Expertise for upright faces improves the precision but not the capacity of visual working memory

Considerable research has focused on how basic visual features are maintained in working memory, but little is currently known about the precision or capacity of visual working memory for complex objects. How precisely can an object be remembered, and to what extent might familiarity or perceptual expertise contribute to working memory performance? To address these questions, we developed a set of computer-generated face stimuli that varied continuously along the dimensions of age and gender, and we probed participants’ memories using a method-of-adjustment reporting procedure. This paradigm allowed us to separately estimate the precision and capacity of working memory for individual faces, on the basis of the assumptions of a discrete capacity model, and to assess the impact of face inversion on memory performance. We found that observers could maintain up to four to five items on average, with equally good memory capacity for upright and upside-down faces. In contrast, memory precision was significantly impaired by face inversion at every set size tested. Our results demonstrate that the precision of visual working memory for a complex stimulus is not strictly fixed but, instead, can be modified by learning and experience. We find that perceptual expertise for upright faces leads to significant improvements in visual precision, without modifying the capacity of working memory.     

Visual working memory capacity for objects from different categories: a face-specific maintenance effect

The capacity of visual working memory was examined when complex objects from different categories were remembered. Previous studies have not examined how visual similarity affects object memory, though it has long been known that similar-sounding phonological information interferes with rehearsal in auditory working memory. Here, experiments required memory for two or four objects. Memory capacity was compared between remembering four objects from a single object category to remembering four objects from two different categories. Two-category sets led to increased memory capacity only when upright faces were included. Capacity for face-only sets never exceeded their nonface counterparts, and the advantage for two-category sets when faces were one of the categories disappeared when inverted faces were used. These results suggest that two-category sets which include faces are advantaged in working memory but that faces alone do not lead to a memory capacity advantage.     

本族和异族面孔的视觉工作记忆容量:面孔特征的影响

采用延迟探测刺激匹配再认任务,考察面孔种族和内外部特征是否会影响面孔视觉工作记忆容量。被试先识记2或4张中国人或白人的真实面孔图片,一半有内部特征(眼睛、鼻子和嘴巴),另一半有内部和外部特征(脸型、发型),再进行面孔再认判断。采用Cowan’s K值的分析显示,我国被试对本族和异族面孔的视觉工作记忆容量没有差异,但面孔数量与特征多少会影响面孔工作记忆容量,表明对本族和异族面孔的视觉加工不同。     

Visual working memory for simple and complex visual stimuli

Does the magical number four characterize our visual working memory (VWM) capacity for all kinds of objects, or is the capacity of VWM inversely related to the perceptual complexity of those objects? To find out how perceptual complexity affects VWM, we used a change detection task to measure VWM capacity for six types of stimuli of different complexity: colors, letters, polygons, squiggles, cubes, and faces. We found that the estimated capacity decreased for more complex stimuli, suggesting that perceptual complexity was an important factor in determining VWM capacity. However, the considerable correlation between perceptual complexity and VWM capacity declined significantly if subjects were allowed to view the sample memory display longer. We conclude that when encoding limitations are minimized, perceptual complexity affects, but does not determine, VWM capacity.     

Visual working memory represents a fixed number of items regardless of complexity

Does visual working memory represent a fixed number of objects, or is capacity reduced as object complexity increases? We measured accuracy in detecting changes between sample and test displays and found that capacity estimates dropped as complexity increased. However, these apparent capacity reductions were strongly correlated with increases in sample-test similarity (r= .97), raising the possibility that change detection was limited by errors in comparing the sample and test, rather than by the number of items that were maintained in working memory. Accordingly, when sample-test similarity was low, capacity estimates for even the most complex objects were equivalent to the estimate for the simplest objects (r= .88), suggesting that visual working memory represents a fixed number of items regardless of complexity. Finally, a correlational analysis suggested a two-factor model of working memory ability, in which the number and resolution of representations in working memory correspond to distinct dimensions of memory ability.     

Familiarity enhances visual working memory for faces

Although it is intuitive that familiarity with complex visual objects should aid their preservation in visual working memory (WM), empirical evidence for this is lacking. This study used a conventional change-detection procedure to assess visual WM for unfamiliar and famous faces in healthy adults. Across experiments, faces were upright or inverted and a low- or high-load concurrent verbal WM task was administered to suppress contribution from verbal WM. Even with a high verbal memory load, visual WM performance was significantly better and capacity estimated as significantly greater for famous versus unfamiliar faces. Face inversion abolished this effect. Thus, neither strategic, explicit support from verbal WM nor low-level feature processing easily accounts for the observed benefit of high familiarity for visual WM. These results demonstrate that storage of items in visual WM can be enhanced if robust visual representations of them already exist in long-term memory.     

A visual short-term memory advantage for faces

What determines how much can be stored in visual short-term memory (VSTM)? Studies of VSTM have focused largely on stimulus-based properties such as the number or complexity of the items stored. Recent work also suggests that capacity is severely reduced for items within the same category. However, the importance for VSTM capacity of more qualitative differences in processing for different categories has not been investigated. For example, faces are processed more holistically than other objects. In Experiments 1 and 2, we show that the processing of faces, objects that are crucial socially and for which we possess considerable expertise, overcomes these limitations. More faces can be stored in VSTM than objects from other complex nonface categories. As in prior studies, at short encoding durations we found that capacity for faces was less than that for other categories. However, at longer encoding durations, capacity for faces exceeded that for nonface objects, and this advantage was specific to upright faces. Because inversion reduces holistic processing, the interaction of orientation with VSTM capacity—which occurred for faces but not objects in Experiment 3—suggests that it is holistic processing that confers an advantage for face VSTM when sufficient encoding time is allowed.     

Face inversion and contrast-reversal effects across development: in contrast to the expertise theory

To determine the role of configural changes on the development of face encoding and memory, we investigated face recognition in an n-back repetition task with upright, inverted and contrast-reversed unfamiliar faces in adults and children (8-16 years). Repetitions occurred immediately (0-lag) or after one intervening face (1-lag). Face recognition continued to develop beyond 14-16 years, as shown with hit rates, d' scores and reaction times that all improved with age. Inversion and contrast-reversal effects were found in all subjects but were not more pronounced with increasing age, suggesting no increased reliance on configural processing and thus arguing against the expertise theory of Diamond and Carey (1986). Recognition improved with age in upright but also in inverted and contrast-reversed faces, suggesting a quantitative rather than a qualitative developmental change in face processing. For all age groups, performances decreased and reaction times increased from 0- to 1-lag conditions similarly, suggesting a similar memory component involved in adults' and children's processing. These data suggest gradual quantitative improvements in face processing with age, mainly due to increasing working memory processing capacity.     

A critical evaluation of c as a measure of mnemonic resolution

It is known that visual working memory capacity is limited, but the nature of this limit remains a subject of controversy. Increasingly, two factors are thought to limit visual memory: an object-based limit associated with so-called "slots" models, and an information-based limit associated with resource models. Recently, Barton, Ester, and Awh (2009) introduced a measure of mnemonic resolution, which they dubbed c. We show here that c is critically flawed, and cannot be interpreted as a measure of resolution. We recommend strongly against its use and interpretation, and suggest alternatives approaches for researchers who want to explore mnemonic resolution.     

The cost of facing fear: Visual working memory is impaired for faces expressing fear

Previous research has identified numerous factors affecting the capacity and accuracy of visual working memory (VWM). One potentially important factor is the emotionality of the stimuli to be encoded and held in VWM. We often must hold in VWM information that is emotionally charged, but much is still unknown about how the emotionality of stimuli impacts VWM performance. In the current research, we performed four studies examining the impact of fearful facial expressions on VWM for faces. Fearful expressions were found to produce a consistent cost to VWM performance. This cost was modulated by encoding time, but not set size. This cost was only present for faces in an upright orientation consistent with this cost being a product of the emotionality of the faces rather than lower‐level perceptual differences between neutral and fearful faces. These findings are discussed in the context of existing theoretical accounts of the impact of emotion on information processing. We suggest that a number of competing effects drive both costs and benefits and are at play when emotional information must be stored in VWM, with the task context determining the balance between them.     

Sustained conscious access to incidental memories in RSVP

In visual search of natural scenes, differentiation of briefly fixated but task-irrelevant distractor items from incidental memory is often comparable to explicit memorization. However, many characteristics of incidental memory remain unclear, including the capacity for its conscious retrieval. Here, we examined incidental memory for faces in either upright or inverted orientation using Rapid Serial Visual Presentation (RSVP). Subjects were instructed to detect a target face in a sequence of 8–15 faces cropped from natural scene photographs (Experiment 1). If the target face was identified within a brief time window, the subject proceeded to an incidental memory task. Here, subjects used incidental memory to discriminate between a probe face (a distractor in the RSVP stream) and a novel, foil face. In Experiment 2 we reduced scene-related semantic coherency by intermixing faces from multiple scenes and contrasted incidental memory with explicit memory, a condition where subjects actively memorized each face from the sequence without searching for a target. In both experiments, we measured objective performance (Type 1 AUC) and metacognitive accuracy (Type 2 AUC), revealing sustained and consciously accessible incidental memory for upright and inverted faces. In novel analyses of face categories, we examined whether accuracy or metacognitive judgments are affected by shared semantic features (i.e., similarity in gender, race, age). Similarity enhanced the accuracy of incidental memory discriminations but did not influence metacognition. We conclude that incidental memory is sustained and consciously accessible, is not reliant on scene contexts, and is not enhanced by explicit memorization.     

Children view own-age faces qualitatively differently to other-age faces

Like most own-group biases in face recognition, the own-age bias (OAB) is thought to be based either on perceptual expertise or socio-cognitive motivational mechanisms [Wolff, N., Kemter, K., Schweinberger, S. R., &; Wiese, H. (2013). What drives social in-group biases in face recognition memory? ERP evidence from the own-gender bias. Social Cognitive and Affective Neuroscience. doi:10.1093/scan/nst024]. The present study employed a recognition paradigm with eye-tracking in order to assess whether participants actively viewed faces of their own-age differently to that of other-age faces. The results indicated a significant OAB (superior recognition for own-age relative to other-age faces), provided that they were upright, indicative of expertise being employed for the recognition of own-age faces. However, the eye-tracking results indicate that viewing other-age faces was qualitatively different to the viewing of own-age faces, with more nose fixations for other-age faces. These results are interpreted as supporting the socio-cognitive model of the OAB.     

Discrimination of faces and houses by rhesus monkeys: the role of stimulus expertise and rotation angle

The face inversion effect, or impaired recognition of upside down compared to upright faces, is used as a marker for the configural processing of faces in primates. The inversion effect in humans and chimpanzees is strongest for categories of stimuli for which subjects have considerable expertise, primarily conspecifics’ faces. Moreover, discrimination performance decreases linearly as faces are incrementally rotated from upright to inverted. This suggests that rotated faces must be transformed, or normalized back into their most typical viewpoint before configural processing can ensue, and the greater the required normalization, the greater the likelihood of errors resulting. Previous studies in our lab have demonstrated a general face inversion effect in rhesus monkeys that was not influenced by expertise. Therefore, the present study examined the influence of rotation angle on the visual perception of face and nonface stimuli that varied in their level of expertise to further delineate the processes underlying the inversion effect in rhesus monkeys. Five subjects discriminated images in five orientation angles. Results showed significant linear impairments for all stimulus categories, including houses. However, compared to the upright images, only rhesus faces resulted in worse performance at rotation angles greater than 45°, suggesting stronger configural processing for stimuli for which subjects had the greatest expertise.     

Do binding deficits account for age-related decline in visual working memory?

Remembering visual material, such as objects, faces, and spatial locations, over a short period of time (seconds) becomes more difficult as we age. We investigated whether these deficits could be explained by a simple reduction in visual working memory capacity or by an impairment in one's ability to form or maintain appropriate associations among pieces of related information. In three experiments, we used recognition and recall tests to address the efficacy with which older adults can create bound object representations by varying the number of features of each object that had to be remembered for a subsequent memory test. Results demonstrated that whereas older adults exhibited reduced memory capacity as compared with that of younger adults, both groups stored integrated object representations in visual working memory. These results are contrasted with other work that suggests that age-related memory decline is due, at least in part, to associative deficits.     

Fluency misattribution and visual hindsight bias

We tested a fluency-misattribution theory of visual hindsight bias, and examined how perceptual and conceptual fluency contribute to the bias. In Experiment 1a observers identified celebrity faces that began blurred and then clarified (Forward baseline), or indicated when faces that began clear and then blurred were no longer recognisable (Backward baseline). In surprise memory tests that followed, observers adjusted the degree of blur of each face to match what the faces looked like when identified in the corresponding baseline condition. Hindsight bias was observed in the Forward condition: During the memory test observers adjusted the faces to be more blurry than when originally identified during baseline. These same observers did not show hindsight bias in the Backward condition: Here, they adjusted faces to the exact blur level at which they identified the faces during baseline. Experiment 1b tested a combined condition in which faces were viewed in a Forward progression at baseline but in a Backward progression at test. Hindsight bias was observed in this condition but was significantly less than the bias observed in the Experiment 1a Forward condition. Experiments 1a and 1b provide support for the fluency-misattribution account of visual hindsight bias: When observers are made aware of why fluency has been enhanced (i.e., in the Backward condition) they are better able to discount it, and as a result show reduced or no hindsight bias. In Experiment 2, observers viewed faces in a Forward progression at baseline and then in a Forward upright or inverted progression at test. Hindsight bias occurred in both conditions, but was greater for upright than inverted faces. We conclude that both conceptual and perceptual fluency contribute to visual hindsight bias.     

Fluency misattribution and visual hindsight bias

We tested a fluency-misattribution theory of visual hindsight bias, and examined how perceptual and conceptual fluency contribute to the bias. In Experiment 1a observers identified celebrity faces that began blurred and then clarified (Forward baseline), or indicated when faces that began clear and then blurred were no longer recognisable (Backward baseline). In surprise memory tests that followed, observers adjusted the degree of blur of each face to match what the faces looked like when identified in the corresponding baseline condition. Hindsight bias was observed in the Forward condition: During the memory test observers adjusted the faces to be more blurry than when originally identified during baseline. These same observers did not show hindsight bias in the Backward condition: Here, they adjusted faces to the exact blur level at which they identified the faces during baseline. Experiment 1b tested a combined condition in which faces were viewed in a Forward progression at baseline but in a Backward progression at test. Hindsight bias was observed in this condition but was significantly less than the bias observed in the Experiment 1a Forward condition. Experiments 1a and 1b provide support for the fluency-misattribution account of visual hindsight bias: When observers are made aware of why fluency has been enhanced (i.e., in the Backward condition) they are better able to discount it, and as a result show reduced or no hindsight bias. In Experiment 2, observers viewed faces in a Forward progression at baseline and then in a Forward upright or inverted progression at test. Hindsight bias occurred in both conditions, but was greater for upright than inverted faces. We conclude that both conceptual and perceptual fluency contribute to visual hindsight bias.     

A bottleneck in face identification: repetition priming from flanker images

There is evidence that face processing is capacity-limited in distractor interference tasks and in tasks requiring overt recognition memory. We examined whether capacity limits for faces can be observed with a more sensitive measure of visual processing, by measuring repetition priming of flanker faces that were presented alongside a face or a nonface target. In Experiment 1, we found identity priming for face flankers, by measuring repetition priming across a change in image, during task-relevant nonface processing, but not during the processing of a concurrently-presented face target. Experiment 2 showed perceptual priming of the flanker faces, across identical images at prime and test, when they were presented alongside a face target. In a third Experiment, all of these effects were replicated by measuring identity priming and perceptual priming within the same task. Overall, these results imply that face processing is capacity limited, such that only a single face can be identified at one time. Merely attending to a target face appears sufficient to trigger these capacity limits, thereby extinguishing identification of a second face in the display, although our results demonstrate that the additional face remains at least subject to superficial image processing.     

Irrelevant objects of expertise compete with faces during visual search

Prior work suggests that nonface objects of expertise can interfere with the perception of faces when the two categories are alternately presented, suggesting competition for shared perceptual resources. Here, we ask whether task-irrelevant distractors from a category of expertise compete when faces are presented in a standard visual search task. Participants searched for a target (face or sofa) in an array containing both relevant and irrelevant distractors. The number of distractors from the target category (face or sofa) remained constant, whereas the number of distractors from the irrelevant category (cars) varied. Search slopes, calculated as a function of the number of irrelevant cars, were correlated with car expertise. The effect was not due to car distractors grabbing attention, because they did not compete with sofa targets. Objects of expertise interfere with face perception even when they are task irrelevant, visually distinct, and separated in space from faces.     

Working memory modulates the perception of time

Recent research has indicated that reentrant feedback from the contents of working memory can enhance neural representations and the perceptual strengths of matching stimuli in the visual field. However, whether the contents of working memory can also distort conscious experiences of perception remains unclear. Our present results show that the durations of perceptual stimuli matching the nontemporal representations in working memory tend to be perceived as longer than those of mismatching stimuli. This is the first demonstration that working memory can lead to distortions of time perception. Our findings are consistent with the ideas that the perceived duration of a stimulus depends on the magnitude of the neural responses to that stimulus in visual cortex and that there is a common system for representing both temporal and nontemporal magnitudes. We conclude that top-down modulation from the nontemporal contents of working memory distorts the perceptual experience of temporal duration.     

Intentionally remembering or forgetting own- and other-race faces: Evidence from directed forgetting

People are better at remembering faces of their own relative to another ethnic group. This so-called own-race bias (ORB) has been explained in terms of differential perceptual expertise for own- and other-race faces or, alternatively, as resulting from socio-cognitive factors. To test predictions derived from the latter account, we examined item-method directed forgetting (DF), a paradigm sensitive to an intentional modulation of memory, for faces belonging to different ethnic and social groups. In a series of five experiments, participants during learning received cues following each face to either remember or forget the item, but at test were required to recognize all items irrespective of instruction. In Experiments 1 and 5, Caucasian participants showed DF for own-race faces only while, in Experiment 2, East Asian participants with considerable expertise for Caucasian faces demonstrated DF for own- and other-race faces. Experiments 3 and 4 found clear DF for social in- and outgroup faces. These results suggest that a modulation of face memory by motivational processes is limited to faces with which we have acquired perceptual expertise. Thus, motivation alone is not sufficient to modulate memory for other-race faces and cannot fully explain the ORB.