Why do photographs seem so real? In particular, if you take a picture
of yourself and put a push pin through one of your eyes in the
picture, you will feel a definite twinge in your stomach. This doesn't
seem to happen at all or not nearly, to the same extent with a
drawing. Both photos and drawings have many distortions from "reality"
or what one would see, say, looking into a mirror. What perceptual
cues cause photos to have more "reality" impact than other forms of
visual representation?

An interesting observation or insight.  Black and White images appear
to me to look more "real" than colour images.  I think perhaps by not
having any colour they are acknowleging their "unrealness" and thus
are accepted by the brain as being more genuine in a strange way. 
Black and white images always hold more depth is some strange fashion.
 I also find it strange that "real" oil paintings (from pre
photographic times) seem more real that reality (in colour).  This I
think is because they are created by a human mind and therefore are an
outer projection of an inner perception, that when reflected (in the
picture) back into a persons mind ... somehow gets amplifed (the
reverse of a photocopy of a photocopy) and appears more real than
reality.  You are also seeing "reality" through another persons mind.

When I read your question, I think of how the eyes of a person in a
photograph seem to follow you around the room.  If this is what you
are wondering about, it is because of the whites of the eyes.  If you
have a friend stand motionless looking straight ahead, and then walk
around him observing his eyes, you will clearly see that he is not
looking at you because the amount of white you can see from different
angles changes as you walk around him.  However, if he were to follow
you with his eyes, there would be an approximately equal amount of
white space on both sides as his eyes move.  Similarly, in a
photograph, the amount of white space never changes because it is on
paper and is 2d.  Therefore, no matter where you go in the room, you
can always see white space on both sides of the eye, reminding your
brain of what someone looks like as they follow you with their eyes. 
This also works for a photograph of someone who is looking left,
right, down, or up.  You will never find a place in the room where it
looks like they are looking at you in this case.

I would imagine it has something to do with the symbolism (whether you
consciously realize it or not). You might get the same effect if you
were to shoot your reflection in a mirror with a gun, or perhaps
vicariously when you see the image of someone you hold in high regard
(even though it doesn't necessarily represent that person in terms of
visual accuracy) burned in effigy. The same might be said of someone
who was offended by having their nation's flag or their scared
religions symbols disrespected, though they don't physically recreate
the image of anyone in particular they are representative of a people
as a whole, and as such, the conscious or subconscious taboo of
defacing them causes that cringing, creepy feeling in some persons. It
isn't the "thing" so much as it is what the "thing" represents.

Just my two cents;
tutuzdad-ga

Photographs *are* real, in a sense. They are a realistic
representation of yourself. They are utterly recognisable, instantly.
In the same way as you would startle if you heard your full name being
read out on the evening news, you react to anything that happens to
your iconic representation, even though you aren't directly affected.
The reaction is sympathetic, just like guys might wince when watching
a movie where a male actor gets kicked in the groin.

In one sense the photograph is more concrete than reality itself,
because by being "an instant, frozen in time" the image escapes the
fleetingness of reality.  Yet the instant it represents is a "real"
one, not an imaginary or idealized one as with most drawings.

-- mathtalk-ga

As far as i believe, the reason that the photograph itself seems so
real, is because in essence it is. When you delve down into the
workings of the mind, the concious mind is controlling your poking the
eye with the pin, meanwhile the sub concious mind is viewing this from
a distance. The simple fact is that the photograph in the first place
is of yourself, and therefore in your memory at some point, is the
thought of you actually standing there posing for the photograph.
Whilst your concious mind is fully aware of the fact that you are
merely looking at a representation of yourself and in your case
prodding it with a pin, your subconcious is delving back into the past
of when that photograph was taken, recalling you standing there, and
then being corrupted by the thought of you poking yourself in the eye
with a pin from the future, and therefore causes you to feel ill. You
are in essence tampering with your own memory. An internal conflict of
opinion, if you will.

Interesting question. My guess is that the answer has several parts,
both perceptual and cognitive.

Probably the most important perceptual component is the accuracy of
the photo in capturing the structure one sees in viewing real objects.
A good photograph viewed under the proper circumstances (with just one
eye, from far enough away that lens accommodation isn't an effective
cue to depth, and with the edges of the photo occluded by viewing it
through an aperture), people cannot actually tell the difference
between the real 3-D scene and the photograph. This sort of thing has
been studied by MH Pirenne (1970: Optics, painting and photography;
Cambridge U. Press).

Yes, there can be distortions in photographs, but most of them are
highly constrained, systematic, and the sort of transformations to
which the visual system is relatively insensitive. People have elastic
enough color constancy, for example, to compensate for poor lighting
conditions, and enough of various spatial constancies to compensate
for perspective distortions. And even when there are distortions of
these sorts (which most picture-takers try to avoid), the accurate
rendering of detail is usually still there.  Drawings have much less
constrained distortions in which the geometry of the images can be
artibraily different from what would actually be seen when viewing a
real face.

Anyone with a digital camera, Photoshop, and an interest in this issue
could conceivably study it. Photoshop provides lots of ways of
introducing systematic changes into photographs of faces so that one
could find out which ones produce changes in the effect you are
referring to. Start with a good photographs of some faces (e.g., your
own, someone you know well, and a stranger) and then use Photoshop to
alter the photo.  Inverting the color would very likely make the
effect go away, but just increasing the redness or greenness probably
wouldn't.  You can also introduce textural transformation using the
various "artistic" filters to make the photos look more like paintings
and less "real." You can also alter the overall geometry by
introducing perspective distortions of various sorts using the
"distortion" filters.

I suspect that there are some cognitive components as well.  Most
people know how photography works and that there is a real object
corresponding to the image it produces. A painting -- even a very
realistic representationalist one -- could be an imagined face of an
imagined person, which doesn't have the same impact as if the viewer
KNOWS it's a real person, particularly him/herself.

Yet another component to the answer is adaptation: the effect will
probably get weaker and go away with repeated exposure to it.  That
will make doing the study suggested above more difficult just on
yourself; you'd have to get some other volunteers.

Finally, there has to be a physiological component to the answer
beyond whatever neural firings underlie the perceptual and cognitive
components mentioned above.  Something (activation of some branch of
the vagus nerve?) produces the twinge in the stomach, which I also
notice whenever I see someone get hurt.  One of the best examples is
the razor slicing through a woman's eye in the Luis Bunuel movie, "Un
Chien Andalou," which give me a big dose of the same twinge.

Hi Barkley-ga,

  The reason why Black and White images appear more realistic is also
due to the physical resolution as perceived by our eyes.The amount of
cells(rods) that perceive B&W images is significantly larger than the
equivalent cells(cones) that perceive color. Also since the absolute
color is formed by using the primary colors(RGB), the actual amount of
color perceiving cells is reduced by approximately(since white is not
exactly one third of RG&B, when it comes to human eye) one third. Also
with B&W, since there are only two colors involved(Black/absence of Black
perceived as white), the cells are effectively used to differentiate the
depth(grayscale) increasing the total depth relative to a color image.

Regards
ldavinci-ga

To my understanding, the brain will works based on concepts and
attached concepts.  While one can consciously poke a pin in the eye of
a picture of themselves, this and the act of doing it in real life is
not so conceptually different in the brain.  In fact, the only
different is your conscious recognition that this object that you are
poking is only a piece of paper.

One should keep in mind that the possibility of you looking in a
mirror and your hand-eye coordination is just very off, as well the
possibility of you looking at a tv that's hooked up to a camera
watching you do this to yourself is a very real possibility on the
lowest logical levels.