Alan P. Rudell

Suppression of visual perception by magnetic coil stimulation of human occipital cortex

Magnetic coil (MC) stimulation percutaneously of human occipital cortex was tested on perception of 3 briefly presented, randomly generated alphabetical characters. When the visual stimulus-MC pulse interval was less than 40-60 msec, or more than 120-140 msec, letters were correctly reported; at test intervals of 80-100 msec, a blur or nothing was seen. Shifting the MC location in the transverse and rostro-caudal axes had effects consistent with the topographical representation in visual cortex, but incompatible with an effect on attention or suppression from an eyeblink. The MC pulse probably acts by eliciting IPSPs in visual cortex. The neural activity subserving letter recognition is probably transmitted from visual cortex within 140 msec of the visual stimulus.

TRANSCRANIAL MAGNETIC STIMULATION IN STUDY OF THE VISUAL PATHWAY

The authors critically reviewed experiments in which transcranial magnetic stimulation (TMS) and repetitive TMS (rTMS) of the higher visual pathway were used. Topics include basic mechanisms of neural excitation by TMS and their relevance to the visual pathway (excitatory and inhibitory effects), TMS and rTMS of calcarine cortex (suppression, unmasking, and phosphenes), TMS of V5 (suppression), TMS and rTMS of higher level temporoparietooccipital areas (perceptual errors, unmasking, and inattention), the role of frontal lobe output in visual perception, and vocalization of perceived visual stimuli (role of consciousness of linguistic symbols).

Measurement of information processing delays in human visual cortex with repetitive magnetic coil stimulation.

Previous work disclosed that single magnetic coil (MC) pulses applied over human calcarine cortex could suppress perception of letters briefly presented, e.g. 80-100 ms earlier. Although individual MC stimuli presented 0-60 ms, or more than 140 ms after the visual stimulus were apparently ineffective, combinations of 2 or 3 MC pulses at such intervals temporarily depressed visual perception. Thus, progressing of such language information could be slowed, without being abolished. By contrast, when the first MC pulse was delivered 120 ms or later, a second MC pulse 40 ms later had no detectable effect, implying that calcarine cortex had already transmitted the information. Perceptual recovery of 5-character words initially occurred no earlier than that of random letters, nor or random letters vs. arbitrary linear patterns, implying that the processing delays in calcarine cortex were similar.

Operant Controlled Neural Event: Formal and Systematic Approach to Electrical Coding of Behavior in Brain

Traditional studies of electrophysiological correlates of behavior contain inherent high variability resulting from the arbitrary choice of behaviors, brain locations, and wave parameters. The operant control of neural events is a formal and systematic approach to the study of prespecified parameters and components of brain activity as they encode behaviors. Two studies in which the electrical activity of brain was the criterion for reinforcement demonstrate the acquisition, under such operant control, of two mutually exclusive behaviors or states which selectively alter evoked potential components.

Operant Control of Neural Events in Humans

Human subjects were trained by traditional methods of instrumental conditioning to change the amplitude of a late component of the auditory evoked potential with and without oscilloscopic feedback of their performance.

The recognition potential, word difficulty, and individual reading ability: on using event-related potentials to study perception.

Ten observers detected words in a stream of random letters. The latency of the recognition potential (RP) was less for easier words. This implicated short latency processes in word detection. Reaction time (RT) and P3 latency decreases with training were attributed to improved motor preparation. The RT decrease with training was correlated with P3 (r = .67), but not RP (r = .04), latency reduction. P3 latency did not predict individual RT (r = .20), but RP latency did (r = .66). Twenty other subjects took the Verbal portion of a Graduate Record Examination to test whether the RP might be a better predictor of individual differences than P3. RP latency predicted a persons reading score (r = -.74), but P3 latency did not (r = .08). The word-difficulty effect and the shorter RP latency observed for superior readers supported the idea that the RP reflects perception that is based on language skill.

The polarity of the induced electric field influences magnetic coil inhibition of human visual cortex: Implications for the site of excitation

Human perception of 3 briefly flashed letters in a horizontal array that subtends a visual angle of 3 degrees or less is reduced by a magnetic coil (MC) pulse given, e.g., 90 msec later. Either a round or a double square MC is effective when the lower windings or central junction region, respectively, are tangential to the skull overlying calcarine cortex and symmetrical across the midline. The modeled, induced electric field has peak amplitude at the midline, but the peak spatial derivatives lie many centimeters laterally. Thus, the foveal representation near the midline is closer to the peak electric field than to its peak spatial derivatives, i.e., excitation of calcarine cortex differs from excitation of a straight nerve. With an MC pulse that induces an electric field which is substantially monophasic in amplitude, the lateral-most letter (usually the right-hand letter) in the trigram is preferentially suppressed when the electric field in the contralateral occipital lobe is directed towards the midline. Inferences from using peripheral nerve models imply that medially located bends in geniculo-calcarine or corticofugal fibers are the relevant sites of excitation in visual suppression; end excitation of fiber arborizations or apical dendrites is considered less likely. This conclusion is supported by the fact that the induced electric field polarity in paracentral lobule for optimally eliciting foot movements is opposite to that for visual suppression, the major bends occurring at different portions of the fiber trajectories in the two systems.

The recognition potential and conscious awareness.

The idea that conscious awareness of a recognizable image is necessary for it to evoke the recognition potential (RP) was tested by asking bilingual subjects to selectively attend to superimposed English and Chinese word images. The subjects detected most of the words in the attended language, but were largely oblivious of words in the non-attended language. Attended word images evoked the RP. Non-attended words did not. RP latency was less for Chinese than for English words. This provided a basis for inferring which language a subject was trying to read when valid English and Chinese words were both present. A subject was looking for Chinese if the latency was short and for English if it was long. The results showed that selective attention had a powerful effect on the RP. They supported the idea that conscious awareness is necessary for evoking it, though they did not rule out the theoretical possibility that some method not yet tested could be found that would block conscious awareness without blocking the RP. The sensitivity of the RP to what a subject is trying to see and its low variance seem to provide advantages for studying visual perception. It provides a short latency indicator of image processing that merits further investigation. Use of it may lead to a better understanding of visual perceptual processes.

Stimulation of the human nervous system using the magnetic coil.

The magnetic coil (MC) is a unique probe that can be used to elucidate basic neurophysiological mechanisms in humans. Either by excitation or inhibition of responding neural elements, we have been able to investigate: (1) the distribution of the electric field induced within isotropic and anisotropic volume conductors by round and figure-eight MCs; (2) the theoretical relationship between electric field distribution and excitation of distal peripheral nerve, nerve root, cranial nerve, and motor cortex; (3) the effect of focal MC stimulation of motor and visual systems; (4) pertubation of sequential digit movements by MC stimulation of human premotor cortex; (5) activation of frontal motor areas related to speech; (6) elicitation of a sense of movement in an ischemic paralyzed limb by focal MC cortical stimulation; and (7) the effect of stimulation of the human visual system to (a) suppress and unmask visual perception using single MC stimuli and (b) prolong visual suppression using short trains of MC stimuli. In the future, prolongation of MC action by using repetitive stimuli should be useful in further investigating functions concerned with language, speech, and cognition.

Does a warning signal accelerate the processing of sensory information? Evidence from recognition potential responses to high and low frequency words.

Electrophysiological and behavioral data were obtained in 12 subjects who detected valid words in a background stream of random letter strings. Behavioral reaction time (RT) showed significant effects of warning signal presentation and the frequency of word usage in printed literature. Cross-correlation functions were used to estimate delays of electrophysiological responses. The critical response was the recognition potential (RP). The RP is a response of the brain that occurs when a person views recognizable images, such as words, pictures, or faces. Its latency is usually less than 300 ms. Both the RP and longer latency activity occurring at approximately 400--600 ms were delayed more for low than for high frequency words. The longer latency responses showed shorter delay if a warning signal was presented, but the RP did not. The results supported the idea that a non-informative warning signal decreases RT by altering response-related processes without facilitating sensory processes.