Ventral Visual Field Research Articles

The role of spatial texture in visual control of bumblebee learning flights

When leaving the nest for the first time, bees and wasps perform elaborate learning flights, during which the location of the nest is memorised. These flights are characterised by a succession of arcs or loops of increasing radius centred around the nest, with an incremental increase in ground speed, which requires precise control of the flight manoeuvres by the insect. Here, we investigated the role of optic flow cues in the control of learning flights by manipulating spatial texture in the ventral and panoramic visual field. We measured height, lateral displacement relative to the nest and ground speed during learning flights in bumblebees when ventral and panoramic optic flow cues were present or minimised, or features of the ground texture varied in size. Our observations show that ventral optic flow cues were required for the smooth execution of learning flights. We also found that bumblebees adjusted their flight height in response to variations of the visual texture on the ground. However, the presence or absence of panoramic optic flow did not have a substantial effect on flight performance. Our findings suggest that bumblebees mainly rely on optic flow information from the ventral visual field to control their learning flights.

How bumblebees use lateral and ventral optic flow cues for position control in environments of different proximity

Flying insects frequently navigate through environments of different complexity. In this study, buff-tailed bumblebees (Bombus terrestris L.) were trained to fly along tunnels of different widths, from 60 to 240 cm. In tunnel widths of 60 and 120 cm, bumblebees control their lateral position by balancing the magnitude of translational optic flow experienced in the lateral visual field of each eye. In wider tunnels, bumblebees use translational optic flow cues in the ventral visual field to control their lateral position and to steer along straight tracks. Our results also suggest that bumblebees prefer to fly over surfaces that provide strong ventral optic flow cues, rather than over featureless ones. Together, these strategies allow bumblebees to minimize the risk of collision and to maintain relatively straight flight paths in a broad range of environments.

Differences in Visual-Spatial Input May Underlie Different Compression Properties of Firing Fields for Grid Cell Modules in Medial Entorhinal Cortex.

Firing fields of grid cells in medial entorhinal cortex show compression or expansion after manipulations of the location of environmental barriers. This compression or expansion could be selective for individual grid cell modules with particular properties of spatial scaling. We present a model for differences in the response of modules to barrier location that arise from different mechanisms for the influence of visual features on the computation of location that drives grid cell firing patterns. These differences could arise from differences in the position of visual features within the visual field. When location was computed from the movement of visual features on the ground plane (optic flow) in the ventral visual field, this resulted in grid cell spatial firing that was not sensitive to barrier location in modules modeled with small spacing between grid cell firing fields. In contrast, when location was computed from static visual features on walls of barriers, i.e. in the more dorsal visual field, this resulted in grid cell spatial firing that compressed or expanded based on the barrier locations in modules modeled with large spacing between grid cell firing fields. This indicates that different grid cell modules might have differential properties for computing location based on visual cues, or the spatial radius of sensitivity to visual cues might differ between modules.

Honeybees (Apis mellifera) exhibit flexible visual search strategies for vertical targets presented at various heights

When honeybees are presented with a colour discrimination task, they tend to choose swiftly and accurately when objects are presented in the ventral part of their frontal visual field. In contrast, poor performance is observed when objects appear in the dorsal part. Here we investigate if this asymmetry is caused by fixed search patterns or if bees can increase their detection ability of objects in search scenarios when targets appear frequently or exclusively in the dorsal area of the visual field.We trained individual honeybees to choose an orange rewarded target among blue distractors. Target and distractors were presented in the ventral visual field, the dorsal field or both. Bees presented with targets in the ventral visual field consistently had the highest search efficiency, with rapid decisions, high accuracy and direct flight paths. In contrast, search performance for dorsally located targets was inaccurate and slow at the beginning of the experimental phase, but bees increased their search performance significantly after a few foraging bouts: they found the target faster, made fewer errors and flew in a straight line towards the target. However, bees needed thrice as long to improve the search for a dorsally located target when the target’s position changed randomly between the ventral and the dorsal visual field. We propose that honeybees form expectations of the location of the target’s appearance and adapt their search strategy accordingly. A variety of possible mechanisms underlying this behavioural adaptation, for example spatial attention, are discussed.

Strategies of the honeybee Apis mellifera during visual search for vertical targets presented at various heights: a role for spatial attention?

When honeybees are presented with a colour discrimination task, they tend to choose swiftly and accurately when objects are presented in the ventral part of their frontal visual field. In contrast, poor performance is observed when objects appear in the dorsal part. Here we investigate if this asymmetry is caused by fixed search patterns or if bees can use alternative search mechanisms such as spatial attention, which allows flexible focusing on different areas of the visual field.We asked individual honeybees to choose an orange rewarded target among blue distractors. Target and distractors were presented in the ventral visual field, the dorsal field or both. Bees presented with targets in the ventral visual field consistently had the highest search efficiency, with rapid decisions, high accuracy and direct flight paths. In contrast, search performance for dorsally located targets was inaccurate and slow at the beginning of the test phase, but bees increased their search performance significantly after a few learning trials: they found the target faster, made fewer errors and flew in a straight line towards the target. However, bees needed thrice as long to improve the search for a dorsally located target when the target’s position changed randomly between the ventral and the dorsal visual field. We propose that honeybees form expectations of the location of the target’s appearance and adapt their search strategy accordingly. Different possible mechanisms of this behavioural adaptation are discussed.

Strategies of the honeybee Apis mellifera during visual search for vertical targets presented at various heights: a role for spatial attention?

When honeybees are presented with a colour discrimination task, they tend to choose swiftly and accurately when objects are presented in the ventral part of their frontal visual field. In contrast, poor performance is observed when objects appear in the dorsal part. Here we investigate if this asymmetry is caused by fixed search patterns or if bees can use alternative search mechanisms such as spatial attention, which allows flexible focusing on different areas of the visual field. We asked individual honeybees to choose an orange rewarded target among blue distractors. Target and distractors were presented in the ventral visual field, the dorsal field or both. Bees presented with targets in the ventral visual field consistently had the highest search efficiency, with rapid decisions, high accuracy and direct flight paths. In contrast, search performance for dorsally located targets was inaccurate and slow at the beginning of the test phase, but bees increased their search performance significantly after a few learning trials: they found the target faster, made fewer errors and flew in a straight line towards the target. However, bees needed thrice as long to improve the search for a dorsally located target when the target's position changed randomly between the ventral and the dorsal visual field. We propose that honeybees form expectations of the location of the target's appearance and adapt their search strategy accordingly. Different possible mechanisms of this behavioural adaptation are discussed.

A stingless bee can use visual odometry to estimate both height and distance

Bees move and forage within three dimensions and rely heavily on vision for navigation. The use of vision-based odometry has been studied extensively in horizontal distance measurement, but not vertical distance measurement. The honey bee Apis mellifera and the stingless bee Melipona seminigra measure distance visually using optic flow-movement of images as they pass across the retina. The honey bees gauge height using image motion in the ventral visual field. The stingless bees forage at different tropical forest canopy levels, ranging up to 40 m at our site. Thus, estimating height would be advantageous. We provide the first evidence that the stingless bee Melipona panamica utilizes optic flow information to gauge not only distance traveled but also height above ground, by processing information primarily from the lateral visual field. After training bees to forage at a set height in a vertical tunnel lined with black and white stripes, we observed foragers that explored a new tunnel with no feeder. In a new tunnel, bees searched at the same height they were trained to. In a narrower tunnel, bees experienced more image motion and significantly lowered their search height. In a wider tunnel, bees experienced less image motion and searched at significantly greater heights. In a tunnel without optic cues, bees were disoriented and searched at random heights. A horizontal tunnel testing these variables similarly affected foraging, but bees exhibited less precision (greater variance in search positions). Accurately gauging flight height above ground may be crucial for this species and others that compete for resources located at heights ranging from ground level to the high tropical forest canopies.

Topographic analysis of the ganglion cell layer in the retina of the four-eyed fishAnableps anableps

Fish of the genus Anableps (Anablepidae, Cyprinodontiformes) have eyes that are adapted for simultaneous aerial and aquatic vision. In this study we investigate some of the corresponding retinal specializations of the adult Anableps anableps eye using retinal transverse sections and wholemounts. The linear dimensions of the retina were found to be asymmetric with a greater representation of the dorsal compared to the ventral visual field. The total number of neurons in the ganglion cell layer of the ventral hemiretina was on average 3.6 times greater than the values obtained in the dorsal hemiretina. Isodensity contour maps revealed a prominent horizontal visual streak in the ventral hemiretina with an average peak cell density of 18,286 cells/mm(2). A second less-well-developed horizontal visual streak was also observed in the dorsal hemiretina. A sub-population of large cells with soma areas between 74 and 188 microm(2) was identified and found to be distributed evenly across both hemiretinas. Together, these results show that the sampling gain of the ventral retina is significantly greater than the dorsal segment, that retinal specializations important for mediating acute vision are present in the parts of the visual field immediately above and below the surface of the water, and that visual functions related with the large ganglion cells require more even sampling across the visual field. The relevance of these retinal specializations to the feeding and other behavioral strategies adopted by Anableps is discussed.

Signals from 'crabworld': cuticular reflections in a fiddler crab colony.

Fiddler crabs inhabit intertidal sand- and mudflats, where they live in dense colonies and are active on the surface during low tide. They exhibit a rich behavioural repertoire, with frequent interactions between animals in the context of territorial and mating activities. Male fiddler crabs have one massively enlarged and conspicuously coloured claw, which they use in waving displays and in fights with other males. The crabs carry their eyes on long, vertically oriented stalks high above the body and, as a consequence, see the bodies of conspecifics in the ventral visual field, below the local visual horizon, and against the mudflat surface as background. We filmed events in a colony of Uca vomeris with a normal video camera and an ultraviolet-sensitive camera placed at the eye height of an average crab, approximately 2-3 cm above ground. We also used a spectrographic imager and linear polarized filters to analyse the cues potentially available to the animals for detecting, monitoring and possibly identifying each other. Areas of high contrast in mudflat scenes include specular reflections on the wet cuticle of crabs that are horizontally polarised. Besides specular reflections, some parts of the cuticle generate high-contrast signals against the mudflat background, both at wavelengths between 400 and 700 nm, and in the ultraviolet region between 300 and 400 nm. Uca vomeris can be very colourful: the different parts of the large claw of the male are white, orange or red. The carapace colours of both males and females can range from a mottled yellowish green brown, to a brilliant light blue. White and blue colours contrast starkly with the mudflat background, especially in the ultraviolet wavelengths. Under stress, the blue and white colours can change within minutes to a duller and darker blue or to a dull white.

Ascending spinal systems in the fish, Prionotus carolinus.

The fin rays of the pectoral fin of the sea robins (teleostei) are specialized chemosensory organs heavily invested with solitary chemoreceptor cells innervated only by spinal nerves. The rostral spinal cord of these animals is marked by accessory spinal lobes which are unique enlargements of the dorsal horn of the rostral spinal segments receiving input from the fin ray nerves. Horseradish peroxidase (HRP) and 1,1;-dioctadecyl-3,3,3', 3'-tetramethylindocarbocyanine perchlorate (diI) were used as anterograde and retrograde tracers to examine the connectivity of these accessory lobes and the associated ascending spinal systems in the sea robin, Prionotus carolinus. The majority of dorsal root fibers terminate within the accessory lobes at or nearby their level of entrance into the spinal cord. A few dorsal root axons turn rostrally in the dorsolateral fasciculus to terminate in the lateral funicular complex situated at the spinomedullary junction. The lateral funicular complex also receives a heavy projection from the ipsilateral accessory lobes. In addition, it contains a few large neurons that project back onto the accessory lobes. Injections of either diI or HRP into the lateral funicular complex label fibers of the medial lemniscus which crosses the midline in the caudal medulla to ascend along the ventral margin of the contralateral rhombencephalon. Within the medulla, fibers leave the medial lemniscus to terminate in the inferior olive and in the ventrolateral medullary reticular formation. Upon reaching the midbrain, the medial lemniscus turns dorsally to terminate heavily in a lateral division of the torus semicircularis, in the ventral optic tectum, and in the lateral subnucleus of the nuc. preglomerulosus of the thalamus. Lesser projections also reach the posterior periventricular portion of the posterior tubercle with a few fibers terminating along the ventral, posterior margin of the ventromedial (VM) nucleus of the thalamus. The restricted projection to the ventral tectum is noteworthy in that this part of the tectum maintains the representation of the ventral visual field, that is, the area in which the fin rays lie. A prominent spinocerebellar system is also evident. Both direct and indirect spinocerebellar fibers can be followed through the dorsolateral fasciculus, with or without relay in the lateral funicular nucleus and terminating in a restricted portion of the granule cell layer of the ipsilateral corpus cerebelli. The similarities in connectivity of the spinal cord between the sea robins and other vertebrates are striking. It is especially notable because sea robins utilize the chemosensory input from the fin rays to localize food in the environment. Thus, although these fish use their spinal chemosense as other fishes use their external taste systems, the spinal chemosense apparently relies on the medial lemniscal system to guide this chemically driven feeding behavior.

Retinal Projections and Retinal Ganglion Cell Distribution Patterns in a Sturgeon (Acipenser transmontanus), a Non-Teleost Actinopterygian Fish

Retinal projections in a sturgeon were studied by injecting biocytin or HRP into the optic nerve. The target areas are the preoptic area, thalamus, area pretectalis, nucleus of posterior commissure, optic tectum, and nuclei of the accessory optic tract. Furthermore, a few labeled fibers and terminals were found in a ventrolateral area of the caudal telencephalon. All retinal projections are bilateral, although contralateral projections were more heavily labeled. Retrogradely labeled neurons were found in the ventral thalamus bilaterally. Retinal projections in sturgeons are similar to those of other non-teleost actinopterygians and chondrichthyans (sharks), in terms of the targets and extent of bilateral projections. Distribution patterns of ganglion cells in the retina were examined in Nissl-stained retinal whole mount preparations. The highest density areas were found in the temporal and nasal retinas, and a dense band of ganglion cells was observed along the horizontal axis between the nasal and temporal areas of highest density. The density of ganglion cells in the dorsal retina is the lowest. The total number of ganglion cells was estimated to be about 5 × 10⁴ in a retina. The existence of a low density area in the dorsal retina suggests reduced visual acuity in the ventral visual field.

Honeybee Vision: Analysis of Orientation and Colour in the Lateral, Dorsal and Ventral Fields of View

The ability of honeybees (Apis mellifera) to learn and recognise peripherally presented patterns was investigated by training bees in a Y-maze which presented patterns on the side walls, the ceiling or the floor. We found that pattern orientation is learnt and recognised in the lateral and frontal visual field, but not in the dorsal or ventral fields. Colour information, in contrast, is used in the lateral and frontal as well as the ventral visual field, but not in the dorsal field. If pattern orientation is different on opposite sides of the visual field during training, both patterns are learned, but each on its own is sufficient for the bees to recognise the learnt stimulus. However, unilaterally learnt pattern information, be it orientation or colour, cannot be accessed when the test pattern is viewed on the other side. That is, interocular transfer of such information does not occur.

The variation of resolution and of ommatidial dimensions in the compound eyes of the fiddler crab Uca lactea annulipes (Ocypodidae, Brachyura, Decapoda)

We studied variations in the optical properties of the compound eyes of Uca lactea annulipes using in vivo optical and histological techniques. The distribution of resolving power in the eyes of this fiddler crab species is typical for arthropods that inhabit flat environments: the eyes possess a panoramic equatorial acute zone for vertical resolution and a steep decrease of resolution away from the eye equator in the dorsal and ventral visual fields. The dimensions of the cellular components of the ommatidia vary accordingly: in the equatorial part of the eyes, facets are larger, and crystalline cones and rhabdoms are longer than in the dorsal and ventral parts of the eyes. Along the eye equator, horizontal resolution is low compared with vertical resolution and varies little throughout the visual field. The eyes of Uca lactea annulipes are unusual in that the gradient of vertical anatomical and optical resolution is steeper in the dorsal than in the ventral visual field. We interpret this difference as indicating that the information content of the world as seen by the crabs differs above and below the horizon line in specific and predictable ways.

The response of the hovering hawk moth Macroglossum stellatarum to translatory pattern motion

1. The European hawk moth Macroglossum stellatarum, while collecting nectar in hovering flight in front of flowers, follows moving stripe patterns in the lateral visual field. This response counteracts a second one, that is the animals' effort to stabilize their distance from dummy flowers. We investigated the response to motion stimuli in the lateral visual field using sinusoidally oscillating stripe patterns (Fig. 1), as well as its interaction with the distance stabilizing response. 2. In both responses moths attempt to compensate for image speed. The balance between the two depends on the number of elementary motion detectors stimulated by the dummy flower and the stripe pattern, respectively. Increasing the diameter of the dummy flower (Figs. 2 to 4) or the spatial frequency of the stripe pattern (Fig. 7) shifts the balance in favour of distance stabilization. The reverse is true when the length of the stripes in the pattern (Fig. 5) or their number is increased (Fig. 6). It does not matter whether the stripe pattern is presented in the lateral (Fig. 4A) or in the dorsal and ventral visual field (Fig. 4B). 3. The gain-frequency relations of the response to the lateral stripe pattern obtained with dummies in two different positions within the drum have their maxima around 3 Hz and decline rapidly towards lower and higher frequencies like the response of a bandpass filter. The distance stabilizing response also has bandpass properties, but with a broad plateau between 0.15 and 5 Hz (Fig. 8). The most likely explanation for this difference is that there is a regional or direction-dependent variation of motion detector properties. 4. The responses to ramp-like stimuli are phasic in accordance with the amplitude frequency characteristics, but the responses to progressive (front to back) and regressive motion of the pattern differ (Figs 9, 10). 5. The response appears to depend on the azimuthal position of the stripe pattern within the visual field (Fig. 11). It is strongest when the pattern covers equally large parts of the frontal and caudal visual fields. The optomotor sensitivity to translational pattern motion is higher in the frontal than in the caudal visual field (Fig. 12, Table 1). 6. When the stripe pattern on one side is removed, the response amplitude is halved. There is no detectable turning response around the vertical axis to the oscillation of the stripe pattern (Fig. 13, Table 2). 7. The possible role of the response to pattern movements parallel to the longitudinal body axis under natural conditions is discussed.

Separation of translation and rotation by means of eye-region specialization in flying gypsy moths (Lepidoptera: Lymantriidae)

A partial functional specialization of eye regions in the visual control of flight was studied in male gypsy moths, Lymantria dispar,under open-loop conditions. When stimulated by means of a rotating striped drum, surrounding either the moth 's longitudinal or its transverse body axis, the induced torque response was always such as to compensate for a simulated translatory disturbance of flight, if the stimulus was restricted to the ventral visual field. If restricted to the lateral visual field(s), the response was always such as to compensate for a simulated rotatory disturbance. Though the conclusions refer to only a limited subset of visual stimuli the moths experience during free flight, the induced responses give reason to suppose that at least some of the simultaneous control of translation and rotation in free flight is based on a regional specialization of the compound eye.

Frogs use retinal elevation as a cue to distance.

We have investigated the role that retinal elevation plays in a frog's (Rana pipiens) estimate of prey distance. We dissociated retinal elevation from other depth cues by artificially increasing the height of the frogs' eyes above the ground. Frogs then snapped short of their prey in their ventral visual field as if their estimate of distance were determined primarily by the retinal elevation of the image of the prey. The data suggest that the frog assumes its eyes to be about 3 cm above the ground. Other cues modify depth judgements when targets are close to this assumed ground-plane.

A Directionally Sensitive Motion Detecting Neurone in the Brain of A Moth

ABSTRACT In the moth, Manduca sexta, a pair of neurones, one on each side of the brain, were characterized morphologically and physiologically as descending interneurones, selective for horizontal motion over a large area of the moth’s visual field. Their cell bodies and dendritic processes are located in the protocerebrum of the brain. Their axons, 12–15μm in diameter, project down the ipsilateral connective, branching profusely on the ipsilateral side of the suboesophageal, prothoracic and pterothoracic ganglia. Each neurone responds to movement over either retina. Their preferred directions are from front to back across the ipsilateral eye and back to front over the contralateral one. Movement in the opposite direction supresses their usual ‘resting’ discharge. The neurones are particularly sensitive to movements within the frontal, ventral visual field. Each neurone responds repeatedly, for up to 5 h, to a stimulus oscillating back and forth across the retinae. The response is not diminished during concurrent wing flapping. An increase in the velocity of stimulus movement produces a proportional increase in firing frequency. For stripes of 2·5 cm wavelength and subtending 32° at the eye, the maximum response occurs at a velocity of 3cm/s which gives a contrast frequency of 1-2 Hz. The latency of the neurone’s response, measured from its axon as it enters the pterothoracic ganglion, depends on at least two factors: light intensity and the speed of stimulus movement. The neurone gives a directional response to stripes of period 6·4° in bright light. The response falls to 16· in dim light. At night, in dim light, the latency of response is much reduced and the threshold light intensity, necessary for a directional response, decreases by two orders of magnitude.

The visual system of the guitar fish (Rhinobatos productus).

The retinal projections in adult and juvenile guitar fish (Rhinobatos productus) were determined with the aid of the Nauta-Fink-Heimer techniques. The visual system was found to be more extensive and more differentiated than in any other elasmobranch studied to date. Massive projections exist to the dorsal and ventral thalamus, tectum and pretectum, in addition to the usual weak contributions to the hypothalamus and the ventral mesencephalic tegmentum. The projection to the lateral tectum is significantly less distinct than that to the medial part of this structure, suggesting that the ventral visual field has a smaller input, perhaps due to the fact that this visual field is normally aimed at the body of this flat fish.

Spectral sensitivity of the red and yellow oil droplet fields of the pigeon (Columba livia).

THE function of the brightly coloured oil droplets situated within the cone receptors of the avian retina has long been the subject of debate1–4. Among other hypotheses it has been suggested that they reduce glare and chromatic abberation1, remove unwanted short-wavelength sensitivity2, and have a role in colour vision3–5. The transmission spectra of the droplets in the visible spectrum are typically those of short wavelength cut-off filters5,6–8, due to the presence in high concentration of carotenoids of dietry origin9–11. Recent evidence from microspectrophotometry suggests that the droplets filter the light reaching the visual pigment in the receptor's outer segment, resulting in the modification of the sensitivity functions of some of the individual receptors, from the broad band sensitivity of the visual pigment alone to a narrow band one12,13. It is also known that marked differences occur in the proportions of the variously coloured droplets between species, and also between different parts of the retina of a given species4,14–16. In the pigeon (Columba livia) the retina is divided into two distinct areas, known as the red and yellow fields, defined by their oil droplet populations. The red field is in the dorsal retina (frontal–ventral visual field), whereas the yellow field is in the ventral retina (dorsal visual field): the two fields differ in both the type and number of broad and narrow band receptors present, as well as in receptor density and the neural complexity of the retina13,17,18. We report here clear differences in the photopic spectral sensitivity of the two fields, determined behaviourally, and suggest that these reflect a difference in the colour vision mechanisms of the two areas.

Visual system ofNotonecta glauca: A neuron sensitive to movement in the binocular visual field

1. Large-amplitude spikes were recorded from a movement-sensitive unit in the optic lobe ofNotonecta. 2. This neuron is sensitive to movement of a small dark spot within the receptive field, in any direction. Movement of a background — e.g., the movement of a large striped pattern — does not elicit discharges. The movement of the striped pattern can even suppress the response to an otherwise effective stimulus. 3. The observed region of high sensitivity to moving spots is large and is found within the binocular field. Excitation involves binocular interaction: a high firing rate can be induced only if both eyes see the moving object. Covering of either the contralateral or the ipsilateral eye reduces the spike frequency to less than one-half the binocular rate (Fig. 15). 4. In the velocity range tested, from 3.5 to 350°/s, the log of the firing frequency varies linearly with the log of the stimulus velocity (Fig. 7). 5. Varying the contrast, above its threshold value, had little effect on the spike rate. At a given weak contrast, however, the neuron responded to movement in the dorsal visual field, but not to movement in the ventral field (Fig. 9). 6. Repetitive stimulation of the same part of the retinal array elicits successively smaller responses. This habituation is partly alleviated if a different group of ommatidia is stimulated (Fig. 6). 7. When a spot of suitable size was moved through the ventral visual field along a path perpendicular to the median plane of the animal, there was a discharge-rate maximum ca. 20° ipsilateral to the median (Fig. 4). A spot moving in the median plane from ventral to dorsal elicited three maxima of excitation (Figs. 3, 5, 11, 15). WhenNotonecta is hanging in its normal posture, upside down below the water surface, its visual environment is divided into three parts: the region of the deep water, the zone of te totally reflecting portion of the surface, and the zone of the transparent water surface. Each of the peaks in the response corresponds to one of these zones of the visual field. 8. The peak corresponding to the region of total reflection changed its position with the angular size of the moving spot; a small spot elicited a peak near the horizon. The larger the spot, the farther the peak shifted ventralwards (i.e., upward; Fig. 11). Due to this effect, an object moving in the plane of the water surface at any distance would excite the neuron maximally if it were of the appropriate size, ca. 1/4 the length ofNotonecta (Fig. 13).