OPT 215: Binocular Depth Perception 1

What is distance perception?

the visual judgement in 3 dimensional space of how far something is from you or from some other reference point in absolute terms of measurement

What is depth perception?

visual judgement in the z dimension of space, of the distance between objects or surfaces expressed in relative terms of measurement

What are monocular cues to depth/distance?

-aerial perspective
-interposition
-shading/shadow
-linear perspective
-texture gradient
-retinal image size (familiarity with object)

What are examples of depth from motion?

-motion parallax
-relative velocity
-looming (size changes)

Describe depth from motion parallax.

-2 point distance information is lost when viewed from a single vantage point
-knowing visual directions and separation between two points provides complete 2 point distance Info
-one moving vantage point gives the same information

T or F: shape from motion and disparity based depth perception share some underlying neural mechanisms.

True; adapting to a given binocular disparity can bias one's percept of shape from motion and vise versa

Since stereo deficient patients can still perceive structure from motion, what can be concluded?

shape from motion is not entirely processed by the same mechanisms as is depth from disparity

T or F: disrupting the normal accommodation convergence relationships can modify our judgements of perceived distance and perceived size.

True; the status in innervation to the various EOMs can be used to approximate the distance of a fixed object

Describe SILO.

small in large out

nearer locations of the apparent target position have binocular parallax of larger magnitude than the original vergence angle (disparity is a positive number which stimulated convergence)

For SILO, what does BO prism do?

signals that the target is getting closer to the observer but the retinal image size is not changing

What is Emmert's Law?

for the same visual angle, closer images are perceived as smaller; as the target appears to be getting closer with increasing BO, the observer sees it as shrinking

Why does SOLI occur?

because the subject expects that nearer objects should look bigger; since convergence signals that the target is getting closer, the patient thinks it should look bigger as it gets closer

What is absolute depth (distance)?

the distance of a given single object from the egocenter of the observer

What does absolute depth determine?

the binocular parallax

What is the binocular parallax?

the angle formed by the intersection of the lines connecting the nodal point of each eye to the object point; does not depend on the vergence angle of the eye

Does the absolute depth change with vergence?

No

What is absolute disparity?

the difference between the binocular parallax of an object and the convergence angle of the eyes

T or F: an object whose image falls on non corresponding retinal points has an absolute disparity with respect to the horopter.

True

How is absolute disparity calculated?

n= aL-aR

What is the stimulus for fusional vergence?

Absolute disparity; presence of absolute disparity will stimulate fusional vergence movements

What is the visual system's sensitivity to absolute disparity?

6 minutes of arc; smaller than this will not stimulate a vergence movement so they cannot give information about target distance

What is relative horizontal disparity?

the difference between the absolute horizontal disparities of two points; two or more objects that are not on the same horopter will have a relative disparity with respect to each other

What is relative horizontal disparity a stimulus for?

stereopsis; only direct means of seeing depth, mediated by special mechanisms in the neural visual system

Why does absolute disparity stimulate vergence?

to bifoveally fixate an object (designates the distance of the horopter with respect to the observer)

What does relative disparity encode?

relative depth between the fixated object and other objects in the binocular field of view

T or F: relative disparity in real space is inversely proportional to the square of the distance from the viewer.

True; this means that the relative disparity of the block at the new distance changes

How much change in depth is caused by fixation drifts and tremors?

7-8 arcmin; we don't see depth changes because these disparities are added equally to all objects in the visual field (relative disparities remain essentially the same; so depth does not change)

What is depth constancy?

the perceptions that the depth within a familiar object is unchanged despite the change in binocular disparity that occurs when the viewing distance changes

When do vertical disparities exist?

-when an object is nearer to one eye than to the other
-when the observer is experiencing the induced effect of aniseikonia
-with vertical or torsional eye misalignment (FD, strabismus, head tilt)
-most natural scenes
-the visual stimulus can be created to have a vertical disparity

What is absolute vertical disparity?

the difference in the elevations of a point for the two eyes
-points in the median plane of the head have zero vertical disparity
-vertical disparity of a point above or below the plane of regard increases from zero as it becomes more eccentric w respect to the median of the head

What is relative vertical disparity?

the difference between the absolute vertical disparities of two points (not affected by the vertical misalignment of both eyes)

T or F: vertical disparities alone do not yield a perception of depth.

True; the information is used to scale perceived depth and size of stereoscopic surfaces if the field of view is sufficiently large

What is the horizontal disparity gradient?

linear change in horizontal disparity across space; encodes the slant in depth

How is disparity gradient calculated?

disparity/separation

[disparity (P1)- disparity (P2)]/ {[location (P1)+ locations (P2)]/2}

What if the disparity gradient of P1 and P2 are on the horopter?

0; aR=aL

When is the disparity gradient infinity?

when P1 and P2 are on cyclopean line of sight; aR=-aL

What is meant by inclination/declination?

tilt with respect to the 180th meridian

What is meant by slant?

tilt with respect to the 90th meridian

T or F: two vertical lines occupying the same location in space but slightly rotated w respect to one another may be perceived as a single line, slanted in depth.

True; likely due to gradually changing horizontal disparity along the lines rather than specific orientation disparity tuned neurons

How are changes in rate of change of disparity perceived?

curvature in the z dimension