128 HUMAN PHYSIOLOGY 



3. The Visual Centre. 



A response to stimulation on the part of the neurons in the 

 occipital lobe of the brain (p. 181) is the physical basis of our 

 visual sensations, and hence this part of the brain is called 

 the visual centre. Usually the visual centre is stimulated 

 by changes in the chain of neurons passing from the retina 

 and set in action by retinal changes ; but direct stimulation 

 of the occipital lobe may induce visual sensations, as is some- 

 times seen in the early part of an epileptic fit. 



The strength of the sensation depends upon the strength 

 of the stimulus, and the smallest difference of sensation 

 which can be appreciated is a constant factor of the degree 

 of stimulation. Thus, to produce a change in visual sen- 

 sation, the strength of the stimulus must vary by about 

 T c75-th of the stimulus. 



The sensation lasts longer tlian the stimulus, and thus, 

 if a series of stimuli follow one another at sufficiently rapid 

 intervals, a fusion of sensation is produced. If a wheel 

 rotating slowly is looked at, the individual spokes are seen, 

 but when it is going more rapidly, the appearance of a con- 

 tinuous surface is presented. If the light is dim, this fusion 

 takes place more readily than when the light is bright. From 

 this it is concluded that a strong stimulus causes a more 

 sudden and acute sensation than a weak one, and, therefore, 

 the individual sensations are distinguished. 



The visual centre of each side must be regarded as a chart 

 of the opposite field of vision, each part corresponding to 

 a particular part of the field. The two centres acting 

 together give the whole field of vision. Since the blind 

 spot is not represented in the centre, it is not perceived 

 in the field of vision. The centre is said to rectify the 

 inverted image formed on the retina, but this simply means 

 that as a result of experience, we have learned that changes 

 in, say the lower part of the retinae and in the corresponding 

 parts of the visual centres, are produced by light from above 

 the head. 



Since the retinal changes differ simply according to the 

 degree of illumination and the rate of the ethereal waves, 

 and since the part of the retina acted on is determined by 



