Perceptual studies make a clear distinction between sensitivity and decision criterion. The former is taken to characterize the processing efficiency of the underlying sensory system and it increases with stimulus strength. The latter is regarded as the manifestation of a subjective operation whereby individuals decide on (as opposed to react reflexively to) the occurrence of an event based on factors such as expectation and payoff, in addition to its strength. To do so, individuals need to have some knowledge of the internal response distributions evoked by this event or its absence. In a natural, behaviorally relevant multistimulus environment, observers must handle many such independent distributions to optimize their decision criteria. Here we show that they cannot do so. Instead, while leaving sensitivity unchanged, lower and higher visibility events tend to be reported respectively less and more frequently than when they are presented in isolation. This behavior is in quantitative agreement with predictions based on the notion that observers represent a multistimulus environment as a unitary internal distribution to which each stimulus contributes proportionally to its probability of occurrence. Perceptual phenomena such as blindsight, hemineglect, and extinction may be, at least in part, accounted for in such a way.decision making ͉ psychophysics ͉ criteria drift ͉ unitary internal distribution I t is well established (1, 2) and by now common knowledge that humans (and nonhumans) can and do set the reference strength of an internal response beyond which internal events will be assumed to represent external events in relation to the probability and͞or the payoff associated with the latter: equally energetic events will be reported more frequently if their occurrence is likely (or the payoff is high) than if it is not. Signal detection experiments also have established that, for a unique stimulus strength and occurrence probability, observers can simultaneously entertain a number of response criteria that translate into different levels of confidence as to the occurrence of that stimulus (1, 2). The literature remains silent, however, on the issue of whether or not humans can entertain multiple criteria based on different probabilities associated with a number of simultaneous, spatially distinct events. Common sense admits the proposition according to which the (yes͞no) report of the occurrence of any among N distinct events can be biased by the a priori likelihood of that event (Fig. 1a). But is this still the case in an environment characterized by a variety of stimulus intensities? A multistimulus environment where different events differ in their likelihood is the typical milieu of most organisms. Characterizing decision making within such an ambience is, then, of critical significance for understanding how humans regulate (or adapt) the point on their sensory continuum they use as a reference for judging ''reality'' given the actual magnitudes and probabilities of the ongoing physical events (3). In th...

It has been commonly reported that the temporal integration of grating contrast proceeds more slowly as spatial frequency is increased. Such results have been based on the critical duration for sensitivity to contrast pulses varying in duration, but the analyses have not assumed full integration at short durations and have neglected the effects of probability summation over time. To take such effects into account, we discuss a class of analytical models based on nonlinear temporal integration. On the assumption that the temporal impulse response of the visual system determines contrast integration over time, we develop both a high-threshold model and a signal-detection approach involving multiple and independent nonlinear signal detectors with a time-limited integration span. The redefined critical durations predicted by the models and verified by the data are about 35 msec and vary by no more than 10 msec across spatial frequency. This variation is entirely attributable to a change in the strength of inhibition with spatial frequency, and the analysis implies that the excitatory component is constant at all spatial frequencies, contrary to previous accounts.

A number of studies have investigated how the visual system extracts the average feature-value of an ensemble of simultaneously or sequentially delivered stimuli. In this study we model these two processes within the unitary framework of linear systems theory. The specific feature value used in this investigation is size, which we define as the logarithm of a circle's diameter. Within each ensemble, sizes were drawn from a normal distribution. Average size discrimination was measured using ensembles of one and eight circles. These circles were presented simultaneously (display times: 13-427 ms), one at a time, or eight at a time (temporal-frequencies: 1.2-38 Hz). Thresholds for eight-item ensembles were lower than thresholds for one-item ensembles. Thresholds decreased by a factor of 1.3 for a 3,200% increase in display time, and decreased by the same factor for a 3,200% decrease in temporal frequency. Modeling and simulations show that the data are consistent with one readout of three to four items every 210 ms.