Perception of Colour
20 important questions on Perception of Colour
Why is colour helpful in the way we see objects?
- info about state of the objects (e.g. the ripeness of a fruit)
What can be some consequences of colour loss?
- loss of color in dreams & imagery i.e. no recollection of colour (this doesn't effect all patients)
- choosing clothes to wear - may be relient on others
- can lead to depression as you always see everything on grey-scale and you are relient on others
What are 2 tests that can identify deficits in colour perception?
2. Colour matching and odd one out detection (Heywood et al., 1987)
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What are 3 preserved functions of cerebral achromatopsia?
2. Luminance discrimination (i.e. the brightness)
3. Colour cognition (e.g. verbal association, colour of known pictures/objects)
What patient demonstrates that not all individuals with cerebral achromatopsia have impaired colour imagery?
- lesion involving the left temporo-occipital sulcus and right middle occipital gyrus
---> developed cerebral achromatopsia = poor performance on Farnsworth-Munsell, colour matching and colour naming tasks
---> Intact visual imagery for colours ( i.e. she can still remember & imagine colour). She has good:
= Colour verbal memory
= Colour-object fluency
= Colour name fluency
= Mental hue comparison
(Bartolomeo et al., 1997)
Who is the patient with the mental imagery defect?
- left occipital and medial temporal infarct
= Right hemianopia (half loss of vision)
Tests
Naming visually presented colours - good performance
Colour naming of objects from memory - poor performance
Recognising if a picture of an object was coloured appropriately - good performance
Recalling appropriate colour of objects - okay performance (13/20)
Fareh et al., 1988
What did Crognale et al., 2013 demonstrate?
Patient:
- 46 year old female
- Bilateral damage to the ventral occipitotemporal cortex
- Cerebral achromatopsia
EEG showed normal chromatic visual evoked potential (Normal Chromatic VEP) demonstrated in patient with achromatopsia
= this EEG response in V1 suggests that the impairment seen in achromatopsia is not in the earlier parts of the visual pathway
= the cones are still intact i.e. light is still being taken in
What does bilateral damage to V4 lead to?
- this is rare
What does unilateral damage to V4 cause?
e.g. damage to right side of the brain means loss of colour in left side of visual field and vice versa
Who converged evidence for the involvement of the V4 complex in cortical colour perception?
Recorded the response of single neurons in monkey V4 to simple coloured visual stimulation
Limitations of study:
- isn't testing human colour perception
Benefits of study:
- exp. knows which neuron they are testing (good control)
- good spatial resolution
What did Heywood et al., (1992) find? What was a development of this initial finding?
Development - Heywood et al. (1995); Cowey et al. (2001):
Lesions that were ventral and anterior to V4 in macaque monkeys did result in impaired colour perception
What did Zeki identify in 1991?
Results: activation in the lingual & fusiform gyri (area V4) = colour centre
What is a limitation of Zeki's colour centre experiment?
Who performed an fMRI during Farnsworth-Munsell 100 hue task? What did they find?
Stimulus conditions:
- Order by colour
- Order by luminance
- Passive viewing of colour
Results: Ventral occipitotemporal activity in colour situations vs. luminance
i.e. when trying to order colour and luminance, there is more activity in anterior V4
What does cerebral achromatopsia tell us? (2 points)
- colour perception is in V4
What is meant by colour blindness?
- Inherited gene abnormality resulting in reduced retinal photoreceptor system (dichromatopsia)
- 3 types of photopigments: sensitive to long-, medium- and short-wavelength light. People who are dichromats only have 2 photopigments (i.e. missing a photo pigment)
- Red-Green colour-blindness = missing photopigment sensitive to either medium or long wavelength light
(more common in males than females)
(p. 201 neuropsychology textbook)
What are the effects of peripheral achromatopsia?
- Colour blindness (achromatopsia)
- Day blindness (light intolerance) - rods are very sensitive to bright light
- Reduced visual acuity
- Nystagmus (rapid involuntary movement of the eyes)
What are the similarities and differences between peripheral achromatopsia and cerebral achromatopsia?
- Perceptual experience of colour (or lack of it)
- Consequential behavioural impairments (some)
Differences:
- Age of onset
- Visual acuity
- Day blindness
- Emotional response
- Peripheral achromatopisa = problems before V4, with cones etc.
What is colour anomia?
- a linguistic problem = patients can't relate the red colour to the word "red"
What are 2 things that people with colour anomia can do and 2 things they cannot do?
- Farnsworth-Munsell test
- Colour matching
Cannot do
- Identify a colour
- Say which colour is red etc
---> they are okay with tasks that are purely perceptual
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