Pupil dilation, or pupillary reflex, is one of the most common cue mentalists look for. Mentalists guide us to recall our memory with words and as we do so, they look into our eyes to see if our pupils dilate. When they spot pupil dilation, they know they are on the right track since what they said correctly connects with our memory. They will continue until they find the piece of information they ultimately seek for. Here, we will look behind the scenes to explain how pupillary reflex gives away the answer.

 

We are familiar with the fact that our pupils dilate upon dimmed environments or fight or flight response. However, the pupillary reflex responds to more than that. It is also closely related to cognitive and emotional processing. Tonic pupillary reflex occurs upon emotional or stress arousal while phasic responses occur upon cognitive processing related to the central nervous system (CNS) (Papesh, 2012). These types of pupillary responses are independent of each other and we will focus on the latter type in this discussion. Pupillary responses due to cognitive processing, or task-evoked pupillary responses (TEPR), are suggested to occur upon parasympathetic nervous system (PNS) inhibition (Goldinger & Papesh, 2012; Papesh, 2012). This inhibition is controlled by the locus coeruleus of the pons in the brainstem. The locus coeruleus and norepinephrine system is known to have significant roles in memory creation, consolidation and retrieval (Goldinger & Papesh, 2012; Papesh, 2012). This may account for the relation between memory retrieval and pupil dilation although the detail biological mechanisms still require further research.

 

An experiment testing subjects with a digit recall task revealed a positive correlation between pupil diameter of subjects and memory load— which in this case is the amount of digits the subjects had to memorize (Kahneman & Beatty, 1966). The demand for cognitive processing may also be an underlying account for the effect of memory load on pupil dilation in this experiment. The experiment shows how subjects experience pupil dilation upon hearing the cue that evokes memory retrieval of the digit (the loading phase), and pupil constriction after reporting the digit (the unloading phase) (Kahneman & Beatty, 1966).

 

Many studies also show that the extent of pupil dilation depends on the subjective strength of memory. There is a positive correlation between pupil diameter and reported confidence of the subjects, which may correspond to the memory strength. (Goldinger & Papesh, 2012; Otero, Weekes & Hutton, 2011; Papesh, 2012; Papesh, Goldinger & Hout, 2011) In addition, experiments involving memory retrieval upon auditory cues reveal that the extent of this pupillary reflex also depend on how similar the auditory cue is to the original sound or voice that created the memory (Papesh, Goldinger & Hout, 2011).