Methods

This page is a work in progress!

Stepping around on the force platform

Defensive freeze to police versus non-police

Exploring trajectory of initial sway

I have recently started exploring peoples' initial sway trajectory upon stimulus presentation. The idea here is that the forward/back initial direction of sway (or lean) in response to a stimulus might capture an approach/avoidance motivation toward that stimulus. For example, when plotting the same police-sway data above in the AP plane and across time we observe an initial forward lean to non-police (approach), and backward lean to non-police (avoid).

Anterior/Forward Lean

Posterior/Backward Lean

Posturography

At standing rest, humans exhibit spontaneous shifts in their center of gravity (they exhibit a "postural sway"). A sizable body of literature demonstrates that the magnitude of postural sway is modulated when viewing affective stimuli. In particular, people automatically exhibit a reduced magnitude in postural sway when viewing threatening compared to non-threatening stimuli. In other words, threatening stimuli elicit a defensive freeze response. One way that I (and others) measure affective modulation of postural sway is by continuously recording a participant's center of pressure using a stabilometric force platform while they view different classes of images.

Take a look at the gif to the left. To demonstrate how force platforms work I recorded myself stepping back and forth between the center of the platform and each of its for corners during separate time intervals (coded by color). In a typical postural sway task, rather than stepping around, a participant stands still during image presentations, and a freeze response is indexed as the variability of sway in the anterior-posterior (forward/backward) plane (AP-SD). Now take a look at the plot next to the .gif. This depicts a grand mean of sway across 3-second image presentations of police (black) and non-police (grey). As you can see, participants exhibited defensive freeze responses to police compared to non-police as evidenced by a smaller magnitude of sway in the AP plane.

Peripheral Physiology

Measures of peripheral physiology capture physical responses emanating from the body. My research often assesses physiological outcomes that index features of the autonomic nervous system designed to prepare the body for defense against physical threat. For example, I have used facial electromyography to capture threat-induced modulation of the startle eyeblink reflex.

The startle eyeblink is a rapid contraction of the orbicularis oculi muscle surrounding the eye (an eyeblink) that occurs in response to visual, tactile, or auditory startle. Think of the "jolt" or recoil you experience when someone unexpectedly sets off fireworks a little too early on the 4th of July. In the lab, we measure the magnitude of the startle eyeblink by using electromyography to index the electrical potential generated by the orbicularis oculi in response to a loud auditory probe.

Research has shown that people blink harder in response to probes when viewing images of people or objects they find physically threatening. We call this a "threat-induced potentiation of the startle eyeblink," which reflects a physical preparedness to respond defensively. Imagine, again, experiencing that "jolt" upon hearing unexpected fireworks. Except this time you are walking down a dark alley on Friday the 13th. In this case, wouldn't the jolt be more intense? I've used this startle eyeblink paradigm to measure people's defensive responding to police officers. Some other physiological measures I have training on include electrodermal activity, respiration, electrocardiogram, and pupillometry

My brother, Tony,
piloting the startle
eyeblink paradigm.

People evince larger startle eyeblinks to noise probes when viewing images of the police compared nonpolice, implying that police evoke a physiological preparedness to respond defensively.