The image dual images are flipped L/R
The relative magnitudes of brightness are somewhat misleading, since they depend on response of film or of digital camera sensors (which are entirely different, the digital camera being more linear (not perfectly so) over range of brightness, while film in not, but has far greater dynamic range).
In addition, human vision is highly nonlinear (logarithmic in response to brightness), so that it resolves far more contrast in the dark range than in the brighter ranges. So small contrast in the SOT (seen naturally), which is relatively bright with a small contrast image, when inverted becomes much higher contrast when seen. It is not really an effect of camera negative, per se, but of inversion making the contrast more visible by the eye, which is more sensitive in the dark ranges. (This will depend on the re-zeroing of the image as inverted.)
If this was done properly, you wouldn’t use film products. You would employ a 256 bit approx. linearly digital camera to obtain accurate physical information over a linear range.
Then you could decide whether to invert the image for human visualization with higher contrast in the darker ranges (with a suitable zero, for max brightness in the original SOT linear image).
You can confirm all this by using Photoshop, taking photos with digital and film cameras, and inverting the results.
I have always been a little puzzled by the effects of using image obtain by film cameras. Especially if the original NASA 3D program was intended to be used with film as opposed to digital camera images.
You can eliminate all this distortion by histogramming output of a digital image of the original, also comparing to a nearby patch which does not show the image of the body.
And so the digital scan found in the iPad app, Shroud 2.0, comes to mind. To what extent have those images been manipulated for presentation value?