Masking of locomotor activity in hamsters

The inhibition of locomotion by light (masking) was investigated in Syrian hamsters. When 1-h pulses of light were presented in the early night, activ ity was strongly suppressed by irradiances of about 1 1x or greater. Ultrad ian light-dark cycles were used as another way to study masking. Hamsters w ere unable to entrain to 3.5:3.5-h light-dark cycles, thus permitting the m asking and the entraining effects of light to be distinguished. Light had g reater suppressive effects on activity in home cages than on activity in no vel running wheels. Moreover, in home cages activity remained very low for about 30 min after lights were turned off. Post-pulse suppression of activi ty was not simply a consequence of reduced running, as shown by experiments in which running was temporarily prevented by locking the wheels. A phase response curve for masking was obtained by placing hamsters in novel wheels for 3-h periods at various times throughout their circadian cycles, and th en superimposing a 30-min light pulse. The suppressive effect of light was maximal around the onset of activity, which normally coincides with dusk in hamsters. This may have adaptive value in limiting foraging to the hours o f darkness.