This article

@article{harmelech2013JNeuroscience,
Title = {The Day-After Effect: Long Term, Hebbian-Like Restructuring of Resting-State fMRI Patterns Induced by a Single Epoch of Cortical Activation},
Author = {Harmelech, Tai and Preminger, Son and Wertmam, Eliahu and Malach, Rafael},
Journal = {Journal of Neuroscience},
Month = {May},
Number = {22},
Pages = {9488-9497},
Volume = {33},
Year = {2013},
Bdsk-Url-1 = {http://dx.doi.org/10.1523/JNEUROSCI.5911-12.2013}
}

featured at
http://wis-wander.weizmann.ac.il/past-brain-activation-revealed-in-scans#.Uc1PHj6OC1m
and
http://www.psychologytoday.com/blog/the-athletes-way/201306/brain-waves-create-archives-experience-and-expectation
strongly suggests the existence of long-lasting eligibility traces. The reverberating activity is observed to last up to 24 hours, a significantly longer time span in comparison to the few seconds time constant of the chemical traces  I’ve  modelled in my recent papers:

http://www.mitpressjournals.org/doi/abs/10.1162/NECO_a_00419 and

http://www.frontiersin.org/Neurorobotics/10.3389/fnbot.2013.00006/full

Reverberating activity could in effect be a higher order type of trace, and  could prove fundamental in the solution of distal reward problems and learning.