Date of Award
nerve inhibition, nerve excitation, limulus lateral eye, inhibitory field, fiber optics illumination system
Spatial distribution of the inhibitory influences exerted by ommatidia in the Limulus lateral eye was measured. The source of inhibition was a small cluster of ommatidia illuminated through a flexible bundle of glass fibers ("fiber optics"). The inhibitory field of the cluster was determined by measuring the decrease it produced in the response frequency of surrounding ommatidia which were illuminated individually through single glass fibers. Applied directly to the corneal facets of the ommatidia, the single fibers provided unusually effective stimulation with a minimum of light scatter into adjacent receptors. The inhibitory field is elliptically shaped with its major axis in the antero-posterior direction on the eye. In the adult 2 animal the field covers an area of 15 mm (about 30% of the eye) and contains approximately 300 ommatidia; however, less than onethird of that number receives the bulk (75%) of the inhibitory effects exerted by the small cluster in the center. The position of maximum inhibition is located at some distance from the center of the field: 0.8 mm or 3 ommatidial diameters in the dorso-ventral direction and 1.3 mm or 5 ommatidial diameters in the anteroposterior. The inhibitory effect tapers off toward the periphery becoming negligible at approximately 2 mm from the center of the field in the dorso-ventral direction and at 3.3 mm in the ventroposterior direction. The configuration of the field was found to be similar for a number of experiments in which the source of inhibition was located in various positions on the eye. Control experiments show that the diminution of the inhibitory effect near the center of the field is not an artifact of the measuring technique and cannot be readily explained by local competing excitatory processes. The ommatidial inhibitory fields enhance visual contrast. Borders and steep intensity gradients in the retinal image are accentuated by maxima and minima (Mach bands) in the response pattern of the optic nerve. A theoretical analysis of the contrast phenomena indicates that the shape of the Mach bands is determined by the configuration of the inhibitory field. Patterns of the optic nerve activity in response to simple, stationary patterns of illumination were measured and compared to theoretically calculated response patterns. The features common to the experimental and calculated response patterns are directly correlated to the most prominent characteristic of the inhibitory field: a diminution in the inhibitory effect near the center of the field. There are, however, some significant discrepancies between theory and experiment resulting most likely from the restriction of the theoretical model to a one-dimensional array of receptors. Preliminary studies using a more realistic twodimensional representation of the eye are in somewhat better agreement with the experimental results.
Barlow, Robert B., "Inhibitory Fields in the Limulus Lateral Eye" (1967). Student Theses and Dissertations. 42.