Scott A. Holmes

The visual coding of grip aperture shows an early but not late adherence to Weber's law

An exemplar metric of goal-directed grasping (i.e., peak grip aperture) has been shown to be refractory to a psychophysical principle governing visuoperceptual estimations of object size (i.e., Webers law). This dissociation suggests that vision for action and vision for perception are mediated by absolute and relative visual information, respectively. The present investigation examined whether aperture shaping elicits a unitary or process-dependent violation of Webers law. Participants grasped differently sized objects (20, 30, 40, 50 and 60 mm of width) and just noticeable difference (JND) scores related to grip aperture were computed at the time of peak grip aperture as well as at normalized deciles of the response (i.e., 10-90% of grasping time). JNDs during the early and middle stages of the trajectory scaled to object size whereas values late in the trajectory (>50% of grasping time and including the time of peak grip aperture) did not. Thus, results show an early, but not late, adherence to Webers law and indicate that movement planning and movement control are supported via relative and absolute visual information, respectively.

Goal-directed grasping: the dimensional properties of an object influence the nature of the visual information mediating aperture shaping.

An issue of continued debate in the visuomotor control literature surrounds whether a 2D object serves as a representative proxy for a 3D object in understanding the nature of the visual information supporting grasping control. In an effort to reconcile this issue, we examined the extent to which aperture profiles for grasping 2D and 3D objects adheres to, or violates, the psychophysical properties of Webers law. Specifically, participants grasped differently sized 2D and 3D objects (20, 30, 40, and 50mm of width) and we computed the just-noticeable-difference scores associated with aperture profiles at decile increments of normalized grasping time. The aperture profiles for 2D objects showed an early through late (i.e., 10% through 90%) adherence to Webers law, whereas the late stages of grasping 3D objects (i.e., >50% of grasping time) produced a fundamental violation of the laws principles. As such, results suggest that grasping a 2D object is a top-down and cognitive task mediated via relative visual information. In contrast, the enriched shape information provided by a 3D object (i.e., stereoscopic vergence and disparity cues) allows for later aperture specification via absolute (Euclidean) visual information. Most notably, our results establish that the dimensional properties of an object influence the visual information mediating motor output, and further indicate that 2D and 3D objects are not representative proxies for one another in understanding the visual control of grasping.

Grasping time does not influence the early adherence of aperture shaping to Weber's law.

The “just noticeable difference” (JND) represents the minimum amount by which a stimulus must change to produce a noticeable variation in ones perceptual experience (i.e., Webers law). Recent work has shown that within-participant standard deviations of grip aperture (i.e., JNDs) increase linearly with increasing object size during the early, but not the late, stages of goal-directed grasping. A visually based explanation for this finding is that the early and late stages of grasping are respectively mediated by relative and absolute visual information and therefore render a time-dependent adherence to Webers law. Alternatively, a motor-based explanation contends that the larger aperture shaping impulses required for larger objects gives rise to a stochastic increase in the variability of motor output (i.e., impulse-variability hypothesis). To test the second explanation, we had participants grasp differently sized objects in grasping time criteria of 400 and 800 ms. Thus, the 400 ms condition required larger aperture shaping impulses than the 800 ms condition. In line with previous work, JNDs during early aperture shaping (i.e., at the time of peak aperture acceleration and peak aperture velocity) for both the 400 and 800 ms conditions scaled linearly with object size, whereas JNDs later in the response (i.e., at the time of peak grip aperture) did not. Moreover, the 400 and 800 ms conditions produced comparable slopes relating JNDs to object size. In other words, larger aperture shaping impulses did not give rise to a stochastic increase in aperture variability at each object size. As such, the theoretical tenets of the impulse-variability hypothesis do not provide a viable framework for the time-dependent scaling of JNDs to object size. Instead, we propose that a dynamic interplay between relative and absolute visual information gives rise to grasp trajectories that exhibit an early adherence and late violation to Webers law.

Distinct Visual Cues Mediate Aperture Shaping for Grasping and Pantomime-Grasping Tasks

ABSTRACT The authors examined whether the top-down requirements of dissociating the spatial relations between stimulus and response in a goal-directed grasping task renders the mediation of aperture trajectories via relative visual information. To address that issue, participants grasped differently sized target objects (i.e., grasping condition) and also grasped to a location that was dissociated from the target object (i.e., pantomime-grasping condition). Just noticeable difference (JND) values associated with the early through late stages of aperture shaping were computed to examine the extent to which motor output adhered to, or violated, the psychophysical principles of Webers law. As expected, JNDs during the late stages of the grasping condition violated Webers law: a result evincing the use of absolute visual information. In contrast, JNDs for the pantomime-grasping condition produced a continuous adherence to Webers law. Such a result indicates that dissociating a stimulus from a response is a perception-based task and results in aperture shaping via relative visual information.

Neer Award 2017: A rapid method for detecting Propionibacterium acnes in surgical biopsy specimens from the shoulder

BACKGROUND Propionibacterium (P) acnes infection of the shoulder after arthroplasty is a common and serious complication. Current detection methods for P acnes involve anaerobic cultures that require prolonged incubation periods (typically 7-14 days). We have developed a polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) approach that sensitively and specifically identifies P acnes in tissue specimens within a 24-hour period. METHODS Primers were designed to amplify a unique region of the 16S rRNA gene in P acnes that contained a unique HaeIII restriction enzyme site. PCR and RFLP analyses were optimized to detect P acnes DNA in in vitro cultures and in arthroscopic surgical biopsy specimens from patients with P acnes infections. RESULTS A 564 base-pair PCR amplicon was derived from all of the known P acnes strains. HaeIII digests of the amplicon yielded a restriction fragment pattern that was unique to P acnes. P acnes-specific amplicons were detected in as few as 10 bacterial cells and in clinical biopsy specimens of infected shoulder tissues. CONCLUSION This PCR-RFLP assay combines the sensitivity of PCR with the specificity of RFLP mapping to identify P acnes in surgical isolates. The assay is robust and rapid, and a P acnes-positive tissue specimen can be confirmed within 24 hours of sampling, facilitating treatment decision making, targeted antibiotic therapy, and monitoring to minimize implant failure and revision surgery.

Pro- and antisaccades: dissociating stimulus and response influences the online control of saccade trajectories.

ABSTRACT The authors examined whether the diminished online control of antisaccades is related to a trade-off between movement planning and control or the remapping of target properties to a mirror-symmetrical location (i.e., vector inversion). Pro- and antisaccades were examined in a standard no-delay schedule wherein target onset served as the movement imperative and a delay cuing schedule wherein responses were initiated 2,000 ms following target onset. Importantly, the delay cuing schedule was employed to equate pro- and antisaccade reaction times. Online control was evaluated by indexing the strength of trajectory amendments at normalized increments of movement time. Antisaccades exhibited fewer online corrections than prosaccades, and this result was consistent across cuing schedules. Thus, the diminished online control of antisaccades cannot be tied to a trade-off between movement planning and control. Rather, the authors propose that the intentional nature of dissociating stimulus and response (i.e., vector inversion) engenders a slow mode of cognitive control that is not optimized for fast oculomotor corrections.

Intracortical inhibition abnormality during the remission phase of multiple sclerosis is related to upper limb dexterity and lesions

OBJECTIVE The impact of inhibitory cortical activity on motor impairment of people with relapsing-remitting multiple sclerosis (RRMS) has not been fully elucidated despite its relevance to neurorehabilitation. The present study assessed the extent to which transcranial magnetic stimulation (TMS)-based metrics of intracortical inhibition are related to motor disability and brain damage. METHODS Participants included forty-three persons with RRMS in the remitting phase and twenty-nine healthy controls. We stimulated the dominant hemisphere and recorded from the dominant hand to assess short-interval intracortical inhibition (SICI) and cortical silent period (CSP) duration. Disability was evaluated with the Multiple Sclerosis Functional Composite (MSFC). Regional cortical thickness and lesion volume were measured. RESULTS RRMS participants with dominant upper limb dexterity impairments had prolonged CSP, but equivalent SICI, compared to participants with preserved function. CSP was not related to walking or cognitive performance. Higher normalized lesion volume correlated with longer CSP duration. When adjusting for normalized lesion volume, longer CSP significantly predicted worse dominant upper extremity impairment. CONCLUSIONS High intracortical inhibition possibly contributes to (or prevents remission from) motor impairment. Lesions may be associated with intracortical inhibition shifts. SIGNIFICANCE CSP duration and lesion burden should be considered when developing interventions aiming to mitigate motor impairment.

Assessing aggression following traumatic brain injury: a systematic review of validated aggression scales.

Background:Every year, millions of people worldwide suffer traumatic brain injuries (TBIs). Aggressive behavior, a known psychological symptom following TBI, has been regarded as an obstacle toward rehabilitation. Having measures that accurately assess aggression during rehabilitation is critical toward proper evaluation. Objective:To undertake a systematic review of the validated scales used to assess aggression in the postacute stage (≥3 months) after sustaining a TBI in the adult population. A comprehensive search was performed and studies meeting the inclusion criteria were reviewed in full. Quality and validity of supporting articles were assessed via the Downs and Black and QUADAS checklists along with their supporting statistics. Results:A total of 1329 articles were reviewed from the literature. Thirty-two were reviewed in detail and 6 studies eventually passed the exclusion criteria. Of these, 6 neuropsychological scales were represented pertaining to the measurement of aggressive behavior; however, only 1 directly addressed the validity of their scales aggression component. Conclusions:Further research is required to establish the validity of scales that specifically address aggression for use in the adult TBI population which could be used to support rehabilitation and social reintegration strategies.

Cortical Damage and Disability in Multiple Sclerosis: Relation to Intracortical Inhibition and Facilitation

BACKGROUND Multimodal research combining biomarkers of intracortical activity and cortical damage could shed light on pathophysiological and adaptive neural processes related to the clinical severity of neurological conditions such as multiple sclerosis (MS). OBJECTIVE Among people with relapsing-remitting and progressive forms of MS, we assessed the extent to which transcranial magnetic stimulation (TMS)-based biomarkers of excitatory and inhibitory cortical activity are related to cortical damage and clinical impairment. METHODS Participants included 18 healthy individuals and 36 people with MS who had a relapsing-remitting or progressive clinical course. Using TMS, intracortical facilitation (ICF), short-interval intracortical inhibition (SICI), long-interval intracortical inhibition (LICI), and cortical silent period (CSP) were obtained. Cortical volume and cortical magnetization transfer ratio (MTR) were quantified. Disability was assessed with Multiple Sclerosis Functional Composite (MSFC). RESULTS Lower mean MTR within the cerebral cortex correlated with shorter CSP among MS participants with a progressive, but not a relapsing-remitting, clinical course. Within the cortical hand knob region targeted with TMS, lower MTR was correlated with lower SICI only among individuals with relapsing-remitting MS. Longer CSP, higher ICF, lower cortical MTR, and sex were all independent significant predictors of poor upper extremity motor performance, while only cortical MTR was a significant independent predictor of total MSFC score among people with MS. CONCLUSIONS Cortical damage and cortical activity (both inhibitory and excitatory) may contribute to the severity of motor disability experienced by people with MS. When interpreting TMS-based outcomes, cortical integrity, clinical course, and symptom type should be considered.

Abnormal functional connectivity and cortical integrity influence dominant hand motor disability in multiple sclerosis: a multimodal analysis.

Functional reorganization and structural damage occur in the brains of people with multiple sclerosis (MS) throughout the disease course. However, the relationship between resting‐state functional connectivity (FC) reorganization in the sensorimotor network and motor disability in MS is not well understood. This study used resting‐state fMRI, T1‐weighted and T2‐weighted, and magnetization transfer (MT) imaging to investigate the relationship between abnormal FC in the sensorimotor network and upper limb motor disability in people with MS, as well as the impact of disease‐related structural abnormalities within this network. Specifically, the differences in FC of the left hemisphere hand motor region between MS participants with preserved (n = 17) and impaired (n = 26) right hand function, compared with healthy controls (n = 20) was investigated. Differences in brain atrophy and MT ratio measured at the global and regional levels were also investigated between the three groups. Motor preserved MS participants had stronger FC in structurally intact visual information processing regions relative to motor impaired MS participants. Motor impaired MS participants showed weaker FC in the sensorimotor and somatosensory association cortices and more severe structural damage throughout the brain compared with the other groups. Logistic regression analysis showed that regional MTR predicted motor disability beyond the impact of global atrophy whereas regional grey matter volume did not. More importantly, as the first multimodal analysis combining resting‐state fMRI, T1‐weighted, T2‐weighted and MTR images in MS, we demonstrate how a combination of structural and functional changes may contribute to motor impairment or preservation in MS. Hum Brain Mapp 37:4262–4275, 2016.