Perception Revised - Lecture notes 1-13 PDF

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Lecture OnePerception● Physical stimuli are converted into nerve impulses by sense organs ● Electrical signals enters the brain and allow individuals to experience a representation of the world ● It mediates learningTypes of psychological disorders and malfunctioningNeuropsychological disorders ● Ap...

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Lecture One

Perception ● ● ●

Physical stimuli are converted into nerve impulses by sense organs Electrical signals enters the brain and allow individuals to experience a representation of the world It mediates learning

Types of psychological disorders and malfunctioning Neuropsychological disorders ● Apperceptive agnosia is the inability to recognise an object ● Associative agnosia is the inability to name an object ○ It can occur within other senses ● Phantom limbs and pain is a sensation that occurs in a part of a body that doesn’t exist ○ The rubber hand illusion Physiological Principles Transduction ● It is the first stage of sensory processing ● Energy is converted into neural signals by receptors ● Electrical impulses travel to terminals which releases neurotransmitters via the axon and synapses to another cell ● Excitatory neurotransmitters are more likely to cause the receiving neuron to generate an impulse while inhibitory neurotransmitters are less likely to cause receiving neurons to generate an impulse Hierarchical processing ● Neural impulses travel ‘up’ to the cortex ● The thalamus is a ‘relay station’ ○ This is not applicable to olfaction ● High cortical areas involves lateral and feedback connections ● Bottom-up processing is the flow of information from sensory receptors to cortical areas ● Top-down processing involves prior knowledge that influences what is perceived ● Both processes can work together simultaneously Selectivity ● A stimuli can vary along various dimensions ○ E.g. location, length and orientation ● Cells are selective and responsive for stimuli with certain characteristics

Organisation ● There is often an orderly stimuli preference progression within the sensory brain regions ● Cortical magnification is when large areas of the cortex processes important senses Doctrine of Specific Nerve Energies ● Each sense is projected to different cortical areas ● Sensations are dependent on which sensory neurons are stimulated ● Many cortical areas are devoted to senses Plasticity ● Neural mechanisms can develop and adapt to changes Noise ● Neural firing can fluctuate and are influenced by stimuli and other factors ● Cells with barely any stimuli can still have spontaneous activity ● It can be caused by random opening and closing of ion channels Perceptual Principles Detectability ● Individuals are more likely to detect intense stimuli ● The absolute threshold (detection threshold) is the minimum intensity required to detect a stimulus ● Sensitivity is the opposite of the absolute threshold Psychophysics: measuring sensory magnitude ● The magnitude estimation technique measure is when a modulus stimulus is allocated a number and other stimuli intensity numbers are determined by comparing them to the modulus stimulus ● Stimuli magnitudes are all a non-linear compressive function ○ E.g. if the intensity is doubled, the sensation is less than double Weber and Fechner ● They laid the foundations for experimental psychology ● The psychophysical linking hypothesis is that neural responses are linked to and underlie perceptual responses Weber’s Law ● “Just noticeable difference”(JND) is the smallest difference between a “standard” weight and a “comparison” weight that can be reliably detected Fechner’s Law ● It is a sequence of equally spaced sensory increases with stimuli that corresponds to JNDs ○ It is not equally spaced in terms of stimulus intensity

Lecture 5

Light Visual stimuli ● It consists of electromagnetic radiation ● Light is measured in nanometres ● Rays travel in straight lines at constant high speeds ● Light particles are known as photons ● “Quanta” is a discrete quantity of energy ○ A quanta is equivalent to a photon Light intensity ● It is measured in luminance on an electromagnetic spectrum ○ Candelas per square metre ● When light falls onto a subject it is known as an incident light Contrast ● Different objects reflect different percentages of incident light ● When max luminance and minimum luminance is equal there is no contrast ● It is important for visibility since luminance does not always make things visible Focusing ● The cornea has a greater refractive power than the lens ● It occurs when multiple rays are combined to form a single point on an image surface Focusing errors ● Emmetropia is when the eye is appropriately focus ● Myopia is short sightedness which can be corrected by concave corrective lenses ● Hyperopia is long sightedness and can be corrected by convex corrective lenses ● Presbyopia occurs from old age as the eye becomes unable to accommodate to changes ● Astigmatism is when there are different focal points in the eye due to correcting lenses or the shape of the eye Eye processes Transduction ● Photoreceptors pass electrical impulses through the bipolar, amacrine and horizontal cells to the ganglion cells ● The optic nerve is a bundle of ganglion cells that exit the eye

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The blindspot has no photoreceptors and is where the optic nerve is located

The retina ● It consists of the fovea (macula) which has many photoreceptors, but no blood vessels ● The optic disc is the blindspot where ganglion cell axons leave the eye through the optic nerve Ganglion cell selectivity ● It is when a ganglion cell has a receptive field made up of many photoreceptors ● The fovea has smaller and densely packed receptive fields, but larger and sparse receptive fields in the periphery of the fovea ● Cortical magnification is when a large area of the cortex is used for processing foveal vision ● Crossover occurs at the optic chiasm The pathway from the eye to the brain ● Retinal ganglion cell axons terminate in the Lateral Geniculate Nucleus (LGN) ● The LGN projects information through optic radiations to the primary visual cortex (V1 or the striate cortex) Extrastriate cortical visual areas ● They are all retinotopic besides the MST ● Cells closer to V1 have closer retinal receptive fields, but have less selectivity compared to areas further away from the V1 ● The V1 projects information to the V2-V5 and MST ● V4 is most responsive to colour changes ● V5 (MT or middle temporal area) is most responsive to simple motion ● MST (medial superior temporal area) is most responsive to complex motion Lecture 6

Centre-surround antagonism ● ● ●

Multiple light sources shining onto a ganglion cell or no light sources shining onto a ganglion cell causes inhibition and excitation to occur It helps individuals identify changes, edges and boundaries within an image or of an object It is sensitive to contrast so higher contrast causes more cell activity

ON-centre ganglion cells ● Light on the inner portion of the eye causes an increase in ganglion cell activity, while light on the outer portion has decreased ganglion cell activity

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It tells us how bright an area is

OFF-centre ganglion cells ● It is the opposite of the ON-centre ganglion cells ● It tells us how dark an area is ● It ensures that dark spots are as easily detected as light spots, since ganglion cells have difficulty performing activities below the spontaneous firing rate Visual illusions Simultaneous lightness contrast

The hermann grid ● The intersection of on-centre ganglion cells gain more light on surrounding regions, thus causes the inhibition of cell activities ● The grey patches are more pronounced in peripheral vision due to the sparse spread of receptive fields Illusory contours ● Hypercomplex cells can detect illusory edges as well as subthreshold edges ○ V2 cells can respond similarly The orientation aftereffect: tilt aftereffect ● Adaptation causes neural activities to become altered as cells are recalibrated ○ There is a decrease in sensitivity and firing rates ● Population coding explains how orientation is normally encoded when selective cells of similar orientation preferences are arranged in a column ● Size aftereffect has the same process Lateral geniculate nucleus (LGN) Properties of the LGN ● It has centre-surround antagonism ● It is on both sides of the brain and consists of 6 layers which are retinotopically organised ● LGN cells are monocular as both eyes have separate inputs to the LGN layers ● The ipsilateral eye transfers its inputs to layers 2, 3 and 5 while the contralateral eye transfers its input to layers 1, 4 and 6

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The ipsilateral eye is the eye on the same side as the stimulus while the contralateral eye is the eye on the opposite side of the stimulus Input from the left side of both eyes get processed in the right LGN and vice versa

Magnocellular cells ● They consist of large receptive fields ● They have high sensitivity, but low resolution ● They are specialised to process motion signals Parvocellular cells ● They consist of small receptive fields and has low sensitivity ● They are specialised to process colour distinctions The primary visual cortex (V1) ● ● ●

It is retinotopic There is cell orientation selectivity in the V1 Ocular dominance consists of a graded location system ranging from numbers 1-7

Visual filters ● Neural signals are tuned specifically for certain stimuli while irrelevant stimuli are filtered out ● It consists of orientation, size and colour filters V1 Organisation: Ice cube model ● Cells within a column have the same preferred orientation ● There are columns of ocular dominance ● The hypercolumn is a small area of visual space that consists of 180 degrees of orientation from both eyes Simple cells ● They respond to oriented stimuli in particular locations within their receptive field ● They are bar or edge detectors ● They are made up of ganglion cells Complex cells ● They respond to stimuli anywhere within their receptive field ● They are built from several simple cells with the same orientation preference, but different edge locations Hypercomplex cells: end-stopped cells ● They prefer stimuli of a particular size anywhere within their receptive field ○ Firing will decrease if the stimuli exceeds the cell’s capacity

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They are built from several complex cells connected together

Gratings and spatial frequency ● ●

Gratings are measured in cycles per degree of visual angle Thin gratings have low high spatial frequency while thick gratings have low spatial frequency

Lecture 7

Light Newton’s prism experiments ● A white light that passes through a prism can split into a spectrum of different lights that appears coloured due to refraction ○ It can be reversed to converge colours into white again ○ It cannot decompose any further The electromagnetic spectrum ● Newton observed the visible spectrum from the broader electromagnetic spectrum ● The visible spectrum ranges from 400-700nm (nanometres) and is the only visible spectrum that appears in different colours Wavelength (λ) ● Crests are the highest point in a wave ● λ is the distance between crests ● Intensity is the height of a wave and it judges the brightness of a light ● λ is an objective property of a stimulus ● Colour is a subjective property of perception because everyone views colours differently Types of λ ● Short λ are purple/blue ● Medium λ are green ● Long λ are red

Colour misconception ● Newton divided visible light into different categories but the light spectrum is a continuum and should not be divided ● There is rarely a single pure λ in nature Light reflection ● Objects can reflect or absorb light ● The colour reflected off objects is what we see the object colour as ● Objects absorb different λ depending on its colour ● E.g. If an object is yellow, all colours are absorbed besides yellow

Additive colour mixtures ● Various pure λ of light can be added together to form new colours however the λ of light has not changed ● Putting blue and yellow plastic together can form green and they can still be separated from one another Subtractive process ● Mixing paint together is a subtractive process ● Most paints reflect more than one pure λ of light ● The mixture of more colours causes the paint to absorb more light and appear to be darker ● ● ●

Blue paint will reflect short λ and other neighbouring λ Yellow paint will reflect yellow light and also green and orange Both paint combined will only reflect green Photoreceptors Rods ○ There is only one type ○ It is not in the central fovea ○ Highly sensitive ○ It is used for scotopic (night) vision

Cones ○ There are different types each tuned to different λs ○ It is located in the fovea ○ Low sensitivity ○ It is used for photopic (day) vision

Colour vision using only one type of receptor (One receptor system) ●

Receptors have broad λ tunings

The principle of univariance ● A receptor’s activity is related to the number of photons captured and not the type of λ of the photon ○ E.g. 450nm and 650 nm at 160cd/m² have the same response ● Any single receptor can only discriminate brightness levels ○ It is one dimensional Photopic colour vision ● Humans can reliably discriminate around 200 wavelengths using their cones A two receptor system ● ●

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Colour is represented by the relative activation of 2 cone channels The pattern of relative activation changes as λ changes ○ Short λs causes more activity in M cones than in L cones ○ Medium λs causes equal activity in both cones ○ Long λs causes more activity in L cones than in M cones Intensity changes will cause both cones to equally change and relative activation will remain the same When a single λ appears “grey” it is known as a neutral point ○ The firing of both cone channels are equal ○ It does not appear grey for most humans

Metamers ● It is when 2 different stimuli are different from one another but appear to be similar or identical with one another Young-Helmholtz trichromatic theory of colour vision ● ● ●

Colour matching experiments explain the three receptor system ○ Young (1802) The theory was developed many decades before neurophysiologists discovered and measured the 3 cone types Perceived colour depends on the relative strength of different cone activations

Colour misconception of primary colours ● When mixing colours, any set of 3 different λs can be mixed to match any λ of light ● Primary colours would be considered as any set of 3 different λs ● The 3 different λs would be short, medium and long Trichromatic metamers ● Colour is encoded by the relative activation of each channel ○ It is similar to population coding ● The colour yellow would be similar to red and green mixed

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Colour deficiency Anomalous Trichromats ● Individuals are deficient in one cone type but still have 3 types of cones ● Protanomaly individuals are deficient in red colours (long cones) ● Deuteranomaly individuals are deficient in green colours (medium cones) ● Tritanomaly individuals are deficient in blue colours (short cones) Dichromats ● Individuals lack one cone entirely ● Protanopia/Protanopes suffer from long cones (red) ● Deuteranopia/Deuteranopes suffer from medium cones (green) ● Tritanopia/Tritanopes suffer from short cones (blue) Colour blindness Monochromats ● Cone monochromats only have one type of cone and lack colour vision for daytime and nighttime ● Rod monochromats have no cones at all and are highly sensitive to light ○ They need dark glasses to be able to see things in daylight Cerebral Achromatopsia ● Individuals have their V4 area damaged ○ Sometimes this area is not completely damaged and allows individuals to see a bit of colour Colour Opponency: Hering (1872) Observation ● Individuals selected 4 “pure” colours over 3 ● People tend to describe the world using 4 colours: ○ Reddish-blue or yellowish-green, but never reddish-green or bluish-yellow Inference ● There are two different colour axes (two dimensions of colour): ○ Red-green and blue-yellow ● A full colour space would include a third dimension/axes:

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black-white

The dual process theory: Hurvich and Jameson (1957) ● ● ● ● ●

It is a cortical process Trichromatic receptors feed into an opponent process Luminance is determined by red(long) + green(medium) Red-green is determined by red(long) ÷ green(medium) Yellow-blue is determined by [red(long) + green(medium)] ÷ blue(short)

Physiology of opponent processing ● The LGN and V1 have cells with antagonistic colour outputs ● Double opponent cells have a centre-surround arrangement ○ They process colour and colours across space The location of the types of cones within the LGN ● Red/green cells are located in the parvocellular layer ● Blue/yellow cells are located within the koniocellular layer ● Luminance cells are located in the magnocellular layer ●

Opponent colour cells are located in the hypercolumns (“blobs”) of V1

Colour constancy ●

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Cells in area V4 exhibit colour constancy ○ Comparing V1 and V4 in the Mondrian pattern demonstrates this Under a variety of lighting conditions, objects would generally be the same colour

Land (1977) ● Isolated coloured patches appear to be a different colour when lit by different λs ● Patches in a Mondrian pattern appear to be the same colour regardless of the illumination The dress ● The visual system needs to be able to estimate the spectral content of the light source to be able to eliminate it ● Problems occur when the light source is unclear

Lecture 8

Motion ● ● ●

It is the displacement of an object overtime It is measured by using: speed = distance/time Motion is sensed directly by specialised mechanisms by adaptation experiments

Reichardt detector ●

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A neighbouring receptor is delayed by a fixed time to arrive at the “AND” unit simultaneously with the receptor ○ The “AND” unit is only fired when both receptors spikes simultaneously No firings would occur for motion in the opposite direction ○ Direction selective

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Motion detectors for opposite directions can be combined by using the same two receptors in either directions

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It can account for stroboscopic motion

Stroboscopic (Apparent) motion ● Motion on TV is illusory as it is created by fast moving shots of images which imitates motion ● Apparent motion stimulates motion detectors in the same way as real motion ○ Reichardt detectors do not care about what happens in between two receptors The wagon wheel effect ● A wheel moving at 90 degree displacements would look identical if it had 4 spokes ● A wheel moving at 45 degree displacements would make it hard to tell whether it is moving clockwise or anti-clockwise ● A wheel moving at 70 degree or 20 degree displacements would look like the wheel is turning anti-clockwise Reason ● Humans tend to perceive motion that correspond to shorter displacements, hence fast motion can appear to be moving “backwards” Motion detectors ●

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The reichardt detector model does not account for simultaneous motion of objects that are identical and swap positions ○ Simultaneous flashes do not signal motion A comparator unit is used to compare responses from each direction-selective motion

detecting unit (“AND” cell) ○ The larger directional signal will be noticeable to individuals to identify the direction of motion ○ Equal directional signal will cause the comparator unit to signal no motion ●

Other popular models that accounts for motion detection all have a type of comparator ○ They suggest the same predictions, but have different processes

Ratio models ● It is any motion system that compares opposite direction responses The motion aftereffect (MAE) ●

It is an example of adaptation as it can cause a biased perception ○ Often motion looks as if it is moving in the opposite direction

Stationary stimulus before ● A reichardt detector with a comparator can respond to ‘updown’ motions when rotated ● The comparator registers no differences between ‘up’ and ‘down’ units...