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
