Speeding up reaction time with invisible stimuli

Normal subjects react more quickly to a pair of visual stimuli than to a stimulus alone. This phenomenon is known as the redundant signal effect (RSE) and represents an example of divided visual attention in which signal processing is carried out in parallel to the advantage of response speed. A most interesting aspect of this phenomenon is that it can occur when one stimulus in a pair cannot be consciously detected because of hemianopia or unilateral extinction resulting from brain damage. Here, we report that a similar dissociation between visual awareness and visually guided behavior is present in normal subjects who show an RSE even when the luminance of one of a pair of stimuli is below detection threshold. The observed RSE cannot be attributed to probability summation because it violates Miller's race inequality and is likely to be related to neural summation between supra- and subthreshold stimuli. Given that a similar implicit RSE is present in hemispherectomy patients, we hypothesize that the site of this summation might be the superior colliculus (SC).     

Carotid baroreflex function during and following voluntary apnea in humans

Muscle sympathetic nerve activity (MSNA) and arterial pressure increase concomitantly during apnea, suggesting a possible overriding of arterial baroreflex inhibitory input to sympathoregulatory centers by apnea-induced excitatory mechanisms. Apnea termination is accompanied by strong sympathoinhibition while arterial pressure remains elevated. Therefore, we hypothesized that the sensitivity of carotid baroreflex control of MSNA would decrease during apnea and return upon apnea termination. MSNA and heart rate responses to -60-Torr neck suction (NS) were evaluated during baseline and throughout apnea. Responses to +30-Torr neck pressure (NP) were evaluated during baseline and throughout 1 min postapnea. Apnea did not affect the sympathoinhibitory or bradycardic response to NS (P > 0.05); however, whereas the cardiac response to NP was maintained postapnea, the sympathoexcitatory response was reduced for 50 s (P < 0.05). These data demonstrate that the sensitivity of carotid baroreflex control of MSNA is not attenuated during apnea. We propose a transient rightward and upward resetting of the carotid baroreflex-MSNA function curve during apnea and that return of the function curve to, or more likely beyond, baseline (i.e., a downward and leftward shift) upon apnea termination may importantly contribute to the reduced sympathoexcitatory response to NP.     

Inter-relationships among anticipatory EMG activity, Hoffmann reflex amplitude and EMG reaction time during voluntary standing movement

In order to understand the process of executing a voluntary standing movement, the parameters latency (AEA-LT), duration (AEA-DUR) and amplitude (AEA-AMP) of the anticipatory electromyographic (EMG) activity (AEA) in the tibialis anterior muscle, Hoffmann (H) reflex amplitude in the soleus muscle (Sol) prior to the onset of EMG activity in that muscle, and EMG reaction time (EMG-RT) were measured during heel raising from the standing position. The following results were obtained: the three parameters of AEA correlated with EMG-RT in each subject; the average values for all nine normal subjects were r = 0.856 for AEA-DUR, r = 0.448 for AEA-LT and r = -0.215 for AEA-AMP; for the group the mean value of AEA-DUR correlated significantly with that of EMG-RT (r = 0.983, P less than 0.01), while no such significant correlation was observed for AEA-LT; the average value of the AEA-DUR in three slower EMG-RT performers (SLOW-PFM) was significantly longer (P less than 0.05) than that in three faster ones (FAST-PFM), while no significant difference in the AEA-LT was observed; and lastly the total area of the anticipatory suppression of the Sol H reflex amplitude in the SLOW-PFM was greater than that in the FAST-PFM. These results suggest that AEA-DUR, representing postural responses, rather than AEA-LT, reflecting cognitive processes, may have had a close link with EMG-RT, and that the increased suppression in Sol H reflex amplitude originated from the increased anticipatory postural requirement, thus bringing about the EMG-RT delay.     

Lateral facilitation of Hoffmann-reflexes prior to voluntary movement in a choice reaction time task.

Hoffmann reflexes (H reflexes) were elicited from both legs simultaneously in human subjects at varying intervals after a reaction signal (RS) in a binary choice reaction time task. A left light RS required a rapid plantar flexion of the left foot and a right light RS required a similar rapid response of the right foot. A large faciliataion of reflex amplitude occurred only in the muscle involved in the movement (right of left soleus). The timing of the facilitation indicated that a decision about the status of the RS occurred within 200 msec and probably was completed somewhat earlier. Furthermore, the facilitation of the H reflexes was shown to be closely linked with the organization required for the contractions of the responding muscle. The results are considered in the light of hypothesized mechanisms regulating voluntary movement.     

The perception of painful and nonpainful stimuli during voluntary motor activity in man

Previous studies have shown that voluntary movement diminishes the transmission of cutaneous afferent input through the dorsal column-medial lemniscal system, and also raises the threshold for detecting nonpainful, cutaneous stimuli (electrical shocks). Although there is some evidence that pain elicited by electrical stimulation is diminished during movement, no studies have tested the effect of movement on the perception of pain produced by natural stimulation. For this reason, we tested the effects of voluntary motor activity on the perception of noxious thermal stimuli in human volunteers. We first developed a motor paradigm in which the thermal stimulation could be applied to the immobile limb (isometric elbow flexion-extension). Both isometric and isotonic muscle contractions about the elbow increased the threshold for detecting weak cutaneous stimuli (electrical shocks) applied to the forearm, and to a lesser extent the detection of stimuli applied to the dorsum of the hand. Afterwards, noxious and innocuous heat stimuli were applied to the forearm during isometric contractions and at rest. Magnitude estimates for the intensity of the pain, as well as latency measures of the onset of pain, were recorded. We found no evidence that isometric motor activity diminished either the threshold for pain or the subjective intensity of the noxious and innocuous thermal stimuli. Thus, motor activity decreases the ability to detect weak low-threshold cutaneous inputs, but has no effect on the perception of warmth and heat pain.     

Phycomyces: electrical response to light stimuli

Electrical signals have been detected in response to light excitation of the fungus Phycomyces blakesleeanus. These signals are related to the wavelength and intensity of the stimulus and the growth stage of the fungus. A relationship between the signals and the possible photoreceptor-pigment system is explored.     

The dependence of simple reaction time on temporal patterns of stimuli

Simple reaction time has been measured using various distributions of interstimulus intervals (ISIs), which differed both in the shape of their envelope and in their degree of discretization. For each of 7 such conditions, and for three mean ISIs (2.0, 3.3, and 5.8 s) 600 responses of two subjects have been accumulated.Reaction times depend significantly on the shape of ISI-distributions but also on their degree of discretization. Within an experimental run they depend on single ISIs back to the third before reaction, and on sequences of ISIs. The latter effects are again influenced by the discretization of ISIs. Finally it was found that some learning of the distribution pattern (not the mere mean value) of ISIs takes place.Our results shed some light on existing hypotheses of RT. Some inconsistencies of earlier experimental results can probably be explained by insufficient consideration of the discretization and the learning effects.