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Summation and inhibition in the frog's retina
Author(s) -
Barlow H. B.
Publication year - 1953
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1953.sp004829
Subject(s) - citation , computer science , retina , information retrieval , combinatorics , library science , psychology , mathematics , neuroscience
Ricco's and Piper's laws relating threshold intensities and the size of a stimulus spot show that there is a mechanism in the visual pathway for summating excitatory effects from neighbouring regions of the retina. The area over which Ricco's law bolds is certainly greater than the area of a single receptor cell, so the summation must be primarily neurological and not simply photochemical. Adrian & Matthews (1927 a, b, 1928) and Hartline (1940b) showed that summa tion occurred if a discharge, of impulses in the optic nerve was taken as the index of activity instead of the subjective sensation of light, so the neuro logical mechanism must lie in the retina. Spatial summation of this sort is an aspect of the activity of nervous ganglia which has been rather neglected, for obvious technical reasons; the bravest neurophysiologist would hesitate to use 1000 stimulators to excite his preparation, but the evidence indicates that a ganglion cell in the retina picks up from a region containing at least 1000 receptor cells. It was hoped that Granit & Svaetichin's (1939) type of single unit preparation in the frog's retina would provide a suitable preparation for investigating this s~mation, using a natural, but nevertheless easily con trollable, stimulus. It was also hoped that something might emerge to indicate the reason for the large receptive fields, up to a millimetre in diameter, which Hartline demonstrated directly. The whole retina could not have an area greater than that of a hemisphere 8 mm in diameter, in which case this single fibre would be picking up from 1/128 of the whole retina. In a frog's retina there are about 30,000 nerve fibres (Breusch & Arey, 1942), and close to a million receptor cells. It would not be in the least surprising to find one fibre picking up from its fair share of receptors, but this would correspond to about thirty rods, or a circle of 0·06 mm in diameter, whereas Hartline's results suggest that one fibre connects to a circle at least ten times this size containing a hundred times as many receptors.