Visual Imagery

Visual Imagery
Most of the research on mental imagery has involved visual imagery, and this will be the principal focus of this chapter. One function of mental imagery is to anticipate how objects will look from different perspectives. People often have the impression that they rotate objects mentally to change the perspective. Roger Shepard and his colleagues were involved in a long series of experiments on mental rotation. Their research was among the first to study the functional properties of mental images, and it has been very influential. It is interesting to note that this research was inspired by a dream (Shepard, 1967): Shepard awoke one day and remembered having visualized a 3-D structure turning in space. He convinced Jackie Metzler, a first-year graduate student at Stanford, to study mental rotation, and the rest is history.
Their first experiment was reported in the journal Science (Shepard & Metzler, 1971). Participants were presented with pairs of 2-D representations of 3-D objects. Their task was to determine whether the objects were identical except for orientation
Participants reported that to match the two shapes, they mentally rotated one of the objects in each pair until it was congruent with the other object. The reaction times are plotted as a function of the angular disparity between the two objects presented. The angular disparity is the amount one object would have to be rotated to match the other object in orientation. Note that the relationship is linear—for every increment in amount of rotation, there is an equal increment in reaction time. Reaction time is plot- ted for two different kinds of rotation. One is for 2-D rotations, which can be performed in the picture plane (i.e., by rotating the page); the other is for depth rotations  which require the participant to rotate the object into the page. Note that the two functions are very similar. Processing an object in depth (in three dimensions) does not appear to have taken longer than processing an object in the picture plane. Hence, participants must have been operating on 3-D representations of the objects in both the picture-plane and depth conditions.
These data seem to indicate that participants rotated the object in a 3-D space within their heads. The greater the angle of disparity between the two objects, the longer participants took to complete the rotation. Though the par- ticipants were obviously not actually rotating a real object in their heads, the mental process appears to be analogous to physical rotation.
A great deal of subsequent research has examined the mental rotation of all sorts of different objects, typically finding that the time required to complete a rotation varies with the angle of disparity. There have also been a number of brain- imaging studies that looked at what regions are active during mental rotation. Consistently, the parietal region has been activated across a range of tasks. This finding corresponds with the results showing that the parietal region is important in spatial attention. Some tasks involve activation of other areas. For instance, Kosslyn, DiGirolamo, Thompson, and Alpert (1998) found that imagining the rotation of one’s hand produced activation in the motor cortex.
Neural recordings of monkeys have provided some evidence about neural representation during mental rotation involving hand movement. Georgopoulos, Lurito, Petrides, Schwartz, and Massey (1989) had monkeys perform a task in which they moved a handle to a specific angle in response to a given stimulus. In the base condition, monkeys just moved the handle to the position of the stimu- lus. Georgopoulos et al. found cells that fired for particular positions. So, for in- stance, there were cells that fired most strongly when the monkeys were moving the handle to the 9 o’clock position and other cells that responded most strongly when the monkeys moved it to the 12 o’clock position. In the rotation condition, the monkeys had to move the handle to a position rotated some number of de- grees from the stimulus. For instance, if the monkeys had to move the handle 90° counterclockwise from a stimulus at the 12 o’clock position, they would have to move the handle to 9 o’clock. If the stimulus appeared at the 6 o’clock posi- tion, they would have to move the handle to 3 o’clock. The greater the angle, the longer it took the monkeys to initiate the movement, suggesting that this task in- volved a mental rotation process. In this rotation condition, Georgopoulos et al. found that various cells fired at different times during the transformation. At the beginning of a trial, when the stimulus was presented, the cells that fired most were associated with a move in the direction of the stimulus. By the end of the trial, when the monkeys actually moved the handle, maximum activity occurred
in cells associated with the movement. Between the beginning and the end of a trial, cells representing intermediate directions were most active. These results suggest that mental rotation involves gradual shifts of firing from cells that en- code the initial stimulus (the handle at its initial angle) to cells that encode the response (the handle at its final angle).
■ When people must transform the orientation of a mental image to make a comparison, they rotate its representation through the inter- mediate positions until they achieve the desired orientation.
■ People suffer interference in scanning a mental image if they have to simultaneously process a conflicting perceptual structure.
■ People experience greater difficulty in judging the relative size of two pictures or of two mental images that are similar in size.
■ It is possible to make many of the same kinds of detailed judgments about mental images that we make about things we actually see, though it is more difficult.
■ Brain regions involved in visual perception are also involved in visual imagery tasks, and disrup- tion of these regions results in disruption of the imagery tasks.
■ Neuropsychological evidence suggests that imagery of spatial information is supported by parietal structures, and that imagery of objects and their visual properties is supported by temporal structures.
■Our knowledge of our environment can be represented in either survey maps that emphasize spatial information or route maps that emphasize action information.
■ Our representation of space includes both allocentric representa- tions of where objects are in the world and egocentric representations of where they are relative to ourselves.
■When people have to work out the relative positions of two loca- tions, they will often reason in terms of the relative positions of larger areas that contain the two locations.