Topic 9a: common visual deficits Flashcards
What do people with dyslexia have problems with?
Where in the brain is this caused?
How does this affect them seeing motion?
What is their coherence threshold for judging direction?
- Problems reading, spelling, and sequencing events
- Magnocellular part of the visual system- dorsal (where) pathway via areas MT + STS (timing + rhythm)
- Less able to see coherent motion amongst random motion (only slightly, needed 3% more dots than controls)
- 75%
If the magnocellular theory explains difficulty reading in dyslexics, what about poor spelling?
What is true dyslexia characterised by?
True dyslexia is characterised by poor temporal processing (impaired visual and auditory sequencing), that is caused by impaired development of magnocellular systems throughout the brain
What is schizophrenia characterised by?
Do they experience poor motion coherance?
- Disorganised thinking and speech, poor memory, auditory (not visual) hallucinations, poor social and emotional function
- Yes
People with autism have low social awareness and heightened sensory awareness with excessive attention to detail and change. What about their coherent motion?
Children with autism show significantly higher motion coherence thresholds than typically developing children (i.e., they show an impaired ability to detect coherent motion).
Dyslexia, schizophrenia and autism have a common visual symptom. Are they caused by the same thing? What are two possible explanations?
- Not necessarily
1) Dorsal system (includes Superior Temporal Sulcus) has a role in processing motion, event timing, and social stimuli that is disrupted in these conditions
2) Dorsal pathway is biologically fragile: affected by multiple biological and developmental problems.
How common is photosensitive epilepsy?
What triggers photosensitive epilepsy?
When are they likely to occur?
What does EEG show for photosensitive epilepsy?
About 4% of population have epileptic seizures at some point in life; in 4% of these, seizures are triggered by visual stimuli - Photosensitive epilepsy.
Visual triggers include:
Stroboscopic flashing in the range 3-60 Hz (flashes per second). Peaking in the population at 15 Hz.
Striped patterns 0.2 – 10 cycles per degree of visual angle. Peaking in the population at 2.5 c/deg (reading text at 12pt at arms length is similar to 2.5 c/deg of visual angle).
Likely to occur:
- Watching videos / using computers.
- Driving when the sun is behind trees.
- Night clubs, concerts.
- Flash photography on the news.
Evidence of cortical excitation
What is a migraine?
What can cause them?
Severe prolonged headache including:
- Nausea
- Photophobia
- Phonophobia
- Preceded by aura
You can have aura without headache
Can be triggered by visual stimuli, the same visual stimuli that affects epilepsy
What causes migraines?
Something (visual stress) triggers excitation in cortex:
The cortex fails to inhibit excessive excitation
Neuron get fatigued (which inhibition is trying to avoid)
Neurons run out of oxygen (so the brain dilates blood vessels to restore depleted oxygen levels) -
you get whopping big headache!
What is migraine aura?
‘Hole’ in fovea (absence of visual) followed by shimmering, oriented, coloured lines slowly moving from centre to edge of vision getting larger as they move out.
Can’t see past the lines.
Activity spreading across visual cortex?
- Visual stress / Meares-Irlen Syndrome shares symptoms with…
- How much of the population is affected?
- What are the key symptoms?
Dyslexia, autism, ADHD, photosensitive epilepsy, photosensitive migraine
- 20% mildly, 5% severely
- Find printed text (and music) difficult to read.
Words seem to jump around on the page.
Coloured blotches appear in text.
Text appears blurred.
Text uncomfortable to read.
Eye strain.
Tiredness.
Difficulty concentrating.
Avoid work involving reading – often with poor behaviour.
Poor handwriting – cannot see problem with own handwriting.
Susceptible to visual illusions more strongly (e.g., enigma stimulus).
How is Meares-Irlen Syndrome treated?
A full eye test is essential to rule out optical and eye co-ordination errors.
Symptoms can be alleviated by:
using coloured overlays for reading and coloured paper for writing.
wearing precision tinted lenses.
Colour overlays have to be very precise and are different for each patient.
Why do colour filters work for Meares-Irlen Syndrome?
Printed text is very similar to the stressful stripe pattern that triggers epilepsy and migraine .
Both epilepsy and migraine involve inappropriate excitation of cortex and spreading of activation.
Size of visually stressful stripes / text is right at the point of maximum visual sensitivity – maximal excitation.
Early cortical areas have retinotopic maps and distinct regions of cortex devoted to different orientations and colours.
If one such patch of cells were over active it could trigger excitation in other cells leading to illusions, reading difficulty, migraine or epilepsy.
Changing the colour of stripes (with filters) may redistribute the activity away from over sensitive (local) areas of cortex.
This is supported by fMRI evidence - Cortical activity reduced in areas V3 (decodes motion) / V4 (decodes colour) in migraineurs.
Similar results now found for Meares-Irlen syndrome.
Meares-Irlen Syndrome is controversial. Evidence against:
Neither Olive Meares or Helen Irlen are scientists: presented anecdotal evidence.
Irlen in particular is very protective of her method – leads to suspicion.
Irlen clinics have a low threshold for treatment and do not screen for other conditions (i.e., dyslexia, autism, eye focussing problems).
Large scope for placebo effects (young children, cool coloured glasses).
Symptoms very broad – overlapping with many other conditions.
Fairly large number of sufferers have some other optical malfunction – when corrected coloured lenses may not be needed.
Use of tinted lenses (chromotherapy) as a general therapy has a long history. A bit like leaches and electrotherapy!!!
Preferred colour for overlays is often different from that for glasses.
Preferred colour for glasses can change with age.
Meares-Irlen Syndrome is controversial. Evidence for:
Arnold Wilkins and colleagues have built up a large body of supporting evidence.
‘Rate of reading’ test shows immediate and significant improvement in reading speed on wearing glasses.
Placebo effects can be ruled out.
Use of special test equipment means that the appropriate colour can be prescribed without the patient being aware of that colour!
Possible to conduct double blind trials where neither the participant nor the experimenter know which of two coloured lenses is the correct one.
Colour constancy & adaptation also explain why lenses differ from overlays.
Increased EEG/fMRI activity in visual stress sufferers is present and alleviated by prescribed lenses, but not control lenses.
Nonetheless, the diagnosis and treatment remains controversial (especially in the US) and is not recognised by most public bodies (e.g., coloured lenses are not available to children on the NHS).
Summary
Magnocellular – dorsal dysfunction has been recorded in dyslexia, autism and schizophrenia.
A separate visual dysfunction exists in some kinds of epilepsy and migraine.
Some stimuli are particularly stressful to look at.
These stimuli are implicated in visual stress or Meares-Irlen syndrome, which has symptoms in common with dyslexia, attention disorders, autism, migraine and epilepsy.
It is primarily a problem with reading printed text – not reading per se, but it can co-occur with dyslexia, autism, epilepsy and migraine.
Although controversial there is reasonably strong evidence that visual stress exists and treated with coloured overlays or precision tinted glasses (but it’s still debated!).
