Eye Movements: Effects of Neurological and Mental Disorders On

Eye movement recordings provide powerful information about the decisions that a person's brain has made about whether something is important. Eye fixations cause the image of an object to fall on the fovea, the cone-rich high-acuity area of the retina, and so provide visual information at a high spatial resolution. The richness of the data provided by eye tracking is revealed in the plethora of dependent measures it provides. Moreover, the neural substrates of eye movement measures are largely known. Therefore, measurement of eye movements may provide insight into the underlying neuropathology of disorders affecting eye movements. Finally, because eye tracking is noninvasive, and tasks can be made simple yet still provide extremely rich data (e.g., participants can merely look at pictures of faces and natural scenes), the methodology is quite well suited to conducting critical developmental studies in young children. This entry describes how the visual scanpaths (the pattern of eye movements that occur when an individual processes a complex stimulus) of affected individuals are altered in some neurodevelopmental and psychiatric disorders. It highlights what the observation of altered scanpaths can reveal about underlying mechanisms in these disorders at the cognitive and neural levels of analysis.

Several basic types of eye movements either shift or stabilize gaze. Movements for shifting gaze direction include saccadic movements, which are rapid, ballistic movements of both eyes in the same direction; smooth pursuit movements that carefully follow a moving object; and vergence movements, which converge or diverge the eyes to focus appropriately on an object at various distances from the viewer. Gaze stabilizing movements include vestibulo-ocular movements (reflexes that stabilize images on the retina during head movements via the production of an eye movement in the direction opposite to the head movement) and the optokinetic reflex, which allows the eye to follow objects in motion when the head remains stationary. Most of the studies of eye movements in psychiatric and neurodevelopmental disorders have focused on movements that shift gaze. Disorders have included attention deficit hyperactivity disorder (ADHD), autism, schizophrenia, Tourette's syndrome, obsessive-compulsive disorder, anxiety, and depression. To illustrate this line of research, findings from studies of ADHD, schizophrenia, and autism are discussed here.

In an antisaccade task, an observer holds his or her fixation on a particular location, then a stimulus appears somewhere within the subject's visual field. The subject must move his or her eyes in the [Page 426]direction opposite of the visual onset. Success on this task requires inhibiting a reflexive saccade to the onset location, and purposefully moving the eye in the opposite direction. Neuroscience studies have revealed a critical role for prefrontal cortical regions in antisaccade tasks. An extensive body of research has demonstrated that ADHD is characterized by an increase of errors on antisaccade tasks (i.e., eye movements made in the wrong direction). These findings further characterize and quantify a cognitive mechanism underlying key aspects of the ADHD phenotype; those having ADHD have difficulties in response inhibition. Further, the work underscores abnormal development of the prefrontal cortex as an etiological factor.

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