Vagus nerve stimulation: a new form of therapeutic brain stimulation

Although the vagus nerve has traditionally been considered to perform efferent functions, in reality it performs significant afferent functions as well, carrying information from the body, head, and neck to the brain. Preliminary studies examining this afferent activity led to the theory that vagus nerve stimulation (VNS) could successfully control seizure activity in persons who are refractory to antiepileptic medications. Unlike other forms of brain stimulation, VNS is unable to directly stimulate multiple discrete areas of the brain; however, through several pathways, it is able to relay sensory information to higher brain regions. An implantable VNS device known as the VNSTM NeuroCybernetic Prosthesis (NCP) System has been used in approximately 9,000 epilepsy patients in Europe and the United States since 1994. The implant has reduced seizure frequency by an average of 25% to 30%, with minimal side effects. Studies underway are also showing some degree of success in the management of treatment-refractory depression. The future efficacy of the implantable system in other disorders may depend on whether the implant can be more precisely focused to affect different brain regions. Research in this area is underway.     

Imagery and olfactory stimulation

The purpose of the present study was to investigate a possible odor-emotion interaction by determining whether humans could differentially detect other humans' odor collected under varying emotional conditions. Odorants were collected from the axillary apocrine glands of four female donors, each of whom underwent different imagery induction procedures of anxiety, relaxation, and sexual arousal. The odorants were chemically preserved and later evaluated by 16 naive white male judges as to the presence of discriminatory odor cues. Judges' imagery was differentially related to the anxiety imagery-produced odorants but only for those produced by the most imaginative donors. The relaxation odorants of all donors were correlated. The judges were unable to detect the sexual imagery arousal odorants.     

Cognitive stimulation in brainstorming

Research on group brainstorming has demonstrated that it is less effective for generating large numbers of ideas than individual brainstorming, yet various scholars have presumed that group idea sharing should enhance cognitive stimulation and idea production. Three experiments examined the potential of cognitive stimulation in brainstorming. Experiments 1 and 2 used a paradigm in which individuals were exposed to ideas on audiotape as they were brainstorming, and Experiment 3 used the electronic brainstorming paradigm. Evidence was obtained for enhanced idea generation both during and after idea exposure. The attentional set of the participant and the content of the exposure manipulation (number of ideas, presence of irrelevant information) influenced this effect. These results are consistent with a cognitive perspective on group brainstorming.     

Brain stimulation and conscious experience

Libet discovered that a substantial duration (> 0.5-1.0 s) of direct electrical stimulation of the surface of the somatosensory cortex at threshold currents is required before human subjects can report that a conscious somatosensory experience had occurred. Using a reaction time method we confirm that a similarly long stimulation duration at threshold currents is required for activation of elementary visual experiences (phosphenes) in human subjects following stimulation of the surface of the striate cortex. However, the reaction times for the subject to respond to the cessation of the visual experience after the end of electrical stimulation could be as brief as 225-242 ms. We also carried out extensive studies in cats under a variety of anesthetic conditions using the same electrodes and parameters of stimulation employed in the human studies to study the patterns of neuronal activity beneath the stimulating surface electrode. Whereas sufficiently strong currents can activate neurons within milliseconds, stimulating currents close to threshold activate sustained neural activity only after at least 350-500 ms. When currents are close to threshold, some neurons are inhibited for several hundreds of millisecond before the balance between inhibition and excitation shifts towards excitation. These results suggest that the prolonged latencies, i.e., latencies beyond 200-250 ms, for the emergence of conscious experience following direct cortical stimulation result from a delay in the sustained activation of underlying cortical neurons at threshold currents rather than being due to any unusually long duration in central processing time. Intracellular records from cortical neurological cells during repetitive electrical stimulation of the surface of the feline striate cortex demonstrate that such stimulation induces a profound depolarizing shift in membrane potential that may persist after each stimulus train. Such a depolarization is evidence that extracellular K+ concentrations have increased during electrical stimulation. Such an increase in extracellular K+ progressively increases cortical excitability until the threshold for sustained activation of cortical neurons is reached and then exceeded. Consequently, the long latency for threshold activation of cortical neurons depends upon a dynamically increasing cortical facilatory process that begins hundreds of milliseconds before there is sustained activation of such neurons. In some cases, this facilatory process must overcome an initial stimulus-induced inhibition before neuronal firing commences.     

Three-space inference from two-space stimulation

Oblique contours sloping at 30° with respect to the horizontal were presented alone, in combination to form chevrons, or with a vertical line to form arrowhead or Y patterns; they were projected onto a screen in the frontal parallel plane and viewed from positions that gave viewing angles of 90° (normal to the screen’s surface), 53°, or 34°. The perceived orientation of the contours, as assessed by a movable arm that the subjects set to be parallel to the obliques, changed monotonically as a function of viewing angle. The change was as great for single obliques as for combinations of obliques within the chevron, arrowhead, and Y patterns. The results of Experiment 1 were extended in Experiment 2, in which obliques at 30° and 50° with respect to the horizontal were presented singly or in combination as chevron patterns. It is argued that the results of both experiments indicate that single two-space oblique lines are immediately interpreted as lying in three-space and that the changes in perceived orientation are a consequence of this perceptual inference.     

Vestibular stimulation affects optic-flow sensitivity

Typically, multiple cues can be used to generate a particular percept. Our area of interest is the extent to which humans are able to synergistically combine cues that are generated when moving through an environment. For example, movement through the environment leads to both visual (optic-flow) and vestibular stimulation, and studies have shown that non-human primates are able to combine these cues to generate a more accurate perception of heading than can be obtained with either cue in isolation. Here we investigate whether humans show a similar ability to synergistically combine optic-flow and vestibular cues. This was achieved by determining the sensitivity to optic-flow stimuli while physically moving the observer, and hence producing a vestibular signal, that was either consistent with the optic-flow signal, eg a radially expanding pattern coupled with forward motion, or inconsistent with it, eg a radially expanding pattern with backward motion. Results indicate that humans are more sensitive to motion-in-depth optic-flow stimuli when they are combined with complementary vestibular signals than when they are combined with conflicting vestibular signals. These results indicate that in humans, like in nonhuman primates, there is perceptual integration of visual and vestibular signals.     

Short-term retention of tactile stimulation

In three experiments subjects were required to reproduce after varying delays the locus of a tactile stimulation delivered to the upper-side of the arm. During the retention periods subjects either performed a subsidiary, arithmetic task or rested. Recall, as measured by accuracy in reproducing the locus of stimulation, decreased as a function of retention interval, asymptoting after approximately 5 s. Performance was poorer in the subsidiary task condition than in the rest condition; however, the effect of the subsidiary task appeared to be more on subject recall strategies than on rehearsal capacity. No evidence of proactive interference effects was found, and a decay interpretation of forgetting of discrete tactile stimuli in the short-term memory distractor paradigm was favoured.     

Conflicting visual and tactual-kinesthetic stimulation

The effects of stimulus and response probability on choice reaction time (RT) were independently investigated. In different conditions, two sets of three stimuli of probabilities of.1, .3, .6 and.3, .3, .4 were assigned to two responses-two of the stimuli assigned to one response. All combinations of S-R assignments were studied, yielding response probabilities of .1-.9, ,3-.7, and ,4-.6. With response probability held constant, variations in stimulus probability led to consistent and substantial effects on both RT and error rates. Response-probability effects were confined to error rates, and repetition effects were negligible. The results support the assumption of recent models of choice behavior that adjustments are made to the absolute (rather than just to the ordinal) values of stimulus probability.