RESUMEN
We have used electron holography to perform quantitative investigations of the leakage flux of thin film tips used as probes in magnetic force microscopy. A method to deduce an arrangement of magnetic domains in a thin magnetic whisker from the knowledge of the stray flux is also described. A simple analytical model of the magnetic properties of the probes allows the extraction of computer images, which simulate the experimental results satisfactory. The reliability of the recorded experimental maps of the magnetic flux arising from these kinds of sensors allows an evaluation of the total flux affecting the sample and the calculation of the magnetic field profile along the tip axis.
RESUMEN
The aim of the present study was to examine the determinants of the non-motoric CNV or E-wave elicited in a complex, variable foreperiod information processing task. Subjects were required to determine whether two sets of tones, one set presented back-to-back (S1/S2), the other separated by a variable foreperiod (S3/S4), were either matching or mismatching. Data were collected over two recording sessions; a baseline and an experimental run. The experimental session comprised three conditions; a match, a mismatch and no-response conditions. Mean amplitude and factor score comparisons showed the parietally-dominant E-wave to be dependent on the task relevance of the stimulus which it precedes. Differing midline distributions for the negative afterwave and the E-wave with increased processing requirements was taken to suggest that the two responses reflect functionally distinct phenomena.
Asunto(s)
Nivel de Alerta/fisiología , Percepción Auditiva/fisiología , Variación Contingente Negativa , Electrofisiología , Tiempo de Reacción/fisiología , Percepción Visual/fisiología , Adulto , Atención/fisiología , Corteza Cerebral/fisiología , Percepción de Color/fisiología , Femenino , Humanos , Masculino , Discriminación de la Altura Tonal/fisiología , Desempeño Psicomotor/fisiologíaRESUMEN
Using a sample of 85 subjects measurements were made of minimum stimulus durations necessary for detection of crossed and uncrossed disparity stimuli which were presented in five positions in the visual field: centre, lower, upper, right, and left field. The results indicated large detection duration differences between the two disparity conditions, with a marked superiority for crossed disparity detection at all positions. A left-right visual field anisotropy was demonstrated for crossed disparity stimuli.
Asunto(s)
Percepción de Profundidad/fisiología , Percepción de Forma/fisiología , Reconocimiento Visual de Modelos/fisiología , Campos Visuales , Fijación Ocular , Humanos , Umbral Sensorial/fisiología , Factores de TiempoRESUMEN
A systematic study was made of visual evoked responses to dynamic random dot stimuli containing controllable, monocularly visible contrast cues. Ratios of dot densities for the centrally presented, disparate figure and the background were varied in steps of 1/8 maximum density between 0/8 and 8/8. The figure was either a square or an equivalent area of nebulous shape. A 30-arc min disparity was compared with binocular nondisparate and monocular conditions. Evoked responses (scalp sites 02, 01, T6, T5) were averaged for each of 24 disparity-contrast-shape conditions. At all contrast levels, response amplitudes and latency over the left hemisphere was significantly greater than over the right hemisphere. For 30 arc min disparity, amplitudes in the 8/8 condition were significantly smaller than in conditions where stimulus/background contrast could afford monocular depth cues. Hemisphere amplitude differences diminished as contrast decreased. Factor analyses isolated two overlapping components in the response to disparate stimuli. The earlier, at 236 ms latency, may index the stereoscopic stimulus features. The later, at 295 ms, peaking at maximum contrast and present in all suprathreshold nondisparity conditions, may index contrast features of the stimulus. The results indicate the importance of controlling dot density ratios in electrophysiological studies of the stereoscopic response to random dot stimuli.