Humans are specially sensitive to mirror symmetry, particularly when the axis of symmetry is vertical. What causes this sensitivity?
Part of the special sensitivity can be explained by the special features of symmetry. In general, symmetrical objects/pictures have 'internal' control, which gives the observer an impression of internal consistency. That would make them special for any observer, not only for humans.
Mirror symmetry has an additional advantage over other point symmetries. This is the fact that the area in which the distance d between a point and the symmetry related point is small is relatively large. For D > d (for some D), the area is L*D, where L is the length of the object along the symmetry axis. For other point symmetries, the area is pi * D * D, which is very small for small D. Again, that would make mirror symmtery special but any observer. However, that does not explain the preference for vertical axis, and does not seem to be enough to explain the strength of the preference.
The preference for vertical axis can probably be explained based on the symmetry of the CNS, but how? Here I suggest the following hypothesis:
The sensitivity to vertical mirror symmetry is a result of a small portion of the neurons in the optical nerves going to the wrong side in the optical chasm, but connect correctly (in the toplogical sense) to the LGN on the wrong side.
We already know that all the neurons from the retinal converge in the optical chasm, and from there neurons from the left of the retina of both eyes continue to the left side of the brain (mostly the left LGN), and from the right side of the retina they to the right side of the brain. In more technical terms, nasal neurons cross to the contralateral optical tract, while temporal neurons continue to the ipsilateral tract. As a result, information from the left of the retina, which corresponds to the right visual field (the image is inverted in the when it passes the lens), reaches the left hemisphere of the cortex. The information is mapped in a topological way from the retina to the LGN and from the LGN to the cortex.
Neurons that 'miss the turn' in the optical chasm (i.e. either nasal neurons that continue to the ipsilateral tract or temporal neurons that do cross), but still connect correctly to the LGN, end up delivering the information to the wrong hemisphere, but in the right position. That means that the information is delivered to the mirror position of the correct place.
In most of the cases, this information is just noise, which blurs the picture. However, when the person focus on the axis of a mirror symmetric object/picture with a vertical axis, this wrong information is actually the same as the correct information. As a result, mirror objects/pictures, with vertical axis, are clearer than any other objects/pictures, and hence easier to perceive.
Obviously, it also requires the person to focus on the axis of symmetry, which adults have already learned to do, while babies do it by chance. Once they happen to fixate close enough to the axis of symmetry, they have a clearer picture (i.e. a more coherent and striner activity in the visual areas), which apparently causes them to keep their gaze longer (*).
The number of stray neurons must be small, because otherwise non-vertical-mirror-symmetrical object/pictures would be very blurred. In this study (Hoffmann et al, Organization of the Visual Cortex in Human Albinism, The Journal of Neuroscience, October 1, 2003, 23(26):8921-8930), Figure 1, comparison between activities in the same hemisphere from the wrong side of the retina an dthe right side in the controls (C1 and C2) suggests significant number of stray neurons, and that they are in the right position. However, this information is by fMRI, which is in general irreproducible, so this result is suspect.
These stray neurons may also explain other phenomena. For example, people with non-functional visual cortex in one hemisphere (or even with a complete hemisphere non-functional) can sometime still show sensitivity to visual input in the affected (contralateral) visual field. This sensitivity is probably mediated by the stray neurons.
While the 'stray neurons' are 'stray', the sensitivity to mirror symmetry may have evolutionary advantage, because it makes detecting animals (which are in general mirror symmteric) easier. It also may be useful in mate selection, because asymmteric mate probably had some problem during development. Thus the existence of these neurons may be 'intentional', in the sense that there is no evolutionary pressure to increase the accuracy of sorting in the optic chasm.
Note that the hypothesis above explain the preference only for vertical symmetry properly located in the visual field. A preference for mirror symmetry in other angles and locations is explained by the general advantage of mirror symmtery (explained in the first two paragraphs), and by learning from experience about its significance. ----------------------------------------------------
(*) I take it for granted that babies look longer at objeects/pictures that look to them clearer.Yehouda Harpaz