From neurons to psychophysics

Question 1

Sensory transduction converts physical energy into electrical signals

Answer 1

• photosensitive
• mechanosensory
• chemosensory

Question 2

Explain how receptor potentials can code stimulus intensity by gradations in amplitude

Answer 2

• Receptor potentials are examples of graded potentials
• Receptor potentials in vertebrate photoreceptors are hyperpolarising
• more commonly, receptor potentials are depolarising

Question 3

Further explain receptor potentials

Answer 3

• Receptor potentials (RP) in invertebrate photoreceptors are depolarising
• RP are generated by selective opening of ion channels causing an ionic current to flow across the membrane
  
  • Exceptions: vertebrate photoreceptors RPs are generated by selectively closing of ion channels

Question 4

Describe sensory coding of stimulus intensity

Answer 4

• Receptor potentials graded with light intensity
• V is plotted as difference from rest, which is here set to zero
• Amplitude v intensity

Question 5

What is RP amplitude limited by??

Answer 5

-by the reversal potential of the ionic current that generates it (near 0mV)

Question 6

Sensitivity or range?

Answer 6

• Large dynamic range comes at cost of less sensitivity to intensity differences
• Fractionation: different receptors with different ranges (or adaptation of same receptor)

Question 7

Psychophysics - How do you measure sensation and perception?

Answer 7

• First step is to separately conceptualise physical stimulus and associated sensation (Advances in physical science made this possible)
• We can measure the physical stimuli - power per unit area
• Can we measure the associated sensations?
• Thresholds = sensitivity
• scaling?

Question 8

Physical and subjective intensity

Answer 8

• Physical intensity: power per unit area
• SI unit: watts per square metre
• Many physical stimuli can be expressed as such (light and sound intensity, etc.)
• in practice other measures may be more convenient

Question 9

What is meant by two alternative forced choice (2AFC)? How is this method thought to mitigate response bias?

Answer 9

Y or N
Respondent learns what is going on

Question 10

Explain how the amplitude of graded potentials is determined and limited by ionic equilibrium potentials, and the implications of this for sensory neural signalling.

Answer 10

x

Question 11

Thresholds vs Scaling

Answer 11

• Thresholds: measuring limits of sensitivity
• Scaling: ordering stimuli along a perceptual dimension

Question 12

ABSOLUTE THRESHOLDS

Answer 12

weakest stimulus intensity

Question 13

DIFFERENCE THRESHOLD

Answer 13

smallest difference between two stimuli that an observer can discriminate/difference limen or just-noticeable difference (JND)

Question 14

What is the smallest change in stimulus intensity that leads to a change in sensation (just-noticeable difference, jnd)?

Answer 14

Weber’s law: jnd is a proportional, not an absolute amount
If i is stimulus intensity and Ai is jnd,
change in і/i= C)
where C is a constant

Question 15

YES OR NO METHOD OF CONSTANT STIMULI

Answer 15

problem? response bias

Question 16

Response bias

Answer 16

• near threshold, stimulus is difficult to detect
• Yes/No - observers may be more or less conservative in deciding
• 2AFC can get round this

Question 17

Pyschophysical correlation

Answer 17

Identify specific patterns of neural activity and correlate with specific sensations and perceptions

Question 18

Microelectrode recordings from cerebral cortex

Answer 18

-Could be regarded as a ‘gold standard’ in so far as individual action potentials can be resolved and recorded
-Analogous to microneurography in peripheral nerve
-Occasionally possible in humans, when microelectrodes may be implanted for clinical reasons prior to neurosurgery
-Over the past few decades numerous reports available of single or multi-neuron recordings from conscious human brain
-Textbook systems-neuroscience content is dependent to a great extent on microelectrode recording of neurons in non-human brains