Nearly 40 years after researchers first asked how some retinal nerve cells could detect the direction of motion independently of the rest of the brain, a team of researchers from The University of Queensland and the Australian National University have solved the problem.
The work appears in today's issue of Science, the prestigious international research journal.
Dr David Vaney, of UQ's Vision, Touch and Hearing Research Centre says, "this work resolves a long-standing problem and shows that nerve cells are much more complex than was originally thought. We have shown that the vast computational power of the brain comes not only from the myriad of connections between nerve cells but also due to processing within each cell."
The work was completed as a collaboration between Dr Rowland Taylor of the John Curtin School of Medical Research, Professor William Levick of the ANU's Department of Psychology, and Dr Vaney and Dr Shigang He from VTHRC.
Certain types of nerve cells in the retina, considered part of the brain, are called direction-selective ganglion cells (DSGCs) and preferentially respond to image motion in one direction. So an object moving, for example, left to right would produce a strong signal but another moving right to left may not be detected at all. The DSGC would be literally blind to such movement.
This response represents a very early form of complex information processing in the visual system but it was not known how this is achieved. The research team has now shown that the crucial computations that underlie the generation of direction selectivity take place within individual processes of the DSGCs.
The problem has come full circle for Professor Levick, who discovered these cells in 1964 with Horace Barlow. They asked how the cells could compute the direction of image motion and now Professor Levick has helped provide the answer.
"The reason why the discovery could be made now but not sooner is because of a combination of new techniques," said Dr Vaney. "These include patch-clamping, in which tiny electrical currents are measured by clamping the cell body to a micro-pipette, and the use of an isolated preparation of the retina that responds normally to visual stimuli."
"In the retina there about 20 distinct types of ganglion cells, which respond preferentially to different features of the visual image, such as local contrast, colour, and the speed and direction of image motion," said Dr Vaney. "The directional information appears to be used by the brain to help orient the eyes and to analyse the relative movement of objects as an animal moves through its environment."
Earlier models of nerve cell function simply required that each cell add up the excitatory and inhibitory inputs coming from other cells in the brain. However, this study shows that there are complex interactions between the excitatory and inhibitory inputs on individual processes of the nerve cell.
"The retina is the easiest part of the brain for us to examine but we expect this to be characteristic of many types of nerve cells in the rest of the brain. These results provide a guidepost, pointing towards new ways of examining nerve cells and understanding how complex brain functions can occur," said Dr Vaney.
For more information contact Dr David Vaney (telephone 07 3365 3759) or Jan King at UQ Communications (telephone 07 3365 1120) or email: communications@mailbox.uq.edu.au
