My comments on the Usher's mail messageabout his paper in nature. My comments are italized
From: M.Usher_ukc.ac.uk Date: Thu, 30 Jul 1998 14:11:45 +0100 To: yeh_harlequin.co.uk Cc: M.Usher_ukc.ac.uk Subject: Re: Your 'synchrony' paper Regarding your paper in Nature (Usher & Donnelly (9 July 1998)
I don't follow the logic of the conclusion about binding _inside_ the brain.. As far as I can see, it assumes that: If humans are sensitive to X in the visual input, than X is used inside the brain. [ And X in this case is simultanousity of the elements of a percpet ] This looks to me totally invalid assumption, which is contrary to the facts (e.g. set X to size of objects, frequency of EM waves etc.). Can you explain this? Yehouda Harpaz
I think that you need to consider our statement about binding _inside_ the_brain, within its context which you eliminated from your nice explanation above. However I think that, first, even according to your presentation, there is a sense in which:
If humans are sensitive to X in the visual input, than X is used inside the brain.This follows from the additional implicit assumtion that human perfornance is mediated by things which happen inside the brain. So for example, following your example, since the brain is not sensitive [meaning doesnt descriminate in its response] to frequencies of EM radio-waves, neither does human performance, and since it descriminates in its responses to the difference between red-blue visible light, so do human subjects.
The theories that this paper tries to support are about using synchrony inside the brain, and I raise this point explicitly in my mail message. Usher simply ignore it, and instead discusses 'sensitivity' and 'descrimination in response', which are clearly different thing, as the brain descriminate in response to light, but light is not used inside the brain.
What I understand that you imply is that, the actual brain mechanism responsible for performance, might be sensitive to the feature in the out_side world (in our case synchrony) only in an implicit, but not explicit way. But this is not the case for synchrony, since brain activity tends to lock on stimulus modulations (If you measure EEG when people watch a flickering display you find a peak in the spectrum at the frequency of the flicker).
1) The last observation does not show that the synchrony is used for anything even in this case, and it certainly does not tell us anything about cases where there is no such strong synchrony in the stimulus.2) More importantly, it does not tell us how do Usher&Donnelly's experiments relate to activity inside the brain. This is an example of the conclusion-validation error (Reasoning errors): Usher tries to support his argument by bringing another (nonsense) argument that 'supports' the same conclusion.
The context in which our paper is stated is essential for the claims. Surely, only by looking inside the brain can one really know that binding is mediated by neural synchrony. However people looked inside the brain and reported those synchonised activities. The question was then, whether it could be relavant for the processes of grouping. Since visual modulation do triger modulations in responses of neurons, it was reasoned that one should be able to detect the effect of such manipulations in tests of grouping with human subjects.
As explained in the the main text {4}, this is broken logic. There is no reason to believe that there is any correlation between the attributes in the stimuli that humans are sensitive to and the internal mechanisms of the brain.Its apriori possible that the performance is mediated by some other mechanism that depends on the temporal manipulations only implicitly. But then one needs to look at the details and compare the support for the alternative mechanisms. Most of our paper was dedicated to rule out such a mechanism (local motion computation) and we know of no other one (except grouping) that could mediate performance in this tasks.
That is irrelevant to the discussion, which is about how grouping is imeplemented inside the brain.
Does this help?
No, because Usher did not explain how his experiments tell us anything on what happens inside the brain.
-Marius