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Evaluates 2 theoretical models suggested to explain studies of the effect of stimulus intensity on the perceived duration of brief light flashes. Some studies found a direct relationship between the 2 variables; others found an indirect relationship. Each model suggests that an additional variable interacts with stimulus intensity. Proposed variables have included the nature of the judgment and the absolute intensity. The present evaluation indicates that both of the proposed variables play a role and that a melding of the models could best account for this phenomenon. The melding incorporates known time- and intensity-dependent characteristics of neural integration into the behavioral performance. The evaluation also indicates that future experiments on this problem will be most informative if they (a) give particular attention to the role of instructions, (b) explore an adequate range of intensities, and (c) strictly control adaptive state.

The carapace characteristics of horseshoe crabs (Limulus polyphemus) have been correlated by others with male mating performance and it has also been suggested that such mating differences might depend on visual differences. The effect of carapace character on horseshoe crab vision was therefore noninvasively investigated using the electroretinogram (ERG) of the lateral compound eye as a measure. Male Limulus were dichotomized on three carapace dimensions: clear versus dark eyes, light versus dark carapaces, and presence versus absence of barnacles. All dichotomizations gave similar results: No crab was ERG-blind. Analyses based on ERG magnitude yielded trivial sensitivity differences between these groups. However, analyses based on ERG latency indicated that response speeds exhibited reliably different trends between groups: Although responses to dim flashes were similar for all animals, increasing flash intensity produced significantly greater changes in the latencies of the ERGs in uniform males than in variegated males. This interactive dependence of ERG speed differences on flash energy cannot be the result of greater light absorption by the variegated specimens'' darkened corneas. The visual capacities of variegated specimens are therefore somewhat altered, but certainly not absent. Equalizing animal size between groups did not measurably affect these results, implying that age was not a factor. The previously reported correlation of an individual male''s appearance with its performance in mating competitions is extended by these data to include its sensory functioning as well.

The neural coding problem is defined and several possible answers to it are reviewed. A widely accepted answer descends from early suggestions that neural activity, in general, is isomorphic with sensation and that the biological signals resident in the axons of neurons, in particular, are given by their frequency of firing. More recent data are reviewed which indicate that the pattern of neural responses may also be informative. Such data led to the formulation of the multiple meaning theory which suggests the neural pattern may encode different information features in single responses. After a period in which attention turned elsewhere, the multiple meaning theory has quite recently been revived and has stimulated novel and careful experimental investigations. A corollary theory, the task dependence hypothesis, suggests that these information-bearing multiple response features are accessed differentially in different behavioral tasks. These theories place stringent temporal requirements on the generation and analysis of neural responses. Recent data are examined indicating that both requirements may indeed be satisfied by the nervous system. Finally, several methods of experimentally testing such coding theories are described; they involve manipulating the biological signals of neurons and observing the effect of these manipulations on behavior.

A new tool that may measure certain absolute temporal properties of information processing in intact organisms is suggested by. investigations of temporal summation in single nerve cells. Two findings have led to this suggestion: One, the form of the temporal summation function (relating the intensity and duration required to evoke a criterion neural signal) depends on the analysis used by the investigator. Corresponding form variations occur in behavioral studies when the observer's task is varied. Two, the critical durations of fixed neural signals depend on the latency of the feature of the signal chosen as criterion; early features yield short critical durations and vice versa. The critical duration also varies in behavioral studies if one varies the observer's task, keeping the stimulus ensemble fixed.These data lead to two inferences: One, the form of a behavioral temporal-summation function expresses the kind of hidden mental analysis mediating that behavior. Two, a behavioral critical duration is an indicator of the absolute timing of the hidden mental analysis mediating that behavior.

Varying stimulus intensity while measuring the perceived duration/visual persistence of brief light flashes has yielded disparate results. Some studies have found a direct relationship between the two variables; others have found an inverse relationship. Several models have been suggested to unify this behavioral literature. They invoke the absolute intensity and the nature ofthe judgment as explanatory variables. We now present physiological data whose analysis was motivated by these models. We measured the duration of photoreceptor potentials as a function of light intensity and response measure. One response measure was the length of time required for the response to decline from the peak by a criterion amount. The other response measure was the length oftime a response stayed above a criterion level. These data suggest that each behaviorally based model captured a different aspect of a single underlying mechanism and that a melding of the two critical concepts would harmonize all of the results: In this melding, the sensory signals that mediate visual perception would have the type of complex intensity-and time-dependent properties found in real neural responses.Visual sensations need not last for the same amount of time as the light flashes that evoke them. If, as is often the case, the sensation outlasts the stimulus, then one can talk about visual persistence: the length of time between the end of the stimulus and the end of the sensation. Alternatively, one can talk about perceived duration; it is usually operationally defined as the time from stimulus onset to the end of the sensation. Sometimes, however, perceived duration is defined as the interval between the beginning and end of the sensation. Whichever measure is used, differences between objective and subjective duration are particularly interesting for two reasons: First, they are quite obviously expressions of the mechanisms that create subjective representations of the external world. Second, they also provide particularly tractable material for quantitative empirical investigation. A large number of studies have therefore measured the effects of stimulus intensity on visual persistence and perceived duration; the results of these studies have not been uniform. Some of these studies have found a direct relationship, whereas other studies have found an inverse relationship.Several hypotheses have been offered to explain these variant results (see

The methods of Fechner and Stevens for assessing sensation quantity usually yield different psychophysical functions even when all other factors are controlled. In this experiment, corresponding differences occurred when different features of the same sensory receptor signals were analyzed. In the visual system, the receptor potential saturated if the peak but not the area was measured; these results match visual psychophysical functions obtained with the methods of Fechner and Stevens, respectively. This result suggests that both methods are equally valid but that each method yields the particular psychophysical function appropriate for a particular kind of information processing. A novel factor in determining sensation quantity, namely the time used by the observer to make a judgment, is implicated by the data.