Biomedical subjects

Nicolaas Prins

Publications and source records attributed to Nicolaas Prins.

7 recordsLinked to original sources

PubMed2006

Direct evidence for the existence of energy-based texture mechanisms.

Two classes of models have been proposed to explain how the visual system processes texture modulations. In 'feature models', abstract representations of the featural properties of local texture regions (eg orientation, spatial frequency, contrast) are first generated, after which differences in individual feature properties across space are detected. In 'energy models', on the other hand, differences across space in the response energies of linear simple-cell-like filters are detected. This model thus processes the existing differences between texture regions directly without generating a full representation of the individual texture regions. We provide here direct evidence for the existence of the second, energy model, using an adaptation paradigm in conjunction with textures simultaneously modulated in two dimensions--orientation and spatial frequency. We found that the mechanism that processed the conjoint modulation was tuned to orientations and spatial frequencies that could not be predicted by any feature model, but which were precisely predicted by the energy model.

Differential Threshold↗

PubMed2003

The role of local grouping and global orientation contrast in perception of orientation-modulated textures.

We explored the contribution to perception of orientation-modulated textures of visual processes selective either for orientation contrast or orientation grouping. To distinguish between these two processes we manipulated the axis of local grouping of texture elements independently of the direction of global orientation modulation. The general question posed was whether visibility of texture structure (measured as threshold for discriminating spatial-frequency of texture structure) is dependent on the magnitude of orientation contrast, strength and direction of local grouping, or some combination of the two. We demonstrated that the factor of primary importance is the amplitude of global orientation contrast rather than the presence of local grouping content. Using orientation-interleaved textures (containing two superimposed textures modulated around orthogonal orientations), we further showed that orientation single-opponent processes are a more likely candidate for detecting orientation contrast than double-opponent processes.

Contrast Sensitivity↗

PubMed2003

Detection and discrimination of texture modulations defined by orientation, spatial frequency, and contrast.

We sought to determine whether the detection and the identification of texture modulations are mediated by a common mechanism. On each trial two textures were presented, one of which contained a modulation in orientation (OM), spatial frequency (FM), or contrast (CM). Observers were required to indicate whether the modulated texture was presented in the first or the second interval as well as the nature of the texture modulation. The results showed that for two of the three pairwise matchings (OM-FM and OM-CM) detection and identification performance were nearly identical, suggesting a common underlying mechanism. However, when FM and CM textures were paired, discrimination thresholds were significantly higher than detection thresholds. In the context of the filter-rectify-filter model of texture perception, our results suggest that the mechanisms underlying detection are labeled with respect to their first-order input; i.e., the identities of these mechanisms are available to higher levels of processing. Several possible explanations for the misidentification of FM and CM at detection threshold are considered.

Contrast Sensitivity↗

PubMed2002

Acoustic orientation via sequential comparison in an ultrasonic moth.

Orientation of female lesser wax moths (Achroia grisella) to male calling song was tested on a locomotion-compensator device that withheld all inter-aural acoustic differences from the insect. Under these circumstances, females remained longer in the vicinity of the sound source if they experienced a variable sound level that increased when approaching the source rather than a level that remained constant at all times. Analyses of orientation paths revealed that greater retention near the source was achieved by enhanced turning when the perceived sound level remained unchanged or decreased but retaining the previous heading when the level increased. These findings suggest that acoustic orientation can be supplemented by mechanisms based on sequential, as opposed to instantaneous, comparison of auditory input. Such mechanisms may be valuable when binaural hearing is impaired or asymmetric or in environments where acoustic differences at the two ears are unreliable indications of direction to the sound source.

Animals↗

PubMed2002

On the perceived location of global motion.

We measured the effects of coherent motion of one set of dots on the perceived location of Gaussian envelopes formed by luminance modulation of a second set of dots. Perceived shifts in envelope location in the direction of coherent motion were obtained even when the dots forming the envelopes did not physically move in the direction of coherent motion. In such cases, perceived shifts coincided with stimulus configurations that permitted motion integration of the envelope dots with the coherently moving dots, for example, when envelope dots moved in random directions as opposed to being static. In subsequent experiments we explored the type of motion integration underlying the positional shifts obtained. We discounted the possibility that the visual system incorrectly attributes motion signals associated with coherently moving dots to envelope dots by demonstrating that positional shifts could be obtained even when the coherent dots were laterally displaced to either side of the envelope dots such that the regions occupied by the dots did not overlap. We also discounted spatio-temporal summation within the receptive fields of low-spatial-frequency motion-sensitive mechanisms by demonstrating that positional shifts persisted even when the dot displays were high-pass filtered. These results, coupled with the observation that the proportion of coherently moving dots required to produce positional shifts correlated well with global motion thresholds measured for the same dot configurations, suggests that visual processes which underlie motion-dependent positional shifts are based at least in part on cooperative interactions of the type implicated in global motion.

Humans↗

PubMed2002

Orientation- and frequency-modulated textures at low depths of modulation are processed by off-orientation and off-frequency texture mechanisms.

Intuitively it may seem likely that orientation-modulated (OM) and frequency-modulated (FM) textures are processed utilizing the first-order channels that are most responsive to the first-order (luminance) information contained in the textures. This assumption would imply that the detection or segmentation of OM or FM textures is accomplished by second-order mechanisms that receive their first-order input from neurons tuned to either the center, or to the peaks in the orientation and spatial-frequency distribution of the texture. Here we show that at low depths of modulation this is not the case. Using an adaptation paradigm, we show that the first-order filters involved in the perception of OM and FM textures are those which maximize the differential response between the different texture regions. Our explanation of this result is similar to that made by Regan and Beverley [J. Opt. Soc. Am. 73 (1983) 1684; J. Opt. Soc. Am. A 2 (1985) 147] for simple grating stimuli. However, we show that whereas Regan and Beverley's results could be accounted for on the basis of the tuning functions of the putative mechanisms involved, our results can be explained in terms of the characteristics of the textures themselves. Some implications of our finding are discussed.

Adaptation, Ocular↗

PubMedDate not supplied

Mechanism independence for texture-modulation detection is consistent with a filter-rectify-filter mechanism.

The ability of the visual system to detect stimuli that vary along dimensions other than luminance or color--"second-order" stimuli--has been of considerable interest in recent years. An important unresolved issue is whether different types of second-order stimuli are detected by a single, all purpose, mechanism, or by mechanisms that are specific to stimulus type. Using a conventional psychophysical paradigm, we show that for a class of second-order stimuli--textures sinusoidally modulated in orientation (OM), spatial frequency (FM), and contrast (CM)--the human visual system employs mechanisms that are selective to stimulus type. Whereas the addition of a subthreshold mask to a test pattern of the same stimulus type was found to facilitate the detection of the test, no facilitation was observed when mask and test were of different types, suggesting mechanism independence for the different types of stimulus. This finding raises the important question of whether mechanism independence is compatible with the well-known filter-rectify-filter (FRF) model of second-order stimulus detection, since FRF mechanisms, in principle, do not discriminate between stimulus types. We show that for all mask/test combinations except those with CM masks, the FRF mechanism giving the largest response to the test modulation is largely unaffected by subthreshold levels of a different stimulus-type mask. For this reason, we cannot rule out the possibility that FRF mechanisms mediate the detection of our stimuli. For combinations involving CM masks, however, we propose that a process of contrast normalization renders the test stimulus insensitive to the mask stimulus.

Contrast Sensitivity↗