Week 6 - topic 1 Flashcards
Sensation and perception
Sensation involves cells of the nervous system that are specialised to detect stimuli from the environment.
Perception is the conscious experience and interpretation of information from the senses
The Ames illusion - sensation and perception
- In the Ames illusion sensation is that the information coming from the environment, such as the location and size of objects, light, colour, enters the eye. The information about the room is sent to the brain to be processed.
- Perception is the conscious experience and interpretation of information from the senses. In the Ames room, the brain has to make sense of the environment and as there are no or limited visual cues for depth, the room looks square not trapezoidal. This manipulates the brain to assume that the person, when they move from left to right, is actually getting larger rather than coming close
Visible light in humans
Visible light is a narrow band of the spectrum of
electromagnetic radiation that receptor cells in our eyes detect
- between roughly 350 and 750, in between ultraviolet and infrared
3 dimensions of perceiving light and colour
- Hue = dominant wavelength
- Saturation = purity
- Brightness = intensity
Hue
Colour = dominant wavelength at a given moment
- around the outside of the colour wheel
Saturation
Is the relative purity of the light being perceived.
- centre to border of the colour wheel, how close to white
Brightness
Brightness is the intensity of the electromagnetic energy.
- light to dark
Sensory receptors
specialized neurons that detect a particular category of physical events (e.g. light)
Sensory transduction
the process by which sensory stimuli are transformed into changes in the cell’s membrane potential
Receptor potentials
electrical changes produced by a receptor cell in response to a physical stimulus
Eye diagram
*look up image
Retina
The retina is located at the back of the eye
• An image must be focused on the retina for us to see because receptor cells for vision are located there.
• Visual receptor cells are called photoreceptors. There are two types (rods and cones)
Three main cellular layers:
• photoreceptive layer
• bipolar cell layer
• ganglion cell layer
Focused image on retina
When an image is focused on the retina, this causes changes in the electrical activity of millions of photoreceptors.
• This results in messages being sent through the optic nerve to the rest of the brain for further processing.
Extraocular muscles
The eyes are held in place and moved by six extraocular muscles
• The extraocular muscles are attached to the sclera (tough, white outer coat of the eye)
• The sclera is opaque and does not let light in
Cornea, iris and pupil
- The cornea is the transparent outer layer at the front of the eye that lets light in to reach the retina
- The amount of light that enters the eye is regulated by the size of the pupil, which is an opening in the iris, the pigmented ring of muscles behind the cornea
Lens
The lens is right behind the iris and consists of a series of transparent, onion-like layers.
• The shape of the lens can be altered by contraction of the muscles called the ciliary muscles.
• Changes in lens shape permit accommodation
Accommodation
Accommodation refers to the ability of the eye to focus images of near or distant objects on the retina.
- far or near
Cones
- most prevalent in the central retina; found in the fovea
- sensitive to moderate to high levels of light
- provide information about hue
- provide excellent acuity
Rods
- most prevalent in the peripheral retina; not found in the fovea
- sensitive to low levels of light
- provide only monochromatic information
- provide poor acuity
Blind spot
• The blind spot occurs is because of the optic disc.
• The optic disk is where the axons conveying visual
information gather together and leave the eye through
the optic nerve = no photoreceptors there to detect light
Photoreceptor, bipolar and ganglion cells
• Photoreceptors form synapses with bipolar cells
• Bipolar cells form synapses with ganglion cells whose axons travel through the optic nerves (second cranial nerves)
*look up image
Transduction
- Transduction is the process by which energy from the environment is converted to a change in membrane potential in a neuron
- First step in the chain of events that eventually leads to visual perception
Transduction - photopigments
- Photopigments are a special chemical responsible for the transduction of visual information embedded in photoreceptors,
- Photopigment molecules contain a protein (opsin) and a lipid (retinal)
- They are embedded into thin plates of membrane that make up the outer segment of photoreceptors (rods and cones)
Splitting photopigments
- When a photopigment is exposed to light, it breaks into two parts (protein and lipid).
- Splitting triggers a chain of intracellular events to occur that hyperpolarize the photoreceptor membrane.
Receptive field
The Receptive Field of a neuron in the visual system is the part of the visual field that an individual cell “sees”—that is, the place in which a visual stimulus must be located to produce a response in the cell
Receptive field location due to photoreceptor location
- Neurons receiving information from photoreceptors in the fovea will have receptive fields in central vision (fixation/central point where the eye is looking)
- Neurons receiving information from photoreceptors in the periphery of the retina will have a receptive field located off to one side - in the periphery of the retina, many individual receptors converge on a single ganglion cell whereas in the fovea these are individual ganglion cells
Photoreceptor and ganglion cell pairings, periphery V fovea
Because a single cell processes information from many receptors in the periphery, vision is less precise
= lower visual acuity
Because a single cell processes information from a single receptor in the fovea, vision is very precise
= higher visual acuity
3 types of eye movements
Because peripheral vision is blurry, we usually want
light to project onto the fovea for detailed processing
of relevant information.
There are three types of eye movements that help
with this:
1. Vergence movements
2. Saccadic movements
3. Pursuit movements
The optic nerves
At the back of each eye, the axons of ganglion cells bundle together to form the optic nerve
• The optic nerve conveys information from the retina to a section of the thalamus called the dorsal lateral geniculate nucleus (LGN).
Optic chiasm
- Optic nerves join together at base of brain to form the X-shaped optic chiasm
- The axons from ganglion cells serving the inner halves of the retina cross through the chiasm and travel to the LGN on the opposite side of the brain (contralateral)
- The axons from ganglion cells serving the outer halves of the retina remain on the same side of the brain (ipsilateral)
Overview of visual pathway
- Vision begins in the photoreceptors of retina
- Next, information from the photoreceptors is sent to rest of brain via optic nerves to lateral geniculate nucleus (LGN).
- Neurons in LGN send axons to primary visual cortex (V1/striate cortex).
- From V1, information sent to visual association cortex (V2/extrastriate cortex), and additional cortical areas (V3, V4, V5).
