Vestibulo-ocular signal transformation in frequency-tuned channels.

Self-generated locomotor activity is accompanied by head movements that cause retinal image displacements with a resultant degradation of visual information processing. To maintain visual acuity, retinal image drift must be counteracted by dynamic compensatory gaze adjustments that derive to a large extent from vestibulo-ocular reflexes (VOR). During head motion, vestibular signals code a wide frequency range from static head position to high acceleration profiles during rapid head turns. This large dynamic range suggests that the sensory-motor transformation occurs in parallel, yet complementary frequency-tuned pathways. In fact, the classic "three-neuronal" VOR pathway is composed of distinct functional subgroups of cells with different intrinsic properties and response dynamics at each synaptic level. This generates sets of neuronal filters that are ideal for particular frequency ranges and signaling patterns, respectively. In second-order vestibular subgroups, different filter functions, and hence a different synaptic processing is facilitated by a coadaptation of intrinsic membrane and emerging network properties. The consecutive assembly and sequential connectivity of pre- and postsynaptic neuronal elements with corresponding physiological properties, generates parallel pathways that allow for separate coding of different dynamic head-motion components during locomotor activity.

Modeling of intrinsic and synaptic properties to reveal the cellular and network contribution for vestibular signal processing.

Computational modeling of cellular and network properties of central vestibular neurons is necessary for understanding the mechanisms of sensory-motor transformation for gaze stabilization. As a first step to mathematically describe vestibular signal processing, the available physiological data of the synaptic and intrinsic properties of frog second-order vestibular neurons (2 degrees VN) were used to create a model that combines cellular and network parameters. With this approach it is now possible to reveal the particular contributions of intrinsic membrane versus emerging network properties in shaping labyrinthine afferent-evoked synaptic responses in 2 degrees VN, to simulate perturbations, and to generate hypotheses that are testable in empiric experiments.

Medical documentation using a gaze-driven camera.

Medical treatments of a surgeon or a dentist are sometimes documented for teaching, telemedicine, or liability issues using a scene oriented video camera. But the most interesting parts of the scene are often covered by the operators hand or body. The best view to the scene is next to the operators field of view or perfectly: Within his head. Head-mounted scene cameras are used to create this exclusive point of view. Eye tracking systems could be used to emphasize the point of gaze within the scene image. The presented system improves classical eye trackers with an additional gaze-driven camera. The resulting scene image maintains the overall context, while the image from the gaze driven camera acts like a magnifying glass and provides a high-resolution image of the gazed detail using an independent exposure, thus creating a high dynamic range image. We show an application in a real dental treatment scenario.

Differential dynamic processing of afferent signals in frog tonic and phasic second-order vestibular neurons.

The sensory-motor transformation of the large dynamic spectrum of head-motion-related signals occurs in separate vestibulo-ocular pathways. Synaptic responses of tonic and phasic second-order vestibular neurons were recorded in isolated frog brains after stimulation of individual labyrinthine nerve branches with trains of single electrical pulses. The timing of the single pulses was adapted from spike discharge patterns of frog semicircular canal nerve afferents during sinusoidal head rotation. Because each electrical pulse evoked a single spike in afferent fibers, the resulting sequences with sinusoidally modulated intervals and peak frequencies up to 100 Hz allowed studying the processing of presynaptic afferent inputs with in vivo characteristics in second-order vestibular neurons recorded in vitro in an isolated whole brain. Variation of pulse-train parameters showed that the postsynaptic compound response dynamics differ in the two types of frog vestibular neurons. In tonic neurons, subthreshold compound responses and evoked discharge patterns exhibited relatively linear dynamics and were generally aligned with pulse frequency modulation. In contrast, compound responses of phasic neurons were asymmetric with large leads of subthreshold response peaks and evoked spike discharge relative to stimulus waveform. These nonlinearities were caused by the particular intrinsic properties of phasic vestibular neurons and were facilitated by GABAergic and glycinergic inhibitory inputs from tonic type vestibular interneurons and by cerebellar circuits. Coadapted intrinsic filter and emerging network properties thus form dynamically different neuronal elements that provide the appropriate cellular basis for a parallel processing of linear, tonic, and nonlinear phasic vestibulo-ocular response components in central vestibular neurons.

Differential inhibitory control of semicircular canal nerve afferent-evoked inputs in second-order vestibular neurons by glycinergic and GABAergic circuits.

Labyrinthine nerve-evoked monosynaptic excitatory postsynaptic potentials (EPSPs) in second-order vestibular neurons (2 degrees VN) sum with disynaptic inhibitory postsynaptic potentials (IPSPs) that originate from the thickest afferent fibers of the same nerve branch and are mediated by neurons in the ipsilateral vestibular nucleus. Pharmacological properties of the inhibition and the interaction with the afferent excitation were studied by recording monosynaptic responses of phasic and tonic 2 degrees VN in an isolated frog brain after electrical stimulation of individual semicircular canal nerves. Specific transmitter antagonists revealed glycine and GABA(A) receptor-mediated IPSPs with a disynaptic onset only in phasic but not in tonic 2 degrees VN. Compared with GABAergic IPSPs, glycinergic responses in phasic 2 degrees VN have larger amplitudes and a longer duration and reduce early and late components of the afferent nerve-evoked subthreshold activation and spike discharge. The difference in profile of the disynaptic glycinergic and GABAergic inhibition is compatible with the larger number of glycinergic as opposed to GABAergic terminal-like structures on 2 degrees VN. The increase in monosynaptic excitation after a block of the disynaptic inhibition in phasic 2 degrees VN is in part mediated by a N-methyl-d-aspartate receptor-activated component. Although inhibitory inputs were superimposed on monosynaptic EPSPs in tonic 2 degrees VN as well, the much longer latency of these IPSPs excludes a control by short-latency inhibitory feed-forward side-loops as observed in phasic 2 degrees VN. The differential synaptic organization of the inhibitory control of labyrinthine afferent signals in phasic and tonic 2 degrees VN is consistent with the different intrinsic signal processing modes of the two neuronal types and suggests a co-adaptation of intrinsic membrane properties and emerging network properties.

Differential intrinsic response dynamics determine synaptic signal processing in frog vestibular neurons.

Central vestibular neurons process head movement-related sensory signals over a wide dynamic range. In the isolated frog whole brain, second-order vestibular neurons were identified by monosynaptic responses after electrical stimulation of individual semicircular canal nerve branches. Neurons were classified as tonic or phasic vestibular neurons based on their different discharge patterns in response to positive current steps. With increasing frequency of sinusoidally modulated current injections, up to 100 Hz, there was a concomitant decrease in the impedance of tonic vestibular neurons. Subthreshold responses as well as spike discharge showed classical low-pass filter-like characteristics with corner frequencies ranging from 5 to 20 Hz. In contrast, the impedance of phasic vestibular neurons was relatively constant over a wider range of frequencies or showed a resonance at approximately 40 Hz. Above spike threshold, single spikes of phasic neurons were synchronized with the sinusoidal stimulation between approximately 20 and 50 Hz, thus showing characteristic bandpass filter-like properties. Both the subthreshold resonance and bandpass filter-like discharge pattern depend on the activation of an I(D) potassium conductance. External current or synaptic stimulation that produced impedance increases (i.e., depolarization in tonic or hyperpolarization in phasic neurons) had opposite and complementary effects on the responses of the two types of neurons. Thus, membrane depolarization by current steps or repetitive synaptic excitation amplified synaptic inputs in tonic vestibular neurons and reduced them in phasic neurons. These differential, opposite membrane response properties render the two neuronal types particularly suitable for either integration (tonic neurons) or signal detection (phasic neurons), respectively, and dampens variations of the resting membrane potential in the latter.