Principales críticas de la Ley de Justicia y Paz

Observer’s prior knowledge significantly influences our visual behavior. Despite the fact that this kind of contribution can not be ruled out, it seems important to underline and define the nature of this contribution. Prior knowledge is a general concept encompassing various aspects such as past experience, education, where we grow up or even social activities. This knowledge which is built progressively and continuously shapes our perception [132]. It then helps the visual system to better interpret the visual image and to resolve visual ambiguities. This prior knowledge about the scene, our familiarity with it have an influence, called contextual influence or scene-schema knowledge [69], on our visual deployment.

Cultural heritage

Regarding the cultural baggage, there is a current debate on whether it influ- ences our visual strategy or not. In 2005, Chua et al. [28] demonstrated eye movement patterns differ between Chinese and Caucasian subjects when look- ing at complex scenes. Chinese participants tend to attend to the background information more than did American participants. However, in 2009, Evans et al. [47] cast doubt on previous results. They indeed did not find out difference in terms of eye movement patterns and recognition memory between American and Chinese participants, suggesting that both populations use the same strategies in scene perception. In 2011, Amatya et al. [3] examined the latency of reflex- ive saccades for a Chineses and Caucasian populations. Reflexive saccades are made in response to a sudden peripheral stimulus onset contrary to voluntary saccades which require additional cognitive processing [51]. Amatya et al. [3] observed that 29% of the Chinese subjects exhibited a high proportion of low latency saccades compared to those of Caucassian sujbects (only 2%). This study indicates that the cultural baggage would influence the visual strategy. Age

Another aspect is related to observers’ age. The older we are the more expert and familiar with our visual environment we are. So, given that prior knowledge depends on the age, the question is whether the age has an influence on the visual deployment or not. This question has been addressed by Acik et al. [1]. In this study they compared the viewing behaviour of three age groups: (1) 7 to 9 years old children; (2) 19 to 27 years old young adults; (3) a last group composed of people above 72 years. Participants have to perform a recognition task while they viewed natural and complex scenes. Results of this experiments showed that the low-level visual features are a more important source guidance for young children than adults. Authors suggest that the pregnancy of bottom-up features decreases with people’s age. Beside, top-down influences which are related to prior knowledge might be more important for adults than young children in the guidance of eye movement. More recently, Musel et al. [129] showed that the nature of visual information extracted from the scene for a rapid categorization varies with age. Young participants tend to process a categorization task in a coarse-to-fine manner with a reaction time smaller than 100ms. For aged participants, the reaction time is longer indicating the presence of an additional visual process. Authors suggest that aged participants might use contextual information to correctly categorize visual scenes.

Number of observers

When the objective of the eye tracking experiment is to test an hypothesis, the number of subjects involved in the experiments is fundamental. Although difficult to define, the goal is to get sufficient sensitivity or statistical power in order to be able to draw significant conclusions. More specifically, the power analysis allows to calculate the minimum sample size required so that one can

be reasonably likely to detect an effect of a given size. The description of the power analysis is out of the scope of this chapter. Readers have to refer to Co- hen’s publications [29] to get a good overview and to Lenth’s publication [116] in which some suggestions for successful and meaningful sample size determina- tion are provided. In this section we propose to illustrate the power analysis by computing the number of observers required to obtain a statistically significant result.

Let’s imagine we want to investigate the role of the global luminance of an im- age on the fixation duration. Two independent groups of observers participated to the experiment. Eye fixation will be collected when observers look at the original image and the modified image. How many observers should be involved in the experiment in order to show a significant effect on the average fixation duration? A t-test is used in order to evaluate whether the difference is statis- tically different or not. If one chooses the significance level (α = 0.05) and the statistical power (1 − β = 0.8), the power of the t-test is given by δ = d√N . δ is associated with the specific degree of power. To determine δ, you just need to read the power table (Table E.5 (Power as a function of δ and significance level α) in [76]). From this table δ must equal 2.8 for power equal to 0.8 and α = 0.05. The d value (called the effect size) is given by d = µ1−µ2

σ and is used

to test the hypothesis on the difference between the sample mean of average fixation durations µ2for the control and the tested group µ1. σ is the standard

deviation of the control population. N is the sample size of the control group. If you want to detect a difference of 10 ms between the two groups given a known standard deviation (here equal to 24), we have d = 10/24 = 0.416. Therefore the sample size should have a size of

n =  δ d 2 (2.3) =  _2.8 0.416 2 (2.4) = 45.3 (2.5)

Finally, if the experimenter wants to have a power of 0.8 to reject the equality between the two populations presenting a difference of 10 ms in terms of average duration of fixations, the number of participants involved in the experiment should be equal to 46.

Basic rules

Keeping in mind the aforementioned factors, it is also necessary to recall the basic rules used when preparing an experiment. Participants who are involved in an eye tracking experiment are most of the time naive to the purpose of the experiment and not familiar with this kind of experiment. They must have normal or corrected-to-normal vision. Note that there is no study, as far as we know, dealing with the influence of factors such as the socioeconomic status

and level of education on the visual strategy. It is however known that video- game playing enhances the capacity of visual attention [61,38]. These studies reveal that video-game players responded more quickly to a stimulation than non video-game players and that with the same accuracy.