Objective: Startle blink reflexes in humans are typically measured as orbicularis oculi electromyographic activity (ooEMG) in response to startling stimuli. When ooEMG activity cannot be measured, responses are scored as 'zero'. However, inhibition of the levator palpebrae is also involved in every blink. The present study examines whether or not during 'zero' startle responses, defined by absence of orbicularis oculi contraction, a startle blink defined by lid movement (i.e. indirect evidence of cessation of tonic levator palpebrae activity) might occur.
Methods: Both ooEMG and vertical electro-oculogram (vEOG) were recorded in 55 children and adolescents during acoustic startle stimulation. Zero ooEMG responses and their accompanying vEOG were tabulated. Fourteen participants who had 3 or more zero ooEMG responses associated with measurable vEOG responses were selected for more detailed study.
Results: For these 14 subjects, of 25.4% of usable trials scored as zero in ooEMG, 86.8% of these were accompanied by measurable vEOG responses (for all 55 subjects, 7.9% of trials had zero ooEMG responses, and 70.8% of these zero trials were accompanied by measurable vEOGs). Mean vEOG amplitude and peak latency were comparable whether associated with small ooEMG responses or ooEMGs scored as zero. These 14 participants had smaller and shorter ooEMG responses than the remainder of the sample, but other parameters of ooEMG and vEOG recordings did not differ. A single-case example shows how recording of ooEMG may have resulted in the decision that this participant was a putative 'startle non-responder' when concomitant recording of vEOG showed blink responding in the form of lid movement on 100% of trials.
Conclusions: These results suggest that in absence of orbicularis oculi contraction, cessation of the tonic activity of the levator palpebrae can occur thereby permitting the eyelid to drop in response to an auditory startle stimulus, as measured by vEOG recordings.
Significance: These findings suggest that some degree of independence in the innervation of the orbicularis oculi and levator palpebrae muscle is demonstrated when the ooEMG is putatively 'zero'. This requires re-evaluation of the construct of startle response probability.