A novel device to measure power grip forces in squirrel monkeys

Abstract

Understanding the neural bases for grip force behaviors in both normal and neurologically impaired animals is imperative prior to improving treatments and therapeutic approaches. The present paper describes a novel device for the assessment of power grip forces in squirrel monkeys. The control of grasping and object manipulation represents a vital aspect of daily living by allowing the performance of a wide variety of complex hand movements. However, following neurological injury such as stroke, these grasping behaviors are often severely affected, resulting in persistent impairments in strength, grip force modulation and kinematic hand control. While there is a significant clinical focus on rehabilitative strategies to address these issues, there exists the need for translational animal models. In the study presented here, we describe a simple grip force device designed for use in non-human primates, which provides detailed quantitative information regarding distal grip force dynamics. Adult squirrel monkeys were trained to exceed a specific grip force threshold, which was rewarded with a food pellet. One of these subjects then received an infarct of the M1 hand representation area. Results suggest that the device provides detailed and reliable information on grip behaviors in healthy monkeys and can detect deficits in grip dynamics in monkeys with cortical lesions (significantly longer release times). Understanding the physiological and neuroanatomical aspects of grasping function following neurological injury may lead to more effective rehabilitative interventions.

Figure 1

(A) Photograph of the grip device. Arrow indicates the manipulandum (grip sensor). Scale bar = 5cm. (B) Close up photograph of grip manipulandum, arrow indicates placement of force sensor within the cylinder. (C) Drawing of a squirrel monkey using the device.

Figure 2

Representative grip force profiles for all three subjects, for all tested reward thresholds. Note MK1’s profile reflects its particular gripping strategy of rapid, multiple flexion grip events, while both MK2 and MK3 tended to perform single flexion grips. X-axis is time in ms (0–900ms), Y-axis is force in grams (0–300g).

Figure 3

Mean maximum grip force (A) and mean peak force deviation from reward threshold (B) for each subject across reward thresholds. All subjects increased grip forces with higher reward thresholds. All subjects also tended to exert forces greater than the required threshold (B), although the amount of this force deviation significantly decreased as reward threshold increased.

Figure 4

Mean grip (A) and release rates (B) for all subjects across all reward thresholds. Reward thresholds did not appear to affect either grip or release rates. Data are means ± S.D

Figure 5

Peak force versus grip rate (top panels) or release rate (bottom panels) for all three monkeys (all trials from all thresholds plotted). Note that MK1 appears to have two “bands” in both the grip and release scatterplots, perhaps consistent with his behavioral strategy differences compared to MK2 and MK3. Data are means ± S.D.

Figure 6

Grip and release rates for MK1 for 8 weeks after a ischemic infarct in M1. Cortical infarct produced a marked reduction of both grip and release rate, although both rates exhibited gradual recovery over time. Asterisk indicates significant reduction (p<0.05) vs. pre-infarct performance. Data are means ± S.E.M