Hecht et al

Question 1

Background

Answer 1

Light consists of quanta, or discrete packets (we know them as
photons)
○ E = hf, h = 6.62607015×10−34 J/Hz
● Visual purple = rhodopsin
○ Your rods’ light sensor
● At low stimulus intensities (the threshold of vision), just a few
molecular ‘activations’ are sufficient for perception
● This work is interested in the ‘precise number of … molecular changes’
○ E.g. how sensitive are the eyes (how many photons?)
○ Specific amount of energy

Question 2

What did they try to measure? Why did they perform similar experiments to those people had done before?

Answer 2

Interested in the ‘precise number of … molecular changes’
○ E.g. how sensitive are the eyes (how many photons?)
○ Specific amount of energy
Seeking to determine the minimum energy for vision.

Previous experiments contained errors; Langley used the wrong light and didn’t take any precautions; results were too high. Other attempts were erroneous in that they did not have direct energy determinations.

Question 3

Table 1

Answer 3

Previous results three sets of direct energy measurements which are free from obvious error. Differ by a factor of three, but can be considered as roughly confirming one another. But want to take the measurements again under best physical and physiological conditions.

Preconditions for maximum retinal sensitivity are dark adaptation peripheral vision, small test fields, short exposures, and selected portions of the spectrum.

Question 4

Methods

Answer 4

● Maximize retinal sensitivity
○ Dark adaptation, peripheral vision,
small test field, short exposure,
green light
● Subject sits inside a dark
chamber, and a light is flashed,
that the subject then reports

Question 5

Figure 1

Answer 5

Optical system for measuring minimum energies necessary for vision. The eye at pupil P fixates the red point FP and observes the test field formed by the lens FL and diaphragm D. The light for this field comes from the lamp L through the neutral filter F and wedge W, thru the double monochromator M1M2 and is controlled by the shutter S.

S is a precision shutter. Permits light to pass thru middle slit for 1/1000 second during each revolution.

Question 6

Figure 2

Answer 6

Shutter for obtaining a single exposure of 1/1000 second.

Question 7

Table 2

Answer 7

● The minimum number of photons necessary to detect a stimulus is roughly 80-100.
● Pretty high variance

Problem – when light travels through the eyes, there are molecules that can cause some of the energy to be lost. Not all the photons are being absorbed and some deflections happening. So miminum energy shown in Table 2 is not fully equivalent to the actual amount of energy required for vision. Must take all the energy lost into account.

Question 8

Reflections and Absorptions

Answer 8

● Energies in Table 2 represent what arrives at the cornea
● However there are lots of energy losses en route to retina
○ Reflection, vitreous humor
● Rhodopsin only absorbs 10% of light
● Important to parse these out to figure out the exact relationship
between photon and photoreceptor

Question 9

Figure 3

Answer 9

You simulate the relationship between the relative absorption and the diff wavelength under different concentrations of rhodopsin (visual purple). Broader range of absorption when the concentration is higher. Simulation results compares to Figure 3, (empirical results); this is the actual absorption relationship with the wavelength. Compare the simulation with the empirical result and can approximate the real concentration of visual purple that mimicks the reality.

Question 10

Key Points

Answer 10

•Demonstration of the minimum amount of quantum/photons/light
necessary to activate the human eye
•Constructedarig with a vibrating slit that reduces the amount of
detectable light to human eye
•Appliedphysical principles to estimate the excitability of visible
purple/rhodopsin
•Based on poissondistribution curves, the relationship between the
intensity and frequency of light detection helps establish the number of necessary quantum.

Question 11

Visual threshold

Answer 11

The minimum level of stimulation that can be detected visually

Question 12

Poisson distribution

Answer 12

Used poisson distribution – even you shine the light that you can artificially show, there is a huge variation in terms of the number of quantas each time you shed the light. You want to measure the accurate number of quantas required for vision.

Question 13

Figure 4

Answer 13

● As concentration increases, a greater amount of frequencies are
caught by Rhodopsin

Question 14

Figure 5

Answer 14

● The fact that the upper curve doesn’t agree with experimental data
suggests that it is the upper bound on Rhodopsin efficacy
● E.g. at most, 20% of light is absorbed
● So what exactly is the energy amount being captured by the eye?

Question 15

Section VII

Answer 15

● About 5-14 photons are likely being absorbed by Rhodopsin
● Likely that each rod is only picking up a photon or two
● E.g. simultaneous activation of multiple rods

Question 16

Figure 6

Answer 16

● Actual photons are poisson-distributed
● Can use this to more accurately determine ‘N,’ the number of quanta that are
being detected by the eye
● The poisson lines up exactly with a simple experiment

Question 17

Figure 7

Answer 17

● The best poisson fits suggest an n of about 6
● E.g., 6 or more photons are the threshold for detection
● This is a pretty ingenious and sophisticated way to determine a threshold,
especially for the 40s

Question 18

Conclusions

Answer 18

● Roughly 50-100 photons hitting the cornea are sufficient for perception
● Regardless of the cornea or inner eye, only about 6 individual molecular
events (photons hitting rhodopsin) are necessary for visual perception
● One rod probably can detect photon
● ‘At the threshold, it is the stimulus which is variable’