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Archive for the ‘memory’ Category

This really gets me freaked out! Martin says I’m just a grumpy old (?) man. So let me lie along the Neurocritic approach just for a minute, and just air my frustration:

HOW CAN YOU GET A SCIENCE PUBLICATION WITH A HUMONGOUS ERROR?

Take a look at this image. It’s from a 2007 article in Science by Depue et al.

It’s supposed to show activation in the hippocampus and amygdala. Looks innocent, right? Let’s take a closer look.

Slice number 3 really provides the best errors:

I spent quite a while figuring the figues out. Did the yellow names indicate the blobs or where the structures actually are? For one thing, the blue blobs don’t fit into amygdala or hippocampus, but rather the entorhinal cortex. But let me comment on two big errors related to this slice.

First, the hippocampus is not present on this slice, so why put the name there? And why put it that lateral? This is really bothering. Do the researchers (and reviewers) really think that the hippocampus has anything to do here?

Second, the rightmost activation blob is centered in white matter. Hmm.. would that not give you the opportunity to speculate whether your coregistration was correct? I would. Related to this, let me just comment briefly on the two leftmost slices:

If you look at the blob, it really looks as if it fits better into the hippocampus. The entorhinal cortex is a thin slice with a whole different orientation. My guess: the hippocampus.

Next slide:

As before, I’m really curisous whether this is only a sign of poorly coregistered (and checked) fMRI images to the structural template brain. Or can it be just another example of why standard spatial normalization in this region is too problematic.

Guys, let’s face it: this is probably one of the bigger, non-spotted errors one can find in visualization of fMRI data. Does it help validate fMRI as a method? NO. How can this error be allowed? I have no idea. But would I trust ANY of the other spatial localizations in this article? NO WAY!

Get a grip, guys! Check your images, and get your medial temporal lobe your anatomy right!

-Thomas

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aniston.jpegHow specific — or sparse — is the neural representation of a memory trace? Quian Quiroga and colleagues now have an article in Neuron (PDF), where they describe their well-known studies using single-cell recordings to well-known faces. As you most likely know, this has given rise to the debate about the “Jennifer Aniston neuron”. Their findings, briefly put, have demonstrated that single cells show quite specific responses to very specific visual stimuli. While one cell may have a preferential response to the Sidney Opera House, another responds dramatically more to Hale Berry, while yet another cell responds to, well, Jennifer Aniston.

The Quiroga studies have re-iterated the debate (if it ever went dead) about how specific the neural coding is in the brain. Is it really so that the brain has such a specific code that one cell can represent one percept? Do we have a grandmother cell, a President Nixon cell and Marilyn Monroe cell?

Today, there is wide agreement that the one-cell-one-percept idea is untenable and unsupported by the literature. Rather than cingle cells, we see that networks represent a percept, rather than single cells. However, the findings by Quiroga et al. have nevertheless stunned the scientific (and global) community with regard to just how specific the neural code can be, and that it can be detected in a single neuron. The findings that we can record how one single neuron responds to one, and only one, percept, is quite surprising.

So forget about the grandmother cell, right? Or maybe not. After all, following the idea from these findings, we should not be surprised that there would in fact be one neuron that responded preferentially to our grandmother. Yes, it would be an expression of a “network code” representing our grandmothers as such, but nevertheless, you may in fact have that one neuron that responds to ol’ granny.

While we leave it at that, it is still surprising that this team of researchers use the term “medial temporal lobe” (or MTL for short). Why doe they say that there is sparse coding in the MTL? It’s a rather big region, and a region packed with qualitatively different regions. Not only are these regions different anatomically, but also functionally, they are thought to be involved in different functions. The perirhinal cortex is involved in processing (and encoding) of complex visual objects as well as novelty processing and working memory, the (posterior) parahippocampal cortex is involved in spatial processing (remember the parahippocampal place area). The entorhinal cortex has a medial and lateral part that deal with spatial and object information, respectively. And in addition to the hippocampus and amygdala, with their quite different functions, we may extend the MTL concept to include the temporopolar cortex, and maybe even the inferotemporal cortex.

Where in this complex system do Quiroga et al. find their sparse coding? Everywhere? My bet is on the perirhinal cortex due to its involvement in complex visual object processing. I’ve come to know that the researchers have not had structural scans available to determine the exact location of their electrodes. The scans have obviously been made, but they have not been able to use that information (hush hush, don’t tell anyone…).

So, while these studies are indeed important to our understanding of the coding of specific information, we’re left with a huge gap in terms of their anatomical properties. While most of the research community focuses on MTL subdivisions to an increasing extent, it is a bit puzzling to me that nobody have ever criticised these studies for their sparsity of anatomical information. Maybe I’ll be the first?

-Thomas

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protein.jpgEver wondered about the neurobiology of memory — how the brain stores information? And, if you know slightly more, how information is stored beyond the hippocampus, or what happens to memory during recall? If you have anything to do with memory — even having a slight interest in the topic — the journal Neurobiology of Learning and Memory now hosts a special issue on the role of protein synthesis in memory. The issue is packed with updates on the findings and controversies on this topic, and it is certain to bring you to up to date on the neurobiology of memory.

As the editor of this issue, Paul E. Gold, notes in his introduction:

The goal of collecting these papers was not to find a single clear view, laying to rest one alternative view or another—a rather delusional goal at best. Instead, the attempt was to provide a venue through which different perspectives could appear together, with the understanding that all contributors are interested in a common purpose, to identify the ways in which brains make and hold new memories.

So, this issue will probably prove important with regard to mapping out the agreements and disagreements. As Gold notes:

Across these papers, there is agreement on the basic findings. All authors agree that proteins and protein synthesis are important to memory formation, but disagree on the question of whether new protein synthesis specifically triggered by an event is important for the formation of memory for that event. Some of the alternatives suggested include protein synthesis needed to maintain cell integrity, to replenish proteins ‘consumed’ by plasticity mechanisms, and to provide particular proteins that might be modified by experience, with long-lasting modification perhaps themselves representing cellular memory.

(…)

The diversity of opinion collected in this special issue, and briefly summarized here, offers an opportunity for readers to examine how different researchers, each sharing a common goal of understanding how memories are made, can view the same data set and come away with disparate opinions. In this way, the readers may find this discourse useful in identifying the important questions, if not the answers, surrounding the roles of protein synthesis in memory.

 -Thomas

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figure1.jpgIs binding the single most important concept in neuroscience? I think it is, even without making the concept too general or vague. On the contrary, binding seems to be a general concept to understand the workings of the brain. No more need for modules of perception, cognition, memory and action. Binding is the solution.

More specifically, what is binding? Or, to reframe the question 100%: what happens when the brain works? To many, the brain binds information together at all levels throughout the brain. If you perceive an object, that particular object is a mixture between colour, form, position, movement etc., that is bound together. Because of you look at the early sensory processes in the brain, we know that the features of an object are treated by separate processes in the brain. Accordingly, they can be lesioned separately, leading to e.g. acquired colour blindless but with intact movement perception.

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brainconnection.jpgIt’s been a while, and whoah! have we been drowning in work or what? The media here in Denmark have caught on both our stories about teenage brains and stem cells in mother’s brains.

Here is a nice demo of how MRI can be used to study not only the brain per se, but also how mental functions work as different functional and physical networks. In a really neat study Takanashi et al. in NeuroImage combined fMRI and Diffusion Tensor Imaging, a scanning technique that basically makes it possible to calculate the brain fibers in the brain, i.e. their homogeneity, direction and so forth.

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In an interesting paper in the latest version of Progress in Neurobiology, Yuri I. Arshavsky from UCSD writes about the epistemological dualism that exists in modern neuroscience. basically, Arshavsky claims that there is a covert dualism in the way that neuroscientists are treating mind-related topics, especially the study of “consciousness”. Indeed, as he claims:

This covert dualism seems to be rooted in the main paradigm of neuroscience that suggests that cognitive functions, such as language production and comprehension, face recognition, declarative memory, emotions, etc., are performed by neural networks consisting of simple elements.

This might initially sound a bit strange. Is not cognitive functions such as face perception due to operational simple elements? Face perception as such is a combination of many simple processes that operate in unison. So what is Arshavsky proposing? Indeed he suggests the existence of a certain kind of brain cells:

(The) performance of cognitive functions is based on complex cooperative activity of “complex” neurons that are carriers of “elementary cognition.” The uniqueness of human cognitive functions, which has a genetic basis, is determined by the specificity of genes expressed by these “complex” neurons. The main goal of the review is to show that the identification of the genes implicated in cognitive functions and the understanding of a functional role of their products is a possible way to overcome covert dualism in neuroscience.

So there should exist a subset of neurons that integrate information from a variety of input. This sounds strange, since all neurons integrate inputs from thousands of inputs, many from a large variety of inputs. So what are complex neurons? Here, we are told that:

(…) neural networks involved in performing cognitive functions are formed not by simple neurons whose function is limited to the generation of electrical potentials and transmission of signals to other neurons, but by complex neurons that can be regarded as carriers of “elementary” cognition. The performance of cognitive functions is based on the cooperative activity of this type of complex neurons.

In this way, complex neurons seem to be integrative neurons, i.e. cells that integrate information from a variety of processes. This could include the multi-modal neurons found in the functional sub-structures of the medial temporal lobe, such as the hippocampus, perirhinal, entorhinal and temporopolar cortex. But would it not mean the colour processing nodes in the visual cortex? Which IMO leads us back to a basic question: what is a functional unit in the brain. yes, the neuron is a basic building block of information processing in the brain. But what is special about language, memory and so forth in the brain?

It is possible that Arshavsky is not radical enough: what we should seek out is to avoid using generalistic and folk-psychological concepts in the first place. We should possibly not study “language”, “memory” or “consciousness”, since these concepts will always allude to fundamental assumptions of “language-ness”, “memory-ness” and “consciousness-ness”, IOW that there is something more to explain after we have found out how the brain produces what we recognize and label a cognitive function.

Maybe neuroscientists are not using a poor strategy after all? Maybe ignoring the past history of philosophy of mind is the best solution. I’m not sure (nor am I sure that I represent Arshavsky’s view properly). But how we choose to label a cognitive function depend on our past historical influence and learning, as well as our current approach.

-Thomas

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Recently, a team in Cambridge has developed a diagnostic tool for prosopagnosia, a rare disorder of face perception where the ability to recognize faces is impaired, although the ability to recognize objects may be relatively intact. While this work has made it easier to sort out between those who are truly prosopagnosic and those who have a more global agnosia, a recent report shows that prosopagnosia is much more common that you would think.

An impressive 2 percent of the population may have some fom of prosopagnosia! That means that millions of people are not only poor but disastrous at recognizing faces, even from famous people.

You can read more about the story at ScienceDaily, visit the faceblind.org website, or read the papers yourself from Ken Nakayama's homepage

So next time you meet a person whos name you've forgotten, don't worry. At least you're not prosopagnosic!

-Thomas 

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