Head movements can give cues as to the depth of different objects and there are other cues that indicate depth such as relative brightness, etc.. (northwestanimaleye.com)
The use of DT40 as a protein evolution platform was demonstrated with the targeted improvement of enhanced green fluorescent protein (eGFP) producing variants displaying relative brightness up to 2.5 fold higher than the precursor eGFP, but without a spectral shift [12]. (journals.plos.org)
Although relative brightness was expected to be stronger, taking into account that Amrose excitation and emission in whole - body imaging is taking place beyond 630 nm there is a significant advantage over the shorter wavelength but brighter orange - red proteins. (journals.plos.org)