Date:Feb/25th/(thu)13:30-15:00 Place:Ew305 Speaker:Dr.Kosuke Hamaguchi(Dept. of Neurobiology, Duke University Medical Center) Title:Synaptic level analysis of corollary discharge mechanisms in neural pathways for vocal learning Abstract: Songbirds learn to sing by using auditory feedback to evaluate vocal performance in reference to a memorized acoustic model, or song template. Part of this process could harness corollary discharge of song motor commands to generate predictions of auditory feedback associated with the resultant vocalization. Intriguingly, a single brain area, HVC, contains one population of projection neurons (HVCRA) that transmit the song motor commands through the song motor nucleus RA and a separate population of projection neurons (HVCX) that transmit corollary discharge to an anterior forebrain pathway (AFP) important to song learning. The output of the AFP, LMAN, is believed to generate trial-to-trial song variability necessary to song learning by injecting gnoiseh into song motor commands at the level of RA. We are interested in how LMAN activity (and hence trial-to-trial variability) is controlled by the motor signals emanating from HVCRA cells. One attractive idea is that HVCX cells transmit precise corollary discharge so that LMAN can control song variability as an exact function of time. However, any corollary discharge presumably must propagate to the AFP from HVCX cells, which integrate input from a variety of sources, including HVCRA cells, local interneurons and auditory afferents. This raises the question of how activity in HVCRA and HVCX cells relates to activity in LMAN neurons. To address this issue, we made simultaneous in vivo intracellular current clamp recordings in HVC and LMAN neurons in urethane anesthetized zebra finches. We used covariance analysis and coherency analysis to quantify the relationship between neurons in these two song areas. We found strong correlations in the subthreshold activity of HVCRA and LMAN neurons, which supports the idea that LMAN receives timing information about the activity of cells that transmit song motor commands. Interestingly, the subthreshold activity of HVCX and LMAN neurons was on average more weakly correlated, even though HVCX cells and not HVCRA cells provide direct input to the AFP. Since HVCX neurons have strong inhibitory inputs from HVC interneurons (HVCI), we investigated the possibility that local inhibition modulates signals transmitted from HVCRA cells. Consistent with this view, we found that the membrane potential correlation between HVCX and LMAN neurons increased significantly when we strongly hyperpolarized the HVCX neuron, a manipulation that renders inhibitory potentials depolarizing. One prediction of the corollary discharge model is that HVC activity drives LMAN activity. However, we found that most HVCRA-LMAN and HVCX-LMAN pairs have correlation peaks around zero time-lag, suggestive of common excitatory inputs to these two song areas. Alternatively, HVC may drive LMAN, but local circuit dynamics may obscure this interaction. To distinguish between these possibilities, we induced bursting activity in HVC with Bicuculline (BMI), a GABAA receptor blocker. We reasoned that if the correlation is due to common input, LMAN activity should not be affected. Instead, when BMI was applied in HVC, a strong HVC leading (~ 40 msec) peak could be detected in the LMAN spike triggered average of HVC multiunit activity. These results support propagation of activity from HVC to LMAN, and also suggest that local circuit dynamics normally obscure this feedforward relationship. More broadly, these experiments support a model in which the information that HVCX cells transmit to LMAN results from an interaction between motor signals from HVCRA cells and inhibitory signals from HVCI cells. Notably, a recent report suggests that putative HVCI cells may encode real time auditory feedback signals. Over time, the interaction between motor and feedback signals converging onto HVCX cells may precisely modify corollary discharge transmitted to the AFP, thus affecting exactly when LMAN injects noise into the song motor command.