Perception & Cognition 1

• 1. Rods
• 2. Cones

The “hole” in the retina where the optic nerve leaves the eyeball & there is no photoreceptors

Rods & Cones

The conversion from 1 form of energy (eg, light), to another (eg, electricity)

A light-sensitive receptor in the retina

Night vision

• 1. Daylight vision
• 2. Fine visual acuity
• 3. Colour

Photopigment molecules

Neurons in the retina that capture light & initiate the act of seeing by producing chemical signals

The light-catching part of the visual pigments of the retina

The distance between the retinal image & the fovea

Dim (scotopic) illumination

Bright (photopic) illumination

• High acuity
• Low sensitivity

• Low acuity
• High sensitivity

They cannot signal differences in colour

• 1. Short wavelength-sensitive cones (Blue)
• 2. Medium wavelength-sensitive cones (Green)
• 3. Long wavelength-sensitive cones (Red)

Specialised retinal cells that contact both photoreceptor & bipolar cells

• 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.

• 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

• 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.

Bipolar retinal cells whose processes are spread out to receive input from multiple peripheral cones.

Small cone bipolar cells in the central retina (fovea), that receive input from a single cone.

Running perpendicular to the photoreceptors, making contact between nearby photoreceptors.

• 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

• 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.

• 1. Photoreceptors
• 2. Bipolar cells
• 3. Ganglion cells

• 1. Horizontal cells
• 2. Amacrine cells

High sensitivity to light, but low acuity

High acuity, but low sensitivity to light

• 1. ON bipolar cell
• 2. OFF bipolar cell

Bipolar cells that respond to an increase in light captured in the cone

Bipolar cells that respond to an decrease in light captured in the cone

They receive visual information from photoreceptors via 2 intermediate neuron types. They process it & then transmit it to the brain & midbrain.

• 1. Bipolar cells
• 2. Amacrine cells

It provides information about the direction of the change in illumination

Small ganglion cells that feed the parvocellular (‘small cell’) layer of 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.

P cells constitute about 70%

Ganglion cells that feed the magnocellular (‘large-cell’) layer of the LGN

About 8-10% of ganglion cells in the human retina are the M variety.

• 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

• 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).

• 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

• 1. ON-centre cell
• 2. OFF-centre cell

• The small circular area in the centre responds to an increase in
• illumination & a surrounding annulus (ring) responds to a decrease in
• illumination.
• 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

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.

• The difference in luminance between an object & the background, or
• between lighter & darker parts of the same object

Total amount of light reflected from a surface

The light falling on a surface

The proportion of incoming light that a surface reflects

• Because each ganglion cell responds best to spots of a particular size
• & will respond less to spots that are bigger or smaller than this

• 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)

• Highlights important information such as contrast, & discounts less
• useful information such as ambient light intensity

The illusory bright & dark stripes that our visual system perceives from the luminance ramp edge of the pattern

• 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)

• 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

Because P cells provide information to the brain mainly about the contrast in the retinal image.

Because M cells signal information to the brain about how the image changes over time

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

The rods

• 1. Problems with peripheral vision
• 2. Problems seeing in low-light conditions

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

Cone bipolar cells synapse directly with ganglion cells

1 foveal cone connects to 2 bipolar cells - 1 ON & 1 OFF midge bipolar cells

100 million, 10 million & 1.25 million, respectively

Spatial resolution

providing inhibition of information in adjacent areas of the space (lateral inhibition)

White surround does

estimates are 40-60%

A modular, parallel & hierachical type of processing

Each photoreceptor is generating electrical current that is proportional to the intensity of the (reflected) light that is falling on it

Highlighting the difference between intensity-based representation & surface-based representation that our visual system has in relation to physically identical images

• 6 layers
• - 3 dark layers of cell bodies
• - 3 light layers of axons & dendrites

• Input Layer, or Input Processing
• (this is where the absorption of electromagnetic energy that enters the eye, takes place)

Middle processing layer

The Output Layer/Output of the Retina

• 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)

The layer of the ganglion cells - the outer layer

• Processing of motion
• + fast conduction rate
• colour insensitive
• prefer transient stimulation

Retinal changes its shape & position & starts a cascade of chemical reaction which makes Opsin [a large protein] absorb electromagnetic energy

In the periphery, there are no rods in the fovea

In the fovea, there are very few cones in the periphery

The visual pigment bleaches (after 10-30mins of exposure)

• Cone pigment regenerates in 6 minutes
• Rod pigment takes over 30 minutes to regenerate

Curve following the time-line of visual pigment regeneration & illumination sensitivity at different points

Primarily the cones

By neurons in a large part of the striate cortex

A smaller part of the striate cortex

A visual distortion of objects in periphery vision

That visual acuity declines in an orderly fashion with eccentricity (distance from the fovea)

• chromatic vision
• +slow conduction rate
• colour sensitive
• prefer sustained stimulation

• They are pattern analysers, implemented by an ensemble of cortical neurons.
• (Each set of neurons is tuned to a limited range of spatial frequencies)

A function describing how the sensitivity to contrast depends on the spatial frequency (size) of the stimulus

A measure of how often a repeating structure (such as a vertical line or bar) appears within a given unit of distance

• Yes, they do.
• They also have ON-centre & OFF-centre cells

At the occipital pole

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

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

• 1. scene segmentation
• 2. signal edibility of food
• 3. social signals (emotions, health)

300 (from 400-700nm)

Surface reflectance X ambient illumination = reflected light

• Brightness is the perceived luminance
• It varies both with changes in illumination & reflectance

• Lightness is the perceived reflectance
• It varies only with changes in reflectance
• (fairly independent of illumination changes)

Because the perceived reflectance (lightness) of the white paper remains the same despite changes in illumination

The ability of a vision system to assign a colour description to an object that does not depend on the illumination environment

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.

Different mixtures of wavelengths that look identical.

• 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.

• 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

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.

• 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.

• 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)

• (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)

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

• Motion blindness (people see the world as if through a strobe light)
• It signals V5 damage

• 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

Where people cannot associate visually-presented objects with their semantic meaning, or organize objects into semantic categories.

The inability to perceive faces

Using stored 2-D viewpoints from different perspective (view invariance does not occur for novel objects)

By decomposing 3D objects into 3D volumetric features that can be combined for a given shape

That object recognition involves matching the 3D model representation constructed from a visual stimulus, against a catalogue of 3D model representations stored in memory.

• 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

• (Occlusion)
• Amodal perception describes the perception of the whole of a physical structure, when only parts of it are actually visible to the sensory receptors

• (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

• 1. Region segmentation
• 2. Grouping
• 3. Parsing

Dividing a single objects (element) into parts

The visual system divides objects into parts where they have abrupt changes in surface orientation toward the interior of the object

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

• 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)

Demonstrates visual system’s decisiveness in resolving ambiguity because it seems impossible to perceive both interpretations at the same time

• A viewing position that produces some regularity in the visual
• image that is not present in the world.

That the lines of low curvature represent redundant information

Determining what part of environment is figure & what part is the background

• 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)

• 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

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)

Objects are perceived based on what is most likely to have caused the pattern.

Quantitative relationship between physical stimuli & sensation & perception

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

• 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.

• Good for how, & how fast information is processed
• Not good for where in the brain this information is processed

• 1. Micro-electrodes in the brain
• 2. EEG
• 3. MEG
• 4. fMRI

Electrical signals from outside

Magnetic fields

Magnetic fields are less distorted than electric fields by the skull, & therefore provide greater spatial precision.

Change in oxygen in the blood flow (BOLD – “blood oxygen level dependency”). Can only measure at the speed of blood flow

Bad temporal resolution

• 3-dimensional units of fMRI
• (little volume elements that make up the 3D fMRI picture, representing a part of the brain)

Using (brain) activity to predict information about the stimuli

• 1. Short-term visual memory
• 2. Mental imagery
• 3. Conscious awareness
• 4. Decision making
• 5. Detect familiar environments

• 1. Lesion studies
• 2. Directly stimulating neurons
• 3. Cortical cooling techniques
• 4. TMS (transcranial magnetic stimulation)

They are often invasive & non-reversible

• TMS is:
• 1. Causative
• 2. Non-invasive
• 3. Precise
• 4. Reversible
• 5. Safe
• 6. Repeatable
• 7. Relatively painless

A method of causation using electromagnetic induction to induce weak electrical currents, causing activity in specific or general parts of the brain

• 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)

• 1. Event related (typically single pulse)
• 2. Block design (typically rTMS)

Seizure induction

Because the sun & the moon have the same visual angles (retinal size)

• Properties of an image that seldom occur by accident within visual scenes:
• 1. Smooth continuation
• 2. Co-termination (eg, arrow)
• 3. Parallelism
• 4. Symmetry

The illusory interpretation of continuous contours in an image, as continuous contours in the 3D environment.

• 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)

Monocular (those cues occurring in the individual eye) & binocular depth cues (both eyes together)

• 1. Occlusion
• 2. Size
• 3. Perspective (geometric, texture, aerial)
• 4. Shading
• 5. Motion parallax

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

• 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)

• The linear perspective
• (convergence of lines that results in perceived depth in a 2D scene)

The horizon moon is surrounded by depth cues, while the moon higher in the sky has none (perceived as a flattened bowl).

Most objects have a textured surface. When elements of the pattern consistently becoming smaller, denser & fainter, then they appear to recede (seem further away)

Objects in the distance appearing less clear, more blurred & lower contrast (because of atmosphere & pollution)

• 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

• Ocular accommodation is focusing
• Good at close range (< 2.2m)

The angle of the eye

The brain taking the 2 separate images from the eyes & fusing them to create experience of 3D depth

• When an object does not project to corresponding regions of the retina
• Fall on outside - crossed disparity
• Fall on insides - uncrossed disparity

Theoretical & actual line where things fall on the same part of the retina (depends on the point of fixation)

• General area on horopter when things appear in depth
• (Outside Panum's fusional area, physiological diplopia occurs)

From the ability to create 3D vision using simple stereograms

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

Given by the difference in retinal co-ordinates between the left & right eyes

• Shorter interpupillary distance = smaller disparity
• Longer IPD = larger disparity

• 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)

• Double vision
• Can occur if the disparity is too big

A phenomenon of visual perception in which perception alternates between different images presented to each eye

Vision will fade - if we don't move our eyes we can't see

• area MT
• (stimulating MT with TMS causes motion perception)

Creating the perception of motion not by changing the object but by changing the position of the shadow (ie, flying, moving diagonally)

A motion detector tuned to a certain speed and direction of movement (ultra-simple version of what's in our brain)

• 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)

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])

• 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)

• 1. Enhanced perception of large moving objects
• 2. Impaired figure-ground discrimination

Moving the eyes to foveate the item to be attended

Attending to items in the periphery without moving the eyes

• Enhanced sensitivity at attended location
• Diminished sensitivity at unattended location

• From Salient stimuli
• It is environmentally relevant, salient, unexpected (pop out)

• Derived from task demands
• It is volitional (conscious), controlled, relevant for more 'complex' reasons

• 1. Imagery is pictorial, like perception
• 2. Mental imagery is not composed of 'images' but mental descriptions of concepts or ideas

Visual imagery is more like perception (eg pictorial) & it overlaps with perception

Integrating awareness from moment to moment

A type of short term visual memory

Active process that can store visual info for times over 20 seconds

No fixed number (but it is a limited resource)

• Both lead to activity in similar visual areas
• Both use memory & visual representations

That people who have stronger imagery are better at working memory tasks

• The qualitative feeling of an experience
• (eg, the redness of red, the wetness of water)

• 1. It is the most efficient way of processing information?
• 2. It acts like a reward (carrot dangling from stick)?

• LGN (small dots)
• V1 (orientation, disparity, some colour)
• V4 (colour, basic 2D & 3D shape, curvature)
• VTC (complex features & objects)

Blindsight

• LGN: 20 ms
• V1: 50 ms
• IT: ~80 ms