Effects of thalamic stimulation in unanaesthetised animals; the arrest reaction and petit mal-like seizures, activation patterns and generalized convulsions.

A cinematographic analysis of the responses of cats to local stimulation of various portions of the thalamus and hypothalamus has been carried out with histological control and with electroencephalography correlation both during operation and post-operatively in an attempt to further investigate the hypothesis of a diencephalic origin for petit mal epilepsy. The results of this investigation may be summarized as follows: 1. (1) Electrical stimulation within the intralaminar nuclei of the thalamus, especially rostrally and bordering upon the dorso-medial and antero-medial nuclei, produced a sudden arrest of responsiveness which we have called the “arrest reaction”. During this arrest the animal would remain immobile, with no loss of tone, and failed to respond to usually effective stimuli. 2. (2) The “arrest reaction” would persist beyond the period of thalamic stimulation in the form of an after-discharge resembling very closely a petit mal type of seizure in man, including the slight twitching movements about the eyes and face so commonly seen in these attacks, and would cease abruptly. 3. (3) More intense and more rapid stimulation of the same regions producing the “arrest reaction” would produce first a petit mal-like attack followed by a major generalized convulsion or “grand mal” seizure. 4. (4) Electrocorticograms showed that stimulation of the intralaminar regions of the thalamus which produced a generalized recruiting type of control of the rhythmic activity of the cortex also produced the generalized changes in behaviour which we have termed the “arrest reaction”. During the petit mal-like seizures a bilaterally synchronous 3 per second wave and spike discharge was recorded from the cortex and was associated with a similar type of wave form in the thalamus. During the grand mal-like attacks the electrocorticogram and electrothalamogram showed rapid high voltage activity simulating closely the form of the EEG at the onset of such attacks in man. 5. (5) Autonomic system responses including a complex defensive pattern of behaviour with sham rage or a fear pattern associated with flight were reproduced particularly from the hypothalamus and occasionally from the overlying midline nuclei of the thalamus. These results corresponded closely with the detailed analysis of such responses as reported by Hess. 6. (6) Extrapyramidal types of movement such as tonic postural states, turning of the head and body, circling movements, alternating movements of a limb, and so forth were reproduced from what was presumed to be the thalamic or sub-thalamic extrapyramidal system. These results also correspond closely to those described by Hess. 7. (7) Control studies in which stimulation of sensory relay nuclei was carried out produced no such reactions but only responses which gave one the impression of a paraesthesia or of a reaction to a sensory stimulus. On no occasion was any loss in general responsiveness or change in emotional behaviour observed when stimulating sensory relay nuclei. 8. (8) It is concluded from observations in the cat that the diffuse thalamo-cortical projection system which we have called the thalamic reticular system has a very widespread and profound effect on behaviour as a whole and may be involved in the mechanism of petit mal and generalized convulsive seizures as seen in man.