Once upon a time
Laura Spinney :: New Scientist, 10 January 2015 :: www.newscientist.com
Summary and review of the above article
INTRODUCTION: The brain is shown to assemble the experience of the specious present from inputs arriving at a variety of times. It is seen to use its inputs to make very short timescale predictions about the future, providing the evolutionary advantage of quicker reaction times. Unusually for something published in a popular science magazine, there is also a challenge to the Libet orthodoxy, with a suggestion that the brain cannot detect the order of events on the timescales that Libet was dealing with.
An accumulation of neuroscientific evidence indicates that the present moment, specious present or ‘now’ lasts for between two and three seconds. This is the period of time which it takes for the brain to organise inputs, while anticipating the immediate future. Neural processing is selective and only some stimuli get included in the specious present. Consciousness is also seen as a filter that focuses attention on some things to the exclusion of others.
A building block within this two to three second specious present is something called the ‘functional moment’, which is the timescale needed to distinguish one input from another. This varies between the different senses. The auditory system can distinguish two sounds that are only two milliseconds apart, whereas there needs to be a ten millisecond gap between visual inputs. Moreover, it is impossible to decide the order of stimuli with less than a 50 millisecond gap.
Organisation of time
The brain has to process these varying inputs into a single specious present. Even auditory and visual signals that are initially noticeably out of synch can come to be processed to appear in synch. It has also been shown that sections of a film story that are randomly shuffled can be put back in a comprehensible order by the brain for periods shorter than 2.5 seconds.
The brain appears to have a mechanism to organise the time sequence of inputs from the external environment. From an evolutionary point of view, this has the advantage of allowing us to make use of complex sequences of events. The specious present is seen as a cohesive concept of the world that allows the brain to make predictions about the very near term. If this was not the case some reactions could be very slow because some stimuli take hundreds of milliseconds to process.
Beyond the experience of the two to three second specious present, there is a argued to be a 30 second period in which a sequence of experienced moments is held together and experienced as continuous in the working memory; this has the function of maintaining and working on a limited range of information for a short time. The sometimes reported ‘stretchiness’ of time, where events such as accidents are experienced as stretched out in time, is now also considered to have a basis in alterations in sensory processing. It is suggested that the brain is taking in more information than it would about a more predictable situation.
Querying the Libet orthodoxy
Unusually for something published in a popular science magazine, the writer queries the orthodoxy of the 1980s Libet experiments that are widely claimed to rule out the possibility of any form of conscious or free will. Libet reported that subjects required to make admittedly trivial actions had detectable readiness potentials in their brains before they were consciously aware of the decision to act. However, some of the material discussed here indicates that the brain is insensitive to the order of events on the short timescales involved in the Libet experiments.Laura Spinney, Libet, present, specious present, time, working memory Posted by