Cells with double vision
Unlike in the peripheral nervous system, where cells are often unable to distinguish which branching pathway an electrical system is travelling from, the central nervous system makes use of sophisticated internal networks not too dissimilar from an IP record, to differentiate between nervous pathways.
Researchers from the Max Planck Institute of Neurobiology in Martinsried, Germany, have been experimenting on fruit flies. Fruit flies for all their obvious differences, are remarkably similar in genetic make-up to humans. Also like humans, and their brains, despite their complexity, are quite small in size.
The researchers have now been able to prove that the brains are capable of a level of complexity previously thought to only be inherent in much larger organic neuro structures, thanks to sophisticated network interactions.
The neurobiologists examined nerve cells that receive motion information in their input region from only a narrow area of the fly's field of vision. Yet, thanks to their linking with neighbouring cells, the cells respond in their output regions to movements from a much wider field of vision.
Now, at first glance, this seems to have little to do with the human brain. The fruit fly brain has 250,000 neurons, in contrast to the 100,000,000,000 of a human brain. Yet, the genetic similarities do come into play again. The 100 billion cell brain follows the self same wiring patterns of the 250,000 cell brain. In other words, our brain is using the same techniques to emulate a brain still much larger than itself.
With the fruit fly, a small network of only 60 nerve cells in each cerebral hemisphere suffices the blowfly to integrate visual motion information. The resulting information is then used in the control and correction of the fly's flight manoeuvres. However, flies clearly demonstrate just how efficient these 60 cells actually are when they dodge obstacles while flying at high speed and land upside-down on the ceiling.
It is obviously much easier to study the interconnections of 120 cells (both hemispheres), and their connections with other cells in the brain in order to learn how these networks function, than it is to study a couple of hundred million cells in the human brain dedicated to the same task. Both yield the same data: a basic understanding of how these routing networks function.
It soon became apparent to the researchers that the 60 nerve cells in a fruit-fly's hemisphere are further sub-divided into several individual cell groups, each of which is responsible for the processing of certain patterns of movement.
A group of ten cells, known as the VS-cells, respond to rotational movements of the fly. Each of these ten cells receives its visual information from only a narrow vertical strip of the fly's eye - the cell's "receptive field". Since the VS-cells are arranged parallel to each other, the fly's field of vision is completely covered by the vertical strips of the ten cells on each side of the fly's brain
"However, the most fascinating aspect of these VS-cells is that the closer we examined the network, the more complex it appeared", group leader Alexander Borst reports. He and his group at the Max Planck Institute of Neurobiology are devoted to investigating the motion vision of flies. Only recently, Borst's co-worker Jürgen Haag showed that VS-cells are connected on two different levels. It was well known that in their input regions, the cells collect incoming signals from nerve cells which represent local motion information coming from the eye. Yet, it came as a surprise that the cells had a second source of information. The scientists found electrical connections between neighbouring VS-cells in the cells' output regions. Computer simulations of this network led to the following assumption: Information received from a VS-cell's "own" receptive field is first compared with the information received by its neighbouring cells. Only then is the information relayed to cells further downstream in the network for the purpose of flight control.
The immediate prediction from this work was somewhat of a surprise. Could a single cell have two different receptive fields, depending on which part of the cell is taken into consideration? In Martinsried, the neurobiologist Yishai Elyada now looked at this question. He examined the reactions of the VS-cells to moving stimuli using a large variety of techniques. The breakthrough came when he used a special microscopy technique which visualises changes in the concentrations of calcium within the cells. The calcium concentration in many kinds of nerve cells, including VS cells, changes when the cell becomes active. Changes in the calcium level therefore reveal when and where a nerve cell reacts to a stimulus.
In order to determine the receptive field of each VS-cell, Elyada presented moving stripe patterns to the flies while simultaneously monitoring the changes in the calcium levels within the cells. The results correlated well with the scientist's predictions. In their input region, VS-cells do indeed respond to movement in only a narrow area of the visual field. In contrast, in the cells' output region, each cell also responds to movement in the receptive fields of its neighbouring cells. The prior assumption that the receptive field of a nerve cell is a single unit must therefore be re-evaluated. In future, such statements need to distinguish between the input and the output regions of the cell - at least when referring to VS-cells. Such spatial separation within a single cell took the scientists by surprise. However, as far as the fly is concerned, it is a very useful attribute. Model simulations demonstrated that a network that is comprised of such "double input cells" can process visual motion information much more efficiently.