The visual flow field provides linear and curved motion perception in which the visual field disposal it is transformed in constant images flow. We know we are moving in a still environment but we perceive objects in space and the ground surface texture as moving in the opposite direction. Actually, we perceive them as lines or stripes which are used as information for determining motion trajectory.
In addition, we must take into account that as we are moving our head constantly to self-orient in the environment, our perception of the mountain visual scene also changes. Our motion creates a visual flow and this flow establishes our motion information that we use to control our actions. The availability of optical flow information does not mean that all skiers take advantage of it.
Optic flow
Optic flow is the term proposed by Gibson (1950) to determine how our eyes transform images on the retina in vectors (lines) of the ground surface during our motion. This illusory images’ displacement is opposite to the direction of our linear motion, including forward (straight run) and lateral sliding (side slipping), as well as in curvilinear motion if our gaze follows our skis’ direction or if our eyes or our head rotate. The optic flow caused by body oscillations is used as feedback for our postural stability and for our speed control since it increases or decreases with motion speed.
Optic flow is classified in three different aspects:
- Radial flow, also called frontal or expansive, which is the flow pattern guiding our linear motion.
- Lamellar flow or lateral, is the pattern produced by curved motion or eyes rotation.
- Contraction flow or posterior,indicating the distance of objects moving backward (the size of objects decreases while moving away as seeing them through a rear-view mirror of a car).
When we move on a linear trajectory and orient our gaze to one side or another, or while moving on a curved path, the optical flow ceases to be radial and turns into lamellar because it includes translation and rotation mechanisms. Both, the optical and the lamellar flow, are essential for controlling our path direction.
Optic flow and adverse weather conditions
It is more difficult to estimate our motion direction and speed when it snows, it is foggy, or there are flat light conditions because the optic flow decreases.
When it snows we perceive not only the optic flow of the ground surface but also the optic flow of the falling flakes. In this situation, we should pay attention to the ground’s optic flow ignoring the snow falling flow, which is not easy if the snowfall is heavy or the flakes are blown by the wind. Here, the difficulty consists in that we tend to follow the snowflakes’ focus of expansion instead of the one of our motion direction.
Fog reduces the ground surface area from where our motion image is available, affecting our balance and influencing the low or non-existent contrast, not allowing to properly detect the optic flow and altering our direction and speed perception.
Optic flow and snow texture
Snow texture influences our speed perception because we detect the optic flow information while sliding on it. We tend to perceive higher speeds when ground irregularities are more frequent or when there is more contrast between them. The greater the texture relevance, the better we will detect visual details, allowing substantial improvements to our direction control.
Skiing in flat light or whiteouts makes it difficult to distinguish snow texture so the optic flow is reduced, inhibiting accurate visual information. Under these conditions, we should depend on vestibular and proprioceptive systems to adjust our balance.
Global optic flow
Global optic flow is the optic flow total value of snow texture determined by our speed and the height of our posture. It increases if we ski faster and closer to the ground. Speed perception is determined mostly by global optic flow (Dryre, 1977). When we get into a tuck (aerodynamic posture) not only we increase our motion speed by decreasing air resistance; in addition, we perceive speed intensification because of approaching the ground’s optic flow.
Focus of expansion
The focus of expansion indicates motion direction and we use it to guide our trajectory. It is the point where all flow lines originate, coinciding with the direction of our motion. We perceive our forward motion by noticing that objects in our visual scene are enlarged from this point, coming radially from the center to the periphery of our visual field.
Our eyes and head rotational movements disturb the focus of expansion detection. If we change our gaze constantly, the focus of expansion is moved to another point (the one of our gaze) and will not coincide with our motion direction. This situation is referred to as the visual rotation problem.
Another arising situation is that when turning, we visually perceive the lamellar flow where visual scene flow lines pass in the opposite direction to the curvilinear path, altering our global visual perception of the turn.
Motion parallax
Motion parallax is an optic flow aspect determining the relative distance between objects. It is generated by our motion, perceiving that lateral images of nearby objects move in the opposite direction faster than more distant ones. This perception provides a reference of the relative distance objects are while in motion in which closer ones pass quickly while more remote seem static.
Sensory systems involved in a skiing motion
The principal sensory organs to detect motion are:
- The visual system: our eyes capture a greater part of the information, assisting our skiing trajectories’ planning and obstacle avoidance.
- The vestibular system: our vestibular organs detect accelerations and are composed of otoliths, which detect linear accelerations, and semicircular canals detecting angular accelerations.
- The somatosensory receptors, which include tactile receptors and the proprioceptive sense. Our tactile receptors detect changes of pressure in postural changes, the haptic sense referred to touch in motion and the proprioceptive sense regarding our limbs’ positioning and their accelerations or decelerations. Berthoz (1997) suggests that the ski racer would use his mental image of a race since the interpretation of sensory information is a slow process and would be used to validate or to correct his mental representation.
