Final answer:
The intrusion of digits from the unattended ear occurs because the brain processes auditory information including the interaural time and intensity differences, which helps in sound localization even when attention is directed to the attended ear.
Step-by-step explanation:
The phenomenon described relates to sound localization and how the brain processes auditory information. The intrusion of digits from the unattended ear likely occurs due to the binaural cues that our brain uses to locate where a sound is coming from. When a sound is played into the attended ear, the intensity and timing of that sound is given prominence, but the unattended ear is still receiving information. Because the brain calculates the interraural time difference and the interaural intensity difference for sound localization, it is possible for the unattended sounds to be processed, especially if the attended sounds are not highly engaging or if there is a significant difference in the content or nature of the sounds.
Cues like the interaural time difference rely on the fact that sound reaches the ear closer to the sound source a few microseconds before the other ear due to the speed of sound and the distance it travels. If a sound comes from the left side, it will reach the left ear first and will also be slightly louder in the left ear. These differences allow the brain to locate the sound horizontally. When focusing on sounds from the left ear, the intrusion of digits from the unattended, or right, ear happens because the auditory system still registers and processes the sounds from both ears and the brain's attention is not completely exclusive to one ear.