Mapping the visual field in mental imagery.

We describe a simple psychophysical technique for measuring the size and shape of visual fields in mental imagery, and use this technique to compare fields in imagery and perception within which bar gratings of various spatial frequencies can be resolved. The first experiment demonstrates that the size of fields of resolution obtained when bar gratings are imagined decreases with increasing spatial frequency of the gratings, in the same way that the size of fields obtained when the gratings are actually observed decreases. This experiment also shows that imagery and perceptual fields are very similar in shape, both being elongated horizontally and extending farther below the point to which one's gaze is directed than above. The second experiment shows that experimental subjects are not able to anticipate the more subtle characteristics of these fields, such as the precise rate at which field size decreases with increasing spatial frequency--characteristics that most clearly reveal the very close correspondence between the imagery and perceptual fields. The third experiment shows that the size of the imagery fields can serve to discriminate between vivid and nonvivid imagers when the task of imagining the gratings is made sufficiently difficult. We interpret these findings as evidence that visual imagery involves the activation of mechanisms in the visual system that are specifically designed to process information about spatial frequency. In particular, we claim that constraints that these mechanisms impose on the resolution of high spatial frequencies in perception are also imposed in mental imagery, restricting how well fine details of an object can be imagined. We then discuss implications of these findings for theories about spatial-frequency analysis in vision and for the general issue of the functional value of mental imagery.