Responses of cells of the dorsal nucleus of the lateral lemniscus to species-specific and other complex sounds
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The sounds animals hear, whether generated by conspecifics, other species, or the environment, typically contain multiple simultaneous frequencies that change over time. However, most studies of the central auditory system have focused on measuring responses to very simple stimuli, namely individual pure tones. This study investigates how one brainstem auditory nucleus, the dorsal nucleus of the lateral lemniscus (DNLL), responds to complex natural and artificial sounds, and whether the responses to complex sounds can be predicted based on a linear model. Single neurons were recorded in awake Mexican free-tailed bats (Tadarida brasiliensis) during acoustic stimulation. Basic response properties, such as threshold, latency, binaurality, temporal response pattern, and frequency tuning were measured with simple stimuli (tone bursts). The responses to complex sounds, including species-specific communication calls, echolocation pulses, other animal calls, and sounds from a human environment were then obtained from the same cells. Data analysis was divided into two sections. First, responses of DNLL cells to complex sounds were measured and quantified in various ways to develop a better understanding of how this nucleus is activated by these more realistic sound stimuli. Second, predictions of how the cells should have responded to the complex sounds were compared to the actual responses to the sounds. The predictions were generated from two models. The first model was based on the cell’s frequency tuning as measured with simple pure tones. The second model, generated by a more complicated reverse correlation technique, was also used to predict responses to complex sounds. Both of these models were fundamentally linear in nature, and thus allowed an estimation of how linearly DNLL cells integrate synaptic inputs. Using these techniques, it was shown that DNLL cells process complex sounds in a simple linear way. Each cell’s responses are primarily determined by its simple frequency tuning. This applies not only to which sounds a cell will respond to, but also when during those sounds the cell will respond. As a population, then, the activity of cells of the DNLL represent the temporal structure of the spectral content of any sound just as that produced by the Fourier transform and visualized in a spectrogram display.