Honeybee vision : analysis of pattern orientation

Honeybees (Apis mellifera) learn the orientation of edges or bars in order to recognise a visual pattern. A number of findings published in recent years suggest that the bee's perception of pattern orientation is independent of other visual cues and is supported by a distinct visual subsystem. The main objective of this thesis is to study the characteristics of this subsystem in detail. A series of behavioural experiments was carried out to investigate both the chromatic and temporal properties of the bee's orientation analysis. Measurements of orientation discrimination using gratings offering different magnitudes of contrast to the bee's three receptor. types revealed that the bee's orientation analysis relies solely on input from the green receptor channel and is therefore "colour-blind" (Chapter 4). Experiments using moving gratings with various velocities and spatial frequencies revealed that the bees discriminate orientation presented at contrast frequencies of up to 50 Hz, irrespective of velocity or spatial frequency. Thus, orientation discrimination appears to be mediated by a neural subsystem that is sensitive to contrast frequency rather than speed (Chapter 5). Three further studies were carried out to examine the role of orientation analysis in honeybee pattern recognition in different behavioural contexts. It was found that the orientation of a pattern is always learned when the pattern is presented in the frontal visual field, regardless of whether it is useful for the discrimination task at hand or not. Furthermore, in recognising and discriminating oriented patterns, the orientation of the pattern seems to be more important than its intensity distribution, when the two cues are available simultaneously (Chapter 6). When information on pattern orientation is confined to a small portion of the frontal visual field, orientation is learned irrespective of its location. However, if the bees are free to move in front of large training patterns, they tend to learn only the orientation offered in the ventral portion of these patterns, ie. in the area below the bees' goal in the centre of the pattern (Chapter 7). If pattern orientation is presented in the lateral visual field, the bees do not associate it with the food source, but seem to treat it as a type of landmark. Interocular transfer of a laterally learned orientation does not seem to occur. This is also true for laterally learned colours. Orientation is not discriminated if it is presented in the dorsal or ventral visual field, while colour is not discriminated when presented in the dorsal visual field (Chapter 8). Electrophysiological data was obtained from a number of neurons in the midbrain of the bee, and possible neural substrates for the bee's orientation analysis are discussed. While there are several intriguing parallels between the cellular responses and the behaviour, it is difficult to find a complete explanation for the behaviour in terms of the response characteristics of the neurons studied so far (Chapter 9). In addition to the studies on orientation analysis, a computer simulation of the bee's optics is described. This simulation is based on anatomical and physiological data from the literature and was developed as a tool for the design of experiments in bee vision (Chapter 2).