Circadian Responses to Light-Flash Exposure: Conceptualization and New Data Guiding Future Directions

A growing number of studies document circadian phase-shifting after exposure to millisecond light flashes. When strung together by intervening periods of darkness, these stimuli evoke pacemaker responses rivaling or outmatching those created by steady luminance, suggesting that the circadian system's relationship to light can be contextualized outside the principle of simple dose-dependence. In the current review, we present a brief chronology of this work. We then develop a conceptual model around it that attempts to relate the circadian effects of flashes to a natural integrative process the pacemaker uses to intermittently sample the photic information available at dawn and dusk. Presumably, these snapshots are employed as building blocks in the construction of a coherent representation of twilight the pacemaker consults to orient the next day's physiology (in that way, flash-resetting of pacemaker rhythms might be less an example of a circadian visual illusion and more an example of the kinds of gestalt inferences that the image-forming system routinely makes when identifying objects within the visual field; i.e., closure). We conclude our review with a discussion on the role of cones in the pacemaker's twilight predictions, providing new electrophysiological data suggesting that classical photoreceptors—but not melanopsin—are necessary for millisecond, intermediate-intensity flash responses in ipRGCs (intrinsically photosensitive retinal ganglion cells). Future investigations are necessary to confirm this “Cone Sentinel Model” of circadian flash-integration and twilight-prediction, and to further define the contribution of cones vs. rods in transducing pacemaker flash signals.

Further Observations on Dipteran Flight: Details of the Mechanism

We took high-intensity flash photographs of the wing base during tethered flight of Lucilia sericata. These show that the radial stop and the pleural wing process separate during the upper part of the wing beat and then mate together on the downstroke. Using tungsten needle probes attached to wire strain gauges, we measured the movements of the parascutal shelf (PSS), of the lateral and dorsal scutum, and of the scutellum during tethered flight. We made a detailed study of the wing basesclerites and associated muscles in order to answer criticisms of our original model (Miyan & Ewing, 1985a,b). The PSS rotates upwards about its hinge with the lateral scutum at the start of the downstroke. The dorsal scutum and medial edge (at the hinge) of the PSS are held down, presumably as a result of dorsoventral muscle activity. As the downstroke progresses, the whole of the PSS and dorsal scutum are lifted together suggesting the action of a locking mechanism. At the bottom of the downstroke there is an opposite, downward rotation of the shelf about its hinge that follows the start of downward scutal movement at the beginning of the upstroke. This is followed by downward movement of the whole PSS and scutum. Movements of the lateral scutum exactly follow scutellar and lateral PSS movements and are probably dictated by the articulation. Scanning electron micrographs illustrate the probable components of the wing base-PSS locking mechanism. Rotation of the first axillary sclerite, brought about by the rising scutellar lever arm, results in mating of its medial arm with a recess in the PSS. This prevents further rotation of the PSS which is now held at two points by the sclerite and is lifted by further movement of the lever. There is no evidence that the third axillary muscles act as wing pronators. Scanning electron micrographs show a mechanism that maintains the line of action of the muscles on the posterior edge of the wing.