Chapter 2: Beginnings Of Perception Flashcards
Define Electromagnetic Spectrum
The continuum of energy produced by electric charges and is radiated as waves (short gamma -> long radio waves) in nanometers
Define Wavelength
The distance between the peaks of electromagnetic waves
What range of the electromagnetic spectrum can be seen by the human eye?
400nm (blues) -700nm (reds)
Light enters the pupil and is focused onto the retina by the Cornea and Lens. What percentage of focusing is each responsible for?
Cornea: 80%
Lens: 20%
What is Accommodation?
The process of the lens adjusting its shape depending on the distance of the stimulus. Ciliary muscles tighten and the lens thickens.
Define Near Point
The distance at which your lens can no longer adjust to focus near objects
Define Far Point
Distance at which the spot of light becomes focused on the retina
What is Presbyopia (“Old Eye”)?
A condition where the distance of the near point increases due to hardening of the lens and weakening of the ciliary muscles
What is Myopia? What are the 2 types of myopia and their causes?
Inability to see distant objects clearly because the image is focused in front of the retina
A. Refractive Myopia: cornea or lens bends the light too much
B. Axial Myopia: eyeball is too long
What is Hyperopia? What is its cause?
Inability to see nearby objects clearly because the focus point is behind the retina, caused by an eyeball that is too short
Define Isomerization
The process of a retinal changing shape, sticking out of the opsin
Define Enzyme Cascade
The idea that one visual pigment activates a chain reaction (of millions of molecules)
What are the Visual Receptors? What are their visual pigments composed of?
The visual receptors are cones and rods. Their visual pigments are composed of:
A. Opsin: a long strand of protein
B. Retinal: one, single light sensitive molecule
Describe the distribution of Rods and Cones on the retina. How many of each are there on average?
There are 120 million rods and 6 million cones on the retina on average.
The fovea contains only cones, while the peripheral retina contains more rods than cones.
What is the Blind Spot? Why don’t humans actually notice it?
The blind spot is where the optic nerve leaves the eye
We don’t notice it because:
- The brain fills in informtation
- One eye covers for the other’s blind spot
- It’s located at the edge of vision
Define Macular Degeneration
The degeneration of the fovea and surrounding area (common in elders)
Define Retinitis Pigmentosa
The degeneration of peripheral rods and potentially foveal cones (genetic)
What is Dark Adaptation?
The phenomenon where extended exposure to the dark increases light sensitivity
Outline the 3 experiments used to test dark adaptation. How long does it take to reach rod and cone max sensitivity?
A. Cones: Observer is light adjusted, lights are turned off, once dark adapted observer adjusts a test light until she can just see it (3-4min max)
B. Rods + Cones: observer looks at a fixation point but pays attention to a test light on the side
- Cone sensitivity reaches max before rod sensitivity, but when dark adapted, cone sensitivity follows rod sensitivity
C. Rods: Observer must be a rod monocromat (no cones) (25min max)
What is the Rod-Cone Break?
The place where rods begin to determine the dark adaptation curve
Define Spectral Sensitivity
An observer’s sensitivity to light at each wavelength across the spectrum
(Threshold for light is lowest in the middle of the spectrum)
How is spectral sensitivity measured? Where on the spectrum are rods and cones most sensitive to light?
Cone: a monochromatic light is used on the fovea
- 560nm (419nm, 531nm, 558nm)
Rods: observer is dark adapted, then a monochromatic light is used
- 500nm
- more sensitive to short wavelength light
What is the Purkinje Shift?
Enhanced sensitivity to short (blue) wavelengths during dark adaptation
What is the Absorption Spectrum?
The plot of the amount of light absorbed vs wavelength
What are the 5 neurons that make up the retina and each of their functions?
(neurons = cell body + dendrites + axons)
A. Receptor cells: respond to specific kinds of energy
B. Bipolar cells: vertical
C. Ganglion cells: long axons, vertical
D. Horizontal cells: connect receptors
E. Amacrine cells: connect bipolar cells, connect ganglion cells
How can you record electrical signals in neurons?
Small electrodes can be used to record electrical signals in neurons. A recording electrode is placed inside nerve fiber and a reference electrode is placed outside the nerve fiber. The electrode inside is -70mV compared to the outside (resting potential) and electrical signals are measured as the difference between the inside and outside charges.
Define Action Potential
Change in electrical charge because of passage of an impulse
size is constant but more frequent firing increases stimulus intensity
Define Refractory Period. How long is a typical refractory period?
Period after firing
1ms
What is the typical firing rate of neurons in impulses/sec?
500-800 imp/sec
What is a Synapse? How do electrical signals travel across synapses?
A small space between neurons
Neurotransmitters are released from the vesicle of one neuron and received by receptor sites of another neuron
What are the 2 types of Neurotransmitters?
Excitatory Transmitters: cause depolarization, so neurons become more positive and likelihood of AP increases
Inhibitory Transmitters: cause hyperpolarization, so neurons become more negative and likelihood of AP decreases
(Excitatory and inhibitory transmitters can be present simultaneously, causing random firing rather consistent firing/no firing)
After a signal moves through all structures, where does it move and why?
A signal goes through all structures, then directly back…possibly to reduce light scatter that would affect rods and cones
What is the make-up of rod and cone:ganglion cell connections?
126 million rods and cones converge to 1 million ganglion cells
- 120 rods to 1 ganglion cell
- 6 cones to 1 ganglion cell
- In fovea, 1 cone to 1 ganglion cell
How does the make-up of visual receptor-ganglion cell connections influence vision?
Because more rods are connected to a single ganglion, there is a higher chance of response, higher sensitivity to light under dim conditions, and vision is blurry
Because there is a 1:1 convergence in fovea, there is high vision acuity
