This chapter focused on the function of neuromodulators on the hippocampal system. And it was packed with information. Acetylcholine, dopamine, norepinephrine, metabotropic glutamate receptors, GABA-B receptors, serotonin all have different effects on excitatory transmission, inhibitory transmission, spike frequency adaptation, resting membrane potential, and plasticity. By the way, all of these modulators may have different effects on each event depending on (more…)
The primate visual system is functionally and anatomically divided into two processing streams. One processes the “what” of the visual field, e.g. the color of an object. The other processes the “where” of the visual field, such as movement of an object and distance of landmarks from each other and from the self. How exactly these two streams meet, and where exactly their information is bound, is unclear. It has been hypothesized that this segregation continues all the way through the entorhinal cortex, in which the medial entorhinal cortex (MEC) computes spatial representations, while the lateral entorhinal cortex (LEC) is responsible for object representation. There is, however, evidence that (more…)
When the Cognitive Map Theory of the hippocampus was originally postulated, it was suggested that spatial representation was the job solely of the hippocampus. However, it has since been discovered that other structures, such as the entorhinal cortex, are integral for establishing spatial representations in the brain. Specifically, this chapter focuses on the medial entorhinal cortex, which, compared to the lateral entorhinal cortex, seems to be preferentially involved in spatial representation rather than object representation.
The entorhinal cortex is home to a particular cell type called grid cells. Grid cells are (more…)
This chapter gives an overview of the functions of two neighboring cortical areas involved in the hippocampal system: the perirhinal (PER) and postrhinal (POR) cortices. These areas are largely thought of as being important for object memory and visuospatial memory, respectively.
Part 1 of this book discusses the inputs to the hippocampus and their roles in spatial processing. It seems each chapter in this book is packed with a ton of data (making it a gold-mine for cool papers), yet the authors (at least Nitz) do a good job of making it highly readable. Rather than go deep into specifics, I’ll present overviews of what I’m taking away from each chapter.
This chapter focuses on the posterior parietal cortex, which, sandwiched between sensory and motor areas makes it an interesting candidate for (more…)
I’ve decided to take a step back from the trace studies and check out this book written by some of the most influential hippocampal researchers. This book was written specifically to allow newcomers to hippocampal research to catch up on the latest trends emerging from the systems view of the hippocampus.
Chapter 1 starts by pointing out that the Cognitive Map Theory (CMT) of the hippocampus is often misconstrued solely as a theory of spatial cognition, but is rather a theory of how the hippocampus is involved in episodic memory. Then, the authors provide an overview of the rest of the book and describe briefly what makes place cells (the physiological foundation of CMT) fire. The hippocampus receives input from the medial entohorinal cortex, an area that seems to convey information about space, and the lateral entorhinal cortex, which, for the most part, conveys object information (though whether or not this is the case will be discussed in a later chapter). Thus, the processing of the hippocampus is made possible through a complex cortical-hippocampal system that includes many areas of specialization, all of which, in the view of CMT, allow for the binding and association of spatial and object information into events which can then later be used to guide behavior in familiar situations. In addition to external sensory information regarding objects and landmarks, spatial computations involve internal input, such as from the vestibular system, to compute how an organism’s position in space changes over time in relation to the world around it.
Since most of the information in this first chapter will be discussed in detail later, I wanted to instead point out an interesting study mentioned about the role of NMDA receptor-dependent LTP and their role in place field formation. It was shown early on that NMDAR’s are necessary for spatial memory recall. However, Kentros et al. (1998) demonstrated that place fields could form in the absence of NMDAR activity, but that these fields would not bind to the cognitive map in the long run. Certainly, this would be an interesting read when revisiting Ryan et al.
OK, so if I’ve learned one thing from the papers reviewed so far, it’s that the population of neurons active during the encoding of a new memory must be reactivated for that memory to be successfully remembered. But, does synaptic plasticity need to occur within the engram circuit for recall?
To be fair, one of these papers reviews the other as well as the Liu, Ramirez, et al. paper I’ve already reviewed. But A) I read both, and B) I need to make up ground in advance for the 6 months I’ll need to re-read, understand, and review Marr’s 1971 paper.
I highly recommend playing the theme music to Inception while reading this paper and/or blog post.
Denny et al. used a different method of memory trace labeling as do the Garner et al. and Liu, Ramirez et al. studies. However, it’s the same principle. Here, instead of removing Dox from the diet to label memory traces, the authors inject ArcCreERT2 mice with an estrogen receptor antagonist (in this case, it was tamoxifen) which in turns induces DNA activation. This only happens, however, in those cells that are actively producing the protein in the promoter region of the transgene: Arc. Arc is an immediate early gene expressed in cells that have recently been highly active. Thus, the authors targeted these recently active cells using this system and were able to express in them a protein of interest, e.g. eYFP. After tagging hippocampal neurons involved in memory encoding with eYFP, (more…)