Visual imagery in hemianopic patients

In this article we report some findings about visual imagery in patients with stable homonymous hemianopia compared to healthy control subjects. These findings were obtained by analyzing the gaze control through recording of eye movements in different phases of viewing and imagery. We used six different visual stimuli for the consecutive viewing and imagery phases. With infrared oculography, we recorded eye movements during this presentation phase and in three subsequent imagery phases in absence of the stimulus. Analyzing the basic parameters of the gaze sequences (known as "scanpaths"), we discovered distinct characteristics of the "viewing scanpaths" and the "imagery scanpaths" in both groups, which suggests a reduced extent of the image within the cognitive representation. We applied different similarity measures (string/vector string editing, Markov analysis). We found a "progressive consistency of imagery," shown through raising similarity values for the comparison of the late imagery scanpaths. This result suggests a strong top-down component in picture exploration: In both groups, healthy subjects and hemianopic patients, a mental model of the viewed picture must evolve very soon and substantially determine the eye movements. As our hemianopic patients showed analogous results to the normal subjects, we conclude that these patients are well adjusted to their deficit and, despite their perceptual defect, have a preserved cognitive representation, which follows the same top-down vision strategies in the process of visual imagery.