Vision (neuro) Flashcards
What are the two photoreceptors?
Rods = most common, sensitive to light and motion, found in peripheral vision Cones = less common, responsible for colour, found in fovea
What are areas V1, V4 and V5 responsible for?
V1 = primary visual area V4 = involved in colour perception V5 = involved in motion perception
What is hemianopia?
Half of visual field is unable, usually effects both eyes, disorder of sensation
What is cortical blindness (blindsight)?
Damage to V1 leaves patients with a lack of vision in part of their visual field and had no knowledge of stimuli that was presented in this area.
Poppel, Held and Frost (1973) asked participants to look towards the lights; participants were confused as they say no light
What is cerebral achromatopsia (colour blindness)?
Damage to V4 left patients with little to no colour perception despite cones being fully intact
often found alongside with an inability to recognise faces and places
fMRI showed V4 area was activated during colour processing; some activation in V1 and V2 (Goddard et al, 2011)
What is akinetopsia (motion blindness)?
Damage to V5 left patients being unable to perceive motion
McKeefry et al (2008) used TMS on V5 and found participants were still able to perceive motion; other areas must be involved
Describe visual perception
Ungerleider and Mishkin (1982) proposed that the ventral system is specialised for object recognition and perception and the dorsal system is specialised for spatial perception
The two streams operate parallel to allow us to address questions of what we are looking at and where in our visual field it is located
theory supported by lesion and anatomical data (Farah, 1990) and functional imaging (Haxby et al, 1991)
Milner and Goodale (1995) studied patient DF who had damage to ventrolateral region of occipital lobe; led to difficulties in recognising and discriminating between object; visual-guided action towards objects was normal
Patients with optic ataxia have object recognition, but are unable to use visual information to guide behaviour
Condition is almost always associated with damage to superior parietal lobe
Goodale and Milner (1992) added a third pathway to the original model; located in superior temporal sulcus area which contains poly-sensory neurons (meaning they respond to multiple sensory channels)
The route is important integration of perceptual information about stimuli arising from different sensory outputs
Taylor et al (2006) found support for this third route with fMRI
What are the three response characteristics of ventral stream
Neurons in posterior regions fire in response to simple stimulus and neurons later in stream respond to more complex stimulus
Neurons further forward in stream are less concerned with physical positions of objects in visual field; cells forward regions have large receptive fields, so no matter where object falls in retina, cortical cells will respond to object in which they are tuned
Cells in stream make use of colour; important for object recognition
Describe visual agnosia
Neurological disorder resulting from damage; apperceptive and associative agnosia both linked to damage at different stages of stream and reflect different types of perceptual disturbance
Apperceptive agnosia = inability to identify object, despite being able to identify features; unable to copy drawings, related to damage to early stages of ventral systems
Associative agnosia = inability to identify specific objects despite being able copy drawings and pick out specific objects
Describe prosopagnosia
Inability to perceive faces, may be unable to match faces, recognise familiar faces or even own face
Bodamer (1947) studied Soldier S, who developed an inability to recognise his own face and was unable to detect facial expressions; unable to detect motion of facial expressions, but couldn’t perceive facial expressions
Bruyer et al (1983) studied Mr. W who couldn’t recognise famous faces or faces of those who he knew personally; able to select faces when given a verbal description and could match faces when shown from different perspectives
Most cases have bilateral damage to occipital or temporal lobes
Most unilateral damage occurs in right hemisphere
Suggests most facial recognition processing occurs in right hemisphere
Co-occurence of different forms of agnosia
Many patients with prosopagnosia show other abnormalities in recognition; led to suggestion that prosopagnosia is a type of object recognition
Farah (1990) conducted a meta-analysis of the co-occurence of prosopagnosia, object recognition and acquired alexia
Found alexia and prosopagnosia could be linked to different deficits in analytical and holistic processing, but object agnosia could result from deficits in either system
Describe spatial perception
Streams run from occipital lobe to parietal lobe; travels via V2 and V3 into V5, then channelled into various modular regions within posterior parietal cortex
Cells in V5 are sensitive to stimuli moving in certain directions, irrespective of location in visual field
Cells in V7 have extensive receptor fields and are selectively responsive to objects moving in a particular direction and at a particular speed
Other cells in parietal area are responsive to a combination of input signalling spatial location of objects in field of vision; this is important as allows viewer to reference object location regardless of eye position
Describe basic spatial processing
Localising points in space = patients with damage to superior regions of parietals cortex have difficulty reaching out towards visual stimulus
Depth perception = ability to detect local depth can be disrupted by right or left hemisphere; ability to detect global depth can be disrupted by right hemisphere only
Motion = usually bilateral damage to parietal lobe, patient MP had damage to mid-temporal gyrus and adjacent regions of parietal in both hemispheres
Describe patient GL
Had form of visual agnosia
Complained couldn’t see as well as previous, after experiencing head trauma, despite having almost normal visual acuity
Unable to recognise objects by sight, but hadn’t lost any semantic knowledge
Objects presented to other modalities could still be recognised
Describe Marr (1982)
Proposed first model of recognition; believed we recognise objects by going through a sequence of stages
- Primal stage = 2D descriptions, information about edges, contours, etc
- 2 1/2 D stage = incorporates depth, orientation of surfaces, view-point dependent
- 3D representation = described 3D shape, relative positions, independent of observer’s viewpoint
