Stephen Van Hooser, Ph.D.
Choosing Which Way to Go: the Development of Direction Preferences in Primary Visual Cortex
At this year's Volen retreat we were pleased to hear from Dr. Stephen Van Hooser, who is one of our more recent additions to the Brandeis neuroscience faculty. Dr. Van Hooser's work focuses on a particular class of neurons within the visual cortex that have the property of responding only to features in the visual field that are moving in a particular direction. Dr. Van Hooser's work has demonstrated that this neuronal characteristic is by no means an intrinsic property, but rather a preference that arises as a result of visual experience.
The proper development of brain circuits critically depends on interactions between processes that are specified genetically and those that require sensory experience. The Van Hooser lab studies these interactions in the ferret visual cortex. At the time of eye opening, neurons in the ferret visual cortex will selectively respond to bars of light or edges that have a particular orientation. However, at this stage there is no selectivity in the direction of motion, a property that arises later as the animal matures. Dr. Van Hooser's question is how directional selectivity arises in visual cortex.
In his presentation, Dr. Van Hooser presented evidence that visual experience is required for the development of direction selectivity. This is evidenced by the fact that animals raised in the dark and not receiving visual stimulation will fail to acquire direction selectivity. Furthermore, exposing animals to a motion-training stimulus causes a more rapid emergence of direction selectivity in visually naive animals. Using advanced microscopy, Dr. Van Hooser and colleagues monitored how the response properties of individual cells were altered by this training stimulus. The direction preference that neurons would eventually acquire could be predicted based upon small initial biases that were present at the time of eye opening. This indicates that processes independent of experience play a role in determining a cell's future direction preference. However, these initial biases are not immutable, as exposure to a single direction of motion causes a majority of neurons to develop a preference for the trained stimulus direction. Finally, Dr. Van Hooser presented preliminary evidence indicating that cells in the layer of cortex that receives the primary projection from the retina, via a structure called the lateral geniculate nucleus, may acquire direction selectivity before cells in other layers. Future experiments will continue to uncover the circuit mechanisms underlying the development of direction selectivity in cortical circuits.