Sensation and Perception Flashcards
The visible part of the electromagnetic Spectrum
Electromagnetic radiation has both an electric and magnetic component which travels through space in the form of a wave. The frequency of these wave going up and down determine what type of electromagnetic radiation it is. Some examples of electromagnetic radiation includes x-rays, radio waves and, most important for our purposes, light.
Electromagnetic Radiation
Wave—> 380 to 760 nm
400 nm —> violet
500 nm —> green
600 nm —> yellow
700 nm —> red
The spectrum of electromagnetic radiation that we are sensitive too is what we call “light”. Specifically, we are sensitive to what is between 380 and 760 nm in length. Differences in color are a result of different wavelengths.
Not all species perceive the same frequencies as “light”. For example, bees are sensitive to ultraviolet light and rattlesnake can perceive infrared rays.
Transduction
Convert environmental energy into neural activity with sensory receptors. Radiant energy —> sensory code.
It converts environmental energy into a neural activity and this important task is accomplished by specialised cells that we call sensory receptors.
Rods (rhodopsin = retinal + scotopsin) and Cones (iodopsin = retinal + photopsin) contain pigments.
Transduction of light into neural energy occurs through absorption of photons. 7 photons of light can produce a visual response.
Pigment isomerizes or changes shape and splits into components (bleached). Pigment regeneration most effective in the dark.
Anatomical Coding
Specific neural circuits signify particular sensory experiences
(doctrine of specific nerve energies).
Specific sensory receptors are specialised to pick up specific types of light information, such as colour.
Temporal Coding
Rate of neural firing signifies stimulus intensity.
Idea that colour or light intensity can be represented by having the sensory receptor become activated frequently for a high intensity light, but less frequently for a low intensity light.
Retina
Screen of photoreceptors extending over most of the interior
Choroid of Pigment Epithelium
Nourish photoreceptors and absorb excess light
Fovea
Region around axis or centre, point of focus for fixated objects.
Blind Spot
Point of exit of ganglion cell axons
Cornea
Transparent bulge in sclera, fixed lens
Pupil
Black hole where light enters the eye, capable of 16 fold change in area.
Iris
Coloured membrane, controls amount of light entering eye
Lens
Focus image on retina through process of accommodation or bending to focus near objects.
The Eye
First, light waves enter the front building, transparent part of the eye, called the cornea. The cornea is important because it not only helps protect the eye, it also is a fixed lens that directs the light towards the back of the eyeball. Direclty behind the cornea is the pupil, which is where the light enters. It’s black because the light enters, but does not leave. Behind the pupil is an adjustable lens. This lens can become flatter or rounder depending on how close or far away the visual object of interest is. Both the fixed cornea and the flexible lens direct the light waves toward the back of the eye ball on a special paper-thin strip of tissue called the retina. It is on this multi-layered tissue that sensory receptors, which we will call photoreceptors, help transduce light waves into neural signals. Surrounding these photoreceptors are support cells that help to nourish the photoreceptors and also absorb light excess. These support cells are called the choroid or pigment epithelium. An especially crucial part of the retina is an area called the fovea. The focus of our visual system is what is presented directly on the fovea.
Structure of the Retina
The retina contains the photoreceptors that transduce light waves into neural signals. There are two main types: Rods and Cones. Cones are specialised photoreceptors that allow for the perception of colour. Rods are photoreceptors that allow us to see in dimly lit situations, but perceive only black, white and shades of grey. The cones work best during the day when there is plenty of light. They don’t work as well when the sun goes down and things get darker.
Some animals only have Rods, like Owls.
Information that is transduced by our photoreceptors is passed to our bipolar cells. Then the information is sent to the ganglion cells, from which the information is taken into the brain. Horizontal cells allow for communication among photoreceptors and amacrine cells allow for communication among ganglion cells.
Rods and Cones #
Rods —> 125 million
Cones —> 6.4 million
Receptors for vision
Bipolar and ganglion
Neurons that transmit information from the rods and cones to the brain
Horizontal
Connect receptors to receptors
Amacrine
Conenct spatially adjacent bipolar and ganglion neurons
Rods
Long, cylindrical
Poor acuity, excellent low-light vision
Cones
Shorter, thicker, tapered
high acuity, poor low-light vision
Duplicity theory of vision
Rods —> scotopic vision (night)
Cones —> photopic vision (day)
Cones responsible for colour vision. Rods more sensitive to light
Formulated over a century ago by Schultz. Because human can see both during the day and night we must have at least two photoreceptors and he was right.
Some individuals have defective rods: day vision fine, sight lost at twilight
Rod monochromats have poor visual acuity and no colour vision but function normally under low light conditions
Cones and Rods with Retina
Photoreceptors not equally distributed across the retina.
Fovea contains only cones. At 20C in periphery maximum rod density .
Only 1 million ganglion cells —> convergence
Axis of eye centred on the fovea.
Cones in fovea generally have 1:1 relationship with a ganglion cell.
Rods and cones outside the fovea have a many to one relationship with ganglion cells.
Dark Adaptation
Gain in sensitivity to light as a function of time spent in the dark.
Curve produces a discontinuous function.
Adjustment in light sensitivity to a dimly illuminated environment. Measure time course of recovery: flashing test light in periphery: decrease intensity until light can just barely be seen: measurements every few seconds for 40 minutes.
Typical observed function: sensitivity increases by 5 log units or 100,000 times more sensitive: two segments or rates of adaptation: initial rapid change stabilises after 5 minutes: more gradual improvement in sensitivity around 10 mins.
Colour
Perception of a colour depends on a mixture of 3 dimensions.
hue —> wavelength of light (green vs blue)
saturation —> purity of light (red vs pink)
brightness —> intensity of light (white vs black)
Subtractive colour mixing
Certain wavelengths of light selectively absorbed by the pigments of paint or filters
Perceived colour reflects combination of wavelengths that were not absorbed
Additive Colour mixing
Different wavelengths of light add together to form the resultant color
red + blue + green = white
Colour matching
Any 3 wavelengths of light (primaries) may be mixed in different proportions to produce all possible colours.
Provided combinations of first 2 wavelengths cannot produce the 3rd wavelength. Red, blue, green often chosen as primaries.
Theory proses that the eye contains 3 colour receptors each differentially sensitive to the various wavelengths of light. For any colour, 3 receptors will produce a unique ratio of activity.
Opponent-process theory
Several colour phenomena are not readily explained by trichromatic theory. Participants describe various colours using 4 rather than 3 primaries (blue, green, yellow, red).
Complementary colours
When arranged in the colour circle, opposite colours when mixed in equal proportions yield neutral grey.
Afterimages
blue and yellow, red and green.
Rod monochromats
Non-functional cones, poor visual acuity, shades of grey.
Protanopia
Defective long, inability to distinguish red and purple
Deuteranopia
Defective medium, insensitive to green
Tritanopia
Defective short, insensitive to blue and yellow
