Spontaneous perception of numerosity in pre-school children.

There is strong evidence that humans can make rough estimates of the numerosity of a set of items, almost from birth. However, as numerosity covaries with many non-numerical variables, the idea of a direct number sense has been challenged. Here we applied two different psychophysical paradigms to demonstrate the spontaneous perception of numerosity in a cohort of young pre-school children. The results of both tasks showed that even at that early developmental stage, humans spontaneously base the perceptual choice on numerosity, rather than on area or density. Precision in one of these tasks predicted mathematical abilities. The results reinforce strongly the idea of a primary number sense and provide further evidence linking mathematical skills to the sensory precision of the spontaneous number sense, rather than to mechanisms involved in handling explicit numerosity judgements or extensive exposure to mathematical teaching.

A low-cost and versatile system for projecting wide-field visual stimuli within fMRI scanners.

We have constructed and tested a custom-made magnetic-imaging-compatible visual projection system designed to project on a very wide visual field (~80°). A standard projector was modified with a coupling lens, projecting images into the termination of an image fiber. The other termination of the fiber was placed in the 3-T scanner room with a projection lens, which projected the images relayed by the fiber onto a screen over the head coil, viewed by a participant wearing magnifying goggles. To validate the system, wide-field stimuli were presented in order to identify retinotopic visual areas. The results showed that this low-cost and versatile optical system may be a valuable tool to map visual areas in the brain that process peripheral receptive fields.

A cortical area that responds specifically to optic flow, revealed by fMRI.

The continuously changing optic flow on the retina provides information about direction of heading and about the three-dimensional structure of the environment. Here we use functional magnetic resonance imaging (fMRI) to demonstrate that an area in human cortex responds selectively to components of optic flow, such as circular and radial motion. This area is within the region commonly referrred to as V5/MT complex, but is distinct from the part of this region that responds to translation. The functional properties of these two areas of the V5/MT complex are also different; the response to optic flow was obtained only with changing flow stimuli, whereas response to translation occurred during exposure to continuous motion.

Saccadic suppression precedes visual motion analysis.

There is now good evidence that perception of motion is strongly suppressed during saccades (rapid shifts of gaze), presumably to blunt the disturbing sense of motion that saccades would otherwise elicit. Other aspects of vision, such as contrast detection of high-frequency or equiluminant gratings, are virtually unaffected by saccades [1] [2] [3] [4] [5]. This has led to the suggestion that saccades may suppress selectively the magnocellular pathway (which is strongly implicated in motion perception), leaving the parvocellular pathway unaffected [5] [6]. Here, we investigate the neural level at which perception of motion is suppressed. We used a simple technique in which an impression of motion is generated from only two frames, allowing precise control over the stimulus [7] [8]. One frame has a certain fixed contrast, whereas the contrast of the other (the test frame) is varied to determine the threshold for motion discrimination (that is, the lowest test-frame contrast level at which the direction of motion can be correctly guessed). Contrast thresholds of the test depended strongly and non-monotonically on the contrast of the fixed-contrast frame, with a minimum at medium contrast. To study the effect of saccadic suppression, we triggered the two-frame sequence by a voluntary saccade. Thresholds during saccades increased in a way that suggested that saccadic suppression precedes motion analysis: when the test frame was first in the motion sequence there was a general depression of sensitivity, whereas when it was second, the contrast response curve was shifted to a higher contrast range, sometimes even resulting in higher sensitivity than without a saccade. The dependence on presentation order suggests that saccadic suppression occurs at an early stage of visual processing, on the single frames themselves rather than on the combined motion signal. As motion detection itself is thought to occur at an early stage, saccadic suppression must take place at a very early phenomenon.

Separate visual representations for perception and action revealed by saccadic eye movements.

Some 30 years ago, Trevarthen [1] introduced the idea of two separate visual systems, a focal system for fine motor acts and an ambient system for gross body movements such as ambulation. More recent developments indicating anatomically and physiologically separate pathways in primate vision [2] have led to a different idea of separate visual systems, one for conscious perception and one for action [3]. It has received empirical support from several studies showing that pointing, reaching, and grasping can remain accurate while the perceived position or size of objects is subject to illusory distortion [4-6]. However, much of this evidence has been challenged on the grounds of methodological flaws, particularly failure to match perfectly the conditions for verbal and motor tasks and failure to replicate results [7-10]. Here we take advantage of the strong compression of perceived position that occurs around the time of saccadic eye movements [11, 12]. Under normal lighting conditions, stimuli flashed briefly over a wide range of spatial positions just before saccadic onset are neither seen nor reached for in their veridical positions, but are compressed toward the saccadic target. We validate the idea of separate systems by showing that, in the dark, subjects are able to point accurately to the correct target position, even though their verbal reports are still subject to compression.

Cardinal directions for visual optic flow.

As we move through our environment, the flow of deforming images on the retinae provides a rich source of information about the three-dimensional structure of the external world and how to navigate through it. Recent evidence from psychophysical [1] [2] [3] [4], electrophysiological [5] [6] [7] [8] [9] and imaging [10] [11] studies suggests that there are neurons in the primate visual system - in the medial superior temporal cortex - that are specialised to respond to this type of complex 'optic flow' motion. In principle, optic flow could be encoded by a small number of neural mechanisms tuned to 'cardinal directions', including radial and circular motion [12] [13]. There is little support for this idea at present, however, from either physiological [6] [7] or psychophysical [14] research. We have measured the sensitivity of human subjects for detection of motion and for discrimination of motion direction over a wide and densely sampled range of complex motions. Average sensitivity was higher for inward and outward radial movement and for both directions of rotation, consistent with the existence of detectors tuned to these four types of motion. Principle component analysis revealed two clear components, one for radial stimuli (outward and inward) and the other for circular stimuli (clockwise and counter-clock-wise). The results imply that the mechanisms that analyse optic flow in humans tend to be tuned to the cardinal axes of radial and rotational motion.

Changes in visual perception at the time of saccades.

We frequently reposition our gaze by making rapid ballistic eye movements that are called saccades. Saccades pose problems for the visual system, because they generate rapid, large-field motion on the retina and change the relationship between the object position in external space and the image position on the retina. The brain must ignore the one and compensate for the other. Much progress has been made in recent years in understanding the effects of saccades on visual function and elucidating the mechanisms responsible for them. Evidence suggests that saccades trigger two distinct neural processes: (1) a suppression of visual sensitivity, specific to the magnocellular pathway, that dampens the sensation of motion and (2) a gross perceptual distortion of visual space in anticipation of the repositioning of gaze. Neurophysiological findings from several laboratories are beginning to identify the neural substrates involved in these effects.

Adaptation to number operates on perceived rather than physical numerosity.

Humans share with many animals a number sense, the ability to estimate rapidly the approximate number of items in a scene. Recent work has shown that like many other perceptual attributes, numerosity is susceptible to adaptation. It is not clear, however, whether adaptation works directly on mechanisms selective to numerosity, or via related mechanisms, such as those tuned to texture density. To disentangle this issue we measured adaptation of numerosity of 10 pairs of connected dots, as connecting dots makes them appear to be less numerous than unconnected dots. Adaptation to a 20-dot pattern (same number of dots as the test) caused robust reduction in apparent numerosity of the connected-dot pattern, but not of the unconnected dot-pattern. This suggests that adaptation to numerosity, at least for relatively sparse dot-pattern, occurs at neural levels encoding perceived numerosity, rather than at lower levels responding to the number of elements in the scene.

Motion deblurring in human vision.

Under normal viewing conditions we are little conscious of blur in moving objects, despite the persistence of vision. Moving objects look more blurred in brief than in long exposures, suggesting an active mechanism for suppressing motion blur. To see whether blur suppression would improve visual discrimination of objects, we measured blur discrimination thresholds for moving Gaussian-blurred edges and bars. The observer's task was to decide which of two moving stimuli, presented successively, was the more blurred. It is known that for stationary objects the just-noticeable difference in blur increases with baseline blur; therefore, if motion increases blur, it would be expected to increase the just-noticeable difference in blur. An active deblurring mechanism, on the other hand, would be expected to counteract the detrimental effects of motion blur on discrimination performance. We found, however, that motion increased thresholds for blur discrimination, both for brief (40 ms) and for longer (150 ms) exposures. We conclude that motion deblurring is a subjective effect, which does not enhance visual discrimination performance. Moving objects appear sharp, not because of some special mechanism that removes blur, but because the visual system is unable to perform the discrimination necessary to decide whether the moving object is really sharp or not.

Development of contrast sensitivity and acuity of the infant colour system.

We have monitored the development of infant colour vision by measuring chromatic contrast sensitivity and acuity in eight young infants over a period of 6 months. Steady-state visual evoked potentials (VEPS) were recorded in response to both chromatic (red-green) and luminance (red-black or green-black) patterns that were reversed in contrast over time. For most infants, no response could be obtained to chromatic stimuli of any size or contrast before 5 weeks of age, although luminance stimuli of 20% contrast gave reliable responses at that age. When responses to chromatic stimuli first appeared, they could be obtained only with stimuli of very low spatial frequency, 20 times lower than the acuity for luminance stimuli. Both contrast sensitivity and acuity for chromatic stimuli increased steadily, more rapidly than for luminance stimuli. As the spectral selectivities of infant cones are similar to those of adults, the difference in rate of development of luminance and chromatic contrast sensitivity and acuity stimuli probably reflects neural development of the infant colour system.

VEP in neglect patients have longer latencies for luminance but not for chromatic patterns.

In patients with unilateral neglect, visual evoked potentials (VEP) to stimuli displayed in the left visual field are delayed compared with responses to right visual field stimuli. In the present study, 10 patients with right brain damage and neglect were tested with contrast-reversed sinusoidal gratings, modulated either in luminance or in chromaticity. For gratings of luminance contrast modulated over relatively high temporal frequencies (4-10.5 Hz), latencies of VEP were about 30 ms longer for stimuli presented to the contralesional (left) visual field than to the field ipsilateral to the lesion. For equiluminant stimuli modulated at relatively low temporal frequencies (1-4 Hz), however, latency was the same for both hemifields. As this condition activates predominately the parvocellular pathway the results are consistent with our previous suggestion that the delay observed with luminance stimuli in neglect patients results from selective disruption of the faster response of the magnocellular pathway.

Visual acuity of neurones in the cat lateral suprasylvian cortex.

The spatial acuity was measured for cells of the posteromedial lateral suprasylvian area (PMLS) of the cat. Acuities were found to be 2 cycles/degree (15 mins arc) at best, and 1 cycle/degree (30 mins arc) on average. Both best acuity and average acuity remained constant with receptive field eccentricity within 20 degrees of the area centralis, and then fell gradually with eccentricity. Acuity was good, given receptive field size, and was not correlated with receptive field size. Comparisons are drawn with other visual structures.

Implications of the Craik-O'Brien illusion for brightness perception.

Measurements are reported for detection thresholds of high and low-pass filtered squarewaves, and for brightness matches of those waveforms. The threshold measurements agree closely with those of Campbell, Howell and Johnstone [J. Physiol., Lond. 284, 193-201 (1978)]: contrasts at which a high-pass squarewave was indistinguishable from an unfiltered squarewave could be well predicted from detection thresholds for an appropriate low-pass squarewave. However, the brightness of high-pass squarewaves (the "Craik-O'Brien illusion") was not related to the threshold measurements. Brightness was virtually constant with spatial frequency, even at spatial frequencies 10 times higher than the region of the low frequency cut. Brightness depended strongly on contrast, being relatively greater at low than at high contrasts. The results can be well accounted for by a recent theory of edge detection, and the existence of parallel channels in vision.

Orientation discrimination depends on spatial frequency.

Thresholds were measured for discriminating the orientation of sinusoidal gratings of varying spatial frequency, and found to decrease monotonically with increasing spatial frequency. For discrimination of high-contrast (10 times threshold) near-vertical gratings, thresholds ranged from about 1 deg at 0.04 c/deg to 0.5 deg at 0.2 c/deg, after which there was little improvement. At lower contrasts and for discriminations around a mean of 45 deg, thresholds varied more so, and continued to improve until 1 c/deg. The variation of orientation discrimination thresholds with spatial frequency follows a similar trend to the variation in orientation bandwidth of visual units over the same range of spatial frequencies. Thus the present results are consistent with recent "opponent-process" models of orientation discrimination, that predict that thresholds to be limited (at least in part) by the maximum slope of orientation selectivity of visual detectors. That thresholds for high contrast vertical gratings did not improve for frequencies higher than 0.2 c/deg implies that orientation bandwidth and noisiness of oriented detectors may not be the sole factor limiting orientation discrimination, and suggests the existence of more central noise sources.

Temporal impulse response functions for luminance and colour during saccades.

Previous work has shown that during saccadic eye movements, contrast sensitivity for low spatial frequency patterns modulated in luminance is selectively reduced by up to one logarithmic unit, while high spatial frequency patterns, and equiluminant patterns of all spatial frequencies are not suppressed at all [Burr et al. (1994). Nature, 371, 511-513]. Here we study the temporal characteristics for sensitivity to luminance and chromatic patterns during saccades, using the two-pulse summation technique. Sensitivity was measured for detecting two successive pulses as a function of stimulus-onset asynchrony, during normal viewing and during saccades. Impulse response functions were estimated from the summation data, for all conditions. For equiluminance, the functions were monophasic during normal viewing and saccades. For luminance modulation, the impulse response functions were di-phasic in both normal viewing and saccades. However, during saccades the impulse responses were faster in normal viewing. This result is consistent with the suggestion that saccadic suppression is mediated by contrast gain control mechanisms, known to occur in M-cells but not P-cells.

Capture and transparency in coarse quantized images.

This study examines the effect of coarse quantization (blocking) on image recognition, and explores possible mechanisms. Thresholds for noise corruption showed that coarse quantization reduces drastically the recognizability of both faces and letters, well beyond the levels expected by equivalent blurring. Phase-shifting the spurious high frequencies introduced by the blocking (with an operation designed to leave both overall and local contrast unaffected, and feature localization) greatly improved recognizability of both faces and letters. For large phase shifts, the low spatial frequencies appear in transparency behind a grid structure of checks or lines. We also studied a more simple example of blocking, the checkerboard, that can be considered as a coarse quantized diagonal sinusoidal plaid. When one component of the plaid was contrast-inverted, it was seen in transparency against the checkerboard, while the other remained "captured" within the block structure. If the higher harmonics are then phase-shifted by pi, the contrast-reversed fundamental becomes captured and the other seen in transparency. Intermediate phase shifts of the higher harmonics cause intermediate effects, which we measured by adjusting the relative contrast of the fundamentals until neither orientation dominated. The contrast match varied considerably with the phase of the higher harmonics, over a range of about 1.5 log units. Simulations with the local energy model predicted qualitatively the results of the recognizability of both faces and letters, and quantitatively the apparent orientation of the modified checkerboard pattern. More generally, the model predicts the conditions under which an image will be "captured" by coarse quantization, or seen in transparency.

Spatial and temporal selectivity of the human motion detection system.

Measurements were made of spatial frequency, orientation and temporal frequency selectivity of the visual motion system. The results suggest: (1) There exists in the motion system mechanisms selective for spatial frequency. The preferred spatial frequency varies considerably and extends down to at least 0.06 c/deg. (2) At all spatial frequencies (from 0.1 to 10 c/deg) there exist detectors selective for orientation which vary in (directed) orientation tuning to encompass 360 degrees. (3) The bandwidth of both spatial frequency and orientation selectivity vary inversely with spatial frequency: the lower the spatial frequency, the broader the bandwidth. (4) There exist two classes of temporally tuned detectors, one lowpass (sustained) and one bandpass (transient), of preferred temporal frequency of 7-13 Hz (depending on spatial frequency).

Contrast sensitivity at high velocities.

Measurements were made of the contrast required to see the direction of motion of drifting gratings (Part 1) and of moving bars (Part 2). The spatial frequency at which least contrast is required to see sinusoidal gratings decreases as their velocity increases, but peak sensitivity is identical at all velocities up to 800 deg/sec. Similarly, the wider a single bar, the higher the velocity at which it is best visible. A bar 80 deg wide is best seen when moving at 300-500 deg/sec, and can be seen, and its direction of motion identified, even when moving at 10(4) deg/sec. These results show that motion does not diminish the visual passband, but instead slides the spatial frequency window along the spatial frequency scale, maintaining peak sensitivity at a temporal frequency of about 10 Hz (at photopic luminances).

Receptive field size of human motion detection units.

Receptive field sizes of motion detector units in the human visual system were determined using a summation technique. Contrast sensitivity was measured for detecting the direction of motion of a drifting (8 Hz) sinewave grating (0.01-30.0 c/deg) multiplied by a stationary Gaussian envelope, for various widths of the Gaussian envelope. For each test spatial frequency, sensitivity increased linearly with aperture width up to a certain limit, and thereafter at a rate consistent with a model incorporating probability summation over space and between channels. The limit of linear summation designates the limit of the receptive field. Results show that the receptive field size varies with spatial frequency, from 2' arc at high spatial frequencies to as large as 7 deg at low frequencies. The change in field size was progressive. The smallest aperture width (delta W) for directional discrimination was also measured. Results show delta W to vary from 0.03 cycles at low spatial frequencies to 0.30 cycles at high frequencies.

Feature-based integration of orientation signals in visual search.

We have measured orientation discrimination in the presence of a variable number of neutral distracters for two distinct tasks: identification of the orientation of a tilted target and location of its position. Both tasks were performed in the presence of visual noise of variable contrasts. Under a range of conditions, subjects could identify the direction of target tilt at thresholds well below those necessary to locate its position. The location thresholds showed only weak dependency on set-size, consistent with a stimulus uncertainty of parallel search of the output of independent orientation analysers, while the identification thresholds showed a much stronger dependency, varying with the square root of set-size over a wide range noise contrasts. The square root relationship suggests perceptual summation of target and distracters. Manipulating the spread of visual noise suggests that the summation is feature-based, possibly operating on the outputs of first-stage orientation analysers. Pre-cueing the target eliminates the effects of set-size, showing that the summation is under rapid attentional control; the visual system can choose between high performance over a limited area and poorer performance over a much larger area.