The brain is apt to sort sensory stimuli into discrete perceptual categories, but the neuronal activity behind this capability has been unclear. Here, the problem has been investigated by presenting zebrafish with different concentrations or types of odours. The results show that the activity of neuronal populations in the olfactory bulb is largely insensitive to changes in odour concentration, but that morphing one odour into another produces abrupt transitions between odour representations

Ultraluminous infrared galaxies are among the most luminous objects in the local Universe and are thought to be powered by intense star formation. In these objects, the rotational lines of molecular hydrogen (H2) observed at mid-infrared wavelengths are not affected by dust obscuration, but the source of excitation has been unknown. Here it is found that H2 emission originates outside the obscured regions; it is proposed that H2 emission traces shocks in the surrounding material that are excited by interactions with nearby galaxies