Source Paper
Functional Neuroanatomy of the Noradrenergic Locus Coeruleus: Its Roles in the Regulation of Arousal and Autonomic Function Part II: Physiological and Pharmacological Manipulations and Pathological Alterations of Locus Coeruleus Activity in Humans
E. Samuels, E. Szabadi
Current Neuropharmacology • 2008
Pupillary Reflex Dilatation Assessment
Objective: Measures pupillary response to noxious stimuli as an autonomic indicator of locus coeruleus activity
Protocol Steps
Baseline pupil diameter measurement
Measure resting pupil diameter in baseline conditions to establish baseline autonomic state
Note: Baseline measurement is essential for detecting changes in pupil diameter following stimulation
View evidence from paper
“The effect on LC activity resulting from these manipulations can often be detected via the measurement of resting pupil diameter”
Deliver noxious stimulus
Present a noxious stimulus to the subject. For cold pressor test, immerse hand in painful cold water
Note: Noxious stimulation increases LC neurone activity which can be detected through pupillary changes
View evidence from paper
“Neurone activity within the LC is increased following the presentation of a noxious stimulus, measurable as an increase in electrical activity”
Measure pupillary reflex dilatation
Record pupil diameter changes in response to noxious stimulus. Pupillary reflex dilatation is observable as an increase in pupil diameter
Note: Pupillary reflex dilatation is the best studied autonomic response to noxious stimulation and reflects LC activation
View evidence from paper
“Pupillary reflex dilatation is observable in both experimental animals and human subjects”
Measure light reflex response
Deliver sudden light stimulus and measure pupillary light reflex amplitude to assess parasympathetic activity
Note: Light reflex attenuation is expected with increased LC activity due to enhanced noradrenergic inhibition of the Edinger-Westphal nucleus
View evidence from paper
“pupillary reflexes to sudden light stimuli, since the LC is so integral to pupillary control”
Record autonomic measures
Simultaneously measure blood pressure and heart rate changes during noxious stimulation as complementary autonomic indicators
Note: Cold pressor test leads to both increase in blood pressure and pupil dilatation
View evidence from paper
“A well-studied experimental paradigm of sympathetic activation by noxious stimulation in human subjects is the cold pressor test, evoked by a painful cold stimulus to the hand, which leads to both an increase in blood pressure and to pupil dilatation”