The effect of viewing distance upon size preference of frogs for prey

Poison frog dietary preference depends on prey type and alkaloid load

The ability to acquire chemical defenses through the diet has evolved across several major taxa. Chemically defended organisms may need to balance chemical defense acquisition and nutritional quality of prey items. However, these dietary preferences and potential trade-offs are rarely considered in the framework of diet-derived defenses. Poison frogs (Family Dendrobatidae) acquire defensive alkaloids from their arthropod diet of ants and mites, although their dietary preferences have never been investigated. We conducted prey preference assays with the Dyeing Poison frog (Dendrobates tinctorius) to test the hypothesis that alkaloid load and prey traits influence frog dietary preferences. We tested size preferences (big versus small) within each of four prey groups (ants, beetles, flies, and fly larvae) and found that frogs preferred interacting with smaller prey items of the fly and beetle groups. Frog taxonomic prey preferences were also tested as we experimentally increased their chemical defense load by feeding frogs decahydroquinoline, an alkaloid compound similar to those naturally found in their diet. Contrary to our expectations, overall preferences did not change during alkaloid consumption, as frogs across groups preferred fly larvae over other prey. Finally, we assessed the protein and lipid content of prey items and found that small ants have the highest lipid content while large fly larvae have the highest protein content. Our results suggest that consideration of toxicity and prey nutritional value are important factors in understanding the evolution of acquired chemical defenses and niche partitioning.

Size discrimination in barn owls as compared to humans

We tested how well barn owls can discriminate objects of different sizes. This ability may be important for the owls when catching prey. We performed a quantitative experiment in the laboratory and trained owls in a task in which the owls had to discriminate whether two rhombi presented simultaneously on a computer monitor were of the same or of different sizes. We obtained full data sets with two experienced owls and one data point with a third owl. For objects being sufficiently larger than the spatial resolution of the barn owl, the angular threshold was related to object size, implying that the discrimination followed Weber's law. The range of Weber fractions we determined was between 0.026 and 0.09. For object sizes close to the spatial resolution, performance degraded. We conducted similar experiments with human subjects. Human thresholds showed the same dependence on object size, albeit down to smaller object sizes. Human performance resulted in a range of Weber fractions extending from 0.025 to 0.036. The differences between owls and humans could be explained by the much higher spatial acuity of humans compared with owls.

The representation of egocentric space in the posterior parietal cortex

The posterior parietal cortex (PPC) is the most likely site where egocentric spatial relationships are represented in the brain. PPC cells receive visual, auditory, somaesthetic, and vestibular sensory inputs; oculomotor, head, limb, and body motor signals; and strong motivational projections from the limbic system. Their discharge increases not only when an animal moves towards a sensory target, but also when it directs its attention to it. PPC lesions have the opposite effect: sensory inattention and neglect. The PPC does not seem to contain a "map" of the location of objects in space but a distributed neural network for transforming one set of sensory vectors into other sensory reference frames or into various motor coordinate systems. Which set of transformation rules is used probably depends on attention, which selectively enhances the synapses needed for making a particular sensory comparison or aiming a particular movement.

A critical interpersonal distance switches between two coordination modes in kendo matches

In many competitive sports, players need to quickly and continuously execute movements that co-adapt to various movements executed by their opponents and physical objects. In a martial art such as kendo, players must be able to skillfully change interpersonal distance in order to win. However, very little information about the task and expertise properties of the maneuvers affecting interpersonal distance is available. This study investigated behavioral dynamics underlying opponent tasks by analyzing changes in interpersonal distance made by expert players in kendo matches. Analysis of preferred interpersonal distances indicated that players tended to step toward and away from their opponents based on two distances. The most preferred distance enabled the players to execute both striking and defensive movements immediately. The relative phase analysis of the velocities at which players executed steps toward and away revealed that players developed anti-phase synchronizations at near distances to maintain safe distances from their opponents. Alternatively, players shifted to in-phase synchronization to approach their opponents from far distances. This abrupt phase-transition phenomenon constitutes a characteristic bifurcation dynamics that regularly and instantaneously occurs between in- and anti-phase synchronizations at a critical interpersonal distance. These dynamics are profoundly affected by the task constraints of kendo and the physical constraints of the players. Thus, the current study identifies the clear behavioral dynamics that emerge in a sport setting.

Does the nervous system depend on kinesthetic information to control natural limb movements?

This target article draws together two groups of experimental studies on the control of human movement through peripheral feedback and centrally generated signals of motor commands. First, during natural movement, feedback from muscle, joint, and cutaneous afferents changes; in human subjects these changes have reflex and kinesthetic consequences. Recent psychophysical and microneurographic evidence suggests that joint and even cutaneous afferents may have a proprioceptive role. Second, the role of centrally generated motor commands in the control of normal movements and movements following acute and chronic deafferentation is reviewed. There is increasing evidence that subjects can perceive their motor commands under various conditions, but that this is inadequate for normal movement; deficits in motor performance arise when the reliance on proprioceptive feedback is abolished either experimentally or because of pathology. During natural movement, the CNS appears to have access to functionally useful input from a range of peripheral receptors as well as from internally generated command signals. The unanswered questions that remain suggest a number of avenues for further research.

Implications of neural networks for how we think about brain function

Engineers use neural networks to control systems too complex for conventional engineering solutions. To examine the behavior of individual hidden units would defeat the purpose of this approach because it would be largely uninterpretable. Yet neurophysiologists spend their careers doing just that! Hidden units contain bits and scraps of signals that yield only arcane hints about network function and no information about how its individual units process signals. Most literature on single-unit recordings attests to this grim fact. On the other hand, knowing a system's function and describing it with elegant mathematics tell one very little about what to expect of interneuronal behavior. Examples of simple networks based on neurophysiology are taken from the oculomotor literature to suggest how single-unit interpretability might decrease with increasing task complexity. It is argued that trying to explain how any real neural network works on a cell-by-cell, reductionist basis is futile and we may have to be content with trying to understand the brain at higher levels of organization.

The effects of linear perspective on relative size discrimination in chimpanzees (Pan troglodytes) and humans (Homo sapiens)

In this study, we tested the corridor illusion in three chimpanzees and five humans, applying a relative size discrimination task to assess pictorial depth perception using linear perspective. The subjects were required to choose the physically larger cylinder of two on a background containing drawn linear perspective cues. We manipulated both background and cylinder size in each trial. Our findings suggest that chimpanzees, like humans, exhibit the corridor illusion.

Archer Fish Learn to Compensate for Complex Optical Distortions to Determine the Absolute Size of Their Aerial Prey

Many animals, including humans, can visually judge the absolute size of objects regardless of changes in viewing distance and thus despite the resulting dramatic differences in the size of the actual retinal images. For animals that have to judge the size of aerial objects from underwater views, this can be a formidable problem; our calculations show that considerable and strongly viewpoint-dependent corrections are needed to compensate for the effects of light refraction. Archer fish face these optical difficulties because they have to shoot down aerial insects over a wide range of horizontal and vertical distances. We show here that these fish can learn to acquire size constancy with remarkable precision and are thus fully capable of taking complex viewpoint dependency into account. Moreover, we demonstrate that archer fish solve the problem not by interpolating within a set of stored views and distances but by learning the laws that connect apparent size with the fish's relative position to the target. This enables the fish to readily judge the absolute sizes of objects from completely novel views.

Recognition of Apertures in Overhead Transparent Barriers by Leopard Frogs

Anurans have independent systems for detecting moving stimuli and stationary opaque objects. We have discovered that leopard frogs will also orient to, and spontaneously and accurately jump through, circular apertures in overhead transparent covers. When given a choice between one large aperture of 3.8 cm diameter, and three apertures of smaller but equal diameter (2.5 cm diameter, 1.9 cm diameter, or 1.3 cm diameter) they choose the larger diameter aperture at a frequency (64, 87 and 97%, respectively) that is statistically greater than chance. In only 1 of 255 attempts was there a jump to the overhead cover that was not directed at an aperture. Atectal frogs are still able to detect and jump accurately through transparent apertures. Frogs cannot distinguish between two apertures of equal diameter if one aperture is covered with clear plastic with high light transmissibility (92% of transmissibility of air). However, if the plastic covering of the aperture has a residue which reduces light through the cover from 92 to 87% of the transmissibility of air, frogs will jump to the uncovered aperture at a frequency that is statistically greater than chance. Our results show that leopard frogs have an extremely well developed ability to detect overhead apertures just as they can vertical obstacles. They are able to jump towards such openings with a small margin of error independent of the tectal visual system.

The three-dimensional word of a toad

The path chosen by a toad ( Bufo viridis ) was examined when a barrier was placed between it and its food. The toad plans its route before it sets out, and so its route reveals something of its perceptions of its immediate environment. If there is a gap in the barrier wider than 3 cm (the width of the toad’s head), the toad aims for the gap, if not, it detours round the end of the barrier. Provided that the toad starts no further than 20 cm from the fence, its choice between detour and direct approach is governed by the physical size of the gap and not by the angle that the gap subtends on the retina, indicating that the toad has size constancy. Similarly, it is less likely to detour round wide barriers than narrow ones and again its behaviour is controlled by the real width of the barrier and not by the retinal subtense.

Responses of retinal and tectal neurons in non-paralyzed toads Bufo bufo and B. marinus to the real size versus angular size of objects moved at variable distance

Toads judge the real size of moving visual objects during prey-catching. But neither ganglion cells of the retinotectal projection nor tectal neurons showed a comparable phenomenon in muscle paralyzed toads. In non-paralyzed toads, however, tectal neurons, unlike neurons, displayed a sensitivity to the real size of an object moving at variable distance to the animal. Interestingly, this property was obtained both in monocular T5 smallfield neurons and in most of the investigated T4 widefield neurons.

Increases of excitatory receptive fields of retinal ganglion cells in common toads under apomorphine are not associated with size preference in prey-snapping

After systemic administration of the dopamine agonist apomorphine (APO) in common toads Bufo bufo spinosus, the excitatory receptive fields of retinal R2 and R3 ganglion cells doubled their size and the response activities of these neurons to moving visual stimuli were generally increased. Comparable investigations of the stimulus-response relationships in the snapping behavior of toads pre- and post-APO showed a general increase in the snapping responses, but no shift in the preferred prey size. This is consistent with the current concept, suggesting that size evaluation in amphibians does not rely on retinal ganglion cell output but on interacting networks beyond the retina.

Do frogs use retinal elevation to measure the distance of a barrier?

Grass frogs, Rana pipiens, will detour around a barrier to reach prey on the other side. However, if the distance between prey and barrier is short, frogs attempt to push through the barrier and reach the prey directly. The relationship between the probability of detouring and the distance between prey and barrier is the same whether the frog's starting position is 4 cm or 8 cm from the barrier. This suggests that frogs measure the absolute separation between the two objects. To discover whether the retinal elevation of the bottom of the barrier contributes to measuring this distance, the relationship between the frequency of detouring and barrier-prey distance was examined in several experiments in which the retinal position of the bottom of the barrier was manipulated. No evidence was obtained that the barrier's retinal elevation helps in gauging distance. On the other hand, retinal elevation influences strongly how far a frog lunges to reach its prey. It is suggested that different cues to distance are applied to the two classes of object because, under natural circumstances, it is difficult to judge where a barrier emerges from the ground. A barrier may be hard to detect below the horizon because of the low contrast between it and the ground, or because vegetation and ground litter mask where the barrier meets the ground. In contrast, the prey's movements make it easily detectable against a stationary background and the prey's short height means that partial occlusion will have little effect on its apparent vertical position in the visual field.

Size constancy in goldfish (Carassius auratus)

This study examines whether fish are able to judge the real size of objects taking distance into account, or if size judgements are based purely on the angle objects subtend on the retina. Goldfish were trained to discriminate between 2 targets which differed from one another only in their diameters (5 cm and 10 cm), presented initially equidistantly from the fish. A choice of the correct target was rewarded either by food or by access to other fish. Following successful training, the targets were shifted so that they both subtended the same visual angle. The fish continued to choose the target to which they had been trained, showing that they display size constancy. This ability was present in both monocular and binocular animals.

A neural model of interactions subserving prey-predator discrimination and size preference in anuran amphibia

The model described is an extension of a previous model of the optic tectum (Arbib & Lara, 1982; Lara, Arbib & Cromarty, 1982; Lara & Arbib, 1982) and takes into consideration anatomical, physiological and behavioral studies in anurans, as well as earlier modelling efforts (Ewert & Von Seelen, 1974; Didday, 1976). Computer simulations were conducted to analyze how interactions among retina, optic tectum and pretectum may give frogs and toads the ability to discriminate between prey and predator stimuli. Results from simulations have allowed us to reproduce empirical observations, to suggest new experiments, and to postulate what neural mechanisms might be involved in some phenomena related to prey-catching orienting behavior, with direction invariance of prey-predator recognition being a consequence of tectal architecture, and size preference and response latency depending on the motivational state of the animal.

A global model of the neural mechanisms responsible for visuomotor coordination in toads

A model of how the nervous system of toads processes visual information to control motor behaviour is proposed. The problem of visuomotor coordination in toads is studied through the integration of two different approaches: a top-down approach through schema theory developed in the studies of cognitive psychology, artificial intelligence and brain theory; and a bottom-up approach through the integration of physiological, anatomical, ethological, and neural modelling. The model proposes that visual information is processed in a parallel distributed way through different brain layers whose interaction defines the proper motor response for that specific situation. It is postulated that visual processing of information is organized into main schemata, which set the goal to be attained and, depending on the specific circumstances of the animal, activates different brain layers; the main schemata may use other schemata to solve a specific subproblem to reach the schemata, and a programme of schema co-ordination. With this model we have simulated how toads plan their route to reach a prey or the route to go away form a predator, depending on the state of their three-dimensional world. The model postulates specific hypotheses that could be tested experimentally on the processing of information in the toad's nervous system.

Eye accommodation during prey capture behaviour in salamanders (Salamandra salamandra L.)

The eyes of fire salamanders were examined refractometrically before, during and after prey capture behaviour. It was ascertained that the refraction makes a myopic shift before the salamander turns to the prey. This means that fire salamanders use accommodation for distance estimation.

Small-field, binocular neurons in the superficial layers of the frog optic tectum

Binocular neurons with receptive fields about 5 degrees across were recorded just beneath the pia. Most of them responded to dark stimuli in the lower half of their receptive field and to light stimuli above. There was almost no vertical disparity between the left and right fields and the modal value of the horizontal disparity of the population of cells was 1.7 degrees. Because frogs do not verge their eyes it is possible to calculate at what distance the receptive fields through the two eyes are superimposed. This calculation suggests that the neurons are tuned to detect features in the external world about 50 cm away. This is too far for the neurons to be involved in the frog's everyday distance vision. It is more likely that they are concerned with assessing the vertical position of a horizontal surface.

New methods for analysis of vision in the gerbil

The visual behavior of gerbils has been studied by adapting natural behaviors such as food pursuit, aperture detection, barrier negotiation and ladder climbing. Methods are described for measuring detection and discrimination thresholds using visual orienting tasks. Furthermore, studies of size-distance constancy, anticipation of a moving target's trajectory, and optimal planning of barrier detours show the usefulness of film records for quantitative analysis of complex perceptual abilities. Finally, preliminary studies of gerbils following ablation of primary visual cortex indicate that some visuomotor abilities are cortically dependent.