Attention is distinct from perception. There are neurological deficits in which a person appears unable to see but in which, on closer examination, one finds that the person can indeed see, but cannot attend to what they see. Such a person is, for many practical purposes, blind even though they are not in fact blind. One example is hemineglect, in which a person can see, but cannot attend to, the left half of the visual field and the left half of individual objects. Such a person will only eat the food on the right half of a plate, and seems unaware of the food on the left side of the plate. But they discover, to their delight and amazement, that if they move to the right around their plate, more food magically appears, and they can continue to eat. Another syndrome highlighting the importance of attention in perception is dorsal simultanagnosia. This is even more severe than hemineglect, for now the whole visual world is gone, and only one part of the visual scene at a time can be seen.
When looking, for instance, at a scene containing a person riding a horse, the dorsal simultanagnosic might first see an arm. This disappears after a few seconds and then they see, say, the horses' head for a few seconds. This disappears to be replaced by yet another part of the scene. After quite some time the patient can sometimes, like a detective, piece together the various parts seen one a time to guess what the whole scene is about. The experience is at once frustrating and magical. It is frustrating because parts appear without invitation, stay for unpredictable amounts of time, then disappear without permission. Systematic exploration of the scene is thus frustratingly out of the question. It is magical because each part appears dramatically out of the fog of the visual field, like a rabbit from the magician's hat. The fog magically gives, and then just as magically reclaims what it has given. Amusing for a time, but crippling for a lifetime.