The time course of stimulus-specific perceptual learning.

Practice on perceptual tasks can lead to long-lasting, stimulus-specific improvements. Rapid stimulus-specific learning, assessed 24 hours after practice, has been found with just 105 practice trials in a face identification task. However, a much longer time course for stimulus-specific learning has been found in other tasks. Here, we examined 1) whether rapid stimulus-specific learning occurs for unfamiliar, non-face stimuli in a texture identification task; 2) the effects of varying practice across a range from just 21 trials up to 840 trials; and 3) if rapid, stimulus-specific learning persists over a 1-week, as well as a 1-day, interval. Observers performed a texture identification task in two sessions separated by one day (Experiment 1) or 1 week (Experiment 2). Observers received varying amounts of practice (21, 63, 105, or 840 training trials) in session 1 and completed 840 trials in session 2. In session 2, one-half of the observers in each group performed the task with the same textures as in session 1, and one-half switched to novel textures (same vs. novel conditions). In both experiments we found that stimulus-specific learning - defined as the difference in response accuracy in the same and novel conditions - increased as a linear function of the log number of session 1 training trials and was statistically significant after approximately 100 training trials. The effects of stimulus novelty did not differ across experiments. These results support the idea that stimulus-specific learning in our task arises gradually and continuously through practice, perhaps concurrently with general learning.

The effect of apparent distance on peripheral target detection.

Previous research suggests that peripheral target detection is modulated by viewing distance and distance simulated by pictorial cues and optic flow. In the latter case, it is unclear what cues contribute to the effect of distance. The current study evaluated the effect of distance on peripheral detection in a virtual three-dimensional environment. Experiments 1-3 used a continuous, dynamic central task that simulated observers traveling either actively or passively through a virtual environment following a car. Peripheral targets were flashed on checkerboard-covered walls to the left and right of the path of motion, at a near and a far distance from the observer. The retinal characteristics of the targets were identical across distances. Experiment 1 found more accurate and faster detection for near targets compared to far targets, especially for larger eccentricities. Experiment 2 equated the predictability of target onset across distances and found the near advantage for larger eccentricities in accuracy but a much smaller effect in reaction time (RT). Experiment 3 removed the checkerboard background implemented in Experiments 1 and 2, and Experiment 4 manipulated several static, monocular cues. Experiments 3 and 4 found that the variation in the density of the checkerboard backgrounds could explain the main effect of distance on accuracy but could not completely account for the interaction between target distance and eccentricity. These results suggest that attention is modulated by target distance, but the effect is small. Finally, there were consistent divided attention costs in the central car-following task but not the peripheral detection task.

Phase integration bias in a motion grouping task.

The perception of the direction of global motion depends on our ability to integrate local motion signals over space and time. We examined motion binding using a task requiring integration of relative phase. Observers completed multiple tasks involving clockwise and counter clockwise motion in a stimulus comprising four sets of linearly arranged dots, two moving horizontally and two moving vertically along sinusoidal trajectories differing in phase. Noise jitter was added along the trajectory perpendicular to each dot's motion. The noise acts as a global grouping cue that improves direction discrimination, but surprisingly, the absence of noise causes consistent below-chance performance (Lorenceau, 1996). We explore this phenomenon and subsequently test the hypothesis that observers perceive reverse motion because their representation of the relative phase of the motion components is systematically biased. We employ a number of different objective and subjective measures of motion integration and measure the phenomenon in both younger and older adults. Taken together, the results presented in the current article demonstrate that noise can promote global grouping in the stimulus and that confident, incorrect responses can be observed in the absence of correct global grouping. Generally, the current result raises the possibility that an integration bias could exist in other motion tasks.

Sensitivity to curvature deformations along closed contours.

Human observers are exquisitely sensitive to curvature deformations along a circular closed contour (Wilkinson, Wilson, & Habak, 1998; Hess, Wang, & Dakin, 1999; Loffler, Wilson, & Wilkinson, 2003). Such remarkable sensitivity has been attributed to the curvature encoding scheme used by V4 neurons, which typically are assumed to be equally sensitive to curvature at all polar angles (Pasupathy & Connor, 2001, 2002; Carlson, Rasquinha, Zhang, & Connor, 2011). To test the assumption that detection thresholds for curvature deformations are invariant across polar angles, we used a novel stimulus class we call Difference of Gaussian (DoG) contours that allowed us to independently manipulate the amplitude, angular frequency, and polar angle of curvature of a closed-contour shape while measuring contour-curvature thresholds. Our results demonstrate that (a) detection thresholds were higher when observers were uncertain about the location of the curvature deformation, but on average, thresholds did not vary significantly across 24 polar angles; (b) the direction and magnitude of the oblique effect varies across individuals; (c) there is a strong association between detecting a contour deformation and identifying its location; (d) curvature detectors may serve as labeled lines.

Evaluating spatiotemporal interactions between shapes.

Spatiotemporal interactions between stimuli can alter the perceived curvature along the outline of a shape (Habak, Wilkinson, Zakher, & Wilson, 2004; Habak, Wilkinson, & Wilson, 2006). To better understand these interactions, we used a forward and backward masking paradigm with radial frequency (RF) contours while measuring RF detection thresholds. In Experiment 1, we presented a mask alongside a target contour and altered the stimulus onset asynchrony between this target-mask pair and a temporal mask. We found that a temporal mask increased thresholds when it preceded the target-mask stimulus by 130-180 ms but decreased thresholds when it followed the target-stimulus mask by 180 ms. Furthermore, Experiment 2 demonstrated that the effects of temporal and spatial masks are approximately additive. We discuss these findings in relation to theories of transient and sustained channels in vision.

The Bandwidth of Diagnostic Horizontal Structure for Face Identification.

Horizontally oriented spatial frequency components are a diagnostic source of face identity information, and sensitivity to this information predicts upright identification accuracy and the magnitude of the face-inversion effect. However, the bandwidth at which this information is conveyed, and the extent to which human tuning matches this distribution of information, has yet to be characterized. We designed a 10-alternative forced choice face identification task in which upright or inverted faces were filtered to retain horizontal or vertical structure. We systematically varied the bandwidth of these filters in 10° steps and replaced the orientation components that were removed from the target face with components from the average of all possible faces. This manipulation created patterns that looked like faces but contained diagnostic information in orientation bands unknown to the observer on any given trial. Further, we quantified human performance relative to the actual information content of our face stimuli using an ideal observer with perfect knowledge of the diagnostic band. We found that the most diagnostic information for face identification is conveyed by a narrow band of orientations along the horizontal meridian, whereas human observers use information from a wide range of orientations.

Implicit Valuation of the Near-Miss is Dependent on Outcome Context.

Gambling studies have described a "near-miss effect" wherein the experience of almost winning increases gambling persistence. The near-miss has been proposed to inflate the value of preceding actions through its perceptual similarity to wins. We demonstrate here, however, that it acts as a conditioned stimulus to positively or negatively influence valuation, dependent on reward expectation and cognitive engagement. When subjects are asked to choose between two simulated slot machines, near-misses increase valuation of machines with a low payout rate, whereas they decrease valuation of high payout machines. This contextual effect impairs decisions and persists regardless of manipulations to outcome feedback or financial incentive provided for good performance. It is consistent with proposals that near-misses cause frustration when wins are expected, and we propose that it increases choice stochasticity and overrides avoidance of low-valued options. Intriguingly, the near-miss effect disappears when subjects are required to explicitly value machines by placing bets, rather than choosing between them. We propose that this task increases cognitive engagement and recruits participation of brain regions involved in cognitive processing, causing inhibition of otherwise dominant systems of decision-making. Our results reveal that only implicit, rather than explicit strategies of decision-making are affected by near-misses, and that the brain can fluidly shift between these strategies according to task demands.

Effects of aging on figure-ground perception: Convexity context effects and competition resolution.

We examined age-related differences in figure-ground perception by exploring the effect of age on Convexity Context Effects (CCE; Peterson & Salvagio, 2008). Experiment 1, using Peterson and Salvagio's procedure and black and white stimuli consisting of 2 to 8 alternating concave and convex regions, established that older adults exhibited reduced CCEs compared to younger adults. Experiments 2 and 3 demonstrated that this age difference was found at various stimulus durations and sizes. Experiment 4 compared CCEs obtained with achromatic stimuli, in which the alternating convex and concave regions were each all black or all white, and chromatic stimuli in which the concave regions were homogeneous in color but the convex regions varied in color. We found that the difference between CCEs measured with achromatic and colored stimuli was larger in older than in younger adults. Our results are consistent with the hypothesis that the senescent visual system is less able to resolve the competition among various perceptual interpretations of the figure-ground relations among stimulus regions.

Personal familiarity enhances sensitivity to horizontal structure during processing of face identity.

What makes identification of familiar faces seemingly effortless? Recent studies using unfamiliar face stimuli suggest that selective processing of information conveyed by horizontally oriented spatial frequency components supports accurate performance in a variety of tasks involving matching of facial identity. Here, we studied upright and inverted face discrimination using stimuli with which observers were either unfamiliar or personally familiar (i.e., friends and colleagues). Our results reveal increased sensitivity to horizontal spatial frequency structure in personally familiar faces, further implicating the selective processing of this information in the face processing expertise exhibited by human observers throughout their daily lives.

Online Modulation of Selective Attention is not Impaired in Healthy Aging.

Background/Study Context: Reduced processing speed pervades a great many aspects of human aging and cognition. However, little is known about one aspect of cognitive aging in which speed is of the essence, namely, the speed with which older adults can deploy attention in response to a cue. METHODS: The authors compared rapid temporal modulation of cued visual attention in younger (Mage = 22.3 years) and older (Mage = 68.9 years) adults. On each trial of a short-term memory task, a cue identified which of two briefly presented stimuli was task relevant and which one should be ignored. After a short delay, subjects demonstrated recall by reproducing from memory the task-relevant stimulus. This produced estimates of (i) accuracy with which the task-relevant stimulus was recalled, (ii) the influence of stimuli encountered on previous trials (a prototype effect), and (iii) the influence of the trial's task-irrelevant stimulus. RESULTS: For both groups, errors in recall were considerably smaller when selective attention was cued before rather than after presentation of the stimuli. Both groups showed serial position effects to the same degree, and both seemed equally adept at exploiting the stimuli encountered on previous trials as a means of supplementing recall accuracy on the current trial. CONCLUSION: Younger and older subjects may not differ reliably in capacity for cue-directed temporal modulation of selective attention, or in ability to draw on previously seen stimuli as memory support.

A robust and representative lower bound on object processing speed in humans.

How early does the brain decode object categories? Addressing this question is critical to constrain the type of neuronal architecture supporting object categorization. In this context, much effort has been devoted to estimating face processing speed. With onsets estimated from 50 to 150 ms, the timing of the first face-sensitive responses in humans remains controversial. This controversy is due partially to the susceptibility of dynamic brain measurements to filtering distortions and analysis issues. Here, using distributions of single-trial event-related potentials (ERPs), causal filtering, statistical analyses at all electrodes and time points, and effective correction for multiple comparisons, we present evidence that the earliest categorical differences start around 90 ms following stimulus presentation. These results were obtained from a representative group of 120 participants, aged 18-81, who categorized images of faces and noise textures. The results were reliable across testing days, as determined by test-retest assessment in 74 of the participants. Furthermore, a control experiment showed similar ERP onsets for contrasts involving images of houses or white noise. Face onsets did not change with age, suggesting that face sensitivity occurs within 100 ms across the adult lifespan. Finally, the simplicity of the face-texture contrast, and the dominant midline distribution of the effects, suggest the face responses were evoked by relatively simple image properties and are not face specific. Our results provide a new lower benchmark for the earliest neuronal responses to complex objects in the human visual system.

On the time course of attentional focusing in older adults.

Many sensory and cognitive changes accompany normal ageing, including changes to visual attention. Several studies have investigated age-related changes in the control of attention to specific locations (spatial orienting), but it is unknown whether control over the distribution or breadth of attention (spatial focus) also changes with age. In the present study, we employed a dual-stream attentional blink task and assessed changes to the spatial distribution of attention through the joint consequences of temporal lag and spatial separation on second-target accuracy. Experiment 1 compared the rate at which attention narrows in younger (mean age 22.6, SD 4.25) and older (mean age 66.8, SD 4.36) adults. The results showed that whereas young adults can narrow attention to one stream within 133 ms, older adults were unable to do the same within this time period. Experiment 2 showed that older adults can narrow their attention to one stream when given more time (266 ms). Experiment 3 confirmed that age-related changes in retinal illuminance did not account for delayed attentional narrowing in older adults. Considered together, these experiments demonstrate that older adults can narrow their attentional focus, but that they are delayed in initiating this process compared to younger adults. This finding adds to previously reported reductions in attentional dynamics, deficits in inhibitory processes, and reductions in posterior parietal cortex function that accompany normal ageing.

Aging and the integration of orientation and position in shape perception.

The current experiments examined the effect of healthy aging on the integration of orientation and position information in shape perception. Following Day and Loffler (2009), conflicting contours were created by sampling the orientations of one shape (e.g., a rounded pentagon) with Gabors, and positioning them on the circumference of a different shape (e.g., a circle). In Experiment 1, subjects judged whether the conflicting contour looked more circular than a rounded pentagon of varying amplitude, which allowed us to estimate the perceived shape of the conflicting contour. The relative amount of position and orientation information was manipulated by varying the number of Gabors comprising the target contour. Orientation information dominated the percept for contours sampled with 15-40 elements, producing a strong shape illusion, but position information determined the shape with denser sampling. The magnitude of this orientation dominance effect was equal in younger and older subjects across all sampling levels. In Experiment 2, subjects discriminated five contours that differed in orientation and/or position information. Both groups showed poor discrimination between conflicting contours and their perceptually equivalent radial frequency patterns, confirming the main finding of Experiment 1. In addition, older subjects showed worse discrimination between two noncircular radial frequency patterns than younger subjects. In sum, integration of orientation and position information in shape perception is preserved with aging; however, older adults are less able to make fine shape discriminations between noncircular sampled contours.

Contribution of internal noise and calculation efficiency to face discrimination deficits in older adults.

Classification images (CIs) measured in a face discrimination task differ significantly between older and younger observers. These age differences are consistent with the hypothesis that older adults sample diagnostic face information less efficiently, or have higher levels of internal noise, compared to younger adults. The current experiments assessed the relative contributions of efficiency and internal noise to age differences in face discrimination using the external noise masking and double-pass response consistency paradigms. Experiment 1 measured discrimination thresholds for faces embedded in several levels of static white noise, and the resulting threshold-vs.-noise curves were used to estimate calculation efficiency and equivalent input noise: older observers had lower efficiency and higher equivalent input noise than younger observers. Experiment 2 presented observers with two identical sequences of faces embedded in static white noise to measure the association between response accuracy and response consistency and estimate the internal:external (i/e) noise ratio for each observer. We found that i/e noise ratios did not differ significantly between groups. These results suggest that age differences in face discrimination are due to differences in calculation efficiency and additive internal noise, but not to age differences in multiplicative internal noise.

The influence of aging on audiovisual temporal order judgments.

UNLABELLED: BACKGROUND/STUDY CONTEXT: The perception of naturalistic events depends on the ability to integrate perceptual information from multiple sensory systems. Currently, little is known about how multisensory integration is affected by normal aging. METHODS: The authors conducted two experiments to investigate audiovisual temporal processing in younger (18-29 years) and older (70+ years) adults. In both experiments, participants were presented with a brief visual stimulus and a brief auditory stimulus separated by various temporal offsets, and participants judged which stimulus was presented first. In Experiment 1, the auditory and visual stimuli were presented from the same perceived location, whereas in Experiment 2 they were presented from different locations. RESULTS: The authors found no effect of stimulus location, and no evidence of age-related declines in performance in either experiment. CONCLUSION: Older adults appear to retain the ability to discriminate the temporal order of audiovisual stimuli and can perform similarly to younger adults.

Peripheral target detection can be modulated by target distance but not attended distance in 3D space simulated by monocular depth cues.

Most studies of visuo-spatial attention present stimuli on a 2D plane, and less is known about how attention varies in 3D space. Previous studies found better peripheral detection performance for targets at a near compared to a far depth, simulated by pictorial cues and optical flow. The current study examined whether target detectability is monotonically related to distance along the depth axis, and whether the attended distance modulates the effect of target distance. We investigated these questions in two experiments that measured how apparent distance and target eccentricity affects peripheral target detection when performed alone during passive simulated self-motion, or during a simultaneous, active central car-following task. Experiment 1 found that targets at an apparent distance of 18.5 virtual meters were detected faster and more accurately than targets at 9.25 and 37 virtual meters, and detectability declined with eccentricity. Experiment 2 examined the effect of the attended location by varying the distance between the viewer and the lead car on which participants were instructed to fixate (i.e. the headway) while equating target distances across headway conditions. Experiment 2 replicated the effects found in Experiment 1, and headway did not modulate the effect of target distance. These results are consistent with the hypothesis that target detection depends non-monotonically on the distance between the viewer and the target, and is not affected by the distance between the target and attended location. However, target detection may also have been affected by stimulus characteristics that co-varied with apparent depth, rather than depth per se.

Cannabis use is associated with sexually dimorphic changes in executive control of visuospatial decision-making.

When the outcome of a choice is less favorable than expected, humans and animals typically shift to an alternate choice option on subsequent trials. Several lines of evidence indicate that this "lose-shift" responding is an innate sensorimotor response strategy that is normally suppressed by executive function. Therefore, the lose-shift response provides a covert gauge of cognitive control over choice mechanisms. We report here that the spatial position, rather than visual features, of choice targets drives the lose-shift effect. Furthermore, the ability to inhibit lose-shift responding to gain reward is different among male and female habitual cannabis users. Increased self-reported cannabis use was concordant with suppressed response flexibility and an increased tendency to lose-shift in women, which reduced performance in a choice task in which random responding is the optimal strategy. On the other hand, increased cannabis use in men was concordant with reduced reliance on spatial cues during decision-making, and had no impact on the number of correct responses. These data (63,600 trials from 106 participants) provide strong evidence that spatial-motor processing is an important component of economic decision-making, and that its governance by executive systems is different in men and women who use cannabis frequently.

Superior identification of familiar visual patterns a year after learning.

Practice improves visual performance on simple tasks in which stimuli vary along one dimension. Such learning frequently is stimulus-specific and enduring, and has been associated with plasticity in striate cortex. It is unclear if similar lasting effects occur for naturalistic patterns that vary on multiple dimensions. We measured perceptual learning in identification tasks that used faces and textures, stimuli that engage multiple stages in visual processing. Performance improved significantly across 2 consecutive days of practice. More important, the effects of practice were remarkably stable across time: Improvements were maintained approximately 1 year later, and both the relative difficulty of identifying individual stimuli and individual differences in performance were essentially constant across sessions. Finally, the effects of practice were largely stimulus-specific. Our results suggest that the characteristics of perceptual learning are similar across a spectrum of stimulus complexities.

Inhibition of target detection in apparent motion trajectory.

Letter discrimination performance is degraded when a letter is presented within an apparent motion (AM) trajectory of a spot. This finding suggests that the internal representation of AM stimuli can perceptually interact with other stimuli. In this study, we demonstrated that AM interference could also occur for pattern detection. We found that target (Gabor patch) detection performance was degraded within an AM trajectory. Further, this AM interference weakened when the differences in orientation between the AM stimuli and target became greater. We also revealed that AM interference occurred for the target with spatiotemporally intermediate orientations of the inducers that changed their orientation during AM. In contrast, the differences in phase among the stimuli did not affect the occurrence of AM interference. These findings suggest that AM stimuli and their internal representations affect lower visual processes involved in detecting a pattern in the AM trajectory and that the internal object representation of an AM stimulus selectively reflects and maintains the stimulus attribute.

Brain structure and function in people recovering from COVID-19 after hospital discharge or self-isolation: a longitudinal observational study protocol.

BACKGROUND: The detailed extent of neuroinvasion or deleterious brain changes resulting from COVID-19 and their time courses remain to be determined in relation to "long-haul" COVID-19 symptoms. Our objective is to determine whether there are alterations in functional brain imaging measures among people with COVID-19 after hospital discharge or self-isolation. METHODS: This paper describes a protocol for NeuroCOVID-19, a longitudinal observational study of adults aged 20-75 years at Sunnybrook Health Sciences Centre in Toronto, Ontario, that began in April 2020. We aim to recruit 240 adults, 60 per group: people who contracted COVID-19 and were admitted to hospital (group 1), people who contracted COVID-19 and self-isolated (group 2), people who experienced influenza-like symptoms at acute presentation but tested negative for COVID-19 and self-isolated (group 3, control) and healthy people (group 4, control). Participants are excluded based on premorbid neurologic or severe psychiatric illness, unstable cardiovascular disease, and magnetic resonance imaging (MRI) contraindications. Initial and 3-month follow-up assessments include multiparametric brain MRI and electroencephalography. Sensation and cognition are assessed alongside neuropsychiatric assessments and symptom self-reports. We will test the data from the initial and follow-up assessments for group differences based on 3 outcome measures: MRI cerebral blood flow, MRI resting state fractional amplitude of low-frequency fluctuation and electroencephalography spectral power. INTERPRETATION: If neurophysiologic alterations are detected in the COVID-19 groups in our NeuroCOVID-19 study, this information could inform future research regarding interventions for long-haul COVID-19. The study results will be disseminated to scientists, clinicians and COVID-19 survivors, as well as the public and private sectors to provide context on how brain measures relate to lingering symptoms.