WEEK 9 LECTURE + TUTORIAL Flashcards
(18 cards)
Visible light
- A narrow band of electromagnetic radiation which is often defined by frequency of wavelength, which is measured in nanometres
-To humans, wavelengths between 360-760nm are visible
-Light travels at a constant speed (300,000kmph)
-wavelength determines its hue
-Intensity = Brightness
Light Rays
-Enters through the cornea
-Progresses through the pupil
-Is bent by the lens
-Continues through the vitreous humour (clear gel)
-Projected onto the retina
Retina
Photoreceptors: convert light energy into neural activity
-Bipolar Cells: Transmit information to ganglion cells
-Ganglion cells: Integrate information and send AP’s to brain
-Horizontal and amacrine cells: lateral neurites influence cells close by
Photoreceptors
Rods
-92million
-Found mainly in the retinal periphery
-Sensitive to light
-Poor acuity
Cones
-4.6 million
- Found in the fovea
-Less sensitive to light
-High acuity
Transduction
-A process that converts an external stimulus to an internal stimulus
-Transduction of light energy into changes in the membrane potential
-Light enters the eye and reaches the photoreceptors, and causes a conformational change in a special protein called opsin
-This change activates a G-protein called transducin, which then activates a protein called phosphodiesterase
Visual Pathways
Receptive field: Is delimited (something with boundaries) medium where a physiological stimulus can evoke a sensory neural response in organisms
-The area of the visual space in which a stimulus must be presented to change the activity of a neuron
-Size of the neurons receptive field determines its acuity (smaller is better) and sensitivity (larger is better)
-Fovea: small as few photoreceptors converge on ganglion cell
-Periphery: large as many receptors converge (magnocellular ganglion cells)
Retinofungal projections
Visual information from the eye transmitted to the visual cortex
Visual hemifield: each eye has an optic nerve, nasal half of axons cross to opposite hemisphere, lateral half of axons stay on same hemisphere
Cortical structure involved in visual processing
-Lateral Geniculate Nucleus:
-Located in the thalamus and one on each hemisphere
-Each LGN receives information from both eyes but only about the contralateral visual field
-Organised into 6 specialised layers, innervated by the contralateral and ipsilateral eye
-Layers 1 and two are magnocellular (large cells)
-Relay information about form, movement, depth, light/dark contrast
-Layers 3 to 6 are parvocellular (small cells)
-Relay information about colour (red and green) and fine detail
-Sublayers are koniocellular
Striate cortex
Primary visual cortex
organised into 6 layers
- LGN
-Striate cortex
-extrastriate cortex
Extrastriate cortex
-Necessary for visual perception
-surrounds the striate cortex/ primary visual cortex
-combines information for perception
-arranged hierarchically
-Info moves up the visual association cortices where it is analysed then passed on to higher centres for further analysis
-Dorsal and Ventral pathways
Perceptions of colour
- Trichromatic coding (cones)
- Opponent-process coding (ganglion cells)
Trichromatic Coding
Retina contains three types of cones responsible for colour vision
1. Red (long nm)
2. Green (medium nm)
3. Blue (short nm)
Opponent processing coding
-three colour system is converted into opponent colour system
2 types, colour sensitive ganglion cells respond to colour pairs
yellow-blue
red-green
Negative afterimage
-Due to an adaption in the rate of firing of ganglion cells
-If ganglion cells are excited or inhibitory for a prolonged time, they will fire less or more relative to baseline activity – a rebound effect
- Looking at the green apple inhibits red-ON green-OFF cells excites green-ON red-OFF cells
-both interpreted by the brain as green
-Looking at a white background (reflecting neutral coloured light containing all colours) makes red-ON green-OFF cells fire more and green-ON red-OFF cells fire less
-interpreted by the brain as red
BPPV: Benign paroxysmal positional vertigo
- An inner ear condition
-symptoms include dizziness, vertigo, loss of balance, nausea and vomiting
-treatment can be canalith repositioning procedure, series of movements
Perception of form
-V1 Striate, V2 striate, Ventral stream of ES cortex
-Damage to ES Cortex has led to understanding of regional functioning
-Visual Agnosia: impaired object recognition
-Areas identified in inferior temporal and lateral occipital complexes activated by object categories
-Prosopagnosia: impaired facial recognition
-Fusi-form face area (FFA) of temporal lobe
Perception of space
-Retina, striate and extra striate cortex (ECS)
-Monocular but also Binocular (stereopsis from retinal disparity)
-Disparity sensitive neurons found in the dorsal stream of ESC
-Dorsal stream, primarily involved, ending in posterior parietal lobe
Perception of Orientation and Movement
-Neurons in striate cortex sensitive to orientation
-Extra Striate Cortex responsible for movement perception
-Medial Superior Temporal (MST) responsible for processing of optic flow across retina
-V5 or MT region
-Receives information from superior colliculus
