Correlates of implicit memory for words and faces in event-related brain potentials

Electrophysiological Correlates of Perceptual Auditory Priming Without Explicit Recognition Memory

The aim of this study was to identify an event-related potential (ERP correlate) of perceptual auditory priming using a method that can dissociate it from explicit memory similar to Rugg et al. (1998) . EEG was recorded during performance of an auditory word recognition test, where 17 participants discriminated “old” from “new” aural words, encoded using either a “deep” or “shallow” levels-of-processing (LOP) study task. A right-lateralized P200 effect was modulated by words’ old/new status but not by accuracy of recognition or LOP manipulation. Because this effect was driven by simple repetition rather than factors known to influence episodic recognition memory, a “bottom-up” perceptual priming function was inferred which was substantiated by its early temporal appearance. A similar ERP amplitude modulation was evident across a broader topographical region during the subsequent N400 time interval. Conversely the late posterior component (LPC; 500–800 ms) for deeply-encoded, correctly-recognized words was of higher amplitude than LPCs for shallowly-encoded and new words, consistent with proposals that this ERP component indexes episodic memory. To our knowledge this is the first report of an ERP correlate of auditory perceptual priming dissociated from explicit episodic memory.

Electrophysiological correlates of object-repetition effects: sLORETA imaging with 64-channel EEG and individual MRI

BACKGROUND

We investigated the electrophysiological correlates of object-repetition effects using an object categorization task, standardized low-resolution electromagnetic tomography (sLORETA), and individual magnetic resonance imaging. Sixteen healthy adults participated, and a total of 396 line drawings of living and non-living objects were used as stimuli. Of these stimuli, 274 were presented only once, and 122 were repeated after one to five intervening pictures. Participants were asked to categorize the objects as living or non-living things by pressing one of two buttons.

RESULTS

The old/new effect (i.e., a faster response time and more positive potentials in response to repeated stimuli than to stimuli initially presented) was observed at 350-550 ms post-stimulus. The distributions of cortical sources for the old and new stimuli were very similar at 250-650 ms after stimulus-onset. Activation in the right middle occipital gyrus/cuneus, right fusiform gyrus, left superior temporal gyrus, and right inferior frontal gyrus was significantly reduced in response to old compared with new stimuli at 250-350, 350-450, 450-550, and 550-650 ms after stimulus-onset, respectively. Priming in response time was correlated with the electrophysiological priming at left parietal area and repetition suppression at left superior temporal gyrus in 450-550 ms.

CONCLUSIONS

These results suggest processing of repeated objects is facilitated by sharpening perceptual representation and by efficient detection or attentional control of repeated objects.

Electrophysiological correlates of face distortion after-effects

When observers are exposed to a distorted face the perceived configuration of a subsequently presented face is altered, a phenomenon called face distortion after-effect (FDAE). We compared the face-related components of the event-related potential (ERP) after adaptation to noise images—veridical and distorted faces. We found large bilateral adaptation effects on the P100 and N170 components that are related to face detection. Moreover, we found smaller adaptation effects on the N170, recorded over the right hemisphere, which can be related to the behavioural distortion after-effect and to face configurations. Our results suggest that the observed ERP adaptation effects are general for various steps of face processing and that the FDAEs similarly to gender after-effects are related to the early face-specific ERP components.

Explicit and implicit memory in female college students with schizotypal traits: an event-related potential study

The explicit and implicit memory of nonclinical individuals with schizotypal traits was investigated using event-related potentials. Explicit and implicit memory was measured with continuous recognition and categorization tasks, respectively. On the recognition task, the control group demonstrated a greater old/new effect in response to the old than to the new words during the 250-750 ms post-stimulus period, whereas schizotypal trait group did not exhibit an old/new effect during the 550-650 ms period. The control group demonstrated faster response times to the old than to the new words, whereas the schizotypal group demonstrated longer response times to the old than to the new words. On the categorization task, both groups showed old/new effects during the 250-550 ms after stimulus onset and responded more rapidly and with fewer errors to the old than to the new words. These results suggest that individuals with schizotypal traits have impaired explicit but preserved implicit memory.

Temporal dynamics of memory-related effects in older and young adults: an event-related potential study

Priming effects on the identification process were examined in young and older adults by using event-related potentials (ERPs). Animals and artifacts were presented in an ascending sequence of filtered images, half of which had been shown in their complete versions in a previous study phase. Each stimulus was represented by a progressively less filtered image (i.e., more complete) until the whole version was revealed in a sequence of frames. Such a paradigm allowed us to record ERPs prior to, and during, the identification of stimuli. Results showed a dynamic interplay between memory, category, and aging effects. At the moment of identification, young adults elicited larger positivity at parietal sites for previously studied stimuli and this effect was not observed for older adults. For stimuli previously studied, a striking effect was observed in both groups at the level just prior to overt identification. In addition, a frontally distributed priming effect was evident in the elderly. Category-related ERP differences emerged between the two age groups. In particular, younger participants elicited an early positive activation at anterior sites upon seeing stimuli of animals. These results are discussed in relation to current models of recognition memory, categorization, and age-related cognitive decline.

Working memory load for faces modulates P300, N170, and N250r

We used event-related potential (ERP) methodology to examine neural activity associated with visual working memory (WM) for faces. There were two main goals. First, to extend previous findings of P300 load modulation to WM for faces. Second, to examine whether N170 and N250r are also influenced by WM load. Between one and four unfamiliar faces were simultaneously presented for memory encoding. After a 1-sec delay, a target face appeared, and participants had to judge whether this face was part of the previous face array. P300 amplitude decreased as WM load increased, and this P300 suppression was observed at both encoding and retrieval. WM load was also found to modulate other ERPs. The amplitude of the N170 elicited by the target face decreased with load, and this N170 decrease leveled off at load 2, reflecting the behavioral WM capacity of around two faces. In addition, the N250r, observed as an ERP difference for target faces that were present in the encoding array relative to target faces that were absent, was also reduced for higher WM loads. These findings extend previous work by showing that P300 modulation by WM load also occurs for faces. Furthermore, we show, for the first time, that WM load affects the N250r and the early visual N170 component. This suggests that higher visual areas play an important role in WM for faces.

Adaptation duration affects the spatial selectivity of facial aftereffects

Adaptation processes in human early visual cortical areas are sensitive to the exposure time of the adaptor stimulus. Here we investigated the effect of adaptation duration at the higher, shape-specific stages of visual processing using facial adaptation. It was found that long-term (5s) adaptation evokes facial aftereffects consisting of a position invariant as well as a position-specific component. As a result of adaptation to a female face, test faces were judged more masculine when they were displayed in the same location as the female adaptor face, as compared to that when they were presented in the opposite visual hemifield. However, aftereffects evoked by short-term (500 ms) adaptation were found to be entirely position invariant. In accordance with these behavioral results, we found that the adaptation effects, measured on the amplitude of the N170 ERP component consisted of a position-specific component only after long-term, but not after short-term adaptation conditions. These results suggest that both short and long exposure to a face stimulus leads to adaptation of position invariant face-selective processes, whereas adaptation of position-specific neural mechanisms of face processing requires long-term adaptation. Our findings imply that manipulating adaptation duration provides an opportunity to specifically adapt different neural processes of shape-specific coding and to investigate their stimulus selectivity.

Discontinuous presentation of ambiguous figures: How interstimulus-interval durations affect reversal dynamics and ERPs

If we observe an ambiguous figure, our percept is unstable and alternates between the possible interpretations. Periodically interrupting the presentation sizably modulates the spontaneous reversal rate. We here studied event-related potential (ERP) correlates of the neural processes underlying these strong modulations. An ambiguous Necker stimulus was presented discontinuously with four randomly varying interstimulus intervals (ISI; 14, 43, 130, 390 ms) while participants indicated perceptual reversals. EEG was selectively averaged with respect to the participants' percept and ISI. ERP traces varied markedly between ISIs. A simple model explained a major part of this variation and showed that the ISI-dependent ERP modulation occurs after disambiguation has already taken place. We suggest that perceptual stability (or reversal) depends on a system state, slowly changing from one reversal to the next. ISI can shift this state on a scale between stability and instability.

Do I know you? Insights into memory for faces from brain potentials

The recognition of faces is central to human social interaction. Recordings of event-related potentials (ERPs) from the brain can shed light on the various processes that occur when a face is recognized and when knowledge related to a specific person is retrieved. ERP contrasts between processing familiar and processing novel faces offer a gateway into investigations of semantic memory for familiar persons. In particular, activity of face recognition units and semantic information units--memory representations of faces and person-related knowledge, respectively--can be indexed by specific ERPs. These potentials thus provide valuable tools for studying the cognitive and neurobiological architecture of person recognition. ERPs have also been found useful for investigating other types of memory for faces. Specifically, important insights have been derived from the study of a category of memory phenomena known as priming. Priming can be revealed in special tests when face recognition is facilitated based on prior experience. Describing the neural processes associated with memory for faces is an exciting focus of research, and future results from this line of inquiry promise to provide further knowledge about face recognition and the various types of memory that can be provoked by a human face.

Neural correlates of perceptual contributions to nondeclarative memory for faces

Face priming is a nondeclarative memory phenomenon that can be observed when recognition is facilitated for a recently encountered face. This data-driven form of priming is distinct from conceptually driven priming. Moreover, it includes two dissociable components, the facilitated access to pre-existing representations and facilitation in perceptual processing of faces. In the present study, we measured neural correlates of perceptual contributions to face priming with event-related brain potentials. Faces appeared two times (separated by 7-17 s), while participants discriminated familiar from unfamiliar faces. Half of the initial face stimuli were inverted, thereby disrupting perceptual face processing and making possible an assessment of perceptual contributions to face priming. Whereas none of the brain waves previously linked to perceptual processing of faces showed indications of priming, such effects were observed between 200 and 600 ms at left occipito-parieto-temporal recording sites. This electrical activity was present for both unfamiliar and familiar faces. The scalp topography of this effect was consistent with sources within the temporal and occipital cortices of the left hemisphere (based on a LORETA source localization). These findings suggest that priming of perceptual face processing is subserved by prolonged neural activity from 200 to 600 ms primarily in the left hemisphere. We propose that this priming reflects facilitated selection based on second-order relations among facial features.

Neural correlates of intentional and incidental recognition of famous faces

Event-related potentials (ERPs) were used to study the relationship between intentional and incidental recognition of famous faces. Intentional and incidental recognition were operationally defined as repeated presentations of targets and nontargets within a modified Sternberg task. These repetitions elicited temporally and topographically distinct ERP modulations. A repetition effect around 300 ms (ERE/N250r) and a preceding modulation did not differ between intentional and incidental recognition, whereas a following repetition effect (LRE/N400) around 500 ms showed differences between incidental and intentional recognition. These results show that during the first few hundred milliseconds intentional and incidental face recognition relate to similar processing, indicating that familiar faces are recognized even when their identification is not required.