and that is different from what it's like to hear something. After Kant's "three irreducible faculties", there appear to be nine qualitatively different forms of sensation -- seeing, hearing, tasting, smelling, feeling touch, feeling warm and cold, and feeling pain. And each of these modalities is associated with the activity of a particular neural system.

Psychophysics

Psychology began as the scientific study of consciousness, even though it quickly lost that interest, and regained it only recently. At the beginning, psychologists relied on introspection -- literally, "looking into" the mind to see what is there.

The  roots of late-19th-century introspection lie in the psychophysics of the early 19th century, which examined the relationship between the physical properties of a stimulus (subject to third-person objectivity) and the psychological properties of the corresponding experience to which that stimulus gives rise (subject to first-person subjectivity).  

The psychophysicists were especially interested in the intensity of stimulation, though they worked the same way for other sensory qualities.  Their essential method was to vary the physical intensity of a stimulus, and then ask the subject to rate, on a numerical scale, the intensity of the corresponding sensory experience.  By this means, they focused their research on two different types of sensory threshold:

Of course, the absolute threshold is really just a special case of the relative threshold -- that is, the difference between "nothing" and "something".

Actually, there were three different psychophysical methods:

• In the method of just-noticeable differences, the experimenter made small variations in the physical intensity of a stimulus, and determined when the subject detected the stimulus, or noticed that it had changed;
• In the method of constant stimuli, the experimenter presented two stimuli that differed slightly in magnitude, and determined when the subject detected a difference between them;
• In the method of adjustment, the subject -- not the experimenter -- varied the magnitude of the stimulus, and indicated when it (or a change) became detectable. 

The Psychophysical Principle

According to the psychophysical principle, every psychological property of a sensation is related to some physical properties of the corresponding stimulus.

In vision, for example, the quality of hue (or color) is related to the wavelength of electromagnetic radiation that falls on the retina of the eye.  Short wavelengths correspond to blue, longer wavelengths to yellow, etc.

Another visual quality, saturation, is related to the amount of gray that is mixed with the color.  For example, pink has more gray than red.

In audition, the quality of pitch is related to the frequency of sound waves falling on the eardrum.  In music, middle C, between the treble and bass clefs, has a lower frequency than third-space C on the treble clef.

Another auditory quality, timbre, is related to the shape of the sound wave, which in turn is related to the number and distribution of harmonics above the fundamental frequency.  The sound of a flute is characterized by a fairly "pure" sine wave, while the raspy sound of an oboe is characterized by a more square wave.

Thresholds for Conscious Awareness

Let's take the simplest psychophysical experiment first, on the absolute threshold for sensation.  The question is: what is the minimum amount of stimulus intensity which will give rise to a conscious sensation?

The Psychophysical Laws

Most early psychophysical research was focused on the dimension of intensity, or the experienced "strength" of a stimulus.  In terms of the psychophysical laws, the intensity of experience was related to the amount of physical energy emanating from the stimulus and falling on the observer's sensory surfaces.  Intensity is coded in the nervous system by the temporal and spatial summation of activity in sensory neurons.  

Weber's Law

This much would seem fairly obvious. But, in fact, there is no isomorphism between physical and sensory intensity. For example, the relative threshold -- the amount of change in physical energy needed before that change is detectable -- depends on the intensity of the original stimulus. If the original intensity is low, even a very small change is noticeable. If it is high, a relatively large change is required.

This principle is represented in Weber's Law:

                         dI/I = C,         where                 I  = physical intensity; and
                                                                            C = a constant.

In other words, the amount of intensity which must be added to a stimulus to produce a JND is a constant fraction of that intensity. To take an example, assume that Weber's Fraction, C, = 1/10. Thus,

 You may wish to do some other problems like this, using different Weber Fractions and different values of I, just to get a feel for the enterprise.

Weber's Fraction differs for each sensory modality, and for each species, and provides a universal index of the sensitivity of a modality. Thus, if a modality is associated with a small Weber's Fraction, it is highly sensitive (small JNDs). In humans, vision and audition are the most sensitive modalities, taste and smell the least. But in cats, Weber's Fraction for smell is 1/300 that of humans.

Fechner's Law

A more general principle relating sensory intensity to physical intensity is Fechner's Law:

                         S = k(logI),        where          S = sensory intensity;
                                                                        I  = physical intensity;
                                                                        k = a constant; and
                                                                    log  = a logarithmic function.

Thus, for every value of I, a corresponding value of S is given by the formula. Ignore the constant k, and assume logarithms to the base 10 (you may wish to get out your calculator and work through some other examples). Thus,

Notice that according to Fechner's Law, 

sensation changes more slowly than stimulation.

As stimulation grows from 1 to 200 units, sensation grows only from 0 to 2.3 units). This follows from Weber's Law: 10 additional units of stimulation makes a big difference to sensation at the bottom end of the scale, but a progressively smaller difference as we move towards the top.

If you re-plot the relation between S and I on a logarithmic scale of I, you get a straight line.  The 19th-century psychophysicists loved findings like this, because the simplicity and elegance of the straight line suggested that they had uncovered a basic law of mental life after all.

Fechner's Law holds across a wide range of situations, but there are some exceptions. For example:

An even more general psychophysical law, Stevens' Law, handles the exceptions.  In fact, Stevens' Law is so general that handles all known psychophysical relations. Stevens' Law states that:

                            S=k(I^N),            where        S = sensory intensity;
                                                                       I  = physical intensity;
                                                                      k  = a constant; and
                                                                             N  = an exponent (either power or root).

In words, Stevens' Law states that 

for any quality of sensation, there is an exponent that relates changes in sensation to changes in stimulation. 

Get out your calculators again, and we'll work through some examples.

In fact, Fechner's Law is a special case of Stevens' Law, where the exponent, N, is less than 1.0 (an exponent of 1/2 means the square root; an exponent of 1/3 means the cube root, etc.). As an example, consider what happens when N = 1/2 (or, alternatively, 0.5):

Again, although the precise values differ from those given in the illustration of Fechner's Law, notice that big changes in I yield small changes in S, just as predicted by Fechner.

Where N = 1, you get the "length" exception to Fechner's Law, where every change in I produces an isomorphic change in S.

Where N is greater than 1.0, you get the "pain" exception. For example, assume that the exponent N = 3/2 (or 1.5:

Notice that physical intensity grew from 1 to 200 units, but sensory intensity grew from 1 to 2828 units. Thus, even a small change in I produces a big change in S.

For a long time, Stevens' Law was held to be a general psychophysical law that applied to all modalities, and all qualities within a modality, under any circumstances. It was assumed to describe the operating characteristic (a term appropriated from engineering) of the sensory transducers: 

These kinds of relations go far beyond the transduction of stimulus energies.  

More recently, classical psychophysics, with its emphasis on thresholds, has been replaced by signal detection theory, which holds that detection is not merely a function of stimulus intensity, but also that the expectations and goals of the observer must be taken into account.

Still and all, psychophysics deserves to be regarded as the beginning of the scientific approach to consciousness, as it took mental states as its subject, and had all the hallmarks of Enlightenment science:

• Empirical studies based on systematic observation and controlled experimentation.
  
• The collection of quantitative data.
  
• Statistical analysis. 
• And the derivation of mathematical functions relating the stimulus environment to conscious sensory experience.

The Visual Analogue Scale and Its Discontents The intensity of a stimulus is often measured with a simple visual analogue scale (VAS) -- subjects are asked to mark a horizontal line to indicate how intense their experience is.  Sometimes the VAS is labeled with numbers, e.g., from 0-10. The VAS may also be open-ended;  For example, in Hilgard's classic studies of hypnotic analgesia, subjects were given a 0-10 scale, where 0 represented "No pain" and 10 "A pain so severe that the subject would wish to terminate it, although he is willing to report pains above 10 when stressed for a while longer" (e.g., Hilgard & Morgan, 1975). In other applications, the VAS is unlabeled, and the investigator has to measure the distance to the patient's mark with a ruler, which is a real pain. VASs are often used clinically, to collect reports from patients of how much pain, anxiety, or depression they're feeling. They are also used by personality and social psychologists -- for example in the "attitude thermometers" commonly used in attitude research. However, some psychophysicists have criticized the standard VAS  on the ground that a rating of "7" may mean one thing to one subject, and something entirely different to another one (though, frankly, Hilgard showed clearly that his open-ended VAS for pain behaved quite well in psychophysical terms).  In order to put subjects on a more common footing, Linda Bartushuk, an expert on the psychophysics and physiology of gustation, have introduced a General Labeled Magnitude Scale (gLMS), in which various points on the scale are labeled with verbal descriptors, as follows (Bartoshuk et al., Current Directions in Psychological Science, 2005; illustration from "Relief: Scientists Harness the Power of Perception to Fade Out Chronic Pain" by Rachel Adelson, APS Observer, 28(9), 11/2015).  Bartoshuk was not, of course, the first to label points on the VAS.  But note that her labels do not correspond precisely to the various numbers" -- or, for that  matter, to our intuitions.  A rating of "Very Strong", for example, is near the midpoint on the scale, somewhere between 5 and 6.  They claim that this form of VAS has better psychometric properties than the usual alternatives.

Structuralism and the 

Physical Dimensions of Consciousness

Classical psychophysics was largely focused on the problem of intensity, but even within the tradition of classical psychophysics, it was clear that there are other measurable attributes of sensation besides intensity.  In principle, these different qualities of sensation are also subject to the psychophysical principle, in that each corresponds to some physical quality of stimulation.  And, in principle, these different qualities of sensation also follow the psychophysical law, such as Stevens' Law.

But before we can get to psychophysical laws and principles, we need to know what these basic, primary, irreducible dimensions are.  To a large degree, that task fell to the members of a school of psychologists known as Structuralism. who employed a method known as introspection to determine what these attributes were -- in other words, to identify what Edwin G. Boring, the great Harvard historian of psychology, called the physical dimensions of consciousness (Boring, 1953). 

Introspection

Introspection was introduced by Wilhelm Wundt, a German psychologist who founded the Structuralist school, and refined by E.B.  Titchener, Wundt's American disciple, who founded the psychological laboratory at Cornell.  Through the method of experimental introspection, stimulus conditions were carefully varied, and subjects were asked to describe their conscious experiences in terms of their constituent elements.  The intended result was a sort of mental chemistry, in which sensory experiences, analogous to molecules, were analyzed into their constituent elements, analogous to atoms.

In their work, the Structuralists argued for a parallel between physics and psychology (they were, after all, psychophysicists!). These are known as the psychophysical parallels.

In classical Newtonian physics, there are three fundamental dimensions:

(According to relativity theory, of course, space and time may constitute a single dimension of spacetime, but the structuralists were working before Einstein came onto the scene.)  

Correspondingly, the Structuralists identified three psychological dimensions analogous to the three fundamental dimensions of physics:

In addition, the Structuralists identified two dimensions that were unique to sensory experience:

While the classical psychophysicists, like Weber and Fechner, focused on sensory intensity, the structuralists focused on sensory qualities.

The sensory qualities, in turn, differed according to modality.

How to Introspect

Some sense of what the structuralists were up to can be gleaned from a modern analog: wine-tasting, in which the taste of the liquid is broken down into its components.  Consider the following example:

"...produced from estate grown grapes at RoxyAnn, the 2004 Syrah displays a dense, purple-black appearance with powerful, forward aromas of grilled plums and cracked black pepper accented with earth and tar notes.  Dark and juicy, this wine is polished and smooth, with ripe integrated tannins layered over a chewy mouthful of concentrated blackberry, cherry, and white pepper.  Lush perfumed spice and loads of ripe blueberry and blackberry flavors reveal lavender blossoms and a gentle touch of spicy oak on the long, lingering finish" (from an ad for RoxyAnn Winery in the Ashland (Or.) Glad Tidings, 09/15/06).

The example isn't perfect, not least because the structuralists would have sought to break down even these elements, such as "grilled plums" and "cracked black pepper" into their constituent elements as well.  Moreover, describing a taste in terms of "blackberry" and "cherry" commits the stimulus error described below.  But, still, you can get the idea.

The general idea of what the Structuralists were up to is given by the color circle, which shows how the four primary hues, varying in saturation, can be combined to yield a whole host of complex colors.  Reversing the process, if you introspect on your experience of seeing a complex color, you can analyze it into basic elements of red, green, yellow, blue, and gray that can't be further reduced.  And in this way you get the basic elements.  These are qualia.

The examples that follow were constructed with PowerPoint presentation software, and they may show up a little differently on your screen, but you'll get the idea.

And so it goes.  The structuralists would present their observers with complex sensory stimuli, and then ask them to analyze their experience into its basic constituent elements.

Introspection, a technique introduced by Wundt and refined by Titchener (who, in turn, was Boring's mentor in graduate school) required careful training of observers, so that they would avoid the "stimulus error" of describing the stimulus, and its meaning, rather than the qualities of the sensory experience to which the stimulus gave rise.  For Wundt and Titchener, the stimulus error reflected a confusion between observation and inference -- and because introspection was construed as the empirical observation of conscious experience, inferences were to be avoided at all costs.  

Of course, you can't report your introspections while you are making them, because the reporting will interfere with the introspecting.  In his 1896 Outline of Psychology, Titchener made clear that all introspection is really retrospection (p. 36):

The rule of experimental introspection, in the sphere of sensation, runs as follows: Have yourself placed under such conditions that there is as little likelihood as possible of external interference with the test to be made.  Attend to the stimulus, and, when it is removed, recall the sensation by an act of memory.  Give a verbal account of the processes constituting your consciousness of the stimulus.

In whichever form it is employed, the introspective method demands the exercise of memory.  Care must therefore be taken to work with memory at its best: the interval of time which elapses between experience and the account of experience must not be so short that memory has not time to recover the experience, or so long that the experience has become faded and blurred.  In its experimental form, introspection demands further an exact use of language.  The terms chosen to describe the experience must be definite, sharp, and concrete.  The conscious process is like a fresco, painted n great sweeps of colour and with all sorts of intermediary and mediating lights and shades: words are little blocks of stone, to be used in the composition of a mosaic. If we are required to represent the fresco by a mosaic, we must see to it that our blocks be of small size and of every obtainable tint and hue.  Otherwise, our representation will not come very near to the original.

Here's how Titchener captured the rules for introspection in his 1898 Primer of Psychology (pp. 33-35):

The rules for introspection are of two kinds: general and special.  the latter refer to the regulation of stimulus, and differ in different investigations; the former refer to the frame of mind, and must be observed in all investigations alike.

Suppose, e.g., that you were trying to find out how small a difference you could distinguish in the smell of beeswax; that is, how much greater the surface of the stimulus must be made if the sensation of smell is to become noticeably stronger.  It would be a special rule that you should work only on dry days; for beeswax smells much stronger in wet than in fine weather. Or if you were trying to discover how well you could call up the smell of beeswax in your mind, without having the wax under your nose, it would be a special rule that you should perform the experiment in a perfectly odourless room, so that the excitation set up from inside the brain should not be interfered with by foreign stimulations set up in the smell-cells of the nose.  Again, if you were trying to distinguish all possible tints of blue, it would be a special rule that you should work always by the same illumination: always by dull daylight, or always by the same electric lights, etc.  For a blue seen in sunlight is different from the same blue seen in dull daylight.

The general rules of experimental introspection are as follows:

1. Be impartial.  Do not form a preconceived idea of what you are going to find by the experiment; do not hope or expect to find this or that process.  Take consciousness as it is.
2. Be attentive.  Do not speculate as to what you are doing or why you are doing it, as to the value or uselessness, during the experiment.  Take the experiment seriously.
3. Be comfortable.  Do not begin to introspect till all the conditions are satisfactory: do not work if you feel nervous or irritated, if the chair is too high or the table too low for you, if you have a cold or a headache.  Take the experiment pleasantly.
4. Be perfectly fresh.  Stop working the moment that you feel tired or jaded.  Take the experiment vigorously.

And, above all, avoid the stimulus error, which Titchener described in his 1905 textbook of Psychology (pp. xxvi-xxvii):

We are constantly confusing sensations with their stimuli, with their objects, with their meanings.  Or rather -- since the sensation of psychology has no object or meaning -- we are constantly confusing logical abstraction with psychological analysis; we abstract a certain aspect of an object or meaning, and then treat this aspect as if it were a simple mental process, an element in the mental representation of the object or meaning....  We do not say, in ordinary conversation, that this visual sensation is lighter than that, but that this pair of gloves or this kind of grey paper is lighter than this other.  We do not say that this complex of cutaneous or organic sensations is more intensive than that, but that this box or package is heavier than this other.  We do not even say, as a rule, that this tonal quality is lower than that, but rather that this instrument is flat and must be tuned up to this other.  Always in what we say there is a reference to the objects, to the meaning of the conscious complex.  It is not the grey, pressure, tone, that we are thinking of; but the grey of leather or paper, the pressure of the box, the pitch of the violin....  What is more natural than to read the character of the stimuli, of the objects, into the 'sensations' with which certain aspects of the stimulus or object are correlated...?

Of course, we could also say, as with James and Brentano, that if the goal of Structuralism is to produce a pure description of experience, then Structuralism misses the point.  If mental states are about things other than themselves, then confining introspection to the mental state, and avoiding the object that it represents, isn't desirable -- or even possible.  But let's set that point aside, and see where the Structuralist analysis of sensory qualia takes us.  It turns out that it takes us to some pretty good places.

Qualia as the "Atoms" of Experience

Titchener argued that the elements of conscious experience could be grouped into two classes: sensations and feelings.  He engaged in a dispute with Kulpe, another Structuralist, over the role of memory images, and whether there were ever imageless thoughts.  Other structuralists argued that feelings were themselves sensations (you can see a shadow of this position in the James-Lange theory of emotion, which asserts that emotional feelings are actually the perception of bodily responses to a stimulus event).  Still others asserted that images were sensations as well -- albeit excited centrally, though thought and memory, rather than peripherally, through stimulation and transduction.  In any event, introspection focused almost exclusively on the qualities of sensation -- although, as we will see, Wundt later instigated a debate about the basic qualities of feeling.

In vision, there is:

In somesthesis, or the skin senses, Boring (1933)` identified four fundamental qualities of:

The Structuralists also debated whether there were additional somesthetic qualities of roughness and wetness.  The question, as always, was whether these were irreducible qualities of somesthetic experience, or whether the qualities of roughness and wetness were actually blends of elementary dimensions such as pressure and pain.  More recently, the list of potential tactile qualia has been expanded to include soft vs. hard, tickle, and hot. 

In  taste, there appear to be four basic sensations of:

In 1916, the Danish structuralist Henning, following up on earlier analyses by Hanig (1901), arranged these four gustatory qualities into a taste tetrahedron.  In principle, according to Henning, the complex taste of any substance could be represented as a point in the space defined by the surfaces of the tetrahedron -- so much sour, so much salt, so much sweet, or whatever.  Think of it the taste tetrahedron as a kind of "color circle" for taste. 

As it happens, recent research (by Japanese investigators) has revealed a fifth basic taste, known as umami, which is a Japanese word meaning savory or deliciousness.  Umami is a taste ingredient found in much Chinese and other Asian food, and is particularly present in monosodium glutamate (MSG) and soy sauce.  It is also present in some meats and cheeses, though perhaps not in Denmark, which is why Henning didn't discover it.

In smell, there appear to be six fundamental sensory qualia: 

Again, Henning arranged a smell prism for representing the complex smell of any substance as a point in the space defined by the surfaces of the prism.

Qualia refer to consciously perceptible qualities of sensation. But in the case of olfaction, there appear to be unconscious olfactory sensations -- namely pheromones (Karlson & Luscher, 1959). Pheromones are odorants, but they don't give rise to conscious olfactory sensations. Still, these odorants are known to affect the social behavior of a wide variety of animals, including humans. Martha McClintock's work on menstrual synchronization among women who live together in close quarters is a famous example.

Although taste (gustation) and smell (olfaction) are basic modalities of sensation, the flavor of any food is given by a combination of taste, smell, and touch.  This is easily demonstrated by tasting a strawberry (without, of course, knowing that it is a strawberry -- otherwise you're liable to commit the stimulus error! -- while holding your nose.  You will taste the sweet of the fruit, but you will not taste the flavor of strawberry until you breathe again.  Similarly, irritants like carbonation, mint, and pepper contribute to flavor through the tactile modality.  What pepper contributes to the taste of food is -- not to put too fine a point on it -- tactile pain.

In somesthesis, or the skin senses, Boring (1933)` identified four fundamental qualities of:

Speaking of touch, Titchener (1920) was inspired by Henning to devise a tentative touch pyramid to map the relations among the various qualities of touch.  As with smell and taste, any tactile sensation could be located somewhere on the surface of this pyramid. A somewhat different list of tactile qualities was advocated by E.g. Boring, Titchener's student, historian of psychology, and longtime professor at Harvard. We don't have to get distracted by the differences between teacher and student. Titchener's touch pyramid is offered here as just another example of the Structuralist approach to the identification of qualia.

Any such proposal, of course, is highly dependent on an accurate determination of just what these basic qualia are, and here the matter can get complicated.  For example, just as there may be a fifth quality of umami in taste, so there may be a seventh quality of alkaline  in smell.  And we still don't know how many somesthetic qualities there are.  Still, this level of uncertainty hasn't prevented the perfume industry from using the basic olfactory elements to synthesize new odors. similarly, the basic elements of taste are used by the food technology industry to synthesize new flavors (see "The New Spice Trade", The Economist, 06/22/02).

The Doctrine of Specific Fiber Energies 

The Structuralists' introspection was not just an abstract exercise.  Rather, it laid the basis for understanding the neural substrates of sensory qualia.  Recall that in the early 19th century, Johannes Muller had proposed the Doctrine of Specific Nerve Energies -- basically, that each modality of sensation was associated with a separate neural system.  Herman von Helmholtz, who had been Muller's student, proposed an extension of Muller's doctrine, the Doctrine of Specific Fiber Energies (1863, 1865) such that different neural systems underlay each quality of sensation with each modality (Helmholtz also came up with the idea of sensory modality itself). 

So, at least in theory, each of different qualities of sensation is mediated by a different neural system which is dedicated to that purpose.  As it happens, the neural bases of these different qualities has actually been worked out -- at least for some qualities, in some modalities.

The classic example, of course, is color, and this is where Helmholtz initially propounded his doctrine.  Following Young (1802), Helmholtz proposed a trichromatic theory of color vision, based on the fact, known from physics, that all visible colors can be produced by a mixture of just three basic colors:

Helmholtz then proposed that there were three types of cones, each sensitive to a different part of the visible electromagnetic spectrum:

Young and Helmholtz had the right idea, but got the details wrong.  The trichromatic theory was eventually replaced by the opponent-process theory of color vision initially proposed in the 19th century by Ewald Hering, and revived in the 20th century by Leo Hurvich and Dorothea Jameson.  According to the theory, color vision is based on three receptor systems, each maximally sensitive to 3 classes of wavelengths, as in the trichromatic theory.  The output of these receptor systems, in turn, becomes input to three opponent processes:

As stated by Hering, and by Hurvich and Jameson, opponent-process theory was a psychological theory, inferred from the data of human performance.  More recently, however, investigators such as the late Russell DeValois at UCB, have identified the physiological substrates of the opponent processes.

Another example, somewhat more speculative at this time, concerns the relation between taste qualities and taste buds on the tongue.  Histological analyses of the tongue have identified three different papillae, or taste buds:

In principle, the output of these papillae could be integrated to yield all the different tastes we can experience, somewhat along the lines of the opponent-process theory.  Note, however two flies in the ointment:

For a long time, it was thought that receptors specific to the four "basic tastes" -- sweet, sour, salty, and bitter -- were located in discrete areas of the tongue.  The sensation of sour is most acute at the sides, least at the tip and base. The sensations of sweet and salty are most acute at the tip, least at the sides, and base. The sensation of bitter is most acute at the base, and on the throat and the palate. This suggests that there may be four different receptors, each sensitive to a different basic taste, and each uniquely distributed on the tongue -- after the manner of the "tongue map" proposed by E.G. Boring in 1942.  

Unfortunately, histological studies have failed so far to reveal differences in the taste buds at various sites on the tongue, so the precise organization of taste remains something of a mystery.  While it's true, to some extent, that different areas of the tongue are differentially sensitive to the various basic tastes, the situation is more complicated than this -- not least by the discovery of umami, that fifth basic taste, which made Boring's "taste map" more than a little quaint, much like maps of the world printed before 1492.  More critically, it appears that clusters of cells, each sensitive to different chemical molecules, are dispersed widely across the tongue.  Much in the manner of color vision, these clusters may be maximally sensitive to particular molecules, they are also responsive to others.  And much in the manner of color vision, the output from these receptors seem to be integrated at high levels of the nervous system to yield the sensation of taste.  

How Do We Know What the Qualia Are?

But before we can go looking for the neural substrates of different sensory qualia, we have to know what those qualia are -- the psychology always precedes the neurology.  Which begs the question: how do we know a basic sensory quality when we see (or hear, or taste, or smell, or touch) it?

Recently, James Cutting (2008) has used the case of color or hue as models for identifying basic qualities in other sensory modalities.  In his view, a basic quality has the following properties:

It would be an interesting exercise to apply Cutting's criteria to proposals for basic qualities in other domains (hint, hint). 

Consider, for example, some recent cross-cultural work on odor-identification by Asifa Majid and her colleagues at the University of Nijmegen in The Netherlands, who have studied the odor lexicons of various Aslian languages spoken by indigenous tribes on the Malay Peninsula (e.g., Majid & Kruspe, Current Biology, 2018). 

Much as Cutting (2008) derived some ideas from the basic colors, Ramachandran and his colleagues have been inspired by synesthesia (e.g., Ramachandran & Hubbard, 2001).  They call these principles New Laws of Qualia:

Dimensions of Feeling

Although the structuralists focused on sensory experience,  in principle the program of introspection, identifying the basic dimensions of conscious experience, could be applied to other elements as well.  In fact, Wundt himself proposed that there were three dimensions of emotional feeling:

  Titchener countered with a different proposal, that pleasantness-unpleasantness was the sole dimension of feeling.

We can see in this system rough analogies to Wundt's three primary psychological dimensions of extensity, intensity, and protensity -- and thus to the fundamental dimensions of space, mass, and time in classical Newtonian physics.  Wundt was nothing if not consistent.  Note, however,  that while the sensory attributes are uni-dimensional (at least, intensity and protensity vary from zero to infinity), Wundt's dimensions of feeling are bipolar. For Wundt, pleasantness and unpleasantness were opposite poles of a single dimension.  A "zero" degree of pleasantness, like a zero" degree of unpleasantness, lies at the midpoint of the dimension.  Similarly, calm is not just the absence of excitement, and relaxation is not just the absence of strain: they are somehow opposites.

Wundt's assertions in this regard were debated by Titchener, who argued that pleasantness-unpleasantness was the sole dimension of feeling.  Titchener agreed, however, that pleasantness and unpleasantness are negatively correlated, comprising opposite poles of a single continuum.. 

This is the conventional way of thinking about the structure of affect: you can't feel bad while feeling good.  For example, Russell summarized research on the affect lexicon -- the language of emotion -- in his affect circumplex.  Technically, a circumplex is a circular arrangement of variables, each represented as a vector in space, where the angular distance between vectors represents the correlation between the variables they represent.  For example, a small angular distance means that the two variables are highly positively correlated; a large angular distance means that the two variables are highly negatively correlated; and a right angle means that the two variables are not correlated at all.  In Russell's circumplex, there are two dimensions: positive-negative, and strong-weak.  In the circumplex, positive and negative affect are opposite ends of a single dimension:

Russell's affect circumplex is essentially Wundt's solution to the structure of affect, imported from the late 19th century into the late 20th.

Later, Watson and Tellegen argued for an alternative circumplex, in which positive and negative affect are independent of each other.  In the Watson-Tellegen circumplex, there are two big dimensions, one of strong-to-weak positive affect, and one of strong-to-weak negative affect.  In principle, it is possible to be in a state characterized by high degrees of both positive and negative affect -- as in states of high arousal, astonishment, or surprise.

Just as the 19th-century structuralists got into endless debates about the structure of sensory experience, so 20th- (and 21st-) century emotion researchers have gotten into a seemingly endless debate about the structure of affect.  

For example, affect circumplexes are generated by a statistical technique known as factor analysis, which is based on the correlation coefficient.  For technical reasons that need not detain us here, there is a certain amount of subjectivity in the placement of principal factors.  Note, for example, that in the Watson & Tellegen affect circumplex there is, in fact, a bipolar dimension of pleasantness-unpleasantness, just as Wundt and Russell maintained.  It's just that Watson & Tellegen believe that the independent factors of positive and negative affect are stronger, and dominate the statistical solution.  A further problem, unfortunately, is that correlation coefficients are not perfectly reliable, and seemingly minor differences from one study to another can lead to major discrepancies in the placement of factor axes.  It does not help, either, to note that Russell and Watson & Tellegen factor-analyzed somewhat different sets of emotion terms.

For these and other reasons, both substantive and non-substantive, an enormously vigorous dispute over the correct structure of affect continues to this day.  

For example, Donald Green, a political scientist interested in political attitude polling, and Peter Salovey, a psychologist interested in emotion, have argued that the Wundtian structure is the correct one after all.  In one study, Green, Goldman, & Salovey (1993) factor-analyzed affect ratings, using a multi-trait multi-method methodology to reduce measurement error, and obtained solid evidence that happiness and sadness were opposites, rather than independent constructs.

Tellegen and Watson (1999a, 199b), for their part, offered a new hierarchical factor analysis that provides yet a third perspective on the structure of affect.  Analyzing the relations among 27 3-item scales measuring 9 content categories of affect, Tellegen and Watson found two strong "secondary" factors, independent of each other, tapping positive and negative activation -- essentially confirming their earlier results.  However, a further analysis yielded a single "tertiary" factor of happiness-unhappiness -- essentially, the bipolar structure favored by Wundt, Russell, and Green and Salovey.  So, the structure of affect appears to depend on the level of analysis.

Determining the basic qualities of affective experience seems a necessary first step in determining the neural substrates of those mental states.

But before we go looking for brain systems, we're going to have to have some idea what we're looking for.  If Ekman is wrong, and there are more than a half-dozen basic emotions, then we'll never find all the brain systems that are involved in our emotional lives.

Consider, for example, the case of pain. Studies of clinical patients (and laboratory subjects) experiencing pain, using such instruments as the McGill Pain Scale (Melzack & Torgerson, 1971; Melzack, 1975) reveal that the pain experience comes in several different kinds (satirized by the cartoonist Chris Noth in the New Yorker magazine, 12/07/2020). Each of these kinds of pain appear to be mediated by a different neural system.

First, there are two components to the pain experience:

Similar studies have identified two different types of sensory pain:

Apparently, however, both fast and slow pain signals end up in the somatosensory cortex. 

These proposals are somewhat controversial; and, as in the Young-Helmholtz theory of color vision, they may be wrong.  But we'll never find the neural substrates of sensory qualia unless we know that the qualia are first.  The psychology comes first.

The Failure of Structuralism

Why did the structuralist program fail? Partly, it was cut short by the onslaught of behaviorism. Partly, it was eaten from within by debates over things like imageless thought. But more important, the structuralists' fundamental assumption -- that conscious experience could be analyzed into its sensory elements -- was undercut by the Gestalt psychologists who showed that, in perception, "the whole is greater than the sum of its parts". But setting aside these debates, the program was sidetracked by its focus on qualia rather than intentionality. By rigorously training introspective observers to avoid the stimulus error, the structuralists neglected the fact that mental states are not pure and abstract experiences; they are always "about" something.

This page last modified 12/10/2020.