Solarized or inverted ("negative") images are products of the technology of photography. They became a fairly major tool of art photography in the 1930s and have continued to be part of the repertoire of post-exposure manipulation of images. Digital emulation of these processes makes them easily to do and easy to control.
Jack Davis and Linnea Dayton have written the how-to book on solarizing for the Photoshop generation (actually just one section from The Photoshop 3 WOW!Book) and the only guidance they give the eager learner about effects is that they are "dramatic" and that when color is involved, the effects vary greatly according to the colors most affected. Which is safe if not very helpful. We will try to sketch in a few specifics.
Solarized images are scarcely a part of daily life; most people, however, have some experience reading inverted images ("negatives"). In fact, they have the experience of brightness inversion every time they look at a very bright point light source (a flashbulb or strobe, the sun) and then away, experiencing a dark afterimage of the light source. These experiences may reinforce a connotation of intensity that inversion seems to have. Inverted images are most commonly encountered as photographic negatives, as of snapshots or xrays. Still, most people do not spend much time each day reading photographic negatives, which are very small and orange-toned (if color). The average camera user is probably able to match negatives to prints for a roll of film and perhaps little more, and we hope that our dental and other body xrays make more sense to our dentists, physicians, surgeons than they do to us. From this association with medical diagnostics comes connotations of the medical gaze. These associations have been available more most of the twentieth century. (One might consult Thomas Mann's Magic Mountain (1924) where Hans Castorp's view of his much desired Claudia Chauchat is disconcerted by her chest x-rays.)
Although achromatic (i.e., black/white) inversion has acquired connotations, chromatic inversion is just starting down that road. The experience is not as uncommon as one might suppose, since it occurs naturally as an afterimage of looking at a pattern of bright, strong colors; if, after looking for 20 seconds or so, you shift your focus from the bright object to a blank area (as from the image to the left to the space below it) and blink once or twice, you will see an afterimage with colors inverted--a ghostly Old Glory. But we do not think that we have the final answer to Mr. Key's anxious query or that it means what he hoped.
Solarization in black and white photography is the result of exposing a print to bright light during its development. This causes a partial reversal ("inversion") of light and dark values and the laying down of a Sabatier line white edging along sharp gradients. The image to the left is Edmund Teske's portrait of Jim Morrison and Pam (1973). Her face is nearly completely inverted and the Sabatier line can clearly be seen (on the larger version!) running along her profile.
Digitally, the effect can be approximated by inverting curves in various ways. At the start, a curve graph is linear and runs from (0,0) diagonally up. Think of the x-axis as input values and the y-axis as output values. At the start they are identical, but moving the line may boost some outputs or lower others. A complete inversion moves the dark corner anchor(x=0, y=0) to dark-light (y=max, x=0) and the light corner anchor (x=max, y=max) to light-dark (x=max, y=0). The line is perpendicular to its initial state. To "solarize", you set a midpoint in the center of the line and graph and pull the two end points to up. You can then experiment with the v-point. In general, the effect is to turn shadow to light, making mid-tones the darkest (though changing them the least). Inverting the other way, so that the V is now upside-down, preserves the dark-to-mid values and inverts the mid-to-high. Pulling the mid-tones lower begins to suppress them and to "posterize" the image.
If you perform these inversions on a color image, the effect will be to invert hue, saturation, and value. Generally no Sabatier line effect occurs. You can decompose an image into channels and solarize or invert one or more of those, and then recompose with the altered channels, or with switched channels. You can control the partial inversions by layering negative over positive (See Photoshop WOW!) and you can perform various Alien color-mappings on one or all of the channels. There are a vast number of possibilities.
It remains to be said, however, what sense such manipulations may be seen to bear. As noted in the McIntyre section, inversion tends to reverse directionality of light (see Krantz on shadow here) which can produce mysterious, non-natural scenes (i.e., dark objects become light sources). A second effect, also useful in making "magic" worlds, is to lower the "modality" of the object (i.e., it may be hallucinated, or some other odd virtual image, such as perception under intense emotion or the very edge of consciousness.) We do after all have one experience of extreme light/dark inversion--that which occurs when we look at a too-bright light (sun, flash, what have you) and experience a dark afterimage, due to sensory depletion. One might add as a third effect that when used with color images of human bodies, the resulting indigo gradients do not project the rosy bloom of health. And as overt manipulation it easily triggers a sense of artiness.
In the end, it very much helps with individual artists to see how they use in it several images. Man Ray, one of the discoverers and certainly the great promoter of solarization used it to give glamor to portraits and stylization, as it were, to nudes. Edmund Teske and Val Telberg seem to associate mystery or some sort of liminal state. With Teske's portrait of Jim Morrison and Pam, their heads are illuminated by a special ring of light that seems related to the roseate pattern behind them. Teske may have wanted to enhance the complementarity of her face and his and make her face continue the arc of his beard, or to strengthen her profile in the extreme light, or make her seem more meditative. With Telberg, inversion seems associated with intensity. In graphic to the left, the hands are solarized (with classic Sabatier outlining, as with Pam) which seems to heighten their reaching toward the sleeping woman as the torments of nightmare. The jagged, triangular piece of glass also seems to alter the lighting. The fingers cast inverted shadows? Where is the light coming from? This is very subjective, expressivist, romantic camera.
One confirmation that inversion/solarization denaturalizes or reduces modality is provided by Diane Fenster's experience with the image at the left, which was published in Risk Management magazine, a journal that has used her work on several occasions. The article was on "domestic partnership insurance" and I remember wondering on first seeing the image why she had inverted/solarized the lad's heads and jeans. If you touch the image with your mouse cursor, you will see an earlier version that was not published. Been in San Francisco a long time, Diane.