Illustration of the experimental apparatus and of the three conditions of vision.

(A) Array configuration used at training and Sagittal test. (B) Array configuration used for the Fronto-Parallel tests. S.P. indicates the chick’s starting position. (C) Binocular condition of vision: both eyes, thus both hemispheres (left in red and right in blue), in use. (D) Right monocular condition with the right eye and the left hemisphere in use, in red. (E) Left monocular condition with the left eye and the right hemisphere in use, in blue. In monocular conditions, the eye not receiving light and its contralateral hemisphere are depicted in gray.

Models and hypotheses explaining SNA.

For each of the three main models, is reported a brief explanation of the hypotheses regarding SNA mechanisms, and the predicted outcomes for experiment 1 and 2.

Descriptive statistics.

For each test in the two experiments, the accuracy of selecting the 4th item in the Sagittal test and the 4L or 4R items in the Fronto-Parallel (FP) tests is reported.

Results of experiment 1.

The average percentage of chicks’ choices (y-axis) as a function of item positions (x-axis), light exposure modulating brain lateralization, and tests. Error bars indicate standard errors. The gray dashed line indicates chance level (10%). (A) Results of the Sagittal test. (B) Results of the Fronto-Parallel Binocular test. (C) Results of the Fronto-Parallel Monocular Left test. (D) Results of Fronto-Parallel Monocular Right test. ***P < 0.001, **P < 0.01, and *P < 0.05.

Results of experiment 2.

The average percentage of chicks’ choices (y-axis) as a function of item positions (x-axis), light exposure modulating brain lateralization, and tests. Error bars indicate standard errors. The gray dashed line indicates chance level (10%). (A) Results of the Sagittal test. (B) Results of the Fronto-Parallel Binocular test. (C) Results of the Fronto-Parallel Monocular Left test. (D) Results of Fronto-Parallel Monocular Right test. ***P < 0.001, **P < 0.01, and *P < 0.05

Time schedule of the experiments.

During the last three days of incubation, eggs were incubated either in darkness (weakly lateralized chicks) or under light exposure (strongly lateralized chicks). When chicks were three days old, the shaping procedure started, followed by Pre-Training and Training. In the following days, chicks were re-trained and then underwent all four tests.

Descriptive statistic experiment 1.

Descriptive statistics of the accuracy in selecting each of the 10 items in the Sagittal test, and in selecting the 5 items on the left (L) and the 5 items on the right (R) in the Fronto-Parallel (FP) tests of Experiment 1.

Descriptive statistic experiment 2.

Descriptive statistics of the accuracy in selecting each of the 10 items in the Sagittal test, and in selecting the 5 items on the left (L) and the 5 items on the right (R) in the Fronto-Parallel (FP) tests of Experiment 2