Ch. 13 : The Occipital Lobes & Networks

Occipital-Lobe Anatomy

main function : vision

• posterior within the brain

  • contains many distinct visual areas & these visual areas extend into the temporal and parietal lobes (occipital lobe is the 1st for vision, but it doens’t do the most)

V1 (primary visual cortex) = surrounds the calcimine sulcus & contains distinct layer & sublayers

contains:

1. Blobs = process colour info

2. interblobs (spaces b/w blobs) = process form (shape) & motion

note: the organization of the primate visual system has been studied extensively in Old world monkeys → helps us understand human vision

Topography of the Human Visual Cortex

• the primary visual cortex is organized in a topographical:

  • fMRI’s show abt 25 different visual topographic regions

→ visual cortex is organized into diff areas

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• cells are in layers in the visual cortex = striate cortex

• move from V1 (blobs & interblobs)→ V2 (contains stripe areas)

Organization of Visual Arcs

→ visual processing starts in V1 and projects to all major visual areas (dorsal stream, ventral stream, STS)

V1 (primary visual cortex) = surrounds the calcimine sulcus & contains distinct layer & sublayers

• heterogeneous

contains:

1. Blobs = process colour info

2. Interblobs (spaces b/w blobs) = process form (shape) & motion

→ processing gets more complex from V1 → V2

→ colour perception ENHANCES our ability to detect form & motion

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V2 = the secondary processing location & it projects onwards to major visual areas

• heterogenous

contains:

1. Thin stripes = involved in colour perception (info from V1 blobs)

2. Thick stripes = process form info (info from V1 interblobs)

3. Pale stripes = involved in motion perception

projects to major visual areas:

• Dorsal stream (how) = visual guidance of movementt

  • goes into parietal lobe

• Ventral stream (what) = object perception & motion perception

  • goes into temporal lobe

• STS stream = object perception & motion perception

  • goes to higher section of temporal lobe

  • where the dorsal & ventral stream communicate + also gets info from V1

Theory of Occipital-Lobe Function

→ V1 and V2 seem to be heterogenous areas processing all types of visual info

→ higher visual areas are more specialized, but can still integrate info from multiple aspects of vision:

1. V3 = processes dynamic form or the shapes of objects in motion - dorsal stream

2. V4 = predominantly processes colour info - ventral stream

3. V5 = involved in motion processing (MT) - dorsal stream

note: not all of these are in the occipital lobe

• damage to these higher visual areas results in defects SPECIFIC to the fxns of those areas

• ppl with V1 damage report being blind, BUT smaller projections from subcortical areas to these higher visual areas provide some limited vision

Visual Networks beyond the Occipital Lobe

• a dorsal & ventral pathway for the visual system was proposed based on macaque monkey studies

→ using resting-state fMRI, new research suggests the ventral stream contains 2 parts : one on the lateral surface & one on the ventral surface of the temporal lobe

note: ventral pathway seems to be related to language

→ visual streams project to many higher visual areas THROUGHOUT the temporal, parietal & frontal lobes

• these visual areas seem to have specific fxns & interact

• these sometimes project back to the occipital lobe

Vision for Action: Using vision to aid actions

→ visual processing is necessary to:

1. direct specific movements (can see what’s there)

ex. like reaching for a mug or phone (uses dorsal stream)

2. to shape the hand to pick up the object (see size/recognize shape using the ventral stream)

→ visual processing can be done with or w/o attention & awareness

→ interaction b/w multiple areas (mainly in the parietal lobe of the dorsal stream) is required to process the visual info abt MOVING targets in order to catch it

Action for Vision

→ when looking at an object, the visual system focuses on ONLY part of the object

ex. when u look at a face, the observer (u) focuses on the eyes & mouth - particularly in the LEFT visual field

• possibly in L-visual field, bc its sent to the R-hemis that specializes in emotions

note: bias to the L-visual field seems to be specific for viewing faces

→ when a subject performs a mental rotation task, the task is often accompanied by EYE movements (bc of feedback?)

Visual Space

→ Objects can be described in terms of their relation to the observer (egocentric space) or to e/o (allocentric space)

• the visual system likely encodes BOTH perspective (egocentric & allocentric) to guide movements appropriately & to remember a visual scene

  • allows us to process/interact quicker in the future (ex. first time going to a class vs ease after a month)

note: women tend to be better at allocentric space (connects to women being better at spatial memory)

Visual Pathways Beyond the Occipital Lobe

→ vision’s primary function is to guide movement, NOT to recognize objects (this just helps guide movements)

• info for visually guided movement projects from V1 → parietal areas via the dorsal stream

→ the dorsal stream provides real-time visual control of action - allows us to modify movement in real time

• thus we dont need to be able to identify the object, just need to know the location & shape to visually control action

pts with damage to visual areas demonstrate the roles of the dorsal & ventral streams:

• damage to lateral occipital area : CANT consciously see, but can reach for an object unseen with correct hand shape to grasp

• damage to dorsal stream : can see objects, but CANT accurately reach for them

Role of Dorsal-Stream Neurons

location : parietal lobe

Posterior parietal visual neurons = activated ONLY by movement (when the brain is acting on visual info)

• just looking at the objects DOESNT cause activity in these cells

• provide a connection b/w the visual world thats perceived + the intent to take action in that world + interacts directly with our movements

→ visual info abt shape, movement & location is sent to the parietal lobe over DIFF pathways:

1. some from occipital : movements & some shape

2. some from ventral stream : location & shape

→ much of this processing occurs below conscious awareness

→ parietal-cortex lesions that impact visual areas are usually characterized as:

1. Visuospatial = difficulty with WHERE something is in space

2. Visuomotor = difficulty MOVING twd something in space

Visual Processing Hierarchy

• V1 send info to the dorsal stream’s V5 (motion) & V3A (form) → sent to parietal visual areas for ACTION

• V1 also send info to V2

  • V2 send info to:

    • V3 (dynamic form) & V4 (colour form) in the ventral stream → sent to temporal visual areas for OBJECT RECOGNITION

    • ALSO send info to parietal visual areas for ACTION

→ V1 is the most important as it sends to dorsal stream for action, sends to V2 for object recognition, AND sends to V2 to be sent to parietal visual areas for action

→ with a damaged V2 : CANT recognize, BUT we can still act because V1 sends info to the dorsal stream→ this displays why visions is MORE for action instead of object recognition

Imaging Studies of Dorsal & Ventral Streams

when comparing 2 faces:

• activity in the temporal lobe (ventral for recognition of facial features) when asked to say which faces were the same

• activity in the parietal lobe (dorsal for action) when asked to consider where on the face a dot or box was superimposed

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• increased activity in V5 (dorsal) when asked to detect motion

• activity in V4 (ventral in temporal lobe) when asked to detect colour

→ fMRI studies in primates also show similar results the these lesion studies

What does Imaging Visual Pathways Tell us?

→ shows that diff areas of the brain tend to specialize in different functions of vision

ex. facial recognition in the temporal lobe

ex. spatial location in the parietal lobe

Disorders of Visual Pathways