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.     

Responses of a looming-sensitive neuron to compound and paired object approaches

The lobula giant movement detector (LGMD) and its target neuron, the descending contralateral movement detector (DCMD), constitute a motion-sensitive pathway in the locust visual system that responds preferentially to objects approaching on a collision course. LGMD receptive field properties, anisotropic distribution of local retinotopic inputs across the visual field, and localized habituation to repeated stimuli suggest that this pathway should be sensitive to approaches of individual objects within a complex visual scene. We presented locusts with compound looming objects while recording from the DCMD to test the effects of nonuniform edge expansion on looming responses. We also presented paired objects approaching from different regions of the visual field at nonoverlapping, closely timed and simultaneous approach intervals to study DCMD responses to multiple looming stimuli. We found that looming compound objects evoked characteristic responses in the DCMD and that the time of peak firing was consistent with predicted values based on a weighted ratio of the half size of each distinct object edge and the absolute approach velocity. We also found that the azimuthal position and interval of paired approaches affected DCMD firing properties and that DCMDs responded to individual objects approaching within 106 ms of each other. Moreover, comparisons between individual and paired approaches revealed that overlapping approaches are processed in a strongly sublinear manner. These findings are consistent with biophysical mechanisms that produce nonlinear integration of excitatory and feed-forward inhibitory inputs onto the LGMD that have been shown to underlie responses to looming stimuli.     

Responses of single units in the monkey superior colliculus to moving stimuli

1. Single unit responses of pan-directional cells to moving and stationary flashing stimuli were studied in the superficial layers of the superior colliculus in paralysed, anaesthetized rhesus monkeys. The aim of this study was to see how far cell responses to moving stimuli fit in with what would be expected from their responses to stationary flashing stimuli. 2. Both the leading and the trailing edge of a moving stimulus evoke a transient response. If the diameter of moving light spots is increased the strength of the leading edge response increases, reaches a maximum and decreases to a constant value which is similar to the behaviour of the on response when the diameter of flashing spots is increased. The strength of the trailing edge response increases and reaches the same strength as that of the leading edge response. If the width of a long moving slit is increased, the strength of the leading edge response is the same at all slit widths, while the strength of the trailing edge response shows a course similar to that of the trailing edge response if the spot diameter is increased. If the length of a wide moving slit is increased both the leading and the trailing edge responses decrease. These results indicate that the strength of both leading and trailing edge responses is dependent on the degree the inhibitory surround is activated. 3. The leading and the trailing edge of a stimulus evoke their responses at the same position in the receptive field independent of the direction of movement. 4. Increasing the velocity of a moving stimulus shows that in general the leading edge response is present up to higher velocities than the trailing edge response independent of the sign of contrast. The burst duration to moving stimuli decreases with increasing stimulus velocity and appears to be determined by the time a moving edge is present in the receptive field centre. When this time becomes shorter than 10--20 ms, the burst duration for moving stimuli is constant and about the same as for flashing stimuli. This indicates that, although spatial receptive field properties can vary considerably, temporal receptive field properties show a strong similarity among different units. 5. The response latencies to light and dark moving edges are the same, which in turn are about equal to the response latencies to stationary flashing stimuli. 6. Stimulation experiments show that the general response characteristics to moving stimuli can be predicted by using a set of receptive field parameters derived from responses to stationary flashing stimuli. The most important variable of moving stimuli appears to be the period of time a moving contour is present within the receptive field centre, besides the degree of activation of the inhibitory surround.     

Arousal facilitates collision avoidance mediated by a looming sensitive visual neuron in a flying locust

Locusts have two large collision-detecting neurons, the descending contralateral movement detectors (DCMDs) that signal object approach and trigger evasive glides during flight. We sought to investigate whether vision for action, when the locust is in an aroused state rather than a passive viewer, significantly alters visual processing in this collision-detecting pathway. To do this we used two different approaches to determine how the arousal state of a locust affects the prolonged periods of high-frequency spikes typical of the DCMD response to approaching objects that trigger evasive glides. First, we manipulated arousal state in the locust by applying a brief mechanical stimulation to the hind leg; this type of change of state occurs when gregarious locusts accumulate in high-density swarms. Second, we examined DCMD responses during flight because flight produces a heightened physiological state of arousal in locusts. When arousal was induced by either method we found that the DCMD response recovered from a previously habituated state; that it followed object motion throughout approach; and--most important--that it was significantly more likely to generate the maintained spike frequencies capable of evoking gliding dives even with extremely short intervals (1.8 s) between approaches. Overall, tethered flying locusts responded to 41% of simulated approaching objects (sets of 6 with 1.8 s ISI). When we injected epinastine, the neuronal octopamine receptor antagonist, into the hemolymph responsiveness declined to 12%, suggesting that octopamine plays a significant role in maintaining responsiveness of the DCMD and the locust to visual stimuli during flight.     

Functional inhibition in direction-selective retinal ganglion cells: spatiotemporal extent and intralaminar interactions

We recorded from ON-OFF direction-selective ganglion cells (DS cells) in the rabbit retina to investigate in detail the inhibition that contributes to direction selectivity in these cells. Using paired stimuli moving sequentially across the cells' receptive fields in the preferred direction, we directly confirmed the prediction of that a wave of inhibition accompanies any moving excitatory stimulus on its null side, at a fixed spatial offset. Varying the interstimulus distance, stimulus size, luminance, and speed yielded a spatiotemporal map of the strength of inhibition within this region. This "null" inhibition was maximal at an intermediate distance behind a moving stimulus: 1/2 to 11/2 times the width of the receptive field. The strength of inhibition depended more on the distance behind the stimulus than on stimulus speed, and the inhibition often lasted 1-2 s. These spatial and temporal parameters appear to account for the known spatial frequency and velocity tuning of ON-OFF DS cells to drifting contrast gratings. Stimuli that elicit distinct ON and OFF responses to leading and trailing edges revealed that an excitatory response of either polarity could inhibit a subsequent response of either polarity. For example, an OFF response inhibited either an ON or OFF response of a subsequent stimulus. This inhibition apparently is conferred by a neural element or network spanning the ON and OFF sublayers of the inner plexiform layer, such as a multistratified amacrine cell. Trials using a stationary flashing spot as a probe demonstrated that the total amount of inhibition conferred on the DS cell was equivalent for stimuli moving in either the null or preferred direction. Apparently the cell does not act as a classic "integrate and fire" neuron, summing all inputs at the soma. Rather, computation of stimulus direction likely involves interactions between excitatory and inhibitory inputs in local regions of the dendrites.     

Spatiotemporal representation of moving luminance edges in human vision

The edges of straight bars in a square-wave luminance grating appear undulating to an observer when the retinal image of this pattern is in motion. The amplitude of the perceived undulations increases linearly with retinal image speed with an average slope of 30 +/- 4 ms. The period of the motion-induced bulges is 2.5 +/- 0.5 degree and shows no consistent variation with the retinal image velocity of the pattern. The close quantitative agreement between the spatiotemporal extent of this effect and recent estimates of the spatiotemporal parameters of human motion-sensitive mechanisms suggests the existence of motion-sensitive cells in the central nervous system that have a fixed time constant but change the shape and size of their retinal support with retinal image velocity.     

Role of arcuate frontal cortex of monkeys in smooth pursuit eye movements. I. Basic response properties to retinal image motion and position

Anatomical and physiological studies have shown that the "frontal pursuit area" (FPA) in the arcuate cortex of monkeys is involved in the control of smooth pursuit eye movements. To further analyze the signals carried by the FPA, we examined the activity of pursuit-related neurons recorded from a discrete region near the arcuate spur during a variety of oculomotor tasks. Pursuit neurons showed direction tuning with a wide range of preferred directions and a mean full width at half-maximum of 129 degrees. Analysis of latency using the "receiver operating characteristic" to compare responses to target motion in opposite directions showed that the directional response of 58% of FPA neurons led the initiation of pursuit, while 19% led by 25 ms or more. Analysis of neuronal responses during pursuit of a range of target velocities revealed that the sensitivity to eye velocity was larger during the initiation of pursuit than during the maintenance of pursuit, consistent with two components of firing related to image motion and eye motion. FPA neurons showed correlates of two behavioral features of pursuit documented in prior reports. 1) Eye acceleration at the initiation of pursuit declines as a function of the eccentricity of the moving target. FPA neurons show decreased firing at the initiation of pursuit in parallel with the decline in eye acceleration. This finding is consistent with prior suggestions that the FPA plays a role in modulating the gain of visual-motor transmission for pursuit. 2) A stationary eccentric cue evokes a smooth eye movement opposite in direction to the cue and enhances the pursuit evoked by subsequent target motions. Many pursuit neurons in the FPA showed weak, phasic visual responses for stationary targets and were tuned for the positions about 4 degrees eccentric on the side opposite to the preferred pursuit direction. However, few neurons (12%) responded during the preparation or execution of saccades. The responses to the stationary target could account for the behavioral effects of stationary, eccentric cues. Further analysis of the relationship between firing rate and retinal position error during pursuit in the preferred and opposite directions failed to provide evidence for a large contribution of image position to the firing of FPA neurons. We conclude that FPA processes information in terms of image and eye velocity and that it is functionally separate from the saccadic frontal eye fields, which processes information in terms of retinal image position.     

Ganglion cell discharge and proximal negativity in the pigeon retina.

1. Proximal negativity has been compared with the spike responses and membrane responses of pigeon ganglion cells. 2. Spike discharge peaks develop during the rising phase of proximal negativity. 3. Proximal negativity is never directional, even when recorded concurrently with directional ganglion cells. 4. To moving stimuli, on-centre cells respond in association with the leading edge of the stimulus, off-centre cells with the trailing edge, and on-off cells with the leading and trailing edges. Proximal negativity is produced in association with the leading and trailing edges. 5. Quasi-intracellular records show that discharge peaks occur in association with depolarizing potentials. The synaptic responses of on-off cells bear a closer resemblance to the proximal negative response than do the responses of on-centre cells, off-centre cells, or directional cells.     

Processing of kinetically defined boundaries in areas V1 and V2 of the macaque monkey

We recorded responses in 107 cells in the primary visual area V1 and 113 cells in the extrastriate visual area V2 while presenting a kinetically defined edge or a luminance contrast edge. Cells meeting statistical criteria for responsiveness and orientation selectivity were classified as selective for the orientation of the kinetic edge if the preferred orientation for a kinetic boundary stimulus remained essentially the same even when the directions of the two motion components defining that boundary were changed by 90 degrees. In area V2, 13 of the 113 cells met all three requirements, whereas in V1, only 4 cells met the criteria of 107 that were tested, and even these demonstrated relatively weak selectivity. Correlation analysis showed that V1 and V2 populations differed greatly (P < 1.0 x 10(-6), Student's t-test) in their selectively for specific orientations of kinetic edge stimuli. Neurons in V2 that were selective for the orientation of a kinetic boundary were further distinguished from their counterparts in V1 in displaying a strong, sharply tuned response to a luminance edge of the same orientation. We concluded that selectivity for the orientation of kinetically defined boundaries first emerges in area V2 rather than in primary visual cortex. An analysis of response onset latencies in V2 revealed that cells selective for the orientation of the motion-defined boundary responded about 40 ms more slowly, on average, to the kinetic edge stimulus than to a luminance edge. In nonselective cells, that is, those presumably responding only to the local motion in the stimulus, this difference was only about 20 ms. Response latencies for the luminance edge were indistinguishable in KE-selective and -nonselective neurons. We infer that while responses to luminance edges or local motion are indigenous to V2, KE-selective responses may involve feedback entering the ventral stream at a point downstream with respect to V2.     

Motion-defined contour processing in the early visual cortex

From our daily experience, it is very clear that relative motion cues can contribute to correctly identifying object boundaries and perceiving depth. Motion-defined contours are not only generated by the motion of objects in a scene but also by the movement of an observer's head and body (motion parallax). However, the neural mechanism involved in detecting these contours is still unknown. To explore this mechanism, we extracellularly recorded visual responses of area 18 neurons in anesthetized and paralyzed cats. The goal of this study was to determine if motion-defined contours could be detected by neurons that have been previously shown to detect luminance-, texture-, and contrast-defined contours cue invariantly. Motion-defined contour stimuli were generated by modulating the velocity of high spatial frequency sinusoidal luminance gratings (carrier gratings) by a moving squarewave envelope. The carrier gratings were outside the luminance passband of a neuron, such that presence of the carrier alone within the receptive field did not elicit a response. Most neurons that responded to contrast-defined contours also responded to motion-defined contours. The orientation and direction selectivity of these neurons for motion-defined contours was similar to that of luminance gratings. A given neuron also exhibited similar selectivity for the spatial frequency of the carrier gratings of contrast- and motion-defined contours. These results suggest that different second-order contours are detected in a form-cue invariant manner, through a common neural mechanism in area 18.     

Latency of adaptive vergence eye movements induced by vergence-vestibular interaction training in monkeys

Clear vision of objects that move in depth toward or away from an observer requires vergence eye movements. The vergence system must interact with the vestibular system to maintain the object images on the foveae of both eyes during head movement. Previous studies have shown that training with sinusoidal vergence-vestibular interaction improves the frequency response of vergence eye movements during pitch rotation: vergence eye velocity gains increase and phase-lags decrease. To further understand the changes in eye movement responses in this adaptation, we examined latencies of vergence eye movements before and after vergence-vestibular training. Two head-stabilized Japanese monkeys were rewarded for tracking a target spot moving in depth that required vergence eye movements of 10°/s. This target motion was synchronized with pitch rotation at 20°/s. Both target and chair moved in a trapezoidal waveform interspersed with random inter-trial intervals. Before training, pitch rotation in complete darkness without a target did not induce vergence eye movements. Mean latencies of convergence and divergence eye movements induced by vergence target motion alone were 182 and 169 ms, respectively. After training, mean latencies of convergence and divergence eye movements to a target synchronized with pitch rotation shortened to 65 and 53 ms, and vergence eye velocity gains (relative to vergence target velocity) at the normal latencies were 0.68 and 1.53, respectively. Pitch rotation alone without a target induced vergence eye movements with similar latencies after training. These results indicate that vestibular information can be used effectively to initiate vergence eye movements following vergence-vestibular training.     

Plasticity in the visual system is correlated with a change in lifestyle of solitarious and gregarious locusts

We demonstrate pronounced differences in the visual system of a polyphenic locust species that can change reversibly between two forms (phases), which vary in morphology and behavior. At low population densities, individuals of Schistocerca gregaria develop into the solitarious phase, are cryptic, and tend to avoid other locusts. At high densities, individuals develop instead into the swarm-forming gregarious phase. We analyzed in both phases the responses of an identified visual interneuron, the descending contralateral movement detector (DCMD), which responds to approaching objects. We demonstrate that habituation of DCMD is fivefold stronger in solitarious locusts. In both phases, the mean time of peak firing relative to the time to collision nevertheless occurs with a similar characteristic delay after an approaching object reaches a particular angular extent on the retina. Variation in the time of peak firing is greater in solitarious locusts, which have lower firing rates. Threshold angle and delay are therefore conserved despite changes in habituation or behavioral phase state. The different rates of habituation should contribute to different predator escape strategies or flight control for locusts living either in a swarm or as isolated individuals. For example, increased variability in the habituated responses of solitarious locusts should render their escape behaviors less predictable. Relative resistance to habituation in gregarious locusts should permit the continued responsiveness required to avoid colliding with other locusts in a swarm. These results will permit us to analyze neuronal plasticity in a model system with a well-defined and controllable behavioral context.     

Spatial distribution of inputs and local receptive field properties of a wide-field, looming sensitive neuron

The lobula giant movement detector (LGMD) in the locust visual system and its target neuron, the descending contralateral movement detector (DCMD), respond to approaching objects looming on a collision course with the animal. They thus provide a good model to study the cellular and network mechanisms underlying the sensitivity to this specific class of behaviorally relevant stimuli. We determined over an entire locust eye the density distribution of optical axes describing the spatial organization of local inputs to the visual system and compared it with the sensitivity distribution of the LGMD/DCMD to local motion stimuli. The density of optical axes peaks in the equatorial region of the frontal eye. Local motion sensitivity, however, peaks in the equatorial region of the caudolateral visual field and only correlates positively with the dorso-ventral density of optical axes. On local stimulation, both the velocity tuning and the response latency of the LGMD/DCMD depend on stimulus position within the visual field. Spatial and temporal integration experiments in which several local motion stimuli were activated either simultaneously or at fixed delays reveal that the LGMD processes local motion in a strongly sublinear way. Thus the neuron's integration properties seem to depend on several factors including its dendritic morphology, the local characteristics of afferent fiber inputs, and inhibition mediated by different pathways or by voltage-gated conductances. Our study shows that the selectivity of this looming sensitive neuron to approaching objects relies on more complex biophysical mechanisms than previously thought.     

Direction selectivity of simple cells in cat striate cortex to moving light bars

Summary Quantitative estimates of the direction selectivities of 118 simple cells in response to moving light bars were expressed as a percentage calculated from the ratio of the response peaks: (preferred minus nonpreferred)/preferred. Virtually all simple cells were direction selective to some degree (mean direction selectivity 73.6%). Static-field plots to a stationary flashing bar were prepared from 74 of the 118 cells. Particular attention was given to the 42 cells with only two subregions in their static-field plot, one subregion responding at light on and the other at light off. It was concluded that interactive effects between subregions, whether synergistic or antagonistic, have little if any influence on the direction selective mechanism when the stimulus is a narrow light bar. Eighty two of the 118 cells were also tested with moving light and dark edges and of these 53 had response profiles with only two response peaks, one to the light edge and the other to the dark edge. Forty one of the 53 cells were each not only direction selective for both a light edge and a dark edge but also had a preferred direction for both edges that was the same as that for a light bar. Only two cells had preferred directions for both light and dark edges that were opposite to the direction preferred by the light bar. With one possible exception, every cell with two response peaks to moving edges and two subregions in the static-field plot showed a one-to-one correspondence between the ordinal sequence of the response peaks and the ordinal sequence of the subregions. Depending upon the polarity of the moving edge and the ordinal sequence of the subregions, the mean level of the direction selectivity to a moving edge was significantly below that to a narrow moving light bar. This reduction in the degree of the direction selectivity appears to be due to an interaction between the subregions leading to a reduction in the amplitude of the response in the preferred direction rather than a suppression of the direction selective mechanism that operates in the nonpreferred direction. Moving edges cause a weak interactive effect between the subregions that seems always to reduce the degree of the direction selectivity, never increasing it.     

Target selection for predictive smooth pursuit eye movements

Previous work has indicated that after exposure to a moving stimulus, people are able to produce predictive smooth eye movements prior to reappearance of the stimulus. Here, we investigated whether subjects are able to extract relevant velocity information from two simultaneously presented targets and use this information to produce a subsequent predictive response. A trial consisted of a series of two or five presentations of moving stimuli, preceded 500 ms earlier by an audio warning cue. In the first one or four presentations, subjects fixated during the presentation of two moving targets and in the final presentation they tracked a single moving target. During fixation, two moving targets were presented concurrently, originating from the fixation point and moving horizontally to the right at differing velocities (10, 20, 30 or 40°/s), with each target being presented at the same velocity throughout a trial. In the tracking presentation, the fixation cross was extinguished and only a single target was presented, which the subjects were required to track with their eyes. To cue which of the two targets would be presented, the appropriate target was presented statically at the same time as the audio warning cue. A significant relationship was found between eye velocity 100 ms after the start of the tracking target (i.e. prior to visual feedback) and the cued target velocity. Thus, subjects were able to make predictive eye movements that were of appropriate velocity for the cued target, despite fixating and being uncertain which target was relevant, during previous exposure.     

Dynamic assessment of disparity vergence ramps

Previous work has shown that the disparity vergence eye movement system responds to inward (i.e., convergent) ramp stimuli with both smooth and step-like behavior depending on target velocity. The responses to diverging ramp stimuli have not been previously studied, but convergence and divergence responses to other stimuli often show different behaviors. Converging and diverging 6 degrees/s ramps were presented to four subjects over a stimulus range of 2 degrees-20 degrees. Step-like behavior was seen in both convergence and divergence responses, but the dynamics was different. For divergent ramps, the peak velocity of each step-like movement decreased as the stimulus moved away from the subject, but no such trend was observed for convergence. The step-like behavior seen in divergence supports the hypothesis that the transient component is active in disparity divergence similar to the transient component proposed for convergent movements. However, the transient component in divergence may be dependent on stimulus position which is not the case for convergence.     

Neural responses to visual texture patterns in middle temporal area of the macaque monkey.

1. We studied how neurons in the middle temporal visual area (MT) of anesthetized macaque monkeys responded to textured and nontextured visual stimuli. Stimuli contained a central rectangular "figure" that was either uniform in luminance or consisted of an array of oriented line segments. The figure moved at constant velocity in one of four orthogonal directions. The region surrounding the figure was either uniform in luminance or contained a texture array (whose elements were identical or orthogonal in orientation to those of the figure), and it either was stationary or moved along with the figure. 2. A textured figure moving across a stationary textured background ("texture bar" stimulus) often elicited vigorous neural responses, but, on average, the responses to texture bars were significantly smaller than to solid (uniform luminance) bars. 3. Many cells showed direction selectivity that was similar for both texture bars and solid bars. However, on average, the direction selectivity measured when texture bars were used was significantly smaller than that for solid bars, and many cells lost significant direction selectivity altogether. The reduction in direction selectivity for texture bars generally reflected a combination of decreased responsiveness in the preferred direction and increased responsiveness in the null (opposite to preferred) direction. 4. Responses to a texture bar in the absence of a texture background ("texture bar alone") were very similar to the responses to solid bars both in the magnitude of response and in the degree of direction selectivity. Conversely, adding a static texture surround to a moving solid bar reduced direction selectivity on average without a reduction in response magnitude. These results indicate that the static surround is largely responsible for the differences in direction selectivity for texture bars versus solid bars. 5. In the majority of MT cells studied, responses to a moving texture bar were largely independent of whether the elements in the bar were of the same orientation as the background elements or of the orthogonal orientation. Thus, for the class of stimuli we used, orientation contrast does not markedly affect the responses of MT neurons to moving texture patterns. 6. The optimum figure length and the shapes of the length tuning curves determined with the use of solid bars and texture bars differed significantly in most of the cells examined. Thus neurons in MT are not simply selective for a particular figure shape independent of whatever cues are used to delineate the figure.     

The enhancement of radiotherapy verification images by an automated edge detection technique.

Adaptive histogram equalization techniques are known to be effective for the enhancement of contrast in portal images acquired during radiotherapy treatments. A significant drawback is the loss of definition on the edges of the treatment field. Analysis of this problem shows that it can be remedied by separating the treatment field from the background prior to the enhancement, and using only the pixels within the field boundary in the enhancement procedure. An edge extraction algorithm has been developed for delineating the treatment field in portal images, and consists of four modules that are applied to the original portal image in sequence. In the first step, edges are enhanced with a derivative of Gaussian operator that assures high response to the field edges relative to anatomical or other edges in the image. Pixels for which the response of the edge operator was the strongest are subsequently connected by an edge following algorithm to produce a raw contour of the field. In the last two steps the contour is refined by converting it into straight line segments and appending to the contour any parts of the field edge that might have been missed out during the initial edge following. The final contour encloses exclusively those pixels that belong to the treatment field, and the adaptive histogram equalization is applied selectively to this region. The combination of edge detection and selective enhancement was shown to produce images of superior contrast on the patient's anatomical features as well as accurate definition of treatment field edges.     

Responses to visual contours: spatio-temporal aspects of excitation in the receptive fields of simple striate neurones

1. The properties of the receptive fields of simple cells in the cat striate cortex have been studied by preparing average response histograms both to moving slits of light of different width and to single light-dark edges or contours.2. The movement of a narrow (< 0.3 degrees ) slit across the receptive field gives rise to average response histograms that are either unimodal, bimodal or multimodal. A slit of light has leading (light) and trailing (dark) edges. By increasing the width of the slit it was shown that a discharge peak in the histogram coincides with the passage of one or other of the two edges over a particular region (discharge centre) in the receptive field. Each edge has its own discharge centre which is fired when the edge has the correct orientation and direction of movement.3. The discharge centres in forty-three simple cell receptive fields were located by using one or more of the following stimuli for each cell:(i) slits of different width;(ii) single light and dark edges;(iii) a wide (3 degrees ) slit moved over a range of different velocities.The same locations were obtained when all three procedures were used on the same cell.4. Most cells (79%) discharged to both edges though not necessarily in the same direction of movement. The majority (72%) fired in only one direction and most commonly (51%) the cells responded to both edges in this one direction. In only 16% of cells did both types of edge excite in both directions of movement. When the one type of edge, light or dark, was considered, 84% of the cells were direction selective and, for these cells, the other edge fired only in the same direction (51%), in both directions (7%), only in the opposite direction (5%) or not at all (21%).5. Cells responding in one direction with a unimodal average response histogram may be responding to both edges, the two responses being concealed in the one discharge peak. The two discharge centres are then either nearly coincident or, more usually, slightly offset with respect to one another. Most commonly the dark edge centre is slightly in advance of the light edge centre.6. The discharge peaks in the bimodal and multimodal types come from discharge centres that are spatially separate, each centre firing to only one type of edge. In the case of the bimodal type the light edge centre always lies ahead of the dark edge centre.7. When a cell responds to a single edge in both directions of movement, the type of contrast effective in one direction is always the reverse of that in the other. When the cell responded in both directions, whether to one or both edges, most commonly a light edge discharge centre in one direction occupied approximately the same location in space as the dark edge centre in the reverse direction and vice versa for the other edge.8. Temporal aspects of the discharge of simple cells have been examined by recording the responses to moving slits and single edges over a wide range of velocities.     

Role of an identified looming-sensitive neuron in triggering a flying locust's escape

Flying locusts perform a characteristic gliding dive in response to predator-sized stimuli looming from one side. These visual looming stimuli trigger trains of spikes in the descending contralateral movement detector (DCMD) neuron that increase in frequency as the stimulus gets nearer. Here we provide evidence that high-frequency (>150 Hz) DCMD spikes are involved in triggering the glide: the DCMD is the only excitatory input to a key gliding motor neuron during a loom; DCMD-mediated EPSPs only summate significantly in this motor neuron when they occur at >150 Hz; when a looming stimulus ceases approach prematurely, high-frequency DCMD spikes are removed from its response and the occurrence of gliding is reduced; and an axon important for glide triggering descends in the nerve cord contralateral to the eye detecting a looming stimulus, as the DCMD does. DCMD recordings from tethered flying locusts showed that glides follow high-frequency spikes in a DCMD, but analyses could not identify a feature of the DCMD response alone that was reliably associated with glides in all trials. This was because, for a glide to be triggered, the high-frequency spikes must be timed appropriately within the wingbeat cycle to coincide with wing elevation. We interpret this as flight-gating of the DCMD response resulting from rhythmic modulation of the flight motor neuron's membrane potential during flight. This means that the locust's escape behavior can vary in response to the same looming stimulus, meaning that a predator cannot exploit predictability in the locust's collision avoidance behavior.