Humans Utilize Multiple Reference Frames In Real-world Visual Search

Humans often need to visually search various objects in their daily lives,and many objects often appear in the physical world with particular orientations.Thus,the human visual system is more experienced in dealing with objects in specific orientations,and humans can more quickly recognize objects with these orientations(aka orientation dependency).Usually,an upright object is easier to identify than a tilted object.In the process of visual search,the observer must recognize the target before making a response.Then does changing the orientation of the target affect performance in visual search tasks? What is the external reference frame of the object’s orientation? Here,Experiment I and Experiment II,respectively,investigated these two questions.In Experiment I,20 participants(7 males and 13 females)were asked to perform a visual search task in a virtual environment under eight conditions of visual stimuli: two levels of set size,two kinds of object orientation,and target absence or presence.The results show the following: 1)Orientation dependency exists in the visual search task.For both levels of set size,the reaction time for upright objects is significantly lower than that for tilted objects.2)The visual search for both upright and tilted objects is a serial search,and there is no significant difference in search efficiency between them.3)For both levels of set size and kinds of object orientation,the reaction time in target-presence trials is significantly lower than in target-absence trials.Experiment II used virtual reality technology and a flight simulator to dissociate the egocentric,visual-contextual and gravitational reference systems.(Usually,these three kinds of reference frames are parallel.)Four conditions of external reference cues were used in this experiment: 1)baseline: the visual context and the body direction of the subjects were parallel to the direction of gravity;2)visual context tilted: the direction of the visual context was rotated 90 degrees clockwise with respect to gravity,while the body of the subject remained upright;3)body tilted: the subject’s body was rotated 90 degrees clockwise relative to gravity;and 4)both visual context and body tilted: the visual context and the subject’s body were rotated 90 degrees clockwise relative to gravity.In these four experimental conditions,subjects were asked to search for upright objects or tilted objects in isolated blocks.Data collected from 10 participants(5 male and 5 female)showed that the orientation dependency in object search tasks was mainly influenced by gravitational cues,with the visual environment and egocentric cues having slight effects.To conclude,this study proves that the human visual system utilizes an internal reference frame to search objects in the visual field,and the internal reference frame is shaped by gravitational,visual-contextual and egocentric cues.