The two players can also play online while using split screen. This game mode is normally exclusive to the Xbox 360 version of Left 4 Dead and Left 4 Dead 2, but may be accessed in the PC versions if you have a controller for the second player and modify some files.
Serial memory processing - Wikipedia. Serial memory processing is the act of attending to and processing one item at a time. This is usually contrasted against parallel memory processing, which is the act of attending to and processing all items simultaneously. https://en.wikipedia.org/wiki/Serial_memory_processing
That's lower level processing. Higher levels of your brain are also activated to make sense of the sensory information. When you initiate a reflex action, like withdrawing your hand from a hot surface, the first part of the response is initiated at the spinal cord. But, as soon as the information reaches your brain, your motor cortex, cerebellum, frontal cortex, parietal cortex, etc, take over, planning and controlling movement.