Lecture 7 - High Level Vision

Question 1

Visual Agnosia

Answer 1

• cannot 8ecognise objects - not a problem with memory but is a problem with linking visual info with memories
• Lissauer (1889-1890) - distinguished between impairments at 2 stages of recognition
  1. apperceptive - can test with simple shapes. cannot name or copy what they are looking at = impairment in conscious visual representations/perception
  2. associative - perception intact but devoid of meaning

Question 2

integrative agnosia

Answer 2

• is there a true distinction between apperceptive and associative agnosia in humans?
• Riddocj & Humphreys (1987) - HJA passes tests for apperceptive agnosia, but shows higher order impairments that are perceptual in nature
• HJA performance at recognising overlapping objects is impaired and slower reaction times. performance at discriminating real/unreal objects is impaired = impaired in integrating objects and identifying features.
• HJA better performance when fewer details present but controls worse = impairment is to do with integrating info and sillouettes have less info to integrate than a line drawing = more complex than 2 types of agnosia
• the term integrative agnosia was introduced to describe HJA. it is a high-level perceptual impairment in integrating the form and features of an object
• birmingham object recognition battery is a series of tests to identify level of processing at which a recognition impairment exists. tasks progressively get more high level

Question 3

associative agnosia

Answer 3

• may be explained by subtle sensory impairments (Bay 1953)
• Ettlinger (1956) tested this in test of basic visual functions
• tests: brightness discrim, flicker fusion, acuity for small objects, local adaptation, tachistoscopic acuity, apparent movement
• but these tasks do not fully capture visual processing and are tests of different visual abilities
• those with brain damage have slightly worse performance. and presence of visual field defect is associated with impairments on visual functions.
• have presence of higher level visual disorders and visual field defects. patients no worse than group B (visual field defect but no agnosia)
• ettlinger showed impairments in visual sensory abilities are associated with visual field defects not [resence of higher perceptual disorders (of which only one pp had true agnosia) BUT tests did not fully account for the functional org of the visual system
• DeHaan et al (1995) replicared ettlinger focussing on agnosia and took into consideration shape discrim, location, colour, light, shape, texture, line discrim. found group B (no agnosia controls) impairments compared to controls than group A. group A shows some impairment but no marked difference in visual tasks = no evidence these visual functions (even shape perception) are necessary or sufficient to cause agnosia

Question 4

form agnosia

Answer 4

• no evidence that associative agnosia is dependent on lower level visuosensory impairments
• one example of apperceptive agnosia is visual form agnosia - MR S unable to recognise objects based on shape
• to do with inability to match objects, faces or letters - more likely to be impairment of visual perception.
• e.g. able to distinguish small changes in luminance and wavelength of test but unable to distinguish between two objects of same luminance, wavelength etc when only difference was shape
• is it an agnosia if there is no loss of knowledge here?
• can categorise perception at Marrs different levels of processing but only when you get a 3D rep can you compare semantic info of objects. If impair at the primal stage = impaired at later stages too as need this info to complete later stages of representation
• possibly caused by low level visuosensory impairments?
• Campion & Latto (1985) - cannot test on lower level visual categories - measured contrast sensitivity in an agnosic patient. had abnormal thresholds relative to controls indicating possible sensory deficit and impairments at spatial frequency levels.
• suggested account based on peppery field defects. agnosia explained not by form-specific impairment but by peppered field defects (scotoma) = small islands of visual field loss (loss of conscious experience in these areas)
• induced presence of small isolated field loss to stimulate what image looks like for someone with visual field loss & found normal threshold = debunked

Question 5

ventral and dorsal

Answer 5

• ungerleider and mishkin (1982) lesioned monkeys in temporal or parietal lobe.
• occipital and temporal = object recognition
• parietal = spatial attention
  > lesion in temporal = impaired object recognition not spatial judgement
  > lesion in parietal = impaired spatial judgement not object recognition
  = visual brain 2 paths: where (dorsal) for recognising spatial location and what (ventral) for recognising objects & properties
• but DF (Milner & goodale 1991, 1992) had visual form agnosia and deficits in low level vision despite still largely intact and damage in ventral stream. performed like appercative agnosia (random drawings but can drae some things by memory) = impaired visual form rep.
• although didnt have conscious visual access to spatial info can use this info to support other tasks e.g. asking them to rotate card to post is impossible but actually posting card is easier = what is impaured is having conscious visual representations rather than actual actions
• suggests 2 stream hypothesis:
  > dorsal - visuomotor interaction, egocentric, no access to memory, unconscious (unimpaired in DF)
  > ventral - damaged in DF. object recognition, access to memory, conscious, allocentric
• suggests DF is based on dif functions of same features. argues against campiano.
• Malach et al - response in lateral occ cortex does not distinguish familiarity. seems to be segregational functions of processing shape and non shape.
• culham et al (2003) - found response in anterior intraparietal sulcus (parietal and intact in DF) which supports action guided movements even without access to visual info
• Aglioti et al (1995) - tested if manipulating object size would get change of grip. found max grip with physical size not perceived size - can change perceptual experience of size but how the person interacts with the object does not change
• some interaction between streams

Question 6

modular processing

Answer 6

• there is some evidence certain kinds of objects are processed in unique way
• faces - Thompson 1980 - people process faces holistically rather than individual features. only for upright faces. does not occur for non-face objects & there are paths in visual system treating faces in specialised way. damge = prosopagnosia
• Kanwisher et al (1997) - FFA lights up for face viewing involved in processing faces
• Gauthier et al (2000) - these modular processes found for specialised features e.g. faces but this is just expertise effect and learned to identify to faces. if people are car enthusiasts get same kind of response in FFA = expertise effects. could be confounded by attention?