Search PubMed⌕ Search

Biomedical subjects

Colin W G Clifford

Publications and source records attributed to Colin W G Clifford.

At least 19 recordsLinked to original sources
PubMed2006

New binary direction aftereffect does not add up.

Neural adaptation and inhibition are pervasive characteristics of the primate brain and are probably understood better within the context of visual processing than with any other sensory modality. These processes are thought to underlie illusions in which one motion affects the perceived direction of another, such as the direction aftereffect (DAE) and direction repulsion. The DAE describes how, following prolonged viewing of motion in one direction, the direction of a subsequently viewed test pattern is misperceived. In the case of direction repulsion, the direction difference between two transparently moving surfaces is overestimated. Explanations of the DAE appeal to neural adaptation, whereas direction repulsion is accounted for through lateral inhibition. Here, we report on a new illusion, the binary DAE (bDAE), in which superimposed slow and fast dots moving in the same direction are perceived to move in different directions following adaptation to a mixed-speed stimulus. This new phenomenon is essentially a combination of the DAE and direction repulsion. Interestingly, the magnitude of the bDAE is greater than would be expected simply through a linear combination of the DAE and direction repulsion, suggesting that the mechanisms underlying these two phenomena interact in a nonlinear fashion.

Adaptation, Physiological↗
PubMed2006

The direction aftereffect is driven by adaptation of local motion detectors.

The processing of motion information by the visual system can be decomposed into two general stages; point-by-point local motion extraction, followed by global motion extraction through the pooling of the local motion signals. The direction aftereffect (DAE) is a well known phenomenon in which prior adaptation to a unidirectional moving pattern results in an exaggerated perceived direction difference between the adapted direction and a subsequently viewed stimulus moving in a different direction. The experiments in this paper sought to identify where the adaptation underlying the DAE occurs within the motion processing hierarchy. We found that the DAE exhibits interocular transfer, thus demonstrating that the underlying adapted neural mechanisms are binocularly driven and must, therefore, reside in the visual cortex. The remaining experiments measured the speed tuning of the DAE, and used the derived function to test a number of local and global models of the phenomenon. Our data provide compelling evidence that the DAE is driven by the adaptation of motion-sensitive neurons at the local-processing stage of motion encoding. This is in contrast to earlier research showing that direction repulsion, which can be viewed as a simultaneous presentation counterpart to the DAE, is a global motion process. This leads us to conclude that the DAE and direction repulsion reflect interactions between motion-sensitive neural mechanisms at different levels of the motion-processing hierarchy.

Adaptation, Ocular↗
PubMed2006

Orientation dependence of the orientation-contingent face aftereffect.

Prolonged exposure to upright and inverted female and male faces produces opposite effects on subsequent judgments of the sex of faces depending on their orientation. We show that the magnitude of this orientation-contingent gender aftereffect can be predicted from simple aftereffects induced separately at the same orientations. The contingent aftereffect can also be induced in faces tilted 90 degrees to the right and left, eliminating any difference in face-processing strategy that may be in operation with upright and inverted faces. This suggests that neurons employing a single face encoding strategy can be activated in an orientation-specific manner.

Face↗
PubMed2006

Dynamics of the influence of segmentation cues on orientation perception.

Contextual effects abound in vision. The tilt illusion (TI) is an example-a tilted surrounding annulus causes a vertical central pattern to appear rotated away from the surround. We investigate the dynamics of this effect by presenting components of the stimulus asynchronously. At equal contrast, the largest illusion occurs when centre and surround are presented simultaneously. We vary the spatial gap between centre and surround, the relative contrast and depth and find that these segmentation cues result in a reduced TI upon simultaneous presentation, but not all other times. This reveals the dynamics of orientation and other segmentation cue interactions.

Contrast Sensitivity↗
PubMed2006

Localization of tactile stimuli depends on conscious detection.

Neurological reports of "tactile blindsight" suggest that the human somatosensory system can extract behaviorally useful information about the location of a tactile stimulus in the absence of conscious awareness that the stimulus occurred (Paillard et al., 1983; Rossetti et al., 1995). However, in a series of psychophysical experiments with neurologically intact subjects, we found no evidence for such a dissociation. Our subjects' ability to name the finger on which a tactile stimulus had been presented was dependent on their ability to consciously detect that stimulus (Harris et al., 2004). The present experiments followed up on this study and specifically sought evidence for a dissociation when subjects were required to indicate the location of the stimulus either by pointing at or moving the stimulated finger, the same response made by the neurological patients. Once again, localization accuracy was correlated with detection, and, crucially, when both detection and localization were measured using equivalent forced-choice tasks, the subjects were completely unable to identify the location of stimuli that they had not detected. These findings are inconsistent with the dissociation implied by the cases of tactile blindsight, but are consistent with other neurological evidence that detection of a tactile stimulus does not depend on localization (Head and Holmes, 1911; Halligan et al., 1995; Rapp et al., 2002).

Adolescent↗
PubMed2006

Determinants of range effects in face recognition.

The effects of test stimulus range on generalization gradients in humans were assessed for discriminations between faces that varied in brightness, faces that varied in orientation in the picture plane, and morphed faces. In Experiment 1, significant range effects, predicted by adaptation level theory, occurred when faces varied along the brightness or orientation dimension, but not for morphed faces. Changing the difficulty of discrimination of both training and test stimuli for Experiment 2 produced range effects in morphed faces. Experiment 3 explored training and testing stimulus factors as determinants of range effects in morphed faces. The results suggest that sufficiently biased testing ranges create shifts in response distributions (generalization gradients), and this may be amplified byusing relatively difficult discriminations between training stimuli.

Discrimination, Psychological↗
PubMed2005

Suppressed patterns alter vision during binocular rivalry.

Binocular rivalry occurs when incongruent patterns are presented to corresponding regions of the retinas, leading to fluctuations of awareness between the patterns . One attribute of a stimulus may rival whereas another may combine between the eyes , but it is typically assumed that the dominant features are perceived veridically. Here, we show this is not necessarily the case and that a suppressed visual feature can alter dominant perception. The cortical representations of oriented gratings can interact even when one of them is perceptually suppressed, such that the perceived orientation of the dominant grating is systematically biased depending on the orientation of the suppressed grating. A suppressed inducing pattern has the same qualitative effect as a visible one, but suppression reduces effective contrast by a factor of around six. A simple neural model quantifies and helps explain these illusions. These results demonstrate that binocular rivalry suppression operates in a graded fashion across multiple sites in the visual hierarchy rather than truncating processing at a single site and that suppressed visual information can alter dominant vision in real-time.

Humans↗
PubMed2005

Contextual modulation outside of awareness.

Contextual effects are ubiquitous in vision and reveal fundamental principles of sensory coding. Here, we demonstrate that an oriented surround grating can affect the perceived orientation of a central test grating even when backward masking of the surround prevents its orientation from being consciously perceived. The effect survives introduction of a gap between test and surround of over a degree even under masking, suggesting either that contextual information can effectively propagate across early visual cortex in the absence of awareness of the signaled context or that it can proceed undetected to higher processing levels at which such horizontal propagation may not be necessary. The effect under masking also shows partial interocular transfer, demonstrating processing of orientation by binocular neurons in visual cortex in the absence of conscious orientation perception. This pattern of results is consistent with the suggestion that simultaneous orientation contrast is mediated at multiple levels of the visual processing hierarchy, and it supports the view that propagation of signals to and, possibly, back from higher visual areas is necessary for conscious perception.

Awareness↗
PubMed2005

Mechanisms selectively engaged in rivalry: normal vision habituates, rivalrous vision primes.

When rivalrous stimuli are presented intermittently, perception stabilises. This indicates the operation of perceptual memory across interruptions in stimulation. Here we show that a percept under non-rivalrous and rivalrous conditions has qualitatively different effects on subsequent rivalrous vision. When an image is perceived under rivalrous viewing, that image is more likely to be perceived in later rivalrous viewing: an effect of stabilisation or priming. When the same image is perceived under non-rivalrous viewing conditions, it is less likely to be perceived again during subsequent rivalrous viewing: an effect of adaptation or habituation. When these stimuli possess different attributes to those in subsequent vision their effect declines. This suggests that visual rivalry might recruit mechanisms that are not engaged in 'normal' non-rivalrous vision but perhaps dedicated to the resolution of competing sensory information.

Awareness↗
PubMed2005

Interactions between luminance and contrast signals in global form detection.

The human visual system is adept at detecting global structure, or form, within a scene. The initial stage of post-retinal processing for all aspects of vision is fed by On- and Off-centre cells sensitive to centred luminance increments and decrements respectively. These cells provide input to two parallel pathways that process variations in local luminance (first-order pathway) and local contrast (second-order pathway). Here, we investigate the contribution of luminance and contrast information to global form detection, a stage between the extraction of local orientation and the recognition of objects. The underlying processes involve two stages. We find that signals in the On-, Off- and second-order pathways are segregated at both stages of processing. Surprisingly, the non-linear stage in the second-order form pathway is different from that in motion processing: the second-order form detectors show an asymmetry in sensitivity to increments and decrements that is not apparent in motion. A functional architecture for global form detection is proposed along with its possible neural substrates.

Contrast Sensitivity↗
PubMed2005

Aftereffect of adaptation to Glass patterns.

Our visual systems constantly adapt their representation of the environment to match the prevailing input. Adaptation phenomena provide striking examples of perceptual plasticity and offer valuable insight into the mechanisms of sensory coding. Here, we describe an aftereffect of adaptation to a spatially structured image whereby an unstructured test stimulus takes on illusory structure locally perpendicular to that of the adaptor. Objective measurement of the strength of the aftereffect for different patterns suggests a neural locus of adaptation prior to the extraction of complex form in the visual processing hierarchy, probably at the level of primary visual cortex. This view is supported by further experiments showing that the aftereffect exhibits partial interocular transfer but complete transfer across opposite contrast polarities. However, the aftereffect does show weak position invariance, suggesting that adaptation at higher levels of the visual system may also contribute to the effect.

Adaptation, Ocular↗
PubMed2005

Inter-ocular transfer of the tilt illusion shows that monocular orientation mechanisms are colour selective.

A vertical grating appears tilted when surrounded by a tilted inducer grating: the tilt illusion. We investigated the inter-ocular transfer of the tilt illusion for gratings modulated along parallel or orthogonal vectors in a L-M and L+M+S cone contrast space. We found that the monocular component of the tilt illusion is entirely colour selective and the binocular component shows only weak colour selectivity. These results suggest that colour and orientation processing interact at monocular stages of visual processing, whereas binocular visual mechanisms code for form in a manner that is largely insensitive to chromatic signature.

Color Perception↗
PubMed2005

Contrast configuration influences grouping in apparent motion.

We investigated whether the same principles that influence grouping in static displays also influence grouping in apparent motion. Using the Ternus display, we found that the proportion of group motion reports was influenced by changes in contrast configuration. Subjects made judgments of completion of these same configurations in a static display. Generally, contrast configurations that induced a high proportion of group motion responses were judged as more 'complete' in static displays. Using a stereo display, we then tested whether stereo information and T-junction information were critical for this increase in group motion. Perceived grouping was consistently higher for same contrast polarity configurations than for opposite contrast polarity configurations, regardless of the presence of stereo information or explicit T-junctions. Thus, while grouping in static and moving displays showed a similar dependence on contrast configuration, motion grouping showed little dependence on stereo or T-junction information.

Contrast Sensitivity↗
PubMed2005

When your brain decides what you see: grouping across monocular, binocular, and stimulus rivalry.

Research suggests that the neural concomitants of visual rivalry are contingent on the stimulus parameters, implying the existence of three different types of rivalry. Binocular rivalry (dissimilar patterns are presented, one to each eye) is seemingly mediated by interactions between pools of monocular neurons. Monocular rivalry (superimposed patterns are presented to one or both eyes) is presumably the result of competition between neural representations of the patterns. Stimulus rivalry (dissimilar patterns are swapped rapidly between the two eyes) is independent of eye of origin. In the experiment reported here, we integrated these three different types of rivalry into one stimulus. We found that perceptual alternations span the three types of rivalry, demonstrating that the brain can produce a coherent percept sourced from three different types of visual conflict. This result is in agreement with recent work suggesting that the resolution of competitive visual stimuli is mediated by a general mechanism spanning different levels of the visual-processing hierarchy.

Brain↗
PubMed2004

Rapid global form binding with loss of associated colors.

Distributed neural processing creates a binding problem: the representations of the various features of an object are distributed across brain areas but must be associated with, or bound to, the same object. Here we determine the limits that binding imposes on the perception of global form in static flow fields defined by oriented dot pairs. The binding of local orientation signals into global form is shown to operate at rapid 20-Hz rates, implying that visual signals at the point of global form extraction retain precise temporal registration. Binding global form with color is limited to rates of 3-5Hz, showing that binding across attributes can impose a severe temporal limit on perception. Judgment of the temporal sequence of the global structures is also limited to slow rates. These results point to a substantial loss of temporal resolution in the visual system following the extraction of global form but preceding visual awareness.

Awareness↗
PubMed2004

Orientation-contingent face aftereffects and implications for face-coding mechanisms.

Humans have an impressive ability to discriminate between faces despite their similarity as visual patterns. This expertise relies on configural coding of spatial relations between face features and/or holistic coding of overall facial structure. These expert face-coding mechanisms appear to be engaged most effectively by upright faces, with inverted faces engaging primarily feature-coding mechanisms. We show that opposite figural aftereffects can be induced simultaneously for upright and inverted faces, demonstrating that distinct neural populations code upright and inverted faces. This result also suggests that expert (upright) face-coding mechanisms can be selectively adapted. These aftereffects occur for judgments of face normality and face gender and are robust to changes in face size, ruling out adaptation of low-level, retinotopically organized coding mechanisms. Our results suggest a resolution of a paradox in the face recognition literature. Neuroimaging studies have found surprisingly little orientation selectivity in the fusiform face area (FFA) despite evidence that this region plays a role in expert face coding and that expert face-coding mechanisms are selectively engaged by upright faces. Our results, demonstrating orientation-contingent adaptation of face-coding mechanisms, suggest that the FFA's apparent lack of orientation selectivity may be an artifact of averaging across distinct populations within the FFA that respond to upright and inverted faces.

Adult↗
PubMed2004

Perceptual grouping of biological motion promotes binocular rivalry.

Investigation of perceptual rivalry between conflicting stimuli presented one to each eye can further understanding of the neural underpinnings of conscious visual perception. During rivalry, visual awareness fluctuates between perceptions of the two stimuli. Here, we demonstrate that high-level perceptual grouping can promote rivalry between stimulus pairs that would otherwise be perceived as nonrivalrous. Perceptual grouping was generated with point-light walker stimuli that simulate human motion, visible only as lights placed on the joints. Although such walking figures are unrecognizable when stationary, recognition judgments as complex as gender and identity can accurately be made from animated displays, demonstrating the efficiency with which our visual system can group dynamic local signals into a globally coherent walking figure. We find that point-light walker stimuli presented one to each eye and in different colors and configurations results in strong rivalry. However, rivalry is minimal when the two walkers are split between the eyes or both presented to one eye. This pattern of results suggests that processing animated walker figures promotes rivalry between signals from the two eyes rather than between higher-level representations of the walkers. This leads us to hypothesize that awareness during binocular rivalry involves the integrated activity of high-level perceptual mechanisms in conjunction with lower-level ocular suppression modulated via cortical feedback.

Dominance, Ocular↗