Spike Burst Coding of Translatory Optic Flow and Depth from Motion in the Fly Visual System

The fly visual system, although tiny when compared to the mammalian visual system, can still perform highly sophisticated spatial tasks like collision avoidance, landing on objects, pursuit of prey, etc. Flies outperform human-made autonomous flying systems in solving such spatial tasks by a long way. This is partly due to their ability to perceive and respond to optical flow generated by motion in the environment. They are also known to take actions that alter the image flow on their eyes. Higher level neurons in the fly visual system respond to different types of complex optical flows due to rotation and translation, by pooling information from local motion detectors called the elementary motion detectors (EMDs) in the lower level. In this sense, neuronal responses (spikes) from these optical flow detectors in the fly carry highly encoded signals: a single spike can represent a complex dynamical pattern of movement in the visual field. In this paper, we investigate how such highly encoded signals can be interpreted and utilized in the fly's brain, while solely operating on the encoded internal spike patterns within their brain and no direct external (unencoded) sensory information, i.e. a form of grounding. With a computational model of the optical flow detectors based on those in the fly, we show that specific pattern of action (or coordinated motor output) is the only way that a fly can decode its internal spikes and generate meaningful, relevant behavior based on that.

Objective To analyze myoelectric activity of the cecum and proximal loop of the ascending colon (PLAC) in cows after spontaneous cecal dilatation/dislocation (CDD) and compare it with that in healthy cows after surgical evacuation of the cecum. Animals 12 cows with spontaneous CDD and 6 healthy cows (group C). Cows with spontaneous CDD were retrospectively assigned to 2 groups: delayed recovery from surgery or recurrence (group A; n = 3), and normal recovery (group B; n = 9). Procedure After surgical evacuation of the cecum, 8 pairs of bipolar, retrievable electrodes were implanted in the ileum, cecum, and PLAC. Cows were evaluated daily from postoperative day 1 to 7, using routine clinical methods and computer-based analysis of myoelectric activity of the cecum and PLAC. Parameters of myoelectric activity included rate of spike bursts, duration of individual spike bursts, duration of overall spike burst activity per electrode, rate of propagated spike burst sequences, and ratio of orally propagated spike burst sequences. Results Rate of spike bursts, duration of cecocolic spike burst activity, and ratio of orally to aborally propagated spike burst sequences did not vary among groups during the 7-day recording period. However, cows with delayed recovery had a typical, uniform pattern of myoelectric activity of the cecum and PLAC at days 1 and 2 after surgery that consisted of repeated, propagated spike burst sequences, made up of spike bursts of significantly (P < 0.05) increased duration at postsurgical day 1 and substantially prolonged duration at postsurgical day 2, interrupted by periods of little or no activity. Conclusion Delayed recovery and recurrence of CDD in cows after spontaneous CDD is not caused by hypomotility of the cecum and PLAC. Clinical Relevance Postoperative treatment, intended to reduce recurrence of CDD or delayed recovery after surgical evacuation of the cecum, should address propagation of digesta in the spiral colon. (Am J Vet Res 1997;58:961–968)

Brief optogenetic stimulation of thalamocortical axons evoked a 400 Hz wavelet, 'ripplet,' in the extracellular field potential in cortical layer 4, together with phase-locked spike bursts in fast-spiking inhibitory interneurons and alternating excitatory and inhibitory synaptic currents in excitatory cells.

There are two types of horizontal cells, the luminosity and the chromaticity cells, in the retina of the kissing gourami, Helostoma rudolfi. Luminosity cells occupy the outermost layer proximal to the receptor terminals, whereas chromaticity cells form a layer proximal to the layer of luminosity cells. Neither type of cell has axons. Responses were evoked by light from red and green light-emitting diodes. The two stimuli were modulated either by a pulsatile or a white-noise signal. The luminosity cell always produced a hyperpolarizing response. The chromaticity cell produced a hyperpolarizing response when stimulated by only one color. However, in the presence of a steady or modulated green input, a red stimulus produced a depolarizing response. Such chromaticity cells were similar to the (spectral) biphasic chromaticity horizontal cells observed in other retinae. The depolarizing phase of the red response was produced by the balance of intensity of the two inputs, red and green. We used white-noise methodology to identify the dynamics of the horizontal cell's modulation response by taking advantage of the fact that a Wiener kernel is a measure of a cell's incremental sensitivity, which includes its response dynamics. Under all conditions, a steady state modulation response by both luminosity and chromaticity cells always was related linearly to the input modulation. The average mean square error (MSE) of the model predicted by the first-order kernel was approximately 8% for both luminosity (n = 116) and chromaticity (n = 23) cells. In some cases, the MSE was a few percent even when the peak-to-peak response amplitude was nearly 30 mV. The ratio of inputs from red and green cones to both types of horizontal cells was variable; the major input for luminosity cells came from red cones, whereas the major input for chromaticity cells came from green cones. First-order kernels generated by the major input were robust in terms of waveform in the sense that the waveform remained unchanged whether or not there was a steady or modulated illumination by the opposing color. The results reported here do not address the question of the neural circuitry that generates horizontal cell responses, in particular, the depolarizing response. However, whatever that circuitry might be, the high degree of linearity of the modulation response by both types of cell under various stimulus conditions imposes restrictions on the performance of any proposed model as well as on mechanisms that underlie the generation of the horizontal cell response.

We studied the effect of motilin on myoelectric activity of the sphincter of Oddi (SO) and upper gastrointestinal tract in conscious opposums. In 17 animals, bipolar electrodes were implanted on the gastric antrum, SO, duodenum, and jejunum. Subsequent 8-h recordings reconfirmed our previous findings that SO spike burst rate changed with interdigestive cycles of the gastrointestinal migrating myoelectric complex (MMC), becoming maximal during passage of phase III activity through the duodenum. In eight animals, peak motilin levels were shown to occur concurrently with maximal SO spike burst rate and MMC phase III activity in the duodenum. Motilin infusion (0.3 and 0.9 micrograms X kg-1 X h-1), given for 30-60 min starting 10 min after duodenal phase III, elicited premature MMC activity that originated in the stomach. Maximal SO activity occurred coincident with passage of premature phase III activity through the duodenum. Pulse intravenous doses of motilin (25-1,600 ng/kg) generally caused an immediate increase in spike burst activity in the gastric antrum, duodenum, and SO that lasted 3-5 min and was often followed by a premature MMC, usually starting in the antrum and progressing through the duodenum and jejunum. Increases in SO spike burst rate also occurred concurrent with motilin-induced, premature duodenal phase III. Motilin given at 5-60% of the duodenal MMC cycle length elicited premature MMCs at 10-60% of the cycle, but no premature MMCs were elicited by any of the motilin doses at the 5% intervals.(ABSTRACT TRUNCATED AT 250 WORDS)

Spatial Distribution of Potential in a Flat Cell: Application to the Catfish Horizontal Cell Layers

1. Horizontal cells of the cat retina were isolated by enzymatic dissociation. Two types of horizontal cells were identified: the axonless (A-type) horizontal cell having four to six thick, long (approximately 100 microns) dendrites, and the short-axon (B-type) horizontal cell having many (> 5) fine, short (approximately 30 microns) dendrites. 2. Membrane properties of isolated horizontal cells were analyzed under current-clamp and voltage-clamp conditions. In the A-type cell, the average resting potential was -55 mV and the mean membrane capacitance was 110 pF, whereas values in the B-type cell were -58 mV and 40 pF, respectively. The A-type cell showed long-lasting Ca spikes, but B-type cells had no Ca spikes. 3. Five types of voltage-dependent ionic currents were recorded: a sodium current (INa), a calcium current (ICa), and three types of potassium currents. Potassium currents consisted of potassium current through the inward rectifier (Ianomal), transient outward potassium current (IA), and potassium current through the delayed rectifier (IK(v)). INa was recorded only from A-type cells. Other currents were recorded from both types of cells. 4. INa activated when cells were depolarized from a holding potential (Vh) of -95 mV, and it was maximal at -25 mV. This current was blocked by tetrodotoxin. Approximately half of the A-type cells had INa, but no B-type cell had this current. 5. L-type ICa, an inward-going sustained current, was activated with depolarization more positive than -25 mV. Current amplitude reached a maximal value near 15 mV and became smaller with further depolarization.(ABSTRACT TRUNCATED AT 250 WORDS)

Intrinsic response properties of bursting neurons in the nucleus principalis trigemini of the gerbil

Last year, we reported a new disparity effect on optic-flow perception. It occurs when a flow field of radially moving random dots is transparently superimposed by a unidirectional motion pattern. Subjects asked to locate the centre of the expansion pattern perceive it shifted away from its true position (Duffy and Wurtz, 1993 Vision Research33 1481 – 1490). The magnitude of this shift depends on binocular disparity between the two superimposed patterns: presenting the translation in front of the expansion causes a massive reduction of the shift, presenting it behind the expansion yields only a slight reduction. To determine possible mechanisms for this interaction, we tried to model our results by modifying a previously developed neural model for optic-flow processing (Lappe and Rauschecker, 1995 Vision Research35 1619 – 1631). We compared three possible mechanisms of disparity influence. The first was based on the assumption that the absolute distance of the stimuli from the observer was known. The second was based on a disparity-based spatial integration, as found in ‘tuned’ neurons in primate visual area MT. Neither could account for our data. Finally we included a disparity-dependent weighting function in the model, assuming that distant flow vectors contribute more strongly than near ones to the processing of optic flow. This reproduced the experimental findings. Surprisingly, the optimal weighting function was similar to the disparity-dependence of the so-called ‘far-neurons’ in area MT. We conclude that the visual system uses disparity in optic-flow processing by putting special emphasis on distant flow vectors, and that far-neurons in MT might serve as a neuronal substrate for this.

The kinetic features of the changes in the cytosolic free Ca2+ concentration, [Ca2+]i, following stimulation by thyrotropin releasing hormone (TRH) were analysed in single cells of a pituitary line (GH3B6) by dual excitation microfluorimetry [Tsien, Rink & Poenie (1985) Cell Calcium 6, 145-157], using fura 2 as intracellular Ca2+ probe. Two phases were observed: initially, [Ca2+]i is raised in a single rapid transient to a maximum averaging 8.0 microM, and in a second phase TRH causes a series of rapid [Ca2+]i oscillations with maxima around 1.0 microM, which are probably due to the enhanced firing of action potentials. TRH triggers both phases independently, i.e. it can elicit either the first or the second phase exclusively. This is also the case in those cells in which [Ca2+]i undergoes rhythmic oscillations due to the firing of spontaneous action potentials [Schlegel, Winiger, Mollard, Vacher, Wuarin, Zahnd, Wolheim & Dufy (1987) Nature (London) 329, 719-721]. The sudden onset of the first phase of TRH action on [Ca2+]i shows that Ca2+ mobilization due to enhanced production of inositol phosphate may occur as rapidly as the firing of action potentials, i.e. in the ms time range. Due to a marked response type heterogeneity and to the randomness of the rapid events, previous monitoring of [Ca2+]i in cell populations had misleadingly suggested small and maintained changes due to TRH. It is concluded that stimulatory regulation of secretion is provided by the generation of rapid [Ca2+]i transients, the frequency of which determines secretory rate. Furthermore, it is demonstrated that the regulation of [Ca2+]i by hormones and neurotransmitters in pituitary and many other cell types will have to be studied at the single cell level in order to appreciate its role in cell activation.

Spike timing dependent plasticity (STDP) is a synaptic learning rule where the relative timing between the presynaptic and postsynaptic action potentials determines the sign and strength of synaptic plasticity. In its basic form STDP has an asymmetric form which incorporates both persistent increases and persistent decreases in synaptic strength. The basic form of STDP, however, is not a fixed property and depends on the dendritic location. An asymmetric curve is observed in the distal dendrites, whereas a symmetrical one is observed in the proximal ones. A recent computational study has shown that the transition from the asymmetry to symmetry is due to inhibition under certain conditions. Synapses have also been observed to be unreliable at generating plasticity when excitatory postsynaptic potentials and single spikes are paired at low frequencies. Bursts of spikes, however, are reliably signaled because transmitter release is facilitated. This article presents a two-compartment model of the CA1 pyramidal cell. The model is neurophysiologically plausible with its dynamics resulting from the interplay of many ionic and synaptic currents. Plasticity is measured by a deterministic Ca(2+) dynamics model which measures the instantaneous calcium level and its time course in the dendrite and change the strength of the synapse accordingly. The model is validated to match the asymmetrical form of STDP from the pairing of a presynaptic (dendritic) and postsynaptic (somatic) spikes as observed experimentally. With the parameter set unchanged the model investigates how pairing of bursts with single spikes and bursts in the presence or absence of inhibition shapes the STDP curve. The model predicts that inhibition strength and frequency are not the only factors of the asymmetry-to-symmetry switch of the STDP curve. Burst interspike interval is another factor. This study is an important first step towards understanding how STDP is affected under natural firing patterns in vivo.

The functional and morphological connectivity between various horizontal cell (HC) types (H1, H2, H3, and H4) and photoreceptors was studied in zebrafish retina. Since HCs are strongly coupled by gap junctions and feedback from HCs to photoreceptors depends strongly on connexin (Cx) hemichannels, we characterized the various HC Cxs (Cx52.6, Cx52.7, Cx52.9, and Cx55.5) in Xenopus oocytes. All Cxs formed hemichannels that were conducting at physiological membrane potentials. The Cx hemichannels differed in kinetic properties and voltage dependence, allowing for specific tuning of the coupling of HCs and the feedback signal from HCs to cones. The morphological connectivity between HC layers and cones was determined next. We used zebrafish expressing green fluorescent protein under the control of Cx promoters. We found that all HCs showed Cx55.5 promoter activity. Cx52.7 promoter activity was exclusively present in H4 cells, while Cx52.9 promoter activity occurred only in H1 cells. Cx52.6 promoter activity was present in H4 cells and in the ventral quadrant of the retina also in H1 cells. Finally, we determined the spectral sensitivities of the HC layers. Three response types were found. Monophasic responses were generated by HCs that contacted all cones (H1 cells), biphasic responses were generated by HCs that contacted M, S, and UV cones (H2 cells), and triphasic responses were generated by HCs that contacted either S and UV cones (H3 cells) or rods and UV cones (H4 cells). Electron microscopy confirms that H4 cells innervate cones. This indicates that rod-driven HCs process spectral information during photopic and luminance information during scotopic conditions.

The effects of sennosides on colonic myoelectrical activity were investigated in man. Spiking activity of the left and sigmoid colons was continuously recorded in seven constipated patients during two sessions from 5:00 PM to 9:00 AM. Each patient received orally at 7:00 PM on two consecutive days 30 mg of sennosides one day (sequence S) and placebo on the other (sequence P) in a random fashion. A significant (P less than 0.05) increase of peristaltic activity (migrating long spike bursts or MLSBs) after sennosides was observed between 1:00 and 7:00 AM, and a corresponding decrease between 7:00 and 9:00 AM. No change was noted in total short spike burst (SSB) activity or in SSBs characteristic of the rectosigmoid area. This study indicates that the main modification of colonic motility induced by sennosides was a stimulation of propulsive activity (MLSBs), which occurred between 6-12 hr after oral administration, the delay required for orocecal transit and metabolism of the drug.

A subset of pyramidal neurons in layer 5 of the mammalian neocortex can fire action potentials in brief, high-frequency bursts while others fire spikes at regularly spaced intervals. Here we show that individual layer 5 pyramidal neurons in acute slices from mouse primary motor cortex can adopt both regular and burst spiking patterns. During constant current injection at the soma, neurons displayed a regular firing pattern at 36–37°C, but switched to burst spiking patterns upon cooling the slice to 24–26°C. This change in firing pattern was reversible and repeatable and was independent of the somatic resting membrane potential. Hence these spiking patterns are not inherent to discrete populations of pyramidal neurons and are more interchangeable than previously thought. Burst spiking in these neurons is the result of electrical interactions between the soma and distal apical dendritic tree. Presumably the interactions between soma and distal dendrite are temperature-sensitive, suggesting that the manner in which layer 5 pyramidal neurons translate synaptic input into an output spiking pattern is fundamentally altered at sub-physiological temperatures.

Spiking Irregularity and Frequency Modulate the Behavioral Report of Single-Neuron Stimulation