Thursday, 17 October 2013

The distinction of mirror images

by Kate Hammond

Despite initial appearances the majority of mirror images are not identical. Each half has a distinct orientation, one left and one right. However, we appear not to retain this orientation information. Massachusetts Institute of Technology research scientist Daniel D.Dilks asked a random selection of people to recall the direction in which the queen was facing on a one penny coin. Only 50% could correctly identify her direction; even though it is an object seen everyday, the results were as if by chance. This would suggest that our memory of an object is actually independent of its orientation. Similarly, an advertising campaign presented two copies of the Mona Lisa to volunteers. In one (the original) she faced to the left and in the other (a manipulated version) she faced to the right. Although all easily identified the picture as the Mona Lisa, very few could say with confidence which was the correct version. The advertising campaign used the slogan ‘don’t mistake familiarity for knowledge’, but what can also be taken from this is further evidence that we represent the identity of an image in our memory independent of its orientation.

Practically, it makes little sense to store information regarding the orientation of objects. Distinguishing mirror images is very rarely needed, and, furthermore, we need to be able to recognize objects from all view points. However, it has also been shown that for a short period of time immediately after viewing an object, orientation information is retained. Looking at an object, looking away, and picturing it immediately afterwards confirms this. This has lead to a major theory concerning object recognition; objects are remembered as seen from a particular point of view, but the brain has adapted so that it stores with it its mirror image. The object can then be matched equally easily to itself or to its mirror image. In this way, the initial orientation information is lost.

Although it is mainly advantageous, this adaptation also has its downfalls. The visual system begins life unable to distinguish mirror images in memory. Consequently, when it is necessary to distinguish between two mirror images, we have to learn to encode each mirror image as an object in its own right. Taking the every day example of b/d and </>, little children struggle to distinguish between the two but over time the mirror images acquire the status of separate objects. This explains why everyone is able to picture b or d in their head, but is unable to picture the orientation of a coin.