Idiosyncratic initiation of saccadic face exploration in humans.

Visual processing and subsequent action are limited by the effectiveness of eye movement control: where the eyes fixate determines what part of the visual environment is seen in detail. Visual exploration consists of stereotypical sequences of saccadic eye movements which are known to depend upon both external factors, such as visual stimulus features, and internal cognition-related factors, such as attention and memory. However, how these two factors are balanced is unknown. One determinant might be the familiarity or ecological importance of the visual stimulus being explored. Recordings of saccades for human face stimuli revealed that their exploration was subject to strong individual biases for the initial saccade direction: subjects tended to look first to one particular side. We attribute this to internal factors. In contrast, exploration of landscapes, fractals or inverted faces showed no significant direction bias for initial saccades, suggesting more externally driven exploration patterns. Thus the balance between external and internal factors in scene exploration depends on stimulus type. An analysis of saccade latencies suggested that this individual preference for first saccade direction during face exploration leads to higher effectiveness through automation. The findings have implications for the understanding of both normal and abnormal eye movements.