Psychophysical Correlation

Question 1

What’s the absolute threshold and how can it be estimated?

Answer 1

The smallest amount of stimulus energy that can detected - a basic way of testing the sensitivity of a sensory modality.

Can be taken as the arbitrary intensity value where detection rate is 50% - can be worked out from a psychophysical function.

Question 2

How can you visualise temporal aspects of neural coding?

Answer 2

Instantaneous frequency

• A trace showing single spikes but on a fast timescale.
• At each spike time (t), you plot the frequency
• Frequency = 1000/T, and T=tn-tn-1

Raster display

• Each dot represents a single spike occurrence
• Each row is a separate stimulus presentation

Post stimulus time histogram

• Shows the firing rate averaged across many stimulus repetitions.
• Time divided into 10ms bins and number of spikes in each bin is added up.

Question 3

Whats the difference threshold?

How do you calculate the value termed just noticeable difference? (JND)

What is it?

Answer 3

The smallest detectable change between two stimuli

JND = stimulus intensity value at 75%- stimulus intensity value at 25% / all divided by 2

• y axis would % correct judgement
• x axis would be stimulus intensity

JND - the amount something must be changed in order for the difference to be noticeable, detectable at least half the time.

Question 4

What did Valbo contributed to the world of psychophysical correlation?

Answer 4

Valbo et al 1979

• Microneurography they found sensation was elicited by a single spike in 6/8 FA1 afferents (corpuscles)
• Sensation was only elicited in SAI afferents (disks) by a train of spikes above some minimum frequency

Valbo 1995
- Compared the psychophysical correlation in RFs in finger and palms and found psychophysical thresholds for detection correspond to neural thresholds in finger tips but are higher in palms.

Question 5

What’s the link between log coding and stimulus intensity?

Advantages?

Answer 5

a) Most senses obey Weber’s and Fechner’s Laws

Weber’s Law:
- The JND between 2 stimuli is proportional to stimulus intensity.
Fechner’s law:
- The law states that the change in a stimulus that will be just noticeable is a constant ratio of the original stimulus.

Therefore AP frequency increases with receptor potential amplitude and is linearly proportional to log of stimulus over the dynamic range.

n.b. absolute sensitivity is less at high intensities but proportional intensity is constant.

b) - Allows representation of the large dynamic range
- Most human senses appear to work logarithmically which allows amplification of smaller signals relative to bigger ones