dc.description.abstracteng | How complex natural sounds such as speech and vocalizations are encoded in the main
converging center of the auditory midbrain is not yet fully understood.
For multi-units, which are composed of several single neurons, the representation of individual
vocalizations is investigated across gradients of neural preferences in the inferior
colliculus. The multi-units allow making inferences about collective neural behavior. The
cross-correlation of averaged spiking responses is used to analyze if the vocalization representation
depends systematically on the spatial distance between multi-units. The measure
of response correlations and correlated variability indicates if interactions between
the multi-units exist. Neural discrimination is employed to examine whether the whole
set of spectrotemporally rich vocalizations can be reliably distinguished. The question is
addressed whether along the frequency gradient of the inferior colliculus, individual vocalizations
are encoded following their specific spectral content. This leads to the question
whether vocalizations are accurately represented by individual multi-units, or, if groups
of multi-units provide a more detailed representation, and if the multi-units interact to
facilitate encoding. Some of these vocalizations display envelope correlations, and neurons
might have neural preferences to these correlations, allowing them to encode the complex
sounds in more detail. Reverse correlation analysis can capture nonlinear neural response
properties. This analysis is used to probe preferences of single neurons to stimulus correlations,
separately for different frequency carriers of the envelope, and to characterize the
neural spectrotemporal tuning.
It is found that similarity of multi-unit spiking responses to vocalizations decreases linearly
with spatial distance across the inferior colliculus, and extends to a few hundred
micrometers. Neural correlations which are due to interactions between the multi-units are
substantial but do not improve separability. Since interactions with other multi-units do
not improve the neural separability, the multi-units act as independent encoders of vocalizations.
However, neural discrimination improves substantially when the responses from
more than one multi-unit are considered. In many cases, combining responses from a few
multi-units results in a perfect discrimination of the whole set of vocalizations. The individual
vocalizations are reliably encoded across wide frequency ranges, thus yielding a
robust representation, which is relevant in a behavioral context. Optimal encoding does
not strictly follow the spectral content of the vocalizations. The analysis of receptive fields
reveals that the majority of the tested single neurons displayed preferences to envelope
correlations. These are mostly found at the preferred frequency of the neuron and exhibit
temporal tuning properties that enhance or modulate the neuron’s linear response.
In conclusion, vocalizations are reliably encoded by independent multi-units across a wide
frequency range of the inferior colliculus. Neural preferences to envelope correlations are
present in the inferior colliculus and are likely to facilitate encoding of the vocalizations. | de |