SENSITIVITY OF RODS AND CONES IN THE PARAFOVEA (REPRINTED FROM ANALESDE LA REAL SOCIEDAD ESPANOL DE FISICA Y QUIMICA, SERIE A, VOL 46, PG 251-282, 1950)

The transition from cone to rod vision was studied by measuring the va riation in absolute threshold across the retina with a small monochrom atic test stimulus (lambda = 490 nm). The shift from one mechanism to the other was determined by means of directional sensitivity. In the r etinal region corresponding to rod vision, the test sensitivity (recip rocal threshold) decreased closer to the fovea. This variation may be attributed to three causes: a) the reduction in the number of rods; b) the increase in absorption of the yellow macular pigment; c) changes in the characteristics of the rods themselves or in the nervous conduc tors, which carry the signal to the brain, resulting in an increased p erceptual threshold. The great importance of the latter cause is discu ssed. The threshold vs. intensity curves of cone vision showed two arm s that may be attributed to the action of cone mechanisms. The existen ce of these two does not agree with what is expected from the simple t richromatic analysis of the results. (C) 1997 John Wiley & Sons, Inc.