PARTIAL DECUSSATION 



321 



receives information from both eyes. This ignores the fact that what 

 reaches each side of the brain is a somewhat lateral view of the object 

 from the temporal half of one retina and a nearly straight-on view of 

 the object from the nasal half of the other retina. If any combination 

 of images in one side of the brain is essential for fusion, it would seem 

 more logical for evolution to have produced a type of partial decussation 

 in which the nasal halves of both retinae were brought to one cerebral 

 hemisphere and the temporal halves to the other. 



The conviction that: "no partial decussation, no fusion" has led to 

 some rather ludicrous corollaries whenever the convincees have been 



Fig. 120 — Ovio's inter- 

 pretation of partial de- 

 cussation. The mental 

 image is 'larger (there- 

 fore better resolved ) ' 

 than where decussation 

 is total (compare Fig. 

 119b). 



Fig. 12] — Completion of Fig 120 (in a) 

 and Fig. 119b (in b) by the addition of 

 the psychic act of inter-hemispheric fusion, 

 showing that with either partial (a) or 

 total (b) decussation, the resulting fusion- 

 image is of the same character (except for 

 Ovio's difference in 'size', which is here 

 allowed for the sake of argument). 



made to face the situation in the non-mammals, with their indisputable 

 urge to attain binocularity despite their total decussations. It is intoler- 

 able for us to observe, centrally, two totally different, unfusible visual 

 patterns with the two eyes independently. Retinal rivalry at once sets 

 in (see Fig. 122, p. 332) and a severe discomfort — powerful headache, 

 or worse — rapidly develops. Having this in mind, it was impossible for 

 the psychologist Wundt to imagine how a lateral-eyed animal, such as a 

 fish or lizard, could possibly attend simultaneously to its two independent 

 visual fields. Wundt believed that consciousness must alternate between 

 them! Yet, we can give ourselves something like the effect of total decus- 

 sation by simply pressing the upright hand flat against the nose. Each 



