1.10.1.2: Vision vs hearing

Because they both tell us so much about the world, it's interesting to compare the performance of the eye and the ear. Both eye and inner ear are remarkable for their performance, sophistication and small size.

In sensitivity, both eye and ear are within an order or magnitude or two of the theoretical maximum: the eye needs about 70 photons focussed on a single receptive field in one integration time to cause us to see a flash. An ear that was much more sensitive than ours would hear the thermal motion of the air.

In intensity, we have a large range in both. The intensity that quickly causes damage is very roughly a thousand billion times the weakest signal we can detect. So both the ear and the eye have a dynamic range of about 120 dB. (See What is a decibel?) (In addition, both have protective reflexes: blinking protects the eye and the acoustic reflex disconnects the ossicles from the tympanum to protect the ear.)

In frequency range, the ear wins easily: young ears have a range exceeding ten octaves compared with the eye's one. In frequency discrimination, the ear wins too: we can distinguish frequencies that differ by a fraction of a percent. Further, the ear is not fooled by mixtures: red plus green looks yellow, but the note C plus the note E sounds like the chord C plus E: it certainly doesn't sound like D, which lies between them on the spectrum.

In angle, the ears have an advantage: we can hear sounds from all directions, whereas our eyes see only about half the possible directions.

It is in spatial resolution, of course, that the eye has an enormous advantage. We do have some directional sense of hearing, due to arrival times, the sound shadow of the head and the directional variation in frequency response of the outer ear. But this is trivial compared to the performance of the eye. Let me put it thus: if someone seated at an organ sounded all pipes in the display rank simultaneously, we certainly couldn't identify the position of all the pipes. The eye, however, not only sees every pipe, but can tell us the shape and the decoration. This is why the eye needs about one hundred times as many nerve channels as the ear – though the ear does much more signal processing before transmission to the brain.

In response time, events closer than a few tens of milliseconds are hard to distinguish with either sense. The two ears, however, can detect arrival times differing by much less: this is part of our directional hearing.

They use somewhat different information codings. The eye codes both position and colour with different channels: what neurophysiologists call place coding. Intensity is coded by the frequency of pulses, both with a number of interesting complications. The ear uses a mix of place and rate codings for both intensity and pitch.

Unless you live in a cave or at the bottom of the ocean, or are nocturnal, it's easy to be persuaded that an eye confers a very big advantage to potential predators and prey. Unless you live in a noisy environment, it's also easy to understand why hearing confers advantages of similar importance. Which is why we have both. Which is more important? That's hard to say. However, my mother, who is both blind and deaf once said that, if asked to choose, she would rather have her hearing back. 'Vision is for things, hearing is for people', she said.

This page supports the multimedia tutorial The eye and colour vision.