CENTRAL CONTROL OF EYE MOVEMENTS 



1099 



phenomenon is called cj|3iokinetic, train or railway 

 nystagmus. It has a latency of about 0.2 sec. (59). 

 It is best elicited in lower animals by surrounding 

 the animal with a revolving drum painted in wide 

 vertical stripes. Tliis is adequate in lower vertebrates 

 (19, 122, 137) but may not be enough in dogs and 

 especially in monkeys to attract the animal's atten- 

 tion. It is said that dogs will ignore rotating stripes 

 but may follow a series of rabbits made to move past 

 the eves. Some believe that the response is not then 

 due to olfactory stimuli. .Mthough vestibular and 

 optokinetic nystagmus use the same final common 

 paths, their neural pathwa)s diverge widely. In pa- 

 tients with complete l)ilateral destruction of vestibu- 

 lar nuclei due to treatment with antibiotics, vestibu- 

 lar nystagmus is abolished, whereas optokinetic 

 nystagmus is unimpaired (29). 



Rademaker & Ter Braak (113) have pointed out 

 that the maximal acceleration of the slow (pursuit) 

 phase of optokinetic nystagmus is lo'- sec. ~' whereas 

 the slow phase of vestibular nystagmus may start 

 with an acceleration of i20°- sec."'. Optokinetic 

 nystagmus will be di.scussed further in connection 

 with the cortical pathways invol\-ed. 



SUPERIOR COLLICULUS 



Anatomy 



Although it has long been known that the superior 

 colliculi (anterior corpora quadrigemina) and their 

 homologues form the principal center for vision in 

 the low-er vertebrates, they are so overshadowed by 

 the increasing importance of the occipital cortex in 

 mammals and especially in primates that very little 

 attention has been paid to them, and even textbooks 

 of ophthalmology almost ignore them. The work of 

 Apter (5, 6) on the cat has led to considerable ac- 

 tivity in this field in the last 10 years. 



Detailed accounts of the fine structure of the 

 superior colliculus in all classes of vertebrates are 

 given by Ramon y Cajal (i 15) and of the optic lobe 

 in birds by P. Ramon y Cajal (114). Descriptions of 

 its afferent and efferent fibers have been given by 

 Huber & Crosby (85-87) for lower vertebrates and 

 by Crosby & Henderson (46) for mammals. The 

 stratification of the cells and fibers of the colliculus is 

 very striking in many species, and there are large 

 differences in the cholinesterase content of difTerent 

 layers. The colliculus is one of the richer sources of 

 cholinesterase and cholinacetylase (76). 



In mammals the afferent tracts from the retina and 

 fi-om the occipital cortex enter the colliculus in the 

 stratum opticum. Many terminal fibers turn su|3er- 

 ficially to end in the stratum griseum superficiale. 

 Cells in this layer send fibers to the stratum griseum 

 profundum perpendicularly to the surface, crossing 

 the stratum opticum. Impulses may spread trans- 

 versely by the most superficial layer, the stratum 

 zonale. The deepest layers of the stratum griseum 

 profundum contain large cells the axons of which 

 form the colliculo-oculomotor pathways and the 

 tectospinal tract. The colliculus receives somatic 

 fibers from the medial lemniscus and the fifth nerve 

 system, and also fibers probably from the vestibular 

 apparatus via the inferior colliculus (73). There is 

 good evidence for a projection from the colliculus to 

 the cerebellum (126). 



The histological evidence for a projection of sepa- 

 rate quadrants of the retina on to separate areas of 

 the superior colliculus is strong, although the Marchi 

 method does not permit degenerated fibers to be fol- 

 lowed to their terminations. Histological studies have 

 been made on fish (2, 90, 99) and, among mammals, 

 on the rat (94), rabbit (23), cat (8) and opossum 

 (17). Brouwer & Zeeman (23) have tried to trace 

 fibers from monkey retina to the colliculus; and al- 

 though they obtained some degenerated fibers from 

 lesions in the periphery of the retina, they failed to 

 observe any degeneration after lesions limited to the 

 macula and concluded that there are no direct 

 macular fibers. 



Elect) ophsioliigical Studies 



Earlier observations on potential changes in the 

 superior colliculus were made with illumination of 

 the whole eye or electrical stimulation of the optic 

 nerve (142), but Apter (5) obtained good localiza- 

 tion of potentials in the cat colliculus with a light 

 subtending 4.5° at the eye. Similar evidence of 

 localization has been obtained by Buser & Dusardier 

 (27) in fish, by Gaze (71) in the frog (fig. 7), Hamdi 

 & Whitteridge (74) in the pigeon, rabbit and goat, 

 and by Daniel & Whitteridge (unpublished ob.serva- 

 tions) in the monkey. In the monkey it was difficult 

 to obtain responses from the peripheral field whereas 

 responses within 10° of the macula were easily ob- 

 tained. In view of Brouwer & Zeeman's difficulty in 

 finding degenerate fibers (23), these physiological 

 responses may be caused by fibers which have been 

 relayed, perhaps in the lateral geniculate body. In 



