Most people realize that being deeply engaged in a task means they are more likely to miss irrelevant (to that task) stimuli and that in order not to miss it they have to amp up the stimulus – make it louder, brighter, bigger, more intense.
The brain – human, canine, feline, murine, etc. – has limited processing power; it must allocate its resources to the task at hand and when we divide our attention, performance suffers and sensory sensitivity diminishes.
A study (van der Wal) looked into the effects of task loading and taste perception. Participants were asked to rate sweet and salty substances under varying cognitive loads. The results revealed that participants experienced reduced taste perception under increasing task loads, a finding consistent with Cognitive and Perceptual Load Theory. There is even evidence that pain perception in animals is modulated by attentional shifts.
Trainers often use high-value treats when working under difficult conditions; these treats are typically smellier/bigger/tastier than the regular food used for reinforcement. When we use “high value” treats because we are working under distracting conditions, the high-value treat may not be working because it is providing a greater motivation to the dog. It may be that it raises the sensory stimulus to compensate for decreased perception due to task loading.
Focus and intentional attention are key to training; if you are breaking out the bacon on a regular basis then attention rather than motivation might be the problem. Hopefully I’ll expand on load theory at a later date; for now the short lesson of load theory is this; keep it simple, do not divide attention, don’t put senses in competition and reduce cognitive load.
REFERENCE
van der Wal RC, & van Dillen LF (2013). Leaving a Flat Taste in Your Mouth: Task Load Reduces Taste Perception. Psychological science PMID: 23722984
Related articles
- Working While Eating Reduces Ability to Taste Salt and Sugar (scienceabstracts.wordpress.com)
Science and Dogs 
I did not know about high value treats, but it makes sense. I remember the reaction to cheese was off the charts with my dogs. They viewed it much more favorably and would even fight each other to get it. I had to stop giving cheese as the Samoyed was too aggressive to the Akita. Not a smart thing on her part.
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