Stereopsis: Understanding the Connection Between VISUAL ANGLES ,FIXATION and BINOCULAR DISPARITY.

Have you ever marveled at how we can perceive the world in three dimensions? It’s all thanks to an incredible process called stereopsis. In this post, we’ll explore the connections between commonly encountered terms in strabismus – visual angle, fixation, and binocular disparity. By unraveling these concepts, we’ll delve into the magic of stereopsis.

Table of Contents

If you would like to watch this as a video, you can watch it from here! Let us start by understanding what is fixation.

What is Fixation?

Fixation is the magical ability of our eyes to maintain focus on a specific point in our visual field, ensuring sharp and detailed vision. In our previous discussions, we explored Panum’s fusional area, where objects within this area appear single, while those outside may cause double vision or diplopia.
To know more about the panums fusional area; check out our video on binocular single vision.

What is Visual Angle?

Visual angle refers to the angle formed by lines extending from the edges of an object to the point of observation – our eyes. Imagine the girl in the above image is looking at a rose. Now the rose shall extend an angle at the girl’s eye. This is known as the visual angle!

It is crucial to note that in the case of binocular vision, each eye receives a slightly different view of the object, resulting in two different visual angles.

This happens because of the horizontal separation of the two eyes and the interpupillary distance.

Understanding Binocular Disparity/Fixation Disparity

Stereopsis begins with binocular disparity
Because are eyes are horizontally seperated, the images formed are also different due to the difference in the visual angle, although this difference might be really minute (few seconds of arc)
Binocular disparity refers to this slight difference in the images received by each eye due to their horizontal separation (disparate retinal images)

BINOCULAR DISPARITY : NORMAL or PATHOLOGICAL

Binocular diaparity is absolutely normal.
Infact it is a physiologic variant of normal binocular vision.
It is important to note that for the binocular disparity to be able to produce stereopsis ;there should be only a minute image displacement/difference, rarely exceeding several minutes of arc of angle between the images of the two eyes
And the object must still lie within Panum’s area and the fusion still maintained

DID YOU KNOW :–>

Vertical disparity doesnot play a major role in stereopsis. It is the horizontal disparity that majorly plays a role in depth perception.

Binocular disparity and Visual Angles

By analyzing the binocular disparity, the brain can calculate the visual angles subtended by the object in each eye’s view and also vice versa.
Its like the brain using complex trigonometry to estimate the distance to the object based on these visual angles. For example, a small visual angle gives the brain information that the object is far away. Whereas a large visual angle tells that the object is near by.
The brain can now find the difference in the two visual angles and based on that calculate disparity and ultimately the depth of the object.

What is depth perception ?

Understanding the depth of an object involves more than just knowing how far or close it is. Depth perception also involves assessing how thick or thin a three-dimensional object appears.

What is Stereopsis?

Stereopsis is the perception of the relative depth of objects on the basis of binocular disparity

What is Stereacuity?

Stereoacuity is a way of measuring stereopsis
It is an angular measurement of the minimum resolvable binocular disparity necessary for the appreciation of stereopsis.
Let us break down this definition further by understanding what is meant by the terms ” angular measurement” and “minimum resolvable angle of binocular disparity“.

Minimum Resolvable Binocular Disparity:

The minimum resolvable binocular disparity is the smallest difference in the positions of two points in the visual field that can be detected by the visual system as separate in depth.
In simple terms, binocular disparity refers to the slight differences in the images received by each eye due to their separation.
Because the eyes are horizontally separated, the images are horizontally separated. This creates a horizontal image disparity .The minimum amount of horizontal disparity that the brain can process to give depth perception and stereopsis is known as the minimum resolvable binocular disparity.

Angular measurement

In the context of stereoacuity, angular measurement refers to the angle subtended by the minimum resolvable binocular disparity.
To further elaborate on that; binocular disparity occurs because each eye has a slightly different viewpoint, resulting in differences in the visual angles for corresponding points in the scene.
The angular measurement of stereoacuity quantifies the difference in the visual angles perceived by each eye for these corresponding points.
It represents the smallest discernible difference in the visual angles of two points that the brain can interpret as separate in depth, leading to the perception of stereopsis or depth perception.
If all this is confusing ; you can check out our video on the same.

Understanding Arcminutes : Tiny unit ; big insights

“Arcminute” (often abbreviated as arcmin ′) is a unit of angular measurement commonly used in optics, astronomy, and vision science.
It comes from the Latin word “pars minuta prima,” which means “first small part.”
The “min” is derived from “minuta,” which refers to a small part, and “arc” denotes the arc of a circle.
Think of it this way: if you’re looking at something far away, like a distant mountain or a star in the sky, and you want to measure how much it appears to move when you move your head slightly, you’d use arcminutes to describe that tiny change in angle.
Similarly, when we’re talking about stereoacuity and trying to measure how well our eyes perceive depth, we’re dealing with really small differences in the angles our eyes see.
Arcminutes help us precisely measure those differences, providing valuable information about the sharpness of our depth perception.

Degrees V/S Arcminutes V/s Arcseconds

Degrees: This is the largest unit of angular measurement. A full circle is divided into 360 degrees.
Arcminutes (arcmin): One degree is equal to 60 arcminutes
Arcseconds (arcsec):One arcminute is equal to 60 arcseconds.

Pizza Analogy & Arcmin

Imagine having a big pizza
The pizza can be divided into 360 slices. (each slice representing a degree).
Now if you take one slice of this pizza and further divide it into 60 parts; one part of those sixty parts will be your arcmin.
Yes! it’s that tiny.
You can now imagine and calculate how tiny an arcsecond would be!

Conclusion

Understanding fixation, visual angles, and binocular disparity is crucial for comprehending stereopsis. By connecting these concepts, we gain valuable insights into the intricacies of depth perception.