I’m not the lucky owner of an Ipad, but I did enter every possible raffle, lottery and prize quiz at this conference to increase my chances of finally getting one. Until then my notepad made of paper will do just fine. You might think that I am thoroughly outdated, but then you would actually be mistaken. You see, tablets are much much older than pen and paper. Around the 8th century BC, the old greeks used a wax tablet and a stylus to take notes on, whereas paper came around in the 2nd century AD.
The tabula (how the Greeks called their wax tablet) was an inexpensive way to take notes and wipe it clean by just heating up the wax, creating a tabula rasa. Neuroscientists think of something very different when they use those words. At least when I hear them I think about experience-dependent and -independent mechanisms that sculpt the developing brain.
This morning, David Fitzpatrick gave a very nice talk on just that. How much of the features encoded in our visual cortex require the circuitry to be sculpted by visual experience? Or is it mostly a fixed developmental program that runs independently of light hitting the retina?
He started with acknowledging the work of Hubel and Wiesel, who while having fun in the lab did so much groundwork on the workings of the visual cortex that afterwards nothing much was left to be discovered. Thanks guys, real great work!
Fortunately, I’m exaggerating and many questions remain. One of the questions that Fitzpatrick asked is:
Do cells in the primary visual cortex require visual input in order to become selectively responsive to certain orientations and directions?
We know from rearing animals in complete darkness that orientation selectivity develops independently of visual input. Direction selectivity on the other hand does not, since it is almost absent in dark-reared and immature ferrets and cats. This seems to be different however in rodents, Nathalie Rochefort showed very recently that mice as soon as they open their eyes have cells properly tuned to direction, albeit initially there is a bias to dorsal and anterior directions. An explanantion for this is that already at the retina there are direction selective ganglion cells, that have shown to project to the thalamus. I believe that in ferrets this has not yet been studied.
Beautiful data collected just weeks ago in the Max-Planck institute in Florida showed how cells in immature ferrets become rapidly direction selective if you train the animals by repeatedly showing them a drifting grating in a certain direction. They followed this in the time course of many hours by chronic Ca2+ imaging using the genetically decoded indicator GCaMP3. It was astonishing to watch the cells bounce back and forth between preferred direction to see them eventually organize into columns.
We’ve come to another interesting difference between mice and cats. Mice’s response properties such as orientation -, direction selectivity and ocular dominance is topographically scattered in a salt and pepper fashion, whereas cats have a very nice columnar organization. I remember distinctly when I entered the field of visual plasticity how disappointed I was to hear this. David made me realize that it just raises an extremely interesting question: What is the difference in their circuitry that gives rise to either a salt and pepper organization or columns? I always thought it was, among other things, a scaling problem. Fitzpatrick proposed that there could be differences in the respective weights of inhibitory and excitatory inputs and that different operations could play a role in determining the tuning of the cells. That sounds to me like there is still a lot for us left to discover. Hubel and Wiesel eat your heart out!