§ 5. Localisation and Projection. — When a sensitive surface is affected by a stimulus, we may attend principally either to the locality of the part affected, and its relation to the spatial map of the organism as a whole, or to spacerelations in an object external to the organ of sense. Thus if a fly crawls across the face, our attention will be directed to the surface of the face and to that part of it in which the tickling irritation occurs. On the other hand, if we actively explore the contours of an external object, we attend chiefly to the spatial relations of the parts of this object, and not to the spatial relations of the sensitive surface of the body. Localisation is the technical term used for perception of the spatial relations of the part of the sensitive surface affected by a stimulus. Projection is the technical word used for perception of the spatial relations of an object external to the organ itself.
Localisation and projection are combined in a peculiarly intimate way, when one part of the tactile surface actively explores another, as when we pass the hand over the face. In relation to the exploring hand, the face is an external object. If the skin of the face were insensible to touch, the face would be just like any other external object. But the face itself feels when the hand touches it, and the successive experiences which thus arise yield perception of the parts of the face successively affected, and not of the parts of the hand which explores it.
It is a noteworthy fact that in the case of sight there is projection but no localisation. When we see a thing we are aware of the spatial relations of the parts of the object seen, but never of the spatial relations of the retina itself. The reason is that we have no spatial map of the retina to begin with. The conditions for obtaining a spatial apprehension of the retina itself are absent. We cannot explore it by active touch, as one hand may explore the other, and of course there is no possibility of seeing it. The impossibility of localising retinal impressions in the retina itself shows that localisation presupposes projection. We can only localise in those parts of the body which have been previously explored by active sight and touch in the same manner as external objects.