Object Processing & Dysfunction I

Object Processing & Dysfunction I: What & Where Pathways

Announcements & Exam Information

• Study Guide: Accessible via Pages > View all pages > Exam 1 Running Study Guide.

• Exam 1 Date/Time: Monday, 4-4:50 in the designated room.

• Materials Needed: Pencils and erasers.

• Coverage: Lectures 1-7 and Discussion sections weeks 1-4.

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Today's Topics

• What vs. Where Pathways

• Origins of What vs. Where Pathways

• V1 and Beyond

What vs. Where Pathways

Parallel Visual Processing Paths

• Cortical vs. Subcortical Paths from Retina:

  • Geniculo-striate (Cortical): The primary visual pathway.

  • Non-striate: Retino-tectal (subcortical), tecal-pulvinar-extra-striate. Involved in reflexive eye movements and attention.

• Parallel Paths within the Cortex:

  • Cortical "What" pathway: Ventral stream, primarily processes object identity.

  • Cortical "Where" pathway: Dorsal stream, primarily processes spatial location and motion.

Anatomical Segregation

• Dorsal stream (WHERE): Primarily involves the Posterior Parietal Lobe.

• Ventral stream (WHAT): Primarily involves the Inferior Temporal Lobe.

• Both streams originate from V1.

Ungerleider & Mishkin (1982) Monkey Experiment

This foundational study provided strong evidence for the two distinct visual streams.

• Task 1: Object Discrimination ("WHAT" task):

  • Monkeys first studied a single object.

  • Then, they had to select the familiar object from a choice.

• Task 2: Landmark Discrimination ("WHERE" task):

  • Monkeys had to pick the food well closer to a specific tower (landmark).

Experimental Design & Results

• Monkey Group 1: Bilateral temporal lesions.

• Monkey Group 2: Bilateral parietal lesions.

• Results for Parietal Lesion Group:

  • Showed a single dissociation: Severely impaired on the Landmark Task (WHERE), but performed normally on the Object Task (WHAT).

  • Performance on Landmark Task was significantly lower than on Object Task.

• Need for Double Dissociation: A single dissociation alone is not sufficient to conclude separate pathways, as one task might simply be harder. Thus, another dissociation is needed.

• Results for Temporal Lesion Group:

  • Showed the complementary deficit, completing the double dissociation: Severely impaired on the Object Task (WHAT), but performed normally on the Landmark Task (WHERE).

  • This ruled out the possibility that the Landmark Task was inherently harder.

Implications of Double Dissociation

1. The Landmark Task (spatial location/WHERE) depends critically on the parietal lobes.

2. The Object Task (object identity/WHAT) depends critically on the temporal lobes.

• Summary: A parietal lesion leads to a selective deficit in "where" processing, while a temporal lesion leads to a selective deficit in "what" processing. This double dissociation confirmed the existence of separate functional pathways.

Why Separate Pathways?

• Knowing what something is (object identity) is a very different function from knowing where something is (spatial location).

• These different functions:

  • Require different types of information.

  • Have different computational demands.

  • Need specialized neural machinery.

Origins of What vs. Where Pathways

Retinal Ganglion Cells

• The differentiation of these pathways begins at the retina:

  • P-cells (Parvocellular):

    • Small receptive fields, dominant input from the fovea (high acuity vision).

    • Project to the parvocellular layers of the LGN.

    • Dominant input to the ventral/what pathway.

    • Properties: Slower conducting, color selective, high acuity.

  • M-cells (Magnocellular):

    • Integrate across multiple cones (larger receptive fields, sensitive to motion).

    • Project to the magnocellular layers of the LGN.

    • Also project to the superior colliculus (involved in eye movements).

    • Dominant input to the dorsal/where pathway.

    • Properties: Faster conducting, no color code, low acuity.

Lateral Geniculate Nucleus (LGN)

• Located in the thalamus, one in each hemisphere.

• Comprised of $6$ layers, each receiving monocular input representing the contralateral visual field.

• Superior 4 layers: Parvocellular (small cells), receiving P-cell input.

  • Layers $6, 5, 4, 3$ receive input from different eyes (e.g., $6$ from contralateral, $5$ from ipsilateral, $4$ from contralateral, $3$ from ipsilateral).

• Inferior 2 layers: Magnocellular (large cells), receiving M-cell input.

  • Layers $2, 1$ receive input from different eyes (e.g., $2$ from ipsilateral, $1$ from contralateral).

V1 and Beyond

Geniculo-Striate Pathway Refined

The visual information flow expands and differentiates:

• Sensory Receptor: Photoreceptors, M & P Retinal Ganglion Cells.

• Thalamic Nucleus: Lateral Geniculate Nucleus (LGN), with Magnocellular & Parvocellular layers.

• Primary Sensory Cortex: Primary Visual Cortex (V1), also known as striate cortex or Area $17$.

• Secondary Sensory Cortex: Extrastriate cortex (V2, V3, V4, V5/MT).

• Association Cortex: Higher-level processing areas.

V1 (Primary Visual Cortex)

• Structure: V1, or striate cortex, is characterized by Nissl staining of cell bodies revealing 6 distinct cortical layers.

• Cytochrome Oxidase Staining: Reveals subregions within V1 called Blobs and Interblobs.

  • Implication: The magnocellular and parvocellular pathways remain segregated within V1.

• Pathway Segregation within V1 Layers:

  • Magnocellular Pathway: Primarily projects to Layer IVB.

  • Parvocellular Pathway: Primarily projects to Layer IVC$_{\beta}$, then further divides into Blobs and Interblobs primarily in Layers II and III.

    • Blobs are involved in color processing.

    • Interblobs are involved in form and orientation processing.

Hierarchical Processing in Visual Subregions Beyond V1

As visual information ascends the processing hierarchy:

• Receptive Fields: Increase in size and complexity, often crossing the midline.

• Specificity: Neurons become sensitive to increasingly complex stimulus characteristics:

  • Colors

  • Direction of motion

  • Simple objects to complex objects

  • Faces

Functional Specialization of Key Visual Areas

Visual processing areas beyond V1 exhibit increasing specialization:

• V1: Processes basic features like speed, direction, spatial frequency, temporal frequency, orientation, and color.

• V2:

  • Thick stripes: Speed, direction, spatial frequency, temporal frequency (contributes to dorsal stream).

  • Thin stripes, Interstripes: Edges, illusory edges, color, border ownership (contributes to ventral stream).

• V4 (Ventral Stream): Processes angles, curvature, perceived color, kinetic contours.

• TEO/PIT (Ventral Stream): Simple shapes.

• TE/AIT (Ventral Stream, Inferior Temporal Cortex): Complex shapes/body parts, object recognition, object invariance (ability to recognize an object despite changes in viewpoint, size, etc.).

• MT (V5) (Dorsal Stream): Specializes in speed, direction, spatial direction, local motion, and temporal direction.

• MST (Dorsal Stream): Processes higher-order motion cues like expansions, contractions, rotations, translations (optic flow), crucial for self-motion perception.

• Parietal Regions (Dorsal Stream): Involved in heading, optic flow, self-motion, and multi-modal integration (combining visual information with other sensory inputs for spatial awareness).

Synthesis: What vs. Where Pathways

• Dorsal (Parietal) Stream - "Where" Pathway:

  • Function: Spatial processing.

  • Information types: Location, movement (speed, direction, temporal frequency), spatial transformations, spatial relations.

  • Origin: Primarily magnocellular pathway input.

• Ventral (Temporal) Stream - "What" Pathway:

  • Function: Object processing.

  • Information types: Shape, color, texture, pictorial detail, size, object recognition, object invariance.

  • Origin: Primarily parvocellular pathway input.

Next Time

• Special-Purpose Modules and Agnosias