Eventually, HTM will need to incorporate surprise. For the purposes of this post, surprising inputs are those which do not fit any possibilities remaining during object or location disambiguation. When surprise occurs, it must add to the list of possibilities.
The surprising input might be from another known object, or the object it has been exploring might be novel. If it is the latter, it should throw out its list of possibilities from before the surprise, but if it is the former, it should put the list away and use it again if it switches back to the first object.
Lower L6 corticothalamic cells could prevent possibility set switching.
- From primary cortex, L6b cells project to higher order thalamus and subnuclei of primary thalamus besides the main sensory relay subnucleus .
- They might only target matrix cells , which could broadcast surprise.
- Some or all matrix cells  and L6 CT cells have facilitating synapses. If that facilitation decays slowly, it could store possibility sets when attention switches and use them again if the facilitated cells fire.
- Acetylcholine depolarizes L6 CT cells . This could make it less willing to switch possibility sets, making it pay attention to something.
L6 corticocortical cells could detect new or surprising inputs.
- L6 excitatory cells fire more rapidly at the start of current injection, especially CC cells .
- L6 CT cells probably form synapses distally on L6 CC cells , so they could predictively depolarize L6 CC cells to prevent their rapid firing. The same could occur in the thalamus, and L6 CC cells receive stronger inputs from the thalamus than L6 CT cells , so L6 CC rapid initial firing could amplify thalamocortical bursts.
L6 corticocortical cells could broadcast surprise signals in the same region and send it to other regions.
- When a single sensory patch is surprised, others might also need to be surprised even if their inputs are expected. This signal could be the same one which does voting, and voting is needed anyway to see if it exhausts the list of possibilities.
- L6 CC cells have transcolumnar axons in L5/6 , so they could vote or at least send surprise signals to other macrocolumns and other regions.
The recent locations paper describes path integrating sets of possible locations, which is an example of possibility set switching. Perhaps this switching uses the same mechanisms for path integration and surprise. One possible shared mechanism is facilitation to temporarily store possibility sets following surprise and do path integration.
Facilitation is also a potential mechanism for disambiguation because it fits evidence accumulation. It could temporarily anchor the location to the feature to help it learn invariance.
Sources and Inferences
 (multiple sources and inferences)
Corticothalamic Projections from the Cortical Barrel Field to the Somatosensory Thalamus in Rats: A Single-fibre Study Using Biocytin as an Anterograde Tracer (Jacques Bourassa, Didier Pinault, and Martin Deschênes, 1995) - Lower L6 cells project to POm and an adjacent part of VPM. There are a couple other articles with similar titles and authors on V1 and S2 with reconstructions which don’t contradict this.
A New Thalamic Pathway of Vibrissal Information Modulated by the Motor Cortex (Nadia Urbain and Martin Deschênes, 2007) - The VPM head subnucleus is adjacent to POm.
Septal Neurons in Barrel Cortex Derive Their Receptive Field Input from the Lemniscal Pathway (Takahiro Furuta, Takeshi Kaneko, and Martin Deschênes, 2009) - The VPM head subnucleus drives the septa in L4.
Subset of Cortical Layer 6b Neurons Selectively Innervates Higher Order Thalamic Nuclei in Mice (Anna Hoerder-Suabedissen et al., 2018) - Lower L6 cells or a genetically labeled subset in many regions project to higher order thalamus and subnuclei of primary thalamus.
Specificity in the axonal connections of layer VI neurons in tree shrew striate cortex: evidence for distinct granular and supragranular systems (W. Martin Usrey and David Fitzpatrick, 1996) - Lower L6 cells in tree shrew (a relative of primates) project preferentially to koniocellular-like layers of LGN and to the most ventral part of pulvinar.
The Laminar Organization Of The Lateral Geniculate Body And The Striate Cortex In The Tree Shrew (Tupaia Gus) (Michael Conley, David Fitzpatrick, And Irving T. Diamond, 1983) - The koniocellular-like layers of tree shrew LGN don’t target L4, only receive small terminals from the retina, and receive input from the superior colliculus.
Sublaminar organization within layer VI of the striate cortex in Galago (Michael Conley and Denis Raczkowski, 1990) - Lower L6 of bush baby V1 targets pulvinar.
Other articles found projections from primary cortex L6 to higher order thalamus in other species. Projections from V1 to pulvinar in macaque haven’t been found, I think, but there are few CT cells in that region and they could be in the white matter below it.
 Differences in projection patterns between large and small corticothalamic terminals (Susan C. Van Horn and S. Murray Sherman, 2004)
 Properties of the thalamic projection from the posterior medial nucleus to primary and secondary somatosensory cortices in the mouse (Angela N. Viaene, Iraklis Petrof, and S. Murray Sherman, 2011)
 Cre‐expressing neurons in visual cortex of Ntsr1‐Cre GN220 mice are corticothalamic and are depolarized by acetylcholine (Sofie Charlotte Sundberg, Sarah Helen Lindström, Gonzalo Manuel Sanchez, and Björn Granseth, 2017)
Excitatory Connections Made by Presynaptic Cortico-Cortical Pyramidal Cells in Layer 6 of the Neocortex (Audrey Mercer, David C. West, Oliver T. Morris, Sarah Kirchhecker, Jane E. Kerkhoff, and Alex M. Thomson, 2005) - This study found that L6 CC cells produce EPSPs in other pyramidal cells a few times more often than L6 CT cells.
Local Connections of Excitatory Neurons to Corticothalamic Neurons in the Rat Barrel Cortex (Yasuhiro R. Tanaka et al., 2011) - Using a virus, this study found that the most numerous nearby input to L6 CT cells is themselves and the most numerous distant input is L6 CC cells.
 Infrabarrels Are Layer 6 Circuit Modules in the Barrel Cortex that Link Long-Range Inputs and Outputs (Shane R. Crandall, Saundra L. Patrick, Scott J. Cruikshank, and Barry W. Connors, 2017)
 Intracortical Axonal Projections of Lamina VI Cells of the Primary Somatosensory Cortex in the Rat: A Single-Cell Labeling Study (Zhong-Wei Zhang and Martin Deschênes, 1997)
 A Third Parallel Visual Pathway to Primate Area V1 (V. A. Casagrande, 1994)