Quote:
Originally Posted by Shiraz
I like the negative version as well - somehow we seem to be able to see detail better when it is reversed - don't know why
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I was thinking about this today on my way to/from classes today, and remembered that I'd read about it during my visual system module at uni last year.
The short answer is that it's physiologically wired into our retinas.
Our retinas have separate and different circuits, retinal ganglion cells, for detecting a "white dot on black background" (so-called ON retinal ganglion cells) versus a "black dot on white background" (OFF cells). The photosensitive rods and cones that we all know about feed their output (eventually) to the retinal ganglion cells.
It sounds a bit absurd, but the different types of cells are used to detect object edges - see the red/green computer simulation diagram at the bottom:
http://en.wikipedia.org/wiki/Receptive_field
When viewing low contrast scenes, ON cells can detect both decreasing light ("grey dot on black background" turns into a "white dot on black background") and increasing light ("black dot on a white background" becomes "grey dot on a white background") changes. However, OFF cells can
only detect decreasing changes in contrast.
Therefore, with low-contrast images - looking at faint structures in astro images - our eyes are more sensitive to light decrements (i.e. black detail on a white background as with inverted astro images) because both the ON and OFF pathways are triggered.
Here's an article that describes the technical detail behind the mechanisms (warning: it's not an accessible read for non-physiologists), but receptive fields in general are discussed in most vision texts.
http://www.jneurosci.org/content/23/7/2645.full.pdf
...some people love'em 
thanks Peter
. Probably will tread warily until I get a better understanding of what I am trying to do
