Midterm 2 - Topic 7 Flashcards by Madelynn Hamilton

3 prerequisites for normal vision

At least one edge
Change over time
High level processes and experience

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What happens when there is no edge present in the visual field?

At least one edge is required, or there is nothing to see but a Ganzfeld

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What does the Ganzfeld demonstrate?

That without edges, there is no vision

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What can an edge/contour be defined as?

An area where there is a sudden change in brightness or colour

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Given the importance of contour, what is our visual system equipped to do? What is this called?

Highlight them
The process that highlights edges is called lateral inhibition

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Formal definition of lateral inhibition

The activation of a given photoreceptor by a light source, produces a reduction of activity in nearby photoreceptors

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What is crucial to emphasize that contour?

Differential inhibition from adjacent neurons for cells on each side of the contour (change in light intensity)

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What does lateral inhibition result from?
At what level?

An interaction between photoreceptors
At the level of horizontal and amacrine cells

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How does lateral inhibition help in perceiving edges?

By exaggerating the physical contrast between the edge and the background

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What does lateral inhibition allow us to see? Example?

Contours that are not there
As in Mach bands

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In receptive fields that overlap two bands, what happens?

The surround generates an inhibitory reaction (that is, lateral inhibition) making these receptive fields see a darker area than other receptive fields positioned entirely in one band

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Explain the prerequisite for normal vision that stimulation must change over time

The area of the retina that is stimulated must change

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How is the fact that stimulation must change over time demonstrated? What happens?

Through stabilized images
The same retinal area is continuously stimulated

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Why might the gradual fading of stabilized images occur?

Due to the length of the refractory period for the photoreceptors

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How does our visual system safeguard against stabilized images?

Equipped to provide these changes over time, by means of involuntary eye movements (microsaccades)

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Microsaccades

These are spontaneous, unconscious, and unavoidable movements and tremors made by our eyes

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High-level processes and experience

Early visual experience seems to favour a better development of neural structures involved in higher level processes (association areas)
People who lost vision for a time and had it restored later experience deficits with complex processes such as face and motion perception

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What does our visual system show with regards to high-level processes and experience?

High level of plasticity in adapting to changes

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Classic experiment demonstrating high-level processes and experience

Stratton’s experiments with inverting lenses in the late 1800’s
Adapted relatively quickly: “after 7 days things felt normal”

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Factors involved in detecting light energy

Adaptation
Time
Size
Locus
Cognition

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Bloch’s law equation

D * I = T
D = duration
I = intensity
T = threshold (constant)

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Explain how size is involved in detecting light energy

We are better able to detect the presence of light when a large area is stimulated than when a small area is stimulated (Ricco’s law)

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Explain how locus is involved in detecting light energy

Detection is easier when the light falls in the periphery of the retina
Light that enters near the centre of the pupil is detected faster (better) than light that enters near the edge of the pupil (Stiles-Crawford effect)

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Stiles-Crawford effect

Light entering the eye near the edge of the pupil produces a lower photoreceptor response compared to light of equal intensity entering near the centre of the pupil

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What is the Stiles-Crawford effect not due to? What is it due to?

Not due to screening of light by iris Due to the shape, size, and refractive indices of the various parts of photoreceptors This includes the fact that light near the edge does not hit photoreceptors head on

Explain how cognition is involved in detecting light energy

Lightness constancy The lightness of an object remains constant despite fluctuations in illumination Unconscious inference/Algorithm theory

How can cognition be involved in detecting light energy

Bright contrast and assimilation

What can brightness assimilation not be explained by?

Lateral inhibition

Visual acuity

Resolution capability of the visual system in terms of the smallest high-contrast detail at a given distance

What is acuity concerned with?

The discrimination of objects in space

What is the best way to specify acuity?

To define it in terms of the size of the retinal image involved or visual angle

Visual angle =

Size of the retinal image; can be defined as 1 cm seen as 57 cm

What is the most common type of measure of visual acuity? What does it involve?

Identification/recognition acuity The use of a Snellan chart

20/20 =

Person has to be at 20' to read what a normal observer can read at 20' (normal)

20/30 =

Person has to be at 20' to read what a normal observer can read at 30' (below or worse than normal)

20/15 =

Person has to be at 20' to read what a normal observer can read at 15' (above or better than normal)

Other acuity measures

Landolt rings Gratings Pattern discrimination Vernier acuity measures

Landolt rings

Good alternative to the Snellan chart for individuals who are not familiar with letters Have to determine where the gap is in each ring

Gratings

What is the direction of the lines in each stimulus?

Pattern discrimination

Provides a non-verbal alternative to the Snellan chart that can be used with infants Which pattern is different from the others?

Vernier acuity measures

Another non-verbal alternative to the Snellan chart How far must one of the bars (A) or spots (B) be laterally displaced in order to be detectably misaligned? Can also be applied to line tilt

Name 4 factors that affect visual acuity

Refractive error Area stimulated Luminance Age

When does refractive error occur?

When the shape of the eye does not bend light correctly

What does refractive error result in?

A blurred image

Name 4 of the most common types of refractive errors

Myopia Hyperopia Presbyopia Astigmatism

Myopia

Nearsightedness

Hyperopia

Farsightedness

Presbyopia

Loss of near vision with age

Astigmatism

Distorted shape of cornea curvature

Where does the light focus in myopia?

Light focuses in front of the retina

Where does the light focus in hyperopia?

Light focuses behind the retina

Where does the light focus in astigmatism?

Light focuses unequally on the retina

Explain area stimulated as a factor affecting acuity

Acuity is best at the fovea and gets worse as we move away from it Distribution of cones on the retina clearly matches the acuity of the different areas on the retina

Explain luminance as a factor affecting acuity

Acuity is best in photopic vision

What happens when luminance is too bright? Give an everyday example.

Results in bleaching Reading a book with small letters in direct sun

Describe age as a factor affecting acuity

Acuity decreases with age: could be due to neuronal degeneration and wear-and-tear on the visual system

What is one of the most basic visual functions?

Our ability to tell what parts of a visual scene belong together as a single object

Stream segregation for vision:

Figure-ground distinction

What creates the figure-ground distinction?

A fundamental organizing tendency

Who studied the organizing tendency closely? What did they find? What did they call them?

Gestalt psychologists Found a # of factors that determine the formations of a figure The Gestalt principles of perceptual organization

Proximity

Objects in close proximity tend to be grouped together

Similarity

Objects that are similar in shape, texture, etc. tend to be grouped together

Closure

Objects that form a more enclosed or complete figure tend to be grouped together

Good continuation

Objects that form a continuous (rather than jagged) pattern tend to be grouped together

Symmetry

Objects that are symmetrical tend to be grouped together

Figural goodness

Law of pragnanz (good figure): Objects that form the simplest configuration tend to be grouped together

Sub-principle of figural goodness

Minimum principle: The organization that will arise from an ambiguous configuration is always the simpler one

Why should we care about basic visual functions?

We have just discussed what we all need to see correctly