Neurobiology

Richard T. Born, M.D.

Professor of Neurobiology

Director of the Program in Neuroscience

Born Website: http://www.hms.harvard.edu/bss/neuro/bornlab/

Our lab is interested in the neural circuitry of the primate visual cortex and how it relates to perception and visually guided behavior. Our current focus is on areas of the brain that make calculations about visual motion. In what direction is it moving? How fast? Is it really moving with respect to its surroundings, or does it just seem to be moving because I'm moving? The calculations that allow an animal to answer these critical questions are performed by populations of neurons which are anatomically organized to form a map representing all possible directions of motion in all parts of the visual field. Superimposed on this map is a coarser organization of neurons whose receptive fields have differing center-surround interactions with respect to moving stimuli. This organization consists of slab-like clusters of cells whose receptive fields have motion opponent surrounds and thus signal local motion contrast interdigitated with clusters of cells whose receptive fields have additive surrounds and thus respond best to wide-field motion. The neurons within these two compartments make segregated connections to higher motion processing areas. We believe that these neurons are performing calculations necessary for distinguishing self- from object-motion. If we electrically activate a direction column within an interband while an animal is visually tracking a moving target, the smooth pursuit eye movements are sped up in the preferred direction of the neurons we're stimulating. If, on the other hand, we stimulate a direction column of wide-field neurons, the pursuit is sped up in the opposite direction; precisely the effect that one gets if the visual background moves in the preferred direction, which causes the target to appear as if it's moving in the opposite direction (so-called "induced motion")

A second major focus of the lab concerns the integration of visual motion signals. Given that MT neurons appear to perform some of the calculations necessary to distinguish and object from the visual background, we might ask how different local motion signals emanating from the same object are combined to form an accurate representation of object motion.

We use combinations of extracellular electrophysiology, 2-deoxyglucose mapping and anatomical tracing in order to learn the nature of the neural circuitry and the sorts of calculations that it makes. This allows us to make predictions about what that circuit is doing for the animal--predictions which we can test by activating parts of the circuits electrically or inactivating them by cooling in order to assess their role in behavior. If we have formulated our theories correctly, we should be able to perturb the behavior in specific and interesting ways.

Correlation of 2-deoxyglucose (2dg) labeling and neuronal receptive field properties in the middle temporal visual area (MT). The gray-level image at the top center is an autoradiograph of a section through MT cut parallel to the cortical surface. The patterns of 2dg uptake were produced by showing the animal a wide-field pattern of random dots (covering ~60 degrees of the visual field) that moved coherently at systematically varied directions and speeds while 2dg was infused intravenously. Regions of high 2dg uptake appear dark (wide-field motion columns), while regions of 2dg uptake equal to that of unstimulated cortex are lighter (local-contrast motion columns). The graphs immediately below the 2dg image depict the responses of two representative neurons to patches of random dots moving in the cellÍs preferred direction and speed as a function of the size of the random dot patch (area response test). Neurons in the dark regions respond more vigorously as the area of the random dot patch increases; neurons in the light regions respond well to small patches of motion but are indifferent to wide-field motion due to the presence of opponent surrounds.