13
$\begingroup$

It seems like 2 eyes is enough “wetware” to do interferometry inside brain. Can you definitely see some reason why this could not be happening, or some way to test if it does happen?

$\endgroup$
6
  • 1
    $\begingroup$ Not quite. You will also need something like a reflecting surface or a thin film. $\endgroup$ Oct 19, 2022 at 12:37
  • 11
    $\begingroup$ I have no idea what you mean by interferometry inside brain. Please clarify. $\endgroup$ Oct 19, 2022 at 12:39
  • 1
    $\begingroup$ The brain could detect the phase difference of the fourier components of the rough features of the image. That would tell how much to cross the eyes to make the phase match between both eyes. $\endgroup$
    – Florian F
    Oct 19, 2022 at 20:41
  • 6
    $\begingroup$ You're looking at the wrong sense transducers. Having two ears allows for forms of interferometry. e.g. the relative phase of a single sound source surrounding at each ear gives an indication of the azimuth of the sound source: en.wikipedia.org/wiki/Sound_localization Meanwhile, hearing two sounds with slightly different frequencies produces "beating": the perception of a non-existent sound that has a frequency related to the difference of the two source frequencies. i.e. this is a sound that "exists" only in the brain, not the world. en.wikipedia.org/wiki/Beat_(acoustics) $\endgroup$ Oct 20, 2022 at 1:14
  • 2
    $\begingroup$ @WillVousden The temporal response of neurons is orders of magnitudes too slow to detect phase differences in electromagnetic signals. Sound waves, however, vibrate at frequencies that are viable for biological systems to process in terms of phase. $\endgroup$ Oct 23, 2022 at 23:09

7 Answers 7

48
$\begingroup$

To do interferometry in post-processing after detection of radiation, the detector must be able to record the phase of the radiation. The eye cannot do this: the photochemical reactions that record the radiation are insensitive to phase.

In instrumentation, radio interferometry may be done post-detection because phase-sensitive radio detectors are practical. Optical interferometry is done pre-detection, using mirrors.

$\endgroup$
4
  • $\begingroup$ Maybe there is some animal that detects phase? $\endgroup$
    – Euphorbium
    Oct 19, 2022 at 13:13
  • 16
    $\begingroup$ @Euphorbium considering the phase changes about 700 trillion times per second per pixel, it's not conceivable. That's why optical interferometers only measure the difference in phase, which doesn't change nearly as quickly. A radio wave might only change phase a few tens of millions times per second per pixel, which is doable with modern electronics. $\endgroup$ Oct 19, 2022 at 23:34
  • 1
    $\begingroup$ @user253751 10s of MHz is considered "low frequency" by radio astronomers. ALMA operates at 100's of GHz. But still, it's 1000x slower than optical. $\endgroup$
    – John Doty
    Oct 20, 2022 at 1:00
  • 3
    $\begingroup$ @JohnDoty the most expensive ground-based telescope in operation, pushing the limits of what's possible with technology :) $\endgroup$ Oct 20, 2022 at 16:02
9
$\begingroup$

You're looking at the wrong sense transducers. Having two ears does allow for forms of interferometry, as the frequencies of auditory signals are in a range that allows for the nervous system to respond to phase differences between signals.

e.g. the relative phase of sound waves from a single source arriving at each ear gives an indication of the azimuth of that source: sound localisation

Meanwhile, hearing two sounds with only very slightly different frequencies, when each is played only in one ear, produces “binaural beats”: the perception of a non-existent sound that has a frequency equal to the difference of the two source frequencies. i.e. this is a sound that “exists” only in the brain, not the world.

$\endgroup$
5
  • 6
    $\begingroup$ This is incorrect. The human ear/cochlea measures the intensity of the magnitude of the spectral spectral without the phase. Direction finding is done by measuring the difference of arrival time s between the two sensors (ears) not their phase difference of arrival. The real mystery is how a female cricket in a tall grass can find her male friend with a separation of two such sensors 1mm apart... $\endgroup$
    – hyportnex
    Oct 20, 2022 at 11:34
  • 1
    $\begingroup$ I can't find anywhere in the linked wikipedia source which supports the claim that the auditory system uses the phase of the sound waves to determine direction. Instead, it seems to support @hyportnex's statement that animals use the difference in time of arrival for the intensity, which wouldn't qualify as interferometry. $\endgroup$
    – craq
    Oct 20, 2022 at 21:04
  • 3
    $\begingroup$ For sounds with a wavelength that is long enough, the phase difference is used for localisation. I.e. this also works with continuous tones, not just short 'pulses'. en.wikipedia.org/wiki/Interaural_time_difference $\endgroup$
    – Compizfox
    Oct 20, 2022 at 21:16
  • 3
    $\begingroup$ @craq You're considering only brief sound pulses. As Compizfox notes, the effect is also possible for a continuous pure tone. In that case, the differential time of arrival at each ear is detectable only via phase differences (plus azimuth-dependent intensity changes due to the shape of the head and pinna (external ear), the individual head-related transfer function en.wikipedia.org/wiki/Head-related_transfer_function ). $\endgroup$ Oct 20, 2022 at 21:38
  • 1
    $\begingroup$ This is a key difference between vision and audition. In vision there is a ~10 order of magnitude gap between frequency and time. But they are on-top of each-other for audition and their interaction produces interesting effects. $\endgroup$ Oct 21, 2022 at 20:39
8
$\begingroup$

Despite the other two answers denying this there is another possibility. The Hanbury Brown - Twiss intensity interferometer uses noncoherent detectors without a common phase reference. It is conceivable that the brain compares the amplitude fluctuations coming from two non-coherent detectors; the optical hardware may allow it I just do not know if that is the case neurologically.

[1] Hanbury Brown & Twiss: Interferometry of the Intensity Fluctuations in Light. I. Basic Theory: The Correlation between Photons in Coherent Beams of Radiation; Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences , Vol. 242, No. 1230 (Nov. 5, 1957), 300-324 [2] https://en.wikipedia.org/wiki/Intensity_interferometer

$\endgroup$
9
  • 2
    $\begingroup$ HBT uses coincidence detectors ... not something the eye/brain can do (bandwidth considerations). .... not too mention separation of the detectors. $\endgroup$ Oct 19, 2022 at 17:28
  • $\begingroup$ @PhysicsDave actually in the original laboratory experiment Hanbury Brown used a mixer/correlator arrangement of which a coincidence detector is a special case, see Hanbury Brown: THE INTENSITY INTERFEROMETER and Its Application to Astronomy, pp 74-78. As regards the separation, that determines the resolution of the system but it is not an issue of "theoretical possibility" of what a human brain may do with it to which my comment intended to relate. $\endgroup$
    – hyportnex
    Oct 19, 2022 at 17:47
  • 3
    $\begingroup$ It's actually pretty interesting to look at how retina cells encode information, they fire at an increasing rate as they are more stimulated, which means that there is an interesting version of a Nyqiust limit problem for encoding information a blinking light source, so as you would have noticed a blinking light somewhere around 15 ~ 20 Hz is difficult to distinguish from a dim light, and a light blinking at 120 Hz is just a dim light, you have no sense of it blinking. So I think it's unlikely that out brain has a was of encoding the phase of even red light at 750 terahertz, but it'd be cool! $\endgroup$ Oct 20, 2022 at 5:01
  • 1
    $\begingroup$ Intensity interferometry is very different from and more limited than more conventional forms of interferometry. It's not really doing interferometry with the light waves, but with fluctuations in the overall light intensity. It's no good for imaging, it's more useful for things like estimating the diameter of a bright star, and that still takes precise, high-speed measurements. If "you can use intensity interferometry" is as close as you can get to a "yes" answer, a more meaningful answer is probably "no". $\endgroup$ Oct 20, 2022 at 19:33
  • $\begingroup$ @ChristopherJamesHuff granted it is not the same phase based interferometry as say as LBI is but who am I to say if intensity interferometry is interferometry or not. If the term was good for Hanbury Brown and Twiss, it is good enough for me. Note too the questioner did not ask for "coherent phase interferometry" only for "interferometry". $\endgroup$
    – hyportnex
    Oct 20, 2022 at 20:31
6
$\begingroup$

I guess you’re imagining an interferometer like that used for long baseline radio interferometry. In this case it is necessary that the detectors can sensing the relative phase difference between waves arriving at each detector. Our eyes (and most visible EM detectors) only detect the amplitude of visible EM waves and not the phase. Therefore we can’t use eyes for optical interferometry no matter the “software” used to analyze the optical nerve signals.

$\endgroup$
5
$\begingroup$

As others have mentioned, for the brain to do optical interferometry the eyes would need to capture phase information from the light waves and send it to the brain, where simultaneous signals from each eye could be combined. But the light-sensitive cones and rods in the retina cannot detect phase, and achieving the required synchronisation at optical frequencies via neural signals is unlikely, if not downright impossible.

However, even if our retinas could detect phase, and used some kind of signal time-stamping to permit synchronisation, there's another major problem. The amount of data that the eyes would need to send to the brain would be enormous. As it is, the retina doesn't simply send raw pixel-like data; it has several layers of neural networking that perform significant data reduction on the signals that get transmitted via the optic nerve.

From the Wikipedia article on the retina;

Although there are more than 130 million retinal receptors, there are only approximately 1.2 million fibres (axons) in the optic nerve. So, a large amount of pre-processing is performed within the retina.

[...]

The centre–surround structures are mathematically equivalent to the edge detection algorithms used by computer programmers to extract or enhance the edges in a digital photograph. Thus, the retina performs operations on the image-representing impulses to enhance the edges of objects within its visual field. 

Please see that article for further details.

$\endgroup$
1
$\begingroup$

Wavelengths of visible light are around 400nm at speed of light, nerve conduction is a few dozen meters per second. Resolution of signals from disparate nerves would not be anywhere near what would be needed for interferometry. With a bit of handwaving, you could consider the effect of different wavelengths of light on the color-sensitive retinal pigments a kind of molecular interferometry: that operates on lengths commensurate with the actual wavelengths in question. Eye distance is a way different scale.

$\endgroup$
1
$\begingroup$

I did not check if the following is actually used, but I believe even one eye should be sufficient, in principle, for some (primitive?) interferometry. What is (visible light) interferometry? Roughly speaking, it's detection of phase differences between, say, two light beams. Let us imagine two different light beams incident on an eye. I assume that the beams have finite width. A human can change the focus distance of the eye at will (within some range), so (under some conditions) (s)he can arrange for the two light beams to overlap on the retina. The intensity registered by the visual receptor cells within the overlap area will depend on the phase difference between the beams.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.