Limulus is tuned into its visual environment.

Figure 1. (A) A single frame of optic nerve imaged under 600-nm illumination. The region of the crush (demarcated with white lines) illuminates poorly and is therefore darker than the rest of the nerve. (B) The activity map of the same nerve during electrical stimulation. Data sets composed of 20 frames, each with a 2-s exposure, were collected during periods chosen randomly in which the nerve was either stimulated or unstimulated. The tissue was allowed to return to baseline for 30 s before the next data set was collected. When the nerve was stimulated, the reflectance of the active regions decreased. Principal components, which represent the best fit to the data, were used to generate the activity map. The strength of activity is denoted by the grey level in the image: a bright pixel denotes a decrease in reflectance. Activity appears to be limited to the region proximal to the crush. (C) The time course of electrically evoked intrinsic signal activity during and after 10 (solid squares) and 100 iterations (open circles) of a brief (10 s) stimulation. The peak (normalized to -1.0) occurs after the offset ofstimulation, and the return to baseline is very slow. (D) The normalized time course of visually evoked intrinsic signal activity.