Biology 202
2003 Second Web Paper
On Serendip

Corollary discharge assists both human and non-human animals distinguish between self-generated (internal) and external motor responses. By sending signals which report important information about movement commands and intension animals are able to accurately produce motor sequences with ease and coordination. When a motor command initiates an electric organ discharge, the signal transmits important information to the brain which serves as a feed-back mechanisms which assist with self-monitoring; this is formally defined as corollary discharge(6).

Although the corollary discharge system is one of the most important systems which animals possess for the control and detection of motor movements, its specific neurological mapping is largely unknown. Many studies which investigate the specific nature of corollary discharge focus on either auditory or visual sensory perception. Current investigations of corollary discharge are commonly associated with the use non-human primates and humans who suffer from Schizophrenia. By using non-human primates for the investigation of the neuronal network of the corollary discharge system both invasive and non-invasive investigation may be explored. In addition, investigating Schizophrenics who suffer from auditory hallucinations, the inability to differentiate between spoken and thought speech (1), may also significantly contribute to the advancement and increased understandings of the corollary discharge systems.

Sommer et. al., proposed a neuronal pathway for corollary discharge in non-human primates. He suggested that this pathway extends from the brainstem to the frontal cortex. Within the pathway the brain was to initiate movement as well as supply internal information which was then used by sensory systems to adjust for resultant changes(5). These adjustments were said to occur within the peripheral receptors and motor planning systems which would then prepare the body for future movements(5). Corollary discharge signals in Sommer's study were identified as movement related activity which projected upstream (up the spinal cord) away from motor neurons, transmitting information but not causing any actual movement (4). By measuring the neuronal firing of the superior colliculus in the frontal cortex during normal and stimulated saccade movements of the eye, Sommer measured the corollary discharge signals in monkeys. The results of Sommer's study suggested that non-human primates did in fact transmit corollary discharge signals during eye saccades which was suggestive of a brainstem to frontal cortex pathway for transmission.

While Sommer's study provided novel and interesting ideas in regard to the specific pathway of corollary discharge, it focused largely of saccadic eye movements in non-human primates. Sommer's non-human primate research findings are consistent with human research; however, they may not be completely generalizable to all human populations. Currently there are various methods for the investigation of corollary discharge which are used in human population. The use of Electroencephalogram (EEG), functional magnetic resonance imagining (fMRI), positron emission tomography (PET) and single photon emission computer tomography (SPECT) all have worked to provide rich and accurate data in regard to both the specific neuronal signaling and function of the corollary discharge system in humans (4). Some of the most promising research, which utilize many of these methods, has been found with the investigation of human subject who suffer from Schizophrenia, a disorder which has been suggested to result from a deficit in corollary discharge function.

Schizophrenic patients with positive symptoms commonly experience auditory hallucinations (2). Although all schizophrenics do not experiences these hallucinations, a modest population report having these experiences (1). The inability to distinguish between covert speech (thoughts) and overt speech (talking) are common characteristics which are said to be a distinguishing marker of auditory hallucinations (1). These deficits of perception have been thought to occur as a result of the dysfunction of the corollary charge system. Without the proper facilitation of overt and covert speech by this system, these different speech mechanisms are easily confused and may result in server neuroses. Normal persons who do not suffer from Schizophrenia are said to be more sensitive to the factors (signals) which help to distinguish overt and covert speech. With these sensitivities, they are better able to make accurate decisions about what speech mechanism they perceive.

The role of corollary discharge in schizophrenia was investigated by Ford, et. al in a 2001 study using Schizophrenics who had reported having auditory hallucinations. Ford recorded subjects responses to covert and overt speech using EEG and sound recordings (1). The study's findings suggested a frontal-temporal pathway for corollary discharge signaling as well as a reduced sensitivity to signaling in schizophrenics providing a basis for the inability to efficiently distinguish between covert and overt speech. While these findings suggest only that Schizophrenics elicit less attention resources (EEG acoustic probe stimulation reduced) to overt speech as apposed covert speech it provides a explanation of why Schizophrenics might experience auditory hallucinations (1).

An additional study by Ford et. al provided a more extensive explanation of the corollary deficits associated with the hallucinations of Schizophrenics. Research findings suggested that Schizophrenics lacked an ability to recognize self-generated speech as their own (2). This inability to distinguish between internal self-general speech and externally generated speech was suggested to result from deficiencies in the corollary discharge systems of Schizophrenics who experienced auditory hallucinations (2).

A recent study by Ray Li, et. al explored the altered performance of schizophrenics in an auditory detection and discrimination task. While the study's findings concurred with the existing literature which suggest a corollary discharge deficit, there was no evidence found which support the idea of the deficit as specific to auditory hallucinations (4). Both positive and negative symptomed Schizophrenics displayed decreases in their sensitivity to perceptual channels during auditory detection tasks as compared to normal subjects. Schizophrenics who experienced hallucinations, however did not significantly differ from non-hallucinating Schizophrenics on their level of perceptual sensitivity (4). These findings pose an interesting contradiction which challenges the original correlation between dysfunctional corollary discharge systems and auditory hallucinations in Schizophrenic patients. It suggested that these hallucinations in fact may not occur as a result of deficits in the corollary discharge systems, but possibly from another underlying mechanism (4).

Both the neurological mapping of the corollary discharge system and its implications in Schizophrenia remain unclear. Sommer et. al provide interesting findings suggesting a brainstem to frontal cortex pathway for corollary discharge in non-human primates. These findings correlate in many ways to the frontal-temporal pathway which Ford proposed in his investigation of Schizophrenic patients. While it is logical that the frontal regions of the brain contributes to the underlying mechanisms of corollary discharge, considering it contains areas such as the Brocas and Wernikies (areas commonly implicated in decision making and planning), the specific neural network of the corollary discharge system other contributing regions requires much more investigation to be determined. Current research regarding the role of corollary discharge and Schizophrenia provide useful avenues for investigation of the specific nature of corollary discharge, if Schizophrenia is truly a disorder which corollary discharge is dysfunctional. While the investigations of both Ford and Ray Li support this idea, various studies propose apposing findings. Further investigation of both the specific deficits of Schizophrenics perceptual incapacities and there relationship to the specific nature of the corollary discharge system should be perform in an effort to better understand and interpret the mapping and functional properties of the corollary discharge system in Schizophrenics and of normal persons.

References

1)Science Direct ,

2)Science Direct,

3)Letters to Nature,

4)Schizophrenia Research, PubMed,

5)Science Magazine,

6)University of Waterloo Cognitive-Neurospych


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