Thomas R. Corwin

The Broca-Sulzer effect in a Ganzfeld.

Abstract Observers judged the brightness of an infinitely large, contourless field of view (Ganzfeld) illuminated by constant luminance flashes of various durations. A Broca-Sulzer effect was found: 50 msec flashes were judged 30–40% brighter than 500 msec flashes of equal luminance. In a second experiment, the same observers demonstrated a Broca-Sulzer effect in estimating the overall brightness of grating targets. The Ganzfeld results show that neither stimulus contours nor edge effects generated by eye movements are necessary for the Broca-Sulzer effect.

Enhancement of apparent contrast in flashed sinusoidal gratings

Abstract The apparent contrast of suprathreshold grating targets was measured as a function of their spatial frequency and exposure duration. For targets of low spatial frequency, apparent contrast reaches a maximum at exposure durations of 80–100 msec relative to its value at shorter and longer durations. Contrast-duration curves thus resemble brightness-duration curves when the Broca-Sulzer effect is present. For gratings of higher spatial frequency, apparent contrast increases monotonically with duration. Thus, temporal contrast enhancement occurs only for low spatial frequency targets. The enhancement effect occurs foveally as well as for peripherally viewed targets, but it is abolished by decreasing the vertical extent of the grating.

The oblique effect in a vernier acuity situation

Observers viewed either vertical or obliquely oriented vernier targets from either an upright position or with their heads tilted. Vernier acuity was consistently better for retinally vertical than for gravitationally vertical targets, even when presented against a background context of vertical stripes designed to aid veridical perception of gravitational orientation. These results indicate that vernier acuity depends on retinal image orientation rather than on perceived orientation. The high contrast of the vernier lines ensures that their gravitational orientation is clearly perceived. Thus the present results provide a stronger basis for ruling out the effects of perceptual orientation than previous studies involving grating contrast-threshold measurements. Since the vernier targets were presented as brief flashes, it is unlikely that the measured oblique effect is attributable to differences in eye-movement patterns.

The effects of temporal waveform upon apparent contrast

Temporal contrast enhancement (TCE) refers to the finding that the apparent contrast of flashed low-spatial-frequency gratings is higher for flashes of intermediate duration [i.e., 50120 msec) than for flashes of longer duration. The present experiment investigated the effect of temporal waveform upon TCE. We measured the apparent contrast of a .6-cycles/deg sinusoidal grating that varied in duration. Four temporal waveforms were used: abrupt onset/ abrupt offset, abrupt onset/gradual offset, gradual onset/abrupt offset, and gradual onset/ gradual offset. TCE was found only for targets with abrupt onsets, regardless of offset waveform. These results have implications about the role of onset transients in the coding of brightness.

Computer-aided estimation of psychophysical thresholds by Wetherill tracking

We describe a microcomputer implementation of an efficient procedure to obtain psycho-physical thresholds at any of three response-probability levels, by Wetherill tracking. The procedure yields reliable 50% thresholds (average coefficient of variation=5.1%) based on about 30 responses. Thresholds obtained in this way correlate highly with constant stimulus measurements requiring six times as many responses. Computer-assisted Wetherill tracking should find wide application in psychophysical screening tests.

An improved circuit for control of LED luminance

The light-emitting diode (LED) is becoming increasingly popular as a luminance source in behavioral research. We describe a simple, inexpensive LED driver circuit that produces luminances almost linearly proportional to an input voltage. Negative feedback provides a stable output. Unlike previous designs, the LED is ground referenced.

The effect of retinal eccentricity on the Broca-Sulzer phenomenon

The Broca-Sulzer phenomenon, in which a flash of intermediate duration appears brighter than a longer flash of the same luminance,’ has received relatively little experimental attention since it was first reported in 1902. Consequently, little is known about the stimulus conditions which affect its magnitude. In particular, the effect of retinal location has not yet been investigated.2 The present study is a preliminary attempt to do this by comparing the shape of brightness vs duration curves for fovea1 and eccentrically fixated targets of equal luminance. A 0.75” circular target was presented at each of three moderate luminances and at eight durations between 10 msec and I sec. Observers judged the brightness of each target flash both when it was fixated foveally and when it was presented 10” vertically below the fovea. Mean brightness estimates were curvilinear functions of duration at all three luminances and at both retinal locations, indicating the presence of a Broca-Sulzer effect. The magnitude of the enhancement effect was larger for fovea1 targets than for eccentric ones, despite the fact that the latter appeared brighter overall than the former.

An encoding multiple-key response unit

Many computer-controlled psychophysical studies allow subjects multiple response alternatives. Encoding the activated response key before signalling the computer makes possible more rapid stimulus/response interaction, simplifies software interrogation of response-key status, and automatically resolves double strikes. We describe an inexpensive unit which uses a priority encoder for up to seven response alternatives.