Chapter 3: 2 visual systems
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 brain identifying two different anatomical pathways within the
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 accuratesized 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 brainsubjects damaged and normal Reveals a basic challenge in cognitive science integrating functional analysis and anatomical analysis Shows that this integration need not be top-down