What was bright and shiny this week? 05.08.09
There were no weekly summaries for a few weeks. I was in Ireland. And England. And I was lazy. Sorry. It’s unfortunate because I didn’t get a chance to add to the swine flu hysteria. But seriously, if you are interested in aggregated coverage of the impending pandemic [shudders!!], click through to the Nature News Special content page. 3…2…1…neuroscience…beneath the fold…GO! (cont.)
Neural progenitors maintain intrinsic properties and do not expand differentiation potential in response to brain injury
We have known for a while that neural precursors can and will move into a damaged brain area following some sort of trauma, like a stroke or hypoxic infarct. Following from this, most would also say that these neural precursors subsequently make brain region-specific neurons. Well, these authors looked at this assumption more in-depth with multiple labeling methods (BrdU & viruses) to “birth date” new neurons and mark them for classification. Damage was induced in the striatum and when SVZ-derived precursors migrated in, they mostly formed calretinin-positive interneurons rather than the medium spiny neurons that were predominantly lost. Thus, it doesn’t seem that neural precursors actually modify their differentiation potential based on the environmental cues within the region to which they migrate. Doesn’t sound very helpful to me…
By not accounting for tuning functions, current spike sorting methods used to assess neural responses may be biased and inconsistent
Most neuroscientists examining circuit and systems level questions are often relating spike activity to behavior in a bid to understand how changes in neural activity translate to functional consequences. This is typically accomplished using some sort of extracellular recording method that listens to the area around the electrode(s). Ultimately, in order to assign a particular spike to a single neuron, “spike-sorting” algorithms are used, which involve clustering the characteristic waveforms of the neuron. Once this spike attribution has been done, researchers typically then move on to assigning more functional parameters to the neuron, like a tuning curve. This new paper suggests that following this serial approach to analysis is faulty and, to quote the authors: “…covariates that modulate tuning functions also contain information about spike identities, and that if tuning information is ignored for spike sorting, the resulting tuning function estimates are biased and inconsistent, unless spikes can be classified with perfect accuracy.” Which, of course, they cannot.
Searching for functionally homologous neural representation areas in human & macaque
Previous single-cell recording studies in macaque have revealed that specific neurons robustly respond to visualizations of body parts, as do “blobs” of neurons in human cortex when given the same stimuli (but measured via fMRI). These authors decided to do a more comparative study in order to determine if these regions are homologous in humans and macaques, using identical techniques. Essentially, they found face- and body part-selective areas in both primates, with some significant overlap. But there were differences too, of course. It is a nice little study and I like the effort to conduct the most direct comparison, but admittedly, we can only ultimately conclude that there are commonalities and differences in the brain organization between the two species. However, the authors note that this is just the first step in identifying functionally homologous category-selective areas.
Zebra finch song culture initiated in complete isolation converges on wild-type song within three generations
This is one of the cooler papers I have seen this year. Okay, I’m biased because I published it, but still. This was a challenging experiment that took a few years to conduct. The authors started with zebra finches raised in isolation and let them grow to maturity. It was already known that when these birds mature without a “tutor” to teach them how to sing like, well….like a zebra finch, they produce a strange song. However, for the isolate female placed with this odd-singing male, the song still induced pure love. Thus, matings produced more birds. But now, rather than maturing in isolation, the juveniles were taught song culture by the original male. So the weird song was “passed on” to the new generation. Here’s where things get cool. Within only 3 generations, the hard-wired circuitry and musculature of the birds were so influential that as the juvenile males learned the song, they adapted it slightly, with those modifications rapidly pushing the song very close to the patterns of the wild-type zebra finch song. Despite no bird in the community having ever heard the wild-type song! Very interesting, but perhaps not nearly as interesting as the question the authors end with for future study: “Because our findings suggest that song culture is the result of an extended developmental process, it would be interesting to examine whether changes in gene expression, neuronal reorganization or neurogenesis associated with song development show orderly multigenerational progression during the evolution of song culture.” Can’t wait for that study (in another 3-4 years)
See other great summaries of this work.
Suck out an astrocyte’s cytoplasm (containing transcriptome), insert into neuron and get an astrocyte
Neurons are post-mitotic, meaning that they no longer divide and therefore, are at an “end-point” in the developmental process. So there is nothing that could coax this cell type to morph into something else, right? Well, the Eberwine lab wanted to challenge this notion doing what they have become world-renowned for: single-cell genetic manipulation. What they did was to take the cytoplasm of a single astrocyte, which contains the transcriptome, or all of the messages expressed in that particular cell type, and transferred this into a neuron. Amazingly, that post-mitotic neuron functionally became an astrocyte. This process was successful 44% of the time, took several weeks to become complete and was stable after full conversion occurred. Quite a striking result, although it would be extremely interesting to go back the other way as well (transfer of the neuronal transcriptome into another cell, astrocyte or whatever) to see if that cell will transform into a post-mitotic neuron…
Long-term experience with music shapes and enhances auditory sensory encoding
The authors measured the brain stem responses to two different musical intervals in musicians as well as those without musical training. Musicians exhibited heightened responses to both harmonics as well as to certain patterns of sounds that were played. In addition, the phase locking of the neural response to the periodicity of the stimulus was also more precise in the musician. These heightened and more accurate responses were also proportional to the years of experience of the musician. Thus, experience with music plays a strong role in enhancing auditory encoding.
Case study examining possible ‘caffeine-induced psychosis’ [PDF]
Got this from Vaughan Bell at Mind Hacks. Caffeine can bind to adenosine receptors and also plays havoc on the Ca2+ regulation system within cells. These caffeine-dependent molecular consequences can sometimes cause enough modification in neuronal firing to subtly modify circuits and in schizophrenics, can cause their symptoms to become worse. However, in this case study, the authors examine a possible induction of psychosis by caffeine in an otherwise normal individual. After “high-caffeine” intake, the 47-year-old man suffered from delusions and paranoia. These symptoms were alleviated by combining anti-psychotics with reduced caffeine intake. A very strange case, and I’m still not sure what to make of it. Nevertheless, for those of you who often suffer from Medical Students’ Syndrome, no need to fear; dude was like drinking 36 cups of joe a day…
The same neurons encode decisions & the certainty underlying that decision
Much research has been dedicated to understanding the neural activity underlying decisions, but only recently have we begun to decipher how the confidence in that decision is encoded. Assessing choice certainty is kind of tricky and previous studies in rodents have measured the willingness of the animal to wait for a delayed reward as a metric for confidence. Here, the authors use primates, and their spin on assessing uncertainty is that if the monkey is unsure as to the validity of its answer, it can “opt-out” of the trial and receive a lesser reward with 100% certainty. If the monkey has confidence, it will hazard an answer in the hopes of receiving a larger reward (only if the answer is correct). By recording from neurons while these decisions were being made with varied degrees of confidence, the researchers found that the same neurons active during the decision are also encoding the degree of choice certainty. These results provide empirical support for the notion of a “Baysian brain”, a concept previously proposed by Alex Pouget.