CENTRAL CONNECTIONS OF THE RETINA The axons of the ganglion cells

receptive fields are circular in shape. Kuffer and later Hubel and Weisel (who were awarded Nobel Prize) showed that, viewed from the functional point, the retinal receptive fields of the photoreceptors, or of the ganglion cells, can be divided info two classes: (i) on center, and (ii) off center In an 'on center' receptive field, when the center is stimulated by a beam of light, but the peripheral part of the field remains non illuminated, the ganglion cell (hence the optic nerve) is stimulated Conversely, in an 'off center' field, when the center is struck by light (the periphery remaining non illuminated) there will be strong inhibition of the ganglion cell (and hence the optic nerve fiber) Because of the presence of on center and off center receptive fields some well known phenomenon can be explained It is known that when a light which is uniformly bright (so that it produces an uniform illumination of retina), falls on the eye, the vision becomes poor The explanation is, that concomitant stimulation of on center and off center receptive fields (due to the uniformity of brightness) results in mutual cancellation of their effects The presence of on center and off center retinal fields help us to make sharp delineation of the objects CENTRAL CONNECTIONS OF THE RETINA The axons of the ganglion cells are collected as the optic nerve, and they make exit from the eye through the optic disc and continue to proceed to their destination as optic nerve (fig 10B2. 3.1) (a) Fibers of the each optic nerve partially decussate at optic chiasm, the fibers from the nasal half of each retina cross to the opposite side but those of the temporal halves do not cross [fig 10B2. 3. 1 (a)] After decussation, what is formed is called optic tract Each optic tract contains fibers from nasal half of the opposite side and fibers from the temporal half of the same side It is to be remembered, that the image that is formed on the nasal half of the retina is from the object at the temporal half of the visual field of the same eye whereas the objects on the nasal half of the field of the vision are focussed on the temporal half of retina of the same eye The right optic tract, for example, therefore, brings information from the left temporal region and right nasal region of visual field that is, left halves of the visual fields of each eye 10B2. 3. 1b). The optic chiasma is in close relation to the pituitary gland. Tumor of the pituitary gland therefore causes compression of the central part of the optic chiasma The optic tract fibers end in the following regions: (i) Lateral geniculate body, LGB, (fig. 10A. 1. 1. C. ) (also called lateral geniculate nucleus, LGN,), this is the most important relay station This is a part of the thalamus (n) Pretectal area, which is just in front of tectum of the mid brain (N B Tectum is the dorsal portion of the midbran) (iii) Superior coliiculi, a pair of elevations, one on either side on the dorsal side of the midbrain (fig. IOA. 1. 1 C). Fig 10B23. 1 (a) Central connections of the optic nerve Note the individual sites, where a lesion produces -(i) blindness of one eye (ii) binasal hemianopia (in) bitemporal hemianopia (iv) homonymous hemianopia (v) scotoma Fig 10B.2.3.1(b) Besides these three mam stations, the fibers from the optic tract also end in the (iv) suprachiasmatic nucleus of the hypothalamus, (v) pulvinar of the thalamus (fig 10A 1 1 C) and (vi) other areas of the thalamus as well as in the brain stem Lateral geniculate body (LGB/LGN) The most important group of fibers from the optic nerve ends in the LGB. LGB is a part of the thalamus LGB itself has six layers Fibers arising from the nasal half of the contralateral retina end in layers, 1,4 and 6 whereas those arising from the ipsilateral temporal half of the retina end in layers, 2, 3 and 5 The cells of LGN also have receptive fields, which are of different sizes and which are also (like those of the photoreceptors), rather circular This is due to obvious reasons Informations of ganglion cells of the retina are thus faithfully transmitted to the LGB cells From the LGB, further order neurons arise and proceed as geniculocalcarme tract also called ('optic radiation'), to terminate on the visual cortex Visual cortex On the occipital lobe, near its posterior pole, lies the visual cortex, popularly called (Brodmann's) area 17 (fig 10A. 1. 1B and fig. 10D. 6. 2) The area is concerned with the integrated vision The neighbouring areas, area 18 and area 19 are visual association areas Visual cortex receives fibers from the LGB The area representing the macula densa lies in the pole of the occipital lobe For a small area of the retina like the fovea centrals, the representative area is rather disproportionately big (the student may recall the sensory homonculus, fig 10B1 3 2) Hubel and Wiesel showed that there are some peculiarities associated with the functions of area 17 Thus, the area 17 cells are readily stimulated when the light (or the object) is (i) in the form of straight lines, (n) the lines are moving, and (in) the lines have a particular orientation Therefore, when one looks at a featureless flat non motile object (e g a featureless big white wall) the cells of the area 17 are not very stimulated, but if on the same wall there are lines, the area 17 cells will be stimulated However, the cells of area 17 which correspond to the demarcations between the individual lines and the wall are particularly stimulated It follows that (i) contrast phenomenon, (n) demarca­tion and (iii) delineation are some of the main features of area 17 Some further issues may also be noted 1 Sup ri singly, even if the area 17 be destroyed still then the person can at least recognize objects to some extent (Recall, pain continues to be perceived even after destruction of sensory cortex) This recognition, however, is so incomplete that it cannot be termed, 'seeing' This 'residual vision' may be due to the activities of superior colliculus or some other area 2 Both the