Daily Rhythms in Sensory Sensitivity
If almost every biochemical, physiological and behavioral function exhibits daily cycles, it is no surprise that such rhythms have been discovered in sensory sensitivity of many sensory modalities – vision, hearing, smell, etc. Even the last lone holdout – electeroreception – has recently been shown to exhibit daily rhythms of sensitivity.
But the phrase “sensory sensitivity” is vague. What it means is that there is a daily switch in the power of detection versus power of resolution. In most senses, the two are incompatible. If the sensory organ is tuned to detect the lowest possible amount of energy, it is incapable of maximal resolution and vice versa. What has been seen so far in all the sense studied to date is that the power of detection is maximal during the time the animal is inactive (e.g., quiescent or asleep), while the power of fine resolution is maximal when the animal is active.
This makes sense from an adaptive perspective. When you are out and about looking for food or mates and on a lookout for enemies, it is a good idea to be able to finely discrimate the details of the environment, be it visual, auditory, olfactory, geomnagnetic, etc. At the same time, lowering the treshold for detection is useful when you are asleep and need to respond by sudden awakening to possible cues of danger in the environment – there is no immediate need for fine discrimination, but there is a need to overpower the sleep mechanism and wake the animal up quickly.
The difference between a pacemaker and a clock
You may already be aware that I am quite obsessed with the concept of circadian organization, i.e., the idea that there is not just one brain pacemaker that times everything, buit an entire circadian system – an organ system on par with all the others you learn about in school (circulatory, endocrine, reproductive, immune, etc.) – in which every cell in the body is a part of it, with different cells and tissues playing different roles. Some tissues are pacemakers (The Master Clocks), while others are peripheral clocks (The Slave Clocks). The criteria for something to be considered a pacemakers are: a) rhytmicity in vivo and in vitro, b) loss of overt rhythmicity of the organism if the pacemaker is lesioned, and c) transplanation of the tissue carries period and phase of the transplant and forces it onto the host.