Rick van der Zwan

Mechanisms of purely subjective contour tilt aftereffects

Neurones tuned for second-order stimuli--those which have edges defined by properties other than luminance and colour--have been frequently observed in prestriate cortex and in area V2 there are neurones which explicitly and unambiguously signal the orientation of purely subjective contours, i.e. contours with no Fourier components at the orientation of the perceived edge [von der Heydt, R. & Peterhans, (1989) Journal of Neuroscience, 9, 1731-1748]. No neurones in area V1 showed similar tuning characteristics. In addition, it has been demonstrated that like real contours, purely subjective test contours are subject to tilt aftereffects following prolonged viewing of an adapting stimulus. whether that stimulus is real or subjective [Paradiso, M. A., Shimojo, S. & Nakayama, K. (1989) Vision Research, 29, 1205-1213]. This result supports the assertion that the cortical processes responsible for real contour perception are also those giving rise to subjective contour perception. The data reported here further examined this hypothesis. Four experiments show that purely subjective contours exhibit both direct and indirect tilt aftereffects and tilt illusions like those observed with real contours. Further, they provide evidence that direct and indirect subjective contour effects, like direct and indirect real contour effects, arise via the operation of two mechanisms: a low level process, possibly lateral inhibition between orientation channels, and a second higher-order process. The data suggest that processing of orientation information beyond the striate cortex is similar to that which occurs in area V1 and the data are consistent with models of contour processing which assume that all perceived contours, both real and subjective, arise from a common mechanism.

Hands as sex cues: sensitivity measures, male bias measures, and implications for sex perception mechanisms.

Sex perceptions, or more particularly, sex discriminations and sex categorisations, are high-value social behaviours. They mediate almost all inter-personal interactions. The two experiments reported here had the aim of exploring some of the basic characteristics of the processes giving rise to sex perceptions. Experiment 1 confirmed that human hands can be used as a cue to an individual’s sex even when colour and texture cues are removed and presentations are brief. Experiment 1 also showed that when hands are sexually ambiguous observers tend to classify them as male more often than female. Experiment 2 showed that “male bias” arises not from sensitivity differences but from differences in response biases. Observers are conservative in their judgements of targets as female but liberal in their judgements of targets as male. These data, combined with earlier reports, suggest the existence of a sex-perception space that is cue-invariant.

An illusion of coherent global motion arising from single brief presentations of a stationary stimulus

We describe a new illusion in which a single stationary stimulus appears to undergo coherent global motion. Contrast relationships between the stimulus elements suggest the illusion arises via processing of Off- and On-channel signals that remain independent until after passing through low-level motion detectors. We propose that patterns of activation resulting from biphasic temporal impulse response functions in the magnocellular pathway are the basis of the illusion, and describe a model to account for the illusory motion percept.

The role of ON- and OFF-channel processing in the detection of bilateral symmetry

We present evidence that grouping for luminance does not take precedence over the detection of bilaterally symmetrical patterns. Using single-axis and double-axis images, we found that element pairs within which luminance is held constant drive symmetry-detection mechanisms more effectively than pairs within which luminance varies. Moreover, the performance decrement observed for patterns defined by element pairs within which luminance varies is not specific to interchannel variation. Luminance variation within the ON and OFF channels has the same effect as variation between the channels on the performance of axis-orientation identification tasks. It is argued that this constitutes possible evidence for subchannels within the ON and OFF channels. One of the characteristics of the subchannels is that each processes only a limited range of luminance steps. The implications of this type of luminance processing for the detection of symmetry in the visual scene are discussed.