Matthew N. Martino¹ (), Jess M. Smith¹, Bradford Z. Mahon¹; ¹Carnegie Mellon University

Research on psychophysical dissociations between magnocellular and parvocellular pathways has largely used screens to present visual stimuli to maintain precise control of all stimulus properties. Here we took a step to advance understanding how different visual pathways contribute to perception of real world stimuli. Parvocellular pathways have differentially high sensitivity for stimuli defined by high spatial frequency red-green isoluminant contrast when compared to magnocellular pathways. We exploited this asymmetry between the parvocellular and magnocellular pathways to test motion detection of a ruler (oriented and dropped vertically, at either 2° or 30° to the right of fixation, monitored with an eye tracker). On each trial, the ruler would drop at a random time point (controlled by PsychoPy), and participants’ task was to push a button as soon as they detected the drop. Care was taken to control and test for any auditory cues associated with the ruler dropping. The experimental design represented the factorial combination of spatial frequency (low vs high), contrast (isoluminant heterochromatic red/green vs black/white), and eccentricity of the ruler (2° vs 30°). Rulers were painted with bands of alternating contrast perpendicular to the direction of motion. Equivalent luminance (reflectance) between the red and green colors on the ruler was ensured with a photometer. A 2x2x2 ANOVA of response times demonstrated a significant main effect of spatial frequency (low < high), contrast (black/white < red/green), eccentricity (center < periphery), as well as a significant 3-way interaction. Follow-up hypothesis testing confirmed the key prediction that the slowest motion detection was for the high-frequency and red-green contrast ruler presented in the periphery. These findings demonstrate sensitivity to differences between the magnocellular and parvocellular pathways, using physical real-world stimuli. Ongoing work is applying this paradigm to study visuomotor function after focal lesions in the geniculostriate and non-geniculostriate pathways.

Acknowledgements: This research was supported in part by the Carnegie Mellon University Summer Undergraduate Research Fellowship