STREET DOGS

Perception requires imagination because the data people encounter in their lives are never complete and always equivocal.

For example, most people consider that the greatest evidence of an event one can obtain is to see it with their own eyes, and in a court of law little is held in more esteem than eyewitness testimony. Yet if you asked to display for a court a video of the same quality as the unprocessed data captured on the retina of a human eye, the judge might wonder what you were trying to put over. For one thing, the view will have a blind spot where the optic nerve attaches to the retina. Moreover, the only part of our field of vision with good resolution is a narrow area of about 1° of visual angle around the retina’s centre. Outside that region, resolution drops off sharply.

To compensate, we constantly move our eyes to bring the sharper region to bear on different portions of the scene we wish to observe. And so the pattern of raw data sent to the brain is a shaky, badly pixilated picture with a hole in it. Fortunately the brain processes the data, combining input from both eyes, filling in gaps on the assumption that the visual properties of neighbouring locations are similar and interpolating. The result — at least until age, injury, disease, or an excess of mai tais takes its toll — is a happy human being suffering from the compelling illusion that his or her vision is sharp and clear.

We also use our imagination and take shortcuts to fill gaps in patterns of nonvisual data. As with visual input, we draw conclusions and make judgments based on uncertain and incomplete information, and we conclude, when we are done analysing the patterns, that out “picture” is clear and accurate. But is it? — Leonard Mlodinow