Julie Schnapf

Research Interests

Phototransduction and signal processing in primate retina

Research Summary

My laboratory records light-evoked responses from single photoreceptor outer segments in macaque and human retinas. We find many parallels between these electrical signals and human vision as a whole. For example, the spectral absorption properties we've measured in photoreceptors quantitatively predict the psychophysically derived color-matching and luminosity functions of human observers. The oscillatory waveform of the cone photon response explains our elevated sensitivity to 5 Hz light flicker. With increasing intensity of background illumination, sensitivity of the human observer to light increments is dramatically reduced. This desensitization can be largely accounted for by a parallel decrease in the signal/noise ratio of single rods and cones.

We also explore the electrical interactions between photoreceptors in primate retina. Both color and form vision derive from comparisons of the signals generated in neighboring photoreceptors. We want to determine how the signals from the three kinds of cones are segregated and compared so that wavelength information may be extracted. How does the amplitude and waveform of the light-responses of retinal neurons vary with the spatial patterning of light stimulation; how does this compare with the spatial and temporal resolution of human vision? Using the whole-cell patch recording in single rods and cones in macaques, we obtained the surprising result that rods are electrically coupled to neighboring cones but not to neighboring rods. We are characterizing the horizontal cell feedback signal to cones, including the spatial and spectral properties and the underlying pharmacology. We are also studying the effects of background adaptation on photoreceptor coupling and sensitivity.

In the coming year we will explore the next step in visual processing in primate vision, bipolar cell signaling. We want to find out which photoreceptors provide input to different subtypes of bipolar cells and how this connectivity shapes color vision. We also plan to look at the temporal characteristics of bipolar cell signals and noise and relate this to visual sensitivity in human subjects.

Selected Publications

DM Schneeweis, JL Schnapf (1995). Photovoltage of rods and cones in the macaque retina. Science 268, 1053-1056.

TW Kraft, JL Schnapf (1998). Aberrant photon responses in rods of the macaque monkey. Vis Neurosci 15, 153-159.

DM Schneeweis, JL Schnapf (1999). The photovoltage of macaque cone photoreceptors: Adaptation, noise, and kinetic properties. J Neurosci 15, 1203-1216.

DM Schneeweis, JL Schnapf (2000). Noise and light adaptation in rods of the macaque monkey. Vis Neurosci 17, 659-666.