-
SENSORY RECEPTOR CELLS TRANSDUCE INFORMATION FROM THE ENVIRONMENT
-
Types of Information Transduced
- -Chemical (taste, smell)
- -Mechanical (touch, hearing)
- -Electromagnetic (vision)
- Information about the physical stimulus is ...
- a) encoded in the form of neural activity (frequency and action potentials), and...
- b) relayed to the brain by specific circuits
-
Psychophysics
- The field that relates the physical characteristics of stimuli to their psychological characteristics.
- JND - just noticeable difference. The minimal amount of physical change in a stimulus can be detected
- Stevens power law - the relation between physical intensity and psychological intensity is linear on a log-log plot, but different modalities have different slopes
-
Signal Detection Theory
- -was designed to handle problems with the measurement of thresholds
- -subjects report the presence or absence of an event that may or may not have occurred
- -subjects can be correct or incorrect in different ways
-
Physical Stimulus for Sound
- Pressure changes (alternating waves of compressed and rarified medium; could be in air or water)
- Relation between Physical and Psychological Characteristics of sound:
- - amplitude: measured in decibels (dB), is related to the loudness of a sound
- - frequency: measured in cycles per second (Hz), is related to the pitch of the sound
-
Frequency of Sound Waves
- Human frequency range: 20-20000Hz
- Low frequency → low pitch
- High frequency → high pitch
- Humans typically have hearing loss that is strongest in frequency as they age
- Ultrasound: higher frequencies above the human frequency range
- Infrasound: lower frequencies below the human frequency range
- Some animals can perceive ultrasound or infrasound
-
The Ear
- Eardrum - vibrates in synchrony with the sound waves
- Middle ear ossicles - small bones, the hammer, anvil and stirrup (in latin, the malleus, incus and stapes). They convey vibrations from the eardrum
- Inner ear - the cochlea (means "snail"); sounds are converted into neural signals in the cochlea, and these signals are conveyed to the brain by the auditory nerve, which is a cranial nerve
-
The Cochlea
- The organ for detection of sounds (the organ of Corti) is contained within the cochlea. The cochlea contains several membranes and is filled with fluid
- The basilar membrane vibrates in response to the physical stimulus of sound; it contains hair cells, which are the sensory receptors for hearing
- Hair cells transduce the physical stimulus into neural signals.
- Hairs cells are mechanoreceptors. They become excited by the mechanical disturbance of their hairs that occurs when the basilar membrane vibrates
-
Theories of Pitch Perception
- Frequency principle (or theory): the frequency of sound is directly related to the frequency of action potentials that are generated
- Problem: neurons cannot fire fast enough for high frequency sounds...this principle only holds up to 200Hz
- Place principle (or theory): different places on the basilar membrane respond to different frequencies of sound. The place principle operates at frequencies higher than 200Hz
-
Vision
- Physical Stimulus: the visible portion of the electromagnetic spectrum
- - although it is only a small portion, the visible spectrum represents about 70% of the electromagnetic energy on the surface of earth
- - wavelength of light is the part of the physical stimulus that is associated with the perception of color
-
The Eye
- Lens - focuses the light onto the retina
- Retina - contains rods and cones, which are the receptor cells for vision. They contain pigments, and transduce the visible light into neural signals. They also contain other cell types.
- Blind spot or scotoma - a blind spot is produces because one part of the retina does not contain receptor cells; axons that come to form the optic nerve exit through this spot
- Optic nerve - contains axons that convey visual information to brain
- Fovea (also called fovea centralis)- center of retina, in well functioning eye it should also be the projection zone for the center of the visual field
- Central scotoma - blind spot produced in the center(fovea) when the background light is low
-
Anatomy of the Retina
- 3 layers of cells
- Inner layer: contains receptor cells(rods and cones); they transduce the light stimulus into neural signals; rods and cones have different pigments
- - rods: very sensitive to the low levels of light, vary activity along the light/dark continuum; color insensitive
- - cones: less sensitive to the low levels of light, color sensitive
Middle layer: bipolar cells convey information from rods and cones to ganglion cells in the outer layer; there also are horizontal and amacrine cells
Outer layer: ganglion cells receive information from bipolar cells; cell bodies are in the retina, but axons bundle together to form the optic nerve for each eye, leaving the eye through the blind spot
-
Anatomy of Visual Connections
- Overall path to primary visual cortex:
- Retina>Thalumus(LGN)>Visual cortex
- - optic nerve: axons from ganglion cells, connects eye to brain
- - optic chiasm: axons from the "nasal" or middle half cross to the other side
- - optic tract: axons passing through chiasm to LGN
- - LGN(lateral geniculate nucleus): relay nucleus in thalamus that conveys info to visual cortex; synapse here, neurons go to primary visual cortex in occipital lobe
Result of optic chiasm crossings: visual image from the left side of the field ends up in the right side of the brain, and vice versa
-
Dark Adaptation
- - the increase in sensitivity to light (i.e. decrease in thresholds) that occurs when a subject is shifted from high to low background light
- There are two components to the dark adaptation processes:
- 1. a cone-dependent process used under high background light
- 2. a rod-dependent process used under low background light
- This is evidence for two visual processes: photopic and scotopic vision
-
Photopic Vision
- "day" vision
- high background light
- high acuity
- good color vision
- low sensitivity
- cone-dependent
-
Scotopic Vision
- "night" vision
- low background light
- poor acuity
- poor color vision
- high sensitivity to low levels of light
- small blind spot in center/fovea (central scomata)
- rod-dependent
-
Visual Phenomena Change with Different Background Lighting and Location
- Light sensitivity
- - better when background light is low
- - better in periphery of visual field
- - best with rods in retinal periphery
- Visual acuity; sharpness of visual image
- - better with high background ligh
- - better in center of visual field
- - more dependent upon cones in the fovea of retina
- Color vision
- - better with high background light
- - better in center of visual field
- - more dependent upon cones in the fovea
-
Theories of Color Vision
- 1. Trichromatic theory - there are three different primary colors
- 2. Opponent process theory - there are two different processes, and within each process there are antagonistic dimensions or processes
- - Red vs. Green process
- - Blue vs. Yellow process
-
Evidence in Favor of the Different Theories of Color Vision
- Trichromatic:
- - there are 3 different types of cones
- Opponent process:
- - color blindness
- - negative after images
- It appears as though both theories are correct, but at different levels
- - cones are trichromatic
- - the rest of the color pathway (e.g. ganglion, thalamic, cortical cells) follows opponent process
|
|