Annular Stimuli Research Articles

Differences between the occipital distribution of upper and lower field pattern-evoked responses in man

(1) The consistent reversal in the polarity of the occipital response to a reversing checkerboard stimulus in the upper and lower visual field, previously reported by the authors, has been investigated further in 9 healthy subjects, using a variety of reference electrodes, to decide between two possible explanations put forward in the earlier paper, viz., that the upper and lower field generators were situated on the lower and upper surfaces of the occipital lobe respectively, or alternatively that both responses, although of opposite surface polarity, came from the same general area of convergent information on the upper convexity. (2) Changing from a two-ear to a midfrontal reference made little difference to the form of the lower field responses, but upper field responses were significantly altered, the characteristics negativity, still present in more anterior channels, being replaced by a positivity at electrodes below the inion. (3) Comparing upper field responses to the central 2° with those to an annular stimulus from 4 to 8° out from the fixation point, the posterior positivity was particularly characteristic of the foveal stimulus, whereas the nagativity, recorded more anteriorly, was seen with the more peripheral stimulus. The difference between the midfrontal and two-ear reference recordings of the full 8° upper field stimulus was contributed almost entirely by the altered response to the central stimulus. (4) The results suggest that the upper field responses arise from the inverted neurones on the under-surface of the occipital lobe, which are sufficiently near to be picked up by the ear reference, whereas the lower field responses arise from the neurones on the upper convexity.

Critical flicker-fusion of solid and annular stimuli.

Critical flicker-fusion thresholds were measured with both solid disks and annular stimuli. The former subtended 1, 4, and 8 degrees of visual angle, and the latter subtended 4 and 8 degrees. The illuminated region of the 4-degree annulus provided the same area of stimulation as that of the 1-degree disk. The illuminated region of the 8-degree annulus provided the same area of stimulation as the 4-degree disk. Annular stimuli significantly reduced critical flicker-fusion thresholds relative to those obtained with disk stimuli of the same area. The effects are not explicable in terms of heterogeneous retinal sensitivity. Laws relating critical flicker-fusion to area seem to be valid only under the restricted condition of homogeneous stimulation.

Stimulus contrast in tactile resolutlon

This experiment was designed to test the influence of contrast on tactile resolution by the use of different modes of distribution of pressure stimuli, with control of various factors such as intensity, area, rate, and time of application. The experimental task was two-point discrimination. The results showed that the use of annular stimuli result in a significantly higher number of resolutions than the use of circular stimuli, the intensity, total stimulating area, and pressure being controlled. The proposed interpretation of the results is that increase of stimulus contrast increases the resolving power of the somesthetic system.

OPTICAL STIMULATOR, MICROELECTRODE ADVANCER, AND ASSOCIATED EQUIPMENT FOR INTRARETINAL NEUROPHYSIOLOGY IN CLOSED MAMMALIAN EYES.

An optical stimulator is described to meet the special requirements of retinal neurophysiology. It is compact and convenient to use, and modular construction confers flexibility of problems to which it may be applied. The maximum stimulus intensity from a Zeiss tungsten light source gives 4.19 log m–c of illuminance at the cat retina. An electromagnetic shutter provides light pulses which rise or fall to within 1/e of the final value in 0.3–0.4 msec. Controls are included for stimulus intensity, color, duration, and frequency. Both spot and annular stimuli of variable size are provided. The stimulus may be positioned anywhere on the retina by a combination of coarse and micrometer controls. Interchangeable stimulus light sources are used for special problems, and a simple attachment permits studying the Stiles-Crawford effect. A background beam is also provided with controls for intensity and color. A direct ophthalmoscopic view of the fundus gives high resolution of retinal details.A modified eye holder provides three needle channels into the fundus of the eye. An improved hydraulic advancer controls microelectrode depth in the retina. A simple head holder is used to position the cat or monkey eye in the stimulator.