DETECTION, SALIENCY OF MOVEMENTS 343 



change in the force or the substance which it records. The most impor- 

 tant changes in visual stimuli are changes in their locations. No sense 

 other than vision is at all reliable for the orientation of animals with 

 respect to the objects in space — bats, with their miraculous ears, again 

 excepted. And, the big reason why it is vital to know where things are is 

 that some of those things, and the animal itself, move. Indeed, if nothing 

 on earth moved, there would never have been such things as eyes. Plants 

 do not have them, and neither do sessile relatives of eyed animals — sea- 

 lilies and barnacles, for example. 



But all vertebrates move about, even if a few, like the ectoparasitic 

 dwarf males of certain fishes, do not do so under their own steam. 

 Always, the vertebrate eye has recorded movement, regardless of the 

 evolutionary ups and downs of its capacities for sensitivity, acuity, and 

 color-reception. We can imagine vision with any of these aspects close to 

 the vanishing point, but not vision without awareness of motion. Psy- 

 chologists are fond of pointing out that a wiggling finger, seen in the 

 extreme periphery of the visual field, is not seen as a finger with a certain 

 brightness, color, and form, but is perceived as pure, disembodied wiggle. 

 Vision in the periphery being crude and 'primitive', the conclusion is 

 often drawn that motion is just about the most ancient and primitive 

 aspect of vision. Motion may persist when all else is lost — an individual 

 with a large scotoma or with hemianopia (v.s.) may see the motion of 

 objects (though not the objects) in what is otherwise a completely blind 

 field. 



Detection versus Saliency — If the biological need for a capacity to 

 perceive movement varies from animal to animal — and it obviously does 

 — we may reasonably look for diflFerences in this capacity. But although 

 we may be able to see morphological and physiological differences which 

 should affect the movement-seeing capacity, we cannot very well assay 

 another set of factors which is of enormous importance. These are the 

 psychic factors which have to do with the conspicuousness in con- 

 sciousness, the saliency, of movements — with their 'attention-value' and 

 importance to the animal, in other words. Animal A may have a far 

 poorer objective basis for detecting movements than species B; yet we 

 may find that species A gives a violent reaction of fear or flight to a slight 

 motion in its surroundings, while animal B calmly contemplates moving 

 objects without making any overt response to them. Here, we can attempt 

 to evaluate only the most nearly objective factors in movement-percep- 

 tion. The subjective factors which endow motion-percepts with their 



