Physical or Retinal Image Size

I am sure that we have an idea about the sizes of the mountains, the birds, people, and etc. Let us take mountain for example to explain the principle of Size Constancy. Will you say that a mountain is a large thing when viewed at a distance of about 5m away? How about 5km? 5miles? 6 miles? 7miles? Etc. I am sure that most of us would still say that a mountain is a large object no matter at which distance we are looking at it. This is the principle of Size Constancy. At varied distances, the principle states that our perception of the size of the object remains relatively constant, though the size of the image on the retina changes. Granrud (2006) study, Size Constancy in Infants: 4-month olds’ responses to Physical vs Retinal Image Size, aimed to answer this question: Do infants respond primarily to the physical size or the retinal image size?
The results of the experiment reveal that infants respond primarily to the physical size, not to the retinal image size. How was the experiment executed? There were two experiments in the study. The first one aimed to determine whether infants prefer novel-sized or familiar-sized objects. There were two groups for this experiment. For the habituation trials, a circular object having 6cm diameter was viewed by one group at a distance of 30cm; whereas, the other group viewed the circular object with 10 cm diameter at the same distance. For the test trials, both groups viewed the two objects put side by side at a distance of 30cm. This experiment did not test size constancy, but served as a prerequisite for the second experiment that is the main part of this study. From the ten infants (10 girls, 6 boys) in the experiment, ten more were tested but were not included for not sufficing the requirements of the experiment, just like not meeting the habituation criterion. For this experiment, the habituation criterion is the two consecutive trials whose combined fixation time is 50% or less than the combined total fixation time of the first two trials. Infants who were judged by a parent, or the observer who scored them using the videotape, to be too fussy or inattentive were also excluded. After the habituation trials, the first test trial is executed. There were two experimenters for the execution of the procedures. Each of them had specific and unique tasks to perform. The first test trial included the presentation of the novel sized object. Generally, the trial started when the object was looked at by the infant, and ended if the infant looked away. The said novel-sized object was positioned on the left side of the infant for half of the participants in a group, and on the right side for another half, randomly. The whole procedure was videotaped. Test-trial fixation times were scored live, and from the videotape, as well. To avoid bias, the score based on the videotape was used in all analyses. The data gathered from live were used for interobserver reliability in which using Pearson’s r revealed a high degree of reliability.
Using t test, they found out that in the first test trial, the infant significantly preferred the novel-sized object than the familiar-sized. In the second trial however, there was no significant difference found.
The second experiment was similar to the first one, except that the distance of viewing the objects were varied to account for size constancy. For the first group, 6-cm in diameter disc viewed at 18cm; second group, 10-cm at 50cm. This was for the habituation trials. For the test trials, the objects put side by side was viewed by each infant at 30 cm. 8 male and 8 female infants were used in the second experiment.
Just like in the first experiment, there was a significant preference for the novel size of the object, but not in the second trial. The results indicate the size constancy of the 4-month olds by perceiving an object despite the change in distance and the retinal image size. As noted, in the second experiment, the objects were presented at different distance from the habituated trials. Physical size is also more salient than the retinal image size. As for the first experiment, the results suggest preference for the novel size. Having this, experiment 2 suggests new physical size to be more novel than the object that had a new retinal image size as supported by the data after the systematic computation was done, comparing the habituation trials and the test trials of the two groups.
It is just so amazing that psychologists and researchers, as well, are able to study the perception of the 4-month old infants. Research really makes people become innovative and resourceful in executing the experiments to gain data and results that are valid.

Reference:
Granrud C., (2006). Size Constancy in Infants: 4-month olds’ responses to Physical vs Retinal Image Size. Journal of Experimental Psychology: Human Perception and Performance. Vol. 32, No. 6, 1398–1404