Continuing the discussion from The evolutionary history of integrated perception, cognition, and action:
Easy, right ? ISI stands for Inter Stimulus Interval, and is sometimes accounted for in researches about pavlovian forms of learning, where often, an originally “neutral” stimulus is paired to an aversive (or to the contrary, liked) one, looking for clues that an association was learned between the two.
Most often during these experiments, the (usually) neutral stimulus above is called CS (for Conditioned Stimulus, since this stimulus is the one which, hopefully, will have formed a new association, after “conditioning”… aka training). And the innately-aversive or innately-liked one, the US (for Unconditioned Stimulus). The amount of learning will then be evaluated based on the observation (or lack thereof) of a behavioral “Response” which was known innately associated with the US, beforehand. During training, the ISI is then the amount of time between the CS and the US.
It is quite striking to me that ISI is an often overlooked problematic: for those scientific endeavors which do care about it, we may further distinguish, among these association experiments, the ones called “trace conditioning” in which there is a temporal gap between the end of the CS and the start of the US.
Depending on the species, some relevant gaps may be measured as short as 0.5 seconds, up to (I guess, but I didn’t dig too far about the longer ones since it was of less concern to me) minutes, still resulting in successful associative learning in the studied species. The original “Pavlov’s dog” realization was an example of trace conditioning indeed, and a quite long one at that.
So, what’s the fuss ?
You’ll find lots and lots of models and theories explaining how associative learning come to be on a functional level. What excites what, what inhibits what, where it happens, how to explain some second-order properties, etc. They seem to be discussed at length, and they have been for decades… While there is an interest alright in discussing those, it’s depressing that we’d still hit same papers while looking for what I’d call… “the ISI problem”.
Research about specifically trace conditioning in mammals currently hint at a fundamental role of the hippocampus (or its homologue in other vertebrates). I’d say it is not so surprising, since it is where episodic memory and measures of temporality are likely to reside, we certainly use that part for great effect there, and we’re even able to correctly predict the timing between the CS and US, provided it was consistent during training.
But mammals don’t interest me so much here.
The thing that troubles me is… associative learning can be accounted for in almost all animal phylums. Chordates alright (comprising us vertebrates), but also Nematodes (roundworms), Cnidaria (sea anemons and the like), Arthropods (insects, arachnids, myriapods, crustacean), etc.
That these organisms also exhibit learning was a recent realization for me (even if it has been known for a long time), but it shouldn’t be too surprising, considering we all have nervous systems, and our neurons are eminently plastic. What is striking however is that they also exhibit trace conditioning.
And it’s like… almost no-one cares. To the point that it is very hard for me to find conclusive online evidence for or against it (I found such conclusive evidence for insects at his point, but I’m still struggling with other lineages).
The crucial point is that, other phylums would have to achieve this without hippocampus. While it’s conceivable that another complex nav-system would be a convergent evolution for insects, and that trace conditioning is, for them also, the result of a high-level organization involving specific temporization loops, short term storages and whatnots, I’d be much more suspicious of such explanations if trace conditioning could be accounted for everywhere. I mean, sea anemons???
Cuz in this case, only explanations at a much more fundamental cell-level would make sense.
And if such widespread cell-levels mechanisms existed, how could we ever hope of building a brain replica without understanding (or for that matter, modelling or simply acknowledging) them?
Mind you… Even if evidence for such a profound issue is hard to find, I’m not guilty here of being simply one single crazy hobbyist, imagining problems where there aren’t. I’ll leave some rare scientists have the final say and finish to expose that problem more vividly than I could:
Linking behavior to underlying physiological mechanisms is one of the central goals of neuroscience, but a major impediment is the large difference between behavioral and physiological timescales. A good example of this discrepancy is provided by an experiment with Drosophila in which Tanimoto et al. (1) found that an odor becomes an aversive stimulus if it is followed by a shock during training but becomes attractive if it is presented after the shock. This finding is tantalizingly reminiscent of spike-timing-dependent plasticity (STDP; reviewed in ref. 2) in which a synapse is strengthened if presynaptic spikes are paired with subsequent postsynaptic spikes but weakened if the order is reversed. Indeed, the plot of learning index versus time interval between odor and shock in Tanimoto et al. (ref. 1; reproduced in Fig. 1B) has a shape and form similar to curves showing the amount of STDP as a function of the time interval between paired pre- and postsynaptic spikes (Fig. 1A). The problem is that the timescale is vastly different, seconds in the case of the behavioral experiment and milliseconds in the case of STDP. As in many similar cases, to account for the behavioral data on the basis of synaptic physiology, we must find a mechanism to span this large gap in timescales.
[…]
For STDP to produce net potentiation over time, pre- and postsynaptic spike sequences must be correlated in such a way that it is more likely for a pre-then-post temporal ordering to occur than a post-then-pre sequence. Such correlations require an appropriate relationship between time-dependent pre- and postsynaptic firing rates. An obvious requirement, shared by any Hebbian mechanism of plasticity, is that pre- and postsynaptic firing must overlap in time, at least to within the interval covered by the STDP window function. When two stimuli are separated by seconds, as is the case between the conditioned stimulus (CS) and the unconditioned stimulus (US) in the classical conditioning paradigm, we consider that this is not an easy requirement to satisfy.
(emphasis mine), from
https://www.pnas.org/content/103/23/8876
)
