Samuel S. Wang

The Wang laboratory does basic research in several areas: (1) information processing in the cerebellum, including its contributions to motor learning; (2) cerebellar roles in cognitive and affective function and autism spectrum disorder; (3) the improvement of tools for awake, in vivo optical imaging; and (4) synaptic learning rules throughout the brain.

We rely on advanced methods for the optical control and imaging of brain activity. Research interests are represented both by published papers and by more recent work. (For Sam Wang's public writings, see WelcomeToYourBrain.com. For Presidential polling meta-analysis see the Princeton Election Consortium.)

Optical imaging of the learning cerebellum in awake mice

Recent research has revealed a broad role for cerebellum as a general processor of unexpected events. We are among the first in the world to make extensive use of multiphoton fluorescence microscopy to probe what cerebellar circuits do during awake behavior. In a central recent finding, external and internal events are encoded by overlapping populations of Purkinje cells in a behavioral state-dependent manner, in the form of synchronous complex spiking. We have also made an analogous observation in granule cells and molecular layer interneurons. Recently, in collaboration with Javier Medina at the University of Pennsylvania, our laboratory implemented head-fixed recording methods for classical eyeblink conditioning in mice, achieving well-timed responses, learning over a time course of days, and consistency from animal to animal. Head-fixed recording is applicable to eyeblink conditioning and a variety of other behavioral and learning tasks.

Autism spectrum disorder

One of the most important unanswered questions in autism research today is the identity of the neural circuit(s) responsible for autistic behavior. We are interested in identifying brain dysfunction that is developmentally “upstream” of the many problems found in autistic brains. The cerebellum is not just a motor structure, but also has cognitive and affective roles. Accumulating evidence suggests that cerebellar abnormalities may play an ongoing role in, or even act as a developmental cause of the core social and cognitive deficits experienced by autistic persons. We are using mouse models to test two questions: (a) In mice with the same genetic disruptions as those found in autistic persons, is cerebellar function also disrupted? (b) Does disruption of cerebellar function during key periods of brain development lead to autistic-like behaviors?

Optical methods and genetically encodable calcium indicator proteins

Central tools in our laboratory include multiphoton in vivo imaging of neural activity, head-fixed awake recording, and the use of fluorescent calcium indicators. In a current project, we are at work on improving GECIs while avoiding the de-optimization of existing beneficial features. In the case of Green fluorescent protein / Calmodulin protein sensor (GCaMP), such features include low degradation, high per-molecule brightness, and large fluorescence changes. We making two improvements. First, we seek to engineer K_D by making targeted changes that vary the affinity but preserve probe performance thus allowing a larger range of activity levels (i.e. firing rates) to be monitored accurately. Second, we are engineering faster kinetics to better track variations in calcium. Dendritic calcium signals can rise in 1 ms and fall in 10-100 ms. Faster variants of GCaMP will allow many laboratories, including our own, to track neuronal signaling events with unprecedented precision.

Synaptic learning rules

In addition, in past years the laboratory has identified fundamental principles by which molecular signaling mechanisms shape learning rules. For example, we have found that calcium signaling mechanisms drive the switchlike strengthening and weakening of single synapses. The likelihood and direction of this change is closely dependent on the precise occurrence of certain presynaptic and postsynaptic spike patterns. In the case of cerebellum, we have identified a learning rule that favors parallel fiber activity that leads the complex spike by tens to hundreds of milliseconds, consistent with order-dependent learning seen in vivo.

Other projects 

In vitro, the laboratory studies how single-neuron function is modified by dynamic changes in neural activity such as complex input patterns of neurotransmitters and neuromodulators. Rapid barrages of dendritic input activation may alter function in a fraction of a second, thus altering circuit function and driving synaptic plasticity. These questions are being pursued using uncaging methods, which allow neurotransmitters such as glutamate to be generated in femtoliter (1 cubic micron) volumes within a millisecond. With rapid beamsteering technology we can uncage at tens of thousands of locations per second. Projects focus on large neurons such as cerebellar Purkinje neurons and pyramidal neurons of the neocortex and hippocampus, all of which receive a large convergence of synaptic input.

Brain scaling and evolution

We use comparative biophysical principles to infer functional principles of brain architecture. For example, the mammalian neocortex (also known as cerebral cortex) shows regularities of structure that suggest that brain structure may be subject to universal design constraints. From shrews to whales, mammalian brains vary over 100,000-fold in volume. Over this range large brains are more folded than small brains: the surface area of the cerebral cortex follows a power law relative to cortical volume greater than simple geometry would predict. We want to understand how these power laws are constructed from the cellular architecture of the cortex. Using electron microscopy, we find that on average, axons are wider in large brains than in small brains. The space demanded by these axons is sufficient to account for the increased folding seen in large brains. This widening of axons may be driven by an evolutionary need to preserve the time it takes for a nerve impulse to cross the brain.