Today I just closed one eye(for finding blind spot), and as usual some of the visual field is lost, but I wonder why is not intensity of light(brightness) reduced to half? Shouldn't it be since our brain is now getting only half impulses?

Does our brain instantly reduces the threshold of rods and cones when one of the eye is closed?

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    $\begingroup$ Never experienced, but removing one earphone surely reduces sound intensity that is perceived. $\endgroup$
    – JM97
    Aug 8, 2017 at 15:02
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    $\begingroup$ The intensity of sound that's experienced by the ear with the earbud would still be the same. The overall experience may change, but if you are experiencing sound at a level of 10 (arbitrary measurement), then after an earbud is removed from one ear, a level of 10 will still be experienced by the other ear that still has the earbud in place. $\endgroup$
    – user22020
    Aug 8, 2017 at 15:06
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    $\begingroup$ Consider two tuning forks are being vibrated one at each ear, now removing one will certainly reduce the intensity of that same frequency. $\endgroup$
    – JM97
    Aug 8, 2017 at 15:25
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    $\begingroup$ Well of course it will because of increased amplification due to the sound waves having the same phase (if I'm remembering my physics correctly). I don't think that analogy applies to this scenario though. $\endgroup$
    – user22020
    Aug 8, 2017 at 15:28
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    $\begingroup$ Okay , take this example, put one ear in water bath and let other in the air. ask someone to make some noise, now remove the ear, the noise will be increased. $\endgroup$
    – JM97
    Aug 8, 2017 at 15:30

2 Answers 2


Think of eyes as cameras producing an image at a given framerate. If you have 2 cameras and shut 1 of them off, that doesn't change the amount of light incoming to the other camera.

Your question is one of signal interpretation rather than receiving signals and adding them. Given that the 2 cameras have different perspectives, adding their respective pixels makes no sense. Instead, the dual images (if both cameras are on) are post-processed for spatial reference relevant to perspective.

In the case where a camera is shut off, then the spatial reference becomes a null picture and the post-processing can be skipped.

(For reference, if you took incoming data from both eyes and added them, the result would be an incoherent and largely unusable mess)


Experiment 2

Aim: Effect of intensity on two eyes

Requirement : Two eyes, a dark transparent glass, an object to glance.

Procedure : 1)Sit in a lit room 2)Look at fan with only right eye 3) Look at fan with only left eye through dark glass. 4) Look from both eyes( keep dark glass in front of left eye)

Observation : Brighter in only right eye, darker in only left and medium brightness with both eyes.

Result: Intensity do add when are different. Or more correctly they rather average out.

In your actual experiment, the intensities in two eyes just averaged out, that was mathematicaly equal to single eye.


Three principle

1) Brightness is transferred via frequency principle. Greater the intensity of light, greater the frequency of AP transmitted.

2) [Action Potential ][1]

Action Potential has a fixed depolarising potential.

3) Each spatial point, in space, of vision, is represented on a single spatial point in occipital cortex.

Two action potential caused by same image pixel reach their representative point on occipital cortex area 17.

They are then sent to visual associative Area 18 for matching.

“Fusion” of the Visual Images from the Two Eyes

To make the visual perceptions more meaningful, the visual images in the two eyes normally fuse with each other on “corresponding points” of the two retinas. The visual cortex plays an important role in fusion. It was pointed out earlier in the chapter that corresponding points of the two retinas transmit visual signals to different neuronal layers of the lateral geniculate body, and these signals in turn are relayed to parallel neurons in the visual cortex Source: Guyton and Hall, chapter 51.

Same impulses from same spatial point are sent to same interneuron.

The action potential cannot rise further.(Principle 1 above)

This fused signal reaches next neuron for further processing. Intensity is judged.

Same amplitude as of one eye reached, so same intensity was percieved by even two eyes.

If fused signal is of intermediate frequency, intermediate brightness is observed.


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