PSYB51 MIDTERM # 1 CHAPTER 1 Perception + reality are products of evolution

- Survival/importance of type of energy in the environment determines which senses have developed/may not see entire reality but we see what is important to our survival

Plato - Reality/perception is dependant on info gathered by our senses - understanding of reality is restricted to things we can perceive - forgot aspect of learning (added by Heraclitus) - truest sense of reality comes from people’s minds and souls (not

from body) - Nativism= idea that mind produces ideas that are not derived from

external source knowledge is born with you Descartes

- Dualist view of the world both mind and body exist - mind-body dualism= the idea positing the existence of 2 distinct

principles of being in the universe: spirit/soul and matter/body Heraclitus

- Can never step into the same river twice “panta rhei”= everything flows

- Idea that perceiver cannot perceive the same thing twice in exactly the same manner experience/learning adaptation alters perception change (we pick out things at change and adapt to things that are constant)

Democritus - World is made up of colliding atoms sensations caused by atoms

leaving object and interacting with our sense organs perception= result of physical interaction between world and our bodies

- *true until today sensory transducers= physical energy to neural energy (hair in cochlea, taste-buds, retina in eyes)

Hobbes - Believed that everything that could ever be known/imagined had to

be learned through our senses Locke

- Sought to explain how all thoughts, even complex ones, could be constructed from experience with a collection of sensations

Fechner - Invented PSYCHOPHYSICS and theory of PANPSYCHISM, thought to be

the true founder of experimental psychology - Followed up Weber’s work on Weber’s laws/fractions

Adaptation: (Heraclitus) a reduction in response caused by prior or continuing stimulation

- Ex. Looking at male/female faces and a hybrid, you perceive the hybrid as the opposite of what you were primed with because you pick out change= OPPOSING ORGANIZATIONS

- Ex. Primed with rat, you see a rat/ primed with man, you see a man due to learning

Monism (opposite of dualism/Descartes)= mind and matter are formed from/reduced to a single ultimate substance or principal of being just mind or just matter

- Materialism= type of monism=physical matter is the only reality and everything including the mind can be explained in terms of matter and physical phenomena

Empiricism (opposite of Nativism/Plato)= the idea that experience form the senses is the only source of knowledge not born with knowledge, learn from your senses

Reality= in btwn Nativism (Plato) and Empiricism born with and gradually learn knowledge

Panpsychism (Fechner)= idea that all matter has consciousness Psychophysics (Fechner)= science of defining quantitative relationships

btwn physical and psychological (subjective) events Weber’s Law= smallest CHANGE in a stimulus, such as weight of an object,

that can be detected is a constant proportion of the stimulus level - (delta)I= kI - delta I= JND/difference threshold, k= Weber fraction, I=stimulus level - JND= smallest detectable difference btwn 2 stimuli, minimum change

in stimuli that can be correctly judged from the reference stimuli two-point threshold: minimum distance at which 2 stimuli can be distinguished (different depending on size of somatosensory cortex, Ex. Finger/shoulder)

Fechner’s Law: principle describing the relationship btwn stimulus magnitude and resulting sensation magnitude such that the magnitude of subjective sensation increases proportionally to the logarithm of the stimulus intensity (S increases less quickly than R)

- S= k log R - S= psychological sensation, k=constant, R=physical stimulus level

Stevens’ Law: principle describing the relationship between stimlus magnitude and resulting sensation magnitude such that magnitude of subjective sensation is proportional to stimulus magnitude raised to an exponent

- Expansion of Fechner’s law as there were some exceptions (Ex. Electric shock)

- S=aI^b - S=sensation, I= stimulus intensity, b=exponent, a= constant - Used magnitude estimation to formulate graph - Exponent of 1 sensation and stimulus intensity rise proportionally,

exponent of <1 S rises slower than I, exponent of >1S increases faster than I (exception to Fechner’s Law)

Absolute threshold= minimum amount of stimulation necessary for a person to detect a stimulus 50% of the time (Ex. Absolute threshold for emotional experience when do u first detect a smile? Whereas JND would be how different do 2 faces have to be in order to notice the difference)

PSYCHOPHYSICAL METHODS 1. Method of constant stimuli (Fechner)

- Many stimuli from rarely to almost always perceivable (or rarely to almost always perceivably different) are presented one at a time. Responses= y/n or same/different. Stimulus levels and responses are graphed and the intensity of the stimulus where it was perceived 50% of the time= absolute threshold.

2. Method of limits (Fechner) - Particular dimension of a stimulus/ difference between two stimuli is

varied incrementally until the participant responds differently repeated trials, absolute threshold is line where 50% of the time was responded yes/no

3. Method of adjustment (Fechner) - Method of limits in which subject controls change in stimulus - *decreasing accuracy from 1-3 but increasing efficiency

4. Magnitude estimation - Participants are asked to assign values according to perceived

magnitudes of stimuli (better if participant decides scale as well) 5. Cross-modality matching

- Similar to magnitude estimation= matching intensities/magnitudes of sensations that come from different modalities

Signal Detection Theory: A psychophysical theory (not method) that

quantifies the response of an observer to the presentation of a signal in the presence of noise

- Calculate d’ (measures of sensitivity) based on hit/false alarm rates - Interprets the importance of internal noise vs. external noise - Influenced by biases (costs of decisions, probability of events,

personality) - For a fixed d’= all you can do is change your pattern of errors by

shifting the response criterion (shift to left= no misses, but increased # of false alarms and vice versa) changing response criterion of a fixed d’= changes hits/false alarms in predictable way

Receiver Operating Characteristic (ROC) curve= in studies of signal detection, the graphical plot of the hit rate as a function of the false alarm rate hits= y axis, false alarm=x axis if hit/false alarm rate is the same= diagonal which means observer can’t tell difference btwn presence and absence of stimuli as the observer sensitivity increases, the curve bows upwards towards the upper left corner (d’ increases).

- Upper left corner= perfect ability to distinguish signal from noise Fourier Analysis= analyses that allow understanding about how complex

sounds (music/speech), motions, images etc. can be decomposed into a set of simpler signals

- Images= broken down into components (spatial frequencies) that capture how often changes from light to dark occur over a particular region in space

- spatial frequency= # of cycles of a grating per unit of visual space (usually specified in cycles per degree= # of pairs of light/dark bars per degree (size of a thumbnail at arms length) of visual angle

- find that many individual neurons have strong preferences for frequency components

MULLER

- Doctrine of specific nerve energies= states nature of a sensation depends not on HOW a nerve is stimulated but on WHICH sensory fibers are stimulated (Ex. Pressing on closed eye produces visual image even without light)

- Not limited to cranial nerves= Ex. Capsaicin= chemical causes warmth fibers in skin to fire creating sense of heat despite stable temperature/ opposite for menthol= both can cause pain sensation if in high quanitity

- Believed we are only aware of the activity of our nerves and we cannot be directly aware of the world itself

- Vitalism- belief that special vital forces drive living organisms HELMHOLTZ

- Didn’t agree with vitalism thought brain + behavior should follow/ be explained by physical forces (vitalism violated law of conservation of energy)

- Showed that activity of neurons obeys normal rules of physics/chemistry

- First to effectively show how fast neurons transmit their signals time passes btwn when u stub your toe and when you feel itnot all neurons transmit at equal speeds (challenges vitalism)

Cranial Nerves (12 pairs)

1. Olfactory Nerves (#1)- smell (olfactory epithelia in nose olfactory bulb 2. Optic Nerve (#2)- vision (ganglion axons of retinaV1 brain) 3. Occulomotor Nerve (#3), Trochlear Nerve (#4) and Abducens Nerve

(#6)= control muscles that move the eye 4. Vestibularcochlear/Auditory Nerve (#8)- connect inner ear with brain

Neurotransmitter- chemical substance used in neural communication at

synapes (previously believed neural communication was all electrical signales) psychoactive drugs work by inc/dec effectiveness of these

Hodgken-Huxley Cycle: electrochemical process involving Na+ moving into the neuron, and K+ moving out via voltage-gated channels causing neurons to fire in all or none fashion

Entire populations of neurons work in concert to process info Electroencephalography (EEG)= using electrodes on the scalp, measures

electrical activity from populations (not individ) of many neurons in the brain roughly localize whole populations of neurons, and to measure their activities with TEMPORAL accuracy single behavioral measurement isn’t very informative, so many measurements are made and averaged

Event-Related Potential (ERP)= measure of electrical activity from a

subpopulation of neurons in response to particular stimuli that is a resulted averaged waveform from may EEG recordings tells us about timing of nervous system responses to stimuli

Magnetoencephalography (MEG)= similar to EEG, measures changes in

magnetic activity across populations of many neurons in the brain maintains good measure of timing with better localization more expensive

Computed tomography (CT)= standard x-rays used to form 3D images Magnetic resonance imaging (MRI)= imaging technology that uses responses

of atoms to strong magnetic field to form images of structures deals with the presence of H (or water) to see water-rich tissue in your head

Functional magnetic resonance imaging(fMRI)= most important MRI for

study of senses shows activity of living brain through BOLD (blood-oxygen level dependant signal= ratio of oxygenated to deoxygenated blood) instead of looking for presence of water, looks for evidence for demand of more resources Drawback: takes a few seconds for BOLD signal to rise after a bit of the brain becomes active so temporal resolution isn’t s good as EEG/ERP and is also loud BUT makes it possible to measure localized patterns of activity in the brain results superimposed on MRI of the brain

CHAPTER 2 Light= wave/stream of photons (tiny particles consisting of ONE quantum of

energy) wavelength of visible light= 400 to 700 nm Violet=400nm, Red=650 nm can sense heat for example from other wavelengths

Light can be: - Absorbed: taken up and transformed into other forms of energy (Ex.

Solar panel= light energy to electrical energy) - Transmitted= opposite of absorption= convey light from one place to

another through a transparent medium (through optic filter) - Scatter= to disperse light in an irregular fashion - Reflected= energy that is redirected when it strikes a surface= not

absorbed or transmitted= Purkinje reflections (light bounces of eyes in 4 reflections sclera + lens)= most light surfaces reflect majority of light which is what makes them look lightly colored

- Diffracted= Bent or having waves that are spread out (waves of sound or light as they encounter an obstacle/ pass through a narrow aperture)

- Refracted: energy whose course is altered as it passes into another medium (ex. Light entering water from air= difference in densities, rainbows) light is refracted in the cornea, lens, aqueous + vitreous fluids of our eyes

Sensing light is important for survival edible foods, view dangers/predators from a distance, social cues, circadian rhythms, trees to change color/adaptation mechanisms etc.

Evolution of the eye: eye spots with light sensitive protein opsin on flat surfaces with photoreceptors= simply region of photosensitive cells depressed/folded area forming cavity with photoreceptor allows more spatial sense (where does light hit and where it doesn’t) PINHOLE eye with water filled chamber allows finer directional sensitivity= smaller hole/ but

downside is very little light enters so low contrast, there is no refraction transparent humor develops in an enclosed chamber which is more hygienic as nothing can enter but light distinct lens and flat cornea develops so pinhole can be larger/smaller iris and cornea develop so the spherical cornea helps with refraction (more power than lens)

Summary: region of photosensitive cells cavity pinhole with water chamber enclosed chamber with transparent humor distinct lens and flat cornea iris and spherical cornea

EYE ANATOMY Cornea= transparent window into the eyeball most light photons are

transmitted through not absorbed or refleted but MAXIMUM refraction power no blood vessels because blood would absorb light rich supply of transparent nerve endings in case of damage form tears and close eye

Aqueous humor= fluid derived from blood, fills space immediately behind cornea and removes waste while supplying Ox and nutrients to cornea and lens watery fluid with refracting power

Crystalline lens= lens inside eye that enables changing of focus, connected to zonules of zinn and then the ciliary muscles ACCOMODATION: change in focus of lens altering its refractory power to focus light rays (ciliary muscles contract, zonules of zinn relax and lens bulges OR ciliary muscles relax, zonules of zinn pull and lens is flat) flatter lens allows for distant vision, blulged lens allows for closer/nearer vision

Pupil= dark circular opening at centre of iris where light enters the eye controls amount of light reaching the retina via pupillary light reflex

Iris= colored part of the eyes, muscle that expands and contracts the pupil Vitreous humor= transparent fluid (egg-white) that fills vitreous chamber in

posterior part of the eyes 80% internal volume of eye last refracting power

Retina= light-sensitive membrane in back of eye containing rod/cone photoreceptors fovea= depression in retina, surrounded by macula (dark spot in fundus view)= no blood vessels in fovea to prevent interference

*much light is lost in space/atmosphere because of absorption and scattering (1/2 light reaches from cornea to retina)

optic disk= where arteries and veins that feed the retina enter the eye, and where ganglion cells leave the eye via the optic nerve blind spot (different for different people) compensate because 2 eyes, fill in the gaps for each other

Problems with Refraction:

- Emmentropia= normal refraction refractive power=length of eye - Myopia= eyeball is too long for optical components image focused in

front of the retina nearsightedness (can’t see distant objects), negative lens

- Hyperopia= eyeball is too short for optical components (babies) image focused behind the reina farsightedness (can’t see near objects), positive lens

- Eyes generally grow to match the power of optical components

- Astigmatism= visual defect caused by unequal curving of one or more of the eyes refractive surfaces (usually cornea is not spherical)

5 classes of cells in the retina 1. Photoreceptors= neurons that capture light + initiate the act of seeing by

producing chemical signals (transducers) light-sensitive receptor photoactivation (activation by light) can pass GRADED potentials unlike other neurons (vary continuously in amplitude due to possible hyperpolerization which is when inner membrane surface is more electronegative than outer) VERTICAL PATHWAY duplex retina:

a. Cones (4-5 mil)= larger, more tightly packed in foveal centre (rod-free area) but density drops with retinal eccentricity (distance from fovea) 3 types of pigments: S (not found in fovea so fovea is essentially dichromatic), M, L differ in accepted wavelengths and therefore color visual acuity (low sensitivity and color processing PHOTOPIC VISION

b. Rods (90 mil.)= contain Rhodopsin pigment nighttime vision (no rods in fovea, so only peripheral vision at night, low acuity, high sensitivty) SCOTOPIC VISION

c. 3rd photoreceptor for circadian rhythm melanopsin (sensitive to ambient light)

d. *check graph for distribution of rods/cones e. *trade-off between acuity and sensitivity f. visual pigment in rods/cones= 2 parts, Protein opsin structure

determines wavelengths of accepted light and chromophore which captures light pigments made in internal segment but stored in membranous discs of outer segment

2. Horizontal Cells= perpendicular to photoreceptors, making contact with multiple nearby photoreceptors implicated in the lateral pathway: interacting via lateral inhibition (antagonistic neural interaction btwn adjacent regions of the retina)

3. Amacrine cells= part of lateral pathway, also perpendicular to photoreceptors receive input from bipolar cells and other amacrine cells and send signals to biopolar, amacrine and ganglion cells

4. Bipolar cells= VERTICAL PATHWAY vertical cells that synapse with either rods or cones (NOT BOTH) and pass the signal to ganglion cells cellular intermediaries

a. Diffuse bipolar cells: receive input from multiple photoreceptors (RODS + PERIPHERAL CONES)= CONVERGENCE increase sensitivity (see in dim light environments) but decrease acuity (so one strip of bright light is the same as multiple strips of dim light)

b. Midget bipolar cells: receive input from a single cone in the fovea (each cone connects to two bipolar cells)= DIVERGENCE

c. ON bipolar cells: respond to an increase in light captured by cones d. OFF bipolar cell: responds to decrease in light captured by cones e. DIVERGENCE each foveal cone connects to one ON bipolar cell and

one OFF bipolar cell

5. Ganglion cells= retinal cells that receive visual info from photoreceptors via either bipolar or amacrine intermediaries VERTICAL pathway final layer of retina pass info to brain axons gather to form part of the optic nerve can fire action potentials even in absence of visual stimulation receptive field= region in space/retina in particular for ganglion cells in which stimuli will activate a neuron (inhibitory or excitatory effect) spatial layout of retinal ganglion RF is COCENTRIC (off center/on center cell, fire most rapidly when size of spot of light matches the center exactly) function as filter (respond best to spots of a particular size and less to smaller/biggers ones) MOST sensitive to CONTRAST differences not to average intensity of light so view things the same despite lighting conditions Example of MACH BANDS= lateral interaction in the retina, neuron in middle transition has more surround antagonism so will appear darker or lighter

a. P ganglion cell= small ganglion cell that receives excitatory input from single midget bipolar cells in the central retina and feeds the parvocellular layer of the LGN (1 cone photoreceptor in fovea midget bipolar cell in central retinaP ganglion cell parvocellular layer of the LGN)= smaller RF, finer spatial resolution/acuity, sustained responses, sensitive to colour= 70% of ganglion cells sustained responses because signal contrast not change over time

b. M ganglion cell= a ganglion cell resembling an umbrella that receives excitatory input from diffuse bipolar cells and feeds the magnocellular layer of the LGN (rods/peripheral cones diffuse bipolar cell (convergence) M ganglion cell magnocellular layer of the LGN)= larger RF, coarser spatial resolution/acuity, transient responses, insensitive to colour= 8-10% of ganglion cells transient responses because signal info about how an image CHANGES over time, with a constant stimuli they return to spontaneous rate

c. Koniocellular cell= neuron located between magnocelluar and parvocellular layers of the LGN (koniocellular layer of LGN)

Dark/Light Adaptations 1. Pupil Dilation

- amount of light entering the eye is proportional to the area of the pupil, which is controlled by the iris muscles 4 fold change in pupil diameter (2 to 8mm) can result in a 16 fold improvement in sensitivity

2. Photoreceptors - Human’s duplex retinas Rods provide a lot of sensitivity at low light

levels, but become overwhelmed when background becomes moderately bright loss in info quality cones much less sensitive than rods but operating range is much larger (10 to 100 to 1000 photons per second) after adapting to bright light= cones recover sensitivity very quickly and then saturate after adapting to low light= rods recover slowly but after 30 mins are very sensitive to dim light

3. Photopigment Replacement

- Dim lighting condition= plently of photopigment available and rods and cones respond to as many photons as they can rod system is capable of detecting a single quantum of light!

- After a photopigment is used to detect a photon (bleached) it must regenerate before it can work again slow regeneration= means in less light all photons are used which increases sensitivity, but in high light some photons are thrown out and others are used

4. Neural Circuitry of the Retina - Accounts for why we are not bothered by variations in overall light

levels visual system regulates amount of light entering the eye by ignoring whatever variation in overall light level is left over cells will still fire at an above spontaneous rate when light falls on the entire receptive field as long as light is brighter on the ON portion than off portion as long as ganglion cells are not FULLY saturated= they will encode pattern of relative light/dark in retinal image pattern of illumination NOT overall light level= primary concern

5. SUMMARY of DARK ADAPTATION: how does visual system deal with variations in overall light level? First, reduce scale of problem by regulating amount of light entering eye (pupil dilation) then by using different photoreceptors in different situations (rods/cones) then by effectively throwing away photons we don’t need (photopigment bleaching) lastly by responding to contrast between adjacent neural regions= ganglions do their best to ignore variation in overall light level

Problems with Vision

- MAN WHO COULDN’T SEE THE STARS= Retinitis Pigmentosa: family of hereditary diseases that involve the progressive death of photoreceptors and degeneration of the pigment epithelium commonly rods affected first so problems with nighttime + peripheral vision eventually spreads to fovea leading to total blindness age of onset/rate of progression varies complex variations of the genetics but recently due to human genome mapping= identified some implicated genes view of fundus: conspicuous clumps in the retinitis pigmentosa fundus that have a characteristic shape= BONE SPICULES 40-60% OF VF is impaired

- Cataracts: opacity (loss of transparency of lens that are usually transparent because the crsytallins proteins that form is are packed very densely and regularly) of lens due to irregularities of the crystalins interfere with vision because they absorb/scatter more light than normal lens

- Heteronymous: different visual field defects for the two eyes (heteronymous= lesion of optic chiasm, tumour in the hypothalamus, both temporal sides of vf is blind)

- Homonymous: same defects for the two eyes (homonymous= lesion after optic chiasm, so same side of each vf is blind)

- Hemianopia: half of the visual field is blind - Quadrantanopia: a quarter of the visual field is blind - Scotoma: a small visual defect

- * lesions in lower visual areas cause (retinotopic) visual field defets often foveal part of the visual field is spared

- GIRL WHO ALMOST COULDN’T SEE STRIPES= cataracts (opacity of the lens) + strabismus (one eye is turned so it receives a view of the world from an abnormal angle, misalignment of 2 eyes so a single object in space is imaged on fovea of one eye and nonfoveal area of another) abnormal early visual experience resulting in permanent consequences (idea= critical period of early visual development when normal BINOCULAR visual stimulation is required for normal cortical dvelopment critical period= cortical neurons still learning, period of neural plasticity so abnormal visual experience can alter normal wiring process

- Congenital cataracts and strabismus (later onset and less severe than c cataracts) can result in amblyopia=developmental disorder characterized by decreased spatial vision in an otherwise healthy eye even with proper correction for refractive error

- Agnosia= object (prosopagnosia)= inability to recognize objects or faces in spite of the ability to see them

Problems with Refraction: - Myopia= eyeball is too long for optical components image focused in

front of the retina nearsightedness (can’t see distant objects), negative lens

- Hyperopia= eyeball is too short for optical components (babies) image focused behind the reina farsightedness (can’t see near objects), positive lens

- Eyes generally grow to match the power of optical components - Astigmatism= visual defect caused by unequal curving of one or more

of the eyes refractive surfaces (usually cornea is not spherical) -

CHAPTER 3 Frequency of visual gratings cycles per visual degree (cpd) other

characteristics of gratings: amplitude + phase Any image can be decomposed into sine wave gratings and other simple

forms! important cues for visual recognition Fourier transform= an operation that breaks down a function/image into

sine waves of different frequencies music can be decomposed into sine waves with different temporal frequencies BUT images can be decomposed into sine waves with different spatial frequencies

- Images= broken down into components (spatial frequencies) that capture how often changes from light to dark occur over a particular region in space

- spatial frequency= # of cycles of a grating per unit of visual space (usually specified in cycles per degree= # of pairs of light/dark bars per degree (size of a thumbnail at arms length) of visual angle

- visual angle= size an object takes up on the retina - (higher spatial frequency means finer details because more cycles per

degree)

- find that many individual neurons have strong preferences for frequency components

Due to statistics of natural images and given the plasticity of the visual system we should expect: certain specialization of the visual system for gratings and spatial frequencies AND filters specialized for edges/lines= natural scene statistics

Visual system codes in terms of oriented stripes limits= narrowness of the

stripes (visual acuity/resolution acuity= fundamental limit of spatial vision depending on spacing of our photoreceptors) acuity= smallest spatial detail that can be resolved measure acuity (opthamologists use distance but vision scientists use smallest visual angle of a cycle of a grating) minimum visual angle of a cycle= 0.017 degrees (1 minute of arc) and minimum spatial frequency= 1/0.017= 60 cycles per degreehigh contrast sine waves on a grey background (ideal condition) can be distinguished from the background as long as adjacent pairs of light/dark stripes are separated by at least 1 min of arc.

- Resolution limited by 2 reasons: - spacing of photoreceptors cones in fovea= 0.5 minutes/0.008

degrees, so 2 photoreceptors= observed acuity limit of 0.017 so 2 cones per grating

- convergence= multiple photoreceptors projecting onto a single bipolar cell (rods/peripheral cones diffuse bipolar cell), so acuity is poorer in the peripheral

- see gratings best when brightest and darkest parts fall on different cones aliasing= misperception of grating due to undersampling entire cycle (light + dark) falls on a single cone= becomes a uniform (Ex. Black +white stripes grey) intermediate= happens if light/dark bars of a cycle are very fine and a photoreceptor falls in between.--> aliasing= refers to artifacts that result when the signal reconstructed from samples is different from original continuous signals

20/20 vision= formula (distance at which a person can just identify the letters)/(distance at which a person with normal vision can just identify the letters) 20ft/15ft= better than normal vision vs. 20/40= need glasses

CONTRAST SENSITIVITY FUNCTION= describes the relationship between contrast and spatial frequencies based on many individually measured absolute contrast thresholds sensitivity= inverse of a threshold (say for 1 cpd threshold is 1% so sensitivity would be 1/0.01= 100) contrast threshold= smallest amount of a contrast required to detect a pattern inverted U shape peak at middle frequencies which means higher sensitivity for medium range spatial frequency why?

- not sensitive to higher frequencies because 1. Spacing of photoreceptors= absolute limit of spatial acuity= cant have better resolution of certain finer details because our photoreceptors are not smaller so limit high end of frequencies 2. Convergence lumps together information at some point so cant see higher frequencies

- not sensitive to lower spatial frequencies because of spatial frequency channels fewer neurons specialized for these lower frequencies (as they’re not as important)

Primary Visual Pathway photoreceptors (rods/cones) bipolar cells (diffuse/midget) retinal ganglion cells (m/p) Lateral Geniculate Nucleus (LGN) (Magnocellular layer, parvocelluluar layer, koniocellular layer) Striate cortex (V1) extrastriate cortex (V2…) other parts of the brain that combine sensory input from different modalities

Retinal ganglion cells= like spots of light tuned to certain spatial frequencies but don’t care about the orientation of gratings) medium frequency yields strong response because centre and surround have different stimuli (light/dark), but weak response to low frequency (same strip encroaches on centre and surround) and weak response to high frequency (multiple strips encroach on centre and surround)are also tuned to phases (in addition to spatial frequency’s)(Ex. ON CENTRE retinal ganglion cell= 0 degrees is a positive response, 90 is no response, 180 is a negative response, 270 is no response) axons of retinal ganglion cells exit the eye and partially cross over to the other side, synapse onto neurons in the lateral geniculate nucleus

LATERAL GENICULATE NUCLEUS input from cortex= attention and arousal

- 6 topographic maps of the visual field organized in layers= layer 1 + 2 Magnocellular, layer 3-6 Parvocellular (high spatial frequency, colour), interlaminar system= koniocellular system (ALL FED BY RELATED retinal ganglion cells)

- Both LGN’S (2 one in each hemisphere) receive input from both eyes layers 2+3+5= ipsilateral eye (same side of the body or brain right VF from right eye, left VF from left eye) layers 1+4+6= contralateral (opposite side of the body or brain)

- EACH LGN CELLS RESPONDS ONLY TO ONE EYE NOT THE OTHER - LGN neurons= cocentric VF field similar to retinal ganglion cells

respons well to spots and gratings - Fibers from LGN project mainly to layer 4 of the V1/striate cortex=

Magnocellular layer to layer 4Ca and parvocellular to layer 4Cb foveal cone photoreceptor (divergence) midget bipolar cell

P ganglion cell Parvocellular layer of LGN (layers 3-6) layer 4Cb of V1

periperhal cones/rod photoreceptors (convergence) diffuse bipolar cell M ganglion cell Magnocellular layer (layers 1 + 2) of LGN 4Ca of V1

Purpose of LGN if it responds to same patterns as the ganglion cells that provide input? LGN isn’t just a stop from retina to cortex= there are many connections between other parts of the brain and LGN. More feedback from visual cortex TO LGN than from LGN to cortex (top-down processing) location where various parts of the brain can modulate input from the eyes

Striate Cortex/V1/Bordmann area 17

- Major transformation of visual info takes place here - 200 million cells!!! Each a processing unit! - 2 important features

1. topographical (retinotopic) mapping= orderly mapping of visual field onto a neural structure (LGN/V1) such that

neighboring points in the visual field are projected onto neighboring patches of neural tissue

2. cortical magnification= dramatic scaling of information from different parts of the visual field cortical representation of the fovea is greatly magnified compared to cortical representation of the peripheral vision large part of V1 works on foveal input due to this visual acuity declines in an orderly fashion with eccentricity, but WHY?= trade-off= high resolution requires an increased # of resources (dense array of photoreceptors, 1-to-1 lines of photoreceptors to ganglion cells, large chunk of striate cortex= not possible to have high acuity for entire field so trade off btwn centre and periphery

- neurons of striate cortex NOT cocentric like retinal + LGN neurons elongated so respond much more vigorously to lines, bars, edges, gratings, moving lines (in one direction), direction, end stoppings, one eye, than to round spots of light

- orientation tuning= selective responsiveness to orientation= striate cortex neurons are tuned to detect lines in a specific orientation (optimally to certain orientation, and less to others almost no response to line tilted more than 30 degrees away from optimal orientation= 12 to 1 on a clock) more cells responsive to horizontal and vertical orientations than to obliques (humans have decreased acuity/contrast sensitivity for oblique targets) HOWEVER population of neurons of the striate cortex as a whole, detect all possible orientations

- how are circular receptive fields of LGN transformed into the elongated receptive fields in V1? LGN cells were lined up and now feed into the elongated arrangement of the striate cortex lateral inhibitions also play role in orientation tuning

- like ganglion cells, LGN cells respond best to gratings that have just the right spatial frequency to fill receptive field center each striate cortex cell is tuned to a particular spatial frequency BUT more specific/narrower range of frequencies= filtering process= allows passage of some frequencies but not of others.

- UNLIKE LGN CELLS majority of striate cortical cells respond to input from both eyes but often cells have a preference and respond more rapidly when a stimulus is presented to one eye as compared to another=OCCULAR DOMINANCE

- 2 types of cortical cells: simple cells= cortical neuron with clearly defined

excitatory/inhibitory regions/ phase sensitive/ respond to edges or (bright/dark) bars of a certain orientation in a certain visual field/receptive field

complex cells= neuron whose receptive-field is characteristics cannot be easily predicted by mapping with spots/stationary bars of light like simple cells are tuned to a particular orientation and spatial frequency and show ocular dominance BUT respond regardless of where on the receptive field the stimuli is presented no modulate like

in retinal ganglion cells, LGN cells, and simple cortical cells=phase insensitive prefer moving stimuli rather than stationary ones

both simple and complex cells have a subset of neurons that are end-stopping: process by which cells in the cortex first increase their firing rate as the bar length increases to fill up its receptive field, and then ecrease their firing rate as the bar is lengthened important for detecting luminance boundaries and discontinuities

Summary of V1 neurons= distinctive set of stimulus properties (act as filters= bars, edges, gratings, orientated within 30 degrees at a particular angle, particular width/spatial frequency)= simple or complex= end stopped or not= respond preferentially to stimulus from one eye (ocular dominance)

complex + simple cell pathways are parallel because both get direct input from LGN neurons

ORGANIZATION OF NEURAL CELLS IN STRIATE CORTEX

- Column= vertical arrangement of neurons, Hubel and Wiesel found the same preffered orientation when testing neurons at different depths/layers of V1 neurons sharing the same eye preference also have columnar arrangement which switches every 0.5mm

- HOWEVER= systematic, progressive change in preferred orientation when testing LATERALLY all orientations (0-180 degrees) were encountered at a distance of 0.5mm

- Hypercolumn= a 1-mm block of striate cortex containg ALL machinery necessary to look after everything the striate cortex is responsible for, in a certain small part of the visual world 2 ocular dominance columns, one for each eye AND many orientation columns uses modular group of neurons to process a small piece (but all aspects) of input from visual world big bank of filters

- CO blobs= functional role is unclear implication in processing color with the interblob regions processing motion and spatial structure additional organizational layer on top of orientation and ocular dominance arrays discovered using cytochrome oxidase (enzyme used to reveal regular array of CO blobs which are 0.5mm aparta

How can be use psychophysics to see if humans have orientation-selective cells? Method of Adaptation diminishing response of a sense organ to sustained stimulus tilt afteraffect= perceptual illusion of tild, provided by adating to a pattern of a given orientation= supports idea that human visual system contains individual neurons selective for different orientation (WORKS ACROSS EYES implication processing after LGN stage) selective adaptations also show that human visual system contains neurons selective for spatial

frequencies= adaptation to high contrast parts results in loss of sensitivity for spatial frequencies of adapting frequency but not of higher or lower ones. OVERALL selective adaptations via tilt afteraffect and selective adaptation to spatial frequencies display evidence for orientation tuning of neurons and preference of neurons for specific spatial frequencies. ALSO adaptation experiments provide strong evidence that orientation and spatial frequency are coded by neurons somewhere in the visual system! spatial frequency channels= a pattern analyzer, implement by an ensemble

of cortical neurons in which each set of neurons is tuned to a limited range of spatial frequencies= different spatial frequencies emphasize different types of information (How many people in a scene= consult low frequency channel that emphasizes broad outlines of face, BUT for viewing a specific person’s facial expression= consult high frequency channel that emphasizes fine details) image of blocks, high spatial frequency introduced by small blocks masks low spatial frequency conveying an underlying portrait squinting eyes, blurs small blocks, minimizing affect of the mask

sensitivity to low spatial frequencies develops more rapidly than high special frequencies (peripheral retina faster than fovea)-->

CHAPTER 4 Cells in V1= interested in basic features of visual image small precise

receptive fieldsTHEORY= but beyond V1 (extrastriate cortex to IT cortex)= increasingly complex/specific stimuli required eventually resulting in grandmother cell

How are different regions of cortex differentiated based on structure (striated or not), function, visual mapping (1 visual map of entire field= 1 area) Why do we have so many areas? Many modules specialized to work on different problems (Ex. Face processing, motion processing, place processing)

2 Main Pathways Where/How= striate cortex to extrastriate cortex of occipital lobe to parietal lobe WHAT pathway= striate cortex to extrastriate cortex of occiptal lobe to temporal lobe= explicit acts of object recognition as you move down into the temporal lobe= receptive fields get bigger (lower part of temporal lobe=IT lobe in monkeys= important for object recognition)

homologous regions= brain regions that appear to have the same function in different species

MIDDLE VISION= combines features into objects= result is object

perception goal of middle vision is to organize elements of a visual scene into groups that we can then recognize

HIGH-LEVEL VISION= object recognition= match perceived object representation to a representation encoded in memory object identification= identify a particular object as the same one previously seen

Recognized objects usually have semantic labels an names assigned to them but that is LANGUAGE.

MIDDLE VISION

Even in simple situations computer algorithms (search for pixels whose luminance is very different from that of pixels around them) lead to gaps

V1 sees edges/lines but what about curved lines and gaps? Early STRUCTURALISM fails= school of thought believing that complex objects or perceptions could be understood by analysis of components (challenged by illusory contours because no edge was found)= whole is sum of parts GESTALT= whole is greater than sum of parts

Illusory contours= Kanizsa figures= contour that is perceived even though nothing changes from one side of it to another in an image because seems likely that something is occluding it due to aligned end stoppings more likely than a coincidence people though V1 was advanced enough to do this but research shows that they are also detected by later visual areas as IC’s make a difference in EEG’s but only relatively late (higher level processing)

Gestalt laws (grouping rules): set of rules describing which elements in an image will appear to group together

- Gestalt law of GOOD CONTINUATION= elements tend to group together if they seem to lie on the same smooth (continuing in the same direction/natural scene statistics) countour favour straight lines over sharp turns, and favour closed shapes and favour lines with the same orientation can be over turned by similarity such as colour

- Gestalt law of SIMILARITY= includes texture segmentation (carving an image into regions of common texture properties)/ segmented by texture rather than edges= two elements group together if they are similar/more similar than others (size, colour, shape etc) doesn’t work that well with combinations (colour + form)

- Gestalt law of PROXIMITY= two elements group together if their distance is small can be overwhelmed by similarity

- Gestalt law of COMMON FATE= group elements moving in the same direction together together movement too much of a coincidence

- Gestalt law of SYNCHRONY= group elements CHANGING at the same time together

- Gestalt law of COMMON REGION= elements perceived to be a part of a larger region group together (overwhelm proximitiy)

- Gestalt law of CONNECTEDNESS= elements that are connected to each other group together (overwhelm proximity)

- WEAKER GROUPING PRINCIPLES= group parallel and symmetric elements together

Perceptual Committee Models= parallel processing (like simple and complex cells of V1)= middle vision similar to a collection of specialist for certain features (values) who vote on their opinios Goal= have a single answer emerge out of diversity physical substrate of committee= massive interconnected set of neurons

- PANDEMONIUM MODEL= selfridge letter recognition demon levels (decision demon, cognitive demons for each letter, feature demons for each line orientation) decision demon decides based on loudest opinion

- Committee rules= honour the laws of physics & biology, Resolve ambiguity, Reject accidental viewpoints!

- Physics/Biology= Concave and Convex rain drops= view one set as concave because the shawdow is above, and view convex ones because physically the 3D image has shadows below due to placement of the sun always see faces as convex because biologically that’s how they are

- Ambiguous figure= visual stimulus that gives rise to 2 or more interpretations of its identity or structure (generally infinite # of possibilities, but narrowed down to 2 of the most likely ones) almost all images are ambiguous in nature but generally perceptual committee agree on one interpretation

- Accidental viewpoint= a viewing position that produces some regularity in the visual image that is not present in the world (EX. 2 independent objects lined up perfectly) any shift in position would destroy the illusion.

Gestalt laws for Figure-Ground Assignment (determines which image region belongs to an object in the foreground and which belongs in the background) find the most likely reality behind retinal image

- Gestalt laws of SURROUNDEDNESS = surrounded image is foreground - Gestalt laws of SIZE= smaller image is foreground - Gestalt laws of SYMMETRY= symmetrical image is foreground - Gestalt laws of PARALLELISM= regions with parallel contours is

foreground figure - Extremal edges= strong figure-ground cue= projections of viewpoint-

specific horizons of self-occlusion on smooth convex surfaces overwhelm surround and size

- *object recognition starts before the figure-ground assignment finishes= can alter/influence assignment

- *object knowledge- contributes to figure ground perception harder seeing the face when its in the background and the foreground is the negative contour

Heuristics (mental shortcuts) for partially occluded features - Relatability= degree to which two line segments appear to be part of

the same contour (can be related across gaps)= 2 edges are related when/if they can be connected by a smooth convex/concave curve but not if the connection requires an S CURVE

- Non-accidental features provide clues to object structure don’t depend on exact viewing position= T junction= occlusion, whereas a Y junction/arrow junction signals a corner these non accidental features only fail to hold at an accidental viewpoint.

- Global superiority effect- finding in various experiments that the properties of the whole object take precedence over properties of parts of the object Big H made up of smaller letters example might indicate that we first carve a retinal image into large-scale parts/objects and THEN work out the details

- Another heuristic= pairs of concavities indicate part boundaries classify 2 objects instead of 1

Summarizing Middle Vision

1. Bring together what should be brought together gestalt grouping principles, processes that complete contours and objects even when partially hidden behind occludents (relatability heuristic)

2. Split what should be split Complimenting group finding processes increase edge-finding processes that divide regions Gestalt rules for figure-ground assignment texture segmentation

3. Use what you know 2D configurations used to indicate 3D corners or occlusion borders (Y, ARROW, T junctions)

4. Avoid Accidents-> Perceptual committee model avoid interpretations that require assumptions of highly specific/accidental combinations of features or accidental viewpoints

5. Seek consensus and AVOID ambiguity every image is ambiguous infinite physical situations but committee ultimately deliver a single solution/1 interpretation

HIGH-LEVEL VISION=OBJECT RECOGNITION