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Pupillometry is the measurement of pupil diameter in psychology. As it is mentioned in Laeng and colleagues paper “the method established itself after the appearance of three seminal studies (Hess & Polt, 1960, 1964; Kahneman & Beatty, 1966) ". The method is a good tool to examine not only humans (including infants) but also animals, because pupillary responses occur from birth and are independent from voluntary control. Changes in pupillary dilation elicited by psychological stimuli are about 0.5 mm and “occur as the result of a neural inhibitory mechanism on the parasympathetic oculomotor complex or Edinger-Westphal nucleus by the noradrenergic system’s locus coeruleus”.

The beginning of pupillometry
Hess and Polt (1960) have presented pictures of semi-naked adults and of babies to adults (four men and two women). Pupils of both sexes dilated after seeing pictures of people of the opposite sex, but in females the difference in pupillary sizes occurred also after seeing pictures of the babies and mother and baby. This examination showed that pupils react not only to the changes of intensity of the light (pupillary light reflex) but also in reaction to the arousal or emotions. Also Simms (1967) also found in his research that pupillary response of males and females are greater to the pictures of opposite sex. In another study, Nunnally and colleagues (1967) found that seeing slides rated as very pleasant is associated with greater pupil dilation as seeing slides rated as neutral or very unpleasant.

Sex and pupillary response
In 1965 Hess, Seltzer and Shlien examinated pupillary responses in hetero- and homosexual males. Results showed a greater pupil dilation in heterosexuals to pictures of opposite sex, and in homosexuals to pictures of the same sex.

Cognitive load
Pupillary responses can reflect activation of the brain allocated to resolving of cognitive task. It is found that greater pupil dilation is associated with increased processing demands in a task. Vacchiaco et colleagues (1968) found that pupillary responses were assiociated with visual exposure of words with high, neutral or low value. Presented low-value words were associated with dilation, and high-value words with constriction of a pupil. In decision making tasks it was found that dilation of pupil was increased before making a decision as a function of cognitive workload. In experiment about short-term serial memory, students repeated the strings of items. Greater pupillary diameter was observed after the items was heard (pupillary diameter depending on how many items were heard), and accordingly pupillary diameter decreased after items was spoken by the participants. The more difficult task is given to participants the greater pupillary diameter is observed in the time of preceding solution and pupil dilatation is maintained untill the solution is found.

Preferential looking
Differences in pupillary diameter were also found in the domain of prefential looking by infants. Infants showed greater pupillary size in tasks in which they saw pictures of faces than to observing geometric shapes , and also greater pupillary dilation after seeing picture of infant's mother that pictures of a stranger.