Most neuroscientists either focus on the molecules that enable neurons to communicate at the most fundamental level, or take a more systemic approach, following the flow of information between sensory input, brain, and motor output. Fred Rieke, however, has a rare ability to unite the two disciplines, striving for a more complete understanding of how the nervous system functions. Add to this his training as a physicist and his full command of biology, and he is uniquely equipped to answer some of the most important questions in neuroscience today.
Rieke wants to know how vision works—specifically, the retina's role in how we see in starlight and continue to see on a bright sunny day. He is pursuing several lines of research. To learn about visual sensitivity, he tracks the signals evoked by visual stimuli across the retina, from sensory structures known as rods and cones through different retinal cell layers. He also hopes to learn how the rods and cones and the retinal circuitry help the eye adapt to changes in light intensity. The unifying theme in this work is relating the mechanistic workings of the retina to the behavioral sensitivity of vision.
Rieke has spent roughly seven years developing techniques to record light-evoked electrical signals from every retinal cell type. Unlike most neuroscientists, who avoid studying hard-to-measure sensory noise, he designs experiments that feature both noise and the sensory signal of interest. In doing so, Rieke gains a broader understanding of how the neural circuits that initiate vision relate to visual behavior—and an appreciation of why the structures work the way they do.
In a recent study, Rieke and colleagues wanted to learn how, in almost complete darkness, a retinal rod can detect a single photon of light and respond with an electrical signal that manages to travel, intact, across a synapse to a rod bipolar cell without getting lost in background noise. To find out, Rieke's team compared single-photon responses in mouse rods with those in rod bipolar cells.
Rieke's team learned that the rod-to-rod bipolar synapse—or the synapse connecting a rod cell to a rod bipolar cell—elegantly regulates which visual signals make it deeper into the brain. Essentially, the synapse lets through high-amplitude signals (bearing a relatively big electrical charge), which tend to be neural responses to visual images, like a star or the moon. However, the rod-to-rod bipolar synapse rejects weaker signals, which tend to be just background noise, the equivalent of sensory static. Functioning this way, the synapse produces what's known as a nonlinear threshold. Like a miniature bouncer at a celebrity party, the synapse opens and shuts its door to the brain, evaluating each neural signal that wants in. This prevents the visual system from being overwhelmed by unwanted noise.
Rieke's lab went on to describe how the rod-to-rod bipolar synapse makes this selection of important and unimportant visual signals. As Rieke continues to study the retinal processing of rod and cone signals, he plans to turn to a new model: primates. By studying the primate retina, he intends to provide a more direct link with human behavior.
