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PubMed1983

The relationship between temporal integration and persistence.

We measured the persistence for gratings of three different spatial frequencies after they had been equated for visibility by manipulating: (1) the contrast of the stimulus, (2) the duration of the stimulus, or (3) both contrast and duration. When we presented gratings of equal apparent contrast and equal duration, persistence increased with increasing spatial frequency. This effect occurred when apparent contrast was equated in terms of extended-duration or brief duration contrast sensitivity. However, when we examined persistence using gratings equated for temporal integration (i.e. equal apparent contrast but unequal durations), no differences in persistence as a function of spatial frequency were observed. Thus, the increase in persistence which is typically found with higher spatial frequencies can be accounted for by knowledge of the spatio-temporal characteristics of the visual system. Our findings are discussed in terms of the notion of different filter time-constants for different spatial channels.

Afterimage↗
PubMed1983

Sleep and the McCollough effect.

Orientation-contingent chromatic ("McCollough") aftereffects (OCCAs) were induced under carefully standardised conditions, in subjects suffering from varying degrees of sleep deprivation. The initial OCCA strength was found to vary systematically with the prior sleep pattern of the subject. In subjects whose sleep normally lasted 7-8 hr, loss of 3 or 4 hr of the previous night's sleep could reduce initial strength by as much as 50%; no further reduction was observed with still shorter sleep durations. Keeping the eye in darkness while awake had in this respect no comparable effects to those of sleep, and sleeping in a lighted room made no difference. The results suggest that the McCollough Effect depends on a form of neural plasticity which requires a normal sleep cycle for its maintenance.

Afterimage↗
PubMed1983

Visible persistence: effects of luminance, spatial frequency and orientation.

Square-wave gratings of various space-average luminances, spatial frequencies and orientations were presented briefly to observers, who were asked to adjust a brief visual probe to coincide with the end of the longer-lasting, gradually decaying sensory trace of these stimuli. Visible persistence of 2 and 7 c/deg gratings increased with increasing luminance, and an interaction was found at approx. 6 cd/m2 between luminance and spatial frequency. The persistence curves showed a low-pass frequency characteristic at low mean luminance levels, whereas at high luminances, they presented a band-pass frequency characteristic. Persistence of a 7 c/deg grating was longer at oblique orientations than at horizontal or vertical orientations. These findings were related to properties of transient and sustained channels in human vision.

Afterimage↗
PubMed1983

Effects of stimulus luminance and duration on responses to onset and offset.

The visual responses to stimulus onset and offset can be isolated psychophysically from the response to steady-state stimulation. Six experiments investigated the perception of onset and offset of dot bigrams within random-dot patterns. Onset and offset persistence were studied at two levels of stimulus luminance (8 and 45ft-L). Similar performance was obtained for targets defined by onset and offset; the lower luminance produced a decrement in response at offset. Onset and offset perceptibility were studied at six stimulus durations between 20 and 1260 msec. Effects of stimulus duration were distinguished from masking effects due to pattern offset. Decreasing stimulus duration had little or no effect on recognition of bigrams defined by onset but produced a large decrement in the perceptibility of bigrams defined by offset. A brief stimulus therefore seems to give a relatively strong on-response and a weak off-response. These psychophysical results are consistent with electrophysiological findings concerning on- and off-responses and contrast with the idea that persistence consists of a decaying trace of the steady-state properties of the stimulus.

Afterimage↗
PubMed1983

Temporal integration following intensification of long-lasting visual displays.

Duration of visible persistence is known to be inversely related to the duration of the inducing stimulus, within a critical interval estimated at between 100 and 150 msec. Stimuli longer than the critical interval yield little or no persistence. Six experiments investigated whether a brief period of intensification at the end of a stimulus longer than the critical interval could restore visible persistence. In the first experiment, a punctate stimulus ceased to give rise to visible persistence at exposure durations longer than the critical interval. The second experiment showed that persistence could be restored to a long display by briefly intensifying the component dots just before the end of the display. The remaining four experiments explored the limits and the distinguishing characteristics of this effect. Two alternative explanations of the results are described and evaluated.

Adaptation, Ocular↗