Mechanisms of Behavioral Modulation

Transcription

1 Feb 19: Rhythms Mechanisms of Behavioral Modulation "Global" modulating mechanisms: act on diverse neural subsystems, changing threshold, selectivity, or strength of many responses EXAMPLES: hormones and biological clocks "Local" modulating mechanisms: act to change specific responses to specific stimuli EXAMPLE: learning

2 Feb 19: Rhythms--rhythms introduced Rhythmic Modulation of Behavior Animals need to modulate behavior in synchrony with environmental cycles Daily light-dark cycle Daily tidal cycle Lunar cycle Annual cycles: temperature/rain/food etc. Plenty of evidence that animals respond to these cycles

3 Feb 19: Rhythms--hypotheses How Do Animals Synchronize Behavior With Environmental Cycles? Two hypotheses: Exogenous cues: animals simply track environmental cycles themselves Endogenous cues: internal oscillators that have evolved to be more or less synchronized with particular environmental cycles Lots of evidence for endogenous oscillators Describing rhythms Continuous variable (e.g., body temperature) Discrete variable (e.g., wheel turn, chirp) Time Time

4 Feb 19: Rhythms: evidence for endogenous rhythms Evidence for endogenous daily rhythms: deprivation experiment Maintain animal in constant conditions, isolated from environmental cues that it might track If behavioral rhythms persist, then they must be based on endogenous oscillator House sparrow activity rhythm Flying squirrels LD LL τ = 24.5 h τ = 23.5 h τ =period length in constant conditions, when animal is free-running Free-running daily rhythms are typically "circadian

5 Feb 19: Rhythms--properties of circadian rhythms Properties of circadian rhythms Stability over time Compensate for temperature Become entrained on environmental cycles Phase shifts (crucial to entrainment) "Zeitgebers" (or "time-setters")--stimuli triggering entrainment or phase-shifting Activity (sleep) Cricket: rhythm in chirping rate Free-runs in constant light Becomes entrained on light-dark cycle Light is Zeitgeber (Alcock Fig. 6.5) Body temperature Splitting of free-running rhythms of a person in a cave May be multiple oscillators controlling different behaviors (evidence from rhythm splitting)

6 Feb 19: Rhythms--Longer-period rhythms Rhythms with Periods Longer Than a Day Circalunar and tidal rhythms Fiddler crabs in constant conditions show 2 peaks of activity every 24 h corresponds to low tide, when they normally become active Circannual rhythms 24 h 12 h 1 yr Ground squirrels in constant darkness go into hibernation about once a year (Alcock Fig. 6.11

7 Feb 19: Rhythms: Other How questions Circadian rhythms: other questions about mechanisms Where in brain? SCN Hamster brain Cockroach brain Optic lobe Lesions of particular structures abolish circadian rhythms Is output from clock carried by neural or humoral (diffusing) factors? Transplant clock from one animal into a lesioned animal This restores rhythmicity in the lesioned animal Clock may strongly influence hormone production

8 Feb 19: Rhythms: Other How questions Circadian rhythms: other questions about mechanisms Genetic/molecular basis of clock: evidence for single genes of large effect The "period" gene in insects (note: same gene influences rhythms on multiple time scales) The "clock" gene in hamsters and mice Alcock Fig. 2.3 Alcock Fig. 6.8

9 Feb 19: Rhythms: Other How questions Mechanisms: using the output of the clock 2 pm in NY Learning time of day Bees are trained to find food at 2 pm in NY They are flown overnight to CA They search at 2 pm NY time (but gradually shift over successive days) 2 pm in CA Thus, bees are selecting flight time based on endogenous signal of training time, not exogenous cues Also, signal from clock must be integrated with information about food (e.g., location, shape, odor, etc.) Time sense can also be integrated with information about sun s position to produce sun compass PST EST TIME