lecture 2 - two visual systems

Question 1

primary goal of perception in
-psychophysics
-evolutionary perspective

-what are the issues with this?

Answer 1

-psychophysics measures detection and discrimination performance of visual stimuli
-the issues with this are that there is no reference to the relationship between visual inputs and motor inputs (i.e what behaviour are perceptual representations used for ) This is the perceptual approach

Evolutionary perspective: 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 approach

Question 2

what is the perception action model
-what is it based on
-origins?

Answer 2

Model 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)

idea - the visual system is divided in two independent parts dedicated to perception and action respectively

Question 3

a mini recap of how visual processing works
mention the key areas involved

Answer 3

Key Areas Involved:

Retina: Converts light into neural signals.

Optic Nerve: Transmits signals to the brain, crossing at the optic chiasm.

Lateral Geniculate Nucleus (LGN): Organizes visual information in the thalamus.

Primary Visual Cortex (V1): Analyzes basic features like edges and motion.

Extrastriate Cortex:
V2, V3, V4, V5: Processes shape, color (V4), and motion (V5).

Dorsal Stream: “Where” pathway for spatial awareness (to parietal lobe).

Ventral Stream: “What” pathway for object recognition (to temporal lobe)

Question 4

lesion study in monkeys

-results

Answer 4

-monkeys used in an experiment
task 1
-used to do object discrimination ‘what’
task 2
-used to do landmark discrimination task ‘where’

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 5

what are the ‘what’ and ‘how’ streams in vision, and what are their properties

Answer 5

what - the ventral stream
vision perception
-object identification and recognition
Properties : allocentric frame of reference, relative metrics, long term
storage, conscious

‘how’ - the dorsal stream
visuomotor control
vision for action
properties: online processing,
metrically accurate, egocentric frame of reference, quick
decay, unconscious
-

Question 6

the pathway in which visual information is processed depends on….

Answer 6

on the intended purpose

Question 7

what is an implication of the perception-action model / two visual stream model
-(Milner & Goodale, 2006)

Answer 7

“A major implication of our model is the counterintuitive idea that people’s perception of the visual world does not always follow the same rules as their behavioural interactions with this world.”

Question 8

evidence for perception action model
-ventral stream

Answer 8

patient with ventral stream damage
-famous neuropsychological patient (DF): bilateral lesions in area LOC
(ventral stream) showing a visual form agnosia (patient left with a lesion laterally in her ventral stream)

-Patient DF struggles to recognise and identify objects (no form or shape perception). Note that
she can identify texture and colour of objects.
if shown a model photo she cant copy it correctly but if asked to draw it from memory she can.
-as soon as she stops drawing it she wont recognise it.

Question 9

evidence for perception action model
-ventral stream
-what is perception action dissociation and what does this suggest

Answer 9

Perception-Action Dissociation:
Patient DF has issues with perceptual tasks
(e.g., can’t report the size of an object) but shows almost normal performance in a corresponding action task (e.g., 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 10

evidence
-dorsal stream

Answer 10

Dorsal stream damage:
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 11

double dissociation
-evidence

Answer 11

Double dissociation means that two cognitive functions are independent of each other, based on evidence from brain lesions or disorders

This double dissociation shows that the ventral and dorsal streams are functionally independent; damage to one affects only specific aspects of perception while leaving others intact. but you don’t realise them being independent as they wotk together

Question 12

example of double dissociation - perception and action

Answer 12

Perception vs. Action:

Patient A: Impaired object recognition (perception) but intact motor skills (can reach for objects).

Patient B: Impaired motor skills (action) but intact object recognition (can name objects).

Question 13

Selective deficit (such as issues with action tasks) cannot be attributed to
differences in…

complementary deficits of patients with ventral stream damage (visual form agnosia) and dorsal stream damage (optic ataxia) supports the idea that…

Answer 13

difficulty
-suggests that the affected process relys on one neural system

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 14

• two different systems
  -visual form agnosia
  -optic ataxia

Answer 14

1. Two different systems- (ventral and dorsal) that process visual information for perceptual and visuomotor tasks respectively. Both systems are assumed to
   work largely independently.

2) 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

3) 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 15

what are the limits of clinical case studies

Answer 15

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 action are distinct in the healthy human brain is needed!

• A number of paradigms have been frequently used to illustrate
  dissociations in neurologically healthy participants (Visual illusions, Garner
  paradigm, Weber’s Law etc.)

Question 16

visual illusions on perception and action
give an example

Answer 16

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

the circle between bigger circles and smaller circles example

Question 17

visual illusions on perception and action
-classic illusion
-manipulation

Answer 17

Classic illusion:
Central disks are the same physical size
→ Perceived as being different (illusion)

Manipulation:
Central disks are of different physical size
→ Perceived as being similar in size

Question 18

visual illusions on perception and action study aglioti et al
-what were they asked to do
-what is the conclusion
-what does the illusion deceive

Answer 18

• “if you think the two
  disks are the same
  size, pick up the one
  on the left. If they are
  different, pick up the
  one on the right.”
• -They are seen as
  equal in size, but
  grip aperture is
  not equal

The illusion deceives perception but not actions → Evidence for differences in perception and action processing
visual illusions reliability affect perception but not necessarily actions

Question 19

what are the issues with illusion studies evidence

Answer 19

Significant effects of the illusion on grasping (only smaller)

• Dichotomous measure in perception task (same/different) –
  continuous measure in action task (size of MGA in mm)
• Problem of matching the perceptual and the motor task (two
  vs. one figures need to be considered) → effects are additive
• Visual feedback of fingers approaching the inner disk can lead
  to adjustment of grip aperture in flight

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

→Results are extremely controversial

Question 20

is the ventral stream or dorsal stream more important in perceptual processes

Answer 20

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

-there is ongoing controversy about how distinct processing mechanisms are in the dorsal and ventral streams

Question 21

modularity in the ventral system
-module
-neurons in v1, in dorsal and ventral areas

examples that support this

Answer 21

Module = brain structure specialised
to process information about a
particular stimulus type.

Neurons in V1 respond to oriented
bars/edges (Hubel & Wiesel)
* Neurons in dorsal and ventral
(extra striate areas) respond to more
complex stimuli
* Neurons responding to similar stimuli tend to be grouped together in one brain area

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)

Example: Patient DF:
“It’s made out of metal
- is it aluminium?
It’s got red plastic on it.”
“Is it some sort of
kitchen utensil?”

Question 22

modularity in the ventral system
-perception of texture, colour and form -cerebral achromatopsia

Answer 22

Patients with damage to areas in the ventral stream that process colour suffer from cerebral achromatopsia. Usually they have no problem processing shape.

Large overlap between regions in
the (medial) ventral stream that
process colour and visual texture

Question 23

modularity in the ventral system
monkey studies
(tsao et al (2006)

Answer 23

response of neurons in
MONKEYs inferotemporal cortex (IT) to 96different images (16 of each group) → 97% of neurons in the face area were face selective. (they responded better to faces)

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 24

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

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

Answer 24

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

object agnosia
-patients lose the ability to recognise objects without any problems in recognising faces

-double dissociation between these two

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

Question 25

modularity in the dorsal stream is based on..

Answer 25

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.)

Question 26

most neurons in the dorsal stream dont fire unless…..

Answer 26

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 27

modularity in the dorsal stream
-AIP neurons are selective to….

Answer 27

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 28

summary