Nervous System and Perception (L6) Flashcards
Visual Design: Stigma Eye
A small, opaque area (stigma) in front of light-sensitive pigments. When pointed toward light the stigma shades the receptor, thus providing animal with directional information.
To remain oriented, protozoan swims in a helix so that its stigma-receptor axis points systematically in a circle about its direction of travel. It compares light coming in from two or more directions. If one part of circle is brighter, it turns slightly in that direction.
When perfectly oriented, all directions should be equally shaded and equally bright.
Typical of many protozoans (e.g., Euglena)
Visual Design: Pinhole Eye
Eye cup with a very tiny opening. Small amount of light scattered in precisely the right direction enters hole. Light passes through opaque barrier and is projected on a unique point on the retina behind the hole.
Receptors in the retina receive a precise but inverted image.
Disadvantages of a pinhole eye: Only a tiny amount of light enters eye. Diffraction or bending of light at edges reduces quality of an already dim image.
Found in chambered nautilus.
Visual Design: Lens Eye
Lens is denser than air and the lens bends and focuses the light arriving over a much larger area than a pinhole. More light falls on the receptors; thus, diffraction is only a minor problem.
Rods: light-sensitive, good at night. High densities in nocturnal animals.
Cones: color-sensitivie, good at daytime. High densities in diurnal animals.
Disadvantages of a lens eye: Unique focal point, compensated for by muscles which alter shape of lens and change focal point. Complicated neural wiring required. Anatomical defects more likely, such as short or far sighted and astigmatism.
Used by most cephalopods (e.g. squid, octopus) and vertebrates.
Visual Design: Compound Eye
Typical of insects. Vast arrays of eye cups (each with a lens), each pointing out in a unique direction.
Ommatidium: individual eye cup. Tube with a lens at one end and layers of receptors at the other end.
Increases the resolution of the eye. Diffraction limits the number of ommatidia. As opening gets smaller, diffraction increases, bending light from other directions into the ommatidial tube. Advantageous in insects because of its low weight and volume. E.g. honeybee.
Flicker-fusion rate: Ability to distinguish two separate images closely spaced in time. About ten times higher than that of more complex eyes. Enables bees to see fluorescent lights flashing on and off 120 times a second; their own wing beats appear as distinct flappings.
Ability to see polarized light (few vertebrates can see polarized light).
Visual Processing: Color Discimination
Three classes of color-sensitive cones (pigments which are color receptors) in bees and humans.
Bees have color receptors tuned to UV, blue, and yellow-green. Humans have receptors tuned to blue, yellow-green, and yellow-orange.
The CNS breaks the resulting visual continuum into arbitrary categories.
Visual Processing: Lateral Inhibition
An interaction between receptors in the retina that causes inhibition in certain receptors in order to emphasize contrasts. E.g. emphasizes contrast between black and white borders.
Certain cells inhibit or prevent their neighbors from firing quite as often as they would otherwise, causing cells on edges of borders to exaggerate the difference to the brain.
Potential Effects of Lateral Inhibition
- Emphasize lines (e.g. edges) in specific orientations.
- Emphasize shapes (e.g. spots) in specific orientations.
- Emphasize movement in particular directions and particular speeds.
Visual Processing: Feature Detectors
Nerve cells in the brain that are wired to sort out species-specific stimuli and exaggerate their differences to the brain through the process of lateral inhibition.
Assist an animal in detecting stimuli important for survival.
Operate at an early stage of neural processing, long before information actually reaches the higher CNS for further consideration.
Likely account for the inexplicable irrationality of releaser detection and innate releasing mechanisms (IRMs).
Auditory Design: What is sound?
Sound is simply the vibration of molecules, whether in air, water, soil or some other medium.
Similarity between LIGHT and SOUND
Both light and sound are wave phenomena with a particle nature.
Light has color
Sound has pitch
Both are subject to diffraction, reflection, refraction and wavelength-specific filtering.
Is the sensory environment more species-specific for vision or hearing?
Animal vision runs from UV to infrared, a factor of 2 in wavelength.
Animal hearing runs from 0.1Hz to 100,000Hz, a factor of a million in wavelength.
Particle-detector ear
Usually consists of thin, low-mass projections attached to solid, high-mass objects.
Molecules rushing back and forth strike the detector and in turn push it back and forth.
Examples of Particle-detector ears
- Antennal hairs of invertebrates. Male mosquito antennae are tuned precisely to the flight sound of females; they are deaf to all other sounds.
- Lateral-line organs of fishes. Small hairs designed to bend with moving water molecules.
- Body hairs of invertebrates. Some moths have hairs tuned to the wingbeat frequencies of the species of wasps that hunt them.
- Subgenual organ of invertebrates. Thin membrane stretched across the nearly hollow legs of arthropods. Detects sound through the ground + deaf to airborne sounds. Extremely sensitive to sound. Roaches are some 100,000 times more sensitive to ground vibrations than humans.
What is a limitation of a particle-detector ear and how can it be overcome?
Limited resonance frequency; thus, deaf to most sounds. This problem may be overcome by using several detector of different receptors.
What are two more limitations of particle-detector ears?
- Cannot distinguish between a relatively quiet sound at peak resonance and a loud sound close to the frequency of peak resonance.
- Deaf to sounds coming from the direction in which the detector is pointed; most sensitive to sounds striking it from the side.
How can limited directional detection be overcome?
Turning to try other orientations for comparison.
Using a second detector with another orientation and performing trigonometry to determine direction.
Pressure Gradient Ear
High mass cavity is not sealed. Membrane responds to difference in pressure between two openings of the cavity.
When tube is along the sound axis, it responds to the pressure difference between the two ends.
The magnitude of the difference depends both on the intensity and wavelength of sound. It is highly directional.
What happens if the tube-like cavity in a pressure-gradient ear lies across the direction of sound propagation?
No pressure difference; thus, the ear is deaf.
What happens if the tube in a pressure-gradient ear is exactly as long as the wavelength?
No pressure difference; thus, the ear is deaf!
Pressure-Gradient Ear #2
If tube is half the wavelength, the sound will be very great.
Frequency and intensity information from a single ear are greatly muddled.
