The data indicate that the ventrobasal thalamus is necessary for the visually-released defensive upright posture and boxing which may be elicited in rats following facial anesthesia. Complete cross-sectional lesions of the ventrobasal thalamus abolished the behavior in seven out of eight animals, and it is possible that the one exception may have been due to incomplete facial anesthesia. No other neural structure besides the ventrobasal thalamus could account for the results in the animals in which visually-released boxing was abolished. The only other structure which was damaged in many cases was the parafascicular nucleus, and control data indicated that this nucleus was not critical.
A particularly effective control procedure consisted of the unilateral thalamic lesions plus closure of one eye. This procedure was made possible by the substantial crossing of the visual system in the rat; therefore, rats with unilateral ventrobasal thalamic lesions could be deprived of visual input to the undamaged side by closing the eye ipsilateral to the lesion. Whereas ipsilateral eye closure abolished the response, contralateral eye closure did not, which showed that the boxing deficit was confined to that function performed by vision and was not, for example, a general motor or motivational deficit.
A complete picture of the neural circuitry of defensive upright posture and boxing is beginning to emerge from this and related studies. Motivating stimuli (pain of footshock) apparently activate a motivational mechanism in the midbrain central gray which can activate a number of alternative defense motor patterning mechanisms including freezing, flight, and upright posture and boxing [5, 10]. The motor patterning mechanism of upright posture and boxing requires a combination of several inputs, a motivational and modulating input from the midbrain central gray [1, 8] and releasing and directing stimuli from facial tactile and/or visual inputs. Under normal circumstances the primary releasing and directing stimuli are facial tactile inputs which are processed by the trigeminal complex and presumably do not require the use of the thalamo-cortical projections of the tactile system; this is indicated by the fact that ventrobasal thalamic lesions do not by themselves produce deficits in upright posture and boxing, even if the animal is blinded.
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