This investigation was conducted according to the principles expressed in the Declaration of Helsinki. All experimental protocols are consistent with the Guide for Experimentation with Humans and were approved by the Institutional Ethics Committee of the Primate Research Institute, Kyoto University.

Participants

The participants were sixty naturally cycling 29- to 30-year-old single women. They were included in the present study after signing informed consent forms. They were healthy, as determined by routine physical and laboratory examinations and had no current psychiatric disorder, as indicated by a structured interview40. They were all right-handed, were free of medication, did not use hormonal contraceptives and reported no history of psychiatric or somatic disease potentially affecting the brain. They were not pregnant and reported no history of pregnancy. They reported that the onset of menstruation had occurred regularly with intervals of 27–29 days over at least the previous 4-month-period.

Each of the 60 participants underwent the visual search experiment (whose detailed protocols are described below) twice during the study. The first and the second testing were separated from one another by a 2- to 3-month-period for each participant. They were randomly assigned to one of three groups. In one of the three groups (Group A), the 20 participants underwent the experiment during the early follicular phase (day 5) and during the luteal phase (day 25) of different menstrual cycles. The 20 women in one of the other two groups (Group B) underwent the experiment during the late follicular phase (day 13) and during the luteal phase (day 25) of different menstrual cycles. The remaining 20 (Group C) underwent the experiment during the early follicular phase (day 5) and during the late follicular phase (day 13) of different menstrual cycles. Concerning the menstrual cycles of the participants when the experiment was conducted, their actual cycle length was indentified retrospectively and was confirmed to be between 27 and 29 days. The order of the two different phases in which the first and second testings were to be conducted was determined randomly for a given participant in each of the three groups.

Materials

For each experiment, we selected 24 photographs in gray-scale for each stimulus category. In a given trial, 9 of these photographs were displayed in a 3-by-3 matrix. Each matrix contained 1 target image from one category and 8 distracter images from the category: a flower matrix would contain a snake target and a snake matrix would contain a flower target. This yielded two combinations: a snake among flowers and a flower among snakes. An RDT151TU (MITSUBISHI) touch-screen monitor was used to present each image matrix on a 38.1 cm (15-in.) screen. Each of the 24 images in the target category served as the target once. Each of the 24 pictures in the distracter category appeared 8 times on average; the different distracters were presented approximately the same number of times across trials. The stimulus order was created by randomly arranging the matrices.

Procedure and analyses

In each experiment, the participant was seated in front of the touch-screen monitor (approximately 40 cm from the base of the screen) and was told to place her hands on a set of handprints. This ensured that her hands were in the same place at the start of each trial, making it possible to collect reliable latency data. An experimenter was seated alongside to monitor and instruct her throughout the procedure.

First, a set of 9 practice trials was given to instruct her how to use the touch screen. In the first 3 trials, a display of 1 target (a puppet from an animated cartoon well known to her) and 8 distracter (another puppet, also well known) images was presented. She was asked to touch the target among distracters as quickly as possible and then return her hands to the handprints. In the next 6 trials, the display consisted of 1 target (a snake or a flower) and 8 distracter (the other) images and she was asked to touch only the target image. All images used in the practice trials were chosen randomly from the original sets of 24.

When the participant had learned the procedure, a series of test trials followed. The task comprised 48 trials in total ordered in 2 blocks of 24 trials. In each trial, a different image matrix containing 1 target (snake or flower) and 8 distracters (the other) was presented. Between trials, an image of a stuffed animal or a popular character appeared on the screen to keep her attention on the screen. A trial was initiated when the experimenter judged that the participant was looking at the image, causing the next matrix to appear in order to ensure that her full attention was on the screen before each matrix appeared. When the first block was over, another block began. If the first block target was snakes, the next target was flowers, or vice versa. Each participant was randomly assigned to one of 2 block orders.

In each trial, the RT of the participant was automatically recorded from the onset of the matrix to when she touched one of the pictures on the screen. The results described in the text were solely based upon analyses of the RT data collected in this manner (RTs of incorrect responses as well as extreme RT scores—defined as values more than 2 standard deviations above or below the mean relative to each participant's mean RT—were excluded from the analyses).