Editing Visual acuity (section)

=== Lateral motion ===
There are acuity limits (<math>\dot\theta_t</math>) of horizontal and vertical motion as well.<ref name="Lappin-2009" /> They can be measured and defined by the threshold detection of movement of an object traveling at distance {{mvar|D}} and velocity {{mvar|v}} orthogonal to the direction of view, from a set-back distance {{mvar|B}} with the formula
<math display="block"> \dot\theta_t \approx \frac{B \cdot v}{B^2 + D^2}. </math>
Because the [[Tangent (trigonometry)|tangent]] of the [[subtended angle]] is the ratio of the orthogonal distance to the set-back distance, the angular time rate ([[radians|rad]]/[[second|s]]) of lateral motion is simply the [[derivative]] of the [[inverse tangent]] multiplied by the velocity ({{math|d<var>θ</var>/d<var>t</var> {{=}} d<var>θ</var>/d<var>x</var> · d<var>x</var>/d<var>t</var>}}). In application this means that an orthogonally traveling object will not be discernible as moving until it has reached the distance

<math display="block"> D \lessapprox \sqrt{\frac{B \cdot v}{\dot\theta_t} - B^2}, </math>

where <math>\dot\theta_t</math> for lateral motion is generally ≥ 0.0087 rad/s with probable dependence on deviation from the [[Fovea centralis|fovia]] and movement orientation,<ref name="Lappin-2009" /> velocity is in terms of the distance units, and zero distance is straight ahead. Far object distances, close set-backs, and low velocities generally lower the salience of lateral motion. Detection with close or null set-back can be accomplished through the pure scale changes of looming motion.<ref name="Schrater-2001" />