Perception and Neuropsychology Flashcards

Intro and Structure of the Eye

• Sensation and Perception:

  • Sensation involves sensory organs absorbing energy.

  • This energy is transduced into a neural signal.

  • The neural signal is sent throughout the brain for further processing.

• Why Study Vision?

  • Vision is the most studied sensory system.

• Light and the Electromagnetic Spectrum

  • Humans respond to a narrow wavelength range called the visible spectrum.

  • Wavelength corresponds to color, and amplitude corresponds to brightness.

Structure and Function of the Eye

• Outer Layer:

  • Cornea: Transparent and involved in focusing the image on the retina.

• Middle Layer:

  • Choroid (vascular tunic): Provides the eye's blood supply, nutrients and waste disposal, not covering every bit of the eye.

  • Fluid in the anterior and posterior chambers also serves this function for the cornea and lens.

• Inner Layer:

  • Iris: Muscle that gives eyes their distinctive color.

  • Pupil: Aperture between the iris muscles controlling light entry.

  • Lens: Allows for accommodation (focusing near and far); cataracts cloud the lens.

  • Retina: Contains photoreceptors (rods and cones) that convert electromagnetic energy into a neural signal; has a seemingly backwards architecture.

Retina

• Photoreceptor Layer -> Bipolar Cell Layer -> Ganglion Cell Layer

• Rods and Cones

  • 120 million rods and 7 million cones perform transduction.

  • Properties:

    • Cones: Color vision, daytime vision, high resolution.

    • Rods: No color vision, nighttime vision, low resolution.

  • Distribution in the retina: Density varies across the retina, with a concentration of cones in the fovea.

• Bipolar cells and retinal ganglion cells engage in processing the visual image.

From Eye to Brain

• Optic Nerve: Axons of the ganglion cells (size of a pencil).

• Blind Spot: Where the axons of the ganglion cells exit the eye.

Learning Objectives

• Understand how visual information is transmitted from the eye further into the brain

• Understand the different types of visual information processing that occur along the different visual pathways

• Understand the coding properties of neurons along the visual pathways

• Understand lateral inhibition and receptive fields

Review of Brain Organization

• Lobes: Frontal, parietal, temporal, occipital.

• Cortex: All the convolutions.

• Subcortex: Everything below those convolutions

Visual Pathways and Functions

• Eyes -> lateral geniculate nucleus (LGN) -> primary visual cortex (V1).

• Note the interesting projection pattern where half of the fibres from each eye remain on the same side and half cross over

• The primary visual cortex (V1) is in the occipital lobe.

Cortical Visual Pathways Beyond V1

• Mishkin and Ungerleider (1982) study with monkeys trained on object discrimination and landmark discrimination tasks.

• Lesions of the Parietal Lobe impaired performance on the landmark discrimination task.

• Lesions of the Temporal Lobe impaired performance on the object discrimination task.

• Double Dissociation.

• Beyond V1, visual information travels along two pathways:

  • Ventral Stream: Involved in pattern/object vision (WHAT pathway).

  • Dorsal Stream: Involved in spatial vision (WHERE pathway).

Neural Properties Along the Visual System

• Electrophysiology studies involve dropping wires into the brain to listen to action potentials of neurons.

• All neurons (cells) fire at a baseline rate.

• If a cell is interested in something it will either increase or decrease its firing rate relative to baseline

• From Eyes up to V1:

  • Rods and Cones: Changes in illumination.

  • Retinal Ganglion (RG) Cells: Spots of light.

  • LGN cells: Spots of light

  • V1 cells: Lines of different orientations.

• Beyond V1 (in the ventral visual stream):

  • IT (inferior temporal) cortex (that area damaged in the Mishkin and Ungerleider (1982) study)

  • Grandmother cells are cells that seem to respond to discrete features. They are also known as feature detectors

Retinotopic Mapping

• Definition: point-to-point mapping of external world onto a brain area.

• V1 and before: There is retinotopic mapping

• After V1: There is NO retinotopic mapping.

• What Does It All Mean?

  • As you go higher into the visual system (away from the eyes) the information that is processed becomes more complex.

  • In other words, the features that drive a cell change from basic illumination levels (rods and cones) to spots of light (RG cells, LGN cells) to lines (V1) to complex features (IT cortex cells)

  • In line with these changes, the complexity with respect to where information has to be changes from small specific areas of space (V1 cells) to very large areas of space (IT cortex cells)

Lateral Inhibition

• What does it mean when you say that a retinal ganglion cell likes "dots"?

  • center-surround architecture

• What is the point of center-surround architecture?

  • Enhances Contrast

  • Brightness Contrast

Herman Grid Illusion

• Why do you see black spots at the intersection?

  • The Herman grid illusion that arises out of the center-surround architecture of RGCs

    • S = Surround RGCs

    • C = center RGCs

• What information does the RGC assembly send forth if bathed in diffuse light or darkness?

  • Nothing!

• What information does the RGC assembly send forth if presented with a light dot or dark dot?

  • Maximum signal!

• What information does the RGC assembly send forth if presented with either the Intersection or Street of the Herman Grid Illusion?

  • your brain is getting the message that the intersection is darker than the street and hence you see a black dot in the intersection

• But why does the dot vanish when you foveate the intersection?

  • at the fovea, the cones are packed more tightly than at the periphery, and so the RGC assembly is smaller, and falls completely within the intersection and street.

• How images are built neurally?

  • Going from dots to a line

  • Going from a line to a face

Disorders of Vision

• Learning Objectives

  • Understand how some forms of blindness arise

  • Understand the different disorders of vision, and what areas of the brain must be damaged to cause these disorders

Blindness

• Review of projections from retina to V1

• How can we accommodate the notion that the right side of the brain looks at the left visual world (and left side of brain at right visual world) if only half of the fibres from the retina cross over and half remain on the same side?

What Damage Causes What Kind of Blindness?

• Follow the pathways back to the retina and shade in the corresponding visual fields to find out

  • Right monocular blindness

  • Bitemporal hemianopia

  • Left homonymous hemianopia

  • Left homonymous hemianopia with macular sparing

  • Blindsight

Patient DB Case Study

• Patient DB had surgical removal of a tumour in his right occipital lobe

• He suffers from left homonymous hemianopia

• Hatched marks represent islands of marginal vision

• Patient DB displays two interesting visual problems

  • Can't identify a static visual object but can localize it in space. Says he can't even see an object.

  • Can't identify a moving visual object but can localize it in space. The difference is that with moving objects he says he has the sensation that something is out there. He does not feel that way with static visual objects.

• How can we accommodate these two aspects of DB's vision?

  • localizing an object that he can't see is a result of information is reaching the dorsal stream

  • sensing a moving object is a result of information reaching V5

  • not being able to identify a static or moving object is because information is bypassing V1

Achromatopsia and Akinetopsia

• Achromatopsia: absence of colour vision

  • damage to V4

  • people with achromatopsia are colour blind.

  • Achromatopsics see only black, white, and shades of gray

  • note that achromatopsia can also arise from missing cone photoreceptors. Indeed, most achromatopsics are colour blind for this reason

• Akinetopsia: absence of motion vision

  • damage to V5 (MT)

  • patients have difficult perceiving objects set in motion

  • extremely rare

Visual Agnosias

• Apperceptive Agnosia

  • Failure of object recognition due to fundamental failure of visual perception

  • Preserved elementary visual function such as colour perception and motion perception

  • Poor matching and copying

  • Neuropathology: bilateral damage to V1

  • Peppery Mask Hypothesis of apperceptive agnosia

    • remember, V1 is retinotopically mapped

    • as a result, damage to V1 results in multiple blind spots known as "scotomas"

    • it's like seeing the world through a peppery mask

    • hence vision is severely impaired

• Dorsal Simultagnosia

  • Failure of object recognition due to a spatial perceptual impairment

  • Preserved elementary visual function such as colour perception and motion perception

  • Can recognize objects but not more than one at a time

  • Neuropathology: bilateral damage to parietal lobes

• Ventral Simultagnosia

  • Failure of object recognition due to a complex perceptual impairment

  • Preserved elementary visual function such as colour perception and motion perception

  • Can recognize objects clearly but not more than one at a time BUT…can see multiple objects (but not clearly)

  • Neuropathology ventral stream beyond V4

• Associative Agnosia

  • Failure of object recognition due to a higher-order complex perceptual impairment

  • Preserved elementary visual function such as colour perception and motion perception

  • Seemingly normal copying

    • gives the illusion that vision is normal…it's not!

    • however, perception is not normal

    • copying is accurate but "slavish"

• When you push visual abilities to limits, it is clear that even associative agnosics have impaired visual abilities

  • incomplete figures test

  • embedded figures test

Pattern Perception

• Theories of Pattern Perception

  • Bottom-Up Theories

    • It is the way the visual system is constructed, starting with analysing low level features (eg, dots, lines) and then building on that until a complex image emerges (eg, face).

    • Relies of sensory information

    • Evidence for Bottom-Up processing: Errors and confusion

      • We are more likely to confuse E and F than E and A. That makes sense because E and F share more features in common (two horizontal lines and one vertical line) than E and A (only one horizontal line)

  • Top-Down Theories

    • Hypothesis testing

    • Relies on knowledge and experience

    • Speed of recognition and speed of reading are so fast it seems unlikely that we are engaging bottom-up processing mechanisms

    • Also, we are faster to recognize an object against a background that one by itself despite the fact that in the former there is more information to process.

  • Application of both Bottom-Up and Top-Down processing

    • Ambiguous and Reversible Figures

  • Interactive Theories of Pattern Perception

    • Theories that accommodate both are correct!

Depth Perception

• Binocular Cues

  • Retinal Disparity

    • Two images (a and b) at different depths will results in different image distances on the retina

    • Your brain interprets that difference as depth

  • Convergence and Divergence

    • objects nearby (a) cause the eyes to converge and objects at a distance (b) causes eyes to diverge

      • Your brain interprets those differing signals as depth

• Monocular Cues

  • interposition, relative size, linear perspective, height in plane, texture gradient, and light and shadow are all monocular cues to depth perception.

  • If these cues aren't used, or used poorly…

  • On the other hand, if these cues are used well…

Colour Perception

• Young-Helmholtz Trichromatic Theory

  • Postulated the existence of three cones in the retina each maximally sensitive to a certain colour

  • Indeed there are three types of cones in the retina

  • But the the Young-Helmholtz Trichromatic Theory struggles to account for:

    • Why colour blindness occur in pairs?

    • Why you get colour after effects?

• Opponent-Process Theory

  • Bipolar and RGCs are opponent process cells

    • For example, a RGC can be blue-ON/green-OFF

    • OR blue-OFF/green-ON

History of Localization of Function

• Learning Objectives

  • Understand the history of the debate of localization of function in the brain

  • Know about the key players in the debate over whether there is localization of function in the brain

• Rene Descartes (early 1600s)

  • Recognized that the brain was symmetrical and every structure on left side was on the right side as well

  • recognizea that we don't see two of everything, so he searched for a structure for which there was just one and that structure must unite everything

    • pineal gland, a pea-sized structure between the two hemispheres

• Gall and Spurzheim (early 1800s)

  • Phrenology

    • pseudoscience in which bumps and depressions on the skull were associated the well-developed and under-developed behaviours (Faculties)

    • one of the first attempts at localization of function!

  • Faculties

    • Many odd "behaviours” were mapped onto the brain!

  • Challenges to Phrenology

    • bumps and depressions on the outer skull do not map onto bumps and depressions on the brain

    • in fact, bumps and depressions on the outer skull bear no resemblance to bumps and depressions on the inner skull, which is for the most part smooth.

    • but Phrenology fails mainly because people are not willing to accept the fact that there is localization of function in the brain

• Paul Broca (1861)

  • Patient called Tan comes under the care of Paul Broca

  • Tan has a severe language problem and is only able to say "Tan"

  • Tan passes away under Broca's care

  • Broca removes Tan's brain and notices a large lesion in the left frontal area (now known as Broca's area).

  • Broca localizes language to the left frontal lobe

  • Broca's aphasia in a difficulty with language output

• Karl Wernicke (1873)

  • Notices a severe language problem in his patient

  • Patient's output is normal but comprehension seems impaired

  • Upon his patient's death, Wernicke removed the brain and notices a lesion in the left temporal lobe

  • Wernicke's finding challenge the notion of localization of function

  • But do Wernicke's findings really challenge the notion of localization of function?

Broca vs Wernicke

• Broca's Aphasia is a difficulty with the motor output of language. Comprehension is fine.

• Wernicke's Aphasia is a difficulty with comprehension of language. Motor output is fine.

• Double Dissociation!

• Fritsch and Hitzig (1870)

  • Noticed that stimulation of a strip of cortex (motor cortex) caused contralateral movement of body parts

  • Very organized topography suggests localization of function and…

  • The brain is an electrical structure!

• Current Views

  • The brain is modular

  • Within each sensory module there is further detailed localization of function

Temporal and Occipital Lobes

• Learning Objectives

  • Understand the behavioural effects of damage to the occipital lobe

  • Understand the behavioural effects of damage to different areas of the temporal lobe

Occipital Lobe

• Damage causes…

  • Blindness and Blindsight

  • Apperceptive Agnosia

Temporal Lobe

• Lateral Surface:

  • Superior, middle, and inferior temporal gyrus

• Medial Surface:

  • Medial temporal lobe

  • Effects of Damage to the Superior Temporal Gyrus

    • Auditory region of the brain

    • Deafness

    • Wernicke's Aphasia

    • Auditory Agnosia

  • Effects of Damage to the Middle and Inferior Temporal Gyrus

    • Achromatopsia

    • Akinetopsia

    • Ventral Simultagnosia

    • Associative Agnosia

  • Effects of Damage to the Right Medial Temporal Lobe

    • Copying is OK

    • Visual memory impaired

  • Effects of Damage to the Left Medial Temporal Lobe

    • Hearing is OK

    • Verbal memory is impaired

Patient H.M.

• Case Study

• Retrograde Amnesia

• Anterograde Amnesia

• Despite profound memory impairments, patients with MTL damage do have some spared memories

  • Mirror-drawing

  • Tower of Hanoi

• Multiple Memory Systems

  • DECLARATIVE (EXPLICIT) MEMORY

    • FACTS

    • EVENTS

  • NONDECLARATIVE (IMPLICIT)

    • SKILLS AND HABITS

    • PRIMING

    • SIMPLE NONASSOCIATIVE LEARNING

    • CLASSICAL CONDITIONING

Parietal Lobes

• Learning Objectives

  • Understand the behavioural effects of damage to the parietal lobe

  • Understand the mechanisms underlying contralateral neglect

Anatomy of Parietal Lobes

• Effects of Damage

  • Impairments in integrating sensory information

  • Impairments in the control of movement

  • Impairments in guiding movements to points in space

  • Impairments in abstract concepts

  • Impairments in directing attention

  • Impairments in Processing Spatial Information

Effects of Left Parietal Damage

• Agraphia

• Acalculia

• Right/Left Confusion

• Dyslexia

• Difficulty in Drawing (details)

Effects of Right Parietal Damage

• Difficulty in recognizing unfamiliar views of objects

• Difficulty in drawing (overall shape)

• Contralateral Neglect

Contralateral Neglect

• Clinical Tests

  • Line cancellation and Letter cancellation

  • Line Bisection

• Is Neglect Pre-Perceptual or Post-Perceptual

  • Burning House Anecdote

  • Milan Piazza Experiment

• What is Being Neglected?

  • Ego-centered vs Object-centered neglect

• Driver, Baylis, Goodrich, & Rafal (1994)

  • Is this triangle pointing 2 o'clock, 6 o'clock, or 10 o'clock?

  • How about these triangles? Driver, Baylis, Goodrich, & Rafal (1994) continued