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Sensory Limitations

Our senses disregard almost all of the virtually infinite amount of information that could be sensed. How do we know this, and why is this so? Let's consider only vision. People have estimated the human visual bandwidth at around 10 megabits/sec by considering things such as how many nerve fibers there are in the optic nerve and how much information each fiber can transmit. How much is there out there to sense that we are missing?

Let's consider only seeing things by reflected sunlight, and not thermal night vision, radio or gamma rays. Sunlight brightness falls to less than 10% of the peak below 300nm and above 1500nm. The response of the human eye is about 380nm to 780nm, which is really a pretty good match. This is not a coincidence, because the eye evolved to sense reflected sunlight. We could push a bit into the ultraviolet and a full octave into the infrared, giving a potential bandwidth of 1.6×1015 bits/sec. This bandwidth allows for all possible color and any changing over time (such as motion.)

We could potentially receive this much at each distinct point that we can see. While it would sometimes be useful to be able to see miles away, let's limit ourselves to an eye the size of the human eye. The optical (diffraction limited) resolution for a 1cm lens at this wavelength over a 90 degree field of view is about one gigapixel.

Multiplying the spatial and time/spectrum resolution gives about 1×1024 bits/sec. It'd also be useful to have at least ten eyes for seeing behind your back, etc., so let's make that 1×1025 bits/sec, which is 1×1018 times more than the actual human visual bandwidth, a vast difference.

Why are our eyes so much weaker than this? Biological possibility is one constraint—this is a theoretical limit that far exceeds the capabilities of the best camera made, let alone what could be achieved using biological goop. But by making the eye a bit bigger, adding more different color receptors and packing the entire field of view as densely as human central vision, it would be possible to increase the bit-rate 100 times or more.

The reason that we have the eyes that we do is that Evolution has made a tradeoff between what it would cost to have better vision and what the benefit would be. We have pretty much the best eye that we can afford. Better eyes didn't give any significant advantage for staying alive in the savannah. A large part of the cost of better eyes would be in the need for more brain to process the data.

A particularly clever economy in the human visual system is giving a high resolution in the center (the fovea), and then moving the eye around to build up a larger mental image. The eye doesn't work at all like a camera, taking in an entire scene in one go. Instead it is more like a paintbrush used to fill in the holes in a virtual canvas in our mind. visual perception and attention are extremely complex brain processes that take place almost entirely automatically and unconsciously. The result is a user illusion so compelling that most people are naive realists who imagine that they directly see the real world.

analysis/physical/sensory_limitations.txt · Last modified: 2013/10/12 14:02 by ram