Dr. Hao Sun
Associate Professor
Department of Optometry & Visual Science
Buskerud University College
Frogsvei 41
PO Box 251
3601 Kongsberg, Norway

I study the human visual system using both psychophysical and physiological methods. In psychophysics, the performance of the human visual system is measured at the system level. In physiology, single cells’ activities are recorded via in vivo preparation in the primate retina. With these two sources of information we are able to make progress towards linking the behavior of the system to cell biology, anatomical circuitry, and neuronal information processing.

At the University of Chicago, I worked in the lab of professor Joel Pokorny and Vivianne Smith. I used psychophysical methods to examine how rod signals interact with visual functions mediated by the cones. Contrary to the standard story from text books, the rod and cone systems are not completely independent; they share the same neural pathways from the retinal ganglion cells to the brain. Rod signals interact with cone mediated visual function, such as color perception, color discrimination, flicker detection. Previous rod-cone interaction studies are usually limited to a small range of luminance and chromaticity, where rod functions can be separated from cone functions. At University of Chicago, we built the first four-primary colorimeter system that allows independent control of the four types of photoreceptors (S-, M-, L-cones and the rod). With this device I can study rod-cone interaction over a wide range of luminance (from scotopic up to photopic levels) and chromaticity. Furthermore, by choosing stimulus conditions that bias the cone detection to the luminance or chromatic pathway, I can study the rod signals in postreceptoral pathways and their contribution to high-level cognitions. My dissertation work suggested that there is stronger rod input to luminance pathways, which is consistent with physiological studies showing rod inputs to primate magnocellular ganglion cells.

I did my postdoctoral research at SUNY State College of Optometry with professor Barry B. Lee. For my postdoctoral work, I combined single unit physiological recording with psychophysics. One question we are interested in is the spatiotemporal information processing in luminance and chromatic pathways in the retina and how the retinal information is utilized by central mechanisms. In psychophysics, we measured vernier thresholds for drifting gratings of various luminance and chromatic contrast at a range of spatial and temporal frequencies. In physiology, we measured magnocellular (MC) and parvocellular (PC) ganglion cells’ response to similar stimuli as in the psychophysical experiments. Comparison of psychophysical vernier performance to the spatiotemporal information delivered by ganglion cells’ spike trains give insight as to how the luminance and chromatic information is utilized by cortical mechanisms and constrain models of spatiotemporal processing. Our data show that psychophysical vernier threshold is determined mainly by the luminance contrast, rather than the chromatic contrast of the vernier targets. The pattern of spatiotemporal precision of MC cells resembled the pattern of human vernier thresholds while those of PC cells did not, which suggests cortical mechanisms must utilize signals from MC ganglion cells.