Brain and Mind Institute Seminar
15.05.09 Friday, 12h15, AAB032
Implications of chaotic dynamics for neural coding
Abstract:
It is well known that neural activity exhibits variability, in the
sense that identical sensory stimuli produce different responses, but
it has been difficult to determine what this variability means. Is it
noise, or does it carry important information -- about, for example,
the internal state of the organism? To address this question, we take
a bottom up approach, and assess whether small perturbations to
networks are amplified in vivo. Combining intracellular current
injections in vivo with theoretical analysis of network dynamics, we
find that, in rat barrel cortex, perturbations are rapidly amplified.
This implies that a substantial component of the variability is noise,
and thus does not carry any information at all. In particular, in the
absence of strong input to reset the circuit, there are intrinsic,
unavoidable stimulus-independent variations in membrane potential on
the order of plus or minus 3-5 mV. For the brain to perform reliable
computations, it must use a code that is robust to such large
variability. This requires either a rate code, or large, fast
depolarizing events, like those proposed by the theory of synfire
chains. In our in vivo recordings, we found that depolarizing events
large and fast enough to produce precisely timed spikes were very
rare, suggesting that rat barrel cortex is likely to use primarily a
rate code.
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