Motion processing at low light levels: Differential effects on the perception of specific motion types Justus Liebig University, Giessen, Germany Jutta Billino Philipps University, Marburg, Germany Frank Bremmer Justus Liebig University, Giessen, Germany Karl R. Gegenfurtner While many aspects of human vision at low light levels have been studied in great detail, motion perception has rarely been investigated so far. Here we address differential effects of light level on the perception of coherent motion, heading from radial flow, and biological motion. We determined detection thresholds under photopic, mesopic, and scotopic conditions. Results indicate that the perception of specific motion types differs in vulnerability to changes in light level. Thresholds for coherent motion and heading from radial flow increased monotonically from photopic to mesopic and scotopic light levels. We suppose that observed deficits are due to temporal pooling under rod-dominated vision. In contrast, detection thresholds for biological motion, which is distinguished by temporal dynamics and a specific spatial distribution of nearby signals, were exclusively elevated under mesopic conditions. Thresholds under scotopic conditions matched those under photopic conditions. Selective constraints under mesopic conditions might be explained by a detrimental interaction of rod and cone vision as well as by activity of different rod pathways. Findings suggest that very early retinal signal processing can have complex effects on the perception of different motion types, which is generally considered to rely on cortical areas. Keywords: motion detection, light level, coherent motion, radial flow, biological motion, rod vision, cone vision Citation: Billino, J., Bremmer, F., & Gegenfurtner, K. R. (2008). Motion processingat low light levels: Differential effects on the perception of specific motion types. Journal of Vision, 8(3):14, 1–10, http://journalofvision.org/8/3/14/, doi:10.1167/8.3.14. Introduction Our ability to process visual information over a wide range of light intensities, from bright daylight to faint starlight at night, reflects a remarkable flexibility of the human visual system. The maintenance of visual perfor- mance despite decreasing light levels relies on temporal and spatial summation which allows to capture more photons and thus preserves the ability to detect visual information. Adaptive information integration is accom- plished by the transition from cone-mediated to rod-mediated photoreception. While many aspects of rod-dominated vision in humans have been studied in great detail (for review, see Hess, Sharpe, & Nordby, 1990), motion perception has rarely been investigated so far. Indeed, specialized processing of motion information in the visual pathways does not occur before striate cortex (Albright, 1984; Albright & Stoner, 1995; Felleman & Van Essen, 1991). Since rods and cones make connections to the same post-receptoral pathways, it might be presumed that cortical processing of their signals is identical (for a review, see Bloomfield & Dacheux, 2001; D’Zmura & Lennie, 1986). van de Grind, Koenderink, and van Doorn (2000) have sophisticatedly compensated for effects of changes in retinal signal transmission by scaling stimuli according to temporal and spatial acuity. Their study has shown robustness of the central motion analysis system at low light levels. However, central motion analysis ultimately relies on input determined by retinal photoreceptors with specific spatio-temporal sensitivities and transmission characteristics. Motion perception is most obviously affected by tempo- ral changes under rod-dominated vision. The response of the visual system becomes more sluggish with decreasing light intensities (Dawson & Di Lollo, 1990; Takeuchi & De Valois, 1997). Several studies have established that perceived speed and speed discrimination are deficient under dim light conditions (Hammett, Champion, Thompson, & Morland, 2007; Raghuram, Lakshminarayanan, & Khanna, 2005; Takeuchi & De Valois, 2000). Some results indicate that impairment is more pronounced for velocities above 4-/s (Hammett et al., 2007; Takeuchi & De Valois, 2000). With regard to global motion detection (see Newsome & Pare ´, 1988), to our best knowledge, only Grossman and Blake (1999) have considered thresholds under scotopic conditions and found thresholds comparable to those under photopic conditions. They argued that motion detection in noise depends on pooling of local motion signals and therefore increased spatial pooling at low light levels should not affect performance. However, applied stimuli were restricted to a relatively low velocity range (3.2-/s to 8.0-/s) in which the effect of temporal averaging might not be evaluable conclusively. Although manipulating light levels represents an ecological valid approach to explore differences in perceptual performance, exact control over Journal of Vision (2008) 8(3):14, 1–10 http://journalofvision.org/8/3/14/ 1 doi: 10.1167/8.3.14 ISSN 1534-7362 * ARVO Received November 15, 2007; published March 18, 2008