V. di Lollo

Inverse-intensity effect in duration of visible persistence.

Duration of visible persistence can vary inversely with stimulus intensity. This inverse-intensity effect is obtained by varying the intensity of the stimuli or of the background, provided that the variations extend into the mesopic range. A similar relationship--known as the Ferry-Porter law--holds for the critical frequency at fusion (CFF). The authors propose that studies of CFF, 2-pulse threshold, and visible persistence can be encompassed within 1 conceptual framework in which the effect is modeled by the progressive reduction in the temporal extent of the positive phase of the systems response as the level of light adaptation changes from scotopic to photopic. In this context, the authors present an integrative scheme in which G. Sperling and M. M. Sondhis (1968) formal model and M. Colthearts (1980) neurophysiological conjecture are shown to be compatible and complementary accounts of the effect.

Temporal integration and segregation of brief visual stimuli: patterns of correlation in time.

Two brief sequential displays separated by a brief interstimulus interval (ISI) are often perceived as a temporally integrated unitary configuration. The probability of temporal integration can be decreased by increasing the ISI or (counterintuitively) by increasing stimulus duration. We tested three hypotheses of the relative contributions of stimulus duration and ISI to the breakdown of temporal integration (the storage, processing, and temporal correlation hypotheses). In the first of two experiments, stimulus duration and ISI were varied factorially, and estimates of temporal integration were obtained with a form-part integration task. The second experiment was a replication of the first at two levels of stimulus intensity. The outcomes were inconsistent with the storage and processing options, but confirmed predictions from the temporal correlation hypothesis. Whether two sequential stimuli are perceived as temporally integrated or disjoint depends not on the availability of visible persistence, but on the emergence of a neural code that is based on the temporal correlation between the two visual responses.

Inverse duration effects in partial report

In partial-report experiments, an array of stimuli is displayed briefly, followed by a probe indicating the items to be reported. When exposure duration of the array is increased, 2 contrasting outcomes have been found: Some experiments find that performance improves (direct-duration effect); others find that it deteriorates (inverse-duration effect). The objective here was to identify the reasons for the discrepant results. This was done by investigating the roles played by 5 factors that differed between the 2 sets of contrasting studies. An inverse-duration effect was obtained in each of 6 experiments; its magnitude was affected by retinal eccentricity and by the number of items denoted by the probe. The effect was independent of array configuration and of number of items in the array. The direct-duration effect was shown to arise from a confounding of exposure duration and brightness. Language: en

Phosphor persistence in oscilloscopic displays: Its luminance and visibility.

Phosphor persistence and its potentially confounding effects in visual experiments were discussed recently in these pages (Groner, Groner, Miiller, Bischof & Di Lollo, 1993). In that report, we showed that phosphor persistence can remain visible for extended periods (in the order of hundreds of msec) on oscilloscopic screens coated with P31 but not with P15 phosphor. In a dissenting research note, Westheimer (1993) dismisses our evidence as stemming from “very indirect exper- iments” and concludes that the persistence of P31 phos- phor, as measured by a photometer, decays within about 2 msec. This 2-msec estimate could be easily taken as applying equally to photometers and to human observ- ers. To obviate this misinterpretation, the generality of Westheimer’s conclusion needs to be qualified. A common reason for measuring the temporal course of phosphor decay is to provide engineering specifica- tions for display oscilloscopes (e.g. Bell, 1970). For this purpose, the measuring instrument of choice is a photo- meter, not the human eye. As pointedly noted by Westheimer (1993), human eyes would supply only indirect estimates. We agree: instead of the direct lumi- nance readings provided by a photometer, visual re- sponses yield only