Abstract

Many living systems demonstrate exquisite sensitivity to small input signals. A tempting hypothesis is that these systems operate close to bifurcation or critical points, where the system's response exhibits a diverging susceptibility to the control parameter and small signals are amplified into a large collective response. A common concern, however, is that proximity to such points requires fine-tuning of parameters, which seems impossible for noisy biological systems. Based on several distinct sensory systems, we have investigated a feedback motif that robustly maintains these systems close to their respective bifurcation point. The key ingredient is that the collective response feedsback onto the control parameter. To illustrate this idea in this talk, I will focus on the case of mammalian hearing. Sound waves entering the cochlea are converted into surface waves along the basilar membrane, whose mechanical properties lead to apposition-dependent resonance. To achieve the cochlea's astonishing frequency resolution and sensitivity to faint sounds, it is brought to the edge of instability by active energy-consuming processes in hair cells that effectively reduce dissipation. However, while collective hair cell activity fine-tunes the entire basilar membrane, individual hair cells only have access to information about local membrane displacement. In this talk, I will show that a simple local feedback mechanism based on self-organized criticality is, in principle, enough to globally tune the cochlea to the edge of instability. However, requiring stability of the system places important limits on possible forms of active feedback in hair cells. Taken together, our work illuminates how and under what conditions individual hair cells can collectively create a critical cochlea.

Biosketch

Isabella Graf is a group leader at EMBL Heidelberg. She is broadly interested in using methods and tools of statistical mechanics, nonlinear dynamics, and information theory to help understand biological phenomena. In particular, she is keen to elucidate the role of bifurcation points and feedback for function. During her Postdoc in Ben Machta’s group at Yale, she focused on signal amplification and information integration in biological sensory systems and on the phase behavior of mixtures with many components. Before joining Yale University, she was a PhD student in Physics in Erwin Frey’s group at LMU Munich, where she worked on collective dynamics and self-organization in the context of the cell cytoskeleton and on non-equilibrium self-assembly of heterogeneous structures.