Studying visually-triggered behaviors and the excitability of underlying neurons during diurnal periods with high/low spontaneous activity in crabs

Daily fluctuations in behavioral activity enable organisms to optimize their interactions with the environment. In the semiterrestrial crab Neohelice granulata, visual processing is essential for predator/prey detection, navigation, etc. Studying spontaneous locomotor rhythms under natural light conditions we described a prolonged rest phase during midday/afternoon and high activity during crepuscular hours/night. Hence, during light hours we can find both periods of high activity and rest. It is not known if factors regulating the activity may also modulate visual pathways and if so, at what level. To test this, two visually-guided behaviors were examined: optomotor responses to different contrasts stimuli and escape responses elicited by a visual danger stimulus. We compared animals tested during periods of high (07:00–10:00 h) vs low (14:00–17:00 h) spontaneous activity under comparable daylight conditions, thereby dissociating activity state from visual responses to changes in illumination. In parallel, intracellular recordings from identified visual neurons belonging to the different optic neuropils taken during both time windows were analyzed considering neuronal excitability and evoked responses. This work provides new insights into the mechanisms by which the nervous system adjusts sensory processing to the physiological and ecological demands of the organism, bridging the activity of identified visual neurons with visually-guided behavior in the intact animal.