The Hoffmann reflex from the flexor pollicis longus of the thumb in left-handed subjects: spinal motor asymmetry and supraspinal facilitation to Cattell's intelligence test.

The spinal motor asymmetry was studied in left-handers. Hand preference was assessed by Geschwind scores (GS), and hand skill by peg moving task, the reflex responses were recorded from the long flexor muscle of the thumb. The H-reflex could be elicited by averaging during cortico-spinal facilitation (voluntary isometric force). The Cattell's Culture Fair Intelligence Test was used to assess the individual differences in mental abilities. H-reflexes were found to be significantly larger on the left than the right side. The amplitude of H-reflex increased linearly with force applied to transducer by the thumb. This facilitation was more pronounced for the left than the right reflexes. Removal of the visual visual input caused facilitation in H-reflex (supraspinal disinhibition). Post-activation potentiation was also observed in H-reflex. There was a positive linear correlation between the degree of left-hand preference (-GSs) and left-hand skill. The correlation for the right-hand skill was not significant. There was a linear correlation between the degree of left-hand preference and the right minus left hand skill. There was an inverse correlation between left-hand skill and H-reflex from left. The correlation for the right side did not reach the 5% significance level. The force-reflex relation did not show any significant change to IQ for the right H-reflex. The left H-reflexes were significantly larger in subjects with high IQ than those with low IQ. The regression line and its slope for the force-reflex relation on the left was found to be higher in subjects with high IQ than those with low IQs. There was a positive linear relationship between IQ and H reflex from left. The correlation for the right side was not as pronounced as that for the left side. The left minus right H reflex was also positive linearly correlated with IQ. These results provide further evidence for the psycho-motor hypothesis (Tan. 1988b) as well as the spinal motor asymmetry to handedness. It was suggested that lateralization of cognitive and motor functions would be essential to create subjects with high psychomotor capacity.