Responsivity and absolute sensitivity of retinal ganglion cells in goldfish of different sizes, when measured under "psychophysical" conditions.
Retinal neurogenesis occurs in adult goldfish, and more rods are added to the retina than any other class of cell as the fish grows. To determine whether the disproportionate addition of rods affects the responsivity and sensitivity of dark adapted retinal ganglion cells, we recorded activity from optic tract fibers in goldfish of different sizes. Experimental conditions were as similar as possible to those used in a separate study in which psychophysical absolute thresholds were measured: large, dim, monochromatic spots 1 sec in duration were projected close to the right eye of alert, self-respiring goldfish. A total of 214 fibers were recorded in small (5.0-5.7 cm), medium (9.5-11.0 cm) and large (13.0-20.0 cm) fish. Neither maintained activity (mean and variance of the discharge rate in darkness) nor responsivity (quantum-to-spike ratios) nor absolute threshold (quantal irradiance required to produce a difference of 1 spike/trial from spontaneous rates) varied reliably with size of fish. However, some Off cells were more active in the dark than On and On/Off cells; these had low QSR's and absolute thresholds, and were found in all sizes of fish. Fifty percent (50%) of Off cells (compared to 8% of On cells) had thresholds comparable to or lower than psychophysical threshold, and Off cell thresholds (but not On cell thresholds) tended to be lower in larger fish. Because psychophysical threshold is closely related to the planimetric density of rods in goldfish, the similarity between Off cell threshold and psychophysical threshold suggests that Off cells may be influenced relatively more than On cells by the addition of new rods to the retina.
Falzett
M
M
Department of Psychology, Vanderbilt University, Nashville, TN 37240.
Nussdorf
J D
JD
Powers
M K
MK
D000200:Action Potentials; D000818:Animals; D001699:Biometry; D003315:Cornea / Q000502:physiology; D003530:Cyprinidae / Q000033:anatomy & histology*; D003623:Dark Adaptation; D006054:Goldfish / Q000033:anatomy & histology*; D009900:Optic Nerve / Q000502:physiology; D010775:Photic Stimulation; D012160:Retina / Q000502:physiology*; D012165:Retinal Ganglion Cells / Q000502:physiology*; D012684:Sensory Thresholds / Q000502:physiology