The operation of connexions between photoreceptors and large second-order neurones in dragonfly ocelli

Applicability of White-Noise Techniques to Analyzing Motion Responses

Motion processing in visual neurons is often understood in terms of how they integrate light stimuli in space and time. These integrative properties, known as the spatiotemporal receptive fields (STRFs), are sometimes obtained using white-noise techniques where a continuous random contrast sequence is delivered to each spatial location within the cell's field of view. In contrast, motion stimuli such as moving bars are usually presented intermittently. Here we compare the STRF prediction of a neuron's response to a moving bar with the measured response in second-order interneurons (L-neurons) of dragonfly ocelli (simple eyes). These low-latency neurons transmit sudden changes in intensity and motion information to mediate flight and gaze stabilization reflexes. A white-noise analysis is made of the responses of L-neurons to random bar stimuli delivered either every frame (densely) or intermittently (sparsely) with a temporal sequence matched to the bar motion stimulus. Linear STRFs estimated using the sparse stimulus were significantly better at predicting the responses to moving bars than the STRFs estimated using a traditional dense white-noise stimulus, even when second-order nonlinear terms were added. Our results strongly suggest that visual adaptation significantly modifies the linear STRF properties of L-neurons in dragonfly ocelli during dense white-noise stimulation. We discuss the ability to predict the responses of visual neurons to arbitrary stimuli based on white-noise analysis. We also discuss the likely functional advantages that adaptive receptive field structures provide for stabilizing attitude during hover and forward flight in dragonflies.

Form vision in the insect dorsal ocelli: an anatomical and optical analysis of the dragonfly median ocellus

Previous work has suggested that dragonfly ocelli are specifically adapted to resolve horizontally extended features of the world, such as the horizon. We investigate the optical and anatomical properties of the median ocellus of Hemicordulia tau and Aeshna mixta to determine the extent to which the findings support this conclusion. Dragonfly median ocelli are shown to possess a number of remarkable properties: astigmatism arising from the elliptical shape of the lens is cancelled by the bilobed shape of the inner lens surface, interference microscopy reveals complex gradients of refractive index within the lens, the morphology of the retina results in zones of high acuity, and the eye has an exceedingly high sensitivity for a diurnal terrestrial invertebrate. It is concluded that dragonfly ocelli employ a number of simple, yet elegant, anatomical and optical strategies to ensure high sensitivity, fast transduction speed, wide fields of views and a modicum of spatial resolving power.

The mapping of visual space by dragonfly lateral ocelli

We study the extent to which the lateral ocelli of dragonflies are able to resolve and map spatial information, following the recent finding that the median ocellus is adapted for spatial resolution around the horizon. Physiological optics are investigated by the hanging-drop technique and related to morphology as determined by sectioning and three-dimensional reconstruction. L-neuron morphology and physiology are investigated by intracellular electrophysiology, white noise analysis and iontophoretic dye injection. The lateral ocellar lens consists of a strongly curved outer surface, and two distinct inner surfaces that separate the retina into dorsal and ventral components. The focal plane lies within the dorsal retina but proximal to the ventral retina. Three identified L-neurons innervate the dorsal retina and extend the one-dimensional mapping arrangement of median ocellar L-neurons, with fields of view that are directed at the horizon. One further L-neuron innervates the ventral retina and is adapted for wide-field intensity summation. In both median and lateral ocelli, a distinct subclass of descending L-neuron carries multi-sensory information via graded and regenerative potentials. Dragonfly ocelli are adapted for high sensitivity as well as a modicum of resolution, especially in elevation, suggesting a role for attitude stabilisation by localization of the horizon.

The mapping of visual space by identified large second-order neurons in the dragonfly median ocellus

In adult dragonflies, the compound eyes are augmented by three simple eyes known as the dorsal ocelli. The outputs of ocellar photoreceptors converge on relatively few second-order neurons with large axonal diameters (L-neurons). We determine L-neuron morphology by iontophoretic dye injection combined with three-dimensional reconstructions. Using intracellular recording and white noise analysis, we also determine the physiological receptive fields of the L-neurons, in order to identify the extent to which they preserve spatial information. We find a total of 11 median ocellar L-neurons, consisting of five symmetrical pairs and one unpaired neuron. L-neurons are distinguishable by the extent and location of their terminations within the ocellar plexus and brain. In the horizontal dimension, L-neurons project to different regions of the ocellar plexus, in close correlation with their receptive fields. In the vertical dimension, dendritic arborizations overlap widely, paralleled by receptive fields that are narrow and do not differ between different neurons. These results provide the first evidence for the preservation of spatial information by the second-order neurons of any dorsal ocellus. The system essentially forms a one-dimensional image of the equator over a wide azimuthal area, possibly forming an internal representation of the horizon. Potential behavioural roles for the system are discussed.

A spatiotemporal white noise analysis of photoreceptor responses to UV and green light in the dragonfly median ocellus

Adult dragonflies augment their compound eyes with three simple eyes known as the dorsal ocelli. While the ocellar system is known to mediate stabilizing head reflexes during flight, the ability of the ocellar retina to dynamically resolve the environment is unknown. For the first time, we directly measured the angular sensitivities of the photoreceptors of the dragonfly median (middle) ocellus. We performed a second-order Wiener Kernel analysis of intracellular recordings of light-adapted photoreceptors. These were stimulated with one-dimensional horizontal or vertical patterns of concurrent UV and green light with different contrast levels and at different ambient temperatures. The photoreceptors were found to have anisotropic receptive fields with vertical and horizontal acceptance angles of 15° and 28°, respectively. The first-order (linear) temporal kernels contained significant undershoots whose amplitudes are invariant under changes in the contrast of the stimulus but significantly reduced at higher temperatures. The second-order kernels showed evidence of two distinct nonlinear components: a fast acting self-facilitation, which is dominant in the UV, followed by delayed self- and cross-inhibition of UV and green light responses. No facilitatory interactions between the UV and green light were found, indicating that facilitation of the green and UV responses occurs in isolated compartments. Inhibition between UV and green stimuli was present, indicating that inhibition occurs at a common point in the UV and green response pathways. We present a nonlinear cascade model (NLN) with initial stages consisting of separate UV and green pathways. Each pathway contains a fast facilitating nonlinearity coupled to a linear response. The linear response is described by an extended log-normal model, accounting for the phasic component. The final nonlinearity is composed of self-inhibition in the UV and green pathways and inhibition between these pathways. The model can largely predict the response of the photoreceptors to UV and green light.

Presynaptic depolarization rate controls transmission at an invertebrate synapse

Second-order neurons L1-3 of the locust ocellar pathway make inhibitory synapses with each other. Although the synapses transmit graded potentials, transmission depresses rapidly and completely so that a synapse only transmits when the presynaptic terminal depolarizes rapidly. The rate at which a presynaptic neuron depolarizes determines the rate at which a postsynaptic neuron hyperpolarizes, and neurotransmitter is only released during a fixed 2 ms long period. Consequently, the amplitude of a postsynaptic potential depends on the rate rather than the amplitude of a presynaptic depolarization. Following a postsynaptic potential, a synapse recovers from depression over about a second. The synapse recovers from depression even if the presynaptic terminal is held depolarized.

Signal processing in a simple visual system: the locust ocellar system and its synapses

The neurons with the widest axons that carry information into a locust brain belong to L-neurons, the large, second-order neurons of the ocelli. L-neurons play roles in flight control and boosting visual sensitivity. Their morphology is simple, and their axons convey graded potentials from the ocellus with little decrement to the brain, which makes them good subjects in which to study transmission of graded potentials. L-neurons are very sensitive to changes in light, due to an abnormally high gain in the sign inverting synapses they receive from photoreceptors. Adaptation ensures that L-neurons signal contrast in a light signal when average light intensity changes, and that their responses depend on the speed of change in light. Neurons L1-3 make excitatory output synapses with third-order neurons and with L4-5. These synapses transmit tonically, but are unable to convey hyperpolarising signals about large increases in light. Graded rebound spikes enhance depolarising responses. L1-3 also make reciprocal inhibitory synapses with each other and transmission at these decrements so rapidly that it normally requires a presynaptic spike. The resolution with which graded potentials can be transferred has been studied at the inhibitory synapses, and is limited by intrinsic variability in the mechanism that determines neurotransmitter release. Electron microscopy has shown that each excitatory connection made from an L-neuron to a postsynaptic partner consists of thousands of discrete synaptic contacts, in which individual dense-staining bars in the presynaptic neuron are associated with clouds of vesicles. Acetylcholine is likely to be a neurotransmitter released by L-neurons.

Neural organization of ocellar pathways in the cockroach brain

A large number of photoreceptors of insect ocelli converge onto a smaller number of second-order neurons. Second-order neurons exit the ocellus and project into the ocellar tract neuropil of the brain. Here, the anatomy and physiology of ocellar interneurons of the ocellar tract neuropil of the cockroach are described. The total number and gross morphologies of ocellar tract neurons were examined by extracellular cobalt impregnations into the neuropil. Morphology and physiology of individual neurons were examined using intracellular recording and stainings. Each ocellar tract neuropil contains at least 25 interneurons comprising: 1) four second-order neurons, 2) 15 third-order neurons that receive synapses from second-order neurons at the ocellar tract and project into a number of target neuropil areas of the brain, 3) two possible efferent neurons, 4) three third-order or efferent neurons, and 5) one neuron still to be characterized. The projection areas of ocellar third-order neurons include 1) visual, olfactory, and mechanosensory centers; 2) the mushroom body (a higher associative center); 3) the posterior slope, a premotor center from which descending brain neurons originate; and 4) the thoracic motor systems. By comparing the present results to those reported from other insects, I conclude that the cockroach ocellar system has two distinctive features. 1) The ratio of convergence at synapses between photoreceptors and second-order neurons is higher than those reported for other insects so far studied. 2) Ocellar signals are transmitted to various target neuropils by third-order neurons, whereas ocellar systems of all other insects possess pathways in which ocellar signals are transmitted to target neuropils by second-order neurons. The functional significance of these features of the cockroach ocellar system is discussed.

S-neurons and not L-neurons are the source of GABAergic action in the ocellar retina

Electrophysiological evidence obtained with current- and voltage clamp experiments from single L-neurons of the ocellar nerve of locust (Locusta migratoria) questions a direct synaptic feedback from these neurons onto the photoreceptors. The synaptic currents recorded under voltage clamp reflected the photoresponse of the L-neuron, despite the fact it developed no synaptic activity under these conditions. This result is contrary to GABAergic feedback models proposed in the literature. Electrophysiological recordings, as well as immunocytochemistry revealing GABA and glutamate decarboxylase, indicated a possible contribution of S-neurons in such a feedback system. A population of probable S-neurons whose somas were in the pars intercerebralis adjacent to the ocellar nerve tracts was heavely labelled. About 10 fibres entered each tract and formed a dense network of fine arborizations within the ocellar plexiform layer. L-neurons showed no GABA-immunoreactivity. Based on these data a new model for GABAergic feedback is proposed and discussed.