title: StartReact restores reaction time in HSP: evidence for subcortical release of a motor program.
abstract: Startling acoustic stimuli (SAS) can accelerate reaction times ("StartReact" effect), but the underlying mechanism remains unclear. Both direct release of a subcortically stored motor program and a subcortically mediated trigger for a cortically stored motor program have been hypothesized. To distinguish between these hypotheses, we examined the StartReact effect in humans with pure hereditary spastic paraplegia (HSP). Delayed reaction times in HSP patients in trials both with and without a SAS would argue in favor of a cortically stored response. We instructed 12 HSP patients and 12 matched controls to respond as rapidly as possible to a visual imperative stimulus, in two different conditions: dorsiflexion of the dominant ankle; or flexion of the dominant wrist. In 25% of trials, a SAS was delivered simultaneously with the imperative stimulus. Before these tests, subjects received five SAS while standing to verify normal function of the reticulospinal tract in HSP. Latencies of startle responses in sternocleidomastoid and tibialis anterior muscles were comparable between patients and controls. During the ankle dorsiflexion task, HSP patients had an average 19 ms delay in reaction times compared with controls. Administration of a SAS accelerated ankle dorsiflexion in both groups, but more so in the patients, which completely normalized their latencies. The wrist flexion task yielded no differences in onset latencies between HSP patients and controls. The reticulospinal tract seems unaffected in HSP patients, because startle reflex onsets were normal. The corticospinal tract was affected, as reflected by delayed ankle dorsiflexion reaction times. These delayed onsets in HSP were normalized when the imperative stimulus was combined with a SAS, presumably through release of a subcortically stored motor program conveyed by the preserved reticulospinal tract.
authors.affiliation: Nijmegen Centre for Evidence Based Practice, Department of Rehabilitation and Donders Institute for Brain, Cognition and Behaviour, Department of Neurology, Radboud University Medical Centre, 6500 HB Nijmegen, The Netherlands, and Sint Maartenskliniek Research, Development and Education, 6500 GM Nijmegen, The Netherlands.
authors.forename: Jorik
authors.initials: J
authors.lastname: Nonnekes
authors.forename: Lars B
authors.initials: LB
authors.lastname: Oude Nijhuis
authors.forename: Mark
authors.initials: M
authors.lastname: de Niet
authors.forename: Susanne T
authors.initials: ST
authors.lastname: de Bot
authors.forename: Jacobus W
authors.initials: JW
authors.lastname: Pasman
authors.forename: Bart P C
authors.initials: BP
authors.lastname: van de Warrenburg
authors.forename: Bastiaan R
authors.initials: BR
authors.lastname: Bloem
authors.forename: Vivian
authors.initials: V
authors.lastname: Weerdesteyn
authors.forename: Alexander C
authors.initials: AC
authors.lastname: Geurts
mesh_terms: D000161:Acoustic Stimulation / Q000379:methods; D000328:Adult; D000368:Aged; D004525:Efferent Pathways / Q000502:physiology*; D004576:Electromyography / Q000379:methods; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009044:Motor Cortex / Q000502:physiology*; D011712:Pyramidal Tracts / Q000502:physiology*; D011930:Reaction Time / Q000502:physiology*; D013216:Reflex, Startle / Q000502:physiology*; D015419:Spastic Paraplegia, Hereditary / Q000175:diagnosis / Q000503:physiopathology*; D055815:Young Adult