I was playing about with my camcorder and it got me thinking about how we see moving images. This is something we often take for granted because its such an integral part of what we do every day. Computer scientists working on visual processing know this intuitively easy skill is far from simple. Anyhow I got asking the question – if you can play a film at half-speed – why doesn’t the same thing happen ot visual processing? My first thought and i’m sure the reader’s will be that this isn’t too clever in the real world. If you see an animal or person moving toward you at half-speed, then you won’t have tim
te to respond quickly enough so it’s not ve yry practical and indeed fa rirly dangerous depending on circumstances. Still, mutations in genes are necessarily clever and in some sens ees are accidental so that it doesn’t matter whether we think they ought to occur. So there is still some use in returning to the subject of why we don’t tend to see in slow motion.
While it might not be immediately obvious, there are at least three ways of seeing in slow motion. In the first, the moving images are seen at say half speed. Using this approach you will get further and further behind present circumstances. In order to do this you would
ahave to develop another skill in terms of hol iding items in memory over ever increasing periods which would cause new problems. With the need for this latter skill, there is some similarity to the problems faced by the famous mnemonist Sheresheveskii who reported that he was unable to forget the events of the day. This in turn negates the ability to sift through the information and retain only what is thought to be important. However its likely that as time proceeds the visual images would degrade fairly quickly so that the utility of visual processing would be degraded beyond the simple point of impracticality.
The second possibility is that there is a ‘place’ or region in the brain where the ‘motion’ is detected. Indeed there is a region in the occipital cortex which may be involved in the processing of motion – the V3 region. So if someone was to see in slow motion it might be that this part of the brain fired half as often as normal and the ‘perceiving’ part of the brain received a signal half as often as a result. Then the person would see at ‘half speed’ if this model of perception and motion detection is viable.
The third possibility is that the perceiving part of the brain operates half as often as usual and so even with the motion detection going on correctly, the experiencing happens half as often. An obvious difficulty with this model is that motion processing and perception might be closely related rather than segregated. It doesn’t really matter though because the interesting thing is that people don’t seem to complain of seeing at half-speed or at least i’m not aware of them if they do.
This is the curious thing though. Why do we so effortlessly see at the ‘right’ speed? The more I think about this, the stranger it seems and it leads to an endless series of other interesting question. Do we all see at the same speed? Are the motions we are detecting continuous or discrete? (Most likely discrete and limited by the firing rates of the rods and cones in the retina) Does our perceptual apparatus fill in the gaps as it does with static images exemplified by certain optical illusions? Do other species see at different rates? Take a fly for instance. If you move your hand towards the fly, it will ahve moved by the time your hand had arrived. They can move very quickly. So quickly in fact that it almost gives the illusion that they are seeing and moving twice as quickly as we are. This might be possible if the firing rate of their rods and cones were twice as fast as ours. However a more straightforward explanation is that the journey from the fly retina to the brain and then back to movement is likely to be much shorter than ours given our size difference. This would mean that a fly could respond more quickly to visual information than we might be able to.
In all likelihood the absence of frequent complaints of slowing of vision probably means that vision works in a completely different way to that of filming devices. It is more likely to be a combination of multiple modular approaches to visual problem solving which have clear objectives and which combine to produce the visual stream of consciousness. Indeed there are various studies such as this and this which hint at more subtle mechanisms for motion processing which occur in widespread brain circuits that may include the cerebellum.
In the process of examining this issue, i’ve taken slowed down footage and compiled several videos. So what does life look like in slow motion? It has lots of birds! Actually birds are really great to see in slow motion. Without slowing them down it is very difficult to get an understanding of the elegance of their movements. However by watching the footage in slow motion it becomes apparent
ly just how remarkable these creatures are in keeping themselves high in the air with a simple combination of muscles and feathers. Slow motion filming is a strange netherworld – not quite a portrait nor is it a real-time representation of the world. To make it interesting there has to be a little bit of movement at least. If you take the opportunity to look around you as you read this you might be surprised at just how still the world is. The multitude of inanimate objects by their very definition are still and given motion through our actions. The main players in the world of motion (well macro-motion that is) are us, animals and machines that have been set in motion by us with the obvious exceptions (wind-associated movements, sea, rain, sun, moon etc).
Perhaps the most interesting finding I had from this footage was that I developed a sense of motion sickness when looking at a boat moving slowly in the sea. Whereas I don’t normally get motion sickness and it couldn’t properly be called such since there was no corresponding motion for a mismatch to occur it did suggest to me that an artefact in visual motion processing might be an interesting lead to pursue in understanding this phenomenon. Maybe this approach might lead to other findings.
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