Showing papers by "Hermann Wagner published in 2000"

Horizontal-Disparity Tuning of Neurons in the Visual Forebrain of the Behaving Barn Owl

Abstract: Stereovision plays a major role in depth perception of animals having frontally-oriented eyes, most notably primates, cats, and owls. Neuronal mechanisms of disparity sensitivity have only been investigated in anesthetized owls so far. In the current study, responses of 160 visual Wulst neurons to static random-dot stereograms (RDS) were recorded via radiotelemetry in awake, fixating barn owls. The majority of neurons (76%) discharged significantly as a function of horizontal disparity in RDS. The distribution of preferred disparities mirrored the behaviorally relevant range of horizontal disparities that owls can exploit for depth vision. Most tuning profiles displayed periodic modulation and could well be fitted with a Gabor function as expected if disparity detectors were implemented according to the disparity energy model. Corresponding to this observation, a continuum of tuning profiles was observed rather than discrete categories. To assess a possible clustering of neurons with similar disparity-tuning properties, single units, and multi-unit activity recorded at individual recording sites were compared. Only a minority of neurons were clustered according to their disparity-tuning properties, suggesting that neurons in the visual Wulst are not organized into columns by preferred disparity. To assess whether variable vergence eye movements influenced tuning data, we correlated tuning peak positions on a trial-by-trial basis for units that were recorded simultaneously. The general lack of significant correlation between single-trial peak positions of simultaneously recorded units indicated that vergence, if at all, had only a minor influence on the data. Our study emphasizes the significance of visual Wulst neurons in analyzing stereoscopic depth information and introduces the barn owl as a second model system to study stereopsis in awake, behaving animals.

A candidate pathway for a visual instructional signal to the barn owl's auditory system.

Abstract: Many organisms use multimodal maps to generate coherent neuronal representations that allow adequate responses to stimuli that excite several sensory modalities. During ontogeny of these maps, one modality typically acts as the dominant system the other modalities are aligned to. A well studied model for the alignment of sensory maps is the calibration of the auditory space map by the visual system in the optic tectum of the barn owl. However, a projection from the optic tectum to the site of plasticity in the auditory pathway that could deliver an instructive signal has not been found so far. We have analyzed the development of the connectivity between the bimodal (visual and auditory) map of space in the barn owl’s optic tectum and the auditory space map in the inferior colliculus with tracing methods and intracellular fills. Neurons in the tectal stratum griseum centrale were found to be suited to deliver an alignment signal from the visual midbrain to the auditory pathway. These neurons are presumably part of the efferent tectal projection pathway that mediates head saccades. The implications of a sensory alignment signal possibly being delivered by a (pre)motor command pathway are discussed.

Disparity tuning as simulated by a neural net.

Abstract: Previous research has suggested that the processing of binocular disparity in complex cells may be described with an energy formalism The energy formalism allows for a representation of disparity by differences in the position or in the phase of monocular receptive subfields of binocular cells, or by combination of these two types We studied the coding of disparities with an approach complementary to previous algorithmic investigations Since realization of these representations is probably not genetically determined but learned during ontogeny, we used backpropagation networks to study which of these three possibilities were realized within neural nets Three types of networks were trained with noise patterns in analogy to the three types of energy models The networks learned the task and generalized to untrained correlated noise pattern input Outputs were broadly tuned to spatial frequency and did not respond to anti-correlated noise patterns Although the energy model was not explicitly implemented, we could analyze the outputs of the networks using predictions of the energy formalism After learning was completed, the model neurons preferred position shifts over phase shifts in representing disparity We discuss the general meaning of these findings and the correspondences and deviations between the energy model, V1 neurons, and our networks

On the perceptual identity of depth vision in the owl

Abstract: Whether owls can be trained to discriminate computer generated random-dot patterns purely on the basis of their figure-ground organisation is an open question. Here I investigate this question by (A) providing an operant shaping technique that prepared four barn owls for behavioural measurements employing a remote computer display (Experiment I) and (B) using the random-dot technique together with tests of discrimination-transfer (Experiment II). Tested were two functions of vision that are thought to be involved in figure-ground segregation: (1) texture perception and (2) motion perception. The computer displays consisted of patterns of randomly positioned black and grey dots, that differed either in their position, textural organisation, or common fate (motion). The rapidness with which the owls learned to discriminate varied between the different individuals and depended heavily on the visual function tested. The here described experiments have been instructive in four ways. First, Experiment I has provided an adequate operant shaping method that prepared barn owls to be behaviourally tested with random-dot patterns that were displayed on a CRT screen. Second, Experiment II provided behavioural data that clearly demonstrated that owls can abstract figure from ground in computer displays of random-dot patterns. Third, signal detection analysis of the measured psychometric curves revealed a consistent response criterion for stimuli above or near threshold level, indicating a high degree of stimulus control. Fourth, the transfer tests provided evidence for generalisation discrimination in owls. 40 On the Perceptual Identity of Depth Vision in the owl