PSYC 1100 Section 1.7 Sensory Processes

• -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

• 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

• -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

• 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

• 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

• 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 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

• 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

• 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 

• 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

• 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

• 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

• - 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

• "day" vision
• high background light
• high acuity
• good color vision
• low sensitivity
• cone-dependent

• "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

• 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

• 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

• 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