Shape Discrimination Task Research Articles

Overlapping activity periods in early visual cortex and posterior intraparietal area in conscious visual shape perception: A TMS study

Parietal cortex is often activated in brain imaging studies on conscious visual processing, but its causal role and timing in conscious and nonconscious perception are poorly understood. We studied the role of posterior intraparietal sulcus (IPS) and early visual areas (V1/V2) in conscious and nonconscious vision by interfering with their functioning with MRI-guided transcranial magnetic stimulation (TMS). The observers made binary forced-choice decisions concerning the shape or color of the metacontrast masked targets and rated the quality of their conscious perception. TMS was applied 30, 60, 90, or 120ms after stimulus-onset. In the shape discrimination task, TMS of V1/V2 impaired conscious perception at 60, 90, and 120ms and nonconscious perception at 90ms. TMS of IPS impaired only conscious shape perception, also around 90ms. Conscious color perception was facilitated or suppressed depending on the strength of the TMS-induced electric field in V1/V2 at 90ms. The results suggest that simultaneous activity in V1/V2 and IPS around 90ms is necessary for visual awareness of shape but not for nonconscious perception. The overlapping activity periods of IPS and V1/V2 may reflect recurrent interaction between parietal cortex and V1 in conscious shape perception.

Component processes in contour integration: A direct comparison between snakes and ladders in a detection and a shape discrimination task

In contour integration, a relevant question is whether snakes and ladders are processed similarly. Higher presentation time thresholds for ladders in detection tasks indicate this is not the case. However, in a detection task only processing differences at the level of element linking and possibly contour localization might be picked up, while differences at the shape encoding level cannot be noticed. In this study, we make a direct comparison of detection and shape discrimination tasks to investigate if processing differences in the visual system between snakes and ladders are limited to contour detection or extend to higher level contour processing, like shape encoding. Stimuli consisted of elements that were oriented collinearly (snakes) or orthogonally (ladders) to the contour path and were surrounded by randomly oriented background elements. In two tasks, six experienced subjects either detected the contour when presented with a contour and a completely random stimulus or performed a shape discrimination task when presented with two contours with different curvature. Presentation time was varied in 9 steps between 8 and 492 ms. By applying a generalized linear mixed model we found that differences in snake and ladder processing are not limited to a detection stage but are also apparent at a shape encoding stage.

Being confident without seeing: What subjective measures of visual consciousness are about

Can observers be confident about the accuracy of a discrimination response without a visual experience of the stimulus? In a series of five experiments, observers performed a masked orientation discrimination task, a masked shape discrimination task, or a random-dot motion discrimination task, followed by two subjective ratings after each trial, in which participants reported either their visual experience of the stimulus or their confidence in being correct. We observed that the threshold for ratings of the perception of the stimulus was above the threshold for ratings of a decision, that decision ratings outperformed stimulus ratings in predicting trial accuracy, and that different decision-related scales were more strongly associated with other decision-related scales than with ratings of stimulus clarity. We propose a taxonomy of subjective measures of consciousness that differentiates between subjective measures relating to the percept of the stimulus and measures relating to a discrimination decision and discuss the relation to type II blindsight.

SU‐F‐500‐08: Correlation of Model and Human Observer Performance On a Lesion Shape Discrimination Task Using Realistic and Repeated CT Scans

Purpose: To investigate how well a channelized Hotelling observer (CHO) can predict the human observer performance in a lesion shape discrimination task using realistic and repeated CT scans for both filtered backprojection (FBP) and iterative reconstruction (IR). Methods: Eight lesion mimicking rods (2 contrasts, 2 sizes, 2 shapes) were inserted into a 35 by 26 cm water phantom and scanned 100 times on a 128‐slice CT scanner (Definition Flash, Siemens Healthcare) with 120 kV at 120, 180, 240, 360 and 480 quality reference mass (careDose4D). CT images were reconstructed using a FBP and an IR method. A region of interest centered in the rod was extracted to construct the 2‐alternative forced choice (2AFC) study with hexagon and circle rod images put side‐by‐side in randomized order. Total 4000 2AFC trials were read by three medical physicists and the CHO with Gabor filter (5 passbands, 5 orientations, 2 phases). An edge‐emphasis binary mask was applied at the CHO input end. Percent correct was calculated and non‐parametric statistical analysis was performed to compare the performance between human and model observers, and between FBP and IR. Results: The Spearman rank correlation coefficient was 0.879 for FBP and 0.916 for IR (P<0.01). Bland‐Altman plots showed excellent agreement between human and model observers for all lesions sizes and contrasts with a mean difference of 0.5%±3.6% for FBP and 0.2%±3.1% for IR. IR significantly increased the human and model observers performances compared to FBP (Wilcoxon signed rank test, P<0.01). Conclusion: This study demonstrates a high correlation between human and model observers in a discrimination task using realistic CT images. Masking the image input to the CHO by an edge emphasis mask mimics the higher weighting applied towards the edge when humans performed the shape discrimination task. IR significantly increased both human and model performance compared to FBP. This work was supported in part by NIH grant R01 EB071095 from the National Institute of Biomedical Imaging and Bioengineering.

Learning to See, but Not Discriminate, Visual Forms Is Impaired in Aging

Despite the central role of learning in visual recognition, it is largely unknown whether visual form learning is maintained in older age. We examined whether training improved performance in both young and older adults at two key stages of visual recognition: integration of local elements and global form discrimination. We used a shape-discrimination task (concentric vs. radial patterns) in which young and older adults showed similar performance before training. Using a parametric stimulus space that allowed us to manipulate global features and background noise, we were able to distinguish integration and discrimination processes. We found that training improves global form discrimination in both young and older adults. However, learning to integrate local elements is impaired in older age, possibly because of reduced tolerance to external noise. These findings suggest that visual selection processes, rather than global feature representations, provide a fundamental limit for learning-dependent plasticity in the aging brain.

Global Shape Processing Deficits Are Amplified by Temporal Masking in Migraine

Individuals with migraine show subtle defects in a range of visual tasks compared to nonmigraineurs. Increased neuronal noise can account for some of these deficits. To examine the generality of increased noise in migraine, masking effects were compared in migraineurs and headache-free controls using a shape discrimination task, thought to involve processing in extrastriate cortical areas. Nine migraineurs with aura, nine migraineurs without aura, and nine headache-free controls participated. observers had to detect deviations in circular shapes with or without a larger contour mask. The nonoverlapping mask was presented at five temporal intervals (stimulus onset asynchronies, SOA): 0 (simultaneous), 66, 100, 133, and 250 ms. Migraineurs with aura performed worse in all tests than migraineurs without aura and controls. Both migraine groups performed poorer than controls at discriminating shapes without masks. Typical masking functions were obtained from all groups, but they were steeper for migraineurs than controls with thresholds raised most dramatically (2.1 and 4.4 times for migraineurs without and with aura relative to controls, respectively) at SOAs where masks had their most detrimental effect (66-100 ms). Modeling the effect of masking showed that raised internal noise alone is insufficient to explain these deficits. Rather, an abnormal nonlinear transducer function (e.g., as part of gain-control) together with increased multiplicative noise is required to capture the data. The findings are consistent with an extrastriate deficit in migraine that cannot be explained completely by defective inhibition.

A functional near-infrared spectroscopy study of sustained attention to local and global target features

Despite a long history of vigilance research, the role of global and local feature discrimination in vigilance tasks has been relatively neglected. In this experiment participants performed a sustained attention task requiring either global or local shape stimuli discrimination. Reaction time to local feature discriminations was characterized by a quadratic trend over time-on-task with performance levels returning to initial levels late in the task. This trend did not occur in the global shape discrimination task. Functional near-infrared spectroscopy (fNIRS) was utilized in this study as an index of cerebral activation. In both tasks there was increased right hemisphere relative to left hemisphere oxygenation and right hemisphere oxygenation increased with time-on-task. Left hemisphere oxygenation, however, decreased slightly in the global task, but increased significantly in the local task as task duration increased. Indeed, total oxygenation, averaging both right and left, increased more with time-on-task in the local discrimination task. Both the performance and physiological results of this study indicate increased utilization of bilateral cerebral resources with time-on-task in the local, but not the global discrimination vigil.

Blind in a virtual world — using distance information to accomplish virtual tasks

Distance information is critical to our understanding of our surrounding environment, especially in virtual reality settings. Unfortunately, as we gage distance mainly visually, the blind are prevented from properly utilizing this parameter to formulate 3D cognitive maps and cognitive imagery of their surroundings. Our purpose is to increase the accessibility of virtual environments to the blind using distance information which they will receive as auditory information. We aim to create a setup which will enable the blind and visually impaired to experience novel environments virtually before travelling to them in the real world, and additionally will enable the blind better accessibility to virtual environments for purposes such as entertainment and education. Here, blind and sighted-blindfolded subjects performed navigation and shape-discrimination tasks in virtual environments, using a simple transformation between virtual distance and sound, based on the concept of a virtual guide cane (paralleling in a virtual environment the ‘EyeCane’, developed in our lab). We show qualitatively that with minimal training it is possible for blind and blindfolded subjects to easily learn this transformation, enabling the discrimination of virtual 3D orientation and shapes and navigation in basic virtual environments using a standard mouse and audio-system.

The time course of shape discrimination in the human brain

The lateral occipital cortex (LOC) activates selectively to images of intact objects versus scrambled controls, is selective for the figure-ground relationship of a scene, and exhibits at least some degree of invariance for size and position. Because of these attributes, it is considered to be a crucial part of the object recognition pathway. Here we show that human LOC is critically involved in perceptual decisions about object shape. High-density EEG was recorded while subjects performed a threshold-level shape discrimination task on texture-defined figures segmented by either phase or orientation cues. The appearance or disappearance of a figure region from a uniform background generated robust visual evoked potentials throughout retinotopic cortex as determined by inverse modeling of the scalp voltage distribution. Contrasting responses from trials containing shape changes that were correctly detected (hits) with trials in which no change occurred (correct rejects) revealed stimulus-locked, target-selective activity in the occipital visual areas LOC and V4 preceding the subject's response. Activity that was locked to the subjects' reaction time was present in the LOC. Response-locked activity in the LOC was determined to be related to shape discrimination for several reasons: shape-selective responses were silenced when subjects viewed identical stimuli but their attention was directed away from the shapes to a demanding letter discrimination task; shape-selectivity was present across four different stimulus configurations used to define the figure; LOC responses correlated with participants' reaction times. These results indicate that decision-related activity is present in the LOC when subjects are engaged in threshold-level shape discriminations.

Using a Modified Shape Discrimination Task to Assess the Interaction Between Low- and Mid-Level Visual Processes as a Function of Development

Using a Modified Shape Discrimination Task to Assess the Interaction Between Low- and Mid-Level Visual Processes as a Function of Development

Shape Perception Is Altered by Normal Aging

To compare the effects of aging on two shape-discrimination tasks: a closed shape task (radial frequency [RF] patterns) and a Glass pattern coherence task (discrimination of global shape signal from textured noise). We hypothesized that aging would impair the extraction of shape from texture more than the discrimination of closed shapes, consistent with evidence that aging impairs the ability to suppress irrelevant information when segmenting contours from noise. Fourteen younger (19-38 years) and 14 older (62-72 years) adults participated. Contrast-detection thresholds were measured for the RF and Glass stimuli, and then shape-discrimination tasks were performed using stimuli of 5-fold each individual's contrast threshold. The threshold sinusoidal amplitude for the discrimination of an RF3 (triangular) versus an RF4 (square) was measured, in addition to the threshold signal coherence level for the discrimination of concentric from radial Glass patterns. Older adults had elevated shape-discrimination thresholds: RF: mean older = 27 second arc, younger = 18 second arc, t(26) = -3.14, P < 0.01; Glass patterns mean coherence: older = 29%, younger = 16%, t(26) = -5.67, P < 0.01. Relative to younger adult performance, the Z-scores for older adult performance on the Glass task were higher than the RF task (paired t-test; P < 0.05). Shape perception is not robust to the effects of aging. Greater deficits were manifest for the discrimination of shape from texture than for the discrimination of closed contours.

Amblyopic deficits in processing structure-from-motion

Amblyopic observers exhibit a range of low- and high-level cortical deficits, and there is strong evidence that the extrastriate cortex is selectively affected for signal/noise tasks but not for simple integration tasks. We tested amblyopic and control observers on a structure-from-motion (SFM) task involving signal integration to gauge whether extrastriate processing is compromised at a level where dorsal and ventral information is combined. SFM tasks require integration of local elements to perceive the global structure using motion-defined depth cues. Observers were monocularly presented with a 2-IFC shape discrimination task and asked to indicate whether two consecutive SFM stimuli represented the same or different depth-defined shapes. Amblyopic observers had higher depth thresholds than control observers, even after controlling for low-level differences in contrast thresholds across eyes and observers. Combined with the presence of the deficit in both the amblyopic and fellow-fixing eyes, this suggests a high-level locus for the SFM deficit.

Finger dexterity and visual discrimination following two yoga breathing practices

Background:Practicing yoga has been shown to improve motor functions and attention. Though attention is required for fine motor and discrimination tasks, the effect of yoga breathing techniques on fine motor skills and visual discrimination has not been assessed.Aim:To study the effect of yoga breathing techniques on finger dexterity and visual discrimination.Materials and Methods:The present study consisted of one hundred and forty subjects who had enrolled for stress management. They were randomly divided into two groups, one group practiced high frequency yoga breathing while the other group practiced breath awareness. High frequency yoga breathing (kapalabhati, breath rate 1.0 Hz) and breath awareness are two yoga practices which improve attention. The immediate effect of high frequency yoga breathing and breath awareness (i) were assessed on the performance on the O′Connor finger dexterity task and (ii) (in) a shape and size discrimination task.Results:There was a significant improvement in the finger dexterity task by 19% after kapalabhati and 9% after breath awareness (P<0.001 in both cases, repeated measures ANOVA and post-hoc analyses). There was a significant reduction (P<0.001) in error (41% after kapalabhati and 21% after breath awareness) as well as time taken to complete the shape and size discrimination test (15% after kapalabhati and 15% after breath awareness; P<0.001) was also observed.Conclusion:Both kapalabahati and breath awareness can improve fine motor skills and visual discrimination, with a greater magnitude of change after kapalabhati.

The role of local features in shape discrimination of contour- and surface-defined radial frequency patterns at low contrast

Shape processing involves a progression from local to global analysis. A key aspect of this is the binding of distributed local features into an overall form followed by the extraction of the shape independently of its local contrast and spatial scales, so enabling the shape to be encoded based on its proportions without reference to its exact size or retinal location. Here we use contour- and surface-defined radial frequency (RF) patterns in a shape discrimination task, previously thought to reflect a global processing stage that has reached contrast and scale invariance. We compare performance across different spatial scales for a wide range of RF patterns (contour spatial frequencies of 0.7–10.0 cpd, pattern radii of 0.5–10.5°), and sharp- and smooth-edged surface-RF patterns, all at low contrast (5× detection threshold). We show that shape discrimination thresholds for RF patterns have a complex series of dependencies on stimulus size (radius), contour spatial frequency (thickness) and contrast, with no scale invariance. Our results are at odds with earlier work showing no effect of radius and spatial frequency on discrimination thresholds. We show that this discrepancy can be accounted for by a differential effect of contrast on shape discrimination, with shape invariance only stabilizing at higher contrasts (10–20× threshold).

Hydrodynamic discrimination of wakes caused by objects of different size or shape in a harbour seal (Phoca vitulina)

Harbour seals can use their mystacial vibrissae to detect and track hydrodynamic wakes. We investigated the ability of a harbour seal to discriminate objects of different size or shape by their hydrodynamic signature and used particle image velocimetry to identify the hydrodynamic parameters that a seal may be using to do so. Hydrodynamic trails were generated by different sized or shaped paddles that were moved in the calm water of an experimental box to produce a characteristic signal. In a two-alternative forced-choice procedure the blindfolded subject was able to discriminate size differences of down to 3.6 cm (Weber fraction 0.6) when paddles were moved at the same speed. Furthermore the subject distinguished hydrodynamic signals generated by flat, cylindrical, triangular or undulated paddles of the same width. Particle image velocimetry measurements demonstrated that the seal could have used the highest velocities and the steepness of the gradients within the wake to discriminate object size, beside the size of counter-rotating vortices and the spatial extension of a wake. For shape discrimination the subject could have used the spatial extension of the whole wake, in addition to the arrangement of the vortices. We tested whether the seal used highest velocities, the steepness of the gradients and the spatial extension of the wake in a second set of experiments by varying moving speed and paddle size, respectively. The subject was still able to discriminate between the respective object sizes, but the minimum detectable size difference increased to 4.4 cm (Weber fraction 3.6). For the shape discrimination task, the seal was only able to distinguish flat from triangular paddles. Our results indicate that the seal's discrimination abilities depend on more than one hydrodynamic parameter.

On the Relationship between Space- and Object-Based Attention

Visual attention research has suggested two distinct and complementary forms of selection: space- and object-based attention. Though attention can be allocated to regions of space as well as perceptual objects, the exact relationship between the two modes of selection is not fully understood yet. Egly, Driver, and Rafal (1994) first demonstrated spatial- and object-based effects within a single paradigm. However, the space- and object-based reference frames overlapped—targets appeared at the cued location inside the cued object on a large majority of trials. The present study dissociated object and location using a variant of the Egly paradigm. Participants performed a shape discrimination task (T- versus L-shaped stimuli) in which the target appeared at a cued or uncued location inside a cued or uncued object. The cue denoted object, but not spatial, validity. We found both spatial- and object-cueing effects, as well as an interaction between spatial and object validity—the object-based effect occurred at both uncued and cued locations, but was smaller when the spatial location was cued. The results suggest that selection is fundamentally location based, which occurs automatically even under conditions of high object validity.

Prestimulus top-down reflection of obsessive-compulsive disorder in EEG frontal theta and occipital alpha oscillations

It has recently been reported that prestimulus electroencephalogram (EEG) frontal theta and occipital alpha oscillations of healthy controls were modulated by the type of upcoming tasks, reflecting prestimulus top-down preparation. The present study explored the differences in dynamics of frontal theta and occipital alpha activities between obsessive-compulsive disorder (OCD) patients and healthy participants in terms of reflection of prestimulus top-down regulation. EEGs were recorded from 16 OCD patients and 16 healthy controls using a color and a shape discrimination task. The power and time course of oscillatory activity were calculated by convolving the EEG signals with Morlet wavelets. Although OCD patients yielded significantly lower total alpha and total theta power results than the normal controls, they demonstrated that significantly higher total alpha and total theta power preceded the difficult task (shape-task) as compared to the easy task (color-task). Furthermore, the frontal region, where OCD patients usually revealed abnormalities, showed significant differences in the prestimulus total theta power between the normal and OCD groups. Taken together, frontal theta and occipital alpha oscillations seem to be potent electrophysiological correlates reflecting impairment in the prestimulus top-down processing of OCD patients.

Object and spatial imagery dimensions in visuo-haptic representations

Visual imagery comprises object and spatial dimensions. Both types of imagery encode shape but a key difference is that object imagers are more likely to encode surface properties than spatial imagers. Since visual and haptic object representations share many characteristics, we investigated whether haptic and multisensory representations also share an object-spatial continuum. Experiment 1 involved two tasks in both visual and haptic within-modal conditions, one requiring discrimination of shape across changes in texture, the other discrimination of texture across changes in shape. In both modalities, spatial imagers could ignore changes in texture but not shape, whereas object imagers could ignore changes in shape but not texture. Experiment 2 re-analyzed a cross-modal version of the shape discrimination task from an earlier study. We found that spatial imagers could discriminate shape across changes in texture but object imagers could not and that the more one preferred object imagery, the more texture changes impaired discrimination. These findings are the first evidence that object and spatial dimensions of imagery can be observed in haptic and multisensory representations.

The eye dominates in guiding attention during simultaneous eye and hand movements

Prior to the onset of a saccade or a reach, attention is directed to the goal of the upcoming movement. However, it remains unknown whether attentional resources are shared across effectors for simultaneous eye and hand movements. Using a 4-AFC shape discrimination task, we investigated attentional allocation during the planning of a saccade alone, reach alone, or combined saccade and reach to one of five peripheral locations. Target discrimination was better when the probe appeared at the goal of the impending movement than when it appeared elsewhere. However, discrimination performance at the movement goal was not better for combined eye-hand movements compared to either effector alone, suggesting a shared limited attentional resource rather than separate pools of effector-specific attention. To test which effector dominates in guiding attention, we then separated eye and hand movement goals in two conditions: (1) cued reach/fixed saccade--subjects made saccades to the same peripheral location throughout the block, while the reach goal was cued and (2) cued saccade/fixed reach--subjects made reaches to the same location, while the saccade goal was cued. For both conditions, discrimination performance was consistently better at the eye goal than the hand goal. This indicates that shared attentional resources are guided predominantly by the eye during the planning of eye and hand movements.

Interpreting local visual features as a global shape requires awareness

How the brain constructs a coherent representation of the environment from noisy visual input remains poorly understood. Here, we explored whether awareness of the stimulus plays a role in the integration of local features into a representation of global shape. Participants were primed with a shape defined either by position or orientation cues, and performed a shape-discrimination task on a subsequently presented probe shape. Crucially, the probe could either be defined by the same or different cues as the prime, which allowed us to distinguish the effect of priming by local features and global shape. We found a robust priming benefit for visible primes, with response times being faster when the probe and prime were the same shape, regardless of the defining cue. However, rendering the prime invisible uncovered a dissociation: position-defined primes produced behavioural benefit only for probes of the same cue type. Surprisingly, orientation-defined primes afforded an enhancement only for probes of the opposite cue. In further experiments, we showed that the effect of priming was confined to retinotopic coordinates and that there was no priming effect by invisible orientation cues in an orientation-discrimination task. This explains the absence of priming by the same cue in our shape-discrimination task. In summary, our findings show that while in the absence of awareness orientation signals can recruit retinotopic circuits (e.g. intrinsic lateral connections), conscious processing is necessary to interpret local features as global shape.