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What are the 2 types of photoreceptors contained in the light-sensitive retina?
What is the optic disc (aka blind spot)?
The “hole” in the retina where the optic nerve leaves the eyeball & there is no photoreceptors
What are the 2 types of photoreceptors contained in the retina?
Rods & Cones
Meaning of 'transduced'?
The conversion from 1 form of energy (eg, light), to another (eg, electricity)
Simple description of a photoreceptor?
A light-sensitive receptor in the retina
What kind of vision are rods specialised for?
What kind of vision are cones specialised for? (3 types)
- 1. Daylight vision
- 2. Fine visual acuity
- 3. Colour
What is contained in the outer segment of a photoreceptor?
Description of photoreceptors (as neurons)?
Neurons in the retina that capture light & initiate the act of seeing by producing chemical signals
What is a chromophore?
The light-catching part of the visual pigments of the retina
What is eccentricity?
The distance between the retinal image & the fovea
Under what kind of illumination do rods function relatively well? (scientific term)
Dim (scotopic) illumination
What type of illumination do cones require to operate efficiently?
Bright (photopic) illumination
What are the levels of acuity & sensitivity for the photopic system?
- High acuity
- Low sensitivity
What are the levels of acuity & sensitivity for the scotopic system?
- Low acuity
- High sensitivity
What are the consequences of rods having all the same type of photopigment?
They cannot signal differences in colour
What are the 3 types of photopigmentation that cones have? (plus corresponding colours)
- 1. Short wavelength-sensitive cones (Blue)
- 2. Medium wavelength-sensitive cones (Green)
- 3. Long wavelength-sensitive cones (Red)
What are horizontal cells?
Specialised retinal cells that contact both photoreceptor & bipolar cells
What is lateral inhibition & what does it enable?
- Lateral inhibition (opponency) is where stimulation excites 1 region & inhibits the other
- It enables the signals that reach retinal ganglion cells to be based on differences in activation between nearby photoreceptors.
What are amacrine cells & what pathway are they part of?
- Retinal cells found in the inner synaptic layer that make synaptic contacts with bipolar cells, ganglion cells, & one another.
- Amacrine cells are part of the lateral pathway
What are bipolar cells & what do they do? (basic)
- Bipolar cells are the intermediaries.
- They are retinal cells that synapse with rods or cones (not both) & with horizontal cells, then pass the signals on to ganglion cells.
What & where are diffuse bipolar cells?
Bipolar retinal cells whose processes are spread out to receive input from multiple peripheral cones.
What & where are midget bipolar cells?
Small cone bipolar cells in the central retina (fovea), that receive input from a single cone.
How are horizontal cells positioned?
Running perpendicular to the photoreceptors, making contact between nearby photoreceptors.
What is Convergence of information via Bipolar Cells is a very important mechanism for & what does it have to do with rods?
- For increasing visual sensitivity
- - The ability of the rod system to function well in dim lighting conditions, is due to most rods communicating with ganglion cells through diffuse bipolar cells
What is Convergence of information via Bipolar cell terrible for & what does it have to do with diffuse bipolar cells?
- Visual acuity
- - Diffuse bipolar cells may fire the same rate in response to 1 bright, or several dim points of light, meaning the receiving ganglion cell is unable to decipher the pattern of light present.
Where are diffuse bipolar cells generally found?
Which neurons/cells of the retina are involved in passing rod or cone information through the vertical pathways? (3)
- 1. Photoreceptors
- 2. Bipolar cells
- 3. Ganglion cells
Which cells of the retina are involved in passing rod or cone information through the lateral pathways? (2)
- 1. Horizontal cells
- 2. Amacrine cells
What is the plus & minus of having a high degree of convergence in peripheral vision?
High sensitivity to light, but low acuity
What is the plus & minus of having a low degree of convergence in the fovea?
High acuity, but low sensitivity to light
Each foveal cone contacts 2 bipolar cells with divergent information. What are these 2 types of bipolar cells?
- 1. ON bipolar cell
- 2. OFF bipolar cell
What are ON bipolar cells?
Bipolar cells that respond to an increase in light captured in the cone
What are OFF bipolar cells?
Bipolar cells that respond to an decrease in light captured in the cone
Ganglion cells are the final layer of the retina, but what do they do?
They receive visual information from photoreceptors via 2 intermediate neuron types. They process it & then transmit it to the brain & midbrain.
What are the 2 intermediate neuron types that pass visual information from the photoreceptors, to the ganglion cells?
- 1. Bipolar cells
- 2. Amacrine cells
What is the benefit of having both ON & OFF bipolar cells?
It provides information about the direction of the change in illumination
What are P ganglion cells?
Small ganglion cells that feed the parvocellular (‘small cell’) layer of the lateral geniculate nucleus (LGN)
What is the lateral geniculate nucleus (LGN)?
- The LGN is the primary processing center for visual information received from the retina of the eye.
- It is found inside the thalamus of the brain, and is thus part of the central nervous system.
What % of the ganglion cells in the human retina are P ganglion cells?
P cells constitute about 70%
What are M ganglion cells?
Ganglion cells that feed the magnocellular (‘large-cell’) layer of the LGN
What % of ganglion cells are M ganglion cells?
About 8-10% of ganglion cells in the human retina are the M variety.
What happens to the Dendrites of P ganglion cells & M ganglion cells with retinal eccentricity, & are the dendrite trees of P or M cells bigger?
- The dendrites of both increase in size with retinal eccentricity
- At all eccentricities P ganglion cells have much smaller dendrite trees than do M ganglion cells
What is the receptive field of a ganglion cell?
- Each ganglion has a small window on the world known as its receptive field.
- It is the region on the retina in which visual stimuli influence the neuron’s firing rate (this influence can be excitatory or inhibitory).
The spatial layout of the ganglion cell’s receptive field is essentially concentric. What does this mean?
- That the receptive field has a common centre.
- A small circular area in the centre responds to an increase in illumination, & a surrounding annulus (ring) responds to a decrease in illumination
Most ganglion cells have one of the 2 types of concentric centre-surround organisation. What are the 2 types?
- 1. ON-centre cell
- 2. OFF-centre cell
What happens in ON-centre cells? (including what affects firing rate)
- The small circular area in the centre responds to an increase in
- illumination & a surrounding annulus (ring) responds to a decrease in
- The ganglion cell fires fastest when the spot size matches the size of the excitatory centre
- It reduces its firing rate when the spot begins to encroach on its inhibitory surround
What happens in OFF-centre cells?
The firing rate decreases when a spot in the centre of the receptive field is turned on & it increases when the spot is turned on in the surround.
What is Contrast? (2 ways to define)
- The difference in luminance between an object & the background, or
- between lighter & darker parts of the same object
What is luminance?
Total amount of light reflected from a surface
What is illuminance?
The light falling on a surface
What is reflectance?
The proportion of incoming light that a surface reflects
How does centre-surround organisation of ganglion cells act as a filter for the information being sent to the brain?
- Because each ganglion cell responds best to spots of a particular size
- & will respond less to spots that are bigger or smaller than this
How does centre-surround organisation of ganglion cells enable perception of contrast?
- Bcos ganglion cells are most sensitive to difference in the intensity of the light in the centre & in the surround & are largely insensitive to the average intensity of the light
- (average intensity fluctuates, contrast remains stable)
What does the filtering system of the visual system change, from the raw image to the final representation?
- Highlights important information such as contrast, & discounts less
- useful information such as ambient light intensity
What are the Mach bands in the Mach pattern?
The illusory bright & dark stripes that our visual system perceives from the luminance ramp edge of the pattern
Why do P ganglion cells have smaller receptive fields than M ganglion cells at all eccentricities ?
- Most likely bcos M cells have larger dendritic fields than P cells
- (therefore they have more incoming messages from photoreceptors via bipolar, horizontal & amacrine cells)
What is a consequence of the different receptive field sizes of P & M ganglion cells?
- M cells are much more sensitive & able to detect visual stimuli under low lighting (like at night).
- Conversely, the smaller receptive fields of P cells enable them to provide finer resolution (greater acuity) than M cells, as long as there is enough light for the P cells
In terms of information signals to the brain, why is there sustained firing of P ganglion cells while light shines on their excitatory regions?
Because P cells provide information to the brain mainly about the contrast in the retinal image.
In terms of information signals to the brain, why do M ganglion cells fire only transiently while light shines on their excitatory regions?
Because M cells signal information to the brain about how the image changes over time
How will an M ganglion cell respond to light shining on its excitatory regions?
An M cell will respond with a brief burst of impulses when the spot is turned on, then quickly return to its spontaneous rate, even if the spot remains lit
Which photoreceptors does Retinitis pigmentosa affect 1st?
What would be the 1st problems that a person with Retinitis pigmentosa would notice regarding their vision?
- 1. Problems with peripheral vision
- 2. Problems seeing in low-light conditions
Do graded changes in photoreceptors & bipolar cells generate action potentials/neural spikes?
No, responses at the photoreceptoral level & bipolar cell level are all in a graded fashion. The graded changes in the potential then get transmitted into spikes & action potentials at the ganglion cell level
Is it rod bipolar cells, or cone bipolar cells that synapse with ganglion cells directly?
Cone bipolar cells synapse directly with ganglion cells
How many foveal cones & bipolar cells normally connect with each other?
1 foveal cone connects to 2 bipolar cells - 1 ON & 1 OFF midge bipolar cells
How many photoreceptors, bipolar cells & retinal ganglion cells are there in the retina?
100 million, 10 million & 1.25 million, respectively
Convergence of bipolar & ganglion cells enables better absolute sensitivity, but what does it compromise?
What do horizontal connections (horizontal & amacrine cells), between receptors, bipolar cells & ganglion cells provide?
providing inhibition of information in adjacent areas of the space (lateral inhibition)
Does white or black surround create more lateral inhibition?
White surround does
How much of our brain is devoted to visual processing?
estimates are 40-60%
What kind of processing does the visual system seem to be organised according to?
A modular, parallel & hierachical type of processing
What kind of activity is happening in the 1st stage of visual processing (image-based stage)?
Each photoreceptor is generating electrical current that is proportional to the intensity of the (reflected) light that is falling on it
What kind of activity is happening in the 2nd stage of visual processing (surface-based stage)?
Highlighting the difference between intensity-based representation & surface-based representation that our visual system has in relation to physically identical images
How many layers does the retina have?
- 6 layers
- - 3 dark layers of cell bodies
- - 3 light layers of axons & dendrites
What is the processing at the level (of the retina) of Photoreceptors known as?
- Input Layer, or Input Processing
- (this is where the absorption of electromagnetic energy that enters the eye, takes place)
Which processing layer of the retina are Bipolar cells at?
Middle processing layer
In which layer of the retina are functioning of the ganglion cells?
The Output Layer/Output of the Retina
What are the 2 modules that arise at the Middle Processing Layer?
- The 2 modules that arise are these pathways that we refer to as On Pathways & Off Pathways.
- (At this level, we have specialised neurons that respond selectively to increases in stimulation, compared to decreases in stimulation)
On what layer of the retina would you find M & P ganglion cells?
The layer of the ganglion cells - the outer layer
What are Magno cells predominantly involved in? (+ conduction rate, colour? & stimulation)
- Processing of motion
- + fast conduction rate
- colour insensitive
- prefer transient stimulation
What happens when the outer segment of the retinal receptor is stimulated by light?
Retinal changes its shape & position & starts a cascade of chemical reaction which makes Opsin [a large protein] absorb electromagnetic energy
Where in the retina are rods predominantly found?
In the periphery, there are no rods in the fovea
Where in the retina are cones predominantly found?
In the fovea, there are very few cones in the periphery
What happens to the visual pigment as a function of exposure to light?
The visual pigment bleaches (after 10-30mins of exposure)
How long does it take for the visual pigment to regenerate after visual pigment bleaching? (Retinal & Opsin must recombine)
- Cone pigment regenerates in 6 minutes
- Rod pigment takes over 30 minutes to regenerate
What is the Dark Adaptation curve?
Curve following the time-line of visual pigment regeneration & illumination sensitivity at different points
What part of the retina does Macular Degeneration affect?
Primarily the cones
Where are objects imaged on or near the fovea, processed?
By neurons in a large part of the striate cortex
Where are objects imaged on the periphery, processed?
A smaller part of the striate cortex
What is cortical magnification & when does it occur?
A visual distortion of objects in periphery vision
What is a consequence of cortical magnification?
That visual acuity declines in an orderly fashion with eccentricity (distance from the fovea)
What are Parvo cells predominantly involved in? (+ conduction rate, colour? & stimulation)
- chromatic vision
- +slow conduction rate
- colour sensitive
- prefer sustained stimulation
What is a spatial frequency channel?
- They are pattern analysers, implemented by an ensemble of cortical neurons.
- (Each set of neurons is tuned to a limited range of spatial frequencies)
What is Contrast Sensitivity Function?
A function describing how the sensitivity to contrast depends on the spatial frequency (size) of the stimulus
What is spatial frequency?
A measure of how often a repeating structure (such as a vertical line or bar) appears within a given unit of distance
Do LGN cells show the same receptive field organisation as ganglion cells?
- Yes, they do.
- They also have ON-centre & OFF-centre cells
Where is Area 17 of Brodmann (primary visual cortex, V1) found?
At the occipital pole
Name 1 difference between simple cortical cells & complex cortical cells in V1..
Simple cortical cells respond best to stationary lines of a particular orientation, complex cortical cells respond best to moving bars of a particular orientation & direction of movement
What are hypercolumns in the Primary Visual Cortex?
A hypercolumn is a 1mm block of primary visual cortex that has all the machinery to look after everything the visual cortex is responsible for, in a certain small part of the visual field
What are the functions of colour vision? (3)
- 1. scene segmentation
- 2. signal edibility of food
- 3. social signals (emotions, health)
Visible light is a small part of electromagnetic spectrum, how many wavelengths does it include?
300 (from 400-700nm)
How is reflected light calculated?
Surface reflectance X ambient illumination = reflected light
What is brightness & what causes it to vary?
- Brightness is the perceived luminance
- It varies both with changes in illumination & reflectance
What is lightness or hue (colour) & what causes it to vary?
- Lightness is the perceived reflectance
- It varies only with changes in reflectance
- (fairly independent of illumination changes)
Why does white paper in shadow appear to have higher lightness than a black paper under intense light?
Because the perceived reflectance (lightness) of the white paper remains the same despite changes in illumination
What is Colour Constancy?
The ability of a vision system to assign a colour description to an object that does not depend on the illumination environment
What is the Trichromatic Theory of Colour Vision?
Theory that the colour of any light is defined in our visual system by the reflectance of electromagnetic wavelengths of different lengths detected by short, medium & long wave cones.
What are Metamers?
Different mixtures of wavelengths that look identical.
What is the Principle of Univariance?
- That an infinite set of wavelength–intensity combinations can
- elicit exactly the same response from a single type of photoreceptor.
- One photoreceptor cannot make discriminations based on wavelengths.
Dichromacy is where a person can only receive 2 pure spectral colours in their vision. What are the causes? (4)
- 1) Deuteranomaly: green shifted toward red
- 2) Deutan Dichromat: no green cones; only red and blue
- 3) Protanomalous: red shifted toward green
- 4) Protan Dichromat: no red cones; only green and blue
What is Opponent Colour Processing Theory?
Perception of colour is based on the output of three mechanisms, each of them based on an opponency between two colours – red/green, blue/yellow, black/white.
What causes colour afterimages?
- After looking at the adapting stimulus for a few seconds, a subsequently achromatic region will appear to take on a colour opposite to the original (negative afterimage).
- Colour mechanisms attempt to reach a “neutral point”, but overshoot this point and cause us to perceive the opposite of the adapting stimulus.
What is Simultaneous Colour Induction & what causes it?
- Where the same colour appears different when given a different backdrop
- (Lateral inhibition between neurons responding to central squares and their immediate spatial surrounds causes one stimulus to be perceived as darker/lighter than another)
What is colour assimilation?
- (opposite of colour induction)
- Colour assimilation is where colours appear to become more like their neighbor instead of less like them (same colours appear different, different colours appear same)
What is the dominant view of information transmission through the cortex?
V1 separates information about the different dimensions (luminance, colour, movement, depth, texture) & dispatches it to different extrastriate regions each specialised for a particular kind of analysis
What is Akinetopsia & what brain area does it signal damage in?
- Motion blindness (people see the world as if through a strobe light)
- It signals V5 damage
What is Cerebral Achromatopsia & what brain area does it signal damage in?
- A type of color-blindness that is caused by damage to the cerebral cortex of the brain, rather than abnormalities in the cells of the eye's retina
- It signals V4 damage
What is Associative Agnosia?
Where people cannot associate visually-presented objects with their semantic meaning, or organize objects into semantic categories.
What is prosopagnosia?
The inability to perceive faces
According to Image Description Models, how do we recognise objects from different viewpoints?
Using stored 2-D viewpoints from different perspective (view invariance does not occur for novel objects)
According to Structural Description Models, how do we recognise objects from different viewpoints?
By decomposing 3D objects into 3D volumetric features that can be combined for a given shape
What is the idea behind Marr & Nishihara's model (1978) of object recognition?
That object recognition involves matching the 3D model representation constructed from a visual stimulus, against a catalogue of 3D model representations stored in memory.
What is the idea behind Biederman's Recognition by Components (RBC) theory?
- That we are able to recognize objects by separating them into geons.
- Geons can be composed of various shapes (i.e. cylinders, cones, etc.) that can be assembled in various arrangements to form a virtually unlimited amount of objects
What is amodal completion?
- Amodal perception describes the perception of the whole of a physical structure, when only parts of it are actually visible to the sensory receptors
What is modal completion?
- (Illusory contours)
- Modal completion is a phenomena in which a shape is perceived to be occluding other shapes even when the shape itself is not drawn
What are the functional aspects of perceptual organisation? (3)
- 1. Region segmentation
- 2. Grouping
- 3. Parsing
What is parsing?
Dividing a single objects (element) into parts
What is the concave discontinuity rule?
The visual system divides objects into parts where they have abrupt changes in surface orientation toward the interior of the object
What is Pandemonium (Selfridge, 1957)?
Attempt to model a complex cognitive process like letter recognition using a computer simulation. The model has 'demons' which are very similar to neurons, & does in fact mimic the brain's structure in some important ways
What are the 4 types of 'demons' in Selfridge's Pandemonium model?
- 1. Image demon (recognises image)
- 2. Feature demon (respond to various features/contours of image)
- 3. Cognitive demon (integrates features into cognitive info, eg words, symbols, numbers)
- 4. Decision demon (decides how to interpret intput)
What does the Necker Cube/Rubin Vase illusion demonstrate?
Demonstrates visual system’s decisiveness in resolving ambiguity because it seems impossible to perceive both interpretations at the same time
What is an Accidental Viewpoint?
- A viewing position that produces some regularity in the visual
- image that is not present in the world.
What did Attneave (1954) demonstrate with his picture of a cat?
That the lines of low curvature represent redundant information
What is Figure-Ground Segregation?
Determining what part of environment is figure & what part is the background
What are the principles of perceptual organisation? (8)
- 1. Similarity (similar elements grouped together)
- 2. Proximity (nearby elements grouped together)
- 3. Common fate (things moving in same direction grouped together)
- 4. Good continuation (intersecting lines are seen as following the smoothest path
- 5. Closure (contours perceived as whole despite gaps in contours)
- 6. Common region (elements in the same region tend to be grouped together)
- 7. Uniform connectedness (connected region of visual properties are perceived as a single unit)
- 8. Synchrony (elements occurring at same time are seen as belonging together)
What were the findings by Field, Hayes & Hess (1993) regarding contour integration in the human visual field?
- It is the alignment (not spacing or angles) of orientations along the curve that is the most important determinant of path detectability.
- (+/-15 degrees reduces performance, +/- 30 degrees makes path almost undetectable)
- Normally adjacent elements will be linked if a smooth curve can be drawn between them
What is the law of Pragnanz?
Fundamental principle of Gestalt perception that says that we tend to order our experience in a manner that is regular, orderly, symmetric & simple. (every stimulus pattern is seen in such a way that the resulting structure is as simple as possible)
What is the likelihood principle?
Objects are perceived based on what is most likely to have caused the pattern.
What is psychophysics?
Quantitative relationship between physical stimuli & sensation & perception
In psychometrics, what is the method of constant stimuli?
Properties of the stimulus are presented randomly (in different levels) so as to prevent the subject from being able to predict the level of the next stimulus. This reduces errors of habituation & expectation
In psychometrics, what are Adaptive Staircases?
- Method for testing perception - usually begin with a high intensity stimulus that's easy to detect.
- Intensity is then reduced until the observer makes a mistake, at which point the staircase 'reverses' & intensity is increased until the observer responds correctly, triggering another reversal.
- The values for these 'reversals' are then averaged.
What are methods of detection & discrimination good for, & not so good for? (psychometrics)
- Good for how, & how fast information is processed
- Not good for where in the brain this information is processed
How can we measure what's going on inside the brain? (psychometrics)
- 1. Micro-electrodes in the brain
- 2. EEG
- 3. MEG
- 4. fMRI
What does EEG measure?
Electrical signals from outside
What does MEG measure?
What is the benefit of using MEG over EEG for measuring inside the brain?
Magnetic fields are less distorted than electric fields by the skull, & therefore provide greater spatial precision.
What does fMRI measure?
Change in oxygen in the blood flow (BOLD – “blood oxygen level dependency”). Can only measure at the speed of blood flow
What is a negative of measurements using fMRI?
Bad temporal resolution
What are voxels?
- 3-dimensional units of fMRI
- (little volume elements that make up the 3D fMRI picture, representing a part of the brain)
What is decoding in fMRI?
Using (brain) activity to predict information about the stimuli
What has decoding been used to show? (5)
- 1. Short-term visual memory
- 2. Mental imagery
- 3. Conscious awareness
- 4. Decision making
- 5. Detect familiar environments
What are some causative techniques in psychometrics? (4)
- 1. Lesion studies
- 2. Directly stimulating neurons
- 3. Cortical cooling techniques
- 4. TMS (transcranial magnetic stimulation)
What is the problem with using lesion studies, directly stimulating neurons & cortical cooling, in order to study causation? (psychometrics)
They are often invasive & non-reversible
What are the benefits of using TMS for causation? (7) (psychometrics)
- TMS is:
- 1. Causative
- 2. Non-invasive
- 3. Precise
- 4. Reversible
- 5. Safe
- 6. Repeatable
- 7. Relatively painless
What is TMS (transcranial magnetic stimulation)?
A method of causation using electromagnetic induction to induce weak electrical currents, causing activity in specific or general parts of the brain
What does TMS do? (psychometrics)
- It can:
- 1. Interfere with ongoing cortical activity (temporary lesion)
- 2. Attenuate subsequent cortical activity (rTMS)
- 3. Cause a perceptual experience (phosphenes - optical phenomenon of perceiving light when there is none)
What are the 2 basic designs of TMS?
- 1. Event related (typically single pulse)
- 2. Block design (typically rTMS)
What is the principle known health risk of TMS?
Why does the moon's disk almost exactly cover the sun during an eclipse, despite being of very different actual size?
Because the sun & the moon have the same visual angles (retinal size)
What are non-accidental properties?
- Properties of an image that seldom occur by accident within visual scenes:
- 1. Smooth continuation
- 2. Co-termination (eg, arrow)
- 3. Parallelism
- 4. Symmetry
What are accidental properties?
The illusory interpretation of continuous contours in an image, as continuous contours in the 3D environment.
What is an anamorphosis? (accidental properties)
- A distorted projection or perspective - especially an image distorted in such a way that it only becomes visible when viewed in a special manner
- (eg, pool drawing on pavement)
What are the 2 visual cues that humans use to gain information about depth in 3D structure?
Monocular (those cues occurring in the individual eye) & binocular depth cues (both eyes together)
What are the monocular sources of depth information?
- 1. Occlusion
- 2. Size
- 3. Perspective (geometric, texture, aerial)
- 4. Shading
- 5. Motion parallax
How does occlusion act as a monocular depth cue?
When one object blocks another object, the object that is blocked is understood (by this cue) to be farther away than the object blocking it
How does size act as a monocular depth cue?
- Through familiar & relative size
- (Familiar size cue tells us that the visual angle of objects becomes smaller with distance, allowing us to calculate the probable depth or distance of objects & then compare relative size with known objects)
What is the Geometric Perspective in monocular sources of depth information?
- The linear perspective
- (convergence of lines that results in perceived depth in a 2D scene)
How does apparent-distance theory explain the moon illusion?
The horizon moon is surrounded by depth cues, while the moon higher in the sky has none (perceived as a flattened bowl).
How does texture perspective contribute to monocular depth perception?
Most objects have a textured surface. When elements of the pattern consistently becoming smaller, denser & fainter, then they appear to recede (seem further away)
What is Aerial Perspective in monocular depth cues?
Objects in the distance appearing less clear, more blurred & lower contrast (because of atmosphere & pollution)
What is Motion Parallax? (monocular depth cues)
- Differences in relative motion of objects located at different distances from the observer
- (objects in the distance appear to move more slowly than objects that are closer
What is Ocular Accommodation & at what range is it best?
- Ocular accommodation is focusing
- Good at close range (< 2.2m)
What is Ocular Vergence?
The angle of the eye
What is the basis of stereoscopic vision?
The brain taking the 2 separate images from the eyes & fusing them to create experience of 3D depth
What is disparity in binocular depth perception?
- When an object does not project to corresponding regions of the retina
- Fall on outside - crossed disparity
- Fall on insides - uncrossed disparity
What is the horopter? (binocular depth perception
Theoretical & actual line where things fall on the same part of the retina (depends on the point of fixation)
What is Panum's Fusional Area?
- General area on horopter when things appear in depth
- (Outside Panum's fusional area, physiological diplopia occurs)
How do we know that having 2 images from slightly different positions is enough for stereopsis (3D vision)?
From the ability to create 3D vision using simple stereograms
Why is fake 3D bad?
Our eyes are normally focused & verged at same location, but during fake 3D our eyes verge in front or behind the screen causing accommodation-vergence conflict
What is the magnitude of disparity? (binocular depth perception)
Given by the difference in retinal co-ordinates between the left & right eyes
How does interpupillary distance (IPD) influence magnitude of disparity? (binocular depth perception)
- Shorter interpupillary distance = smaller disparity
- Longer IPD = larger disparity
What is Chromostereopsis? (binocular depth perception)
- Colour given depth
- (short wavelengths refract more than long wavelengths, this causes disparity between each eye's view that is interpreted perceptually as difference in depth)
What is Diplopia & what causes it to occur?
- Double vision
- Can occur if the disparity is too big
What is binocular rivalry?
A phenomenon of visual perception in which perception alternates between different images presented to each eye
What would happen to our vision without motion?
Vision will fade - if we don't move our eyes we can't see
Where is motion processed in the brain?
- area MT
- (stimulating MT with TMS causes motion perception)
What is induced motion?
Creating the perception of motion not by changing the object but by changing the position of the shadow (ie, flying, moving diagonally)
What is a Reichardt detector?
A motion detector tuned to a certain speed and direction of movement (ultra-simple version of what's in our brain)
What are the different types of eye movements? (4)
- 1. Saccades (rapid intermittent eye movements)
- 2. Smooth pursuit (needs something to track)
- 3. Vergence (simultaneous movement of both eyes in opposite directions to obtain or maintain single binocular vision)
- 4. Micro saccades (mall, jerk-like, involuntary eye movements - prevents adaption & fading)
What is corollary discharge theory?
According to CDT, the perception of movement occurs only when the comparator receives just 1 input (either Image Movement Signals (IMS) or Corollary Discharge Signals [caused by eyes moving])
What is centre-surround suppression in motion processing?
- The phenomenon in which the perception &/or neural response to a target is reduced by the presence of a surrounding annulus
- (kicks in only at high contrasts)
What are 2 of the functional consequences of centre-surround suppression? (motion processing)
- 1. Enhanced perception of large moving objects
- 2. Impaired figure-ground discrimination
What is overt attention?
Moving the eyes to foveate the item to be attended
What is covert attention?
Attending to items in the periphery without moving the eyes
What are the effects of covert attention? (2)
- Enhanced sensitivity at attended location
- Diminished sensitivity at unattended location
What is bottom-up attention?
- From Salient stimuli
- It is environmentally relevant, salient, unexpected (pop out)
What is top-down attention?
- Derived from task demands
- It is volitional (conscious), controlled, relevant for more 'complex' reasons
What are the opposing arguments in the debate about pictorial mental imagery?
- 1. Imagery is pictorial, like perception
- 2. Mental imagery is not composed of 'images' but mental descriptions of concepts or ideas
How can imagery change perception?
Visual imagery is more like perception (eg pictorial) & it overlaps with perception
What is visual memory traces important for?
Integrating awareness from moment to moment
What is Iconic memory?
A type of short term visual memory
What is visual working memory?
Active process that can store visual info for times over 20 seconds
What is the fixed number of objects for visual working memory?
No fixed number (but it is a limited resource)
What is the overlap between working memory & mental imagery? (2)
- Both lead to activity in similar visual areas
- Both use memory & visual representations
What is the evidence that people use imagery for visual working memory?
That people who have stronger imagery are better at working memory tasks
What is qualia?
- The qualitative feeling of an experience
- (eg, the redness of red, the wetness of water)
What are 2 possible reasons for having qualia?
- 1. It is the most efficient way of processing information?
- 2. It acts like a reward (carrot dangling from stick)?
What is the hierarchy in visual processing areas?
- LGN (small dots)
- V1 (orientation, disparity, some colour)
- V4 (colour, basic 2D & 3D shape, curvature)
- VTC (complex features & objects)
What might damage in V1 cause?
How long is each delay from the retina to the LGN, V1, & IT?
- LGN: 20 ms
- V1: 50 ms
- IT: ~80 ms