Friday, July 22, 2011

The Direct Pathway and Winner Take All

The direct pathway conveys motor signals governing speech from regions within the cerebral cortex through the basal ganglia. The basal ganglia is responsible for choosing from among competing motor signals the one that best conveys the intent of the speaker. This process may involve, for example, the selection of a single word from among a number of competing alternatives.

The manner in which this selection is facilitated by the basal ganglia circuits is through a ‘winner-take-all’ mechanism. And this is where the indirect pathway comes into play. As indicated in the previous post, the indirect pathway provides inhibitory signals that effectively screen out the unwanted motor signals.

A more complete representation (than the one presented in the previous post) of the signals from the direct pathway is shown in the figure below. The “preferred” speech motor signals are the first, fourth, fifth, and ninth signals in the diagram. The remaining competing signals are less preferred and have substantially smaller amplitudes.
If the amplitudes of the inhibiting signals from the indirect pathway are at level A, then the less preferred competing signals will be screened out. There then may be a sufficient amplitude of the preferred direct pathway signals above the inhibiting “noise” of the indirect path to execute the speech motor function. On the other hand, if the inhibiting signal amplitudes are at point B, some of the preferred signal amplitudes may be too weak to execute the associated motor functions.

We indicated in the previous post that possible areas of brain dysfunction leading to disfluency may reside in the midbrain, namely various components of the basal ganglia, or in the cerebral cortex itself. Weak signals emanating from the cortex or the inability of the basal ganglia to process adequate incoming signals may result in disfluency.

We can liken the basal ganglia to a radio receiver and the cerebral cortex to a radio station. A particular radio station may be sending weak signals and the gain* of the receiver’s amplifier may not be adequate to increase the amplitude of this signal. On the other hand, the signal from the radio station may be adequate, but the amplifier may be faulty in that it generates a lot of static noise that effectively blocks the signal.


* The gain of an amplifier is the ratio of output amplitude to input amplitude

No comments: