What is perception?
- a largely unconscious, automatic process, based on ‘unavailable’ neural events (to our conscious mind), together with ‘unconscious’ inferences from specific cues (eg. depth perception cues)
- (no introspection of the process. Can't say "how am I seeing right now")
- but, at times conscious effort is needed to interpret sensory data (e.g., when data are ambiguous and incomplete)
- effortlessness of perception disguises the complex nervous system mechanisms operating (behind the scenes)
The perceptual process
- Stimulus -> environmental stimulus -> attend stimulus-> by bringing stimulus to receptors (eg. to look at something, move eyes the stimulus)
- Stimulus is turned into something the brain understands eg. electricity or physiology
- This process involves transduction: the change of energy from one state to another state. Eg. change light to electricity or energy in the form of pressure changes to electricity (sound)
- Transmission: transmitting electricity from one place to another through neurons where we can process it.
Experience and action: after processing, comes perception. Identify what we are seeing/hear/touch/taste, is there something we need to do?
Two Interacting Aspects of Perception
- Bottom-up processing
- – Processing based on incoming stimuli from the environment
- – Also called data-based processing
- Top-down processing
- – Processing based on the perceiver’s previous knowledge (cognitive factors)
- – Also called knowledge-based processing
Approaches to the Study of Perception
- Observing perceptual processes at different stages in the system:
- Psychophysical approach (PP) - the stimulus perception relationship
- • Physiological approach (PH1) - the stimulus physiology relationship
- • Physiological approach (PH2) - the physiology and perception relationship
- These stages are interconnected and communicate with one another
- The three boxes represent the three major components of the perceptual process.
- The three relationships that are usually measured to study the perceptual process are the psychophysical (PP) relationship between stimuli and perception, the physiological (PH1) relationship between stimuli and physiological processes and the physiological (PH2) relationship between physiological processes and perception.
Psychophysical approach (PP)
- the stimulus-perception relationship
- In experiment, will want to control stimulus as we cannot control perception.
Physiological approach (PH1)
- The stimulus-physiology relationship
- About direct measurement of the physiological process
- We control stimulus and measure the physiological process
Physiological approach (PH2)
- The physiology-perception relationship
- Eg. fMRI experiment- we are measuring physiology (where is active in the brain) for the stimulus
- We're asking their experience of the perception
Psychophysics - Overview of Methods of Measurement
- Qualitative Methods
- – Describing
- – Recognising
- • Quantitative Methods
- – Detecting
- – Perceiving Magnitude
- – Searching
Qualitative Methods of Psychophysical Measurement
- Indicating characteristics of a stimulus
- First step in studying perception
- Called phenomenological method
- Placing a stimulus in a category by identifying it (eg. name it)
- Categorisation of stimuli
- Used to test patients with brain damage
Quantitative Methods - Classical Psychophysics- Absolute threshold
- Absolute threshold - smallest amount of
- energy needed to detect a stimulus
- – Method of limits
- • Stimuli of different intensities presented in
- ascending and descending order
- • Observer responds to whether she
- perceived the stimulus
- • Cross-over point is the threshold
- The dashed lines
- indicate the crossover point for each sequence of stimuli. The threshold - the average of the crossover values.
- – Method of adjustment
- • Stimulus intensity is adjusted continuously
- until observer detects it
- • Repeated trials averaged for threshold
- – Method of constant stimuli
- • Five to nine stimuli of different intensities
- are presented in random order
- • Multiple trials are presented – detection
- ‘yes or no’
- • Threshold is the intensity that results in
- detection in 50% of trials.
- (takes out predictability of 2 previous methods)
1. the absolute threshold is the:
2. The difference threshold is the:
- 1. a statistically determined minimum level of stimulation necessary to detect a stimulus
- 2. the level of change in a stimulus necessary to notice a change in that stimulus
Quantitative Classical Psychophysics - Difference Threshold or Limen
- Difference Threshold or Limen (DeltaL) - smallest difference between two stimuli a person can detect
- – Same methods can be used as for absolute threshold
- – As magnitude of stimulus increases, so does DL
- – Weber’s Law describes this relationship
- DL / S = K
- Amount of difference you can detect/magnitude of stimulus= webbers fraction
- As you get brighter light, your ability to detect difference in stimulus decreases
- • Weber’s Law doesn’t hold over whole range
- of sensation magnitudes
- (a) The person can detect the difference between a 100-gram standard weight and a 102-gram weight but cannot detect a smaller difference, so the DL is 2 grams.
- With a 200-gram standard weight, the comparison weight
- must be 204 grams before the person can detect the difference, so the DL is 4 grams. Weber’s Law predicts the ratio of DL to the
- weight of the standard is constant.
Quantitative Methods - Modern Psychophysics
- Magnitude estimation (scaling)
- – Stimuli are above threshold.
- – Observer is given a standard stimulus and a
- value for its intensity.
- – Observer compares the standard stimulus to test stimuli by assigning numbers relative to the standard.
- – Response compression
- • As intensity increases, the perceived magnitude increases more slowly than the intensity.
- – Response expansion
- • As intensity increases, the perceived magnitude increases more quickly than the intensity.
- – Relationship between intensity and perceived magnitude is a power function
- The relationship between perceived magnitude and stimulus intensity for electric shock, line length, and brightness.
- – Steven’s Power Law
- • P = kS^n
- n= a function of those lines
- The three functions from earlier plotted on log-log coordinates.
- Taking the logarithm of the magnitude estimates and the logarithm of the stimulus intensity turns the functions into straight lines.
- n is related to the gradient
- n will be small for response compression
- and large to response expansion
Difference threshold for car seat vibration…
- Figured out the diff thresholds for a group of people's sensitivity to changes in vibrations ins the car
- It tells car manufacturers how much suspension or cushion car needs
Weber Fractions for a number of different sensory dimensions
- We are more sensitive to changes to an electric shock than we are to changes in light intensity
- We need to be more sensitive in pain intensity because diff in damage done to body could be very large
Basic Brain Structure
- The brain has modular organization
- – The sensory modalities have primary receiving areas
- • Vision - occipital lobe
- • Audition - temporal lobe
- • Tactile senses - parietal lobe
- – Frontal lobe coordinates information received
- from two or more senses
Neurons: Transduction & Transmission
- Key components of neurons:
- – Cell body
- – Dendrites
- – Axon or nerve fiber
- • Receptors - specialized neurons that respond to specific kinds of energy
- The neuron on the right consists of a cell body, dendrites, and an axon, or nerve fiber. The neuron on the left that receives stimuli from the environment has a receptor in place of the cell body.
- Each of these receptors is specialized to transduce a specific type of environmental energy into electricity.
- Arrows indicate the place on the receptor neuron where the stimulus acts to begin the process of transduction.
- Electrical signals or action potentials occur
- permeability of the membrane changes
- Na+ flows into the fiber making the neuron more positive, then,
- K+ flows out of the fiber making the neuron more negative
- Finally Na+ pumped out of axon, to restore normal cell level.
- • This process travels down the axon in a propagated response
- (a) When a nerve fiber is at rest, there is a difference in charge of -70 mV between the inside and the outside of the fiber. This difference is measured by the meter on the left; the difference in charge measured by the meter is displayed on the right.
- (b) As the nerve impulse, indicated by the red band, passes the electrode, the inside of the fiber near the electrode becomes more positive. This positivity is the rising phase of the action potential.
- (c) As the nerve impulse moves past the electrode, the charge inside the fiber becomes more negative. This is the falling phase of the action potential.
- (d) Eventually the neuron returns to its resting state.
Properties of Action Potentials
- Action potentials:
- – show propagated response.
- – remain the same size regardless of stimulus intensity.
- – increase in rate to increase in stimulus intensity.
- – have a refractory period of 1 ms - upper firing rate is
- 500 to 800 impulses per second.
- – show spontaneous activity that occurs without
- Response of a nerve fiber to (a) soft, (b) medium, and (c) strong stimulation.
- Increasing the stimulus strength increases both the rate and the regularity of nerve firing in this fiber.
• Neurotransmitters are:
- released by the presynaptic neuron from vesicles.
- received by the postsynaptic neuron on receptor sites.
- matched like a key to a lock into specific receptor sites.
- used as triggers for voltage change in the postsynaptic neuron.
Types of Neurotransmitters
- Excitatory transmitters - cause depolarization
- – Neuron becomes more positive
- – Increases the likelihood of an action potential
- • Inhibitory transmitters - cause hyperpolarization
- – Neuron becomes more negative
- – Decreases the likelihood of an action potential
Neural Circuits - in the eye
- Groups of neurons connected by excitatory and inhibitory synapses
- A simple circuit has no convergence and only excitatory inputs.
- – Input into each receptor has no effect on the output of neighboring circuits.
- – Each circuit can only indicate single spot of stimulation.
- Y shaped synapse: excitatory
- Useful for basic detection but become inefficient when stimulus getting bigger
- Convergent circuit (excitatory connections)
- – Input from each receptor summates into the next neuron in the circuit.
- – Output from convergent system varies based on input.
- – Output of circuit can indicate single input and increases output as length of stimulus increases.
- Tells us presence/not and how big it is through the output of B
Convergent circuit with excitatory and inhibitory connections
- – Inputs from receptors summate to determine output of circuit.
- – Summation of inputs result in:
- • weak response for single inputs and long stimuli.
- • maximum firing rate for medium length stimulus.
- Because stimulation of the receptors on the side (1, 2, 6, and 7) sends inhibition to neuron B, neuron B responds best when just the center (3 - 5) are stimulated.
- Good at detecting edges
- Area of receptors that affects firing rate of a given neuron in the circuit. Receptive fields are the areas of the retina that, when stimulated, produce a change in the firing of cells in the visual system.
- Receptive fields are highly specialised and differ in their sensitivity to specific features of a line, such as its position, length, movement, colour and intensity.
- • Receptive fields are determined by monitoring single cell responses.
- • Research example for vision
- – Stimulus is presented to retina and response of cell is measured by an electrode.
- All the numbers are part of B's receptive field
Center-Surround Receptive Fields
- Receptive fields in the retina are often circular with a centre-surround arrangement.
- Cells fire when stimulated in the centre of their receptive field but do not fire when stimulated outside the centre area.
- Excitatory and inhibitory effects are found in receptive fields.
- • Center and surround areas of receptive fields result in:
- – Excitatory-center-inhibitory surround
- – Inhibitory-center-excitatory surround
- (a) Response of a ganglion cell in a cat’s retina to stimulation: outside the cell’s receptive field (area A on the screen); inside the excitatory area of the cell’s receptive field (area B); and inside the inhibitory area of the cell’s receptive field (area C). (b) The receptive field is shown without the screen.
Can detect mouse's location by the movement of the mouse into the centre receptive fields into different neurons
- Output of center-surround receptive fields changes depending on area stimulated:
- – Highest response when only the excitatory area is stimulated
- – Lowest response when only the inhibitory area is stimulated
- – Intermediate responses when both areas are stimulated
Cross section through the retina
- 1. Light passes between the ganglion cells and the bipolar cells, reaching roads and cones at the back of the retina.
- 2. The rods and cones, which are sensitive to light, respond by transmitting information to the bipolar cells
- 3. Now, the bipolar cells transmit this information to the ganglion cells
- 4. The axons of the ganglion cells gather together forming the optic nerve, which transmits the messages from both eyes to the brain, where they are interpreted as sight.
Bipolar cells: interneuron between neurons and rods/cones
- • 126 million rods and cones converge to 1 million ganglion cells.
- • Higher convergence of rods than cones
- – Average of 120 rods to one ganglion cell
- – Average of six cones to one ganglion cell
- – Cones in fovea have one to one relation to ganglion cells
Retinal processing- rods and cones
- Shape: rods are large and cylindrical
- Cones are small and tapered
- Distribution on retina: fovea consists solely of cones
- Peripheral retina has both rods and cones
- More rods than cones in periphery
- Blind spot: thats where all the info goes through the optic nerve. No receptors
Convergence and Sensitivity
- Rods are more sensitive to light than cones.
- – Rods require less light to respond
- – Rods have greater convergence which results in summation of the inputs of many rods into ganglion cells increasing the likelihood of
- – Trade-off is that rods cannot distinguish detail
Rods: low light detectors. Highly sensitive to light
Convergence and Detail
- All-cone foveal vision results in high visual acuity
- – One-to-one wiring leads to ability to discriminate details
- – Trade-off is that cones need more light to respond than rods
- a) convergence says there is light but doesn't tell you there are two whereas the cones do
- b) the spot of light moved however convergence does not detect this whereas cones do.
Lateral Inhibition of Neurons
- Or inhibition between neurons
- • Experiments with eye of Limulus (Horseshoe Crab)
- – Ommatidia allow recordings from a single receptor.
- – Light shown into a single receptor leads to rapid firing rate of nerve fibre.
- – Adding light into neighbouring receptors leads to reduced firing rate of initial nerve fibre
- Signals between these neurons are acting like the inhibitory neurons in the convergent circuit
Lateral Inhibition and Lightness Perception
- Three lightness perception phenomena
- explained by lateral inhibition
- – The Hermann Grid: Seeing spots at an intersection
- – Mach Bands: Seeing borders more sharply
- – Simultaneous Contrast: Seeing areas of different brightness due to adjacent areas
- People see an illusion of gray images in intersections of white areas.
- • Signals from bipolar cells cause effect
- – Receptors responding to white corridors send inhibiting signals to receptor at the intersection
- – The lateral inhibition causes a reduced response which leads to the perception of gray.
- (a) Four squares of the Hermann grid, showing five of the receptors under the pattern. Receptor A is located at the intersection, and
- B, C, D, and E have a black square on either side. (b) Perspective view of the grid and five receptors, showing how the receptors
- connect to bipolar cells. Receptor A’s bipolar cell receives lateral inhibition from the bipolar cells associated with receptors B, C, D,
- and E. (c) The calculation of the final response of receptor A’s bipolar cell starts with A’s initial response (100) and subtracts the
- inhibition associated with each of the other receptors.
Sensory Code: Representation of Environment
- Sensory code - representation of perceived objects through neural firing
- Specificity coding - specific neurons responding to specific stimuli
- • Leads to the “grandmother cell” hypothesis
- • Recent research shows cells in the hippocampus that respond to concepts such as individual celebrities.
- – Problems with specificity coding:
- • Too many different stimuli to assign specific neurons
- • Most neurons respond to a number of different stimuli.
- Distributed coding - pattern of firing across many neurons codes specific objects
- – Large number of stimuli can be coded by a few neurons.
- How faces could be coded by distributed coding. Each face causes all the neurons to fire, but the pattern of firing is different for each face. One advantage of this method of coding is that many faces could be represented by the firing of the three neurons.