Mod 2 - Sensation

SENSATION AND PERCEPTION Summary 1: The process of sensing Sense – A system that translates information from outside the nervous system into neural activity Sensations – Messages from the senses that make up the raw information that affects many kinds of behaviour and mental processes Steps in the process of detecting information in the environment:

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Physical stimulus / energy – eg light, heat, sound – person talking, flashing light

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Accessory structures – Structures, such as the lens of the eye, that modify a stimulus

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Transduction – The process of converting incoming energy into neural activity through receptors.

o Coding - Translating the physical properties of a stimulus into a pattern of neural activity that specifically identifies those properties

o Temporal codes – Coding attributes of a stimulus in terms of changes in the timing of neural firing. Eg. faster response to bright light than dim

o Spatial codes – Coding attributes of a stimulus in terms of the location of firing neurons relative to their neighbours. Eg. touched hand or foot

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Sensory receptors – Specialized cells that detect certain forms of energy

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Sensory nerves – carry the output from receptors to the central nervous system (spinal cord / brain)

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Neural signals – thalamus  cerebral cortex  sensory cortex

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Receptor potential – the physical stimulus makes the receptor cell more permeable to electrically charged particles, and the movement of these particles through the membrane creates a changed electrical charge, this is called receptor potential.

Summary 2: The auditory system Sound is a repetitive fluctuation in the pressure of a medium. such as air and water. Acoustic stimulus is its physical stimulus. •

Sound wave

o Wavelength – the distance from one peak to the next in a wave form. o Frequency – the number of complete waveforms, or cycles, that pass by a given point in space every second. Measured in Hertz, this is the pitch (high / low) of a tone. Human auditable range is 20Hz to 20kHz.

o Pressure amplitude – the difference between the peak and the baseline of a wave form. This is the loudness and measured in decibels (dB), which is a log scale. Increase of 10dB is doubling the loudness.

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Timbre (“tamber”) is the quality of sound; it’s determined by the complex wave pattern added onto the lowest (fundamental) frequency. Eg. flute and clarinet sounded different.

o Phase full cycle of a sound wave = 360 degree phase angle. When the same wave is 180 deg out of phase, it eliminates each other, this principle is used in active noise suppression systems. Eg. headphones used in aircrafts •

The Ear

o Outer ear (accessory structures) – pinna and auditory (ear) canal funnels and channels sounds into the middle ear. The sound waves strikes the eardrum (tympanic membrane) setting up vibrations.

o Middle ear (accessory structures) – The ossicles are the three smallest bones in the human body. They are contained within the middle ear space and serve to transmit sounds from the air to the fluid-filled labyrinth (cochlea). They are the malleus (hammer), incus (anvil) and stapes (stirrup). They amplify the changes in pressure by focusing the vibrations of the eardrum onto a smaller membrane, the oval window. o

Inner ear – where transduction occurs.

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Oval window – the membrane interacts between the middle and inner ear. After sound passes thru, it reaches the cochlea (a coiled spiral).

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Basilar membrane forms the floor of the long tube that coils into the cochlea. When the sound wave passes thru the fluids in the tube, it moves the basilar membrane, and this movement bends hair cells of the organ of Corti, a group of cells that rests on the membrane.

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These hair cells connect with fibres from the auditory nerve.

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The tectorial membrane is on top of the layer of hair cells.

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The auditory neurons / nerve are connected to the hair cells, and the bundle of axons that goes into the brain. When the hair cells bend, they stimulate neurons in the nerve to fire in a pattern that sends the brain a coded message about the amplitude and frequency of the incoming sound wave.

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Because mechanical movement of the hairlike projections (cilia) produce the changes in the membrane of the hair cells that create the (electrical) receptor potential, this is known as mechano-electrical transduction.

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Deafness

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Conduction deafness – caused when the three ossicles fuse together, preventing accurate reproduction of vibrations. Can surgically break the bones apart, or replace with plastic ones. Hearing aids to amplify incoming sound will also help.

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Nerve (sensorineural) deafness – the nerve, or more commonly the hair cells are damaged. Damages could be old age or resulted by extended exposures of noise. Potential to regrow

human hair cells. Hearing aids are artificial cochlea as implants that stimulate the auditory nerve. •

Coding of sound

o Place theory / travelling wave theory – hair cells at a particular place on the basilar membrane respond most to a particular frequency of sound. Supports high freq hearing loss in older people – the cells nearer the middle ear are responsible for higher freq, and they wear out first because they are involved in all sounds.

o Frequency matching theory / volley theory / temporal / timing – the view that some sounds are coded in terms of the frequency of neural firing. o

Frequency matching at low frequencies; place mechanisms at high frequencies.

Summary 3: The visual system Light is a form of energy known as electromagnetic radiation (spectrum). Visible light has wavelength b/w 400 nanometres (violet) to 750 nanometres (red) (10-6). Wavelength changes in colour and intensity is experienced as brightness. •

The eye o

Cornea – the curved, transparent, protective layer through which light rays enter the eye.

o Iris – the colourful part of the eye, which constricts or relaxes to adjust the amount of light entering the eye, passing to the pupil.

o Pupil – an opening in the eye, just behind the cornea, through which light passes. o

Lens – the part of the eye behind the pupil that bends light rays, focusing them on the retina

o Retina – the surface at the back of the eye onto which the lens focuses light rays. This is the ‘net’ of cells.

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Photoreceptors – nerve cells in the retina that code light energy into neural activity. They contains photopigments, chemicals that break when light strikes, changing the membrane potential of the photoreceptor cells.

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Rods – contains rhodopsin (row-DOP-sin), only 1 pigment so cannot discriminate colours. More sensitive to light, this is what we see with in the dark.

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Cones – contains varieties of iodopsin, they provide the basis of colour vision. They are concentrated in the centre of the retina, called fovea. Concentrated cones also allow for details.

Analysing before passing to the brain

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Bipolar cells – They can synapse with either rods or cones (but not both), and they also accept synapses

from horizontal cells. The bipolar cells then transmit the signals from the photoreceptors or the horizontal cells, and pass it on to the ganglion cells through action potentials.

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Interneurons – they are cells that make sideways, or lateral, connections between photoreceptors. Used for comparisons between the photoreceptors – lateral inhibition amplifies the differences by reducing the responsiveness of its neighbour.

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Ganglion cells – to travel long distance to the brain, these cells generate the action potential.

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As people get older, the lens loses its flexibility, making accommodation more difficult. Hence older people become “farsighted”.

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Cones allow for details but they are not good in dim lights. Rods are, but the colour and details of the vision is scarified.

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Dark adaptation – for 30 mins in a darken room, the ability to see in the dark increases.

o Blind spot – there are no photoreceptors at the point where optic nerve exits the eye ball, hence a blind spot. o

Colour blindness

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Trichromatic theory (Young-Helmholtz theory) – only 3 type of cones – blue (short), green (medium) and red (long wavelengths). Not a single, but all three in ratios indicates what colour is sensed. This describes the properties of the photometers.

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Opponent-process theory – a theory of colour vision stating that colour sensitive visual elements are grouped into red-green, blue-yellow, and blackwhite elements. Mixing lights of complementary colours produces grey (cancelling each other). This describes the properties of the ganglion cells.

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Colour-blindness – people who are born with only 2 of the 3 possible coloursensitive pigments. They discriminate fewer colours than other people.

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Neural processing – preparation for perceiving objects

o Contrast enhancement – coding to detect edges via lateral inhibition o

Feature detectors – sensing orientation and visual texture

o Perceiving contours – recognise objects. Snow blindness; Hermann grid with grey smudges. Summary 4: Touch – the skin senses •

Skin receptors o

free nerve endings, without small bulb nor capsules on the end near the epidermis.

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Encapsulated endings, with small capsules on the end nearest the epidermis.

Theories o

Specificity theory states that each of the different kinds of receptors responds exclusively to only one kind of physical stimuli (eg pain), and each kind of receptor is therefore responsible for only one kind of sensation.

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Pattern theory suggests that the pattern of nerve impulses determines sensation. Each kind of receptor responds to many different kinds of stimulation, but it response more to some than to others.

o Melzack and Wall (1962) incorporated aspects of each theory into their proposal. Their paper is now regarded as a landmark in the theory of skin sensitivity, and the basic assumptions of the theory still appear correct. •

The four categories based on combination of speed and adaptation o

Rapidly adapting (RA) receptors good at picking up vibrations on the skin.

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Slowly adaption (SA) receptors are good at pickup up constant pressure.

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Size large / small

o Pacinian corpuscles are the largest sensory end organs on the body. It is a large RA best to detect vibrations, but with high density in some areas, it can also detect the roughness of the surfaces. •

Haptic perception is the perception of objects by touch. It is best with active touch (moving fingers around the object), and regardless of where the passive touch is against, the result is very similarly poor.

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Visual information takes precedence over touch

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Reading Braille requires active touching

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Using the Tadoma method for speech perception, a deaf or deaf-blind person places their hand on the lips and jaw of the speaker to pick up tactile sensations of speech such as airflow, lip and jaw movement, and vibration.

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Smell (olfaction) – stimuli are molecules in the air, suspect a lock-and-key process with the receptors. These neurons are continuously replaced with new ones, with a life of only 2 months.

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Taste (gustation) – made up of four primary qualities – sweet, sour, bitter and salty and can be mapped to different regions of the tongue. Receptors normally respond to two or more of these stimuli, but some are stronger over the others. Two other taste qualities umami, a taste enhancer associated with proteins and monosodium glutamate (MSG) and astringent, a taste associated with tannis in teas. Spicy / hot are actually pain stimuli.

Summary 5: Methods underlying the study of perception Physiologic methods – are used to investigate processes in the sensory receptors and the brain that are associated with sensory stimulation and perceptual experience.

Psychophysical methods – are used to investigate the relationship between the physical attributes of the sensory stimuli and the sensations or perceptual experiences produced by these stimuli. •

Absolute threshold refers to the faintest detectable stimulus

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Difference threshold refers to the smallest detectable change in a stimulus. o

Method of limits

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Method of constant stimuli

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Method of adjustment

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The threshold is a statistical concept, rather than a distinct boundary between detection and non-detection.

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Weber’s (“VAY-ber”) law states that the smallest detectable difference in stimulus energy is a constant fraction of the intensity of the stimulus. o

Just noticeable difference (JND) = KI (K is Weber’s constant for a particular sense) (I is the amount / intensity of the stimulus)

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Signal detection theory – a mathematical model of what determines a person’s report that a near-threshold stimulus has or has not occurred. The proportions of false alarm, hit, miss and correct rejection depends not only on a person’s sensitivity, but also their response biases (criterion).

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Fechner’s law: S (sensation magnitude) = c (constant) log M (physical quantity) attempts to address the relationship between changes in stimulus magnitude and the magnitude of the resultant sensation. However, the sensation / magnitude cannot be measured directly.

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Steven’s power low: S = cMp where p is the power to which the magnitude must be raised. If p=1 then the sensory experience directly matches the magnitude of the stimulus. if p<1 then S increase slower as M increase (eg. perceived brightness). If p>1 then S increase more rapidly with small change in M (eg. pain with electric shock)

Summary 6: Perceiving patterns and recognising objects •

Bottom-up processing – aspects of recognition that depend first on the information about the stimulus that comes to the brain from the sensory receptors.

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Top-down processing – aspects of recognition that are guided by higher-level cognitive process and psychological factors such as expectations.

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Perceptual organisation o

Gestalt principles (relatively ‘bottom’ level) 

Proximity – closer objects or events

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Similarity – similar elements (pattern)

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Continuity – create continuous form

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Closure – fill in missing contours to complete objects

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Common fate – objects moving same direction

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Synchrony – occur at the same time

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Common region – located within some boundaries

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Connectedness

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Cognitive approach – effect of the past experience creates a perceptual set

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Depth perception

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Binocular depth cues are convergence and retinal or binocular disparity. •

Convergence is a cue from the eye muscles when they turn the eyes inward to focus on a nearby object.

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Monocular cues comprise interposition (or occlusion), relative size, linear perspective, motion parallax, height on the visual field, texture gradient, linear perspective, aerial perspective (clarity), and highlights and shadows.

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Depth cues help transport this image into a 3D mental representation.

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For some process in perception of depth occur before object recognition takes place. Eg. used in 3D movies.

o Movement perception. Movement/motion can be detected: 

From the optical flow of information.

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When there’s a change in the relative displacement between objects

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Can e simulated by appropriate temporal sequencing of stationary images, eg. phi phenomenon, stroboscopic; the apparent movement effect; or as a consequence of continual movement giving rise to after effects of movement in stationary objects.

o Perceptual constancies – the perception of objects as constant in size, shape, colour and other properties despite changes in their retinal image. 

Size – estimates of size and distance are related in a process called constancy scaling. Emmert’s Law: perceived size is a function of the retinal size multiplied by its perceived distance.

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Shape

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Colour & brightness – retinal illuminance of an object is due to external illuminance (source) and reflectance (proportion of light reflected). •

Brightness (lightness) constancy is based on the ratio principle of light b/w different regions of illuminance on the retina.

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Colour constancy is due to the nature of the illuminance falling on the object and on surrounding areas.