Picture the classic I Love Lucy episode where Lucy begins a new job dipping and packaging chocolates that come to her via an conveyor belt.  When the belt runs relatively slowly at first, Lucy is quite capable of handling the load of the job.  Once the belt’s speed increases significantly, however, Lucy’s capacity for processing and appropriately packaging the chocolates is exceeded.  Chocolates begin going by her and begin falling uselessly to the floor off the end of the belt.   Attempts to eat the chocolates, place them in her hat, or place them in her blouse result in the chocolates being stowed nearly everywhere but where they belong.

Rewrite this scene and add to the mayhem a bit. Imagine Lucy being simultaneously responsible for two separate conveyor belts that are filled with candies.  It is not hard to picture her vain attempts to switch back and forth between the relentlessly moving belts.  Our beloved Lucy would inevitably  fail even more miserably. Chocolate after chocolate from each belt would fail to reach the desired destination of being neatly packaged in the smartly wrapped box.

A high school chemistry student's brain is not unlike Lucy in her attempts to dip and box the candies in the chocolate factory.  Information can come at a student through auditory, visual,  tactile, taste, and olfactory sensory inputs much like the streams of chocolates that came unabatedly at Lucy.  The chemistry student's brain has a limited amount of working memory capacity and can only attend to a few (traditionally about seven plus or minus two-- but probably fewer)  items at a time.  If the chemistry content is new to the student, all (or nearly all) of this working memory capacity will need to be utilized in attempts to understand, in attempts to make connections with existing knowledge, and in attempts to conduct enough mental rehearsals to move the knowledge into long term memory in a retrievable form.

Competition from a secondary source of information can, at the very least, take up a significant amount of the finite quantity of working memory capacity that the student has available.

Sweller’s concept of cognitive load (Merrienboer & Sweller, 2005) proposes that individual’s working memory does indeed have a limited capacity; limited in the amount of information that it can manage, the speed at which the information can be handled, and the length of time that it can be maintained before it is lost (Barrouilet, Bernardin, Portrat, Vergauwe, & Camos, 2007).

Simultaneous sensory inputs are indeed a reality of life.  The two most common types of sensory inputs come in the form of visual information and audible information.  When presented sympathetically, an audio and visual form of the same message can reinforce the message and facilitate more effective learning.  

Kalyuga, Chandler, and Sweller (1999) examine both sides of this equation, researching the interference (cognitive load) that can be introduced by additional sensory inputs and the benefit that can be gained when the secondary channel of input works in concert with the primary message.  Moreno and Mayer (1998) also contribute to this understanding.

 In the chemistry classroom situation described above, the messages that the student is attempting to attend to are not in concert with each other.   They are in competition with each other over any of the mental processing power that the student's brain has to offer.  With the magazine's information being attended to in the visual channel and with the lecture probably being attended to in the audio channel alone, it is unavoidable that "some of the chocolates are going to go past on the conveyor belt".
 
In some regards, the student's comment may have a grain of truth.  Students today seem to be more comfortable "multi-tasking".   Even though they may seem more comfortable with switching from one task to another, it inevitably introduces a cost upon working memory function (Liefooghe, Barrouillet, Vandierendonck, & Camos, 2008).  This cost inevitably will cause a drop in learning performance and at least some information from each source is going to be missed, misunderstood, cognitively orphaned, just very likely to be forgotten in the near future.

The direction of attention is such a key component of any learning situation.  In this classroom context as it is presented it seems as though there is an issue of motivation.  The student in question apparently is missing the motivation that drives him to direct his attention upon the academic content at hand.