Spatiotemporal integration of light by the cat X-cell center under photopic and scotopic conditions.

Visual responses to stimulation at high temporal frequency are generally considered to result from signals that avoid light adaptive gain adjustment, simply reflecting linear summation of luminance. Under conditions of high photopic illuminance, the center of the receptive field of the cat X-cell has been shown to expand in size when stimulated at high temporal frequency, raising the possibility that there is spatiotemporal interaction in luminance summation. Here we show that this expansion maintains constant the product of the center's luminance summing area and the temporal period of luminance modulation, implying that spatial and temporal integration of luminance can be traded for one another by the X-cell center. As such the X-cell has a spatiotemporal window for luminance integration that fuses the classical concepts of a spatial window of luminance integration (Ricco's Law) with a temporal window of luminance integration (Bloch's Law). We were interested to determine whether this tradeoff between spatial and temporal summation of luminance occurs also at lower light levels, where the temporal-frequency bandwidth of the X-cell is narrower. We found that it does not. Center radius does not expand with temporal frequency under either low photopic or scotopic conditions. These results are discussed within the context of the known retinal circuitry that underlies the X-cell center for photopic and scotopic conditions.

Effects of remote stimulation on the mean firing rate of cat retinal ganglion cells.

Visual stimulation outside the classical receptive field can have pronounced effects on cat retinal ganglion cells. We characterized the effects of such stimulation by varying the contrast, spatial frequency, temporal frequency, and spatial extent of remote drifting sinusoidal gratings. We found that the mean firing rate of some X-cells and most Y-cells increased to remote gratings of low spatial frequency and high temporal frequency and decreased to ones of high spatial frequency and low temporal frequency. At least 10-20% contrast was required to see either effect, which quickly saturated at higher contrasts. Both effects were substantial, raising or lowering the mean rate of some cells by over 40 impulses/sec. Classical receptive field mechanisms were not involved because the remote gratings caused little or no response modulation. We conclude that, in addition to a mean-increasing mechanism known from previous work, a mean-decreasing one operates in the cat retina. This mechanism prefers slower motion and resolves finer patterns than the mean-increasing one. We incorporate these findings into a model consisting of pools of small and large rectifying subunits of opposite polarity. Model estimates of subunit radius were primarily independent of eccentricity and averaged approximately 0.15 and approximately 0.60 degrees for the mean-decreasing and mean-increasing mechanisms, respectively. This makes the subunits approximately the center size of central X- and Y-cells. Because smooth movements of the eyes, head, or body should engage these mechanisms under natural conditions, we propose that the mean rate changes that would ensue are functionally relevant to cat vision.

Cell-based model of the Limulus lateral eye.

We present a cell-based model of the Limulus lateral eye that computes the eye's input to the brain in response to any specified scene. Based on the results of extensive physiological studies, the model simulates the optical sampling of visual space by the array of retinal receptors (ommatidia), the transduction of light into receptor potentials, the integration of excitatory and inhibitory signals into generator potentials, and the conversion of generator potentials into trains of optic nerve impulses. By simulating these processes at the cellular level, model ommatidia can reproduce response variability resulting from noise inherent in the stimulus and the eye itself, and they can adapt to changes in light intensity over a wide operating range. Programmed with these realistic properties, the model eye computes the simultaneous activity of its ensemble of optic nerve fibers, allowing us to explore the retinal code that mediates the visually guided behavior of the animal in its natural habitat. We assess the accuracy of model predictions by comparing the response recorded from a single optic nerve fiber to that computed by the model for the corresponding receptor. Correlation coefficients between recorded and computed responses were typically >95% under laboratory conditions. Parametric analyses of the model together with optic nerve recordings show that animal-to-animal variation in the optical and neural properties of the eye do not alter significantly its response to objects having the size and speed of horseshoe crabs. The eye appears robustly designed for encoding behaviorally important visual stimuli. Simulations with the cell-based model provide insights about the design of the Limulus eye and its encoding of the animal's visual world.