lecture 2- two visual systems

Question 1

what is the primary goal of perception?

Answer 1

-psychophysics
- evolutionary perspective

Question 2

evolutionary perspective

Answer 2

guidance of actions

• primary role of the visual system is to guide our actions
• vision is needed to explore and manipulate the world around us
• ensure survival and reproduction
  -active vision

Question 3

psychophysics

Answer 3

perceptual representation of the world

-measures detection and discrimination performance of visual stimuli
- no reference to the relationship between visual inputs and motor outputs (ie what behaviour are perceptual representations used for?)
- perceptual approach

Question 4

quick preview on perception-action model

Answer 4

• model based on how perception and action relate in the human brain
• based on cortical processing mechanisms
• origin in animal and neuropsychological (patient) research
• one of the most influential theories on higher visual processing
• formulated by A.D. Milner & M.A. Goodale (1995)

Question 5

lesion studies in monkeys

Answer 5

• ungerleider and Mishkin, 1982
• lesion to the ventral stream impairs recognition and identification of objects (“what”)
• lesions to the dorsal stream impairs perception of spatial relations between objects (“where”)

Question 6

ventral stream and properties

Answer 6

-vision for perception

properties
- allocentric frame of reference, relative metrics, long term storage, conscious

Question 6

dorsal stream and properties

Answer 6

-vision for action

properties
-online processing, metrically accurate, egocentric frame of reference, quick decay, unconscious

Question 7

assumption:

Answer 7

the pathway in which visual information is processed depends on the intended purpose

^^^^^
Milner & Goodale, 1995; 2006

Question 8

perception action model in a nutshell

Answer 8

” a major implication of our model is the counterintuitive idea that peoples perception of the visual world does not always follow the same rules as their behaviour interactions with this world” (Milner & Goodale, 2006)

note: (different names-same thing)
perception action model is also called:
- “two visual-stream model”
- “two visual-systems hypothesis”

Question 9

evidence: patient with ventral-stream damage I

Answer 9

• classic and famous neuropsychological patient (DF): bilateral lesions in area LOC (ventral stream) showing a visual form agnosia
• patient DF struggles to recognise and identify objects (no form or shape perception). note that she can identify texture and colour of objects

Question 10

patient ventral damage - perception action dissociation

Answer 10

-patient DF has issues with perceptuak tasks (eg cant report the size of an object) but shows almost normal performance in a corresponding action task (eg when grasping she adjusts the opening of her hand to object size [which she is unable to report perceptually]
- suggests that ventral stream (which is damaged) is important for perception- such as recognising size and shape of objects

Question 11

evidence: double dissociation

Answer 11

• a “a double dissociation” of functions (eg perception and action), observed between the different disorders, is considered an especially strong and convincing piece of evidence for the existence of neurologically segregated functional systems (rossetti and revonsuo, 2000)
• selective deficit (such as issues with action tasks) cannot be attributed to differences in difficulty
• complementary deficits of patients with ventral stream damage (visual form agnosia) and dorsal stream damage (optic ataxia) inspired/supported the idea that the ventral stream mediates “vision for perception” and the dorsal stream “vision for action”

Question 11

what are limits of clinical case studies

Answer 11

• neuropsychological evidence on its own cannot unequivocally prove that the perceptual system is not involved in the visual guidance of actions => patients may rely on different information / may have developed different strategies or may use alternative processing systems (brain plasticity)
  => evidence that vision for perception and vision for actions are distinct in the healthy human brain is needed
• a number of paradims have been frequently used to illustrate dissociations in neurologically healthy participants (visual illusions, Garner paradim, Webers law ect”

Question 11

patient with dorsal stream damage

Answer 11

• associated with optic ataxia- a disorder where patients struggle to perform visually guided actions but have no problem with perceptual tasks
• opposite pattern to visual form agnosia patients
• suggests that dorsal stream (which is damaged) is important for visual guidance of actions

Question 12

basic ideas of PAM

Answer 12

• two different systems (ventral and dorsal) that processes visual information for perceptual and visuomotor tasks respectively. both systems are assumed to work largely independently
• visual form agnosia is a disorder that is caused by damage to ventral stream areas. patients have particular problems with perceptual tasks but show (nearly) normal behaviour in visuomotor tasks
• optic ataxia is a disorder that is caused by damage to dorsal stream areas. patients have particular problems with visually guided action tasks but show (nearly) normal behaviour in perceptual tasks.

Question 13

visual illusions on perception and action

Answer 13

• visual illusions introduce a dissociation between conscious perception of size, and the real metrics of the object which are assumed to guide actions

Question 14

classic illusion:

Answer 14

central disks are the same physical size => perceived as being different (illusion)

Question 15

manipulation:

Answer 15

central disks are of different physical size => perceived as being similar in size

Question 16

issues with evidence and claims

Answer 16

• significant effects of the illusions on grasping (only smaller)
• dichotomous measure in perception task (same/different) - continuous measure in action task (size of MGA in mm)
• problem matching the perceptual and the motor task (two vs one figures need to be considered) => effects are addictive
• visual feedback of fingers approaching the inner disk can lead to adjustment of grip aperture flight

=> results are extremely controversial

Question 17

interim summary II- perception action dissociations in the healthy brain

Answer 17

• visual illusions reliably affect perception but not necessarily actions
• potential indication for distinct processing strategies and independent functional systems (suggested by PAM)
• issues of making valid comparisons between action and perception tasks- different strategies and constraints in “matched” tasks (do we really measure the same in both tasks?)
• evidence is controversial: often differences between perception and action tasks can be explained without the assumption of distinct processing mechanisms

Question 17

visual processing in dorsal and ventral streams

Answer 17

• ongoing controversy about how distinct processing mechanisms are in the dorsal and ventral streams
• however it is undisputed that ventral stream areas are relatively more important for perceptual processes and dorsal stream areas are important for the planning and guidance of actions
• degree of communication between the two streams remains controversial

note- model is overall controversially discussed in literature

Question 18

modularity in the ventral stream

Answer 18

• neurons in V1 respond to oriented bars/edges (Hubel & Wiesel)
• neurons in dorsal and ventral (extrastriate areas respond to more complex stimuli
• neurons responding to similar stimuli tend to be grouped together in one brain area
• module = brain structure specialised to process information about a particular stimulus type

-patient DF has specific deficits in perceiving form/shapes/ orientation (but intact texture and colour perception)=> not all of her ventral stream is damaged (lesion primarily affects area LOC)

Question 19

modularity in the ventral stream- perception of texture, colour and form

Answer 19

• large overlap between regions in the (medial) ventral stream that process colour and visual texture
• note: patients with damage to areas in the ventral stream that process colour suffer from cerebral achromatopsia. usually they have no problem processing shape.

Question 20

modularity in the ventral stream- perception of faces, places and bodies

Answer 20

OFC= occipital face area
FFA= Fusiform face area
LO= lateral occipital complex
EBA = extrastriate body area
PPA = parahippocampal place area

Question 20

modularity in the ventral stream - moneky studies

Answer 20

• Tsao et al (2006)- response of neurons in Monkeys inferotemporal cortex (IT) to 96 different images (16 of each group) => 97% of neurons in the face area were face selective
• response of face selective neurons in MONKEYS ventral stream => cells respond to front view of monkey or human face but responds is weaker for jumbled or disturbed faces or other body parts

Question 21

modularity in the ventral stream- perception of faces, places and bodies cont

Answer 21

neuropsychological evidence for areas in the ventral stream specialised for faces and objects

Prosopagnosia:
Patients are unable to recognise faces (identify people based on their voices, movements, colour of clothes, etc.) → usually patients have no general
agnosia for objects!

double dissociation ^

Object agnosia:
* Patients lose the ability to recognise
objects without any problems in recognising faces

Object agnosia patient CK looked at
painting and saw the face straight
away – however was unable to
identify any of the fruit/ vegetables
the face is made of…
Many patients will show
simultaneous face and object
agnosia (e.g., patient DF)
→ stimuli are processed in
anatomically nearby areas…
Modularity in the ventral stream
Perception of faces, places and bodies
Neuropsychological evidence for areas in the
ventral stream specialised for faces and objects

Question 22

modularity in the dorsal stream

Answer 22

-Modularity in the dorsal stream is based on the nature of the actions that are guided by visual objects (reaching, saccades, smooth pursuit, grasping, locomotion etc.)!
-Human homologs of areas in the
monkey brain specialised for
different kinds of visually guided
movements
- Most neurons in the dorsal stream do not fire unless some sort of action is required towards a seen object → modularity of certain movements

Question 23

modularity in the dorsal stream - grasping monkey

Answer 23

AIP Neurons in the dorsal stream
of the monkey:
- Neurons are selective to shapes
and specific grasp postures
(note certain neurons fire
independent of grip posture)
- Example neuron fires strongest
when monkey grasps a vertically
oriented square plate (Murata
et al., 2000)

Question 24

summary

Answer 24

• Dorsal/ventral distinction has been useful (inspired lots of research in the
  area) but has its limits
• Correct to regard visual processes as fractured into multiple components
• Incorrect to make only a single division
• Probably perception and action processes are not as strongly divided as
  originally suggested (specialisation of dorsal and ventral streams is relative,
  not absolute)