Failure to handle more than one internal representation in visual detection tasks

Abstract

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...

Decision criteria in dual discrimination tasks estimated using external-noise methods

“…Hence, observers do not place their criteria optimally when handling two different tasks. This decisional behavior in the DD tasks replicates the behavior observed in previous studies with close to threshold stimuli (i.e., where performance was limited by internal noise only; Gorea & Sagi, 2000, thereby generalizing the notion of "criteria attraction" to the case where performance is limited by external noise. To verify that these criteria shifts are due to decisional processes per se and not to responses to the unprobed blob, we measured linear correlations between P("High") and the unprobed blob amplitude, finding r < .09 across all conditions and observers (see the Appendix, Fig.…”

“…Given that the internal response is monotonically related to stimulus contrast (for contrast > 0), a comparison between criteria expressed in stimulus luminance units maps onto the comparison of the corresponding internal evoked responses. Hence, in contrast to previous experiments that could not discriminate between criteria equality being due to a uc or to an FA equality (Gorea & Sagi, 2000Kontsevich et al, 2002), the present external-noise paradigm does permit such a distinction. This is so because the performance-limiting noise, σ EX , is known and can be manipulated; given that, in the present conditions, c 0 zFA × σ EX (assuming σ EX >> σ IN ) observers cannot equalize at the same time their FA rates and c when facing two tasks with a different σ EX .…”

“…Previous results of dual detection and dual discrimination tasks (Gorea & Sagi, 2000, 2002 provided evidence for the use by observers of a unique decision criterion, uc, even though an optimal behavior requires (and the experimental design permits) the use of distinct criteria as actually observed in single (detection or discrimination) tasks. In Sagi's (2000, 2001) terminology, this uc is defined as a value on the sensory continuum, measured in σ IN units referenced to the mean of the noise, μ IN 0 0.…”

Continuous theta burst transcranial magnetic stimulation reduces resting state connectivity between visual areas

“…The main idea of the modeling was that, if constant decision criteria between conditions are assumed (Gorea and Sagi 2000), then one expects different effects of decreased signal strength and increased variability. Figure 3B depicts visually a situation where a subject discriminates between two stimulus alternatives (i.e., S1 vs. S2).…”

“…The last assumption is justified by our use of a wagering procedure that encourages constant placement of decision criteria, as well as by previous research that demonstrated that subjects used unified decision criteria for different conditions within an experiment, even when such unified criteria were clearly suboptimal (Gorea and Sagi 2000).…”