Effects of string length on the organization of rat string-pulling behavior

Information-theory analysis of mouse string-pulling agrees with Fitts's Law: Increasing task difficulty engages multiple sensorimotor modalities in a dual oscillator behavior

Mouse string pulling, in which a mouse reels in a string with hand-over-hand movements, can provide insights into skilled motor behavior, neurological status, and cognitive function. The task involves two oscillatory movements connected by a string. The snout oscillates to track the pendulum movement of the string produced by hand-over-hand oscillations of pulling, and so the snout guides the hands to grasp the string. The present study examines the allocation of time required to pull strings of varying diameter. Movement is also described with end-point measures, string-pulling topography with 2D markerless pose estimates based on transfer learning with deep neural networks, and Mat-lab image-segmentation and heuristic algorithms for object tracking. With reduced string diameter, mice took longer to pull 60 cm long strings. They also made more pulling cycles, misses, and mouth engagements, and displayed changes in the amplitude and frequency of pull cycles. The time measures agree with Fitts's law in showing that increased task difficulty slows behavior and engages multiple compensatory sensorimotor modalities. The analysis reveals that time is a valuable resource in skilled motor behavior and information-theory can serve as a measure of its effective use.

Impairments and Compensation in String-pulling After Middle Cerebral Artery Occlusion in the Rat

Stroke is a leading cause of long-term disability in humans, and it is frequently associated with impairments in the skilled use of the arms and hands. Many human upper limb impairments and compensatory changes have been successfully modeled in rodent studies of neocortical stroke, especially those that evaluate single limb use in tasks, such as reaching for food. Humans also use their hands for bilaterally coordinated movements, dependent upon interhemispheric cortical projections, which are also compromised by unilateral stroke. This study describes middle cerebral artery occlusion (MCAO) dependent changes in the bilaterally dependent hand use behavior of string-pulling in the rat. The task involves making hand-over-hand movements to pull down a string that contains a food reward attached to its end. MCAO rats missed the string more often with both hands than Sham rats. When the string was missed on the contralateral to MCAO body side, rats continued to cycle through subcomponents of string-pulling behavior as if the string were grasped in the hand. Rats also failed to make a grasping motion with the contralateral to MCAO hand when the string was missed and instead, demonstrated an open-handed raking-like motions. Nevertheless, with repeated attempts, rats performed components of string-pulling well enough to obtain a reward on the end of the string. Thus, string-pulling behavior is sensitive to bilateral impairments but is achieved with compensatory adjustments following MCAO. These aspects of MCAO string-pulling provide a foundation for studies that investigate the efficacy of therapeutic intervention which might enhance neuroplasticity and recovery. DATA AVAILABILITY: The datasets generated during the current study are available upon request.

Therapeutic hypothermia demonstrates sex-dependent improvements in motor function in a rat model of neonatal hypoxic ischemic encephalopathy

Neonatal hypoxic ischemic encephalopathy (HIE) is a neurological disease caused by restricted oxygen and blood flow to the brain at or around the time of birth. Long term cognitive and motor sequelae are common and demonstrate sexual dimorphism in animal studies. Therapeutic hypothermia (TH) is the standard of care for HIE, but provides incomplete neuroprotection. Using the Vannucci model of neonatal HIE, term-equivalent 11-day old rat pups were subjected to mild-moderate hypoxic-ischemic injury (HII), and a subset of animals were treated with TH. Sex-dependent neuroprotection was measured with gross and fine motor control assays, and functional deficits detected with these assays were correlated to injury in specific brain structures. At the equivalent of human adolescence and adulthood (P51-89), accelerod and beam walking tests were used to assess gross motor function, and string-pulling and food handling tests were used to assess fine motor function. At necropsy (P94-97), brain lesions were primarily focused to the posterior cerebrum and characterized by variable reduction in cortical, thalamic and hippocampal regions and glial scarring. Gross motor impairment was detected in male rats with untreated and TH-treated HIE in the accelerod test, but beam walk test data was confounded by the lower body mass of untreated male rats. HIE animals of both sexes demonstrated deficit in the forelimb contralateral to ischemic surgery, observed as unilaterally impaired food handling behaviors, and in string pulling as decreased string contacts and increased in bracing behavior. However, kinematic analyses revealed sex-specific decreases in peak speeds in string reaching and pulling movements. In both sexes, treatment with TH improved body mass, some measures of contralateral forelimb impairment, and the severity of brain lesions to levels not different to Sham surgery rats. Unique differences in behavior following TH were observed in female rats, who took longer to consume food items but traversed beams and approached strings faster than untreated and Sham females. Future use of these motor assays may unravel the subtle, sex-specific differences in HIE outcomes and in developing a customized therapeutic approach to neonatal brain injury.