According to the traditional inferential theory of perception, percepts of object motion or stationarity stem from an evaluation of afferent retinal signals (which encode image motion) with the help of extraretinal signals (which encode eye movements). According to direct perception theory, on the other hand, the percepts derive from retinally conveyed information only. Neither view is compatible with a perceptual phenomenon that occurs during visually induced sensations of ego motion (vection). A modified version of inferential theory yields a model in (...) which the concept of extraretinal signals is replaced by that of reference signals, which do not encode how the eyes move in their orbits but how they move in space. Hence reference signals are produced not only during eye movements but also during ego motion (i.e., in response to vestibular stimulation and to retinal image flow, which may induce vection). The present theory describes the interface between self-motion and object-motion percepts. An experimental paradigm that allows quantitative measurement of the magnitude and gain of reference signals and the size of the just noticeable difference (JND) between retinal and reference signals reveals that the distinction between direct and inferential theories largely depends on: (1) a mistaken belief that perceptual veridicality is evidence that extraretinal information is not involved, and (2) a failure to distinguish between (the perception of) absolute object motion in space and relative motion of objects with respect to each other. The model corrects these errors, and provides a new, unified framework for interpreting many phenomena in the field of motion perception. (shrink)

Recently it has been argued that a model of directed perception provides an alternative to both indirect and direct accounts of the nature of vision. An examination of this proposal serves as a basis for challenging the meaningfulness and empirical import of the theoretical and ontological differences said to separate these models. Although focusing on James Cutting's work, the analysis is meant to speak more generally to the supposed significance of the distinctions among indirect, direct, and directed theories of perception.

In terms of physics, mass is the fixed attribute of an object while weight varies with the accelerative force. Neither weight nor mass are simple sensory stimuli as both involve the integration of sensory and motor information with higher cognitive processes. Studies of apparent heaviness yield only vague information about sensorimotor mechanisms.

Psychophysical scales can be constructed under suitable restrictions from appropriate data, but they still do not justify privileged internal sensations.

In his target article Lockhead calls attention to numerous complications that prevent a valid straightforward or Fechnerian interpretation of psychophysical data. Here I describe three additional sources of bias, all involving the influence of stimuli presented before the start of an investigation.

In contrast to Lockhead's view it is argued that psychology as a genuine science must not be based on other sciences and that psychological measurements have to be validated inside psychology. It is pointed out that psychological scalings, unaffected by judgment contexts, can be obtained if the experimental setting is compatible with everyday situations.

Given that psychophysical responses are not a function of a single property but vary with a variety of stimulus and context variables, Lockhead has little hope for laws relating behavior to the environment. However, progress can be made with tasks that manipulate multiple sources of information to test formal information-processing models.

People cannot make independent judgements of stimulus attributes and so (Lockhead 1992, p. 551) rather than in terms of stimulus features. The new commentaries here further this statement and also support the observations in the target article that psychophysical scaling methods allow us to measure (1) how context determines judgments and (2) what people remember about prior stimuli.