Week 6 - Visual Processing

Brain areas contributing to perception of shape

<ventral stream>

V1
↓
V2
↓
V4
↓
Lateral Occipital Complex (LOC)
↓
Inferior Temporal (IT) Cortex

Brain areas contributing to perception of location

<dorsal stream>

V1
↓
V2
↓
Middle Temporal Area (V5) / Dorsomedial Area (V6)
↓
Posterior parietal cortex

Dorsal stream

“Where + how” ⇒ spatial location, using vision to guide action

• Can operate without conscious awareness

• Begins at occipital pole / calcarine sulcus (occipito-parietal)

• From primary visual cortex (V1) → posterior parietal lobe

• Diverge into 3 streams beyond PPC:

  1. To prefrontal cortex (PFC) = where
     (spatial working memory, location)

  2. To premotor cortex (PMC) = how
     (action, visual-motor integration)

  3. To medial temporal lobe (PHC, PHG, MPC, RSC) = where
     (navigation, broader environmental / contextual scale)
     
     

Ventral stream

“What” ⇒ shape & object identity, conscious perception & awareness

• Begins at occipital pole / calcarine sulcus (occipito-temporal)

• From primary visual cortex (V1) → inferior temporal cortex

• Hierarchical organisation

  • Progressively increases in complexity

  • Small receptive fields become larger

What visual information is represented at each stage of the ventral pathway?

1. Early visual areas (V1, V2) = simple information

   • i.e., edges, points of light

   • Small receptive fields

   • Retinotopically organised (points in space on the retina translate spatially to their corresponding cortical areas)
     
     

2. Intermediate areas (V4) = moderately complex features

   • Integrates smaller pieces of information

   • Larger receptive fields
     —but restricted to 1 quadrant
     i.e., upper right visual field)

   • Less specificity for the location of a stimulus capable of driving a neuronal response
     
     

3. High-level areas (inferior temporal (IT) cortex)
   = responds to complex objects

   • i.e., faces, hands, common objects, individuals within a category

   • Very large receptive fields (little retinotopic organisation, respond to almost anywhere)
     → but still contralaterally limited, i.e., right visual field = left IT

Lateral Occipital Complex (LOC)

• The ‘object area’

• Spatially biased (despite no retinotopic organisation)

  • Objects are processed more efficiently in their typical location

  • aka how ‘normal’ its spatial location is relative to its environmental context

  • Encodes info on statistical regularities of a given environment
    
    

SUBDIVISIONS:

1. Dorsal-caudal subdivision: LO / LO-proper (lateral surface)
   → LO = objects (responds more to whole objects)

2. Ventral-anterior subdivision: vOT / pFs (ventral surface)
   → vOT = textures (more early visual, responds to smaller details)

How do the 2 visual pathways contribute to perception vs vision in action?

How do we test for perceptual awareness?

Modules that code specific object classes

Theoretical explanations of the function(s) of modules

Object representation coding in the LO vs VOT

fMRI adaptation paradigm to understand different object representations

Intraoccular suppression (spatial bias in the LOC)

1. Present an object to one eye

2. Follow-up with a dynamic image flickering at a high freq. (Mondrian, very strong stimulus) to the other eye

3. Harder to see object of interest as Mondrian dominates perception

4. Gradually, object of interest emerges into consciousness
   → measuring how long it takes to see object of interest

Apperceptive agnosias

Perception deficit

The brain cannot build a coherent visual shape by piecing together basic visual features like shapes, lines, contours

• cannot copy or draw a simple figure

• linked to damage in the parietal or occipital cortex

Associative agnosias

Recognition or meaning deficit

The brain sees and forms shapes correctly but cannot connect these visual images to stored memory, meaning, or names

• can accurately copy or draw, but don’t know what object it is

• linked to damage in the bilateral inferior occipitotemporal cortex

Case example: C.K. (Kevin Chappell)

How does studying object recognition disorders help understand visual processing?

Case example C.K.

Object and face recognition tasks

Effectiveness of different stimuli and testing methods for diagnosing visual agnosias

How well do they engage the face recognition system?

Cognitive mechanisms underlying object recognition and face perception

(based on neuropsychological evidence)