Week 4 L7 211 Flashcards
Sight has been studied extensively,
Perception is still rather ambiguous though, interacts with experience.
But they have found how energy is converted
Ok
Video of Visual agnostic
See fine, cannot recognize objects.
Damage to visual cortex.
But can identify throug touch, using memories and emotions
The Stimulus
§ Our eyes detect the presence of light.
§ The eyes respond to waves of electromagnetic energy between
380 ad 760 nm in length.
§ Wavelength is determined by three dimensions:
- Hue is an attribute of perceived colour (e.g. red)
- Brightness provides intensity to the colour
- Saturation is the relative purity of the perceived light
Movement of the eyes
Eyes are suspended in the orbit: bony sockets in the front of the skull
Six extraocular muscles move and hold the eye in place
§ Vergence movements
keep both eyes fixed on the same target.
§ Rapid, jerky saccadic movements shift your gaze from one point to another.
§ Pursuit movements allow us to maintain a moving object.
Movement of the Eye
Anatomy of the Eye (
I) The conjunctiva is a mucous
The sclera is opaque and does not permit entry of light
membranes that line the eyelid
The cornea is the outer, front layer of the eye. It is transparent and admits light
The iris is a pigmented ring of muscles
The lens consists of several transparent layers. The ciliary muscles can change the shape of the pupil to allow the eye to focus, a process known as accomodation
The pupil regulates the
amount of light entering the eye. It is an opening in the iris
Anatomy of the Eye (II)
The interior lining of the eye is the retina. Photoreceptors called rods and cones are located here. Rods are sensitive to low light intensity (i.e. vision in the dark). Cones are essential for colour vision therefore useful in bright light and daytime vision.
Light passed through the lens and crosses the vitreous humor, a clear, gelatinous fluid.
The central region of the retina is the fovea. It contains the highest number of cones. Give us sharp vision
Site of blind spot is the point at which the optic nerve exits through the back of the eye. It has no receptors
The Route within the Retina
Optic Nerve
§ Light passes through transparent cells and stimulate the photoreceptors located at the back of eye.
§ Photoreceptors then send messages to bipolar and ganglion cells located closer to the centre of the
§ The ganglion cells’ axons loop around each other and travel back to the brain
Retina is kind of backwards us light does not travel directly to brain,
Example of evolution does not explain everything.
Why light go backward and move forward.
Horizontal and amacrine cells are interneurons without axons. They combine messages from adjacent neurons.
§ Amacrine cells connect adjacent ganglion cells with bipolar cells.
§ The horizontal cells interconnect adjacent photoreceptors with bipolar cells.
Retina may have 55 different types of neurons
Fovea and Periphery of Retina
§ The fovea is a tiny area specialised for visual acuity (sharpness of an image).
§ Cones are prevalent in the fovea
§ Each cone in the fovea connects to a single bipolar cell which in turn connect to a single ganglion cell
§ Thus, receptors in the fovea can register the exact location of the input.
§ In the periphery, several receptors converge onto the bipolar and ganglion cells.
§ Precise location and shape of input is heavily impeded.
§ However, the periphery enables the perceptions of faint lights
v Foveal vision is sensitive to detail v Peripheral vision is sensitive to dim light so can see more stars.
Eyes like a hawk
Human eyes only 1/5 of head yet most of head for a bird.
§ Most birds have two fovea’s per eye, one pointing ahead and one pointing to the side.
§ The extra fovea enables perception of detail in the periphery.
Gerardo number of receptors on top part of retina for looking down. Must turn head round to see up.
§ Most birds have two fovea’s per eye, one pointing ahead and one pointing to the side.
§ The extra fovea enables perception of detail in the periphery.
When looking up, the bird has to turn its head almost upside down to see above itself
Hawks and other predatory birds have a greater density of visual receptors on the top half of their retina (looking down) than on the bottom half (looking up).
This enables the bird to see below them in great detail during flight.
A behavioural consequence of how receptors are arranged on the retina
What are photoreceptors and the two types
§ Photoreceptors are light sensitive neurons located in the retina. Their function is to transduce light (or photic energy) into electrical potentials.
Rods
Prevalent in peripheral retina Sensitive to low light intensity Are monochromatic
Provide poor acuity
Cones
Prevalent in fovea
Sensitive to moderate-high light intensity Are trichromatic
Used mostly in daytime
Provide excellent acuity
The photoreceptors are made up of an inner and outer segment.
§ The inner segment contains the cell body and axon-like process.
§ The outer segment contains several hundred lamellae (thin membranes).
§ Photopigments are molecules embedded in the membranes of the lamellae. Photopigments release energy when struck by light.
§ Photopigments are made up of opsin (a protein) and retinal (a lipid), e. g. rhodopsin.
§ When rhodopsin is exposed to light, is breaks down into opsin and retinal.
§ The rhodopsin changes colour from pink to pale yellow, i.e. the light bleaches the photopigment.
§ The splitting of the photopigment causes a change in the membrane potential of the photoreceptor which will cause the release of the neurotransmitter, glutamate.
Translation of light into neural signals (I)
Photoreceptors membrane is different, ion channels are normally open unlike other neurons,
In the dark…
§ Ion channels on the photoreceptor membrane are normally ‘open.’
§ The ion channels admit cations
§ The ion channels are held open by molecules of
cGMP (cyclic guanosine monophosphate), a
second messenger.
§ The entry of cations depolarises the membrane
which results in a continuous release of glutamate.
In the light…
§ The rhodopsin molecule will split.
§ A chemical reaction involving a G protein and a
phosphodiesterase enzyme will destroy the
cGMP. This will close the ion channel.
§ Cations can no longer enter the cell and the release of glutamate decreases.
The hyperpolarising effect of light on the photoreceptor membrane reduces the release of glutamate.
§ Reduced glutamate depolarises the bipolar cell.
§ Depolarisation of bipolar cell leads to increased glutamate release causing it to increase rate of firing in the ganglion cell, because only the ganglion cell fires action potentials.
Color vision and Wavelengths of Light
§ A red apple does not emit red light. It absorbs all the visible frequencies shining on it except for the frequencies perceived as red that are reflected.
§ Cone photoreceptors are sensitive to different portions of the visible spectrum
§ No single neuron can simultaneously signal brightness and colour so our perceptions must rely on a combination of responses by different neurons.
§ Two major theories of colour vision:
- The Trichromatic Theory of Colour Vision
- The Opponent-Process Theory
The Trichromatic Theory of Colour Vision (Young-Helmholtz Theory)
Colour perception is a function of the relative rates of response by three types of cone photoreceptors, each sensitive to a different set of wavelengths.
- People can match any colour by mixing appropriate amounts of three wavelengths
- Each cone cell is sensitive to short (blue), medium (green) or long (red) wavelengths
- Wavelengths are discriminated by the ratio of activity across the three types of cones
But it is ambiguous, huge overlap of colors though.
This theory is used in TV, but it is incomplete to explain humans.
Transfer AFTERIMAGE
Suggests trichromatic theory not quite right,
Fails to explain blends of colors,
Or opposite o a color.
Trichromatic theory cannot explain:
1. Why we have negative afterimages
2. Why we see blends of colours (e.g. bluish-green but not bluish-yellow)
Opponent-Process of Colour Vision (Hering)
We perceive colour in terms of paired opposites
§ There are three pairs:
§ Red vs. Green; Yellow vs. Blue; Black vs.
White.
§ Some neurons are excited by one set of
wavelengths and inhibited by another
§ The bipolar cell is excited by the blue light but inhibited by red, green and yellow.
§ An increase in the bipolar cells activity leads to a “blue experience” but a decrease produces a “yellow experience.”
§ Extended blue light stimulation fatigues the bipolar cell.
§ If blue light is substituted with white light, bipolar cell becomes more inhibited and produces a “yellow experience.”
Oshawa test
Note on color blindness, not necessarily rattled to genetic defect of malfunctioning cones.
Colour Vision Deficiency
Normal Vision Protanopia Deuteranopia Tritanopia
§ For genetic reasons, some people lack one or two cone types.
§ The most common deficiency is distinguishing red from green (protanopia) because they lack red cones
