ABSTRACT
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.
Subject(s)
Cyprinidae/anatomy & histology , Goldfish/anatomy & histology , Retina/physiology , Retinal Ganglion Cells/physiology , Action Potentials , Animals , Biometry , Cornea/physiology , Dark Adaptation , Optic Nerve/physiology , Photic Stimulation , Sensory Thresholds/physiologyABSTRACT
A computationally straightforward method is described for determining the latency, duration and magnitude of stimulus-evoked single-unit neural activity. A unique feature of the method is its ability to define the neural response without reference to stimulus parameters. First, the temporal component of the spike train that represents the response is located and then that component is analyzed to determine the magnitude of the response. Intensity-response functions can then be constructed, using the number of extra spikes above baseline activity as a measure of response magnitude. Threshold can be defined as any point on the intensity-response function.
Subject(s)
Electrophysiology/methods , Neurons/physiology , Neurophysiology/methods , Action Potentials , Animals , Dark Adaptation , Goldfish/physiology , Physical Stimulation , Retinal Ganglion Cells/physiologyABSTRACT
Stationary and moving target forms were composed of 5 equally spaced dots embedded in a background of 600 noise dots; the spatial and temporal separations between the target dots were varied independently. Target detectability decreased linearly with both spatial and temporal separations between the target dots. Detectability of both stationary and moving targets obeyed the same quantitative dependence on total separations, invariant under orientation in space-time. Detection also depended primarily on the relative density of the target and noise rather than on the absolute spatial or temporal separations between target dots. Thus, space and time had interchangeable effects on the detection of both stationary and moving targets.
