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The detection model is used to describe the detection of scarcely discernible noiseless signals by human observers. Detection is hampered by an internal noise generated by the sensory system itself. In the present study, it appears that increments of loudness of a tone and increments of intensity of a light can be detected independently and simultaneously. Here, the internal noise is specific for the system responsible for this detection. On the other hand, detection of an increment in duration of visual and auditory signals is completely correlated. This correlation and the similarity of the detectability of the two kinds of signals indicate that a “duration center” exists, serving to estimate the duration of both visual and auditory signals. The methods developed are useful for identification of separate or common parts of sensory systems.

Two similar visual flashes spatially separated were used as stimuli in a simple RT experiment. The RT distributions of the single stimuli were compared to cases with paired stimuli applied simultaneously (or nearly simultaneously). Spatial separation of simultaneous stimuli induces parallel processing. After comparing single-stimuli RT distributions with double-stimuli RT distributions, it was concluded that a race model with independent parallel processing provides the better fit to the data as compared to dependent parallel processing or partially parallel processing. Scanning models as well as signal summation models had to be rejected. The serial structure of the processing of single stimuli allows for a description of an exponential and a residual distribution. The analysis presented in this study is of importance for the concept of stages in serial processing of more complicated stimuli.With visual signal processing, it is possible to induce parallel processing by using spatially separated stimuli. Because of the convergence of multiple inputs onto a single output, internal signals necessarily interact. The general notion is that with dualstimulus simple reactions, the same subprocesses are involved as with the single stimulus simple reactions for each separate stimulus. But with multiple-stimulus simple reactions, some of the subprocesses occur separately but in parallel, while others are in common. A number of interesting questions now arise: (l) How is subprocess interaction accomplished? (2) To what extent does parallel processing occur? (3) Do parallel processes act independently?In thinking about subprocesses interaction, one can assume that the internal representation of activities caused by a temporally punctate stimulus gives rise to temporally punctate response, such as breaking a contact.When the internal representation is temporally punctate, communication between subsystems may be regarded to occur with pulses. In the first part of the psychophysical system, pulses are transmitted along parallel channels-serial subsystems-each channel contributing a specific latency for the first pulse to arrive. Somewhere in the system, pulses will be guided into a common channel to emit a response through the motor system. An emitted response is therefore viewed as the outcome of a race between pulses transmitted through different channels. The first pulse to arrive at the common channel will emit a response; later parts of the signal will be without consequence. Following Kornblum (1973), such models will be referred to as "race models." The outcome of the race may be influenced by the experimenter through the time differences between 41 stimuli within the multiple stimulus condition.The general hypothesis of independent parallel processing has been proposed and supported in some studies. Kantowitz (1974) indicates that no sensory interaction is found among multiple stimuli. Lappin and Eriksen (1966) studied reaction time for effective inhibition of the response by a second stimulus. OIlman (1973) foun...

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