Our human interface with reality

What is the maximum bit rate at which we can Recognise familiar things?

The rate at which we can absorb truly novel information is remarkably low, a few tens of bits per second maximum. However, we already know a lot about the world out there.  Most of our experience involves recognising things that we are already broadly familiar with, and learning what if anything is different.

Our ability to recognise visually is limited by the maximum information rate of our eyes. This total information rate can be determined by measuring our ability to resolve fine structured patterns falling on our retinas, the speed of response to changes in brightness, and our ability to resolve different shades and colours. Though our eyes may excel in these parameters individually, when any one is tested near the limit, the others fall dramatically. For example, at the limit of our spatial resolution we can only resolve black or white, no intermediate shades of grey.

The maximum information rate between our eyes and our brain is estimated to be somewhere around 6 Megabits per second, taking into consideration the actual measured resolution and speed of response of the eye. In The Oxford Companion to the Mind” [1], Robert Ditchburn describes the reasoning as follows:

“Accurate data is available only for the central region of the retina (a circle whose diameter is 2 degrees in the visual field) and the total information capacity of the visual system is about ten times that of this central circle, the fovea. The number of points that can be resolved within this central circle is about 104 (i.e. 10,000 pixels). When the eye is working near the limit of resolution, contrast discrimination is poor and at the limit of resolution, a person can distinguish black from white but not discriminate any intermediate shades of grey, therefore he associated just one bit of information with each of the resolved points. For the central region the highest rate that flicker can be observed is about 50 cycles per second, so that the information capacity is 50 × 104 = 500 kilobits per second”.

Measurements of contrast sensitivity with coarser sinusoidal patterns with various shades of grey gave almost identical results [2,3]. Neither of these estimates includes colour information, but it is estimated that the additional information due to colour is no more than about 20 per cent, making 600 kilobits per second in all for the central region. Given the estimate that the total information capacity of the visual system is about ten times that of this central circle, the fovea, this gives a total figure (per eye of around) 6 Megabits per second. This is roughly a million times greater than our learning capacity, and a million times smaller our subjective experience of reality (compared for example with the 15 Terabits per second that Jim Crowe had speculated was “the bandwidth of the optic nerve”)[4].

Ditchburn also commented that:

“Jacobson estimated the capacity for the ear at about 10,000 bits per second [5]. Thus the central region of the retina has about 50 times the capacity of the ear and the whole visual system has about 500 or 600 times the aural capacity.…… In the situation involved in the evolution of the higher animals, decisions vital to survival had to be made mainly or solely on visual information. A wide variety of situations was encountered, so a vast information capacity was needed. Yet the amount used in making a decision had to be limited to the minimum required for a correct decision. This limited amount had to be processed to yield an action as rapidly as possible. If too little information was processed so that there was a considerable chance of a wrong decision, or if too much was processed so that the decision came too late, the animal did not survive. Those species which did survive usually had a large visual information capacity but were able to select a small number of bits for processing towards an action decision”.- Robert William Ditchburn.

1/. The Oxford Companion to the Mind”, (1987), Edited by Richard L. Gregory. ISBN: 0198662246, (2004).

2/. “Perception of structure in flashes and in afterimages”, Ditchburn, R. W., and Drysdale, A. E., Volume 13, Issue 12, December 1973, Pages 2423–2433.

3/. “Information capacity of the human eye”, Jacobson, H., Science 113, 292–293 (1951).

4/. Wired magazine: Issue 6.11, Nov. 1998

5/. “Information and the human ear”, Jacobson, H., J. Acoust. Soc. Amer. 23, 463-471 (1951).

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