Introduction to Vision

Implications for the developing child

What is Vision?

• Our ability to see helps us to gather information on:

• Form

• Colour

• Movement

• Depth

Vision: Key Questions

• How do we detect depth given that the back of the eye is a two dimensional sheet and that depth perception is not directly preserved from the outset?

Vision: Key Questions

• How do we register shape in a way that allows us to recognise familiar objects under varying circumstances – including when parts are missing or added?

Vision: Key Questions

• What is the relationship between vision and mental imagery?

• What aspects of visual perception are learned?

• What does it mean to ‘see’ something?

Sequence of visual learning

• First Awareness

• First Attention to Light

• First Fixation

• Then Attention• Then Understanding• Then People Then

Objects

• Then Tracking

Sequence of visual learning

• First interest in distant objects

• First Peripheral

• Then near objects

• Then Central Vision

Sequence of visual learning

• First preference for familiar

• First viewing part of an object

• Then preference for novel stimuli

• Then viewing whole of object

Sequence of visual learning

• First interest in simple items and designs

• First interest in large items

• Then interest in complex items and designs

• Then interest in small items

Beginnings

• The eyeballs develop during the 12th week of gestation

• The eyes are part of the brain

• Seeing is a learnt and dynamic process

• Seeing is highly personal

Beginnings

• Vision is affects and is affected by, the whole body and personality

The importance of vision

• Vision is only one of the five senses but greater reliance is placed on it than all the other senses put together.

Function

• Human eyes evolved to give good eyesight in daylight

Eyes

• Eyeballs are spherical - diameter 23mm

• Eyeballs maintain their shape by means of a fluid called aqueous at the front and a jelly like substance called vitreous in the main body

Functional Vision

• Relies on:

• Spatial analysis

• Temporal analysis

• Cognitive analysis

Seeing Believing and Knowing

• Perception: The foundation of coming to know about our world

• Memory: The ways we retain and build on knowledge

• Mediation: The bridges between aspects of perception

Parts of the Eye

• 1 The Front or “window” - cornea, iris,lens

• 2 Middle - globe

• 3 Innermost coat - the retina which contains rods and cones

Visual acuity

• The ability to discriminate high-contrast fine detail at a distance

• The power of the eye to distinguish form

• The sharpness and clarity of vision

Assessment

• Distance

• Near

• Field of vision

• Colour vision

Implications

• Near vision – N number (usually smallest size child can read)

• Field – the area that can be seen from all parts of the eye when looking ahead

• Colour – common red/green confusion

• Contrast sensitivity – need for clear definition (central vision issues)

Visual Systems

• Two streams in development– where and what

• Three streams of processing – where, what and how

Ventral and Dorsal Streams

• One stream for perceptual processing and one stream for controlling actions

• Ventral Pathways – contain specialised areas for face perception (who)

• Dorsal Stream – controls eye movements, reaching and grasping (how)(action)

A Typical Visual Child: 3-5 years

• Can:

• See in detail with good contrast sensitivity and accommodation

• See in colour

• See in depth

• Keep eyes straight

A Typical Visual Child: 3-5 years

• See all around whilst maintaining interest on target centrally

• Move the eyes in controlled fashion – Further refinement takes place 8-12 years

• The critical period: Up to 9-10 months

Intervention

• 1. Front of Eye – intervention +

• 2. Eye globe – intervention +-

• 3. Back of globe/neural path --?

Overall Impact of Visual Impairment

• Range and variety of experience

• The ability to get about

• The control of the environment and self in relation to it

Individual Differences Related To

• Degree of vision

• Cause of visual impairment

• Age and type of onset

• Present condition

• Personality

DEFINITIONS

• Snellen (1869) – acuity and field loss

• VI – visual acuity less than 6/18 but better or equal to 6/60

• Severe VI – visual acuity of less than 6/60 but better or equal to 3/60

• Blind – visual acuity of less than 3/60

Degree of Vision

• Total blindness

• Partial Sight/Low Vision

Causation

• Ocular

• Cerebral

Age and Type of Onset

• Congenital

• Acquired

• Sudden

• Gradual

• Stable

• Degenerative