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?
- 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
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)
+slow conduction rate
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?
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
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
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)
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)
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)
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?
3. Perspective (geometric, texture, aerial)
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