Visual Pattern Recognition in Animals

Abstract

Some two thousand million years ago living organisms initiated the invention of means for detecting optical signals to be reacted to by purposeful actions. This decisive evolutionary step already took place at the primitive organizational level of the bacteria, as may be inferred from present life. In collecting light energy with pigments, some of these micro-organisms swim about, “looking” for properly illuminated places. Rhodospirillum, for instance, driven on its spiral course by rotary flagellar action on both of its terminal poles, may suddenly cross the boundary into a shadow. At this instant the sensitive basal area of the leading flagella is suddenly darkened. This particular optical signal sets the switch for reversing the flagellar propulsion, and from then on the opposite pole is leading (Fig. 1). This, the most primitive photosensory system known, is selective as well as adaptive in only responding to the fast decline from average light intensity or to the sudden rise towards damaging intensities. Chromatium’s manoeuvre to avoid darkness is still less efficient than that of Rhodospirillum. This unipolar micro-organism first jumps backward after passing into a shaded area, then it remains immobile for a short period, while Brownian movement and micro-eddies push it about randomly. When it resumes swimming, it may be fortunate enough to remain in the illuminated area whence it came, otherwise the whole procedure has to be repeated after another penetration into the dark area (Fig. 1) (references: Clayton, 1964).