Oxygen tension changes evoked in the brain by visual stimulation

Localized changes in oxygen tension were recorded with platinum cathodes placed in the lateral geniculate nucleus in both anesthetized and awake cats. The amplitude of the responses increased with increasing stimulus intensity, but decreased with increasing flash rate. Both increases and decreases in cathode current were produced by steady illumination. The characteristics of the responses suggest that the responses reflect localized variations in blood flow, produced in turn by changes evoked in the tonic neural activity of the lateral geniculate nucleus.     

Encoding predictive reward value in human amygdala and orbitofrontal cortex

Adaptive behavior is optimized in organisms that maintain flexible representations of the value of sensory-predictive cues. To identify central representations of predictive reward value in humans, we used reinforcer devaluation while measuring neural activity with functional magnetic resonance imaging. We presented two arbitrary visual stimuli, both before and after olfactory devaluation, in a paradigm of appetitive conditioning. In amygdala and orbitofrontal cortex, responses evoked by a predictive target stimulus were decreased after devaluation, whereas responses to the nondevalued stimulus were maintained. Thus, differential activity in amygdala and orbitofrontal cortex encodes the current value of reward representations accessible to predictive cues.     

Entrainment of circadian rhythms by sound in Passer domesticus

The circadian locomotor rhythm of house sparrows was entrained by a sound stimulus. The birds were maintained at a constant temperature in, dim green light. The entraining agent was 4 (1/2) 12 hours of tape-recorded bird song ,played each day. Variations in the response to this stimulus have been correlated with individual variations in free-running period. This is the first clear demonstration that a biological clock can be influenced by sound stimuli.     

Visual evoked response correlates of unconscious mental processes

Average evoked responses and accompanying free associations elicited by subthreshold visual stimuli were studied to determine if a differential discrimination between two stimuli would be reflected in either or both of these responses. The results indicate that the effects of subliminal perception are encoded in the average evoked response and also influence the content of free associations.     

Metacontrast: its relation to evoked potentials

Electrophysiological correlates of metacontrast were studied by means of averaged evoked potentials recorded from the scalp in man. Under conditions in which the brightness of the first of two successive stimuli appears diminished there is no accompanying attenuation of the evoked potentials to that stimulus. The results suggest that the amplitude and latency of evoked potentials correlate with stimulus intensity but not with brightness.     

Auditory evoked response: meaningfulness of stimuli and interhemispheric asymmetry

Interhemispheric asymmetries of different magnitudes were observed in human cortical auditory evoked responses to speech and sound-effect stimuli. The wave with peak asymmetry occurred 100 milliseconds after signal onset. The amount of asymmetry of the amplitude of this wave was related to the meaningfulness to the subject of the auditory stimulus rather than to the mere use of verbal versus nonverbal materials.     

Self-stimulation alters human sensory brain responses

Human electrocortical potentials evoked by self-administered auditory and visual stimuli manifest much smaller amplitude and faster poststimulus timing than do average brain responses evoked by identical machine-delivered stimuli. Auditory evoked potentials show this "self-stimulation effect" to a greater degree than do visual responses. For visual evoked potentials, the effect appears greater at the vertex association area than over the occipital cortex. Individual differences in the magnitude of the "self-stimutlation effect" relate to level of intelligence.     

Disinhibition of visually masked stimuli

Backward-masking conditions were established for a pair of circularpatch stimuli. A third stimulus was then selected so as to mask the second when the second and the third were presented in the absence of the first. When all three stimuli were presented in serial order, the first and third were reliably detected but the second was not. Apparently, by masking the second flash, the third "disinhibited" the first.     

Evoked response and behavior in cats

Electroencephalographic averaged evoked responses to flashing lights of four different intensities were recorded in ten cats and correlated with behavior. Animals showing a high degree of exploratory behavior, aggressiveness, and activity and little withdrawal showed relatively large increases in amplitude of the averaged evoked response with increases of stimulus intensity. Those showing opposite behavioral traits had small increases or decreases of average evoked response amplitude with increases of stimulus intensity. These findings are compatible with those reported for human subjects. Inference is made about a neurophysiological mechanism for stimulus intensity modulation.     

Pavlovian conditioning with heat reinforcement produces stimulus-directed pecking in chicks

In a cooled chamber, chicks approached and pecked a small disk whose illumination preceded heat lamp activation, even when pecks prevented heat lamp onset. These behaviors did not occur when the disk and heat stimuli were randomly presented. Approach and contact of conditioned stimuli may develop even though these behaviors are not (i) evoked by the reinforcing stimulus, (ii) necessary for reinforcer reception, or (iii) ever followed by the reinforcer.     

Information delivery and the sensory evoked potential

The waveform of evoked responses recorded from human scalp is not determined solely by the physical eliciting stimulus, but also varies as a function of the effective information provided by the stimulus. There is a positive component whose latency is determined by the point in time at which ambiguity is reduced, and whose shape and amplitude are influenced by whether it is the presence or absence of an external event which delivers the information.     

CORTICAL EVOKED POTENTIALS AND PERCEPTION OF PAIRED FLASHES

Digital computer techniques have been employed to extract cortical evoked potentials to paired visual stimuli. Changes in the evoked potentials have been related to perceptual phenomena varying as a function of the interval between flashes. Evoked potentials to paired stimuli, which gave rise to perceptual interactions, could be approximated by algebraic summation of the responses to the stimuli when presented separately.     

Delay of constant light-induced persistent vaginal estrus by 24-hour time cues in rats

The normal ovarian cycle of female rats is typically replaced by persistent estrus when these animals are housed under constant light. Evidence presented here shows that the maintenance of periodicity in the environment can at least delay (if not prevent) the photic induction of persistent vaginal estrus. Female rats in constant light were exposed to vaginal smearing at random times or at the same time every day. In another experiment, female rats were exposed to either constant bright light, constant dim light, or a 24-hour photic cycle of bright and dim light. The onset of persistent vaginal estrus was delayed in rats exposed to 24-hour time cues even though the light intensities were the same as or greater than those for the aperiodic control groups. The results suggest that the absence of 24-hour time cues in constant light contributes to the induction of persistent estrus.     

Conditioned Approach and Contact Behavior toward Signals for Food or Brain-Stimulation Reinforcement

When presentation of a retractable lever always preceded food delivery, rats licked or gnawed the lever. They also approached but seldom orally contacted a lever signaling brain-stimulation reinforcement; instead, subjects sniffed, pawed, or "explored" the lever. Therefore, a Pavlovian conditioned stimulus evoked directed skeletal responses whose specific form depended on the forthcoming unconditioned stimulus.     

Neuroimaging evidence for dissociable forms of repetition priming

Repetition priming has been characterized neurophysiologically as a decreased response following stimulus repetition. The present study used event-related functional magnetic resonance imaging to investigate whether this repetition-related response is sensitive to stimulus familiarity. A right fusiform region exhibited an attenuated response to the repetition of familiar stimuli, both faces and symbols, but exhibited an enhanced response to the repetition of unfamiliar stimuli. Moreover, both repetition effects were modulated by lag between successive presentations. Further experiments replicated the interactions between repetition, familiarity, and lag and demonstrated the persistence of these effects over multiple repetitions. Priming-related responses are therefore not unitary but depend on the presence or absence of preexisting stimulus representations.     

Evoked potential correlates of signal recognition between and within modalities

Electrical responses evoked by clicks, flashes, changes in noise level, and changes in light level were recorded from the scalps of human subjects set to detect one of the stimuli. An early negative component of the evoked responses reflects selection between sensory modalities, whereas the later positive component reflects a more complex intramodal discrimination.     

Responses of human somatosensory cortex to stimuli below threshold for conscious sensation

Averaged evoked responses of somatosensory cortex, recorded subdurally, appeared with stimuli (skin, ventral posterolateral nucleus, cortex) which were subthreshold for sensation. Such responses were deficient in late components. Subthreshold stimuli could elicit sensation with suitable repetition. The primary evoked response was not sufficient for sensation. These facts bear on the problems of neurophysiological correlates of conscious and unconscious experience, and of "subliminal perception."     

Epinephrine,norepinephrine, and acetylcholine as conditioned stimuli for avoidance behavior

Conditioned leg-flexion responses in dogs were developed with electric shock as an unconditioned stimulus and intestinal stimulation or the effects of injections of various drugs as conditioned stimuli. It is concluded that physiological effects can play a role in the development and maintenance of conditioned avoidance behavior.     

Effects of visual form on the evoked response

The average visual evoked potentials elicited from relaxed human subjects are different for a blank visual field and one containing a geometric form, are different for different geometric forms of equal area, are similar for versions of the same geometric fortn of unequal area, and are different for two printed words equated for total letter area. These findings suggest that the waveform of evoked responses is not determined solely by the set of peripheral receptors which is stimulated, but it also reflects the perceptual content of the stimulus.     

Rod-cone interaction in human scotopic vision

Thresholds of a test flash were measured at various time intervals from onset of a conditioning flash under parafoveal scotopic conditions; rods or cones were selectively stimulated by utilizing either 420- or 680-nanometer light. Rod-cone interaction was indicated because conditioning flash presentation increased test threshold above control level for heterochromatic as well as for homochromatic stimulus pairs. The time course of these t.. reshold changes indicates that the rod system has a longer latency than the cone system.