[Show abstract][Hide abstract] ABSTRACT: 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 ms to 150 ms, the timing of the first face-sensitive responses in humans remains controversial. This controversy is partially due 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 categorised images of faces and noise textures. The results were reliable across testing days, as determined by test-retest assessment in 74 of the participants. Further, 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. This article is protected by copyright. All rights reserved.
European Journal of Neuroscience 10/2015; DOI:10.1111/ejn.13100 · 3.18 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Although regional alpha power and asymmetry measures have been widely used as indices of individual differences in emotional processing and affective style in younger populations, there have been relatively few studies that have examined these measures in older adults. Here, we examined the short-term test-retest reliability of resting regional alpha power (7.5-12.5 Hz) and asymmetry in a sample of 38 active, community-dwelling older adults (M age = 71.2, SD = 6.5 years). Resting electroencephalogram recordings were made before and after a perceptual computer task. Pearson and intra-class correlations indicated acceptable test-retest reliability for alpha power and asymmetry measures in all regions. Interestingly, alpha asymmetry appeared to be less affected by the task than was alpha power. Findings suggest that alpha asymmetry may reflect more enduring, "trait-like" characteristics, while alpha power may reflect more "state-like" processes in older adults.
[Show abstract][Hide abstract] ABSTRACT: Perceptual learning (PL) is often due to observers becoming more sensitive to the diagnostic information in the specific stimuli used during training. Here, we asked how diagnostic information affects PL in a texture identification task. Specifically, we examined whether learning and stimulus generalization differed when diagnostic information was presented alone or embedded in an uninformative context. Texture stimuli were generated from low-frequency band-pass filtered white noise; to create stimuli with restricted diagnostic information, we applied a ±45 degree orientation filter centered on 0 (HORZ) or 90 (VERT) degrees. Explicit stimuli comprised HORZ and VERT orientation-filtered textures as well as an unfiltered FULL texture. Embedded stimuli comprised HORZ and VERT filtered textures summed with non-informative complementary orientation information from the average of all our texture stimuli. We measured accuracy in a 1-of-6 identification task: Observers were trained with either HORZ-explicit or HORZ-embedded stimuli, and tested with all stimulus types. When trained with HORZ-embedded stimuli, performance improvements could result from: i) increased reliance on the informative horizontal band, manifesting as improved accuracy for all conditions in which HORZ structure is informative, or ii) decreased reliance on the uninformative vertical band, manifesting as reduced accuracy for both VERT-embedded and VERT-explicit stimuli. When trained with HORZ-explicit stimuli, performance improvements can only result from increased sensitivity to horizontal structure. Our results reveal higher pre-training accuracy with explicit than embedded stimuli, demonstrating that uninformative structure impaired identification. Training with HORZ-embedded stimuli eliminated this effect, producing post-training accuracy similar to pre-training explicit stimuli. Further, accuracy for VERT stimuli was not decreased, suggesting that learning was not at a cost to the uninformative band. Finally, embedded and explicit learning were largely stimulus-specific, with minimal transfer to other conditions. Together, these results support an increased reliance on the trained orientation band, without suppression of non-informative orientations, but high context-specificity. Meeting abstract presented at VSS 2015.
Journal of Vision 09/2015; 15(12):1297. DOI:10.1167/15.12.1297 · 2.39 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Figure-ground (FG) perception involves segmenting adjacent regions sharing a border into figure and background. Border convexity is one static cue that influences FG perception in a context-dependent manner: The probability of perceiving the figure on the convex side of a border increases with the number of alternating convex and homogeneously filled concave regions (Peterson & Salvagio, J Vision, 2008). This Convexity Context Effect (CCE) is reduced in older adults compared to younger adults (Lass, et. al., VSS, 2013). The reduced CCE in older observers may result from decreased competition resolution in FG patterns, making it more likely that the stimuli are interpreted as flat patterns. If so, then adding cues that indicate depth in the stimulus may enhance the CCE in seniors. We examined this hypothesis by testing younger (M=22.1 years) and older (M=65.9 years) observers in a FG task using 100 ms static displays consisting of 2 or 8 alternating lighter and darker regions of random dot textures, and dynamic displays in which the textures in adjacent regions moved in opposite directions. Froyen, Feldman, and Singh (J Vision, 2013) found that such motion evokes a strong percept of depth in younger observers. The FG task was to indicate the colour of the region that appeared to be in the foreground. Contrary to the hypothesis, the CCE exhibited by older observers was not larger for moving stimuli compared to static stimuli. This result is consistent with the idea that healthy aging alters the way that configural cues influence FG organization, even when additional cues signal depth in the displays. Currently, we are examining the possibility that seniors require longer presentation times to accurately perceive the motion (Bennett, Sekuler & Sekuler, Vis Res, 1997). We are also investigating the relationship between individual differences in perceived depth and CCEs. Meeting abstract presented at VSS 2015.
Journal of Vision 09/2015; 15(12):339. DOI:10.1167/15.12.339 · 2.39 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Observers preferentially process information conveyed by the horizontal orientation band when identifying faces (Dakin and Watt, J Vis 2009; Goffaux and Dakin, Front Psychol 2010). However, ideal observer analysis reveals that such horizontal selectivity is optimal in face identification tasks (Pachai et al, Front Psychol 2013). Therefore, it remains unclear whether horizontal selectivity results from a flexible system tuned to the most diagnostic band for a given task, or a general bias present during all face-related tasks. To disambiguate these hypotheses, we asked observers to perform two face-related tasks for which the diagnostic orientation band differed. On each trial, one of six identities was presented with the head turned slightly to the left or right. Observers were asked on different trials, either blocked or intermixed, to judge the stimulus identity or viewpoint direction. Stimuli were masked with high-contrast orientation-filtered noise (horizontal or vertical, bandwidth = 90 deg) and a low-contrast white noise to enable ideal observer analysis. The dependent measure was the d'=1 RMS contrast threshold, which should be elevated from baseline proportionally to the weight placed by the observer on the masked orientation band during the task in question. A simulated ideal observer confirmed the differential diagnosticity of orientation bands in the two tasks: more masking produced by horizontal noise in the face identification task, and more masking produced by vertical noise in the viewpoint direction task. However, human observers exhibited more masking from horizontal noise in both the identity and direction tasks, regardless of whether these tasks were blocked or intermixed. This result demonstrates an inability to preferentially process vertical facial structure even when it is optimal for the task at hand, and suggests that horizontal selectivity may represent a general face processing strategy. Meeting abstract presented at VSS 2015.
Journal of Vision 09/2015; 15(12):693. DOI:10.1167/15.12.693 · 2.39 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Studies of motion binding often examine the integration of components such as orientation and speed. We examined motion binding using a task requiring only integration of relative phase (Cali et al., VSS, 2014). Observers discriminated 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. Across conditions, noise jitter could be added along the trajectory perpendicular to each dot's motion. Interestingly, as originally showed by Lorenceau (Vision Res., 1996), noise improved discrimination accuracy, consistent with the notion that noise acts as a grouping cue encouraging perception of global motion. Furthermore, when noise was absent from the stimulus, accuracy was not at chance, but significantly below chance; observers consistently reported motion in the incorrect direction. Here we test the hypothesis that observers perceive reverse motion because their representation of the relative phase of the motion components is systematically biased. We asked observers to adjust the relative phase of motion components to produce the most compelling clockwise or counter-clockwise motion with stimuli that did or did not contain noise. We also measured discrimination accuracy for clockwise and counter-clockwise motion. We found that i) phase adjustment error was significantly greater with no noise; ii) discrimination accuracy was significantly below chance with no noise; and iii) the correlation between phase adjustment error and discrimination accuracy were significant in both noise conditions. Our results support the hypothesis that observers misperceive the direction of motion without noise because their representation of the relative phases of motion components is biased. This bias may occur because observers sample the motion components sequentially in the zero noise condition and simultaneously in the high noise condition. More generally, this result suggests the presence of an integration bias in other motion tasks. Meeting abstract presented at VSS 2015.
Journal of Vision 09/2015; 15(12):1182. DOI:10.1167/15.12.1182 · 2.39 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Stimulus-specific perceptual learning (PL) often has been found in relatively simple tasks that require observers to discriminate stimuli that vary along a single dimension. Such learning has been interpreted as showing that training alters the response properties of mechanisms in early visual cortical areas. Stimulus-specific PL also has been found in situations that are arguably more complex, namely identification tasks using filtered random textures that varied on multiple dimensions: PL does not transfer to contrast-reversed versions of the training textures or to textures rotated by 180 deg [Hussain et al., J Vis, 2009, 9(4):20]. Such findings suggest that PL may alter the response properties of mechanisms in higher-level visual areas such as inferotemporal cortex. One potential criticism of the Hussain et al. findings is that stimulus specificity was assessed using transformations that altered the spatial distribution of the black and white blobs in the textures, and essentially created a new set of stimuli. According to this argument, PL in a texture identification task may generalize to textures presented in different spatial locations, without altering the spatial distributions of features. We tested this idea by measuring accuracy in a 1-of-5 texture identification task. Practice on Day 1 consisted of interleaved trials with two different sets of textures: one set was presented slightly above the fixation point and the other was presented below the fixation point. On Day 2, subjects continued the task either with stimuli in the same positions seen during Day 1 or with the top-bottom positions switched. We found evidence for both generalized and stimulus-specific learning: presenting textures in new positions on Day 2 caused accuracy to decrease significantly, but not to levels seen at the beginning of Day 1. Hence, a component of learning in this relatively complex texture identification task is stimulus specific. Meeting abstract presented at VSS 2015.
Journal of Vision 09/2015; 15(12):1135. DOI:10.1167/15.12.1135 · 2.39 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: One possible explanation for the face inversion effect (FIE) is that inversion swaps the eye and mouth locations relative to fixation, and attention typically is directed to the top of a stimulus for faces. As the eye region is the most informative for face discrimination, automatically attending to the upper-half of a face would cause observers to use less diagnostic regions for inverted faces. Consistent with this hypothesis, cueing attention to the eyes modulates the FIE measured both behaviourally (Hills et al. , JEP: HPP 2011) and with EEG (de Lissa et al., Neuropsychologia 2014). However, past studies used old/new recognition or gender discrimination tasks rather than identification tasks, and they did not consider the effects of stimulus size. The size manipulation is interesting in light of a recent suggestion that specialized face processing is engaged only by large stimuli (Yang et al., J Vis 2014). To address these issues, we measured accuracy and ERPs in a 6-AFC identification task that varied fixation location (center, left eye, right eye, mouth), orientation (upright or inverted), and face width (3.2 or 8.1 deg). Behavioural results showed significant main effects of: i) face size (higher accuracy for large faces), ii) fixation location (lower accuracy for mouth fixations), and iii) orientation (lower accuracy for inverted faces). However, we observed no fixation x orientation interaction, thus fixation location did not modulate the FIE. The size x orientation interaction also was not significant, which is inconsistent with the suggestion that small and large faces differentially recruit face-specific mechanisms. Finally, we found a significant N170 latency FIE that, consistent with previous studies, was larger with eye fixations. Together, these results clarify the roles of size and fixation in identification tasks, and further implicate the eyes in both behavioural and electrophysiological markers of face processing. Meeting abstract presented at VSS 2015.
Journal of Vision 09/2015; 15(12):694. DOI:10.1167/15.12.694 · 2.39 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Horizontal orientations comprise the primary diagnostic structure for face perception (Dakin & Watt, J Vis 2009), and young adult observers' ability to selectively use horizontal structure more than vertical structure (horizontal tuning) predicts both face identification accuracy and the face inversion effect magnitude (Pachai et al., Front Psychol 2013). Here, we ask if the age-related decline of face identification accuracy (Konar et al., Vis Res 2013) is associated with age-related changes in horizontal tuning. Because the N170 ERP component is delayed and smaller in response to faces lacking horizontal structure (Hashemi et al., VSS 2014), we also examined whether age-related behavioural changes in horizontal tuning were associated with age-related changes in both the N170 and the N250. We measured identification accuracy and ERPs in younger (M=22±3.8 years) and older (M=73±7.3 years) adults with a 6-AFC face identification task. Base stimuli were generated using a ±45 deg orientation filter centered on either the horizontal or vertical orientation, with additional stimuli generated by increasing orientation bandwidth by ±9 deg steps, totalling 10 orientation-filtered conditions plus 1 unfiltered condition that included all orientations. In younger observers, adding horizontal structure improved accuracy significantly more than adding vertical structure. However, in older adults, the addition of horizontal and vertical structure produced similar, small increases in accuracy. Thus, older adults exhibited less horizontal selectivity than younger adults. Interestingly, orientation filtering affected the N170 similarly in the two age groups, but the N250 was modulated by orientation filtering only in younger adults,consistent with the notion that the two ERP components represent different stages of face processing.The decreased behavioural horizontal tuning seen with age is reflected in the N250's decreased sensitivity to orientation filtering, providing behavioural and electrophysiological support for the idea that face identification relies on effective use of horizontally oriented structure. Meeting abstract presented at VSS 2015.
Journal of Vision 09/2015; 15(12):681. DOI:10.1167/15.12.681 · 2.39 Impact Factor
[Show description][Hide description] DESCRIPTION: Categorization is common in a wide variety of human behaviors but the degree to which categories are the result of perception versus other factors (e.g., cognitive or linguistic) is less well understood. We investigated whether simple visual stimuli (sinusoidal grating mixtures) exhibit categorization. Previous investigators have found that 180 degree relative phase shifts in f + 2f gratings are discriminated when the sine or cosine contrast of the shift exceeds some criterion. Three observers performed traditional categorical perception tasks of classification and discrimination tasks with f + 2f gratings.
[Show abstract][Hide abstract] ABSTRACT: Previous work on spatial contextual interactions between shapes defined by radial frequency (RF) contours has demonstrated that such interactions depend on the rotational phase alignment between a mask and target stimulus (Habak et al., 2004; 2009). When the points of maximum curvature in target and masking RF contours are aligned (i.e., zero phase difference), thresholds for detecting deviations from circularity are significantly elevated relative to a baseline condition that contains no masking stimulus, and the strength of masking declines as the relative phase difference increases (Habak et al., 2004; 2009). The current study extended this previous work by examining the effect of RF number on the magnitude of threshold elevation observed across seven target-mask relative phases. We measured detection thresholds for five RF contours (RF3, 5, 6, 8, 11) in the presence of a surrounding mask of the same RF number as the target in seven target-mask relative phase combinations (0°, 30°, 60°, 90°, 120°, 150°, 180°). Although the amount of masking varied considerably across RF contours, we found that the magnitude of masking declined with increasing relative target-mask phase difference for all RF combinations. However, we also found a significant interaction between RF number and phase such that the rate at which masking declined with phase differed across RF contours. Overall, these results suggest significant differences exist in how the frequency of local curvature affects the interference observed as a function of the alignment between two shapes. The results of this study serve as a foundation for interpreting the effect of rotational phase alignment between spatially separated shapes.
[Show abstract][Hide abstract] ABSTRACT: Although binocular rivalry has been examined thoroughly in young adults, we know relatively little about its developmental trajectory. To address this issue, we created a child-friendly task, in which we presented pairs of orthogonal, oblique sine wave gratings to 7-, 9-, 11-year-olds, and young adults (mean = 21.25 years). Stimulus size (diameter = 1.4 or 4.4) and contrast level (0.2 or 0.8), factors with well-known effects on rivalry in young adults, varied across trials. On each trial, participants recorded their alternations among percepts (each of the two exclusive, mixed, and fading/other) with a handheld button box. To measure accuracy of reported percepts, we intermixed pseudo-rivalry with experimental trials. For experimental trials, dependent measures included the average duration and proportion of time participants reported viewing each type of percept. Children spent a significantly greater proportion of time viewing exclusive percepts and less time viewing mixed percepts compared to young adults, a finding that provides evidence against the prediction of increased mixed percepts in children (Kovacs & Eisenberg, 2005). Sequential patterns of alternations between percepts also varied between children and young adults. For example, the proportion of return transitions increased from childhood to adulthood, specifically for low contrast conditions. Average durations for exclusive percepts did not differ significantly across age groups, contrary to previous reports suggesting faster alternation rates in children (Kovacs & Eisenberg, 2005; Hudak et al., 2011). Average durations for mixed percepts were shorter in children compared to young adults. No differences were observed between 7-, 9-, and 11-year-olds for any dependent measure. These are the first reports of several characteristics of binocular rivalry in children, specifically measures of mixed percepts and sequential transitions. Meeting abstract presented at VSS 2015.
Journal of Vision 12/2014; 14(15):47-47. DOI:10.1167/14.15.47 · 2.39 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Background / Purpose:
In order to examine the processing underlying figure/ground perception, we examined neural signals associated with convexity context effects (Peterson and Salvagio, 2008). During a convexity task with 2- and 8-region black and white stimuli that involved indicating which central region was perceived as figure, we recorded electroencephalography of the posterior region of the cortex that was time-locked to stimulus onset. One sample of observers was used to characterize the evoked response potential (ERP) profile and determine spatiotemporal location(s) sensitive to how the stimulus was perceived and a second sample was used to directly test those findings in a replication.
We observed a robust effect on mean amplitude of the ERP that was sensitive to how the stimulus is perceived (convex vs. concave as figure interpretations) in the medial parietal/occipital cortex between 160-180ms post stimulus onset. In combination with the other findings presented, these results are consistent with the idea that the early visual system is sensitive to figure/ground perception and highlight the importance of perceived, 3-dimensionality in the processing underlying it.
Vision Sciences Society Annual Meeting 2014; 06/2014