Ralf Wessel

Synaptic dynamics mediate sensitivity to motion independent of stimulus details

Humans and other animals generally perceive motion independently of the cues that define the moving object. To understand the underlying mechanisms of this generalization of stimulus attributes, we have examined the cellular properties of avian wide-field tectal neurons that are sensitive to a variety of moving stimuli but not to static stationary stimuli. This in vitro study showed phasic signal transfer at the retinotectal synapse and binary dendritic responses to synaptic inputs that interact in a mutually exclusive manner in the postsynaptic tectal neuron. A model of the tectal circuitry predicts that these two cellular properties mediate sensitivity to a wide range of dynamic spatiotemporal stimuli, including moving stimuli, but not to static stationary stimuli in a tectal neuron. The computation that is independent of stimulus detail is initiated by tectal neurons and is completed by rotundal neurons that integrate outputs from multiple tectal neurons in a directionally selective manner.

Developmental regulation of active and passive membrane properties in rat vibrissa motoneurones

We characterized the electrophysiological properties of vibrissa motoneurones (vMNs) in rat. Intracellular recordings of vMNs in brainstem slices from animals aged P4 to P5 and P9 to P11, i.e. newborn animals, showed that the subthreshold membrane impedance has the form of passive decay. In particular, the impedance follows the 1/√f signature for long dendrites beyond a cut‐off frequency of fc= 8 Hz. In contrast, the impedance has the form of a resonant filter in vMNs from slices prepared from animals aged P17 to P23, i.e. young animals. The resonance has a peak near 4 Hz and an amplitude of 1.2 times that at low frequencies (f∼ 0.1Hz). The low frequency onset of the resonance is shown to depend on a hyperpolarization‐activated depolarizing current, Ih. This current functions as a high‐pass filter. The high frequency cut‐off of the resonance results from passive decay in long dendrites, similar to the case with newborn animals but with fc= 20Hz. In addition to a resonance in subthreshold properties, an enhanced resonance in spiking is observed in young as opposed to newborn animals. The transition from solely passive decay in vMNs from newborn animals to resonance in young animals coincides with the onset of whisking. Further, the width of the resonance encompasses the 4–15Hz range of exploratory whisking. Nonetheless, it remains to be shown if there is a causal relation between the regulation of currents in vMNs and the onset of whisking. In particular, we further observed that the membrane impedance of hypoglossal motoneurones from both newborn and young animals exhibits a subthreshold resonance that also peaks near 4Hz. The amplitude of this resonance increases from 1.1 to 1.4 times that at low frequencies in newborn versus young animals. We conjecture that resonance properties in vibrissa, hypoglossal, and potentially other motoneurones, may serve to transiently and purposely synchronize different orofacial behaviours.

Recurrent Antitopographic Inhibition Mediates Competitive Stimulus Selection in an Attention Network

Topographically organized neurons represent multiple stimuli within complex visual scenes and compete for subsequent processing in higher visual centers. The underlying neural mechanisms of this process have long been elusive. We investigate an experimentally constrained model of a midbrain structure: the optic tectum and the reciprocally connected nucleus isthmi. We show that a recurrent antitopographic inhibition mediates the competitive stimulus selection between distant sensory inputs in this visual pathway. This recurrent antitopographic inhibition is fundamentally different from surround inhibition in that it projects on all locations of its input layer, except to the locus from which it receives input. At a larger scale, the model shows how a focal top-down input from a forebrain region, the arcopallial gaze field, biases the competitive stimulus selection via the combined activation of a local excitation and the recurrent antitopographic inhibition. Our findings reveal circuit mechanisms of competitive stimulus selection and should motivate a search for anatomical implementations of these mechanisms in a range of vertebrate attentional systems.

Variational calculation of the limit cycle and its frequency in a two-neuron model with delay

We consider a model system of two coupled Hopfield neurons, which is described by delay differential equations taking into account the finite signal propagation and processing times. When the delay exceeds a critical value, a limit cycle emerges via a supercritical Hopf bifurcation. First, we calculate its frequency and trajectory perturbatively by applying the Poincaré-Lindstedt method. Then, the perturbation series are resummed by means of the Shohat expansion in good agreement with numerical values. However, with increasing delay, the accuracy of the results from the Shohat expansion worsens. We thus apply variational perturbation theory (VPT) to the perturbation expansions to obtain more accurate results, which moreover hold even in the limit of large delays.

Winner-take-all selection in a neural system with delayed feedback

We consider the effects of temporal delay in a neural feedback system with excitation and inhibition. The topology of our model system reflects the anatomy of the avian isthmic circuitry, a feedback structure found in all classes of vertebrates. We show that the system is capable of performing a ‘winner-take-all’ selection rule for certain combinations of excitatory and inhibitory feedback. In particular, we show that when the time delays are sufficiently large a system with local inhibition and global excitation can function as a ‘winner-take-all’ network and exhibit oscillatory dynamics. We demonstrate how the origin of the oscillations can be attributed to the finite delays through a linear stability analysis.

Response properties of visual neurons in the turtle nucleus isthmi.

The optic tectum holds a central position in the tectofugal pathway of non-mammalian species and is reciprocally connected with the nucleus isthmi. Here, we recorded from individual nucleus isthmi pars parvocellularis (Ipc) neurons in the turtle eye-attached whole-brain preparation in response to a range of computer-generated visual stimuli. Ipc neurons responded to a variety of moving or flashing stimuli as long as those stimuli were small. When mapped with a moving spot, the excitatory receptive field was of circular Gaussian shape with an average half-width of less than 3°. We found no evidence for directional sensitivity. For moving spots of varying sizes, the measured Ipc response-size profile was reproduced by the linear Difference-of-Gaussian model, which is consistent with the superposition of a narrow excitatory center and an inhibitory surround. Intracellular Ipc recordings revealed a strong inhibitory connection from the nucleus isthmi pars magnocellularis (Imc), which has the anatomical feature to provide a broad inhibitory projection. The recorded Ipc response properties, together with the modulatory role of the Ipc in tectal visual processing, suggest that the columns of Ipc axon terminals in turtle optic tectum bias tectal visual responses to small dark changing features in visual scenes.

GENERATING OSCILLATORY BURSTS FROM A NETWORK OF REGULAR SPIKING NEURONS WITHOUT INHIBITION

Avian nucleus isthmi pars parvocellularis (Ipc) neurons are reciprocally connected with the layer 10 (L10) neurons in the optic tectum and respond with oscillatory bursts to visual stimulation. Our in vitro experiments show that both neuron types respond with regular spiking to somatic current injection and that the feedforward and feedback synaptic connections are excitatory, but of different strength and time course. To elucidate mechanisms of oscillatory bursting in this network of regularly spiking neurons, we investigated an experimentally constrained model of coupled leaky integrate-and-fire neurons with spike-rate adaptation. The model reproduces the observed Ipc oscillatory bursting in response to simulated visual stimulation. A scan through the model parameter volume reveals that Ipc oscillatory burst generation can be caused by strong and brief feedforward synaptic conductance changes. The mechanism is sensitive to the parameter values of spike-rate adaptation. In conclusion, we show that a network of regular-spiking neurons with feedforward excitation and spike-rate adaptation can generate oscillatory bursting in response to a constant input.

Sparse Spatial Sampling for the Computation of Motion in Multiple Stages

The avian retino-tecto-rotundal pathway plays a central role in motion analysis and features complex connectivity. Yet, the relation between the pathway’s structural arrangement and motion computation has remained elusive. For an important type of tectal wide-field neuron, the stratum griseum centrale type I (SGC-I) neuron, we quantified its structure and found a spatially sparse but extensive sampling of the retinal projection. A computational investigation revealed that these structural properties enhance the neuron’s sensitivity to change, a behaviorally important stimulus attribute, while preserving information about the stimulus location in the SGC-I population activity. Furthermore, the SGC-I neurons project with an interdigitating topography to the nucleus rotundus, where the direction of motion is computed. We showed that, for accurate direction-of-motion estimation, the interdigitating projection of tectal wide-field neurons requires a two-stage rotundal algorithm, where the second rotundal stage estimates the direction of motion from the change in the relative stimulus position represented in the first stage

Motion repulsion arises from stimulus statistics when analyzed with a clustering algorithm

Abstract.Motion repulsion is the perceived enlargement of the angle between the directions of motion of two transparently moving patterns. An explanation of this illusion has long been sought for in the neural circuitry of the brain. We show that motion repulsion already arises from the statistical properties of the motion transparency problem when analyzed with a clustering algorithm.

Pulsed high-power mid-infrared gas lasers

We present a survey in the spectral region of 200-700 cm-1( 14-50 \mu m) of pulsed mirrorless gas lasers pumped by a TEA CO 2 laser. 55 lines are found covering the wavelength range 15.8-57 cm-1in the gases NH 3 ,15NH 3 , D 2 O, CH 3 OH, CD 3 OH, and CD 3 OD. Most of these lines are reported for the first time. One of the strongest lines (26.4 μm in NH 3 ) emits a considerable pulse energy of 100 mJ. No lines were found in C 2 H 5 OH and C 3 H 6 O.