Psychophysics and Neural Coding in Perception

psychophysics

a scientific approach for establishing quantitative relations between physical and perceptual events

Magnitude Estimation

the subject rates an aspect of a stimulus (how bright something appears or how loud it sounds)

Magnitude Estimation Task

the subject rates an aspect of a stimulus (how bright something appears or how loud it sounds)

detection/discrimination

measures thresholds, the minimum that can be perceived

Method of Limits

starting from a high intensity, decrease intensity until the participant doesn't detect it, THEN vice versa

absolute threshold

smallest stimulus level that can just be detected (the stimulus is detected 50% of the time)

thresholds

measure the limits of sensory systems (measures of minimums, means of crossovers)

Method of Adjustment

same as method of limits, but participants adjusts a knob

Method of Constant Stimuli

same as method of limits, but the levels and orders are randomized

bias

overall tendency to report one option over another, regardless of the stimulus

forced choice task

present a stimulus on some trials, no stimulus on other trials; participant must respond on every trial either 'present' or 'absent'

false alarm rate

how often did the participant say 'present' when the stimulus was absent

Discrimination Task

the participant's task is to compare two stimuli and indicate which stimulus has a greater intensity (brighter, louder, etc.)

difference threshold

just noticeable difference (JND) - the smallest detectable difference

Weber's Law

simple, universal relationship between a stimulus level and the smallest difference that can be detected from that JND level

JND

a constant proportion of the stimulus level; JND = kS

internal responses

responses that are variable and can be affected by internal variability, also called 'noise'

Internal variability

Explains why a person may make different responses on different trials even when the same stimulus is presented.

Intrinsic (dark) light

Spontaneous sense of dim light caused by residual retinal signals arising in the complete absence of light stimulation.

Signal Detection Theory (SDT)

Models how we detect a signal in noise.

d-prime (d')

Quantifies the separability of signal from noise; calculated as d'= distance/STD.

Criterion (c)

Quantifies the response required to say the signal is present; placed to the right indicates a more conservative criterion, while placed to the left indicates a more liberal criterion.

Sensitivity

The ability to distinguish signal from noise.

Higher sensitivity results in

More correct answers (hits and correct rejections) and fewer incorrect answers (misses and false alarms).

Simple decision rule

Determines response based on the criterion; to the left reports absent, to the right reports present.

Criterion changes

More liberal criterion leads to more 'yes' responses and fewer 'no' responses.

Difference threshold

The minimum difference that must exist between two stimuli before we can tell the difference between them.

Weber fraction

Remains constant for a particular sense, but each type of sensory judgment has its own Weber fraction.

Response compression

Increase in perceived magnitude is smaller than the increase in stimulus intensity.

Response expansion

As intensity is increased, perceptual magnitude increases more than intensity.

Power functions

Relationship between intensity of a stimulus and perception of its magnitude.

Steven's power law

Equation: P=KS^n, where n=2 and k=constant of 1 results in P=100.

Response criterion

Each person has a different perception affecting their responses.

Signal detection approach

Used to take differing response criteria into account with multiple stimulus intensities.

Receiver operating characteristic (ROC) curve

Indicates a person's sensitivity; identical ROC curves imply equal sensitivities.

Signal Detection Theory

1. The percentage of hits and false alarms depends on a person's criterion. 2. A person's sensitivity to a stimulus is indicated by the shape of the person's ROC curve.

Signal

Stimulus presented to the person (tone).

Noise

All the other stimuli in the environment that can be mistaken for the signal.

False alarms

Caused by noise.

Neurons and Neural Coding

Perception is not a perfect copy of a stimulus due to variable internal responses.

Internal responses

Variable, making it difficult to distinguish signal from noise.

Brain composition

What is the brain made of?

Camillio Golgi

Believed the nervous system was a single network, not separate cells.

Santiago Ramon y Cajal

The idea that the nervous system is comprised of discrete cells.

Golgi stain

Labels a small subset of neurons to enable visualization.

Cajal drawing

Illustrates that the nervous system has discrete cells, which communicate in circuits.

Neuron

A cell that is part of the nervous system.

Dendrite

The part of a neuron that serves as input.

Axon

The part of a neuron that serves as output.

Myelin sheath

Allows for signal transmission along the axon.

Micro-electrode

Stabbed within the cell membrane to find the electrical signal.

Amplifier

Reads out the voltage of the signal from the neuron.

Action potential

Depolarization (increase in voltage) that propagates down an axon.

Action potential measurement

Starts at around -70 millivolts, increases to +40, and back down to -70.

Electrical signal

Carried by Na+ and K+ ions during the action potential.

Propagation of action potential

Ions flow in and out of the cell membrane segment by segment.

Spontaneous activity

Neurons regularly fire at a low rate even when no stimulus is present.

Firing rates

Used by neurons to transmit information, increasing with stronger stimuli.

Properties of action potentials

They propagate down an axon, remain the same size regardless of stimulus intensity, and show spontaneous activity.

Neural responses

Variable and not identical every time, but with some common structure.

Synapse

The gap between neurons through which information is transmitted.

Neurotransmitters

Chemicals released by the presynaptic neuron to transmit information across the synapse.

Triggers voltage change in the post synaptic neuron

Ex. SSRIs causes serotonin to linger in the synapse

Excitatory neurotransmitters

Depolarize the neuron, increasing membrane potential making it more likely for the post synaptic neuron to fire.

Inhibitory neurotransmitters

Hyperpolarize the post synaptic neuron, decreasing the membrane potential, decreasing its likelihood of firing.

Action Potential (AP)

Occurs only when the membrane potential reaches a certain threshold level.

Synaptic transmission

Synapses can send different amounts of excitatory and inhibitory neurotransmitters.

Cancellation of inputs

Both excitatory and inhibitory inputs can cancel each other out.

Neural signaling flexibility

Synapses make neural signaling more flexible.

Amplification in synapses

Increase weak signals at the site of transmission.

Sign reversal in synapses

Lets neurons add and subtract, helping to integrate and summarize information from neurons.

Learning

Strengthens and weakens the connection between two neurons (continue to change)

Cortical neurons

Communicate by spikes; all spikes are pretty much the same.

Law of specific nerve energies

Allows us to know when neurons fire.

Neural code

The scheme that neurons use to carry information.

Specificity coding

A neural code where 1 neuron specifies a particular stimulus.

Sparse coding

A neural code where a few neurons specify a particular stimulus.

Population coding

A neural code where many neurons specify a particular stimulus.

Visible light

A form of electromagnetic radiation that ranges from 400-700nm.

Wavelength

The length of one cycle (from peak to peak).

Light focusing

Light rays reflect from nearby points on an object and contact a surface at corresponding nearby points.

Blurry image

Occurs when divergent light rays are not fully refocused.

Human eye

Built to form an image on the retina.

Convex lens

Bends light rays to converge on a focal point.

Focal point

Closer to the light source = farther focal point; farther light source = closer focal point.

Focal length

Depends on the lens strength.

Less convex lens

Light bends less, resulting in a farther focal point (for farther objects).

More convex lens

Light bends more, resulting in a closer focal point (for closer placed objects).

Accommodation

The lens can change its convexity to adjust the focal point depending on the distance to the object.

Emmetropia

Correct focus.

Myopia

Nearsightedness; failure to see farther away, fixed by concave lens.

Hyperopia

Farsightedness; failure to see close, occurs when the eyeball is too short, fixed by convex lens.

Presbyopia

Also known as 'old eye'; can't focus on nearby objects due to a stiff lens.

Blind spot

Where the optic nerve exits the eye; there are no cells to detect light in the blind spot.

Retina

Contains photoreceptors that transduce light into neural signals and retinal circuitry that processes images.

Retinal ganglion cells

Send output to the brain.

Photoreceptors

Light detecting cells in the retina that transduce light.

Opsins

Molecules in the outer segments of photoreceptors that absorb light and generate a signal.

Transduction

Converting light into neural signals.

Rods

Photoreceptors sensitive to dim light in the periphery that cannot support color vision.