Announcements Study Guide available on web by 8 PM tonight Study Guide available on web by 8 PM tonight Quiz next week for first hour of class. Quiz next week for first hour of class. Multiple choice, fills in, identifications Multiple choice, fills in, identifications Bring pencils for scantrons Bring pencils for scantrons

Last Lecture Projections maps and the homuculi Projections maps and the homuculi Organization of the Visual System Organization of the Visual System

This Lecture Organization of the Visual System continued Organization of the Visual System continued Blindsight Blindsight  what it is; how it ‘s studied; what it tells us about human visual information processing. What/Where pathways What/Where pathways  Their discovery, their neural origins, their manifestations in humans

Sensitive areas are “magnified” Field of View Cortical Map of Visual Field

Receptive Field: the region of the sensory surface that makes a cell fire. Cortical Magnification Each neuron in area 17 has a retinal receptive field More neurons have foveal receptive fields

Pathway from Eye to Brain Geniculo-Striate pathway Optic nerve carries signals from retina. Optic nerve carries signals from retina. Decussation at optic chiasm (optic tract) Decussation at optic chiasm (optic tract) Synapse at LGN of thalamus Synapse at LGN of thalamus Optic radiations to occipital lobe Optic radiations to occipital lobe AREA 17; Striate Cortex AREA 17; Striate Cortex

The connections Left Eye Left Eye Nasal hemiretina- LVF- projects to right hemisphere Nasal hemiretina- LVF- projects to right hemisphere Temporal hemiretina- RVF- projects to left hemisphere Temporal hemiretina- RVF- projects to left hemisphere To cross at optic chiasm uncrossed Right Eye Right Eye Nasal hemiretina- RVF- projects to left hemisphere Nasal hemiretina- RVF- projects to left hemisphere Temporal hemiretina- LVF- projects to right hemisphere Temporal hemiretina- LVF- projects to right hemisphere nasal LVF input to RH RVF input to LH

Vision requires cortex… or does it? Reports of residual vision in animals with striate lesions : Reports of residual vision in animals with striate lesions : Recovery after experimental field defects Recovery after experimental field defects spared light/dark discrimination spared light/dark discrimination spared localization abilities spared localization abilities Implication: Other structures (tectum) can compensate for striate function. Implication: Other structures (tectum) can compensate for striate function. LGNStriate extra Striate LORE of neurology until the early 70's...

Re-Evaluate vision in human scotomas: Poppel, Held & Frost (1973). Residual visual function after brain wounds involving the central visual pathways in man. Nature. Poppel, Held & Frost (1973). Residual visual function after brain wounds involving the central visual pathways in man. Nature.Task: light flashed in scotoma light flashed in scotoma Patient looks at (saccades) to flash location Patient looks at (saccades) to flash location NOTE: Patient protests: “How can I look at something I can't see?" Experimenter: GUESS! NOTE: Patient protests: “How can I look at something I can't see?" Experimenter: GUESS! Results : Saccadic endpoint related to target location Saccadic endpoint related to target location Text notes case D.B. (Weiskrantz) Text notes case D.B. (Weiskrantz) TARGET LOC.

Why Does Blind Sight Occur? One theory (non-striate hypothesis): Due to non geniculo-striate pathways Due to non geniculo-striate pathways  Retinotectal pathway --> direct route for saccadic eye movement control.  Tecto-pulvinar-extrastriate --> indirect route for manual and guessing responses. LGNStriate extra Striate S.C. tectopulvinar pathway Pulvinar Retinotectal pathway

Alternative to Nonstriate Hypothesis: Cortical (Striate) Islands Fendrich, et al. (1992). Fendrich, et al. (1992). detailed perimetry, retinal stabilization detailed perimetry, retinal stabilization revealed islands of vision in scotoma revealed islands of vision in scotoma Implications: Implications: Spared tissue in Area 17 can explain residual vision. Spared tissue in Area 17 can explain residual vision. Colliculus may not mediate all blindsight Colliculus may not mediate all blindsight

WHY is consciousness lacking? Two possibilities Non striate theory: primitive SC circuitry SC is inadequate for "awareness". Non striate theory: primitive SC circuitry SC is inadequate for "awareness".  Cortex necessary for awareness Both theories: The weak visual signal is below threshold for consciousness. Both theories: The weak visual signal is below threshold for consciousness.  non-striate theory- SC only gets weak retinal input.  Cortical Islands- tissue fragments are insufficient for consciousness-- NOTE: cortex does not equal consciousness.

Lessons from Blindsight Implicit vs Explicit (conscious) Knowledge. Parallel operations Parallel operations Yield products (representations) for different behaviors Yield products (representations) for different behaviors Human residual visual is “nonfunctional” compared to animals. humans "residual" capacities seen only with experimental methods. humans "residual" capacities seen only with experimental methods. nonhuman species may depend more on tectum nonhuman species may depend more on tectum Encephalization of visual (WHAT & WHERE) function in primates and humans. Encephalization of visual (WHAT & WHERE) function in primates and humans.

Announcements Study Guide available on web by 8 PM tonight Study Guide available on web by 8 PM tonight Quiz next week for first hour of class. Quiz next week for first hour of class. Multiple choice, fills in, identifications Multiple choice, fills in, identifications Bring pencils for scantrons Bring pencils for scantrons

The What and Where Pathways

The Ungerleider & Mishkin (1982) Experiment Task 1: Object discrimination study an object study an object select the familiar object* (reward) select the familiar object* (reward) parietal lesions impair LANDMARK TASK temporal lesions impair OBJECT TASK Task 2: Landmark discrimination select foodwell closest to the TOWER select foodwell closest to the TOWER

Conclusion: VENTRAL stream processes identity information VENTRAL stream processes identity information DORSAL stream processes spatial information DORSAL stream processes spatial information Logical basis for this inference: 1) Focal parietal lesion --> selective deficit, dissociation: “where” impaired, “what” intact 2) Focal temporal lesion --> selective deficit, dissociation: “what” impaired, “where” intact DOUBLE DISSOCIATION

Inadequacy of a single dissociation Consider the following hypothetical outcome: Parietal & Temporal lesions impair the landmark task, but neither impaired the object task Parietal & Temporal lesions impair the landmark task, but neither impaired the object task What does this result mean? What does this result mean? A) both brain areas contribute to spatial processing? A) both brain areas contribute to spatial processing? B) the tasks differ in difficulty? B) the tasks differ in difficulty?

Double Dissociation: Experimental strategy Experimental strategy 2 brain areas (X,Y) 2 brain areas (X,Y) 2 different tests (T1,T2). 2 different tests (T1,T2). Result Result Lesion of X impairs T1 not T2 Lesion of X impairs T1 not T2 Lesion of Y impairs T2 not T1 Lesion of Y impairs T2 not T1 Locus XLocus Y 1 2 DOUBLE DISSOCIATION PERCENT CORRECT TASK BRAIN REGION

Double Dissociation Logic Applied Locus XLocus Y 1 2 DOUBLE DISSOCIATION PERCENT CORRECT TASK What/Where DISSOCIATION TemporalParietal Object Spatial PERCENT CORRECT TASK

Why do these separate pathways exist? Different functions: Different functions: NEED special purpose machinery. NEED special purpose machinery. have different computational demands. have different computational demands. require different types of information. require different types of information. How does the input to these pathways differ? How does the input to these pathways differ? RETURN to the Retina: 1. photoreceptors (rods and cones) perform transduction 2. Signals sent to RETINAL GANGLION CELLS (RG). 3. Convergence: many photoreceptors --> one RG cell

M and P Retinal Ganglion Cells RG Cells are not just light detectors. RG Cells are not just light detectors. Cells classified by what "turns them on" Cells classified by what "turns them on" receptive field properties of cells receptive field properties of cells M cells M cells a) project to Magnocellular layers of LGN b) dominant in DORSAL/WHERE stream c) also go to SC d) fast conducting, colorblind, low acuity P cells P cells a) project to Parvocellular layers of LGN b) dominant in VENTRAL/WHAT stream c) slow, color selective, high acuity

M cells --> Magnocellular pathway M cells --> Magnocellular pathway P cells --> Parvocellular pathway P cells --> Parvocellular pathway Their output remains segregated in V1 & beyond Their output remains segregated in V1 & beyond Cytochrome oxidase stain reveals subregions in V1

A Schematic of What vs. Where

What vs. Where in Humans Focal lesions produce selective color or motion blindness. Focal lesions produce selective color or motion blindness. cortical color blindness (achromatopsia) cortical color blindness (achromatopsia) motion blindness (akinetopsia) motion blindness (akinetopsia) Positron Emission Tomography (PET) studies in Humans reveal: Positron Emission Tomography (PET) studies in Humans reveal: Motion Area distinct from color area Motion Area distinct from color area (e.g. Zeki and colleagues, 1990.) (e.g. Zeki and colleagues, 1990.)

Artwork pre and post achromatopsia

Achromatopsia Inability to perceive color-- color knowledge and color naming can be intact. Inability to perceive color-- color knowledge and color naming can be intact. acquired after cortical brain damage acquired after cortical brain damage distinct from congenital color blindness (photoreceptor abnormality) distinct from congenital color blindness (photoreceptor abnormality) Subjective reports... Subjective reports... Lesion locus: bilateral occipito-temporal lesions associated w/upper visual field deficits. Lesion locus: bilateral occipito-temporal lesions associated w/upper visual field deficits.

Akinetopsia Acquired inability to motion Acquired inability to motion Subjective reports... Subjective reports... Lesion locus: bilateral occipito-parietal involving area MT/V5. Lesion locus: bilateral occipito-parietal involving area MT/V5.

PET Activation Study Logic Perceptual/cognitive activity mediated by localized neural activity. Perceptual/cognitive activity mediated by localized neural activity. Blood flow changes accompany regional changes in neural activity. Blood flow changes accompany regional changes in neural activity. Blood flow changes measured by monitoring distribution of radioactive tracer. Blood flow changes measured by monitoring distribution of radioactive tracer.

Regions active in response to COLOR Regions active in response to MOTION