Chapter 9 Flashcards
Blindsight and Perception
Patient can’t identify objects in blind area but can accurately tell about changes in visual field
Not conscious that they can register changes in visual field
NS constructs images from bits of information + brain must bind it together to create PERCEPTION
Selective awareness → can only access some part of the info our brain is processing
Nature of sensation and Perception
Only input brain receives: series of action potentials originating from external energy and is transduced by sensory receptors → info is passed along Sensory neurons that form pathways
Collective sensory input → transduction→ perception: Perception (how one set of nerve impulses = sound and others = vision) is unknown
Diverse sensory systems: vision, touch, taste, etc.
↳ organized similarity but all leads to different perceptual experience
Sensory receptors
Specialized cells that transduce (convert) external sensory energy into neural activity
Each sensory receptors are designed to respond to a narrow band of energy
- vision: light energy → chemical energy
- auditory: air pressure → mechanical energy
- somatosensory → mechanical energy
-taste and olfaction → chemical molecules
Receptive field
Region of sensory space that selectively responds to external stimulation → stim. modifies receptors activity
What open eye sees: receptive field
→ each photoreceptor in eye points in slightly different direction: creates a broader sense of perception
RF can help sample sensory info and locate events in space→ very tightly packed
Sensory receptor RF can contrast info each receptor is providing→ many overlap and form a network of communication → contrasting responses and levels of activation: helps localize sensations
Spatial dimension of sensory info produces cortical patterns and maps→form reality
Receptor density and sensitivity
Sensory receptors are not evenly distributed across body or organs
Density is important for determining the sensitivity of a sensory system
↳ ex. Visual receptors packed towards Center of visual field = poor peripheral vision
Differences m receptor density determine the special abilities of many animals
Neural relays
All receptors connect to cortex through a sequence of intervening neurons
(Visual): Retina → thalamus → V1 area → other cortical regions
(Auditory): Auditory receptors (ear)→ hindbrain → midbrain → thalamus → cortex
Sensory info is modified at each relay stage: each region constructs different aspects of sensory experience
Neural relays allow sensory systems to interact
Sensory coding
After transduction: sensory info is encoded by action potentials that travel along peripheral nerves to CNS → action potentials travel on nerve tracks within CNS → to whichever area is needed
Unless a reflex: most people need CNS to process info and input
Presence of a stimulus can be encoded by an increase or a decrease in discharge rate
↳amount of increases/decreases encode stimulus intensity
Changes in visual field encoded by activity in different neurons or different levels of discharge within a Neuron
Sensory coding and representation
Neocortex represents sensory field of each sensory modality as a spatially organized neural representation of external world
Topographic map: spatially organized rep. Of external world
Each sensory system has at least one primary cortical area → main relay station where sensory input comes first: MOST input arrives here
May project to secondary areas
Topographic map in sensorimotor cortex
Size of its features represent relative proportions of the parts of the human brain responsible for motor and somatosensory function
Features that are exaggerated have largest correlate representations in the brain
Sensation vs. Perception
Sensation: registration of physical stimuli from the environment by the sensory organs → no conscious registration
Perception: subjective interpretation of sensations by the brain → visual experience is not an objective reproduction of what is out there; rather a subjective construction of reality manufactured by brain → human senses are incredibly limited and perception is very flawed
Structure of Retina
Retina → consists of neurons and photoreceptor cells (cones and rods)
- translates light into action potentials
- discriminates wavelengths (colours)
- works in wide range of light intensities
Fovea→ region at the center of the retina that is specialized for high acuity: Sharp vision
↳ receptive field at the center of the eyes visual field: can see more clearly at center of visual field
Blind spot (optic disk)
- region of retina where axons forming optic nerve leave the eye and where blood vessels enter and leave → no photoreceptors
Acuity across visual field
Vision is better in the Center of the visual field than in periphery
Visual system is colour based at Center and black and white in peripheral → Brain just compensates so all vision is cohesive
Photoreceptors
Light energy → chemical energy → neural activity
Light arrives at photoreceptors →series of chemical reactions → change in membrane potential → change in release of neurotransmitters onto nearby neurons
Rods → light levels: more numerous than cones, sensitive to low levels of light, one type of pigment only, used mainly for night vision
Cones →colour: highly responsive to bright light, specialized for colour and visual acuity, in fovea only, 3 types of pigment
Cones
3 types of cone pigments (absorb over a range of frequencies, but their maximal absorptions are:
→ 419 nm (blue, or short wavelength)
→ 531 nm (green, or middle wavelength)
→ 559 nm (red, or long wavelength)
Equal numbers of red and green cones but fewer blue cones→ random distribution throughout fovea
Retinal neutrons → 4 types
Bipolar cell: receives input from photoreceptors into ganglion cells
Horizontal cell: links photoreceptors and bipolar cells
Amacrine cell: links bipolar cells and ganglion cells
Retinal ganglion cell (RGC): gives rise to the optic nerve
Ganglion cells → 2 types
Magnocellular cell (M-cell): magno- ‘large’
- receives input primarily from rods
- sensitive to light and moving stimuli
Parvocellular cell (P-cell): parvo- ‘small’
- receives input primarily from cones
- sensitive to colour→ encode features from stimulus such as colour
Visual pathways → simplified
Left visual field → processed in right side of brain
Right visual field → processed in left side of brain
Optic Chiasm
Junction of the optic nerves from each eye
Axons from the nasal (inside) half of each retina cross over to opposite side of brain
Axons from the temporal (outer) half of each retina remain on the same side of the brain
3 routes to the visual brain
2 main pathways lead to visual cortex in occipital lobe:
→ Geniculostriate pathway for processing the objects image: made by ALL P ganglion axons and some M ganglion axons
→ Tectopulvinar pathway for directing rapid eye movements: made by remaining M ganglion axons
Smaller pathway tracks into the hypothalamus:
→ hypothalamic tract: sleep and circadian rhythm’s
Geniculostriate System
Projections from the retina to the lateral geniculate nucleus to layer IV of the primary visual cortex
Bridges the thalamus (geniculate) and the striate cortex (primary visual processing)
Lateral geniculate nucleus → striate cortex → other visual cortical areas
Striate Cortex
The primary visual cortex (V1) in the occipital lobe
→ shows striped (striations) when stained
Two visual paths emerge from the striate cortex:
→ one route goes to vision-related regions of the parietal lobe
→ one route goes to vision-related regions of the temporal lobe
Tectopulvinar System
Projections from the retina to the midbrain’s superior colliculus→ to the pulvinar (part of the thalamus) → to the parietal and temporal visual areas (processed to create perception: colour, motion, etc)
Retinohypothalamic tract
Synapses in the tiny suprachiasmatic nucleus in the hypothalamus
Roles in regulating circadian rhythms and in the pupillary reflex
Contains photosensitive RGC’s → ex. Blue light suppresses the release of melatonin
Dorsal visual stream
Visual processing pathway from V1 to the parietal lobe → movement relative to objects
Known as the ‘how’ pathway → how action is to be guided towards objects
→ orients perception to create movement
Ventral visual stream
V1 to the temporal lobe: object identification and perceiving related movements
Known as the ‘what’ pathway → identifies what an object is
All geniculostriate → tectopulvinar doesn’t engage in this pathway***
Geniculostriate pathway
Lateral geniculate nucleus (thalamus) → ganglion cell fibers will eventually distribute to thalamus
↳ right LGN: input from right half of each retina
↳ left LGN: input from left half of each retina
Contains 6 layers: projections from the eyes go to different layers
↳ layers 1,4, and 6: input from contralateral retina
↳ layers 2,3, and 5: input from ipsilateral retina
↳ layers 1 and 2: input from mangocellular cells
↳ layers 3 to 6: input from parvocellular cells
Visual info has to stay segregated on pathways: each pathway can specialize and do its job well
P (colour) and M (movement) retinal ganglion cells can send separate pathways to thalamus→ segregation continues in striate cortex
Left and right eyes also send separate pathways to the thalamus→ remain segregated in striate cortex
Maintaining separate visual input
After info is segregated in LGN it maintains segregation in primary visual cortex
Pattern → alternating regions of the left and right eye
Ocular dominance columns » each region is responsible for ocular dominance (segregated and processed selectively)
Tectopulvinar pathway → tectum: roof of midbrain
Formed by the axons of remaining M cells
Magnocellular cells are sensitive to light but not colour → sensitive to movement but not fine detail
Magnocellular cells from the retina project to the superior colliculus (in the midbrain’s tectum)
↳ function: orienting movements to detect and focus the eyes towards stimuli
Medial pulvinar → sends connections to the parietal lobe
Lateral pulvinar → sends connections to temporal lobe
Provides info regarding location: damage to this pathway prevents individual from identifying location of stimuli
Superior colliculus sends connections to the pulvinar region of the thalamus
Occipital Cortex
Composed of at least 6 visual regions (V1, V2, V3, V3A, V4, and V5)
Primary visual cortex (V1; Striate cortex)→ receives input from the lateral geniculate nucleus
Secondary visual cortex (V2 to V5; extrastriate cortex) → visual cortical areas outside striate cortex, each region processing specific features of visual input
Heterogenous laying in V1 area
Blob (V1) → region in visual cortex that contains colour-sensitive neurons
Interblob (V1) → region that separates blobs: participates in perception of form and motion
Within V1 region input is segregated by type: colour, form, motion → then projected into V2 region
Heterogeneous layering in V2
Thick and thin stripes mixed with pale zones
↳ thick stripes receive input from movement-sensitive neurons in region V1
↳ thin stripes receive input from V1’s colour- sensitive neurons
↳ pale zones receive input from V1’s form-sensitive neurons
Visual pathways proceed from V2 to other occipital/temporal regions
Coding location in retina
Each RGC responds to stimulation on a small circular patch of the retina→the cells receptive field
↳ represents outer world as seen by a single cell: think pixel of photo
↳ visual field composed of thousands of receptive fields → very overlapped, influenced by intensity of stimuli
Coding Location → light falling on one place on retina will activate one ganglion cell
Location in LGN and Region V1
Cells in later geniculate nucleus also have receptive fields
Each LGN cell represents a particular place
Projects to V1 (striate cortex) forming a topographic map→ pattern of neural activation
↳Info in retina retains spatial position: info in top of one topographic map is on top of the next
Topographic organization of region V1
Receptive fields of cells in cortex are typically larger than those of retinal ganglion cells
More functional cortical tissue devoted to cells in the fovea than in the periphery →higher acuity because density = receptiveness
Neural tissue vs. Function
Amount of neural tissue responsible for a particular function is proportional to the amount of neural processing the function requires
Sensory areas that have more cortical representation provide a more detailed construct of the external world
Receptive -field hierarchy
Ganglion cells → single LGN cell → single V1 cell
Receptive fields combine to form next cell level
Retinal ganglion cells (RGC’s)
On-center vs. Off-center cells
RGC: respond only to presence or absence of light, not to shape → concentric circle arrangement
On - Center cells: excited when light falls on the Center portion of the receptive field; inhibited when light falls on the periphery of the receptive field
Off-center cells: excited when light falls on the surround of the receptive field; inhibited when light falls on Center
Luminance contrast
Ganglion cells tell the brain about the amount of light hitting a certain spot on the retina relative to the average amount of light falling on surrounding retinal regions
Allows input from RGC’s to tell brain about shape
Processing shape in V1
V1 neurons receive input from multiple RGC’s
↳ have much larger receptive field than RGC’s
↳ respond to stimuli more complex than light on/off
Cells behave like orientation detectors → excited by bars of light oriented in particular directions
Simple cells: receptive field with a rectangular on-off arrangement
Complex cells: maximally excited by bars of light moving in a particular direction through receptive field
Hypercomplex cells: maximally responsive to moving bars but also have a strong inhibitory area at one end of receptive field
V1 receptivity
RGC respond maximally to spots of light (not orientation)
V1 input comes from ganglion cells aligned in a row
Processing shape → temporal cortex
Maximally excited by complex visual stimuli (ex. Face or hands) → very selective
Stimulus equivalence: recognizing that an object is the same across different viewing orientations
Complex features are necessary for activation of most neurons in area TE → include a combination of characteristics such as orientation, size, colour, and texture
Neurons with similar but not identical responsiveness to particular features tend to cluster in columns → functional column
Subtractive colour mixing → seeing colour
Obtains entire range of colours by mixing only 3 colours→ property of the cones in the retina
Light of different wavelengths stimulates the 3 cone receptor types in different ways
Ratio of activity of these 3 receptor types forms our impressions of colours
Trichromatic theory
Explanation of colour vision based on the coding of the 3 primary colours
Colour we see is determined by the relative responses of the different cone types → all equally active = we see white
Can explain different types of colour blindness » lacking one cone receptor type = colour blind
Limitation: cannot explain afterimages and 4 basic colours: red, green, yellow blue
Opponent-process theory
Explanation of colour vision that emphasizes the importance of the opposition of colours
→ red vs. Green
→ blue vs. Yellow
Opponent processing occurs in retinal ganglion cells
↳ on-off and center-surround receptive fields
↳about 60% of retinal ganglion cells
Cells excited by red are inhibited by green and vice versa ( red-green and blue - yellow = colour opponents)
Opponent-colour contrast response
Center of receptive field is excitatory in some cells and inhibitory in other cells
Stimulation to the periphery has the opposite effect → Center is responsive to one wavelength and the surround is responsive to another
Opponent process → V4
Neurons in cortical region V4 do not respond to particular wavelengths but are responsive to various perceived colours → Center is excited by certain colour, and surrounding is inhibited
May be important for colour constancy → perceived colour is constant relative to other colours, regardless of changes in illumination
Neuronal activity in dorsal stream
Posterior parietal cortex → involved in processing visual information for action » the ‘how’ stream
↳ neurons in this area are silent to visual stimulation when a person is under anesthesia
Some cells in this area process the visual appearance of an object to be grasped
↳these cells will fire even when a monkey watches another monkey picking up an object
Monocular blindness
Destruction of the retina or optic nerve of one eye, producing loss of sight in that eye → some info still travels into both hemispheres
Optic nerve is very difficult to damage
Homonymous hemianopia
Blindness of an entire left or right visual field
→ cuts in the optic track, LGN, or V1
Quadrantanopia
Blindness of one quadrant of the visual field
Scotoma
Small blind spot in the visual field caused by a small lesion or migraines of the visual cortex
Visual white noise essentially
Injury to ‘what’ pathway → 3 types
Visual-form agnosia → inability to recognize objects or drawings of objects
Color agnosia (achromatopsia) → inability to recognize colours
Face agnosia (prosopagnosia) → inability to recognize faces
Injury to the ‘how’ pathway
Optic ataxia → deficit in the visual control of reaching and other movements
Damage to the parietal cortex
Retention of the ability to recognize objects normally
Dorsal and ventral visual stream damage
People with damage to the parietal cortex in the dorsal visual stream can see perfectly well→ cannot accurately guide their movements on the basis of visual information
People with damage to the ventral stream cannot perceive objects because object perception is a ventral stream function → these same people can guide their movements to objects on the basis of visual information
