Their studies concentrated on a region of the motor cortex they have dubbed the polysensory zone, which Graziano and other researchers had shown could be induced to trigger flinching by electrical stimulation.
However, Graziano and Cooke sought to test the hypothesis that the polysensory zone controls the natural behavioral flinch; motor cortex is usually not thought to control full, natural movements. To do this, they investigated how increasing or decreasing neuronal activity in the region affected the flinch response.
In their experiments with monkeys, the researchers targeted the region with drugs that either enhanced or reduced neuronal activity. They then recorded how the monkeys responded to a mild puff of air to the face.
The researchers found that the activating drugs enhanced the flinch response--which included a squint, a blink, lifting of the upper lip, flattening of the ears, and a protective arm and shoulder movement.
Flinching consists of two phases--an initial, general startle response, followed by a longer defensive response more directed toward protection from a threat. By measuring the electrical activity of the animals' eye muscles during the response, the researchers could follow it in greater detail. Significantly, they found that the initial startle response was unaffected by the drug, as were the nondefensive movements the animals made between trials. Only the sustained, directed phase of the flinch was affected.
Conversely, the researchers found that a drug that reduced neuronal activity in the polysensory zone selectively reduced the sustained, directed phase of the flinch.
The researchers' analysis indicated that the effects on the polysensory zone were partly due to alteration of the muscle response as well as to effects on the sensory response to the air puff.
The researchers concluded that their findings help reveal the role of the polysensory zone (PZ) as a "hotspot" that is relatively specialized for processing a particular set of stimuli--in this case those that represent a sudden approach of objects to the body.
"PZ may provide an example in the motor system of a relative hotspot for the ethologically important function of defense of the body surface, although PZ may of course have other sensory and motor functions not tested in the present experiment," they wrote.
Dylan F. Cooke and Michael S.A. Graziano: "Super-Flinchers and Nerves of Steel: Defensive Movements Altered by Chemical Manipulation of a Cortical Motor Area"
Publishing in Neuron, Volume 43, Issue 4, 19 August 2004, pages 585-593.
Journal
Neuron