Perception PSYU2247 All leacture notes PDF

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Summary

Perception- Physical stimuli are “transduced” into nerve impulses by our sense organs - We experience these as a reconstruction or representation of the world - Perception is the only way information enters your brain - Senses; vision, audition, chemical senses (gustation + olfaction), body senses (...

Description

Perception -

Physical stimuli are “transduced” into nerve impulses by our sense organs We experience these as a reconstruction or representation of the world Perception is the only way information enters your brain Senses; vision, audition, chemical senses (gustation + olfaction), body senses (somatosensation; taction, proprioception, haptics + equilibrioception)

GENERAL PRINCIPLES Physiological Principles Transduction -

First stages of sensory process, receptors turn energy into neural signals Impulses travel along axons to terminals which release neurotransmitters across synapses to be received by another cell

Hierarchical processing -

Neural impulses travel “up” system to the cortex ‘Relay station’ in the Thalamus (except for olfaction) Bottom up; flow of information from sensory receptors towards “higher” cortical areas with increasing levels of complexity Top down; Prior knowledge influences what is perceived Not a dichotomy; there is always influences of both Forward, lateral & backward connections demonstrate information can flow in all directions

Selectivity -

Within each sense, stimuli can vary along various dimensions Cells are selective for stimuli with certain characteristics Response are smaller the more stimulus differs from the preferred stimulus

Organisation -

Cortical magnification; most important stimulus is processed by a larger amount of cortex

Specific nerve energies -

Each sense projects to a different cortical area of the brain The nature of a sensation depends on which sensory fibers are stimulated

Plasticity -

Neural mechanisms are modifiable, development, recovery from brain injury

Noise -

Neural firing is stochastic, precise firing rate determined by stimulus & other random factors Spontaneous Activity; cells fire a little even with no stimulus

Perceptual Principles

Detectability -

Detection threshold; intensity required for detecting a stimulus (lower is better) Sensitivity; higher sensitivity is better

Sensory Magnitude -

More intense stimulus  higher magnitude of sensation Measure with magnitude estimation technique (subject estimates magnitude of stimulus by assigning it a position on a numerical scale) Compressive non-linear functions e.g. if you double intensity, sensation is less than double

Discrimination -

Discrimination threshold; difference between two stimuli required for successful discrimination (just noticeable difference) Lower is better

Adaptation -

A change in response of a system to a sustained stimulus Prolonged stimulation results in a decrease in the rate of firing Various perceptual consequences;  Increased detection thresholds for same/similar stimuli  Reduction of perceived intensity for similar suprathreshold stimuli  Perceived properties of other similar stimuli can appear biased (motion aftereffect)

Studying techniques Anatomical Methods (dead brains) -

Visible differences in brains; white/grey matter Staining; reveals axons/connections, reveals cell body density & size, reveals activity

Recording techniques (live brains) -

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Invasive (mostly animals);  Single cell recording; anaesthetised, microstimulation, high spatial and temporal resolution, difficult to get the “big picture”  Optical imaging; blood flow changes, small area of cortical surface, slow response Non-invasive (mostly humans);  Visually evoked potential & magnetoencephalography; measures electric currents or magnetic fields from cortex with sensors on the scalp  PET & fMRI

Lesions -

Animal studies; Neurotoxins or surgery Human Neuropsychology; usually diffuse damage & often varying patterns of deficit Problems; damage of fibres passing through can affect areas far from lesion, brains recover from damage, need to know the right test

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Transcranial Magnetic stimulation; temporary, magnetic field knocks out cells over a broad area, temporally precise, spatially imprecise

SOUND, EAR & BRAIN What is sound -

Sound consists of pressure waves carried by vibrating air molecules

Sound waves -

A graph of pressure changes over time is a “waveform” Compression; positive cycle Refraction; negative cycle

Properties of waves -

Sine wave; waveform for pure tone (equal positive & negative deviations) Frequency; number of cycles (higher frequency, higher the pitch) Amplitude; height of the wave, loudness, expressed in decibels Phase; point on waveform, corresponds to changes to perceived quality of sound

Complex Sounds: adding waves together -

Sound waves are linear (add together logically) Natural sounds; collection of simple sine waves added together Add sine waves to make complex waveforms & decompose to make less complex Fundamental frequency; lowest frequency in complex wave (gives sound its pitch) Harmonic frequencies; a frequency that is an integer multiple of the fundamental frequency

Fourier analysis -

Decompose a complex sound into its frequency (sine wave) components Displayed in a spectrogram (graphical representation of changes in frequency overtime) Time is plotted horizontally, frequency = vertically & amplitude = darkness of the plot

Fourier Filters - Filters separate things on the basis of a given property - Allow certain frequency components to pass while blocking others - e.g. sound travelling past the head, head obstructs high frequencies, this acts as a low-pass filter Linear & Non-linear filters -

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To Fourier theory, filter must be linear & follow 3 rules:  Output must not contain any frequency not present in input  If amplitude of input changed, output must change by same factor  Total output of multiple signals (A, B, C) must be equal to the output of A + B + C Non-linear filters; filters that violate these assumptions (often add distortions to a signal)

Is the ear a Fourier analyser? -

Auditory process from outer ear to inner ear is not linear

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Maybe a Fourier Analyser; Auditory filters separate frequencies into different channels, responds to amplitude within each channel & can encode phase information Due to non-linearities between input & output, 2 tones should result in two peaks, but often 3 peaks merge, outer hair cells amplify small intensities

The Ear -

Divided into 3 parts; outer ear, middle ear, inner ear

Outer ear -

Pinna; flexible flap on outside of ear, funnels frequencies into middle ear Shape & size have effect of amplifying medium sound frequencies

Middle ear -

Small bones (ossicles) transmit sound energy from eardrum to oval window

Inner ear -

Oval window  Cochlea (fluid filled, divided in two) Sound waves displace fluid along cochlea & cause a wave to travel along basilar membrane

Transduction: inner hair cells -

Organ of Corti contains basilar membrane (3500 inner hair cells protrude) f Fluid displacement causes vibration in basilar membrane (moves in shearing motion) This deflects stereocilia of inner hair cells, generating impulses Base of each hair cell contacts afferent fibres of auditory nerve (50 000 fibres) Ear transduces mechanical stimulus into electrical nerve impulse that travels to brain

Frequency-to-Place Conversion in the Cochlear -

Wave along the basilar membrane peaks at a particular location (due to width & stiffness gradient) High frequencies = largest vibration near base (stapes) Low frequencies = largest vibration near the apex Place code; hair cells have characteristic frequency sound frequency can be encoded by which cells are most active Frequency code; hair cell impulses coincide with certain phase of the wave (phase locking)

Coding intensity -

Range of sound levels for human hearing is >100 dB Individual auditory nerve fibres have a range of only 20-60 dB Dynamic range; diff between min & max intensity a fibre responds To cover full dynamic range two groups of auditory fibres have different roles:  High spontaneous rate fibres respond to low intensities  Low spontaneous rate fibres respond to high intensities

Auditory nerve & brain -

Nerve cells send signals along auditory nerve to brain Fibres fire a little to frequencies close to their characteristic frequency filters Auditory nerve filters; auditory nerve cells filter on basis of frequency Band-pass filter; responds to a specific range (band) of frequencies

Ascending auditory pathway -

Auditory nerve fibres terminate in cochlear nucleus Binaural neurons found in superior olive take inputs from both ears Thalamic relay is called the medial geniculate nucleus

Auditory cortex -

Tonotopic organisation; primary cortex organised in terms of frequency (orderly progression of preferred frequencies across cortex)

Outer hair cells & descending auditory pathway -

Descending fibres run from auditory cortex to cochlea (synapses in reverse order to ascending projections) Descending projections involved in auditory attention & outer hair cell amplification functions More outer hair cells than inner hair cells, yet only 5-10% send signals upwards Outer hair cells receive impulses from higher areas & respond by changing their length (motile response) Serves as a cochlear amplifier, making early auditory processes highly non-linear

Summary -

Three critical parameters of sound wave are frequency, amplitude & phase Complex sounds consist of simple sine waves at different frequencies (Fourier theory) Hair cells at different places in cochlea encode different sound frequencies (frequency-toplace conversion) Information sent through auditory nerve to brain & back down to cochlear

AUDITORY PERCEPTION: PITCH, LOUDNESS & LOCALISATION Pitch perception -

Pitch: perceptual attribute of a sound that corresponds to its frequency

Theories of pitch perception (four theories) Place theory -

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Based on pitch perception of sine waves Auditory system is an approximate Fourier Analyser (Frequency-to-place conversion);  Resolves complex sounds into sine waves through frequency-tuned filters  Frequencies given by location of most active fibres Predicts pitch discrimination for pure tones should depend on bandwidth of auditory filters

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Critical bandwidths provide good psychophysical method of estimating filter properties Critical bandwidths increase with frequency Frequency discrimination good at low frequencies & worse at high frequencies Frequency discrimination performance not constant proportion of bandwidth, as it should be according to place theory

Pitch perception of complex tones -

Contain series of harmonic frequency components spaced at intervals equal to frequency or repetition rate of fundamental Pitch heard in a complex tone determined by fundamental frequency Missing fundamental; if fundamental frequency is removed, pitch is still heard at a frequency that corresponds to fundamental frequency (problem for place theory)

Timing (Rate) theory -

Auditory nerve responses are phase-locked to a sound-waves frequency below 4-5kHz Response rate of neural impulses carries information about sound frequency Listener discriminates pitch by differences in time intervals between neural firings Evidence; important for low/mid-range frequency, poor for high frequencies

Temporal theory -

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Phase locking occurs for tones up to 1kHz Resolved harmonics; volley principle important for tones up to 4kHz Unresolved harmonics; for higher frequencies, two harmonics fit into wider bandwidths of single auditory filters & can produce beats as their waveforms overlap (residue pitch; same pitch as fundamental) If nerve firing becomes phase locked, temporal theory can account for missing fundamental? When lower (resolved) harmonics & higher (unresolved) harmonics specify different fundamentals, humans rely more on resolved harmonics

Patterns rejection theory (Goldstein 1973) -

Auditory system analyses frequencies using place code Then analyses harmonically frequencies that fit resolved components Pitch is determined by fundamental of best-fitting harmonic series Explains missing fundamental effect (pitch of fundamental defined by harmonics present)

Loudness perception -

Perceptual attribute that corresponds to intensity Different intensities can have equal loudness due to frequency Two techniques to compare loudness (loudness matching & loudness scaling) Loudness matching; subject adjust intensity of sound until it matches a standard stimulus  Sensitivity poor at low frequency, mid-intensity; low-frequency sounds lack loudness  This is why some stereos have a bass boost

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As intensity increases, curves flatten out

Models of loudness -

Individual Auditory Nerve Fibres; tuned to characteristic frequencies (CF) At detection threshold only fibres with appropriate CF fire above spontaneous rate As intensity increases, neighbouring neurons will become excited Higher the intensity, louder sound (more neurons are firing)

Binaural sound localisation -

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Both ears receive different sound waves depending on source location Differ in time of arrival (Interaural Time Difference - ITD) & intensity (Interaural Level Difference - ILD) Duplex theory;  ITD is good for localising low frequency sounds  ILD is good for localising high frequency sounds Direction of source specified by azimuth angle relative to straight ahead

Interaural Time Difference -

Difference in time of arrival at each ear depends on azimuth angle ITD = time left ear – time right ear Maximum ITD; 650 microseconds for a stimulus directly to right or left ITD = 0 for stimulus directly in front or behind; Any time difference could be two possible azimuths (e.g. 0.4ms delay may be 50° or 130°) Processed in Medial Superior Olive in brainstem

Interaural Level Difference -

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Ear furthest from sound source lies in acoustic shadow cast by head Difference in intensity at each ear depends on frequency  For low frequencies, differences are small (not useful)  For high frequencies, differences are large (very useful) ILD = 0 for stimulus directly in front or behind; Processed in Lateral Superior Olive in the brainstem

Cone of confusion -

ITD & ILD can be ambiguous (corresponds to two possible azimuths) Binaural cues give no information on elevation A sound producing a particular ILD & ITD can originate anywhere on surface of a cone Head movements can resolve this confusion (also frequency filtering at pinna)

Monaural sound localisation -

Monaural; involving one ear, pinna filters sound waves Important for sound source localisation on vertical plane

Summary -

Pitch of a sine wave coded by frequency-to-place conversion (place theory) & rhythmic firing pattern of cochlear cells (timing theory)

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Pitch of complex tones can be explained by temporal & pattern recognition theories Loudness is proportional to total neural activity evoked by sound in auditory nerve

BODY SENSES The somatosensory system -

Touch; physical properties of surfaces e.g. texture, warmth & softness Thermoception; temperature Proprioception; position of body parts Kinesthesis; movement of body parts

Somatosensory transduction -

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Tactile receptors;  Superficial receptors (Meissner corpuscles/Merkel disks) respond to light touch  Deep receptors (Pacinian & Ruffini corpuscles) respond to pressure & stretch Thermoceptors; Free Nerve Endings  Warmth Fibres; Signal increase in skin temperature  Cold Fibres; Signal decrease in skin temperature Proprioception/Kinaesthesia  Muscle Spindles; Respond to muscle length & rate of stretch  Golgi Tendon Organs; Respond to muscle tension  Joint Receptors; Respond to joint position

Somatosensory hierarchy -

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Axons follow two routes to the brain;  Spinothalamic pathway; carries slow temperature signals from free nerve endings  Lemniscal pathway; carries fast somatosensory signals from mechanoreceptors Both pathways have branching circuits in spinal cord that mediate reflex responses

Cortical organisation -

Primary Somatosensory Cortex is a thin strip that runs across the head (from ear to ear) Somatotopic Organisation; specific body parts associated with distinct location in cortex Cortical Magnification; some parts (lips, hands) occupy much greater cortical area Neurons connected to receptors on left half of body project to right hemisphere Vertically downwards though cortex; cells have RFs originating from same region of body Horizontal across cortex; systematic cell mapping of entire body (somatosensory homunculus; described by Penfield during microsimulation surgery for epilepsy)

Discrimination: Two-Point Acuity -

Our ability to discriminate differences in perceived touch stimulation can be measured by using a pair of callipers & placing one or both points on the skin Subjects report if they feel a single point or a pair of points Callipers adjusted until subject can just report presence of a pair of points reliably (discrimination threshold or JND) Performance varies markedly in different regions of the body (cortical magnification)

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JNDs smallest for mouth & fingers, largest for neck & back Receptive fields small for mouth & fingers, largest for neck & back

Selectivity: cortical receptive fields -

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Receptive Fields  Each cortical cell responds to stimulation in a small area of the body surface  Receptive field: area where touch affects activity in a given cell Centre-Surround Antagonism (Lateral Inhibition)  Two concentric zones in receptive field (RF) o Cell activity is increased by stimulation in the centre o Activity is decreased by stimulation in the surround o Stimulation in both areas hardly results in any change at all 

Amplifies responses to differences in stimulation within the RF

Equilibrioception -

Forces; gravity & acceleration Sensory apparatus; vestibular system (inner ear)

Principal planes & axes of body -

The body can move linearly along 3 possible axes: x, y, z Body can also rotate around these axes: x (roll), y (pitch), z (yaw) These movements define 6 independent degrees of freedom Natural movements combine 2 or more of these components Stimuli for vestibular system:  Linear acceleration along or rotational acceleration around axes  Tilt with respect to gravitational vertical

The Vestibular System -

Signals head's acceleration & inclination relative to gravitational vertical 2 otolith organs; Utricle & saccule 3 semi-circular canals; posterior, anterior & lateral (horizontal) Each filled with fluid & small patch of sensory hair cells Movements result in fluid flow, which displaces hair cells & leads to sensory responses

Vestibular transduction -

Each canal contains a bundle of hair cells (cupula) projecting across the canal Utricle & saccule; patch of hair cells (macula) covered in gelatinous carpet (otolithic membrane)

Canal responses to rotational head movements -

Rotational acceleration causes fluid movement As fluid lags (inertia), fluid deflects the cupula & causes responses in hair cells The three canals a...