Jonathan B. Demb, Ph.D.
Research Projects
Dr. Jonathan B. Demb’s overall goal is to understand how vision arises from synaptic interactions in the neural retina. The retina contains approximately 60 types of neurons, with great diversity in the inner retina – approximately 10 types of bipolar cells, 30 types of amacrine cells and 15 types of ganglion cells (which form the optic nerve). However, it is not known how these various cell types connect with each other to form functional “circuits” and create messages that travel along the optic nerve to the rest of the brain.
Dr. Demb is particularly interested in how the eye adjusts its neural sensitivity to adapt to different lighting conditions. For instance, in dim light the retina increases sensitivity so small signals can be picked up; whereas, in bright light, the retina decreases sensitivity so neural responses do not become saturated and thus lose information. The retina adapts to the mean intensity through several outer retinal mechanisms, including the switch between rods and cones and intrinsic properties of rods and cones. However, a second process adapts to the range of intensities relative to the mean, or the contrast of the visual signal. Contrast adaptation is absent in recordings from photoreceptors and horizontal cells, and thus, it must arise in the circuitry of the inner retina.
Dr. Demb’s studies use an in vitro preparation of the flattened mammalian retina, which remains attached to the pigment epithelium. Cones are stimulated with the projected image of a computer monitor while ganglion cells are recorded intracellularly. Studies use the same visual stimuli commonly used in vivo. Future studies focus on a specific cell type found in all mammals, the “alpha” ganglion cell, which is highly sensitive at low contrast and, thus, must adapt strongly so as not to saturate at high contrast. The alpha ganglion cell has the thickest axon in the optic nerve and therefore may be particularly vulnerable in glaucoma. Thus, these studies could lead to a better understanding of the visual changes that accompany glaucoma or other eye diseases and lead to techniques that monitor ganglion cell health.
