Perceiving visual expansion without optic flow

When an observer moves forward in the environment, the image on his or her retina expands. The rate of this expansion conveys information about the observer's speed and the time to collision. Psychophysical and physiological studies have provided abundant evidence that these expansionary motions are processed by specialized mechanisms in mammalian visual systems. It is commonly assumed that the rate of expansion is estimated from the divergence of the optic-flow field (the two-dimensional field of local translational velocities). But this rate might also be estimated from changes in the size (or scale) of image features. To determine whether human vision uses such scale-change information, we have synthesized stochastic texture stimuli in which the scale of image elements increases gradually over time, while the optic-flow pattern is random. Here we show, using these stimuli, that observers can estimate expansion rates from scale-change information alone, and that pure scale changes can produce motion after-effects. These two findings suggest that the visual system contains mechanisms that are explicitly sensitive to changes in scale.

Velocity constancy in a virtual reality environment

During everyday life the brain is continuously integrating multiple perceptual cues in order to allow us to make decisions and to guide our actions. In this study we have used a simulated (virtual reality--VR) visual environment to investigate how cues to speed judgments are integrated. There are two sources that could be used to provide signals for velocity constancy: temporal-frequency or distance cues. However, evidence from most psychophysical studies favours temporal-frequency cues. Here we report that two depth cues that provide a relative object--object distance--disparity and motion parallax--can provide a significant input to velocity-constancy judgments, particularly when combined. This result indicates that the second mechanism can also play a significant role in generating velocity constancy. Furthermore, we show that cognitive factors, such as familiar size, can influence the perception of object speed. The results suggest that both low-level cues to spatiotemporal structure and depth, and high-level cues, such as object familiarity, are integrated by the brain during velocity estimation in real-world viewing.

Efficient search for size targets on a background texture gradient: is detection guided by discontinuities in the retinal-size gradient of items?

Six visual search experiments were carried out to investigate the processing of size information in early vision. The apparent size of display items was manipulated independently of their retinal size by placing items on a textured surface which altered the perceived distance in depth of the items. Overall, these experiments demonstrate that a target item differing from non-target items in terms of apparent size can be detected efficiently. However, the pattern of results indicates that, rather than deriving apparent-size information, target detection is guided by discontinuities in the 'retinal-size gradient' of items, in particular between items at the same 'depth'. Although the arrangement of items on the texture surface strongly influenced search, this was largely due to the retinal size of items and the retinal separation between items. The implications of these experiments for the nature of the pre-attentive representation of size are discussed.

Speed perception is affected by the Ebbinghaus-Titchener illusion

We examined whether the apparent extent of motion affects speed perception. On the first presentation of each trial, a light dot travelled horizontally across a central circle of one of the Ebbinghaus configurations (with either small or large inducing elements). On the second presentation, observers adjusted the speed of a dot moving within the central circle alone so as to match the speed perceived in the first presentation. For all stimulus speeds (1.3, 2.1, and 5.5 deg s-1), the matched speed with small inducing circles was systematically less than that with large inducing circles. The findings indicate that the perceived speed depends on the apparent extent of motion: the larger the apparent size of a frame, the slower the apparent speed. These results are consistent with the predictions of transposition effects in visual motion.

The spatial spread of attentional modulation of the motion aftereffect

The spatial spread of attentional modulation of selective adaptation was investigated in four experiments in which the duration of the movement aftereffect (MAE) was measured with and without processing of intermittently changing digits at the fixation point. In the first experiment, the effects of diverting attention on MAE duration were found to reduce as the distance between the fixation digits and the inner edge of the surrounding adapt/test grating was increased. A second experiment suggested that eye movements were unlikely to underlie the attentional effects. In experiment 3, the attentional effect stayed constant as the outer diameter of the adapt/test gratings was increased. In experiment 4 (as in experiment 1) the modulatory effects of attention were larger the closer the adapt/test gratings were to the locus of attention, when the area of the grating was held constant but its eccentricity varied. In experiments 1 and 4, an intermittently changing fixation digit was found to reduce MAE durations more than an unchanging digit, even when subjects were not required to process it, suggesting that exogenous as well as endogenous attentional processes modulate early motion processing.

Emmert's law in the dark: active and passive proprioceptive effects on positive visual afterimages

The relationship between apparent size and apparent distance is given by Emmert's law, which states that a retinal image is proportional in size to the distance of the surface it is projected upon. This principle also applies to retinal afterimages in that they, too, will change in apparent size if distance cues suggest that the location of the object onto the retinal image has been altered. It has also been known for some time that non-retinal cues can produce quantitative and qualitative effects on an afterimage when it is viewed in the dark. In the present two studies, positive afterimages of an observer's hand, as well as objects held by that hand, were used as targets to investigate the effects on size-constancy scaling of moving the hand to and fro along the line of sight for different distances in the dark. Results show that, when observers focus on a held object, the changes in size predicted by Emmert's law occur in response to both active and passive proprioceptive or haptic cues. The most intriguing result consisted of the finding that, when only the hand is the target, there appears to be a limit to the decrease in apparent hand size. It appears that the visual system 'refuses' to size-scale the hand below a limit it accepts as representative or acceptable of 'its' hand.

Identification of facial images in peripheral vision

Contrast sensitivity for face identification was measured as a function of image size to find out whether foveal and peripheral performance would become equivalent by magnification. Size scaling was not sufficient for this task, but when the data was scaled both in size and contrast dimensions, there was no significant eccentricity-dependent variation in the data, i.e. for equivalent performance both the size and contrast needed to increase in the periphery. By utilising spatial noise added to the images we found that in periphery information was utilised less efficiently and peripheral inferiority arose completely from lower efficiency, not from increased internal noise.

Short-latency disparity vergence in humans

Eye movement recordings from humans indicated that brief exposures (200 ms) to horizontal disparity steps applied to large random-dot patterns elicit horizontal vergence at short latencies (80.9 +/- 3.9 ms, mean +/- SD; n = 7). Disparity tuning curves, describing the dependence of the initial vergence responses (measured over the period 90-157 ms after the step) on the magnitude of the steps, resembled the derivative of a Gaussian, with nonzero asymptotes and a roughly linear servo region that extended only a degree or two on either side of zero disparity. Responses showed transient postsaccadic enhancement: disparity steps applied in the immediate wake of saccadic eye movements yielded higher vergence accelerations than did the same steps applied some time later (mean time constant of the decay, 200 ms). This enhancement seemed to be dependent, at least in part, on the visual reafference associated with the prior saccade because similar enhancement was observed when the disparity steps were applied in the wake of saccadelike shifts of the textured pattern. Vertical vergence responses to vertical disparity steps were qualitatively similar: latencies were longer (on average, by 3 ms), disparity tuning curves had the same general form but were narrower (by approximately 20%), and their peak-to-peak amplitudes were smaller (by approximately 70%). Initial vergence responses usually had directional errors (orthogonal components) with a very systematic dependence on step size that often approximated an exponential decay to a nonzero asymptote (mean space constant +/- SD, 1.18 +/- 0.66 degrees ). Based on the asymptotes of these orthogonal responses, horizontal errors (with vertical steps) were on average more than three times greater than vertical errors (with horizontal steps). Disparity steps >7 degrees generated "default" responses that were independent of the direction of the step, idiosyncratic, and generally had both horizontal and vertical components. We suggest that the responses depend on detectors that sense local disparity matches, and that orthogonal and "default" responses result from globally "false" matches. Recordings from three monkeys, using identical disparity stimuli, confirmed that monkeys also show short-latency disparity vergence responses (latency approximately 25 ms shorter than that of humans), and further indicated that these responses show all of the major features seen in humans, the differences between the two species being solely quantitative. Based on these data and those of others implying that foveal images normally take precedence, we suggest that the mechanisms under study here ordinarily serve to correct small vergence errors, automatically, especially after saccades.

Burst and tonic response modes in thalamic neurons during sleep and wakefulness

Thalamic neurons can exhibit two distinct firing modes: tonic and burst. In the lateral geniculate nucleus (LGN), the tonic mode appears as a relatively faithful relay of visual information from retina to cortex. The function of the burst mode is less understood. Its prevalence during slow-wave sleep (SWS) and linkage to synchronous cortical electroencephalogram (EEG) suggest that it has an important role during this form of sleep. Although not nearly as common, bursting can also occur during wakefulness. The goal of this study was to identify conditions that affect burst probability, and to compare burst incidence during sleeping and waking. LGN neurons are extraordinarily heterogenous in the degree to which they burst, during both sleeping and waking. Some LGN neurons never burst under any conditions during wakefulness, and several never burst during slow-wave sleep. During wakefulness, <1% of action potentials were associated with bursting, whereas during sleep this fraction jumps to 18%. Although bursting was most common during slow-wave sleep, more than 50% of the bursting originated from 14% of the LGN cells. Bursting during sleep was largely restricted to episodes lasting 1-5 s, with approximately 47% of these episodes being rhythmic and in the delta frequency range (0.5-4 Hz). In wakefulness, although visual stimulation accounted for the greatest number of bursts, it was still a small fraction of the total response (4%, 742 bursts/17,744 cycles in 93 cells). We identified two variables that appeared to influence burst probability: size of the visual stimuli used to elicit responses and behavioral state. Increased stimulus size increased burst probability. We attribute this to the increased influence large stimuli have on a cell's inhibitory mechanisms. As with sleep, a large fraction of bursting originated from a small number of cells. During visual stimulation, 50% of bursting was generated by 9% of neurons. Increased vigilance was negatively correlated with burst probability. Visual stimuli presented during active fixation (i.e., when the animal must fixate on an overt fixation point) were less likely to produce bursting, than when the same visual stimuli were presented but no fixation point present ("passive" fixation). Such observations suggest that even brief departures from attentive states can hyperpolarize neurons sufficiently to de-inactivate the burst mechanism. Our results provide a new view of the temporal structure of bursting during slow-wave sleep; one that supports episodic rhythmic activity in the intact animal. In addition, because bursting could be tied to specific conditions within wakefulness, we suggest that bursting has a specific function within that state.

Size perception in hemianopia and neglect

Some debate remains as to whether underestimation of the horizontal size of objects in the left part of visual space is a general disturbance in spatial neglect. The issue is unclear because size perception may be influenced by factors other than neglect, e.g. visual field defects such as hemianopia. To disentangle these effects, we compared the performance of patients with pure neglect, pure hemianopia or both on the same size-comparison test. Whereas pure neglect was accompanied by misperception of horizontal object size, patients with pure hemianopia showed an even greater impairment of size perception. Accordingly, the area of maximum lesion overlap of these patients with impaired size perception was not centred around the parietotemporal cortex, which is typically associated with spatial neglect, but rather was found in the occipital lobe (Brodmann areas 17 and 18). The results suggest that spatial neglect is not an inevitable consequence of distorted size perception. The perceptual distortion of objects in the leftward parts of visual space is not sufficient to account for the occurrence of visuospatial neglect.

The consequences of opponent rectification: the effect of surround size and luminance on color appearance

Smith and Pokorny (Vision Res. 36 (1996) 3087.) described conditions under which chromatic contrast induction can reveal a hiatus, a region of chromaticity space which appears neither reddish nor greenish when presented in a chromatic equiluminant surround. The current study investigated the effect of varying the size and the luminance of the inducing surround. The color appearance of test stimuli in chromatic surrounds was assessed by asymmetric color matching to a comparison display. Equiluminant (12 cd/m(2)), 1 degrees square stimuli were generated on a CRT display and presented haploscopically. Ten test fields varied in their L-cone excitation along a constant S-cone line. The chromatic surrounds were of either high (red) or low (green) L-cone excitation on a constant S-cone line. In Experiment 1, surrounds were 1.1 degrees, 1.5 degrees, 2.0 degrees, or 3.0 degrees square (surround widths of 3', 15', 30', 1 degrees ). In Experiment 2, the test and comparison surrounds were at higher (16.7 cd/m(2)) or lower (8.3 cd/m(2)) retinal illuminance than the test field. Contrast induction reached an asymptote for surround widths of 30' or larger. The amount of induction decreased for the surround widths of 15' and 3'. The hiatus was present for the larger surrounds and decreased as surround size decreased. The use of a higher or lower surround luminance did not affect the magnitude of induction or the size of the hiatus.

The effects of blur and size on monocular and stereoscopic localization

Monocular localization of non-abutting stimuli and stereoscopic localization of the same second-order targets are performed with the same precision (Wilcox, L.M. & Hess, R.F. (1996) Is the site of non-linear filtering in stereopsis before or after binocular combination? Vision Research, 36, 391-399). Further, both tasks show a similar dependence on the scale of the stimulus. Since prior studies used Gaussian-enveloped stimuli, modifications of stimulus scale produced concurrent changes in edge blur. The experiments reported here assess the relative contributions of size and blur to the observed dependence on envelope scale for both monocular localization and stereoacuity. Stereoacuity for first-order targets was found to be an order of magnitude better than stereoacuity for second-order targets and monocular acuity for both first- and second-order targets. Further, while first-order stereopsis was found to depend solely on blur, second-order stereoacuity and monocular acuity were affected by both size and blur. These results suggest that while stereoacuity for first-order stimuli may be determined by a correlative process limited by early additive noise, stereoacuity for second-order stimuli and monocular acuity for non-abutting targets are more likely limited by stimulus-dependent spatial subsampling.

[Apparent size of stereoscopic images with disparity in relation to the function of accommodation]

PURPOSE

We investigated experimentally the relationship between the apparent size of stereoscopic images with disparity and the accommodative function.

METHODS

The judgement of the apparent stereoscopic image size used the subject reply. The size of the forward image, which was produced by crossed visual lines with binocular disparity and by a time-sharing type stereoscopic three-dimensional display using liquid crystal shutter glasses, was compared with the size of the plane image. The size of the backward image produced by uncrossed visual lines was also compared with the size of the plane image. Sixteen normal volunteers were requested to subjectively compare each image with the original plane image on the display screen in relation to its size. Accommodation was measured using an infrared optometer to record the step responses (from far to near, and near to far). The subjects were divided into two groups, a fast response group and slow response group.

RESULTS

It was found that the forward image was smaller than the plane image and the backward image was larger than the plane image, and this tendency was remarkable in the fast response group.

CONCLUSION

From these results, it appears that the state of accommodation affects the perceived size of stereoscopic images with disparity.

Invariance of angular threshold computation in a wide-field looming-sensitive neuron

The lobula giant motion detector (LGMD) is a wide-field bilateral visual interneuron in North American locusts that acts as an angular threshold detector during the approach of a solid square along a trajectory perpendicular to the long axis of the animal (Gabbiani et al., 1999a). We investigated the dependence of this angular threshold computation on several stimulus parameters that alter the spatial and temporal activation patterns of inputs onto the dendritic tree of the LGMD, across three locust species. The same angular threshold computation was implemented by LGMD in all three species. The angular threshold computation was invariant to changes in target shape (from solid squares to solid discs) and to changes in target texture (checkerboard and concentric patterns). Finally, the angular threshold computation did not depend on object approach angle, over at least 135 degrees in the horizontal plane. A two-dimensional model of the responses of the LGMD based on linear summation of motion-related excitatory and size-dependent inhibitory inputs successfully reproduced the experimental results for squares and discs approaching perpendicular to the long axis of the animal. Linear summation, however, was unable to account for invariance to object texture or approach angle. These results indicate that LGMD is a reliable neuron with which to study the biophysical mechanisms underlying the generation of complex but invariant visual responses by dendritic integration. They also suggest that invariance arises in part from non-linear integration of excitatory inputs within the dendritic tree of the LGMD.

Role of familiar size in the control of grasping

The present study examined whether the learned pictorial depth cue of "familiar size" could be used to plan a reaching and grasping movement in the absence of binocular vision. Sixteen right-handed subjects were presented with two different arrays, under monocular and binocular viewing conditions, in which a range of different "grasp-sized" spheres that were lit from within could be presented in an otherwise darkened environment. In the "familiar-size" presentation array, only one "standard" sized sphere was presented, which gave subjects an opportunity to learn the relationship between the standard sphere's retinal image size and its distance. In the "multiple" spheres presentation array, subjects could not learn such a relationship because on any one trial, one of four different sphere sizes could be present. In a second experiment, the effects of this paradigm on six subjects' perceptual reports of distance were examined by having subjects slide their index fingers apart along a horizontal rod to indicate the estimated distance of the spheres. When familiar size could not be used as a cue to distance, subjects produced more on-line corrections in their reaching and grasping movements to the standard-sized spheres--but only under monocular viewing conditions. It appears that subjects are able to exploit the learned relationship between an object's distance and its projected retinal image size to help program and control reaching and grasping movements when binocular vision is not available. Although the influence of familiar size on subjects' perceptual estimates is less clear, it is clear that subjects' perceptual estimates show poor absolute scaling for distance. This result further supports the notion that under normal viewing conditions the visuomotor system uses binocular information to program and control manual prehension, but is able to use pictorial information when binocular vision is denied.

Characterizing visual performance fields: effects of transient covert attention, spatial frequency, eccentricity, task and set size

We investigated whether transient covert attention would differentially affect 'performance fields' (shape depicted by percent correct performance at particular locations in the visual field) for orientation discrimination, detection and localization tasks, while manipulating a number of visual factors. We found that although attention improved overall performance, it did not affect performance fields. Two patterns were observed regardless of the presence of a local post-mask, the stimulus orientation, or the task. A horizontal-vertical anisotropy (HVA) became more pronounced as spatial frequency, eccentricity and set size increased. A vertical meridian asymmetry (VMA) became more pronounced as spatial frequency and eccentricity increased. We conclude that performance fields are determined by visual, rather than by transient attentional, constraints.

Neural response to perception of volume in the lateral occipital complex

Projection of a 3D scene onto the 2D retina necessarily entails a loss of information, yet perceivers experience a world populated with volumetric objects. Using simultaneous behavioral and neural (fMRI) measures, we identify neural bases of volume perception. Neural activity in the lateral occipital cortex increased with presentation of 3D volumes relative to presentation of 2D shapes. Neural activity also modulated with perceived volume, independent of image information. When behavioral responses indicated that observers saw ambiguous images as 3D volumes, neural response increased; when behavioral data revealed a 2D interpretation, neural response waned. Crucially, the physical stimulus was identical under both interpretations; only the percept of volume can account for the increased neural activity.

How vertical disparities assist judgements of distance

The ratio of the vertical sizes of corresponding features in the two eyes' retinal images depends both on the associated object's distance and on its horizontal direction relative to the head (eccentricity). It is known that manipulations of vertical size ratio can affect perceived distance, size, depth and shape. We examined how observers use the vertical size ratio to determine the viewing distance. Do they use the horizontal gradient of vertical size ratio, or do they combine the vertical size ratio itself with the eccentricity at which it is found? Distance scaling (as measured by having subjects set an ellipsoid's size and shape to match a tennis ball) was no better when the judged object was 30 degrees to the right of the head (where vertical size ratios vary considerably with distance) than when it was located straight ahead. Distance scaling improved when vertical disparities were presented within larger visual fields, irrespective of where this was relative to the head. Our results support the proposal that subjects use the horizontal gradient of vertical size ratio to estimate the distance of an object that they are looking at.

Visual and tactile size distortion in a patient with right neglect

One typical feature of the neglect syndrome in patients with right hemisphere damage is that they bisect horizontal lines to the right of centre. It has been argued that to a large extent these bisection errors can be attributed to a perceptual change whereby the patient experiences the left half of a line as shorter than the right half, causing them to set the midpoint of the line towards the right. We describe here a patient with a left hemisphere lesion and rightward neglect, who consequently makes bisection errors in a leftward direction. We carried out a series of tests which confirmed that he shows a subjective visual distortion in the converse direction, i.e. a perception of horizontal extents on the right as shorter than extents on the left. We also found that he shows a similar distortion in his tactile perception. The association of visual and tactile distortions in this patient is compatible with the view that the distortion effects have a rather high-level origin. Multiple single-case studies will, however, be required to establish whether this association of deficits is typical, or whether visual and tactile size distortions are separable symptoms associated with neglect.

The size of the visual size cue used for programming manipulative forces during precision grip

We used a perturbation technique to quantify the contribution of visual size cues to the programming of target force when lifting an object. Our results indicate that the nervous system attaches a reasonable weight to visual size cues when programming the target grip force for a novel object. In a subsequent lift of the same object, however, the confidence attached to the visual size cue fell dramatically. It is not clear whether the decrease in the use of size information was accelerated by the presence of a cue conflict or whether the fall represents the normal shift towards the use of a memory-based representation for programming grip force. In a second experiment, we used the "size-weight illusion" to explore the relationship between the verbal report of an object's weight and the programming of the grip and load force. We found that erroneous motor programming (as indexed by a number of measures) was neither necessary nor sufficient for the size-weight illusion to occur. These findings call for a re-evaluation of a previous explanation for the size-weight illusion. We suggest that the illusion arises because the cognitive system attempts to rationalise the fact that objects of apparently equal density but different size feel as if they have the same weight.

Changes in the vertical size of a three-dimensional object drawn in weightlessness by astronauts

The purpose of this study was to investigate the effects of weightlessness on mental representation of spatial cues. Two astronauts drew two groups of three-dimensional cubes with their eyes closed, one on Earth (preflight) and the other under weightless conditions during a 7-day orbital flight (inflight). Differences in the average height of the two groups of cubes were observed. The ratio of average length of the horizontal vs. the vertical lines of the inflight cubes increased significantly compared to that of the preflight cubes. The disappearance of the gravitational reference system, which determines on Earth the vertical direction, seems to influence the internal representation of the vertical dimension, (i. e. the height) of a three-dimensional object.

Discriminating the role of binocular information in the timing of a one-handed catch. The effects of telestereoscopic viewing and ball size

Previous work using prediction-motion tasks has indicated that time-to-contact estimates are based exclusively on binocular information when the approaching object is small. The aim of this study was to examine the influence of object size on the use of binocular information sources in specifying time-to-contact in one-handed catching. Subjects (n = 10) were required to time their grasp to catch different-sized balls (60, 80, and 100 mm in diameter) approaching with a constant spatial trajectory and constant velocity of 2.0 m/s. Binocular information was manipulated with a telestereoscope that increased the individuals' effective interocular separation. Subjects performed six trials with each of the different-sized balls in normal and telestereoscopic viewing conditions. It was found that subjects closed the hand earlier when catching the small and medium balls under telestereoscopic viewing compared with normal viewing; furthermore, that subjects closed their hand earlier for the small and medium balls compared with the large ball under telestereoscopic viewing. With regard to hand aperture, there was an effect of ball size, with the hand being opened to a wider aperture for each increase in ball size. Subjects also opened the hand to a wider aperture when catching the small and medium balls under telestereoscopic viewing. These findings are consistent with the notion of an increased reliance on the use of binocular information when the approaching object is small. Furthermore, they indicate that binocular and monocular sources of information are not used in isolation, but rather in combination to support the timing of one-handed catching.

The effect of learned perceptual associations on visuomotor programming varies with kinematic demands

The present set of experiments investigated the possibility that learned perceptual information can, under certain circumstances, be utilized by visuomotor programming. In Experiment 1 (N = 28), an association was established between the color and size of square wooden blocks (e.g., red = large; yellow = small, or vice-versa). In Experiment 2 (N = 28), an association was established between the shape and size of plastic objects (e.g., hexagon = large; circle = small, or vice-versa). It was expected that the learned associations would change the perceived size of two probe objects halfway in size between the large and small objects (the probe object matched by color or shape to the large group of objects would appear smaller than the probe object matched to the small group of objects as a result of within-group relative size comparisons). In both experiments, half of the participants grasped the target objects, and the other half estimated the size of the objects by opening their thumb and finger a matching amount. For Experiment 1, it was predicted that an influence of the learned association on the treatment of the probe objects would be seen in manual estimations and in grip scaling because the kinematics of the grasping movement were very similar across trials. As predicted, the learned association between size and color was as easily incorporated into visually guided grasping as it was into visual perceptions. In Experiment 2, it was predicted that an influence of the learned perceptual association would be seen only in manual estimations, and not in grip scaling, because the variability in target object shape from trial to trial would demand changes in precontact finger posture across trials. Despite the significant effect of the size-shape association on size estimations, no influence was seen in preparatory grip scaling, probably because varying shape increased the metrical demands on visuomotor programming from those in Experiment 1. Together, the results suggest that visuomotor programming can make use of learned size information under some, but not all, conditions.

Role of chromaticity, contrast, and local orientation cues in the perception of density

We compared the role of the red-green, blue-yellow, and luminance post-receptoral mechanisms in the perception of density. The task requires the comparison of densities between two stimuli composed of oriented bandpass elements, pseudo-randomly scattered across an area of constant size. The perception of density differences was measured by a temporal 2AFC procedure for all pairs of mechanisms and for four possible densities. We found that stimuli of identical physical densities are not perceived equally: there is a consistent bias in favour of blue-yellow stimuli which are perceived as significantly more dense than red-green and achromatic stimuli. We considered three factors that could have differentially affected the density perception of blue-yellow stimuli: an increase in the perceived size of the individual blue-yellow elements, a perceived contrast difference, and the presence of local orientation cues. We found that the increased perceived density of the blue-yellow stimuli occurred despite the fact that there was no increase in perceived size of the individual elements, and remained despite corrections for the two other factors. We conclude that the significant increase in perceived density for the blue-yellow mechanism is a global effect, associated with a perceived colour 'melting' of the elements in the array. Our data were fitted with the occupancy model of Allik and Tuulmets (1991, Perception & Psychophysics 49 303-314) and we found that blue-yellow stimuli have a greater 'occupancy' than red-green or achromatic stimuli.

Alzheimer disease constricts the dynamic range of spatial attention in visual search

A cued visual search task was used to examine the dynamic range over which spatial attention affects target identification during visual search. Precues varied in validity (valid, invalid, or neutral) and in precision (cue size) of target localization. Participants were "young-old" (65-74 years) and "old-old" (75-85 years) elderly adults and individuals in the mild stage of dementia of the Alzheimer type (DAT). For all participants, search was speeded as the precision with which a precue surrounding the location of a subsequently appearing target increased (precue size decreased). The cue size effect was evident in both feature and conjunction search, but was greatly reduced in both old-old and DAT groups compared to the young-old. However, whereas all non-demented adults showed a progressive modulation of search efficiency over the entire range of cue sizes, the dynamic range of spatial attention was restricted to the most precise cue in the DAT group. The restriction in the dynamic range of spatial attention may represent an underlying component of the impairment in perceptual and memory functioning found in early-stage DAT.