Here we describe methods for bringing the real-world into the laboratory. How to prepare and present real objects and match two-dimensional images of the same items under tightly controlled viewing conditions. Current knowledge of human vision is derived from studies that have relied on stimuli in the form of computerized two-dimensional images.
However, it's unclear whether images of objects trigger similar underlying cognitive and neural processes as to real-world solid objects. Although working with real objects in experimental context presents numerous practical challenges, the approach will yield important new insights into the mechanisms that underlie naturalistic vision. Here we compare real objects and image vision in a decision-making context.
However, the general approach can be extended to study other cognitive processes, such as perception, memory, or attention. Begin by creating a circular wood base for the turntable that is two meters in diameter and has a round central core with 20 slots. Create 20 dividers and slide each divider into the central core of the turntable to form 20 cells.
Place the core on top of a rotating cylinder, allowing for easy rotation. Create a vertical partition between the turntable and the participant and place it 26 centimeters from the turntable, allowing room for an LCD computer monitor behind the partition. Construct an aperture in the partition and ensure the width of the aperture is adjustable so that in the final setup, the participant can see only one item on the turntable at a time.
Next, place a sliding platform and the participant monitor between the turntable and the partition to allow for rapid transitions between display format conditions. Use a small desk or shelf for the experimenter's monitor. Attach a keyboard tray to the turntable base for the mouse, directly underneath the aperture in the partition.
Then, attach a curtain between the sides of the turntable and the wall to prevent the participant from viewing the stimuli and the experimenter during the experiment. After the turntable has been assembled, acquire 60 popular snack food items. Open the packaging for each food and place both the package and some of the food on a plate.
Next, place a plate of food on the turntable and prepare to photograph the stimulus on the turntable so that the background of the stimulus in the 2-D image matches the real food counterpart. To do this, position the camera on a tripod 50 centimeters from the edge of the turntable. The real objects and matched images must be within the reaching distance of the participant.
Finally, while holding the source of illumination in the testing room constant, photograph the real foods on the turntable using a camera with a constant f-stop and shutter speed. Match the overall luminance, shading patterns, and specular highlights across display formats as closely as possible. Begin by creating a script using experimental software that will randomly interleave real and image trials.
Have the script list which real items to place on the turntable and in what order prior to the start of the experiment. Then, place the real items on the turntable in the correct order. Place the monitor in the aperture and ensure all other items and the experimenter are masked from the participant's view.
Next, seat the participant to approximately 50 centimeters from the turntable and play white noise in the testing room. Give the participant the occlusion glasses to put on and make sure that they are in the closed opaque state. Look at the experimenter monitor to see what type of condition the upcoming trial will be.
On real object trials, retract the participant monitor from the viewing aperture via the sliding platform so that the real object is visible to the participant on the turntable. Make a computer command via a button press to trigger the opening and closing of the glasses, allowing for the real food to be visible on the turntable for three seconds. Once the glasses close, position the participant monitor back in front of the aperture and press a key to open the glasses to allow the participant to make a response.
Have the glasses automatically close once the participant enters his or her response. Then, check the experimenter monitor to prepare the stimulus for the next trial and press a key to advance to the next trial. For 2-D image trials, place the LCD monitor within the viewing aperture.
Leave the monitor in the viewing aperture and press a key to open the glasses for the participant to make a response. Ensure the next stimulus is ready for viewing and then press a key to advance to the next trial. Finally, after the testing is complete, thank the participant for their involvement in the study and ask whether they have any questions about it.
Results indicate a strong positive relationship between monetary bids and food preference ratings with higher bids for food that were more strongly liked. Importantly, there was a significant main effect of display format in which bids for real foods were greater than for matched food images. Likewise, there was a significant positive relationship between bids and actual caloric density, with higher bids for foods of higher caloric density.
There was no significant interaction between the effective display format and caloric density. It's important to match as closely as possible the appearance of the real objects and images, as well as the timing of events during each trial. Future studies could assess the impact of stereopsis by presenting real objects under monocular viewing conditions, or examine the mechanism for the effect by manipulating the reachability of the stimulus.
For example, using similar methods, we've shown that real objects capture attention more so than matched 2-D or 3-D images, but only when the real objects are within reach.
