The visual accommodation response during concurrent mental activity

Concurrent mental activity seems to be a significant, nonvisual factor affecting the human accommodation response. Two experiments were conducted to determine the direction and magnitude of this accommodation response. Experiment 1 employed a concurrent, written backwards counting task. Experiment 2 employed a concurrent, mental imagery task of “thinking near” and “thinking far.” In both experiments, the concurrent secondary task effected a cumulative accommodative shift toward the visual far point of from .25 to .75 diopter away from a near (3.0 diopter) target. This accommodative shift was observed only in the presence of a stimulus field and not in open-loop (analogous to empty-field) conditions. In addition, a long-term instability in the open-loop method of obtaining the dark focus was observed. Similarities between this accommodative shift and the pupillary response are noted. The accommodation response is discussed in relationship to both an attention-sharing and an involuntary autonomic response model.