chapter 13, Study Notes on Occipital Lobes and Visual Processing

Chapter 13: The Occipital Lobes and Networks

Learning Objectives

  • Visual Processing Stages: Describe the main stages involved in visual processing from the retina to the cortex.

  • Anatomy of the Occipital Lobe: Explain the anatomy and functional organization of the occipital lobe.

  • Visual Pathways: Compare the roles of the ventral (“what”) and dorsal (“where/how”) visual pathways.

  • Visual Agnosias: Recognize key visual agnosias and other disorders affecting visual processing.

The Eye and the Retina

  • Retina: A light-sensitive layer at the back of the eye that converts light into neural signals.

    • Choroid: The layer of blood vessels between the retina and sclera.

    • Retinal Veins: Blood vessels that carry blood away from the retina.

    • Optic Nerve: Transmits visual information from the retina to the brain.

    • Vitreous: The gel-like substance filling the space between the lens and retina.

Image Processing Stages

  • Light passes through several layers of cells before reaching rods and cones.

  • Ganglion Cells: Have axons that leave the retina through the optic disc (blind spot).

  • Optic Disc:

    • Location: Where ganglion cell axons leave the eye.

    • Contains no light-sensitive cells: This results in a blind spot in the visual field.

Blind Spot Experiment

  • Instructions:

    1. Fixate on the cross while closing your right eye.

    2. Hold the figure about 1.5 feet from your face.

    3. Note the disappearance of the filled circle as it projects on the blind spot.

    4. Again, fixate on the lower cross to observe the continuity of the line despite its interruption at the blind spot.

Visual Pathways

  • Path of Image Processing:

    • Light rays enter through the cornea, pass through the pupil, and get focused by the lens onto the retina.

    • Signals travel via the optic nerve to the brain's visual cortex.

    • Visual processing pathway: Eye → Optic Disc → Macula → Signals travel via Optic Nerve to applicable areas in the brain.

  • Visual Pathways:

    • Retino-geniculate-striate Pathway:
      Eye → optic chiasm → lateral geniculate nucleus (LGN) of thalamus.

    • Tectopulvinar Pathway:
      Eye → optic chiasm → superior colliculi → lateral posterior and pulvinar nuclei of thalamus.

Occipital Lobe Anatomy and Function

  • Primary (V1) Visual Cortex: Processes basic features such as edges, orientation, spatial frequency.

  • Secondary (V2/V3) Visual Cortex: Involved in form, depth, and binocular vision.

  • Association Cortex (V4): Involved in complex processing of visual information.

  • Middle temporal region (MT; V5): Specialized in processing motion.

Damage to the Occipital Lobe

  • Consequences of lesions in various regions of the occipital lobe include:

    • Hemianopia: Loss of vision in half of the visual field.

    • Scotoma: Loss of vision in one specific area of the visual field.

    • Quadrantanopia: Loss of vision in one quadrant of the visual field.

Ventral and Dorsal Visual Streams

  • Ventral Stream (“What” pathway): Responsible for recognizing objects, including their names and functions, regardless of location.

  • Dorsal Stream (“Where/How” pathway): Identifies the location of objects but not their names or functions. Informs how to interact with objects.

Case Studies of Visual Stream Damage

  • Patient D.F.: Demonstrates impairments resulting from lesions in regions connected to the ventral stream.

  • Patient V.K.: Illustrates effects associated with the dorsal stream.

Inferotemporal (IT) Cortex

  • Located in the lower portion of the temporal lobe, the IT cortex is critical for object recognition.

  • Damage to the IT cortex can result in:

    • Agnosias: Inability to recognize objects.

    • Prosopagnosia: Inability to recognize faces; often leads to difficulty in identifying familiar people.

Neuron Response in IT Cortex

  • Neurons in the IT cortex respond to complex visual properties. Examples include:

    • Some neurons respond to simple stimuli like spots or lines.

    • Other neurons specialize in more complex stimuli such as hands, faces, and objects.

  • Research with macaque monkeys indicates that certain cells are particularly responsive to specific stimuli like monkey faces.

Specialized Areas of the Cortex

  • Parahippocampal Place Area (PPA): Responds to places, such as photos of landmarks or houses.

  • Fusiform Face Area (FFA): Responds preferentially to faces compared to objects.

Double Dissociation Findings

  • Some patients lose the ability to recognize faces (prosopagnosia) yet retain object recognition.

  • Conversely, other patients lose object recognition while retaining face recognition.

  • These findings stress that facial and object processing are distinct tasks within the brain.

Face Recognition Abilities

  • Humans exhibit advanced skill in recognizing faces, even in complex visual contexts.

  • FFA activation is noted when processing face-related information, indicating its importance in facial recognition tasks.

Pareidolia

  • A psychological phenomenon where people perceive patterns, such as faces or objects, in ambiguous stimuli, leading to continued perception of recognizable patterns in unrelated contexts.

Visual Perceptual Disorders

  • Characterized by challenges in interpreting visual information despite having normal vision. The following terms define various disorders:

    • Agnosias: Inability to recognize objects or patterns despite intact sensory function.

    • Prosopagnosia: Specific inability to recognize faces while having normal visual capabilities.

    • Blindsight: Ability to respond to visual stimuli without conscious awareness of them.

    • Akinetopsia: Difficulty in perceiving motion despite normal vision.

Apperceptive Visual Agnosia

  • Describes an impairment in forming a coherent visual perception of objects.

    • Patients fail to recognize objects due to the inability to assemble visual features into a complete perception.

    • Basic sensory abilities, such as acuity, color, and shape detection, remain intact.

Associative Visual Agnosia

  • Characterized by impaired linking of visual perception to understanding.

    • Individuals can see objects but cannot recognize them.

    • Patients can accurately copy and describe objects but struggle to identify or name them despite normal vision and perception.

Tests for Visual Agnosias

  • Common tests include:

    • Task of copying drawings, discerning “real or not” images, or evaluating “impossible objects.”

    • Apperceptive Agnosia: Patients struggle to determine truthfulness of objects because they cannot form a proper percept.

    • Associative Agnosia: Those who can tell the difference between real and impossible objects still cannot recognize or name real objects.

Prosopagnosia

  • Impairment in recognizing familiar faces despite having normal vision.

  • Often linked to damage in the fusiform face area (FFA) located in the IT cortex.

  • Patients may identify individuals through cues unrelated to face recognition, such as:

    • Voice

    • Clothing

    • Body image

    • Contextual information

    • Interesting fact: Both Brad Pitt and neurologist Oliver Sacks experienced prosopagnosia.

Famous Faces Test

  • Assessment used to investigate recognition abilities of familiar faces in prosopagnosia patients.

Video References

  • Face Processing in Patients: Documented research actual activations of face processing during a specific task.

  • Explore differences in processing between individuals with prosopagnosia and those with normal recognition abilities.

Blindsight

  • Defines the capacity to respond to visual stimuli without conscious awareness of seeing them, typically resulting from damage to the V1 region.

  • Patients may deny seeing any objects in certain visual fields but can detect aspects like movement, location, or orientation above chance levels.

Mechanisms of Blindsight

  • Blindsight occurs when visual information is processed through alternative pathways that do not engage the primary visual cortex (V1).

  • Highlights include:

    • Eye → Optic Nerve → Lateral Geniculate Nucleus (LGN) → Visual Cortex pathways.

    • Not all visual processing relies on the geniculostriate pathway, thus supporting phenomena observed in blindsight.

Akinetopsia

  • A condition characterized by the inability to perceive motion, despite normal static vision.

  • Patients may experience the world as composed of static “frames,” creating a disjointed perception of movement.

  • Cause of akinetopsia originates from damage to the visual area V5/MT.

Take Home Message

  • Vision initiates in the retina, subsequently passing through the LGN before reaching the visual cortex.

  • Emerging from the primary visual cortex (V1) are two distinct routes for further visual processing:

    • Ventral Stream: Focuses on identifying ‘what’ a stimulus is.

    • Dorsal Stream: Concentrates on understanding ‘where’ a stimulus is located.

  • Specialized areas within the IT cortex, including the FFA and PPA, cater specifically to different categories of stimuli.

  • Knowledge and insights gained from visual disorders highlight significant aspects of the visual processing system and its organization.