Imagery effects on early visual processing

Three experiments were carried out to test the hypothesis that visual imagery activates patternprocessing mechanisms in the visual system. This hypothesis may be contrasted with one which posits that the effort required to maintain an image impairs performance on subsequent visual tasks. In Experiment 1, subjects imaged vertical, square-wave gratings confined to one quadrant of the imaginai visual field. This raised thresholds for detecting subsequently presented gratings in all four quadrants, relative to a control condition in which no imaging was required. The orientation specificity of this effect was tested in two further experiments by varying the orientation of adapting and test gratings orthogonally (vertical and horizontal orientations were used). In Experiment 2, a faint grating was present during adaptation to help subjects localize their images and a small fixation bar was scanned to prevent the formation of retinal afterimages. The presence of the faint grating itself produced orientation-specific and location-specific threshold elevation. However, imagining that this grating was of high contrast did not produce any further elevation of the thresholds. In Experiment 3, high-imagery subjects adapted to more easily imaged centrally located gratings. No threshold elevation was found. These results suggest that imagery affects detection thresholds as a result of the effort required rather than by adapting orientation-specific mechanisms in the visual cortex.