From Neurones to Psychophysics

Question 1

What is sensory transduction?

Answer 1

the conversion of physical stimulus into electrical signals, which then via neural processing can be used to construct mental representations, which is the brain’s hypothesis of what is out there in the world

Question 2

What are the different sensory transduction processes?

Answer 2

• Photosensory transduction (rods and cones)→ visible light
• Mechanosensory transduction → mass, force, vibration
• Chemosensory transduction → olfaction, taste

In all cases, the end result is to produce an electrical signal

Question 3

What is a receptor potential?

Answer 3

transmembrane potential difference produced by activation of sensory receptor
-type of graded potential

Question 4

Are most receptor potentials depolarising or hyperpolarising?

Answer 4

Most commonly, receptor potentials are depolarising

• opening of ion channels is linked to the sensory transduction in a graded manner
• a weak stimulus will open a few ion channels, generating a small current leading to a small depolarisation
• progressively stronger stimuli will open more ion channels, generating larger current, larger depolarisatoins in a graded manner until it reaches a point of saturation (amplitude can’t keep growing)

Question 5

What is an exception in which the receptor potential hyperpolarises in repsonse to a stimulus?

Answer 5

Receptor potentials in vertebrate photoreceptors (rods and cones) are hyperpolarising

• in dark, cell is relatively depolarised
• in light (stimulus), cell will hyperpolarise and as you increase stimulus the magnitude of hyperpolarisation will grow and receptor potential will be more negative

Question 6

How is the receptor potential amplitude limited?

Answer 6

The receptor potential amplitude is limited by the reversal potential of the inward ionic current that it generates
-reversal potential is typically at 0mV

Question 7

What is the reversal potential of an ion?

Answer 7

The reversal potential of an ion is the membrane potential at which there is no net overall flow of that particular ion from one side of the membrane to the other

All ionic currents have a reversal potential

Question 8

Example: Explain the Limiting depolarising receptor potential

Answer 8

Resting potential is -70mV and reversal potential is 0mV.

Weak stimulus opens small number of ion channels and inward cation current flows but not all the way to the reversal potential because there are other background currents competing with this weak current

Stronger stimulus opens more ion channels and larger current produced, with the membrane potential moving closer to the reversal potential

The receptor potential will approach the value of the reversal potential asymptomatically, but it will never actually cross it, even if we keep applying a stronger stimulus, meaning the receptor potential will not keep growing

*the value of the reversal potential limits the range over which graded potentials can code stimulus intensity

Question 9

What would the receptor potential response be if the resting potential was +50mV (and not -70mV)?

Answer 9

If the resting potential is positive to the reversal potential, the receptor potential would go negative (hyperpolarise) to the reversal potential
-net current flow is outward (positive charge leaving the cell)

Question 10

Where is the range in which the receptor can usefully operate?

Answer 10

Between threshold and saturation, we have a dynamic range in which which the receptor can usefully operate

Changes in intensity above the saturation level cannot be detected

Question 11

Explain the relationsgip between Receptor potential sensitivity and range.

Answer 11

There is a trade-off between receptor sensitivity and range

-the more sensitive a receptor is to changes in intensity, the quicker it reaches saturation level, meaning it has a smaller dynamic range and it saturates at lower stimulus intensities

Question 12

What is range fractionation?

Answer 12

It is the idea that different receptors have different thresholds for firing

-this covers more of the dynamic range by having neuronal specialisation

Question 13

Which receptors display range fractionation?

Answer 13

Rods and cones

• rods have a low absolute sensitivity but saturate at low light levels
• cones have moderate absolute sensitivity but saturate at a higher level

Question 14

How can a receptor shift along a stimulus intensity axis and respond to higher stimulus intensities?

Answer 14

By Adaptation

• receptor saturates
• receptor gradually adapts to this stimulus intensity
• receptor goes back to non-responding, but at a new level of adaptation where it can respond to higher intensities

Question 15

How do we measure sensation and perception?

Answer 15

1) Separately conceptualise the physical stimulus and associated sensation
2) Physical stimulus can be measured: power per unit area (e.g. Watts per square metre)
3) We can’t directly measure the physiological sensations but there are two basic approaches to measuring sensations to stimuli:
- Thresholds: measuring limits of sensitivity
- Scaling: ordering stimuli along a perceptual dimension

Question 16

What is the Psychophysical threshold?

Answer 16

the lowest level of a stimulus that causes a change in the mental state of an observer

Question 17

What are the Different psychophysical thresholds?

Answer 17

Absolute Threshold

Difference Threshold

Question 18

What is the Absolute Threshold?

Answer 18

weakest stimulus intensity that is detectable (e.g. weakest flash of light distinguished from absolute dark)

Question 19

What is the Difference Threshold?

Answer 19

smallest change in stimulus intensity that leads to a change in sensation (also called difference limen, or just-noticeable difference/JND)

Weber’s law states that the minimum value difference that you can detect is not an absolute value, but a percentage value of the original stimulus

Question 20

What is a possible limitation in an experiment measuring psychophysical thresholds?

Answer 20

RESPONSE BIAS
-observers may be more or less conservative in deciding if they have detected the stimulus when you get near the threshold

Question 21

How can we circumvent response bias when measuring psychophysical thresholds?

Answer 21

Using a two-alternative forced-choice (2AFC)
-if there is a response bias, the bias will apply equally (e.g. your bias towards believing you detected the stimulus when you weren’t certain would cancel out because your bias would be the same for the intervals with and without the stimulus)

Question 22

What is Psychophysical correlation?

Answer 22

Psychophysical correlation identifies specific patterns of neural activity and correlates them with specific sensations and perceptions

Question 23

What is microneurography?

Answer 23

Neurophysical method employed by scientists to visualise and record the normal traffic of nerve impulses that are conducted in peripheral nerves of waking human subjects (extracellular recording)

Question 24

How does microneurography work?

Answer 24

Tungsten needle electrodes coated with insulating material up to tip are inserted into the nerve (e.g. ulnar, median), and stimulus applied (e.g. finger touched)

These electrodes can pick up small current due to passing action potential in a nearby axon

Question 25

What pathway mediates tactile sensation?

Answer 25

The Dorsal Column-Medial Lemniscal System (DCML) of the somatosensory system

Question 26

Explain the Process of tactile sensation

Answer 26

Primary sensory afferents (Aβ fibres) are large myelinated fibres which convey tactile sensation and innervate sensory endings in the skin.

1) Aβ afferents may send out some local branches within the grey matter of the spinal cord, but the main axon branch ascends in the dorsal columns (white matter) of the spinal cord to the dorsal column nuclei (gracile and cuneate nuclei) in the medulla
2) Axons synapse at these nuclei, and second order neurones originate from the dorsal column nuclei. Second order neurones immediately cross the midline and ascend in a fibre tract called the medial lemniscus on the contralateral side
3) They then go through the midbrain and synapse in the ventral posterior nuclear complex of the thalamus
4) The thalamic neurones are third order neurones and their axons project to the cerebral cortex, specifically to the primary somatic sensory cortex. This would be the first reception/sensation of tactile information in the cerebral cortex

Question 27

What are the different classes of tactile afferents?

Answer 27

Superficial:

• Merkel’s disks
• Meissner corpuscles

Deep:

• Pacinian corpuscles
• Ruffini’s corpuscles

*what we know of the physiology of these different specialised nerve endings is largely due to microneurography recordings