Size preferences for prey-catching in frogs: relationship to motivational state

Auditory responses in a rodent model of Attention Deficit Hyperactivity Disorder

A central component of Attention Deficit Hyperactivity Disorder (ADHD) is increased distractibility in response to visual and auditory stimuli, which is linked to the superior colliculus (SC). Furthermore, there is now mounting evidence of altered collicular functioning in ADHD and it is proposed that a hyper-responsive SC could mediate symptoms of ADHD, including distractibility. In the present study we conducted a systematic characterisation of the intermediate and deep layers of the SC in the most commonly used and well-validated model of ADHD, the spontaneously hypertensive rat (SHR), building on prior work showing increased distractible behaviour in this strain using visual distractors. We examined collicular-dependent orienting behaviour, local field potential (LFP) and multiunit activity (MUA) in response to auditory stimuli in the anaesthetised rat, and morphological measures, in the SHR in comparison to the Wistar Kyoto (WKY) and Wistar (WIS). We found no evidence of increased distractibility in the behavioural data but suggest that this may arise due to cochlear hearing loss in the SHR. Furthermore, the electrophysiology data indicate that the SC in the SHR may still be hyper-responsive, normalising the amplitude of auditory responses that would otherwise be reduced due to the hearing impairment. The morphological measures of collicular volume, cell density and ratios did not indicate this potential hyper-responsiveness had a basis at the structural level examined. These findings have implications for future use of the SHR in auditory processing studies and may represent a limitation to the validity of this animal model.

Leopard frog priorities in choosing between prey at different locations

Frogs are able to respond to a prey stimulus throughout their 360° ground-level visual field as well as in the superior visual field. We compared the likelihood of frogs choosing between a more nasally located, ground-level prey versus a more temporally located ground-level prey, when the prey at the nasal location is further away from the frog. Two crickets were presented simultaneously at 9 pairs of angles that included both crickets in the binocular visual field, both crickets in the monocular visual field, or one cricket in the binocular field and one in the monocular field. Frogs chose the more nasally located prey at least 71% of the time when the more temporal prey was in the monocular field; and 64% of the time when both prey were in the binocular field. Frogs tended to choose the more nasally located prey, even though it takes the frog longer to reach the prey. In addition, when given a choice between a prey located at ground level versus a prey located in the superior field, frogs tend to choose the prey at ground-level. These results suggest that there is a neural mechanism that biases frogs' responses to prey stimuli.

Neuroethology of releasing mechanisms: Prey-catching in toads

“Sign stimuli” elicit specific patterns of behavior when an organism's motivation is appropriate. In the toad, visually released prey-catching involves orienting toward the prey, approaching, fixating, and snapping. For these action patterns to be selected and released, the prey must be recognized and localized in space. Toads discriminate prey from nonprey by certain spatiotemporal stimulus features. The stimulus-response relations are mediated by innate releasing mechanisms (RMs) with recognition properties partly modifiable by experience. Striato-pretecto-tectal connectivity determines the RM's recognition and localization properties, whereas medialpallio-thalamo-tectal circuitry makes the system sensitive to changes in internal state and to prior history of exposure to stimuli. RMs encode the diverse stimulus conditions referring to the same prey object through different combinations of “specialized” tectal neurons, involving cells selectively tuned to prey features. The prey-selective neurons express the outcome of information processing in functional units consisting of interconnected cells. Excitatory and inhibitory interactions among feature-sensitive tectal and pretectal neurons specify the perceptual operations involved in distinguishing the prey from its background, selecting its features, and discriminating it from predators. Other connections indicate stimulus location. The results of these analyses are transmitted by specialized neurons projecting from the tectum to bulbar/spinal motor systems, providing a sensorimotor interface. Specific combinations of such projective neurons – mediating feature- and space-related messages – form “command releasing systems” that activate corresponding motor pattern generators for appropriate prey-catching action patterns.

The effect of stimulus features on the visual orienting behaviour of the salamander Plethodon jordani

The effects of the visual features of prey-like objects on the orienting behaviour of the salamander Plethodon jordani were studied. Two stimuli (cricket dummies, rectangles), moving in opposite directions, were presented simultaneously on a computer screen. They differed in size, contrast, velocity and movement pattern of the entire body or the body appendages. Size and velocity appeared to be the dominant features; shape was of lesser importance. Contrast and movement pattern were of intermediate importance and local motion of little importance. This rank order was the same when the probability of a response to the different stimuli was estimated by means of the maximum-likelihood method. Cluster analysis revealed that in all animals stimuli could be grouped into five clusters. Among individuals, the rank order of stimuli was similar for high- and low-ranking stimuli and varied for those of intermediate rank; stimuli could be grouped into 3–5 clusters. Our findings favour the view that, in amphibians, prey recognition is guided by a number of visual features acting either alone or in combination and depending on internal motivational or attentional states and individual experience.

Movie available on-line: http://www.biologists.com/JEB/movies/jeb3864.html.

Quantitative study of the tectally projecting retinal ganglion cells in the adult frog. II. Cell survival and functional recovery after optic nerve transection

It is known from previous work that ganglion cells disappear from the retina in significant numbers during optic nerve regeneration in the adult frog. In the present study, the population size of surviving ganglion cells that have returned axon terminals to the correct tectal loci was estimated by counts of retrogradely labeled cells in retina-flat-mounts after tectal injections of HRP. Bilaterally symmetric injections were delivered to allow comparison of the normal and affected retinas. The frogs studied had regenerated the left optic nerve and had visually guided behaviors initiated by the recovered eye (see below). The proportion of tectally projecting ganglion cells in the normal retinas and in retinas of normal frogs studied in parallel ranged from 83-86% (Singman and Scalia: J. Comp. Neurol. 302:792-809, 1991). In the affected retinas, the subpopulation of tectally projecting cells was reduced by 40-90% after regeneration, and the relative size of this subpopulation ranged from 67-86%. The optic tectum was injected unilaterally in one specimen, on the side ipsilateral to the regenerated (left) optic nerve. The HRP-labeled ganglion cells in the ipsilateral (left) retina accounted for only 0.8% of the surviving ganglion cells in this animal, whereas it was previously found that the ipsilateral tectally projecting ganglion cells normally amount to 0.9-2.3% (Singman and Scalia, op. cit.) In frogs recovering from transection of the left optic nerve, the frequency, latency, and accuracy of the prey-acquisition responses initiated by the recovering eye were compared with those initiated by the normal eye. Mealworms or lure dummies were used to stimulate prey acquisition. In one experiment, the stimuli were presented unilaterally in the monocular fields of frogs permitted to use both eyes. Prior to the fourteenth postoperative week, the affected eye initiated responses of abnormally long latency and low frequency. In contrast, responses initiated by the affected eye after 14 weeks appeared to be normal in all respects. In another experiment, the normal eye was sutured shut in some frogs recovering for at least 24 weeks and then the affected eye was retested. The affected eye was capable of consistently initiating brisk and accurate prey acquisition. In a final experiment, two stimuli were presented simultaneously in bilaterally symmetric regions of the monocular fields of frogs surviving at least 42 weeks. These fully recovered frogs showed no preference for using either the normal or the recovered eye. Despite severe loss of tectally projecting ganglion cells during optic nerve regeneration, frogs are capable of apparently normal visual responses in prey acquisition tests.

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.