According to Pulfrich's phenomenon, if a neutral density filter is placed over a patient's left eye, how will the pendulum appear to move?

In Pulfrich's phenomenon, the perception of motion can be influenced by differences in the visual processing of each eye, particularly when one eye is subjected to reduced light input—like when a neutral density filter is placed over it. When the left eye is filtered, it receives less light and thus transmits visual information more slowly compared to the unfiltered right eye.

As the pendulum swings, the brain interprets the signal from the left eye as delayed in comparison to the right eye. This asynchronous visual information leads to the perception of motion appearing to deviate. Specifically, the pendulum will appear to move in a circular motion, potentially creating an illusion of depth or an outward arc.

The effect will cause the pendulum to appear to move in a clockwise direction from the observer’s perspective, as the brain compensates for the delay in the visual information from the left eye. This results in a cohesive but distorted perception of the pendulum's trajectory, aligning with the correct interpretation of Pulfrich's phenomenon in the given scenario.