An unperceived acoustic stimulus decreases reaction time to visual information in a patient with cortical deafness

Abstract

Responding to multiple stimuli of different modalities has been shown to reduce reaction time (RT), yet many different processes can potentially contribute to multisensory response enhancement. To investigate the neural circuits involved in voluntary response initiation, an acoustic stimulus of varying intensities (80, 105, or 120 dB) was presented during a visual RT task to a patient with profound bilateral cortical deafness and an intact auditory brainstem response. Despite being unable to consciously perceive sound, RT was reliably shortened (~100 ms) on trials where the unperceived acoustic stimulus was presented, confirming the presence of multisensory response enhancement. Although the exact locus of this enhancement is unclear, these results cannot be attributed to involvement of the auditory cortex. Thus, these data provide new and compelling evidence that activation from subcortical auditory processing circuits can contribute to other cortical or subcortical areas responsible for the initiation of a response, without the need for conscious perception. Responding to a stimulus with a voluntary action is typically thought to involve several processes that are mediated by cortical structures. For example, a sprinter will "get set" to launch their body out of the starting blocks, and once the starting pistol fires the sprinter must recognize that the signal has occurred and initiate that planned response. The processes involved in planning a response (preparation) and holding it ready until the go-signal occurs (storage) have been ascribed principally to cortical motor and premotor areas 1-3. Similarly, processes involved in stimulus recognition (detection/discrimination) and response initiation are thought to occur in cortical sensory and association areas as well as frontal cortical regions 4. However, data from a paradigm involving the presentation of an unpredictable high-intensity auditory stimulus that is capable of eliciting a startle reflex has suggested that lower subcortical structures may play a role in response initiation processes 5. Typical mean reaction time (RT) to the onset of an auditory stimulus is approximately 140-150 ms, with reaction to a visual stimulus being slightly longer at 180-200 ms 6. Yet, when a loud startling acoustic stimulus (SAS) is presented, RT is dramatically shortened to approximately 70-80 ms on average, while the characteristic features of the response remain largely unchanged 7,8. The startle reflex is primarily mediated by brainstem nuclei 9 and elicits a pattern of generalized flexion in various muscles with latencies of 50-100 ms. Given the short latency of both the startle reflex and prepared response, early explanations of RT speeding by startle were that the "voluntary" reaction was too short to have involved the normal cortically mediated stimulus-response processing pathways and was somehow driven at the speed of the startle response. In order for this to occur, it was argued that storage and/or initiation of the response must have involved subcortical struc...