Neural Basis of Stereopsis Flashcards
What is stereopsis?
The perception of depth.
(Stereo= Solid and Opsis=sight in greek)
Do you need two eyes to perceive depth?
Technically no, even from one eye using monocular pictorial clues such as shadows, occlusion, relative size, etc, one can determine depth
Why do we get binocular stereopsis - give a general answer?
The disparities in images seen between the two eyes leads to two slightly different retinal images. These are interpreted by the striate cortex,V1 and extra-striate areas, via single neurones which are ‘tuned’ to act as ‘Disparity Detectors’, from which the basis of depth is perceived.
What is the benefit of having two eyes in regards to depth perception?
It greatly enhances depth perception (we use both monocular and binocular cues)
How can we test a patient’s ability to discriminate depth?
Stereo-acuity tests
In what sitaution can images from the eyes have disparities yet be corresponding?
When viewing an image on the horopter/Veith Muller circle.
Disparities - because the two eyes are not in the same place therefore there will be disparities between the images produced from each eye.
Retinal Correspondence- this is when the image corresponds to the retina of both eyes- i.e. if the left eye is using its fovea to focus the right eye is too or if the left eye is using nasal retina, the right eye will be using temporal retina by the same amount- this happens when the image being viewed is on the theoretical horopter/Veith Muller Circle.
In what situation do we get disparities and (complete) non retinal correspondance?
When viewing an image outside of panum’s fusuional area
What are the two things to note about retinal disparities when their is non-retinal correspondance (i.e. diplopia bc vision falls outside of the panum’s fusional area)?
True or False- cells above layer 4C recieve binocular input (i.e. are innervated by input from both eyes)
True - follow the red and blue lines in the diagram.
(every layer other than 4C recieves binocular input)
How are the receptive fields of binocularly driven cortical cells similar and different in each eye?
They are similar in respect to:
Spatial location
Preferred orientation, frequency etc,
Organisation (simple, complex, hypercomplex).
Because they are analysing the same stimulus as seen by each eye. However, they differ in their:
Preferences for horizontal stimulus disparities
Spatial ‘offsets’ - i.e. basically positions of the receptive fields between the eyes may overlap or be slightly further away than normal in order to pick up positional disparities i.e. that crossed or uncrossed disparity where an image is further or closer to you than what you are focusing on (the horopter).
What are the different types of disparity tuned cells (i.e what does each type of ‘disparity detector’ looks for)?
Different Disparity tuned cells show seectivity for different signs (i.e. positive or negative disparity) and different size disparities (including zero).
While others may be only tuned for a specific disparity sign, responding to a rnage of crossed (near) or uncrossed (far) disparities.
Describe the Six classes of Disparity Tuned Cells
All six are labelled in the picture.
Tuned ones are cells that respond to both size and sign.
Near or far ones just respond to sign (i.e. whether the stimulus is infront or behind the horopter).
The excitatory zero one responds to no disparity between images.
True or False- the response of tuned disparity cells to binocular input is far far greater than it’s repsonse to stimulus from a single eye
True - (this is called facillitating in VP terms).
What’s the difference between a tuned and only near or far disparity cells?
Tuned ones have a distinct peak.
Tuned cells facilitate.
Far or near cells summate responses from both eyes.
In which pathway, the parvo or magno, does a lesion cause a significant reduction in stereoacuity?
A lesion in the parvo pathway causes a significant reduction in stereoacuity.
What are the three clinical grades and what are their neural requirements?
How do random dot stereograms work?
The px wears filtered glasses e.g. redgreen glasses and so each eye views a slightly different image.
Some dots seen by each eye are slightly displaced while others remain in the same place giving rise to the appearance of depth.
