Swallowing involves many complex sensorimotor events that are not thoroughly understood. Normal swallowing has both volitional (oral) and non-volitional (pharyngeal) components, which couple seamlessly to move ingested material safely into the esophagus when swallowing. Swallowing is associated with cortical activity when elicited spontaneously (response to saliva build up) and in response to a command [1
]. However, to broaden the understanding of volitional control of sensorimotor motor tasks, such as swallowing, investigations of intention, internal selection, and inhibition are becoming more frequent in the literature. Others have argued that intentional action may involve decisions of what and when to act and, in more recent studies, whether
to act [3
]. Studies on decisions whether
to act center on activation of the medial frontal cortex. Neural imaging studies of swallowing indicate multiple cortical regions are involved in healthy younger and older adults [4
]. However, cortical changes with neurological damage might cause under-activity in neurodegeneration, such as Alzheimer’s Disease (AD) [6
], prompting further research of swallowing within healthy aging adults and those with neurological disease.
Most of our current understanding of sensorimotor behavior requires participants to perform the task of interest. However, a growing body of knowledge has enriched our view of motor perception, planning and performance by examining, not only self-executed motor tasks, but also intentional off-states, imagination and observation of an action [7
]. For example, the left inferior frontal gyrus (IFG) or Broca’s area is critical for initiating a speech task, whereas the right IFG is active when healthy adults comply with instructions to avoid producing speech [10
]. Findings such as these have helped to establish models of hemispheric dominance for planning speech initiation and inhibition. They also provide a baseline for gauging alterations caused by healthy aging or neurological disease. For swallowing, the go, no-go paradigm was used with functional magnetic resonance imaging (fMRI) in healthy young adults to dissociate BOLD responses to the visual swallowing cues from actual swallowing [11
]. This was not a traditional response-inhibition study because the no-go task did not involve a bolus stimulus for the participant to suppress, possibly because it could be unsafe for participants to hold liquid in the posterior oral cavity and inhibit swallowing while lying supinely. Nonetheless, as expected, discrete cortical regions (pre- and post-central gyri, insula, anterior cingulate cortex, frontoparietal opercula) were active during the “go” task (saliva swallow) and not during the “no-go” state (no swallow performed), effectively separating the visual command to swallow from the sensorimotor behavior of swallowing.
Impaired executive functioning is often reported with healthy aging and, to a greater degree, in neurodegenerative diseases such as AD. More specifically, compliance with a “do not” command for motor tasks involves executive control over motor regions for initiation, which also changes with age and in the early phases of AD [12
]. Studies of response-inhibition show that AD patients have impairments beyond those observed in healthy aging for both behavior and BOLD response [14
We have shown that healthy aging and early AD are associated with BOLD differences for execution of various swallowing conditions [6
]. Central control of motor-related tasks can inform models for diagnosis and treatment of dysphagia (swallowing impairment). In this preliminary study we examined age- and AD-related differences in BOLD response for an intentional swallowing “off-state” or compliance with performing a “Do Not Swallow” (DNS) command during fMRI. The goal was to understand the intentional swallowing off-state differences among healthy young and old adults and in early AD, as they have known differences in swallowing [6
]. We propose that BOLD differences for an intentional swallowing off-state will involve (a) similar cortical regions as in other traditional response inhibition fMRI studies in our healthy groups, although with greater BOLD response in the older group and (b) differences in cortical activation patterns with disease. A secondary goal was to determine whether laterality differences exist in the intentional swallowing off-state across the three groups. We expected that activation in the young group would be more lateralized to the left than in the healthy old or AD group.