2 visual systems - kalamazoo collegegeza.kzoo.edu/~erdi/cogsci/12/twovis.pdf · cognitive science...

Chapter 3:2 visual systems

Cognitive Science José Luis Bermúdez / Cambridge

University Press 2010

Overview

•Explain the significance of the “turn to the brain” in cognitive science

•Explain Mishkin and Ungerleider’s hypothesis that there are two distinct visual systems

•Outline additional evidence and other interpretations



Classical CogSci and the brain

•Neuroscience did not feature prominently in the early days of cognitive science

• Widespread view that neural details are “merely implementational”

• Top-down approach (e.g. Marr)

• Functional analysis of cognitive abilities



The turn to the brain

•1. Development of new technologies for studying cognition in the brain (as opposed to neuroanatomy)

• single-neuron recording

• PET and fMRI

•2. Neurally-inspired models of information-processing

• connectionism

•3. The “AI winter”



2 visual systems hypothesis

•Originally proposed by Mishkin and Ungerleider

•Draws on both functional and anatomical data

• • functional data derived from lesion studies on the brain

•Illustrates a bottom-up approach to studying cognition - and illustrates how “boxology” can connect up with the brain



Primary visual pathway

• Contralateral organization (relative to visual field)

• Projects to primary visual cortex (a.k.a striate cortex/V1)

• What happens next?



2 visual pathways

• Dorsal – carries information relevant to object location

(the “where” pathway)

• Ventral – carries information relevant to object identification (the “what” pathway)



Two different levels of analysis

•Functional analysis

• identifying two different and dissociable types of visual information-processing

•Anatomical analysis

• identifying two different anatomical pathways within the brain



Preliminary evidence 1

•Schneider 1969

• after ablation of prestriate cortex hamsters were unable to learn simple pattern discriminations

• but remained able to orient towards visual stimuli

• ablating the tectum preserved pattern discrimination, but abolished visual orientation

•But the tectum is a sub-cortical structure (part of the mid-brain)



Preliminary evidence 2

• Brain-damaged patients – Damage to parietal and temporal lobes produces different types of impairment

• parietal = problems acting on and locating objects

e.g. visuospatial neglect

• temporal = problems identifying objects

e.g. agnosia



Visuospatial neglect

• Caused by damage to parietal cortex

• Inability to respond to stimuli in contralesional field

• Compatible with some forms of processing of neglected information



Visual agnosias

• Associated with damage to the temporal and/or occipital lobe

• Basic features of vision are preserved (acuity, brightness discrimination, color vision etc.)

• Impairment in copying, drawing or naming objects



General questions

• How do we move from functional analysis to anatomical analysis?• dangerous to infer function of neural areas directly from

what happens when they are damaged• is the impairment due to the damaged area? Or to the

fact that information fails to reach another area

• How do we get a model of an information-processing pathway?



Cross-lesion disconnection experiments - background

• The cerebrum is divided into two hemispheres

• Major cortical areas are duplicated in each hemisphere

• Hemispheres can commmunicate through the corpus callosum in order to compensate for damage in one hemisphere



Basic idea

• Remove one of the stations on a postulated pathway from one hemisphere

• The corresponding area in the other hemisphere will typically compensate

• Subsequent transection of the corpus callosum allows experimenters to identify whether the station lies on the pathway



Ventral lessions• crossed striate and inferior

temporal lesions

• pathways preserved via corpus callosum

• but performance on pattern discrimination tasks abolished by transection of corpus callosum



Dorsal lesion study



Two versions of the hypothesis

• Mishkin and Ungerleider:

Dorsal = location (“Where?”

Ventral = identification (“What?”)

• Milner and Goodale

Dorsal = vision for action

Ventral = vision for identification



Evidence from monkeys

• Bilateral temporal lobe lesions result in severe impairments of recognition, but basic visuomotor skills are preserved

• object avoidance

• judging distances when jumping

• Electrophysiological studies revealed neurons in parietal cortex sensitive to different types of reaching movement

– neuronal enhancement



Evidence from brain-damaged patients

•Double dissociation between visual recognition and visuomotor control

• Balint’s syndrome – visuomotor deficit (optic ataxia) with recognitional impairment

• Visual form agnosia - impaired recognition with preserved visuomotor skills



Visual form agnosia (DF)

• Damage to ventral stream• Preserved ability to calibrate grip size



Contrast with optic ataxia• Optic ataxia = deficit in reaching

not explicable by motor, somatosensory or visual field deficits

• Grasp lines (i.e. where thumb and index finger make contact with shape

• No significant difference between DF and control



Evidence from normal subjects

• Ebbinhaus illusion is very robust for normal subjects

• But if asked to reach subjects typically make an accurate-sized grasp response

• Dissociation between action and conscious visual awareness?



Important points• Reveals interdisciplinary nature of cognitive science

• lesion experiments on monkeys• single neuron neurophysiology• cognitive psychology experiments on brain- damaged and normal

subjects

• Reveals a basic challenge in cognitive science – integrating functional analysis and anatomical analysis

• Shows that this integration need not be top-down

2 visual systems - kalamazoo collegegeza.kzoo.edu/~erdi/cogsci/12/twovis.pdf · cognitive science...

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