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The pathways of interoceptive awareness
Khalsa, SS,* Rudrauf, D,* Feinstein, JS, and Tranel, D.
Supplementary Information
* These authors contributed equally to this work.
Participants
The participants were 11 healthy males and patient 1951 (aka ‘Roger’), a rare
neurological patient drawn from our patient registry (see Supplementary Table 1 for
complete demographics). Roger acquired focal bilateral brain damage following a severe
episode of herpes simplex encephalitis in 1980. Briefly, his brain injury involves
extensive bilateral damage to the insular, anterior cingulate and orbitofrontal cortices,
basal forebrain, hippocampus, amygdala and temporal poles (Fig. 1; for detailed
descriptions of Roger’s profile see1,2). All healthy comparison participants were screened
for the presence of any neurological, psychiatric, cardiac or respiratory disease. None of
the participants were smokers, and all participants demonstrated a normal 12 lead
electrocardiogram (EKG), as assessed by a board certified cardiologist or neurologist..
Tasks
Using a previously developed protocol3, participants rated the experience of
heartbeat sensations following bolus intravenous infusions of isoproterenol (a beta
adrenergic agonist similar to adrenaline) and saline. This protocol was adopted after
preliminary testing with Roger on another heartbeat detection task4,5 (modeled after6)
revealed chance heartbeat detection performance within the normal range (Roger’s
average detection rate = 53%; 14 age–matched male healthy comparison’s average
detection rate = 61%, SD 12%). After each bolus, participants turned a dial to track their
ongoing, moment–to–moment experience of the overall intensity of heartbeat sensations.
The dial could range from 0 (“normal, i.e., no change in intensity”) to 10 (“most ever”).
The dial was always set to zero at the beginning of each infusion, and participants were
specifically instructed to keep the dial at zero if they did not notice any increase in the
intensity of heartbeat sensations above baseline levels. Drug administration was
randomized and double blinded with the assistance of a skilled nurse. Participants were
informed ahead of time that they would be receiving both isoproterenol and saline
infusions at some point during the challenge, and were told what the isoproterenol
sensations might feel like (e.g., “you may notice your heart beating harder and faster,
and/or may feel an increase in your breathing sensations”). Each infusion period lasted
approximately 2 minutes. At the onset of each infusion period participants were verbally
notified (e.g., “infusion starting”). All infusions were delivered a minimum of 3.5
minutes apart.
Since Roger has an anterograde memory impairment, his task instructions were
repeated prior to each bolus. During extensive testing sessions in our laboratory, Roger
has demonstrated an intact ability to follow experimental instructions over prolonged
periods of time (on the order of several minutes and longer). The moment–to–moment
nature of the dial rating protocol employed in the current study was designed to alleviate
potential memory confounds associated with retrospective ratings7,8. Moreover, verbal
reports during all infusion periods indicate that Roger understood the task instructions
and was following them appropriately.
All participants received two challenges of isoproterenol and saline infusions.
During the first challenge, participants rated their experience of heartbeat sensations
immediately following bolus infusions of isoproterenol and normal saline. During the
second challenge, participants again rated their experience of heartbeat sensations
following application of a topical lidocaine anesthetic to the body surface where the
heartbeat sensation was felt maximally during the previous challenge (Fig. 3c), and
participants once again rated their experience of heartbeat sensations.
Infusion protocol
The first challenge consisted of 14 infusions: 7 isoproterenol (0.1, 0.25, 0.5, 0.75,
1.0, 2.0 and 4.0 micrograms (mcg)) and 7 saline. These doses were used to establish the
chronotropic dose 25 (CD25), which is the dose necessary to increase the participant’s
heart rate by 25 beats per minute above baseline, a commonly reported measure of beta
adrenergic receptor sensitivity9–13. The CD25 was calculated by extrapolation from the
slope of a linear regression at each individual’s isoproterenol induced heart rate
response9,12–14.
The second challenge consisted of four infusions: 2 isoproterenol (2.0 and 4.0
mcg) and 2 saline. We only selected the two highest doses for the second challenge, as
we have previously found that all participants receiving the 2.0 mcg dose experience
report changes in heartbeat sensations3.
Each infusion (isoproterenol and saline) consisted of two 3 milliliter (ml) bolus
infusions delivered sequentially through an intravenous catheter. During isoproterenol
infusions, a 3 ml bolus containing the specified dose was delivered, immediately
followed by a 3 ml bolus of saline to flush the line. During saline infusions, a 3 ml bolus
of saline was delivered, immediately followed by an additional 3 ml bolus of saline.
Both bolus volumes were administered in entirety within a 15 second period by a nurse.
This method of delivery minimized the participant’s ability to use external cues to
distinguish between the different infusion types, and ensured rapid and standardized
systemic introduction of isoproterenol.
Anesthetic protocol
Sixty grams of 4% lidocaine topical anesthetic cream (L.M.X.4TM, Ferndale
Laboratories, Inc.) were applied to the body region where each participant reported
maximally feeling their heartbeat sensations during the first infusion challenge (Fig. 3c).
Anesthetic administration was single blinded. To ameliorate possible placebo effects, all
participants were informed that the topical cream would either (a) increase skin
sensitivity to heartbeat sensations, (b) decrease skin sensitivity to heartbeat sensations, or
(c) would not change skin sensitivity to heartbeat sensations at all. In order to achieve an
anesthetic effect the topical cream was left in place for a minimum of 30 minutes prior to
the start of the second infusion challenge. To maintain adequate anesthesia, the anesthetic
was left in place throughout the second infusion challenge.
Anesthetic body surface area coverage was equivalent across all participants
(Roger: 1.3%, healthy comparison participants: mean 1.7%, SD 0.7%)15. After the
conclusion of the second infusion set, quantitative pinprick testing was conducted to
determine whether an anesthetic effect had been obtained. Throughout the testing each
participant was instructed to keep their eyes closed. A series of 18 stimulations were then
applied to the participant’s body, 50% in the anesthetized area and 50% within a 10 cm
radius of adjacent (non–anesthetized) skin. Participants were asked to spontaneously
report whether they had experienced a sharp or dull sensation. A report of “dull” in the
anesthetized area was considered correct, whereas a report of “sharp” in the same area
was incorrect. The opposite criteria were applied to the adjacent non–anesthetized skin.
Any participant achieving > 13 out of 18 trials correct or > 72% correct (p < .05 per
binomial test) was considered to have demonstrated a satisfactory anesthetic effect. The
quantitative sensory testing results showed that all participants demonstrated a
satisfactory anesthetic effect according to this criterion (Roger: 83% correct, healthy
comparison participants: mean 83% correct, SD 11%).
Procedure
The study involved one visit, which always started between 7 and 8am in the
General Clinical Research Center at the University of Iowa. After completing the
consent process a nurse measured each participant’s height and weight. The nurse then
placed a 22 gauge intravenous catheter into the participant’s non dominant dorsal hand
vein, and administered a 12 lead EKG. A physician evaluated the EKG, and the
experiment proceeded only if the EKG was considered normal (all participants displayed
normal EKGs). The participant was led to a quiet room, seated in a comfortable chair,
and was attached to leads for measuring heart rate (lead II EKG). At this point the
participant’s non dominant hand was placed outstretched on a pillow at chest level. A
curtain was positioned with the participant on one side and the nurse and the
experimenter on the other side, to prevent the participant from viewing the preparation
and administration of each infusion. The nurse then measured the participant’s blood
pressure and began the infusion protocol. Participants were instructed not to recline in
the chair during each infusion period, in order to prevent them from using the back of the
chair as an external source to help them detect heartbeat sensations. The entire procedure
lasted approximately 5 hours.
Psychophysiological measures
Physiological data including heart rate were recorded continuously during all
infusions with an MP100 acquisition unit (Biopac Systems, Inc). Dial ratings were
collected with a custom built dial that consisted of a rotating potentiometer with a
continuous range of 0.000 to 5.000 Volts. The average heart rate response during each
infusion was calculated across a 120–second interval immediately following the onset of
each infusion. The average heart rate response was obtained by subtracting the average
heart rate during the 30–second post–infusion window (i.e., before isoproterenol–induced
heart rate changes had occurred) from the average heart rate during the subsequent 90–
second window (i.e., when the isoproterenol induced heart rate changes were most likely
to occur). These time windows were carefully chosen to coincide with the typical delays
observed in the onset of isoproterenol–induced heart rate changes (mainly due to the slow
rate of venous drainage to the heart)9,11-14. All artifacts affecting the instantaneous heart
rate waveform (e.g., movement related, or due to premature ventricular contractions)
were manually identified and removed.
Cross correlations for each participant were calculated in Matlab (Mathworks,
Inc.) from mean centered dial ratings and instantaneous heart rate changes occurring over
the 120–second interval following the onset of each infusion. Dial ratings and
instantaneous heart rate changes for each dose were mean centered by subtracting the
120–second mean for each infusion interval from each time point within that interval.
Supplementary Results
See Supplementary Fig. 1a,b for full results of heart rate response and
interoceptive awareness testing of all seven doses (0.1, 0.25, 0.5, 0.75, 1.0, 2.0 and 4.0
mcg) during the first challenge (i.e., without lidocaine anesthetic). All participants
correctly detected increases in heartbeat sensations at the two highest doses (2.0 and 4.0
mcg), as indicated by a positive dial rating during the infusion period (Supplementary
Fig. 1c). Roger correctly detected increases in heartbeat sensations at the four highest
doses (0.75, 1.0, 2.0 and 4.0 mcg), as further indicated by his verbal responses
(Supplementary Table 2). Cross correlation analysis of the time course of subjective
and objective interoceptive changes revealed comparable zero order and maximum cross
correlations between Roger and the comparison participants (Supplementary Fig. 1d).
On average, Roger tended to generate ratings that were somewhat delayed with respect to
the heart rate changes, with dose–dependent ratings that were similar in amplitude to the
comparison participants.
See Supplementary Fig. 2a,b for full results of heart rate response and
interoceptive awareness testing of the two highest doses (2.0 and 4.0 mcg), following
topical anesthetic application. Under this condition, Roger no longer detected any
increase in heartbeat sensations, as indicated by the absence of dial ratings
(Supplementary Fig. 2b,d,e), and as further indicated by his verbal responses
(Supplementary Table 3). On the contrary, healthy comparison participants’ ratings
were unaffected by anesthetic application, although it remains possible that there could
have been subtle reductions in interoceptive awareness that were not picked up by our
measurement. For example, anesthetic seemed to abolish sensation in one healthy
comparison subject at the 2.0 mcg dose (91% of healthy participants reported a change in
sensation), but this was not the case for the 4.0 mcg dose (100% of participants reported a
change in sensation) (Supplementary Fig. 2d).
Supplementary Table 1. Participant demographic data. Means +/– SD. Chronotropic
dose 25 (CD25) is the isoproterenol dose necessary to increase the participant’s heart rate
by 25 beats per minute above baseline.
Age (yrs)
Sex
Body Mass Index
CD25 (mcg)
Roger
55
Male
29.4
8.1
Healthy comparison
54 +/– 9.6
11 Males
25.8 +/– 5.1
7.8 +/– 3.0
Supplementary Table 2. Verbal reports obtained during the non–anesthetized condition,
4.0 mcg dose. This particular healthy comparison participant used a numerical scale when
verbalizing his sensation that went from 0 (“normal, i.e., no change in intensity”) to 10
(“most ever”). HR change refers to increase above baseline heart rate at the start of the
infusion.
Roger
Comparison participant
HR (bpm)
90
HR change
13
Verbal rating
“Still pretty
low”
HR (bpm)
80
HR change
8
Verbal rating
“3 maybe”
103
26
“It’s
increasing
some”
86
14
“4…a sense of
fullness…from
going up a
couple flights
of stairs”
91
14
78
6
“2…maybe 1”
87
10
“Not beating
real hard or
fast”
“Now it’s
closer to
normal”
72
0
0
Supplementary Table 3. Verbal reports obtained during the anesthetized condition, 4.0
mcg dose. This particular healthy comparison participant used a numerical scale when
verbalizing his sensation that went from 0 (“normal, i.e., no change in intensity”) to 10
(“most ever”). HR change refers to increase above baseline heart rate at the start of the
infusion.
Roger
Comparison participant
HR (bpm)
HR change
Verbal rating
HR (bpm)
HR change
Verbal rating
94
10
“I haven’t
noticed hardly
any…any
increase in
heartbeat
85
2
“1”
107
23
“Not great
heartbeat”
97
14
“2. Within my
chest, It’s
slightly faster,
the beat has
more power to
it, akin to a
quick walk or
maybe a full
flight of stairs”
109
25
“Pretty quiet,
calm”
95
12
[Where in your
body are you
feeling it?] “In
the chest under
the gel area.”
[Are you feeling
it in your skin?]
“No. Not in the
skin, all within
the chest cavity.
It’s like a
pulsing within”
105
21
[Would you
describe it as
normal?]
“Yes”
80
–3
“0”
99
15
“I think it’s
pretty normal”
Supplementary Figure 1. Objective and subjective interoceptive changes during
isoproterenol infusions. (a) Mean heart rate response to isoproterenol and saline. (b)
Mean time course of heart rate response and subjective dial rating. Bolus infusions
occurred at time zero. (c) Percent of participants correctly detecting increases in heartbeat
intensity. A participant was considered to have correctly detected an increased heartbeat
intensity if they turned the dial above zero at any point during the latter 90 seconds of the
infusion period, when isoproterenol induced heart rate changes are most likely to occur.
(d) Mean zero lag and maximum cross correlations between heart rate response and
subjective interoceptive rating as a function of absolute lag time. Error bars = SE.
Supplementary Figure 2. Objective and subjective interoceptive changes during
isoproterenol infusions following topical anesthetic application. (a) Mean heart rate
response to isoproterenol and saline. (b) Mean time course of heart rate response and
subjective dial rating. (c) Overlaps showing area of topical anesthetic application,
corresponding to the region of maximal heartbeat sensation. (d) Percent of participants
correctly detecting increases in heartbeat intensity. (e) Mean zero lag and maximum cross
correlations between heart rate response and subjective interoceptive rating as a function
of absolute lag time. All comparison data depict mean values, except (c). Error bars = SE.
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