PSYC 306 exam 2 study guide

What are the main four optical components of the eye?

Cornea: first tissue that light encounters, provides a window to the world because it is transparent

Aqueous humor: fluid derived from blood that fills the space immediately behind cornea and supplies oxygen and nutrients to, and removes waste from, both cornea and lens

lens: structure inside eye that enables changing of focus

Vitreous humor: transparent fluid that fills vitreous chamber in the posterior part of eye

What is accommodation in the eye?

when the lens alters the refractive power of the eye by changing its shape.

Emmetropia, myopia, hyperopia, astigmatism similarities and differences

• Emmetropia is when all four optical components of eye are perfectly matched

• Myopia: if eyeball is too long relative to power of four optics

  • Need minus lenses

• Hyperopia: if eyeball is too short relative to power of four optics

  • Need positive lenses

• Astigmatism: if the cornea is not spherical, but shaped like a football.

  • Need lenses with two focal points or LASIK surgery

What are photoreceptors?

light-sensitive receptors located in the retina, including rods and cones.

What is age-related macular degeneration (AMD)?

AMD is a disease associated with aging that gradually destroys sharp central vision and has two forms: wet and dry.

• sensory substitution: an attempt to correct AMD by attempting to use touch to substitute sight, did not work

• 3 promising measures to cure AMD

  • Substituting an electronic prosthesis into the retina, using a camera to convert light into energy

    • Has technical challenges to implanting them and suffer low spatial resolution

  • Gene therapy to express light-activated channels in surviving photoreceptors using adeno-associated viral (AAV) vectors. 

    • Successfully used in several clinical trials 

  • Chemically modify endogenous channels in retinal ganglion cells to make them light-sensitive. Basically adds a synthetic small molecule “photoswitch” to confer light sensitivity onto retinal ganglion cells.

    • Shown to reinstate light sensitivity in blind people

    • One limitation is their low light sensitivity and inability to adapt to changes in ambient lighting. Recent work suggests medium wavelength cone opsin overcomes these limits.

What is the fusiform face area (FFA)?

an area in the extrastriate visual cortex that specializes in the perception of human faces.

Light

A narrow band of electromagnetic radiation that can be conceptualized as a wave or stream of electrons

Photon

Quantum of visible light (or other form of ER) demonstrating particle and wave properties both

What can happen to light?

Absorbed: energy taken and not transmitted at all

reflected: redirected when it strikes surface

transmitted: passed on through a surface

refracted: altered as it passes into another medium

transduced: converted from one form of energy to another

Cornea

transparent “window” into the eyeball (refraction)

Lens

Structure inside the eye that enables the changing of focus (refraction)

Retina

A light sensitive membrane in the back of the eye that contains rods and cones (transduction)

Photo receptors

cells in the retina that initially transduce light energy into neural

Rods

photoreceptors for night vision

• respond well in low luminance condition

• do not produce color

• more of them

• on edges of eye

Cones

Photoreceptors for daytime vision, fine visual acuity, and color

• respond best in high luminance conditions

• less of them

• center of eye

Mach bonds

Exist in part due to center-surround antagonism:

• retina is composed of receptive fields that have opposite responses to the same light stimulus at center and edges

  • serves to increase contrast between different light and intensities

receptive field

region on the retina in which visual stimuli influence a neurons firing rate

Blind spot

Where optic nerve leaves the eye (optic disc), are no receptors

• Why don’t we see a hole?

  • brain fills in

• Where does this integration occur?

  • likely visual cortex

How visual system deals with large variations in overall light levels

• One: reduce the scale of the problem by regulating the amount of light entering the eyeball by using different types of photoreceptors in different situations, and by effectively throwing away photons we don't need.

• Two: by responding to the contrast between adjacent retinal regions, ganglion cells do their best to ignore whatever variation in overall light level is left over

OFF center vs ON center cells:

• ON: increases firing in response to an increase in light intensity in its receptive field center

• OFF: increases firing in response to a decrease in light intensity in tis receptive-field center

Ganglion center-surround organization:

A ganglion cell fires fastest when the size of the spot of light matches the size of the extrasensory center

Kuffler, 1953

Mapped out the receptive fields of individual retinal ganglion cells in the cat, using small spots of light

Acuity

smallest spatial detail that can be resolved at 100% contrast

Sinha et al 2017

• central vision considerably lower than peripheral

• Suggests peripheral cones respond about twice as quickly to light as foveal cones because of their longer axons for denser packing in central fovea

Amblyopia

developmental disorder characterized by reduced spatial vision in an otherwise healthy eye, even with proper correction for refractive error (LAZY EYE)

Types of visual acuity

• finest tripes that can be resolved (minimum resolvable)

• smallest letter that can be recognized (minimum recognizable)

• detection of a feature (minimum visible)

• discrimination of a change in a feature (minimum discriminable)

Structure of brain axons in the retinal ganglion cells synapse in

Synapse in the two lateral geniculate nuclei (LGN)

Koniocellular cells:

a neuron located between the magnocellular and parvocellular layers of the LGN

Ipsilateral vs Contralateral

Ipsilateral: same side of body (brain), layers 2, 3, 5 of LGN get input from right side

Contralateral: opposite side of body (brain), layers 1, 2, and 4 of LGN get input from left side

Primary visual cortex (V1), area 17, or striate cortex

area of cerebral cortex of the brain that receives direct inputs from LGN as well as feedback from other brain areas

properties:

• Orientation tuning: tendency of neurons in striate cortex to respond optimally to certain orientations and less to others

• An individual neuron will not respond equivalently to just any old stripe, but rather it responds best when the line or edge is just at the right orientation, hardly at all if tilted more than 30 degrees off

• Circular receptive fields in LGN are transformed into elongated receptive fields in striate. The arrangements of LGN inputs is crucial but also neural interactions are important

Simple vs complex cells:

Simple: a cortical neuron who’s receptive field has clearly defined excitatory and inhibitory regions

complex: does not have clearly defined regions

Adaptation

a reduction in response caused by prior or continuing stimulation

Selective adaptation

 adaptation to a limited set of stimuli

• Orientations of stimuli can show how this works bc it fires best at certain stimuli

• Site of selective adaptation effects:

  • Can localize orientation-selective and spatial frequency selective neurons in humans

Tilt aftereffect

perpetual illusion of tilt, produced by adaptation to a pattern of a given orientation

Spatial frequency channels

a pattern analyzer, implemented by an ensemble of cortical neurons, in which each set of neurons is tuned to a limited range of spatial frequencies

Human visual systems contain neurons selective for spatial frequency

Robert Franz on babies

Infants when shown 2 scenes, will stare at the more complex one (lines vs a blob)

Norcia et al 1990

• Sensitivity to low spatial frequencies develops much more rapidly than sensitivity to high spatial frequencies

• Thus at low spatial frequencies,  contrast sensitivity may reach adult levels around 9 weeks, whereas sensitivity at higher spatial frequencies develops dramatically continuously

Lesion

in reference to neurophysiology, a region of damaged brain

Agnosia

failure to recognize objects in spite of the ability to see them, typically from brain damage

Extrastriate body area EBA

region of EVC in humans that is activated by images of body (not face)

Parahippocampal place are (PPA)

a region of EVC in human that is activated more by places

Mid level vision

loosely defined stage of visual processing that comes after basic features have been extracted from the image and before object recognition and scene understanding

principles of mid level:

• good continuation:1. A Gestalt grouping rule stating that two elements will tend to group together if they seem to lie on the same contour. For example, sounds will tend to group together as continuous if they seem to share a common path, similar to a shared contour for vision.

• Similarity: similar objects more likely to group

• Proximity: closeness means grouped more likely

• Parallelism and symmetry: weaker, pretty self explanatory

Structuralists

a school of thought that believed that complex objects or perceptions could be understood by analysis of the components

Gestalt grouping rules

a set of rules describing which elements in an image will appear to group together. Original list was assembled by gestalt schools

Accidental viewpoint

 a viewing position that produces some regularity in the visual image that is not present in the world

• example : optical illusion of four squares, person “holding up” eiffel tower

• Perceptual committees don’t bet on them but still get fooled

Gallant et al and Connor and Pasu:

Warm colors indicate more response from one particular cell in area V4. Darker circles indicated more response also

Prosopagnosia

inability to recognize faces

Haxby face pathways

• 1. Invariant property: same face from past times 

• 2. Dynamic: what is she saying? emotion?

Inverted glasses experiments

people who wear glasses that make world appear upside down adapt quickly

• where does integration occur

  • likely multi-sensory integration in high order brain cells

Case studies of visual systems:

Blindsight: people can sometimes report correctly on visual info they cannot see

• 2 studies: someone could predict shapes better than chance though she couldn’t see them, and another had a blind person navigate an obstacle course

Akinetopsia: neurophysiological disorder in which the affected individual cannot perceive motion

Visual agnosia: failure to recognize objects despite being able to see them

• Dr. P mistook his wife for a hat

Sequential vs. Parallel processing of visual information

Sequential: additive processing where each layer builds on what comes before

• disadvantages

  • brainstorming research analogy: one sequential layer offering competing info which muddies next layer

  • reduced info processing: non-independent sources gather less info

Parallel: processing where layers simultaneously evaluate info separately

Parvocellular and Magnocellular Pathway

Parv:

• Fovea (cones) → P ganglion cells → Parvocellular layers 3-6 LGN → Striate cortex

Mag

• Periphieral (Rods) → M ganglion cells → Magnocellular layers 1-2 LGN → Striate cortex

Parvocellular Pathway

Function: fine visual acuity, color, and shape processing

Poor temporal resolution but good spatial

P ganglion cells: provide info mainly about contrast in retinal image

“what” pathway: helps you define and recognize objects

Magnocellular Pathway

Functions: location of objects in visual field relative to each other, finding objects to interact with, and motion detection

Excellent temporal resolution but poor spatial resolution

M ganglion cells: provide info about how an image changes over time

the “where” pathway: helps you figure out where objects are

Properties in LGN

LGN neurons have concentric receptive fields, very similar to those of the retinal ganglion cells

Topographical mapping: orderly mapping of the world in the LGN and visual cortex

• organizes the visual fields and thus begins process of producing larger images

• if two neighboring neurons are identified in the retina, they will be fixed next to each other in the LGN, and their projection then will be found next to each other in the striate cortex

Parvocellular layer

smaller cells

top four layers of LGN (3-6)

Receive input from P ganglion cells

respond best to fine spatial details of stationary objects

Nishimoto et al

Participants watched movie while hooked up to fMRI

computer nets “learned” to associate fMRI activity with movie images

neural nets predicted stuff

Wang shows lots of reconstruction happens in striate cortex

Eccentricity

distance from retinal image and the fovea

• as eccentricity increases, less cortex is devoted to processing and visual acuity decreases

Parvocellular regions of the extra striate cortex

V2

• topographically mapped

• receptive fields are larger, ab 3 times as large as V1

• illusory contour: a contour that is perceived even though nothing changes from one side to the other

V4

• topographically mapped

• receptive fields are larger than V2

  • responds to stimuli such as fans, spirals, and pinwheels

Infernal temporal (IT) cortex:

part of cerebral cortex respon in lower part of temporal lobe, important in object recognition to part of memory system

Gross et al and Quiroga

some cells respond to monkey faces while others specific objects (Gross)

Jennifer anniston cell in temporal lobe (Quiroga)

some neurons fire for abstract concepts and concrete objects or person (quiroga)

Precognition

Bem (2011)

• Replicated classic psychology effects in reversed time

  • people remember words better that they reheasrsed AFTER DV collected