Dom V. Finocchio

Correlations between activity of motor cortex cells and arm muscles during operantly conditioned response patterns.

SummaryMonkey motor cortex cells were recorded during isolated, isometric contractions of each of four representative arm muscles — a flexor and extensor of wrist and elbow — and comparable response averages computed. Most cells were coactivated with several of the muscles; some fired the same way with all four and others with none. Results suggest that many precentral cells have a higher order relation to muscles than motoneurons.Operantly reinforced bursts of cell activity were associated with coactivation of specific muscles, called the cells “motor field”; the most strongly coactivated muscle was usually the one whose isolated contraction had evoked the most intense unit activity. During active elbow movements most cells fired in a manner consistent with their isometric patterns, but clear exceptions were noted. Differential reinforcement of unit activity and muscle suppression was invariably successful in dissociating correlations.The strength of each unit-muscle correlation was assessed by the relative intensity of their coactivation and its consistency under different response conditions. Several cells exhibited the most intense coactivation with the same muscle during all conditions. Thus, intensity and consistency criteria usually agreed, suggesting that strong correlations so determined may operationally define a “functional relation”.However, correlations in the sense of covariation are neither necessary nor sufficient evidence to establish anatomical connections. To test the possibility of direct excitatory connections we stimulated the cortex, but found lowest threshold responses in distal muscles, even from points where most cells had been strongly correlated with proximal muscles. Post-spike averages of rectified EMG activity provided scant evidence for cell-related fluctuations in firing probabilities of any muscles.

Smooth pursuit in 1- to 4-month-old human infants

The ability of human infants < or = 4 months of age to pursue objects smoothly with their eyes was assessed by presenting small target spots moving with hold-ramp-hold trajectories at ramp velocities of 4-32 deg/sec. Infants as young as 1 month old followed such target motions with a combination of smooth-pursuit and saccadic eye movements interrupted occasionally by periods when the eyes remained stationary. The slowest targets produced variable performance, but targets moving 8-32 deg/sec produced consistent pursuit behavior, even in the youngest infants. By the fourth month, eye-movement latency decreased and smooth-pursuit gain and the percentage of smooth pursuit per trial increased for all target velocities, though these measures had not yet reached adult levels.

Saccadic eye movement deficits in the MPTP monkey model of Parkinson's disease

Saccadic eye tracking was studied in a monkey given i.v. injections of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The Parkinson-like symptoms which appeared in the animals general motor behavior (akinesia, bradykinesia, hypokinesia) were also observed in its eye tracking. Similar oculomotor deficits are seen in patients with idiopathic Parkinsonism. The MPTP model offers excellent possibilities for studying the mechanisms underlying the motor disabilities of Parkinsons disease.

Obtaining a quantitative measure of eye movements in human infants : a method of calibrating the electrooculogram

We have developed a calibration procedure that combines the measurement of the EOG voltage with the concurrent assessment of the actual direction of gaze as revealed by the corneal reflection of a target light. Using this method, we have been able to calibrate the eye-position signal recorded from 2- and 3-month-old infants. Our results show that in young infants (1) the EOG is linearly related to eye position to at least +/- 20 deg; (2) the slopes of the calibration lines measured early and late in the same test session were not significantly different at the 0.1 level; (3) at the most eccentric eye position, the calibration was accurate to within +/- 1 deg; and (4) an abbreviated calibration at 0 and +/- 15 deg, which took less than 2 min, produced essentially the same slope (t-test not significant at the 0.1 level) as a longer procedure that tested at every 5 deg between +20 and -20 deg.

Characteristics of complex voluntary mandibular movements in the monkey before and after destruction of most jaw muscle spindle afferents

Summary To study the role of proprioception in voluntary movement, two monkeys were trained to use their mandible to control the position of a lever that electronically loaded the jaw. They “tracked” stationary and moving targets both with and without visual feedback. Lesions of the tract of the mesencephalic nucleus of the fifth nerve, which destroyed most jaw muscle proprioceptors, did not interfere, even transiently, with the ability to perform the basic requirements of the jaw tracking tasks. There was an increase in low-frequency jaw tremor, and a small increase in the tracking error when the visual feedback signal was present, but these effects may have resulted from damage to other neural pathways. We conclude that muscle spindles are not necessary to perform well-practiced movements.

Motor Fields of Precentral Cells Elicited by Operant Reinforcement of Unit Activity

Motor responses in which a given motor cortex cell may play a functional role can be elicited by operantly reinforcing bursts of cell activity and observing the correlated behavioural responses. Under isometric conditions, operant bursts were often repeatedly correlated with EMG bursts in specific contralateral arm muscles. These EMG bursts broadly coincided with the operant unit bursts, but their onset and peak usually followed the onset and peak of the precentral unit burst. One may refer to the set of muscles co-activated with operant bursts of a motor cortex cell as the cell’s “motor field”. Under isometric conditions the motor field of a given cell generally remained stable over many bursts, not only with respect to the set of co-activated muscles, but also with respect to the relative intensity of their activation. Different units in the same cortical region could have quite different motor fields. Many of the unit-muscle correlations observed when the unit was reinforced were replicated during other reinforced response patterns.