week 6 Vision and Hearing Flashcards
What is a receptor cell?
A cell responding with an action potential due to a sensory input.
What is meant by the notion that a particular neuron codes for (or represents) a particular type of physical information? How does this notion differ from Descarte’s notion that the brain needs to represent objects literally within its structure?
This is known as the Law of Specific Nerve Energies. Action potentials from auditory n. are interpreted as sound, and from optic n. as vision. Demonstrated by firm pressure applied to eyeballs will generate sight of “colour flashes”. There is only “one interpretation “ of what action potentials signify. This differs to Descartes who believed the images would be an almost visual representation of themselves, and not coded.
Sketch and label a schematic representation of the eye and retina.
3
Identify on this representation the following structures (and include a Understand of their functions):
a) the iris and pupil,
b) the cornea and lens (How are they similar? How are they different?),
c) the retina,
d) the fovea.
4
Draw a (rough) schematic representation of the retina and label all the various retinal neurons (rods, cones, bipolar cells, horizontal cells, amacrine cells, ganglion cells).
5
Compare and contrast the differences (in terms of wavelength selectivity and light sensitivity) between the two types of photoreceptors (rods vs cones) in the eye. What is it about cones that makes them selective to particular wavelengths of light?
6 Cones are more responsive to particular wavelengths as they contain specific photopigments substances which generate action potentials when triggered by light of specific wavelength. Specific proteins called opsins are bound to the photopigments, and determine which wavelength of light the photopigment will respond most to.
The brain is able to judge colour by comparing the activities of short, medium, long -wavelength tuned cones, as well as rods.
Rods respond to low level light but we have fewer of them and more of them are on the retina’s periphery. Cones respond better to higher levels of light and are of 3 types, correlating to wavelength of light. Long wavelength attuned cones respond maximally to green/yellow light, medium to green, and short to deep indigo.
What is the significance of ganglion cells with regards to the information that is sent to the brain?
If a bipolar cell receives signals from many cells, the cns gets more of a total assessment but less accurate, whereas if the bipolar cell synapses with just one receptor cell, the information assessed by the cns is very accurate and specific.
There are many types of ganglion cells. The axons of many ganglia run alongside each other as the Optic n, and into the CNS.
Light must actually travel through several retinal layers (comprising ganglion, bipolar and other cells, before it reaches the receptor cells).
Midget ganglion cells are located in the Fovea, where accurate detailed sight is best, and each midget ganglion cell responds to just a single cone. These midget ganglion cells account for 70% of the visual input to the brain.
Explain the nature of the blind spot in anatomical terms.
Many ganglia axons come together and exit the back of the eye. These means at this point at the back of the eye, there are no receptor cells, and there is a “blind spot”. Because we have 2 eyes though, usually what one “blind spot” cannot detect, the other eye can.
Relate the approximate wavelength of light (in nm) to perceived colour (for blue, green, and red).
Blue 350-500 nm, green 450-550 nm, red 600-700nm
Be able to provide a brief description of the Trichromatic theory of colour vision.
Also known as Young-Helmholtz Theory. Despite being able to discern many colours, we only have three colour receptors. By comparing the relative rates of responses of all these three receptor types, allows us to interpret many colours. The colour receptors have maximal response rates to ceratin wavelengths of colour.
Be able to provide a brief description of the Opponent-Process theory of colour vision.
The brain perceives colours incontinuums of red/green, yellow/blue and white/black. After staring at a bright shade for a period, when then move gaze to white, see an after image of the opposite colour to what were first seeing. This is because those cells being firing, have become exhausted and so then stop firing (= same effect as if seeing opposite colour).eg. A bipolar cell might be excited by short wave cone (excited by blue light) and inhibited by yellow light, thus when fatigued, is same as if had seen yellow light.
Understand the retinex theory and use this theory to explain colour constancy (by the way, why is colour constancy so important?).
Colour Constancy= the ability to recognise colours despite different light levels. The brain (cortex) compares all inputs from different parts of the retina to each other, and interprets/recognises the same colour under different light levels. These inferences are built into all of how assessments of object/situation etc .
To where in the thalamus do most axons of retinal ganglion cells project?
Lateral Geniculate Nucleus
Very briefly, compare and contrast the properties of parvocellular and magnocellular cells in the lateral geniculate nucleus (LGN).
These are all types of ganglion cells;
a) Parvocellular; small cell bodies, small receptive fields, responds to colour and detailed shape, receive input from midget cells in or near fovea.
b)Magnocellular;large cell body, large receptive field, located throughout retina (the receptive field is located throughout retina, the cell is in the thalamus), responds to movement and broad outlines, and not colour.
c). Koniocellular; small cell body, usually small receptive firel but some exceptions, located throughout the retina, some respond to colour, various responses to other specifics.
To where in the brain do most axons of LGN cells project?
Other parts of thalamus and visual cortex.
