An interesting contrast between the prefrontal cortex (PFC) and medial temporal lobe (MTL) in encoding temporal order.  PFC neurons showed stronger “mixed selectivity” type encoding. They responded to a combination of an item and the order in which in appeared, only responding to specific items at specific times.  By contrast, MTL neurons were mainly item-selective.  They typically responded to an item, regardless of its order, but their firing rate was modulated by order.

Naya, Y., Chen, H., Yang, C., & Suzuki, W. A. (2017). Contributions of primate prefrontal cortex and medial temporal lobe to temporal-order memory. Proceedings of the National Academy of Sciences, 201712711.

Further reading on mixed selectivity:
Rigotti, M., Barak, O., Warden, M.R., Wang, X., Daw, N.D., Miller, E.K., & Fusi, S. (2013) The importance of mixed selectivity in complex cognitive tasks. Nature, 497, 585-590, doi:10.1038/nature12160. View PDF »

Fusi, S., Miller, E.K., and Rigotti, M. (2016) Why neurons mix: High dimensionality for higher cognition.  Current Opinion in Neurobiology. 37:66-74  doi:10.1016/j.conb.2016.01.010. View PDF »

A Meta-Analysis Suggests Different Neural Correlates for Implicit and Explicit Learning
Roman F. Loonis, Scott L. Brincat, Evan G. Antzoulatos, Earl K. Miller
Neuron, 96(2): p521-534, 2017.

Preview by Matthew Chafee and David Crowe:
Implicit and Explicit Learning Mechanisms Meet in Monkey Prefrontal Cortex

Increased theta synchrony between the prefrontal cortex and hippocampus when subjects encoded unexpected study items.  This is further evidence that theta-band (6-10 Hz) oscillations orchestrate communication between these brain areas.

Gruber, M. J., Hsieh, L. T., Staresina, B., Elger, C., Fell, J., Axmacher, N., & Ranganath, C. (2017). Theta Phase Synchronization Between The Human Hippocampus And The Prefrontal Cortex Supports Learning Of Unexpected InformationbioRxiv, 144634.

For further reading:

Brincat, S.L. and Miller, E.K. (2015)  Frequency-specific hippocampal-prefrontal interactions during associative learning.  Nature Neuroscience. Published online 23 Feb 2015 doi:10.1038/nn.3954. View PDF »

Brincat, S.L. and Miller, E.K (2016) Prefrontal networks shift from external to internal modes during learning  Journal of Neuroscience. 36(37): 9739-9754, 2016 doi: 10.1523/JNEUROSCI.0274-16.2016. View PDF

A recent review by the late, great Howard Eichenbaum.  You’ll be missed, Howard.

Eichenbaum, H. (2017). Memory: organization and control. Annual review of psychology, 68, 19-45.

As we learn about items in our environment, their neural representations become increasingly enriched with our acquired knowledge. But there is little understanding of how network dynamics and neural processing related to external information changes as it becomes laden with “internal” memories. We sampled spiking and local field potential activity simultaneously from multiple sites in the lateral prefrontal cortex (PFC) and the hippocampus (HPC)—regions critical for sensory associations—of monkeys performing an object paired-associate learning task. We found that in the PFC, evoked potentials to, and neural information about, external sensory stimulation decreased while induced beta-band (∼11–27 Hz) oscillatory power and synchrony associated with “top-down” or internal processing increased. By contrast, the HPC showed little evidence of learning-related changes in either spiking activity or network dynamics. The results suggest that during associative learning, PFC networks shift their resources from external to internal processing.

Brincat, S.L. and Miller, E.K (2016) Prefrontal networks shift from external to internal modes during learning  Journal of Neuroscience. 36(37): 9739-9754, 2016 doi: 10.1523/JNEUROSCI.0274-16.2016. View PDF

Matt Wilson and colleagues describe how oscillatory cycles can be viewed as functional units, how different oscillation phases can represent distinct computations, and how all this can be organized across cycles.  Phew!

Wilson, Matthew A., Carmen Varela, and Miguel Remondes. “Phase organization of network computations.” Current opinion in neurobiology 31 (2015): 250-253.

Craig and McBain review the role of oscillations in understanding the functional circuitry of the hippocampus with an eye toward bridging in vitro and in vivo studies.

Craig, Michael T., and Chris J. McBain. “Navigating the circuitry of the brain’s GPS system: Future challenges for neurophysiologists.” Hippocampus (2015).

An excellent review by Matt Shapiro and crew on an important topic.  They discuss complementary roles and bidirectional interactions between the prefrontal cortex and hippocampus.

Miguel Remondes and Matt Wilson show that theta synchrony between the hippocampus and anterior cingulate during a task in which rats chose one of four trajectories.  Theta coherence at 8 Hz lowers slightly (by 1 Hz) when rats enter the decision phase of the task and hippocampal processing of trajectory information precedes that of the anterior cingulate.  Thus, lowering of theta synchrony may coordinate the integration of hippocampal information by neurons in the anterior cingulate.

A review in Science of Sue Corkin’s book on the famous neuropsychology patient H.M., who could no longer form memories after his hippocampus was removed.

Permanent Present Tense The Unforgettable Life of the Amnesic Patient, H.M. by Suzanne Corkin Basic Books, New York, 2013. 400 pp. $28.99, C$32. ISBN 9780465031597. Allen Lane, London. £20. ISBN 9781846142710.

Jutras et al find a relationship between hippocampal theta and visual exploration via saccadic eye movements.  Saccades caused a theta reset that was predictive of subsequent recognition of visual images.  Enhanced theta power before stimulus onset was also predictive of recognition.

A flurry of articles about Picower Institute’s Susumu Tonegawa’s paper implanting false memories in the mouse brain. They identified and tagged a memory engram for one environment, then activated that engram in a different environment while pairing it with shock.  Later, the animals showed fear in the first environment as if they were shocked there.
The Guardian
The New York Times
The cover of Science

The paper: Creating a false memory in the hippocampus

Kraus et al required rats to run on a treadmill during a working memory task.  They dissociated distance traveled vs time spent running by requiring the rats to run for a fixed distance or a fixed amount of time.  This revealed “time cells” in the hippocampus that reflect  the passage of time.