Biomedical subjects

F C Rind

Publications and source records attributed to F C Rind.

10 recordsLinked to original sources

PubMed2000

Immunocytochemical evidence that collision sensing neurons in the locust visual system contain acetylcholine.

The lobula giant movement detector (LGMD1 and -2) neurons in the locust visual system are parts of motion-sensitive pathways that detect objects approaching on a collision course. The dendritic processes of the LGMD1 and -2 in the lobula are localised to discrete regions, allowing the dendrites of each neuron to be distinguished uniquely. As was described previously for the LGMD1, the afferent processes onto the LGMD2 synapse directly with each other, and these synapses are immediately adjacent to their outputs onto the LGMD2. Here we present immunocytochemical evidence, using antibodies against choline-protein conjugates and a polyclonal antiserum against choline acetyltransferase (ChAT; Chemicon Ab 143), that the LGMD1 and -2 and the retinotopic units presynaptic to them contain acetylcholine (ACh). It is proposed that these retinotopic units excite the LGMD1 or -2 but inhibit each other. It is well established that ACh has both excitatory and inhibitory effects and may provide the substrate for a critical race in the LGMD1 or -2, between excitation caused by edges moving out over successive photoreceptors, and inhibition spreading laterally resulting in the selective response to objects approaching on a collision course. In the optic lobe, ACh was also found to be localised in discrete layers of the medulla and in the outer chiasm between the lamina and medulla. In the brain, the antennal lobes contained neurons that reacted positively for ACh. Silver- or haematoxylin and eosin-stained sections through the optic lobe confirmed the identities of the positively immunostained neurons.

Acetylcholine↗

PubMed1999

Seeing what is coming: building collision-sensitive neurones.

The image of a rapidly approaching object has to elicit a quick response. An animal needs to know that the object is approaching on a collision course and how imminent a collision is. The relevant information can be computed from the way that the image of the object grows on the retina of one eye. Firm data about the types of neurones that react to such looming stimuli and trigger avoidance reactions come from recent studies on the pigeon and the locust. The neurones responsible are tightly tuned to detect objects that are approaching on a direct collision course. In the pigeon these neurones signal the time remaining before collision whereas in the locust they have a crucial role in the simple strategy this animal uses to detect an object approaching on a collision course.

Animals↗

PubMed1999

Using a mobile robot to study locust collision avoidance responses.

The visual systems of insects perform complex processing using remarkably compact neural circuits, yet these circuits are often studied using simplified stimuli which fail to reveal their behaviour in more complex visual environments. We address this issue by testing models of these circuits in real-world visual environments using a mobile robot. In this paper we focus on the lobula giant movement detector (LGMD) system of the locust which responds selectively to objects which approach the animal on a collision course and is thought to trigger escape behaviours. We show that a neural network model of the LGMD system shares the preference for approaching objects and detects obstacles over a range of speeds. Our results highlight aspects of the basic response properties of the biological system which have important implications for the behavioural role of the LGMD.

Animals↗

PubMed1998

Local circuit for the computation of object approach by an identified visual neuron in the locust.

The lobula giant movement detector (LGMD) neuron in the locust visual system is part of a motion-sensitive pathway that detects objects approaching on a collision course. Here we show that the retinotopic units presynaptic to the LGMD make synapses directly with each other and these synapses are immediately adjacent to their outputs onto the LGMD. Synapses occur along the fine dendrites of the LGMD in the distal lobula, often in large numbers and completely covering the LGMD processes. Gamma aminobutyric acid (GABA) was eliminated as a possible neurotransmitter at these synapses when immunogold-tagged monoclonal GABA antibody did not specifically label the afferent processes. We used a histochemical method to demonstrate that acetylcholine esterase, the enzyme that hydrolyses acetylcholine at cholinergic synapses, was present in the synaptic clefts between the retinotopic units and along the membrane of the LGMD. It is well established that acetylcholine has both excitatory and inhibitory effects and we propose that these retinotopic units excite the LGMD, but inhibit each other; and that the synapses form the substrate for a critical race between excitation caused by edges moving out over successive photoreceptors, and inhibition spreading laterally. This results in the selective response to objects approaching on a collision course.

Acetylcholine↗

PubMed1997

Signaling of object approach by the DCMD neuron of the locust.

The locust descending contralateral motion detector (DCMD) responds to movements anywhere within a wide visual field, but responds most strongly to the images of approaching objects. It has been claimed that the response peaks before the end of an approach movement, providing a signal that anticipates collision. However, we find that when the locust eye is presented with appropriate computer-generated images of approaching objects, the response builds up until after movement has stopped. Premature peaking in the response is due to failure to stimulate the eye with sufficiently small and frequent jumps in image edges. We conclude that the DCMD signals impending collision by tracking edge motion throughout object approach.

Animals↗

PubMed1996

Neural network based on the input organization of an identified neuron signaling impending collision.

1. We describe a four-layered neural network (Fig. 1), based on the input organization of a collision signaling neuron in the visual system of the locust, the lobula giant movement detector (LGMD). The 250 photoreceptors ("P" units) in layer 1 are excited by any change in illumination, generated when an image edge passes over them. Layers 2 and 3 incorporate both excitatory and inhibitory interactions, and layer 4 consists of a single output element, equivalent to the locust LGMD. 2. The output element of the neural network, the "LGMD", responds directionally when challenged with approaching versus receding objects, preferring approaching objects (Figs. 2-4). The time course and shape of the "LGMD" response matches that of the LGMD (Fig. 4). Directionality is maintained with objects of various sizes and approach velocities. The network is tuned to direct approach (Fig. 5). The "LGMD" shows no directional selectivity for translatory motion at a constant velocity across the "eye", but its response increases with edge velocity (Figs. 6 and 9). 3. The critical image cues for a selective response to object approach by the "LGMD" are edges that change in extent or in velocity as they move (Fig. 7). Lateral inhibition is crucial to the selectivity of the "LGMD" and the selective response is abolished or else much reduced if lateral inhibition is taken out of the network (Fig. 7). We conclude that lateral inhibition in the neuronal network for the locust LGMD also underlies the experimentally observed critical image cues for its directional response. 4. Lateral inhibition shapes the velocity tuning of the network for objects moving in the X and Y directions without approaching the eye (see Fig. 1). As an edge moves over the eye at a constant velocity, a race occurs between the excitation that is caused by edge movement and which passes down the network and the inhibition that passes laterally. Excitation must win this race for units in layer 3 to reach threshold (Fig. 8). The faster the edge moves over the eye the more units in layer 3 reach threshold and pass excitation on to the "LGMD" (Fig. 9). 5. Lateral inhibition shapes the tuning of the network for objects moving in the Z direction, toward or away from the eye (see Fig. 1). As an object approaches the eye there is a buildup of excitation in the "LGMD" throughout the movement whereas the response to object recession is often brief, particularly for high velocities. During object motion, a critical race occurs between excitation passing down the network and inhibition directed laterally, excitation must win this race for the rapid buildup in excitation in the "LGMD" as seen in the final stages of object approach (Figs. 10-12). The buildup is eliminated if, during object approach, excitation cannot win this race (as happens when the spread of inhibition laterally takes < 1 ms Fig. 13, D and E). Taking all lateral inhibition away increases the "LGMD" response to object approach, but overall directional selectivity is reduced as there is also a lot of residual network excitation following object recession (Fig. 13B). 6. Directional selectivity for rapidly approaching objects is further enhanced at the level of the "LGMD" by the timing of a feed-forward, inhibitory loop onto the "LGMD", activated when a large number of receptor units are excited in a short time. The inhibitory loop is activated at the end of object approach, truncating the excitatory "LGMD" response after approach has ceased, but at the initiation of object recession (*Fig. 2, 3, and 13). Eliminating the feed-forward, inhibitory loop prolongs the "LGMD" response to both receding and approaching objects (Fig. 13F).

Animals↗

PubMed1996

Intracellular characterization of neurons in the locust brain signaling impending collision.

1. In response to a rapidly approaching object, intracellular recordings show that excitation in the locust lobula giant movement detecting (LGMD) neuron builds up exponentially, particularly during the final stages of object approach. After the cessation of object motion, inhibitory potentials in the LGMD then help to terminate this excitation. Excitation in the LGMD follows object recession with a short, constant latency but is cut back rapidly by hyperpolarizing potentials. The timing of these hyperpolarizing potentials in the LGMD is variable, and their latency following object recession is shortest with the highest velocities of motion simulated. The hyperpolarizing potentials last from 50-300 ms and are often followed by re-excitation. The observed hyperpolarizations of the LGMD can occur without any preceding excitation and are accompanied by a measurable conductance increase. The hyperpolarizations are likely to be inhibitory postsynaptic potentials (PSPs). The behavior of the intracellularly recorded inhibitory PSPs (IPSPs) closely parallels that of the feed forward inhibitory loop in the neural network described by Rind and Bramwell. 2. The preference of the LGMD for approaching versus receding objects remains over a wide range of starting and finishing distances. The response to object approach, measured both as membrane potential and spike rate, remains single peaked with starting distances of between 200 and 2,100 mm, and approach speeds of 0.5-2 m/s. These results confirm the behavior predicted by the neural network described by Rind and Bramwell but contradicts the findings of Rind and Simmons, forcing a re-evaluation of the suitability of some of the mechanical visual stimuli used in that study. 3. For depolarization of the LGMD neuron to be maintained or increased throughout the motion of image edges, the edges must move with increasing velocity over the eye. Membrane potential declines before the end of edge motion with constant velocities of edge motion. 4. A second identified neuron, the LGMD2 also is shown to respond directionally to approaching objects. In both the LGMD and LGMD2 neurons, postsynaptic inhibition shapes the directional response to object motion.

Animals↗

PubMed1992

Orthopteran DCMD neuron: a reevaluation of responses to moving objects. I. Selective responses to approaching objects.

1. The "descending contralateral movement detector" (DCMD) neuron in the locust has been challenged with a variety of moving stimuli, including scenes from a film (Star Wars), moving disks, and images generated by computer. The neuron responds well to any rapid movement. For a dark object moving along a straight path at a uniform velocity, the DCMD gives the strongest response when the object travels directly toward the eye, and the weakest when the object travels away from the eye. Instead of expressing selectivity for movements of small rather than large objects, the DCMD responds preferentially to approaching objects. 2. The neuron shows a clear selectivity for approach over recession for a variety of sizes and velocities of movement both of real objects and in simulated movements. When a disk that subtends > or = 5 degrees at the eye approaches the eye, there are two peaks in spike rate: one immediately after the start of movement; and a second that builds up during the approach. When a disk recedes from the eye, there is a single peak in response as the movement starts. There is a good correlation between spike rate and angular acceleration of the edges of the image over the eye. 3. When an object approaches from a distance sufficient for it to subtend less than one interommatidial angle at the start of its approach, there is a single peak in response. The DCMD tracks the approach, and, if the object moves at 1 m/s or faster, the spike rate increases throughout the duration of object movement. The size of the response depends on the speed of approach. 4. It is unlikely that the DCMD encodes the time to collision accurately, because the response depends on the size as well as the velocity of an approaching object. 5. Wide-field movements suppress the response to an approaching object. The suppression varies with the temporal frequency of the background pattern. 6. Over a wide range of contrasts of object against background, the DCMD gives a stronger response to approaching than to receding objects. For low contrasts, the selectivity is greater for objects that are darker than the background than for objects that are lighter.

Animals↗

PubMed1992

Orthopteran DCMD neuron: a reevaluation of responses to moving objects. II. Critical cues for detecting approaching objects.

1. We examine the critical image cues that are used by the locust visual system for the descending contralateral motion detector (DCMD) neuron to distinguish approaching from receding objects. Images were controlled by computer and presented on an electrostatic monitor. 2. Changes in overall luminance elicited much smaller and briefer responses from the DCMD than objects that appeared to approach the eye. Although a decrease in overall luminance might boost the response to an approaching dark object, movement of edges of the image is more important. 3. When two pairs of lines, in a cross-hairs configuration, were moved apart and then together again, the DCMD showed no preference for divergence compared with convergence of edges. A directional response was obtained by either making the lines increase in extent during divergence and decrease in extent during convergence; or by continually increasing the velocity of line movement during divergence and decreasing velocity during convergence. 4. The DCMD consistently gave a larger response to growing than to shrinking solid rectangular images. An increase compared with a decrease in the extent of edge in an image is, therefore, an important cue for the directionality of the response. For single moving edges of fixed extent, the neuron gave the largest response to edges that subtended 15 degrees at the eye. 5. The DCMD was very sensitive to the amount by which an edge traveled between frames on the display screen, with the largest responses generated by 2.5 degrees of travel. This implies that the neurons in the optic lobe that drive this movement-detecting system have receptive fields of about the same extent as a single ommatidium. 6. For edges moving up to 250 degree/s, the excitation of the DCMD increases with velocity. The response to an edge moving at a constant velocity adapts rapidly, in a manner that depends on velocity. Movement over one part of the retina can adapt the subsequent response to movement over another part of the retina. 7. For the DCMD to track and continue to respond to the image of an approaching object, the edges of the image must continually increase in velocity. This is the second important stimulus cue. 8. Edges of opposite contrasts (light-dark compared with dark-light) are processed in separate pathways that inhibit each other. This would contribute to the reduction of responses to wide-field movements.

Adaptation, Physiological↗

PubMed1984

A chemical synapse between two motion detecting neurones in the locust brain.

The LGMD is the major source of visual input from the compound eye to the ipsilateral DCMD. Inactivating the LGMD or hyperpolarizing it, so it no longer spikes, abolishes the response of the DCMD to the visual stimulus. Synaptic transmission between the LGMD and DCMD neurones is chemical. A spike in the LGMD terminals induces a postsynaptic potential in the DCMD dendrites, with a transmission delay of 1 ms. There is a conductance increase in the DCMD during an LGMD-mediated PSP. The conductance increase occurs at membrane potentials when the current/voltage relationship of the DCMD membrane is linear, and at several different membrane potentials. The LGMD-mediated PSP within the dendritic region of the DCMD has a rise time of 1.3 ms, a half-time for decay of 2.2 ms and a total duration of 8.3 ms. In the cell body it has a rise time of 3.3 ms, a half-time for decay of 8 ms and a total duration of 21.3 ms. The amplitude of the LGMD-mediated PSP depends on the membrane potential of the DCMD. The PSP amplitude is increased by membrane hyperpolarization and decreased by membrane depolarizations. At a membrane potential 30 mV more positive than resting potential the extrapolated size of the PSP is zero. The synaptic efficiency of the LGMD-DCMD connection is usually 1.2. (formula; see text) There is a threshold of 13 mV in the LGMD before synaptic transmission occurs. Currents less than 13 mV are not transmitted in either direction across the synapse although they do reach the synaptic region if they are injected at the extremities of the neurones within the brain. Length constants for the LGMD are 0.36 mm between points c and d in the protocerebrum and 0.63 mm between point b in the optic lobe and point d in the protocerebrum. The length constant measured between the dendrite region of the DCMD and its cell body is 1.34 mm. DCMD spikes and PSPs follow spikes in the LGMD at a constant latency at frequencies up to 400 Hz. Usually a spike in the LGMD induces a spike in the DCMD.

Action Potentials↗