Infrared detection is not phototaxis

[See ref 13, ref 16, ref 17 ]

Cells reach out to a second light source while exposed to the high intensity of the first that they reached

One may try to explain the cellular detection of infrared light sources by simple phototaxis. In other words, it seems possible that the test cells attracted to the light extended the new surface projections towards the area of highest intensity. Cellular 'vision', on the other hand, would require a considerable more complex response, namely the ability of the cells to make out several light sources individually.

In order to distinguish between these possibilities we offered the cells 2 light spots of equal intensity. If the attraction of the test cells was merely phototactic they would either steer exactly between the light spots while being attracted with equal strength to both. Alternatively, should they ever stray off the midline they would experience increasing intensity from the closer light spot and approach it while ignoring the other which appears the weaker the closer the cell approaches the first. In contrast, we recorded many video sequences which showed that the cells were able to extend first to one and subsequently to the other. The following 2 live cell recordings show examples of this finding.

(The illustration is animated.Click here for a minimal strip of frames.)

(The illustration is animated.Click here for a minimal strip of frames.)
To download a better quality video of the above sequences Click here
Based on these observations that cells can identify 2 (and presumably more) light sources individually, we feel justified to use the term cellular infrared 'vision' rather than the term infrared phototaxis. There are other reasons that suggest that the seeking out of the pulsating near-infrared light sources by the cells is far from an automatic response to the light intensity. Several video sequences may illustrate this point, including cases where cells do not take the shortest distance to the light source.

Significance for cell intelligence:

Cells may be able to generate and read optical 'images' of their surrounding.
Optical images map the direction of many different light sources around the eye regardless of their intensity. The mapping process may not require more than a lens or a pair of centrioles, but responding to the map requires considerable data processing cpacities. Since cells could extend to a weak light source while exposed to a much stronger source in a different direction they fulfilled a major criterion of intelligent systems that can generate and read optical images of their surrounding.