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Previous research has shown that conditioned aversive
stimuli (light air puffs) potentiate startle responses in rhesus
monkeys (Winslow Parr Davis, 2002). However in recent research
(Davis unpublished, 2003) it has been shown that the presentation
of unconditioned aversive stimuli (video clips of rhesus threats)
effectively inhibits the startle response. It was hypothesized
that this inhibition effect was due to either A) the subject very
strongly attending to the stimulus and consequently decreasing the
startle response; or B) the subject becoming severely afraid of
the stimulus and consequently ‘freezing’ the startle
response. By administering startle tests using both aversive
(threats) and neutral (video clips of calm rhesus monkeys) it was
shown that the aversive stimuli inhibited startle responses more
than the neutral stimuli but that both aversive and neutral stimuli
produced inhibited startle responses. From these results it
is concluded that unconditioned stimuli in general will inhibit
startle responses but that unconditioned aversive stimuli will inhibit
startle more than unconditioned neutral stimuli.
Previous research has shown that conditioned aversive
stimuli potentiate startle responses in rhesus monkeys (1). However
in recent research (2) the presentation of unconditioned aversive
stimuli (video clips of rhesus threats) effectively inhibited the
startle response. It was hypothesized that this startle inhibition
effect was due to either A) the subject very strongly attending
to the stimulus and thus decreasing the startle response; or B)
the subject becoming severely afraid of the stimulus and ‘freezing’
the startle response. More specifically were rhesus faces in general
causing the startle inhibition (the attention hypothesis) or were
the specific threat faces causing it (the freezing hypothesis)?
To determine the cause of this startle inhibition effect further
testing was conducted using both aversive (rhesus threat videos)
and neutral (calm rhesus videos) stimuli. Startle responses were
monitored for both types of stimuli. Baseline physiology tests were
conducted both before and after testing to determine if the testing
itself had any significant effect on the subjects.
1—Parr L; Winslow J; Davis M. (2002). Rearing
experience differentially affects somatic and cardiac startle responses
in rhesus monkeys (Macaca mulatta); Behavioral Neuroscience vol.
116 378-386. 2—Unpublished personal data and communication;
Davis J. May 2003.
None given
Subject Rearing Groups
- 12 male rhesus macaque subjects (approximately 24 months of
age)
- 6 socially reared (SR): reared for first 12 months in large
social group
- 6 nursery reared (NR): removed from natal group within 48 hours;
then reared in a nursery with an age matched peer
- After 12 months of SR/NR all subjects were placed in a colony
room in a cage with an age and rearing-matched peer.
Baseline Physiology and Hormone Testing
Prior to startle testing CSF and blood plasma were
taken from each of the 12 subjects for baseline level analyses of
oxytocin testosterone and cortisol.
- Oxytocin and Cortisol levels were positively correlated. Before
and after startle testing baseline measurements (BS1 BS2) of heart
rate (HR) respiratory sinus arhythmia (RSA) and respiratory rate
(RR) were measured for all subjects while inside the testing apparatus.
- Oxytocin was positively correlated with RSA in both BS1 and
BS2. - Oxytocin levels were higher in the SR subjects in both
BS1 and BS2.
- Cortisol levels were higher in the SR subjects in both BS1
and BS2.
- HR was positively correlated to RSA in both BS1 and BS2.
- RSA in BS1 was positively correlated with RSA in BS2.
Fear-potentiated Startle Testing
The fear-potentiated startle of 6 ‘beta’
subjects (3 NR 3 SR) to a 120 dB noise was tested on three separate
testing sessions. The subject was placed in a Lexan primate restraint
chair inside a wooden startle box. From within the box the subject
was shown one of four video stimuli for a period of 8 seconds out
of every 60 (during the interim periods the screen remained blank).
The stimulus was followed by the 120 dB noise. Startle responses
were measured with an acclerometer. The four video stimuli were:
1) Red screen 2) Control video (Star Wars movie clip) 3) Rhesus
threat video (the threat stimulus) 4) Calm rhesus video (the neutral
stimulus)
For each session of testing there were 6 blocks of 6 trials. Each
6 trials included one of each of the four video stimuli and two
blank screens. In addition each session included four randomly placed
red screen/air puff pairings. The order of blocks and trials was
counterbalanced. The ‘alpha’ subjects (3NR 3SR; Davis
J. unpublished data May 2003) received similar testing procedures
(slightly different sequencing methods were used); but the beta
group was tested ~2 weeks after the alpha group.
- The threat stimuli inhibited startle on NR day 2.
- Neutral and control inhibit startle marginally.
- Overall red potentiates startle.
Startle Behavior Analysis
Video footage of the startle tests for the 6 beta
subjects (3 NR 3 SR) was analyzed for two behavioral categories:
struggle behaviors and fear-associated behaviors (FAB’s).
FAB’s were counted as either lip-smacking or fear grimace
behaviors.
- On startle test day 1 NR subjects displayed a significantly
higher number of FAB’s for the threat video stimuli than
was displayed with any other stimuli.
- Due to high individual subject variance some other FAB/struggle
measurements did not reach significance level despite changes
in the behaviors (ex: the blank measurement in the graph pictured
below).
- The order of startle testing (alpha vs. beta) did not significantly
affect BS2.
- In NR subjects the initially high-FAB reaction to the threat
stimuli decreased on day 2 and day 3. This suggests that the subjects
learned that the threat video stimulus was not an actual threat
to them.
- The NR subjects were more behaviorally responsive to the stimuli
than were the SR subjects; however the SR subjects had non-significantly
higher startle responses overall.
- Threat stimuli inhibit startle.
- Neutral stimuli also inhibit startle (to a slightly lesser
degree though).
- Rather than ‘freezing’ in response to a fearful
stimuli the subjects show evidence of startle inhibition with
exposure to both aversive and neutral stimuli.
- Based on these findings it is concluded that the startle inhibition
effect is caused by an increased attending of the subject to the
stimuli.
- Due to high individual variability within the testing groups
further testing should be conducted with larger samples to more
clearly elucidate the startle inhibition effect and its relations
to rearing groups and the type of stimuli presented.
I would like to thank Jeniffer Fugate, Aeneas Murnanae, Shelia
Ster,k Kasey Lyons, and Pamela Noble for their assistance with this
research. This project has been funded in part by Howard Hughes
Medical Institute grant number 52003727.
The current research project investigated the degree
to which a certain stimulus causes an animal to startle. By measuring
the amount of body movement when the stimulus is presented (using
an accelerometer) we are able to quantify this startle response.
In this particular project we are interested in determining if there
is a difference in the amount of startle response when the subject
(a rhesus macaque) is presented with A) an aversive stimulus--a
video clip of a rhesus threat or B) a neutral stimulus--a video
clip of a rhesus sittting around in his cage.
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