Asymmetry pays: visual lateralization improves discrimination success in pigeons

Abstract

Functional cerebral asymmetries, once thought to be exclusively human, are now accepted to be a widespread principle of brain organization in vertebrates [1]. The prevalence of lateralization makes it likely that it has some major advantage. Until now, however, conclusive evidence has been lacking. To analyze the relation between the extent of cerebral asymmetry and the degree of performance in visual foraging, we studied grain-grit discrimination success in pigeons, a species with a left hemisphere dominance for visual object processing [2,3]. The birds performed the task under left-eye, right-eye or binocular seeing conditions. In most animals, right-eye seeing was superior to left-eye seeing performance, and binocular performance was higher than each monocular level. The absolute difference between left- and right-eye levels was defined as a measure for the degree of visual asymmetry. Animals with higher asymmetries were more successful in discriminating grain from grit under binocular conditions. This shows that an increase in visual asymmetry enhances success in visually guided foraging. Possibly, asymmetries of the pigeon's visual system increase the computational speed of object recognition processes by concentrating them into one hemisphere while preventing the other side of the brain from initiating conflicting search sequences of its own.

Unihemispheric memory in pigeons-knowledge, the left hemisphere is reluctant to share

“…The results of the grain grit task make this explanation very unlikely. As usual for most pigeons right eye performance was higher than left eye performance in the grain grit task [11]. However, the left-eye scores of the animals of the present study are at the upper half of what pigeons usually achieve in this task [11].…”

“…Parallel to the monocular tests in the experimental chamber, a grain-grit discrimination was performed to analyze the individual degree of visual lateralization with an easy and established method [11]. The pigeons had to discriminate brownish-white common vetch (Vicia sativa) grains of approximately 2 )/3 mm axial length from small pebbles of varying size.…”

The impact of asymmetrical light input on cerebral hemispheric specialization and interhemispheric cooperation

“…These hemispheric specializations can be easily tested just by occluding one eye with an eye cap as the avian optic nerves cross completely. However, pigeons exhibit a quantitative advantage of using both eyes as compared with monocular performances 18 and this advantage increases with task complexity 19 . Accordingly, hemispheric-specific information is exchanged and integrated to acquire optimal visuomotor responses.…”

“…This functional organization shows that the presence of a corpus callosum-like forebrain commissure is not a prerequisite neither for the generation of hemispheric asymmetries nor for the ability to integrate information, as discussed in human research 13,20 . Binocular performance, in turn, correlates with the degree of visual lateralization 18 and strongly lateralized chicken demonstrate a better ability to carry out representational learning than less-lateralized individuals 21 . Visual asymmetries develop in response to asymmetrical light stimulation as indicated by the absence of asymmetrical differentiation processes in darkincubated birds 16,22,23 .…”

“Let There be Light!” pigeon eggs are regularly exposed to light during breeding

“…Lateralised animals benefit from being left-right different since birds with higher asymmetries were shown to be significantly more successful in discriminating grain from grit (Güntürkün et al, 2000). This means that a rise in asymmetry results in a concomitant rise of foraging efficiency.…”