Vision: Temporal Factors

Roles of Temporal Factors in Brightness Perception

The perceived brightness of a stimulus is not only determined by physical intensity but is also affected by duration. That is, a stimulus can be perceived as more intense by increasing either its physical intensity or its duration. Thus, the literature often refers to the time-integrated contrast energy, c, of a stimulus instead of its physical intensity.

One such relationship is conveyed in Bloch's law. Bloch's law (also called the time-intensity reciprocity law) states that a short-duration visual stimulus that is high in physical intensity (I) can appear as bright as a longer-duration stimulus of lower intensity (so long as both stimuli are of a duration, t, that is shorter than a critical duration, τ): for t ≤τ, c = I × t. The critical duration that temporally limits Bloch's law, τ, becomes shorter as the overall intensity of the stimuli increases. Bloch's law presumably operates due to integrative action of the visual system. However, the neural correlates are not currently known. Some reports suggest that a simple decay of neural activity can account for the effect.

In the Broca-Sulzer effect, perceived brightness also depends on stimulus duration. As the duration of a flashed stimulus increases, so does its perceived brightness, but then it decreases. Brücke and Bartley individually reported that the brightness of a flickering stimulus varies as a function of flash duration (an effect now referred to as the Brücke-Bartley effect). Later studies also investigated the role of flicker rate on the perceived brightness of flickering stimuli. Taken together, flicker rate, stimulus duration, interstimulus interval, and flicker on-off ratio (duty cycle) are all temporal factors that impinge upon the perceived brightness of a stimulus.

Flicker Fusion

The temporal factors that affect the appearance of a stimulus include the stimulus duration, but also its temporal interactions with other stimuli. Flicker fusion is the perceived continuous appearance of a flickering light. Although artificial electric lights appear to be stable light sources, in fact their light emission turns on and off cyclically at a rate of 50 to 60 Hz. Movies, computer monitors, TVs, and other presentation devices similarly flicker, despite the fact that they appear continuous. The apparent continuity of these devices is of central interest to the field of temporal visual processing, as well as to understanding how humans interact with the many devices and environments that make up modern society. For flicker fusion to occur, the rate of flicker of the source must be higher than the “critical flicker fusion threshold.” This rate is typically 30 Hz or more, but it may vary as a function of context and lighting conditions.

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