Cache consensus: rapid object sorting by a robotic swarm

We present a new method which allows a swarm of robots to sort arbitrarily arranged objects into homogeneous clusters. In the ideal case, a distributed robotic sorting method should establish a single homogeneous cluster for each object type. This can be achieved with existing methods, but the rate of convergence is considered too slow for real-world application. Previous research on distributed robotic sorting is typified by randomised movement with a pick-up/deposit behaviour that is a probabilistic function of local object density. We investigate whether the ability of each robot to localise and return to remembered places can improve distributed sorting performance. In our method, each robot maintains a cache point for each object type. Upon collecting an object, it returns to add this object to the cluster surrounding the cache point. Similar to previous biologically inspired work on distributed sorting, no explicit communication between robots is implemented. However, the robots can still come to a consensus on the best cache for each object type by observing clusters and comparing their sizes with remembered cache sizes. We refer to this method as cache consensus. Our results indicate that incorporating this localisation capability enables a significant improvement in the rate of convergence. We present experimental results using a realistic simulation of our targeted robotic platform. A subset of these experiments is also validated on physical robots.     

How bees detect coloured targets using different regions of their compound eyes

Honeybees Apis mellifera detect coloured targets presented to the frontal region of their compound eyes using their colour vision system at larger visual angles (α > 15°), and an achromatic visual system based on the long-wave photoreceptor type at smaller visual angles (5° < α < 15°). Here we examine the capability of the dorsal, ventral and frontal regions of the eye for colour detection. The minimum visual angle α_min at which the bees detect a stimulus providing both chromatic contrast and receptor-specific contrasts to the three receptor types varies for the different regions of the eye: 7.1 ± 0.5° for the ventral region, 8.2 ± 0.6° for the dorsal region and 4.0 ± 0.5° for the frontal region. Flight trajectories show that when the target was presented in the horizontal plane, bees used only the ventral region of their eyes to make their choices. When the targets appeared dorsally, bees used the frontodorsal region. This finding suggests that pure dorsal detection of coloured targets is difficult in this context. Furthermore, α_min in the ventral plane depends on receptor-specific contrasts. The absence of S-receptor contrast does not affect the performance (α_min = 5.9 ± 0.5°), whilst the absence of M- and L-receptor contrast significantly impairs the detection task. Minimal visual angles of 10.3 ± 0.9° and 17.6 ± 3°, respectively, are obtained in these cases. Thus, as for many visual tasks, the compound eye of the honeybee shows a regionalisation of colour detection that might be related to peripheral or central specialisations. Accepted: 28 September 1999     

Chromosome aberrations induced in human bone-marrow cells by therapeutical local irradiation. Time and dose relationships

Bone-marrow cell chromosome aberrations were studied in 14 unirradiated control patients with malignant tumors and 132 comparable patients after a single localised exposure to ⁶⁰Co γ irradiation. In control patients the chromosome break frequency was 0.01 per cell and aberrant cell frequency was 1.1%.     

Receptor noise as a determinant of colour thresholds.

Inferences about mechanisms at one particular stage of a visual pathway may be made from psychophysical thresholds only if the noise at the stage in question dominates that in the others. Spectral sensitivities, measured under bright conditions, for di-, tri-, and tetrachromatic eyes from a range of animals can be modelled by assuming that thresholds are set by colour opponency mechanisms whose performance is limited by photoreceptor noise, the achromatic signal being disregarded. Noise in the opponency channels themselves is therefore not statistically independent, and it is not possible to infer anything more about the channels from psychophysical thresholds. As well as giving insight into mechanisms of vision, the model predicts the performance of colour vision in animals where physiological and anatomical data on the eye are available, but there are no direct measurements of perceptual thresholds. The model, therefore, is widely applicable to comparative studies of eye design and visual ecology.