The tactile perception of stimulus orientation

Studies of the visual system suggest that, at an early stage of form processing, a stimulus is represented as a set of contours and that a critical feature of these local contours is their orientation. Here, we characterize the ability of human observers to identify or discriminate the orientation of bars and edges presented to the distal fingerpad. The experiments were performed using a 400-probe stimulator that allowed us to flexibly deliver stimuli across a wide range of conditions. Orientation thresholds, approximately 20 degrees on average, varied only slightly across modes of stimulus presentation (scanned or indented), stimulus amplitudes, scanning speeds, and different stimulus types (bars or edges). The tactile orientation acuity was found to be poorer than its visual counterpart for stimuli of similar aspect ratio, contrast, and size. This result stands in contrast to the equivalent spatial acuity of the two systems (at the limit set by peripheral innervation density) and to the results of studies of tactile and visual letter recognition, which show that the two modalities yield comparable performance when stimuli are scaled appropriately.     

Motion adaptation distorts perceived visual position

After an observer adapts to a moving stimulus, texture within a stationary stimulus is perceived to drift in the opposite direction-the traditional motion aftereffect (MAE). It has recently been shown that the perceived position of objects can be markedly influenced by motion adaptation. In the present study, we examine the selectivity of positional shifts resulting from motion adaptation to stimulus attributes such as velocity, relative contrast, and relative spatial frequency. In addition, we ask whether spatial position can be modified in the absence of perceived motion. Results show that when adapting and test stimuli have collinear carrier gratings, the global position of the object shows a substantial shift in the direction of the illusory motion. When the carrier gratings of the adapting and test stimuli are orthogonal (a configuration in which no MAE is experienced), a global positional shift of similar magnitude is found. The illusory positional shift was found to be immune to changes in spatial frequency and to contrast between adapting and test stimuli-manipulations that dramatically reduce the magnitude of the traditional MAE. The lack of sensitivity for stimulus characteristics other than direction of motion suggests that a specialized population of cortical neurones, which are insensitive to changes in a number of rudimentary visual attributes, may modulate positional representation in lower cortical areas.     

Matched filtering in motion detection and discrimination

When humans detect and discriminate visual motion, some neural mechanism extracts the motion information that is embedded in the noisy spatio-temporal stimulus. We show that an ideal mechanism in a motion discrimination experiment cross-correlates the received waveform with the signals to be discriminated. If the human visual system uses such a cross-correlator mechanism, discrimination performance should depend on the cross-correlation between the two signals. Manipulations of the signals' cross-correlation using differences in the speed and phase of moving gratings produced the predicted changes in the performance of human observers. The cross-correlator's motion performance improves linearly as contrast increases and human performance is similar. The ideal cross-correlator can be implemented by passing the stimulus through linear spatio-temporal filters matched to the signals. We propose that directionally selective simple cells in the striate cortex serve as matched filters during motion detection and discrimination.     

Size as a parameter for tuning visual attention]

The ability of visual attention to tune to the stimulus size (when this size could not be described by spatial frequencies) was studies. Sinusoidal gratings with frequencies of 1.5, 3, and 6 cycle/degree were used as test stimuli. All these stimuli consisted of 3 periods, consequently, they had different sizes: 2 x 2, 1 x 1, and 0.5 x 0.5 degrees. Three reference stimuli had the same sizes but were constructed as a superposition of all the test frequencies. The reference stimulus of suprathreshold contrast was displayed for 400 ms to the left or to the right of a fixation point at a distance of 3 degrees. After that, the test stimulus of threshold contrast was for 100 ms displayed symmetrically to the fixation point on the other side. Subjects were instructed that the sizes of the reference and test stimuli were the same. It was found that the probability of test detection decreased with increase in the difference between the sizes of the reference and test stimuli. Since in our experiments the spatial frequency could not be used for tuning visual attention, the obtained results suggest that there are specialized mechanisms in the visual system for estimation of the general image size.     

A new perceptual bias reveals suboptimal population decoding of sensory responses

Several studies have reported optimal population decoding of sensory responses in two-alternative visual discrimination tasks. Such decoding involves integrating noisy neural responses into a more reliable representation of the likelihood that the stimuli under consideration evoked the observed responses. Importantly, an ideal observer must be able to evaluate likelihood with high precision and only consider the likelihood of the two relevant stimuli involved in the discrimination task. We report a new perceptual bias suggesting that observers read out the likelihood representation with remarkably low precision when discriminating grating spatial frequencies. Using spectrally filtered noise, we induced an asymmetry in the likelihood function of spatial frequency. This manipulation mainly affects the likelihood of spatial frequencies that are irrelevant to the task at hand. Nevertheless, we find a significant shift in perceived grating frequency, indicating that observers evaluate likelihoods of a broad range of irrelevant frequencies and discard prior knowledge of stimulus alternatives when performing two-alternative discrimination.     

Visual processing levels revealed by response latencies to changes in different visual attributes.

Visual latencies, and their variation with stimulus attributes, can provide information about the level in the visual system at which different attributes of the image are analysed, and decisions about them made. A change in the colour, structure or movement of a visual stimulus brings about a highly reproducible transient constriction of the pupil that probably depends on visual cortical mechanisms. We measured this transient response to changes in several attributes of visual stimuli, and also measured manual reaction times to the same stimulus changes. Through analysis of latencies, we hoped to establish whether changes in different stimulus attributes were processed by mechanisms at the same or different levels in the visual pathway. Pupil responses to a change in spatial structure or colour are almost identical, but both are ca. 40 ms slower than those to a change in light flux, which are thought to depend largely on subcortical pathways. Manual reaction times to a change in spatial structure or colour, or to the onset of coherent movement, differ reliably, and all are longer than the reaction time to a change in light flux. On average, observers take 184 ms to detect a change in light flux, 6 ms more to detect the onset of a grating, 30 ms more to detect a change in colour, and 37 ms more to detect the onset of coherent motion. The pattern of latency variation for pupil responses and reaction times suggests that the mechanisms that trigger the responses lie at different levels in cortex. Given our present knowledge of visual cortical organization, the long reaction time to the change in motion is surprising. The range of reaction times across different stimuli is consistent with decisions about the onset of a grating being made in V1 and decisions about the change in colour or change in motion being made in V4.     

Transitions between central and peripheral vision create spatial/temporal distortions: a hypothesis concerning the perceived break of the curveball

Background

The human visual system does not treat all parts of an image equally: the central segments of an image, which fall on the fovea, are processed with a higher resolution than the segments that fall in the visual periphery. Even though the differences between foveal and peripheral resolution are large, these differences do not usually disrupt our perception of seamless visual space. Here we examine a motion stimulus in which the shift from foveal to peripheral viewing creates a dramatic spatial/temporal discontinuity.

Methodology/Principal Findings

The stimulus consists of a descending disk (global motion) with an internal moving grating (local motion). When observers view the disk centrally, they perceive both global and local motion (i.e., observers see the disk''s vertical descent and the internal spinning). When observers view the disk peripherally, the internal portion appears stationary, and the disk appears to descend at an angle. The angle of perceived descent increases as the observer views the stimulus from further in the periphery. We examine the first- and second-order information content in the display with the use of a three-dimensional Fourier analysis and show how our results can be used to describe perceived spatial/temporal discontinuities in real-world situations.

Conclusions/Significance

The perceived shift of the disk''s direction in the periphery is consistent with a model in which foveal processing separates first- and second-order motion information while peripheral processing integrates first- and second-order motion information. We argue that the perceived distortion may influence real-world visual observations. To this end, we present a hypothesis and analysis of the perception of the curveball and rising fastball in the sport of baseball. The curveball is a physically measurable phenomenon: the imbalance of forces created by the ball''s spin causes the ball to deviate from a straight line and to follow a smooth parabolic path. However, the curveball is also a perceptual puzzle because batters often report that the flight of the ball undergoes a dramatic and nearly discontinuous shift in position as the ball nears home plate. We suggest that the perception of a discontinuous shift in position results from differences between foveal and peripheral processing.     

Local spatial scale for three-dot alignment acuity

Three-dot alignment discrimination thresholds were determined for blobs with Gaussian spatial and temporal contrast envelopes. The stimuli were presented at detection threshold luminance contrast. Thresholds were determined as a function of the blur parameter of the stimuli. This was done for a range of eccentricities in the visual field (from 45 degrees nasal to 65 degrees temporal). The thresholds were corrected for variations of the stimulus extent with the blur parameter. The results were used to estimate the local spatial scale for three-dot alignment acuity. This was done by a method recently introduced by Watson (1987). It was found that the local spatial scale for three-dot alignment acuity is approximately linearly proportional to eccentricity.     

Spatial frequency response functions obtained from cat visual evoked potentials

Visual evoked potentials (VEPs) were obtained from the surface of teh cat visual cortex in response to contrast reversing sinusoidal gratings. Gratings of different spatial frequency were presented either separately, using signal averaging to increase the signal-to-noise ratio, or as a spatial frequency sweep, in which spatial frequency was sequentially increased every 5 sec during a 40 sec trial (3.99 Hz) or every 3 sec during a 24 sec trial (6.65 Hz). The second harmonic amplitude- and phase-spatial frequency functions derived from averaging or from sweep trials were similar, indicating that the swept stimulus method can be used to provide a rapid and reliable measure of the VEP-spatial frequency function. Intravenous administration of physostigmine, an acetylcholinesterase inhibitor, evoked a spatial frequency-dependent change in VEP amplitude. At 3.99 Hz, responses to low spatial frequencies were enhanced to a greater extent than were responses to high spatial frequency stimuli. At 6.65 Hz, responses to mid-range spatial frequencies were enhanced to a greater extent than were responses to low and high spatial frequency stimuli. VEP phase at both 3.99 and 6.65 Hz was advanced to a greater degree at the higher spatial frequencies. These results indicate that the swept spatial frequency method may be useful in studying spatial frequency-dependent pharmacological effects on the VEP and support the possibility that pharmacological disruption of the cholinergic visual system can produce such changes.     

Noise,not stimulus entropy,determines neural information rate

In the quest for deciphering the neural code, theoretical advances were made which allow for the determination of the information rate inherent in the spike trains of nerve cells. However, up to now, the dependence of the information rate on stimulus parameters has not been studied in any neuron in a systematic way. Here, I investigate the information carried by the spike trains of H1, a motion-sensitive visual interneuron of the blowfly (Calliphora vicina) using a moving grating as a stimulus. Stimulus parameters fall in two classes: those that have only a minor effect on the information rate like increasing the frequency bandwidth or the maximum amplitude of the stimulus velocity, and those which dramatically affect the neural information rate, like varying the spatial size or the contrast of the visual pattern being moved. It appears that, for a broad range of complex stimuli, the neuron covers the stimulus with its whole response repertoire regardless of the stimulus entropy, with the information rate being limited by the noise of the stimulus and the neural hardware.     

A comparison of vernier acuity for narrowband and broadband stimuli

This study investigates the influence of contrast and exposure duration on vernier acuity thresholds for abutting and separated narrowband stimuli, and asks whether these data can predict broadband vernier performance. Vernier thresholds were determined for sinusoidal grating stimuli at two spatial frequencies (1 and 8 c/deg) across a range of contrasts (0.05-0.8) and exposure durations (35-2100 ms). Performance was assessed for the abutting configuration, and when a gap equivalent to 0.5 to 1.5 times the spatial period of the grating was introduced between the upper and lower halves of the grating. Vernier thresholds were also determined for a square-wave stimulus as a function of contrast (0.06 to 0.78). Exposure duration was fixed at 2100 ms. In addition, thresholds were determined at the appropriate contrast levels for the fundamental frequency (1.8 c/deg) of the square-wave, and for a number of the harmonics (3F, 5F, 7F, 9F). Our results provide support for filter models of vernier acuity by showing that vernier performance for abutting and closely-separated broadband stimuli represents the envelope of vernier sensitivity of those spatial frequency mechanisms that are activated by the broadband stimulus. In the case of high frequency grating stimuli presented for long exposure durations, vernier performance can be invariant across much of the contrast range. Despite this, however, contrast independence is not exhibited for abutting broadband stimuli because, within the broadband stimuli, the contrast of the higher harmonic components never reaches a level to reveal this plateau.     

The spatial frequency discrimination function at low contrasts

Spatial frequency difference thresholds for sinewave gratings near contrast threshold were measured using a two-alternative forced-choice technique, and the threshold frequency differences were plotted as a proportion of standard frequency for standards from 2 to 7 cycles/degree. This function shows reliable local maxima and minima, and these features are more pronounced than they are when stimuli of 30% contrast are used. This result is consistent with the notion that at low contrasts, fewer spatial frequency channels are above threshold in the area of the visual field covered by the stimulus than when the stimulus is at high contrast.     

Aerial and aquatic visual acuity of the grey bichir Polypterus senegalus,as estimated by optokinetic response

The present study assessed the aerial and aquatic visual abilities of juvenile grey bichir Polypterus senegalus, fish capable of terrestrial locomotion, by measuring the optokinetic response to stimuli of varying speed and spatial frequency. In water, fish tracked slow-moving (2° s⁻¹) stimuli moderately well and fast-moving stimuli very poorly. Spatial acuity was very low compared with many other species, with maximum response observed at 0.05–0.075 stimulus cycles per degree of visual arc; however, it should be noted that adult fish, with their larger eyes, are likely to have somewhat improved spatial acuity. Low spatial acuity and limited stimulus tracking ability might be expected in a nocturnal ambush predator such as P. senegalus, where gaze stabilization may be less crucial and other sensory inputs may have greater importance in perception of the environment. In air, spatial and temporal acuity were both poorer by every measure, but some visual ability persisted. As the eye shows no anatomical specialization for aerial vision, poor vision was expected; however, the large decrease in saccade velocity observed in air trials was unexpected. Stimulus parameters typically have little effect on the characteristics of the saccade, so this finding may suggest that the function of the reflex system itself could be compromised in the aerial vision of some fishes capable of terrestrial locomotion.     

Visual performance fields: frames of reference

Performance in most visual discrimination tasks is better along the horizontal than the vertical meridian (Horizontal-Vertical Anisotropy, HVA), and along the lower than the upper vertical meridian (Vertical Meridian Asymmetry, VMA), with intermediate performance at intercardinal locations. As these inhomogeneities are prevalent throughout visual tasks, it is important to understand the perceptual consequences of dissociating spatial reference frames. In all studies of performance fields so far, allocentric environmental references and egocentric observer reference frames were aligned. Here we quantified the effects of manipulating head-centric and retinotopic coordinates on the shape of visual performance fields. When observers viewed briefly presented radial arrays of Gabors and discriminated the tilt of a target relative to homogeneously oriented distractors, performance fields shifted with head tilt (Experiment 1), and fixation (Experiment 2). These results show that performance fields shift in-line with egocentric referents, corresponding to the retinal location of the stimulus.     

Fovea-Periphery Axis Symmetry of Surround Modulation in the Human Visual System

A visual stimulus activates different sized cortical area depending on eccentricity of the stimulus. Here, our aim is to understand whether the visual field size of a stimulus or cortical size of the corresponding representation determines how strongly it interacts with other stimuli. We measured surround modulation of blood-oxygenation-level-dependent signal and perceived contrast with surrounds that extended either towards the periphery or the fovea from a center stimulus, centered at 6° eccentricity. This design compares the effects of two surrounds which are identical in visual field size, but differ in the sizes of their cortical representations. The surrounds produced equally strong suppression, which suggests that visual field size of the surround determines suppression strength. A modeled population of neuronal responses, in which all the parameters were experimentally fixed, captured the pattern of results both in psychophysics and functional magnetic resonance imaging. Although the fovea-periphery anisotropy affects nearly all aspects of spatial vision, our results suggest that in surround modulation the visual system compensates for it.     

Visual discrimination of objects differing in spatial depth by goldfish

Training experiments were performed to investigate the ability of goldfish to discriminate objects differing in spatial depth. Tests on size constancy should give insight into the mechanisms of distance estimation. Goldfish were successfully trained to discriminate between two black disk stimuli of equal size but different distance from the tank wall. Each stimulus was presented in a white tube so that the fish could see only one stimulus at a time. For each of eight training stimulus distances, the just noticeable difference in distance was determined at a threshold criterion of 70% choice frequency. The ratio of the retinal image sizes between training stimulus and comparison stimulus at threshold was about constant. However, in contrast to Douglas et al. (Behav Brain Res 30:37–42, 1988), goldfish did not show size constancy in tests with stimuli of the same visual angle. This indicates that they did not estimate distance, but simply compared the retinal images under our experimental conditions. We did not find any indication for the use of accommodation as a depth cue. A patterned background at the rear end of the tubes did not have any effect, which, however, does not exclude the possibility that motion parallax is used as a depth cue under natural conditions.     

Dynamic characteristics of spatial mechanisms coding contour structures

Psychophysical thresholds for the detection of luminance targets improve significantly when the targets are presented in a specific context of spatially separated, collinear inducing stimuli defining visual contours. This phenomenon is generally referred to as a special case of detection facilitation called spatial facilitation. Spatial facilitation has been observed with luminance-defined. achromatic stimuli on achromatic backgrounds as well as with targets and inducers defined by colour contrast. This paper reviews psychophysical results from detection experiments with human observers showing the conditions under which spatially separated contour inducers facilitate the detection of simultaneously presented target stimuli. The findings point towards two types of spatial mechanisms: (i) Short-range mechanisms that are sensitive to narrowly spaced stimuli of small size and, at distinct target locations, selective to the contrast polarity of targets and inducers. (ii) Long-range mechanisms that are triggered by longer stimuli, generate facilitation across wider spatial gaps between targets and inducers, and are insensitive to their contrast polarity. Spatial facilitation with chromatic stimuli requires a longer inducer exposure than spatial facilitation with achromatic stimuli, which is already fully effective at inducer exposures of 30 ms. This difference in temporal dynamics indicates some functional segregation between mechanisms for colour and luminance contrast in spatial coding. In general, spatially induced detection facilitation can to a large extent be explained by mechanisms involving from-short-to-long-range interactions between cortical detectors.     

Eye position affects orienting of visuospatial attention

The ability to detect an incoming visual stimulus is enhanced by knowledge of stimulus location (orienting of visuospatial attention). Although the brain mechanisms at the basis of this enhancement are not yet fully clarified, there is evidence that orienting of attention is accompanied by the activation of oculomotor circuits. It remains unclear, however, whether this oculomotor activity is an epiphenomenon or is functionally related to the attentional process. Attentional benefits are usually measured by the classical Posner paradigm. When subjects fixate centrally and are requested to detect a visual stimulus that could appear in an attended or unattended location, they react faster to stimuli appearing in the attended one. Here, we demonstrate that in monocular vision visuospatial attention was significantly modulated by the position of the eye in the orbit. When the screen was placed 40 degrees to the right or to the left of subjects' sagittal plane, attentional benefits for stimuli appearing in subjects' temporal spatial hemifield dramatically decayed, even if the retinal stimulation was exactly the same as in the classical paradigm. The finding that eyes and attention show a common limit stop point supports their close functional coupling.     

Visual perception of texture regularity: Conjoint measurements and a wavelet response-distribution model

Texture regularity, such as the repeating pattern in a carpet, brickwork or tree bark, is a ubiquitous feature of the visual world. The perception of regularity has generally been studied using multi-element textures in which the degree of regularity has been manipulated by adding random jitter to the elements’ positions. Here we used three-factor Maximum Likelihood Conjoint Measurement (MLCM) for the first time to investigate the encoding of regularity information under more complex conditions in which element spacing and size, in addition to positional jitter, were manipulated. Human observers were presented with large numbers of pairs of multi-element stimuli with varying levels of the three factors, and indicated on each trial which stimulus appeared more regular. All three factors contributed to regularity perception. Jitter, as expected, strongly affected regularity perception. This effect of jitter on regularity perception is strongest at small element spacing and large texture element size, suggesting that the visual system utilizes the edge-to-edge distance between elements as the basis for regularity judgments. We then examined how the responses of a bank of Gabor wavelet spatial filters might account for our results. Our analysis indicates that the peakedness of the spatial frequency (SF) distribution, a previously favored proposal, is insufficient for regularity encoding since it varied more with element spacing and size than with jitter. Instead, our results support the idea that the visual system may extract texture regularity information from the moments of the SF-distribution across orientation. In our best-performing model, the variance of SF-distribution skew across orientations can explain 70% of the variance of estimated texture regularity from our data, suggesting that it could provide a candidate read-out for perceived regularity.