Research Seminar - May 31, 2002

Topographic maps in the brain: biology vs theory

Abstract:

One of the most common characteristics of neuronal connectivity patterns in the brain is topography, whereby neighbouring points in one structure project to neighbouring points in a target structure. The organisation allows the integration of incoming information and coordination of responses within the organism. The most extensively studied model system in this regard is the visual projection from the retina to a brain region known as the optic tectum. While theoretical and computational models abound, the biological mechanisms that axons use in order to achieve such precise targeting are only now beginning to be understood.

Experiments carried out in the 1960s to 1980s to manipulate the visual system rapidly concluded that multiple mechanisms are involved in establishing topography. One of these, the "chemoaffinity hypothesis", proposed by Nobel laureate Roger Sperry, predicted that orthogonal and complementary gradients of proteins expressed in the retina and tectum would be sufficient to establish topography. Recently, the discovery of gradients of Eph receptors and their ligands, the ephrins, has focussed attention on this single mechanism proposed by Sperry. However, current debate largely ignores previous work showing that gradient matching alone is an incomplete explanation, and neuronal activity itself has an important role to play.

In my seminar, I will describe the topographic arrangement of the visual system and the mechanisms thought to be involved in its formation and regeneration following injury. Our current understanding of the biological mechanisms involved in the establishment of topographic connections should allow the elaboration of improved theoretical and computational models.