Abstract
SEVERAL studies have indicated that the sensitivity of small areas of the retina can be changed by stimulating neighbouring retinal regions. In 1955 Fiorentini, Jeanne and Toraldo di Francia1 showed that the threshold of a small test probe was greatly increased in the vicinity of the brighter side of an illumination gradient. More recently, Westheimer2 has determined the sensitivity of the centre of a small, uniformly illuminated patch of the human cone retina by means of a small brief test spot. He found that as the illuminated patch of the retina increased in size the adaptation state of the centre of the patch was initially raised, and then lowered beyond a certain background diameter. Westheimer attributed these results to excitatory and inhibitory retinal interactions; as the background diameter was increased beyond 5′ an increase of inhibition was thought to decrease the sensitivity of the tested area, thus requiring more light for the test spot to be seen in order to offset the increased inhibition by added excitation.
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