Temporal discrimination was investigated by the methods of production, reproduction, constant stimuli, single stimuli, and estimation. The Weber function was found to give a good fit to the relation between AT and T; evidence was obtained that a dip in the Weber function at a short interval is not an essential feature of time estimation but may be due to the development of rhythmic modes of response or other factors. Productions or reproductions of intervals tend to lengthen during the course of a session at a rate proportionately greater for short intervals; but estimates tend to shorten. A model for the "internal clock" is described, based on a pacemaker, counter, store, and comparator with functional relations tending to reduce error, and it is shown to provide explanations for the Weber function, the indifference interval, overestimation of short and underestimation of long intervals, the features of "lengthening," assimilation, and other findings.

Three models of sequential effects in psychophysical tasks are defined and experimental results described. These appear most consistent with a model in which the momentary value of the criterion is modified by memory traces, an independent trace being retained for each relevant past event. On this basis, a theory of criterion setting is developed: A long-term process determines an initial value for the criterion, and two short-term processes adjust the criterion (a) to match current changes in the expected probabilities of signals and (b) to maximize the information transmitted by the subject's responses. The theory is applied to results in the literature.

Evidence for the proposition that human time perception is determined by an internal clock is largely indirect. It would strengthen the case for this hypothesis if a model for the internal clock were available from which predictions could be derived and tested, and if the basic parameter of such a model, the frequency at which the clock runs, could be estimated. A model for an internal temporal pacemaker is briefly described and its properties are explored by computer simulation. Results are obtained that provide a basis for predicting that, under appropriate conditions, interference between an imposed rhythm and the frequency of a temporal oscillator may cause perturbations in temporal judgment which are related to the characteristic frequency of that oscillator. Experimental data are reported which appear to demonstrate such an interference pattern. These results allow some estimates of the characteristic frequency of the temporal oscillator to be obtained.

Since the occurrence of vomiting as a response to motion is both widespread and apparently disadvantageous, it presents a problem for evolutionary theory. An hypothesis is proposed suggesting that motion sickness is triggered by difficulties which arise in the programming of movements of the eyes or head when the relations between the spatial frameworks defined by the visual, vestibular, or proprioceptive inputs are repeatedly and unpredictably perturbed. Such perturbations may be produced by certain types of motion, or by disturbances in sensory input or motor control produced by ingested toxins. The last would be the important cause in nature, the main function of the emesis being to rid the individual of ingested neurotoxins. Its occurrence in response to motion would be an accidental by product of this system.