Reaction Time: A Determinant of Photoreceptor Location

Scott Tusa and Henry Vogt

 

CU Boulder, Fall 2006

 

 

         Are cones predominant in the human fovea and are rods predominant in the peripheral region around the fovea? The fovea is an area in the center of the retina where light is focused by the lens. The retina is also where the photoreceptors of the eye, the rods and cones are located.  Rods have evolved to allow humans to see black and white in low light conditions, specifically at night. Cones have evolved to allow humans to see color in bright light conditions, specifically during the day. If rods are primarily found in the peripheral region around the fovea and cones are primarily found in the fovea, our experiment testing a personsÕ reaction time to a light directly in their vision and to a light in their peripheral vision should determine if this statement is correct.

         To test this hypothesis we used a reaction timer to measure individualsÕ reaction times to four different stimuli: Lights On(Direct)-Lights On(Periphery) and Dark(Direct)-Dark(Periphery). With the lights on, we measured individualsÕ reaction times when looking directly at a white light and when only using their peripheral vision. With the lights off, we again measured individualsÕ reaction times when directly looking at the white light and when only using their peripheral vision. We ran these tests on fourteen individuals. If cones are primarily found in the human fovea, we predict the reaction time to the Lights On(Direct) to be significantly faster than the Lights On(Periphery). We also predict that the reaction time to the Dark(Periphery) will be significantly faster than the Dark(Direct) if rods are primarily found in the peripheral region around the fovea.

         The mean reaction times to Lights On(Direct) vs. Lights On(Periphery) were 0.523 and 0.539 respectively (p=0.309). The mean reaction times to Dark(Periphery) vs. Dark(Direct) were  1.22 and 1.21 (p=0.466). 

         Our results were inconsistent with our predictions based on our hypotheses. There was not a significantly faster reaction time to Lights On(Direct) vs. Lights on(Periphery), indicating that cones are not primarily found in the fovea. Also, there was not a significantly faster reaction time to Dark(Periphery) vs. Dark(Direct), indicating that rods are not primarily found in the peripheral region around the fovea. One potential problem with our experimental design was that we did not get the room dark enough to accurately test our rods. We would not change our hypotheses or predictions but rather alter the experiment to better fit our hypotheses. Since cones detect color in bright light conditions and we used a white light, our conclusions may not be as valid as possible. This would mean we actually tested our rods in the bright light condition instead of our cones. We should have used red light in the light to measure reaction time of the cones and a white light in the dark to measure reaction time of the rods.