Exam 2: Vision Flashcards
To study and ACE Exam 2!
What characteristics do all senses share?
- Physical stimulus
- Stimulus transformed to neural code
- Perception of or response to stimulus
What is the purpose of senses?
Convey information from environment.
What is meant by “Perception is not reality”?
Information perceived does not match environment because we perceive certain parts of stimulus and interpret in terms of experience.
- Colors, tones, smells, tastes are constructs of our brain.
- Perception limited by capabilities of sensors and neural systems.
What is modality?
An attribute of stimulus.
- Type of energy transduced
What is location (stimulus attribute)?
Which receptors active (topography) and helps define size of object.
What is intensity (stimulus attribute)?
Size of response of each sensor.
What is timing (stimulus attribute)?
When receptor starts and stops signaling.
What is sensory transduction?
- Sensory receptor transduces stimulus energy into electrical signal, the receptor potential.
- Intensity and timing of stimulus related to amplitude and duration of receptor potential.
What is the idea of Labeled Lines?
- “Law of Specific Sense Energies”
- Each sensory receptor maximally sensitive to one type of energy and wired to specific area of brain.
- Modality perceived depends on which sensory pathway was activated.
- Action potentials only encode stimulus intensity and duration.
What is mechanical (category of receptor)?
Somatosensory, inner ear
What is chemical (category of receptor)?
Pain, itch, taste, smell
What is thermal (category of receptor)?
Body temperature, temperature of objects
what is electromagnetic (category of receptor)?
Vision
Describe mechanoreceptors and its function.
- Physical deformation of tissue opens ion channel and generates receptor potential.
- Found in skin for pressure, light touch, vibration.
- Found in inner ear for responding to sound waves.
What is useful about secondary messengers and what is effected?
- Chemoreceptors and photoreceptors
2. Benefit from signal amplification
What is meant when receptors are narrowly sensitive?
- Receptors not sensitive to full range of energies within a modality. EG. Different colors, tastes, pitches.
- Modlities divided into sub modalities based on receptor sensitivities.
- Each receptor has tuning curve.
Explain the neuronal tuning curve.
In terms of auditory, a neuron is able to elicit a response for certain frequencies depending on the intensity of the sound. EG. At lower sound frequencies (.5 kHz), the sound intensity must be high (100dB) in order for a neuron to respond and register the sound. At 2 kHz, a neuron can respond at sound intensities lower than 25 dB etc.
Receptive fields
- Similar in bipolar ganglion cells, LGN
- Circular
- Center surround organization
- Detect and encode contrast differences.
What is glutamate released by and do?
Glutamate is released by cones and inhibit on-center bipolar cells and depolarize off-center bipolar cells.
On-center ganglion cells fire with _______.
Rapid increase in illumination
Off-center ganglion cells fire with _______.
Rapid decreases in illumination.
What are M-Cells?
Magnocellular cells
- Project to LGN 1-2
- Color insensitive
- Identify general shape and movement of object.
What are P-Cells?
Parvocellular cells
- Project to LGN 3-6
- Detect color
- Identify form and color of object
Divisions of the Retina
Brain needs to combine the information from the nasal hemiretina and right/left temporal hemiretina.
What percentage of axons does the retina-geniculate-striate pathway have?
~ 90% of axons of retinal ganglion cells.
What is retinotopic organization?
Information from the receptive field is mapped accordingly in the brain.
How are activated receptors organized and what do they do?
- Receptors arrayed topographically. EG. A hand has many receptors that send specific information.
- Information about size and location in space (visual system, somatosensory system)
- Information about pitch (auditory), taste (gustation) or smell (olfaction).
Describe receptive fields.
- Neuron only responds to stimulation within receptive field.
- Receptor density varies in different regions.
- Receptor density affects level of detail of information received.
What happens with more receptors available?
More receptors = better image, higher sensitivity to something
Describe psychophysics
- Relationship between characteristics of stimulus and quality of perception.
- “Just noticeable difference” related to size of reference stimulus.
- Sensory threshold is stimulus that can be detected accurately 50% of time.
What is “Just noticeable difference”?
Detecting change based on the initial or reference stimulus. EG. Holding a soda can and then adding a 50lb bag of rice = noticeable difference.
How is the sensory threshold determined?
- Evaluated clinically.
- Minimum energy stimulus energy to generate action potential.
- Modifiable by psychological factors.
Dorsal stream
“Where” pathway
Ventral stream
“What” pathway
V5/MT and MST
Detect motion
V1, V2, V3
Sensitive to binocular disparity
V2
Detects illusory and actual edges
V4
Detects form (also color)
Inferior temporal cortex
Recognizes faces and objects
Posterior parietal lobe
Integrates vision and motion
Motion detection pathway
M cells -> V1 layer 4B -> MT/V5
How is motion detected?
As objects move across retina and as eye tracks them in space.
MT cells sensitive to what?
Direction of motion
Damage to MT impairs what?
Ability to detect non-coherent motion
Stimulation to MT biases what?
Perceived motion
MST detects what?
Global motion in visual field (Starship going light speed)
When are monocular cues used and what are they?
When judging distance of objects more than 100 feet away.
- Familiar size
- Occlusion (one object behind another)
- Linear perspective (parallel lines converge)
- Size perspective
- Distribution of shadows and illumination
- Motion parallax (close things move in opposite direction, things that are far seem to be moving along with you)
What is binocular disparity?
When you fixate on an object causing objects further or closer than fixation point to be represented at different locations on retina.
- Cells in V1, V2, V3, MT sensitive to disparity
V2 responds to what?
Illusory Edges: V2 completes images by filling in the lines. (Responds to actual and illusory contours in the same manner)
V1 is insensitive to what?
Illusory edges: only sees what is there.
How is orientation and light related?
Cells are responsive to different angles of light. = Systematic mapping of orientation.
What do blobs do?
Color processing on cortical module
What types of cells have receptive fields?
Ganglion cells, LGN cells, simple cells, complex cells
How does adaptation to light work?
- Due to slow decreases in Ca2+ concentration.
- Slow depolarization back to -40 mV
- Receptor desensitization.
What happens in the dark for transduction?
- Dark current
2. In the dark, the concentration of cGMP is increased, opening cation channels.
What are the stages of transduction?
- Light activation of receptors.
- Activated receptors stimulate cGMP phosphodiesterase.
- Decrease in cGMP closes channels and cell hyperpolarizes.
What is frequency Coding?
- Receptor potentials converted to action potentials of a given frequency.
- Absolute refractory period limits firing rate.
- Large amplitude stimulus can generate action potential during relative refractory period, thereby increasing firing rate.
- Larger stimuli depolarize more neurons (population coding)
Slowly adapting receptor
Frequency code tells us intensity
Rapidly adapting receptor
Most are ionotropic
What is the function of relay nuclei?
- Process and filter information before it ascends to CNS.
2. Have large receptive fields.
Inhibition _______ contrast.
Increases
1. Gives a sharper perception vs. broad reception.
What types of inhibition are there?
- Feed-forward
- Feedback(wards)
- Top-down (Brain sends info down)
How are images perceived in the brain (orientation)?
Upside down and reversed (L to R)
Pathway of light entering eye.
- Cornea
- Pupil
- Lens
- Fovea
What is the fovea?
Rod-free area (cones are most dense in this area) allows for accurate vision.
- Center of eye where light falls.
Organization of retina (Inner to outer)
- Ganglion cells
- Amacrine cells
- Bipolar cells
- Horizontal cells
- Cones
- Rods
- Pigmented part of retina
Describe rods
- Low acuity (low detail vision)
- Color insensitive
- Threshold: single photon (more sensitive to light)
- Low temporal resolution (little timing detail)
- High convergence (Many rods converging to one point. Consequence: specificity is vague)
Describe cones
- High acuity (HD)
- Color vision (3 cones for wavelength of color)
- Threshold: 100s photons (needed before response)
- High temporal resolution
- Low convergence (1 to 1) (Very specific for location of signal)
Describe Color Perception
- One aspect of vision
- Considered in context of brightness, background, lighting (how you see something affects the color)
- Visual system detects light reflected from surface (wavelengths of visible light are either absorbed or reflected)
How do cones help with color perception?
- Three cones (S,M,L) maximally sensitive to different wavelengths of light.
- Cones do no transmit about wavelength of light detected.
- Relative activity of all three cones interpreted as color. (combined output)
What is the opponent process theory?
- Perceive color in terms of paired opposites
- Color perceived in brain on continuum from red to green, from yellow to blue, from light to dark.
- Bipolar cells may be excited by one set of wavelengths and inhibited by another.
Short wavelength cone responds best to _____.
Blue
Medium wavelength cone responds best to _____.
Green, less to yellow
Long wavelength cone responds well to _____.
Red or yellow
What does the horizontal cell do to the bipolar cell?
Inhibits it after being excited from cones.
What does the short wavelength cone do to the bipolar cell?
Excites it
P cells project to ______ and ______.
- V1 layers 4Cbeta and 4A
- Most of these cells are color insensitive
- Some sensitive to opponent colors.
Describe blobs and color perception.
- Blobs -> V2 thin stripes -> V4
2. Color-sensitive cells seem clustered in blobs
Describe congenital color blindness.
- Loss of one of three cones.
- Dichromats have less specificity in surface reflectance functions, therefore more likely to confuse colors
- Most common form looses differentiation between L and M cones
Describe acquired color blindness.
- Loss of color vision often associated with eye disorders.
2. Some cortical damage affects color vision.
