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Abstract

During linear self-motion at constant speed, the retinal speeds of stationary objects vary as
a function of their declination angle (the angle between the line of sight and the horizontal
plane). Nevertheless, when we move in our environment, we do not feel that different places
move at different speeds: a compensation mechanism is thought to mediate between angular
velocity and perceived linear speed so that velocity constancy is achieved. In a recent study
[1] it has been shown that the perceived speed is altered when driving with a reduced fieldof-
view (FOV). The explanation proposed in that study leads us to the hypothesis that, when
moving at constant speed, humans might not be able to compensate for the different velocity
signals coming from various declination angles when only a limited portion of the visual field
is visible. Here we tested this hypothesis using a Virtual Reality (VR) setup that provides a
230×125 (H×V) FOV. We measured the visual perceived speed at eye-height (1.7m) while
simulating fast walking speeds on a virtual open field. We manipulated the FOV (full field vs.
limited field corresponding to an aperture of 40×6) and the gaze declination angle (12, 20
and 28 degrees), corresponding to positions on the plane located at a distance of 8, 4.7, and
3.2 m, respectively. We used a two alternative forced choice (2AFC) with constant stimuli
method in a 2×3 within subjects design. We tested eight different speeds ranging from 0.67
to 6 m/s. The reference stimulus appeared always in the intermediate declination angle at the
speed of 2 m/s. A fixation cross appeared at the desired declination angle 500 ms before each
stimulus. At every trial, subjects had to select which of the two presented stimuli indicated
a faster linear forward speed. The results of four observers show that when looking with a
different declination angle in the test, the perceived speed appeared either higher or lower than
the reference speed. This effect was accentuated in the limited FOV condition, suggesting that
limiting the FOV impairs the compensation mechanism. Interestingly, while two observers
could not fully compensate for the perceived retinal speed even within a full FOV condition,
the other two showed a reliable over-compensation independently of the FOV. This indicates
that a veridical speed estimation cannot be achieved in VR and with limited FOV and that speed
estimation is not independent of gaze direction.