Previous studies revealed other anisotropies in discrimination and detection of visual motion. For instance, centripetal motion can be detected and discriminated better than centrifugal motion (Giaschi, Zwicker, Young, & Bjornson,
2007). Directional anisotropies are opposite at low- and high-speed conditions (Naito, Sato, & Osaka,
2010). At low speed (
Display Formula\(4\ \rm deg\ {s^{ - 1}}\)), centrifugal directional anisotropy was observed, while at high speed (
Display Formula\(16\ \rm deg\ {s^{ - 1}}\)), centripetal directional anisotropy was observed in the peripheral upper visual field. The perceived depth of moving random dots depends on its motion directions, and this preferred direction is usually either downward or rightward (Mamassian & Wallace,
2011). Direction discrimination of moving random dots depends on the axis-of-motion, with the response being more precise for objects moving cardinally compared to oblique motion stimuli (Matthews & Qian,
1999). Instead, no systematic differences across the cardinal directions have been reported in direction detection and discrimination experiments (Gros, Blake, & Hiris,
1998). A recent study investigated whether motion direction produced a speed bias (Manning, Thomas, & Braddick,
2018). The authors found that stimuli moving along an oblique axis are perceived on average as faster than those moving along cardinal directions, with some differences in this result between the four experiments of the study. Instead, the authors did not find any systematic difference between downward motion and the other cardinal directions. Two possible reasons may explain the difference with our findings. Manning et al. (
2018) used two sets of random dots presented side by side for a short time window, equal to 300 ms. Instead, we presented gray-shaded disks moving across a circular aperture that, when moving downward, might evoke the sensation of a falling object. Accordingly, previous studies showed that effects related to the representation of gravity are modulated by the realism of the visual scene (Miller et al.,
2008; Moscatelli & Lacquaniti,
2011). Additionally, in Manning et al. (
2018) the repetition of vertical and oblique directions in the reference stimuli may have led to adaptation inducing shifts in perceived speed. Instead, in our protocol the reference and the comparison stimulus appeared each in 50% of the stimuli; hence, a putative adaptation affected the two motion directions equally. The neural basis of motion anisotropies has been deeply studied; for a review of the literature see Maloney and Clifford (
2015). Interestingly, anisotropies in the activity of early visual areas depends on stimulus contrast: Maloney and Clifford (
2015) reported an orientation preference for vertical orientations at low contrasts, which instead shifted toward oblique orientations at high contrast. To the best of our knowledge, our study is the first showing a speed bias associated with downward motion. We hypothesized that this downward bias may depend on prior expectations on the effects of gravity on object's motion. Previous studies involving perceptual and motor tasks provided strong evidence about the role of prior knowledge of gravity in motion processing in vision (La Scaleia et al.,
2014; McIntyre et al.,
2001; Moscatelli & Lacquaniti,
2011; Senot et al.,
2005; Zago et al.,
2004). Adaptation to downward visual motion produces a tactile motion aftereffect, which is stronger than after upward visual motion adaptation (Konkle, Wang, Hayward, & Moore,
2009). Humans take gravity into account to estimate the stability of a pile (Battaglia, Hamrick, & Tenenbaum,
2013), and in shape judgment tasks our visual system partially relies on a gravitational frame of reference where the light-source is assumed as roughly overhead (Adams,
2008; Adams, Graf, & Ernst,
2004). Imaging studies shed light on the neural correlates of the representation of gravity with respect to target motion (Indovina et al.,
2005; Lacquaniti et al.,
2013). Because vision is weakly sensitive to accelerations, prior knowledge accounting for the effects of gravity is derived from graviceptive information, is stored in the vestibular cortex, and is activated by visual motion that appears to be coherent with natural gravity (Indovina et al.,
2005). Additionally, the over-representation of downward direction in mammals' visual cortex may also partially explain anisotropies in motion perception (Konkle et al.,
2009; Ribot, Tanaka, O'Hashi, & Ajima,
2008).