Photoreception 



435 



optic nerve discharge^' or optic ganglion response^'^^" in time; it has also 

 been shown that a nerve impulse mav appear on illumination (usually low 

 levels) in the absence of detectable retinal action potential. In response to 

 high intensity flashes of light the train of nerve impulses may last for several 

 seconds, whereas the retinal action potential is over in a fraction of a second 

 (Wulff, unpublished). These observations do not permit an answer; future 

 observations will undoubtedly serve to clarify this problem. 



LIGHT ADAPTATION 

 t= 10 SEC. 



!■ 10 



20 



10 



5MIN. 



lOMIN. 

 TIME IN DARK 



IS MIN. 



20MIN. 



Fig. 139. Dark adaptation of single visual elements of Limulus after exposure to lights 

 of different intensities (indicated on the curves in relative units). The response of the 

 eye is measured by recording spike potentials of single visual elements in response to a 

 constant intensity, constant duration test flash. Note the de.pression of the recovery 

 curve after adaptation to more intense illumination. From Hartline and McDonald." 



The Central Visual Mechanism 



Central Visual Pathways. A vertical line drawn through the center of 

 the fovea of the human retina demarcates the two hemiretinas, the fibers of 

 which follow different pathwavs to the brain. Fibers from the temporal 

 hemiretina of the left eye pass through the optic chiasma and without 

 crossing pass to the lateral geniculate body of the left side of the brain; 

 those of the temporal side of the right eye enter the right optic tract. Fibers 

 from the nasal half of each retina cross in the optic chiasma, enter the optic 

 tract of the opposite side of the brain, where they join the uncrossed fibers 

 from the temporal half of the other eye, and end in the contralateral genicu- 

 late body. In the geniculate bodies the visual fibers enter into synaptic rela- 

 tions with the fourth order neurones which continue on to the occipital lobe 

 of the cortex. As a result of the regrouping of fibers in the chiasma, the ef- 

 fect of lesions of the chiasma or central to the chiasma causes visual defects 

 which are different from those produced by lesions of the optic nerve. 



The Visual Cortex. Knowledge of the manner in which the visual fibers 

 terminate in the optic cortex is obtained by three principal methods: (1) 



