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    864—Animal Learning and Memory: Gamma and Theta Activity

    Wednesday, November 13, 2013, 1:00 pm - 5:00 pm

    864.02: Slow and fast gamma oscillations support distinct spatial coding modes in hippocampal place cells

    Location: Halls B-H

    ">K. W. BIERI, K. N. BOBBITT, *L. L. COLGIN;
    Ctr. for Learning and Memory, Univ. of Texas At Austin, Austin, TX

    Abstract Body: Gamma oscillations (~25 - 140 Hz) synchronize the activity of distributed groups of neurons and are thought to create transient ensembles unique to given cognitive processes. The hippocampus exhibits two distinct types of gamma oscillations that selectively couple hippocampal subregion CA1 to two of its primary inputs (Colgin et al., Nature 462, 2009). Fast gamma (~65 - 140 Hz) couples CA1 to the medial entorhinal cortex, a region that supplies the hippocampus with current sensory information. Slow gamma (~25 - 55 Hz) couples CA1 to CA3, a neighboring subfield thought to store previously learned representations of the environment. These findings raise the possibility that fast gamma promotes encoding of ongoing experiences and slow gamma supports retrieval of stored memory representations. In the hippocampus, the principal neurons are ‘place cells’. A place cell only fires when an animal is in a specific location, called that cell’s ’place field’. Collectively, place cells are thought to code the ‘where’ component of memory. If fast gamma promotes ongoing memory encoding, then place cells firing during fast gamma episodes would be expected to represent recent locations (“retrospective coding”). If slow gamma supports memory retrieval, then place cells firing during slow gamma periods would be expected to predict upcoming locations (“prospective coding”). To test these hypotheses, we investigated the relationship between place cell firing patterns and the different gamma oscillation subtypes in freely behaving rats. Retrospective and prospective spatial coding modes alternated on sub-second time scales at the single unit and ensemble levels, resembling alternations between fast and slow gamma oscillations. Additionally, fast gamma power and phase-locking of spike times were heightened during retrospective coding, whereas slow gamma power and phase-locking were enhanced during prospective coding. These findings suggest that fast gamma coordinates place cells during encoding of recently visited locations, while slow gamma coordinates place cells during retrieval of stored representations.

    Lay Language Summary: Many of us leave our cars in a parking garage on a daily basis. Every morning, we create a memory of where we parked our car, which we retrieve in the evening when we pick it up. Memory involving location is stored in an area of the brain called the hippocampus. The neurons in the hippocampus that store spatial memories (such as the location where you parked your car) are called place cells. Interestingly, the same set of place cells are activated both when a new memory of a location is stored and, later, when the memory of that location is recalled, or retrieved. How then do our brains distinguish between current location and the memory of a location? Our new findings suggest a mechanism for distinguishing these different representations. Specifically, information about present locations and previously stored spatial memories are transmitted by different frequencies of gamma waves, a specific type of rhythmic electrical activity occurring across a wide range of frequencies.
    When the hippocampus forms a new spatial memory, it receives sensory information about your current location from a brain region called the entorhinal cortex. When recalling a particular location, the hippocampus retrieves the stored spatial memory from a subregion of the hippocampus called CA3. The entorhinal cortex and CA3 transmit these different types of information using different frequencies of gamma waves. The entorhinal cortex uses fast gamma waves, which have a frequency of about 80 Hz. In contrast, CA3 sends its signals on slow gamma waves, which have a frequency of about 40 Hz. Based on these observations, we hypothesized that fast gamma waves promote encoding of recent experiences, while slow gamma waves support memory retrieval. We tested these hypotheses by recording gamma waves in the hippocampus, together with electrical signals from place cells, in a group of 5 rats navigating a simplistic spatial environment. We found that place cells represented the rat’s current location when cells were active on fast gamma waves. On the other hand, when cells were active on slow gamma waves, place cells represented locations where the rat was heading. These findings suggest that fast gamma waves promote current memory encoding, such as the memory of where we just parked. However, when we need to remember where we are going, like when finding our parked car later in the day, the hippocampus tunes into slow gamma waves.
    One of the next steps in our research is to apply technologies that induce different types of gamma waves in rats performing memory tasks. We will use a task in which previously stored memories interfere with the ability to form new memories. We imagine that we will be able to improve new memory encoding by inducing fast gamma waves. Conversely, we expect that inducing slow gamma waves in this circumstance will be detrimental to the encoding of new memories. Slow gamma waves should trigger old memories, which would interfere with new learning.
    Because gamma waves are seen in many areas of the brain besides the hippocampus, our findings may generalize beyond spatial memory. The ability for neurons to tune into different frequencies of gamma waves provides a way for the brain to traffic different types of information across the same neuronal circuits. Our findings may provide insight into the cognitive and memory disruptions seen in diseases such as schizophrenia and Alzheimer’s, in which gamma waves are disturbed.