Visual priming within and across symbolic format using a tachistoscopic picture identification task: a PET study

Validity of an eyetracking method for capturing auditory-visual cross-format semantic priming

INTRODUCTION

Semantic priming paradigms are important for understanding lexical-semantic processing and the nature of linguistic deficits accompanying language performance in neurologically impaired individuals such as people with aphasia. Reaction-time-(RT)-based traditional semantic priming tasks entail potential confounds, especially problematic when applied to people with aphasia, who may have concomitant neurocognitive challenges that limit task performance. Some of these confounds include requirements of following complex instructions, making metalinguistic judgments, and using speech or limb-based motor actions to indicate overt responses. Eyetracking methods have great potential for avoiding some of these confounds. We tested the validity of an eyetracking method in capturing semantic priming in an auditory-visual cross-format priming paradigm (auditory word prime-visual image target).

METHOD

A total of 72 neurologically unimpaired adults participated in two phases: a stimulus development phase using traditional priming (n = 32) and an experimental eyetracking phase (n = 40). Each phase included two conditions, representing distinct levels of prime-target semantic relatedness: unrelated and related. Mean RT data from the traditional priming (stimuli development) phase guided image selection for the eyetracking experiment. Eyetracking indices of fixation duration and latency of fixation were recorded to capture semantic priming in the eyetracking experiment.

RESULTS

Eye fixation data indicated that images related to auditory primes were attended to earlier and attracted significantly greater visual attention than unrelated images. These results mirrored RT data from the traditional priming method, which showed faster RT latencies and more accurate naming performance for related images than for unrelated images.

CONCLUSIONS

Results support the validity of eyetracking indices of semantic priming and offer a robust testing protocol for future studies in this line of research. Current clinical relevance for people with aphasia is highlighted. Further empirical testing of the psychometric properties of the eyetracking measures in various semantic priming contexts is recommended.

Does N200 reflect semantic processing?--An ERP study on Chinese visual word recognition

Recent event-related potential research has reported a N200 response or a negative deflection peaking around 200 ms following the visual presentation of two-character Chinese words. This N200 shows amplitude enhancement upon immediate repetition and there has been preliminary evidence that it reflects orthographic processing but not semantic processing. The present study tested whether this N200 is indeed unrelated to semantic processing with more sensitive measures, including the use of two tasks engaging semantic processing either implicitly or explicitly and the adoption of a within-trial priming paradigm. In Exp. 1, participants viewed repeated, semantically related and unrelated prime-target word pairs as they performed a lexical decision task judging whether or not each target was a real word. In Exp. 2, participants viewed high-related, low-related and unrelated word pairs as they performed a semantic task judging whether each word pair was related in meaning. In both tasks, semantic priming was found from both the behavioral data and the N400 ERP responses. Critically, while repetition priming elicited a clear and large enhancement on the N200 response, semantic priming did not show any modulation effect on the same response. The results indicate that the N200 repetition enhancement effect cannot be explained with semantic priming and that this specific N200 response is unlikely to reflect semantic processing.

Phonological facilitation of object naming in agrammatic and logopenic primary progressive aphasia (PPA)

Phonological processing deficits are characteristic of both the agrammatic and logopenic subtypes of primary progressive aphasia (PPA-G and PPA-L). However, it is an open question which substages of phonological processing (i.e., phonological word form retrieval, phonological encoding) are impaired in these subtypes of PPA, as well as how phonological processing deficits contribute to anomia. In the present study, participants with PPA-G (n = 7), participants with PPA-L (n = 7), and unimpaired controls (n = 17) named objects as interfering written words (phonologically related/unrelated) were presented at different stimulus onset asynchronies (SOAs) of 0, +100, +300, and +500 ms. Phonological facilitation (PF) effects (faster naming times with phonologically related interfering words) were found for the controls and PPA-L group only at SOA = 0 and +100 ms. However, the PPA-G group exhibited protracted PF effects (PF at SOA = 0, +100, and +300 ms). These results may reflect deficits in phonological encoding in PPA-G, but not in PPA-L, supporting the neuropsychological reality of this substage of phonological processing and the distinction between these two PPA subtypes.

Validity of eye movement methods and indices for capturing semantic (associative) priming effects

PURPOSE

The purpose of this investigation was to evaluate the usefulness of eye movement methods and indices as a tool for studying priming effects by verifying whether eye movement indices capture semantic (associative) priming effects in a visual cross-format (written word to semantically related picture) priming paradigm.

METHOD

In the stimuli development phase, words semantically associated to an array of pictures were generated based on 100 adults' association data for each picture. A total of 40 additional adult participants with normal language engaged in an eye movement experiment using the word-picture associations developed previously. The design consisted of each prime preceding a display showing 1 high-association (target) and 2 low-association (nontarget) images. Fixation durations, locations, and latencies were measured.

RESULTS

Images semantically related to the prime showed greater fixation durations and shorter latencies compared to nontargets. Eye movement and traditional reaction time measures were found to correlate for some of the experimental conditions.

CONCLUSIONS

Results showed that free-viewing eye movement measures, in which participants are not instructed to look at anything in particular, hold promise as valid indicators of priming effects. Further research in this area will help to advance language-processing theories in individuals with and without language impairment.

Seeing and hearing meaning: ERP and fMRI evidence of word versus picture integration into a sentence context

Understanding language always occurs within a situational context and, therefore, often implies combining streams of information from different domains and modalities. One such combination is that of spoken language and visual information, which are perceived together in a variety of ways during everyday communication. Here we investigate whether and how words and pictures differ in terms of their neural correlates when they are integrated into a previously built-up sentence context. This is assessed in two experiments looking at the time course (measuring event-related potentials, ERPs) and the locus (using functional magnetic resonance imaging, fMRI) of this integration process. We manipulated the ease of semantic integration of word and/or picture to a previous sentence context to increase the semantic load of processing. In the ERP study, an increased semantic load led to an N400 effect which was similar for pictures and words in terms of latency and amplitude. In the fMRI study, we found overlapping activations to both picture and word integration in the left inferior frontal cortex. Specific activations for the integration of a word were observed in the left superior temporal cortex. We conclude that despite obvious differences in representational format, semantic information coming from pictures and words is integrated into a sentence context in similar ways in the brain. This study adds to the growing insight that the language system incorporates (semantic) information coming from linguistic and extralinguistic domains with the same neural time course and by recruitment of overlapping brain areas.

Increased neural efficiency in the temporal association cortex as the result of semantic task repetition

We examined the effect of semantic task repetition and of alternating between tasks on cerebral blood flow in three H(2) (15)O positron emission tomography experiments. We found that repeatedly performing semantic tasks resulted in a reduction in cerebral blood flow to the left temporal association cortex similar to that found in priming experiments even though here there was no repetition of stimuli. Although the same effect was found in two different tasks (word meaning judgments and picture naming), it was only present when the same task was repeated on consecutive scans and not when the subjects alternated from scan to scan between tasks. We propose that there is a neural efficiency which develops in the association cortex of the temporal lobe as a result of repeatedly performing a semantic task. This efficiency is abolished by interruptions such as performing a different task.

Spoken word memory traces within the human auditory cortex revealed by repetition priming and functional magnetic resonance imaging

Previous neuroimaging studies in the visual domain have shown that neurons along the perceptual processing pathway retain the physical properties of written words, faces, and objects. The aim of this study was to reveal the existence of similar neuronal properties within the human auditory cortex. Brain activity was measured using functional magnetic resonance imaging during a repetition priming paradigm, with words and pseudowords heard in an acoustically degraded format. Both the amplitude and peak latency of the hemodynamic response (HR) were assessed to determine the nature of the neuronal signature of spoken word priming. A statistically significant stimulus type by repetition interaction was found in various bilateral auditory cortical areas, demonstrating either HR suppression and enhancement for repeated spoken words and pseudowords, respectively, or word-specific repetition suppression without any significant effects for pseudowords. Repetition latency shift only occurred with word-specific repetition suppression in the right middle/posterior superior temporal sulcus. In this region, both repetition suppression and latency shift were related to behavioral priming. Our findings highlight for the first time the existence of long-term spoken word memory traces within the human auditory cortex. The timescale of auditory information integration and the neuronal mechanisms underlying priming both appear to differ according to the level of representations coded by neurons. Repetition may "sharpen" word-nonspecific representations coding short temporal variations, whereas a complex interaction between the activation strength and temporal integration of neuronal activity may occur in neuronal populations coding word-specific representations within longer temporal windows.

MNESIS: towards the integration of current multisystem models of memory

After a brief description of the "diseases of memory" which have made the greatest contribution to theoretical developments in the past years, we turn our attention to the most important concepts to have arisen from the dissociations brought to light in different neuropsychological syndromes. This is followed by a critical review of the tasks currently used to assess each memory system. We then describe the monohierarchical model proposed by E. Tulving (Philos Trans R Soc Lond, B, Biol, Sci, 356:1505-1515, 2001), together with other recent concepts, notably Baddeley's model of working memory with its latest component, the episodic buffer. Lastly, we attempt to reconcile these models with several other theoretical propositions, which we have linked together in a macromodel--the Memory NEo-Structural Inter-Systemic model (MNESIS).

Magnetic Stimulation of the Right Visual Cortex Impairs Form-specific Priming

Recent evidence suggests that priming of objects across different images (abstract priming) and priming of specific images of an object (form-specific priming) are mediated by dissociable neural processing subsystems that operate in parallel and are predominantly linked to left and right hemispheric processing, respectively [Marsolek, C. J. Dissociable neural subsystems underlie abstract and specific object recognition. Psychological Science, 10, 111–118, 1999]. Previous brain imaging studies have provided important information about the neuroanatomical regions that are involved in form-specific and abstract priming; however, these techniques did not fully establish the functional significance of priming-related changes in cortical brain activity. Here, we used repetitive transcranial magnetic stimulation (rTMS) in order to establish the functional role of the right occipital cortex in form-specific priming [Kroll, N. E. A., Yonelinas, A. P., Kishiyama, M. M., Baynes, K., Knight, R. T., & Gazzaniga, M. S. The neural substrates of visual implicit memory: Do the two hemispheres play different roles? Journal of Cognitive Neuroscience, 15, 833–842, 2003]. Compared to no TMS and sham TMS, rTMS of the right occipital cortex disrupted immediate form-specific priming in a semantic categorization task. Left occipital rTMS, on the other hand, had no converse effect on abstractive priming. Abstract priming may involve deeper semantic processing and may be unresponsive to magnetic stimulation of a single cortical locus. Our TMS results show that form-specific priming relies on a visual word-form system localized in the right occipital lobe, in line with the predictions from divided visual field behavioral studies [Marsolek, 1999].

Brain correlates of negative and positive visuospatial priming in adults

A balance of inhibitory and facilitatory mechanisms is essential for efficient and goal-directed behaviors. These mechanisms may go awry in several neuropsychiatric disorders characterized by uncontrolled, repetitive behaviors. The visuospatial priming paradigm is a well-established probe of inhibition and facilitation that has been used to demonstrate behavioral deficits in patients with Tourette syndrome and obsessive-compulsive disorder. However, the brain correlates of this visuospatial priming paradigm are not yet well established. In the present study, we used a visuospatial priming paradigm and event-related functional MRI, to probe inhibitory and facilitatory brain mechanisms in healthy adult women. When subjects performed the negative priming (i.e., inhibitory) task, several regions of the prefrontal cortex were selectively activated relative to the neutral condition. Non-overlapping regions of the prefrontal cortex were deactivated in the positive priming condition. These results support the notion that the prefrontal cortex is involved in both inhibitory and facilitatory processing and demonstrate that this visuospatial priming task shares brain correlates with other positive and negative priming tasks. In conjunction with functional MRI, this visuospatial priming task may be useful for studying the pathophysiology of neuropsychiatric disorders in which deficient inhibitory processing or excessive facilitation is a feature.

Selecting among competing alternatives: selection and retrieval in the left inferior frontal gyrus

It has been widely argued that the left inferior frontal gyrus (LIFG) is involved in the control of retrieval of information from long-term memory. Recent claims that the LIFG is involved in selecting among semantic alternatives have been challenged on the grounds that the manipulation of selection demands may have been confounded with controlled retrieval. The current study used an event-related functional magnetic resonance paradigm to re-examine the possibility that LIFG activation is involved in selection processes. In order to minimize potential confounding effects of controlled retrieval, we used an automatic retrieval task (picture naming) and held retrieval demands constant, while varying selection demands by way of competitor priming from earlier semantically related trials. We found significant activation in LIFG as a function of increased selection demands; activation centred on two peaks, one in anterior LIFG and a second more superior and posterior region. These data support the view that LIFG plays a role in selection among semantic information, even in the absence of controlled retrieval processes.

Differential activation of object-selective visual areas by passive viewing of pictures and words

Functional imaging has shown that pictures of faces (N. Kanwisher, J. McDermott, M.M. Chun, The fusiform face area: a module in human extrastriate cortex specialized for face perception, J. Neurosci. 17 (1997) 4302-4311) and buildings (R. Epstein, N. Kanwisher, A cortical representation of the local visual environment, Nature 391 (1998) 598-601) activate different regions within the lateral occipital complex (LOC). To investigate effects of stimulus format on activation in these areas, we used event-related fMRI to measure brain activity during the passive viewing of pictures showing buildings and faces, and of words identifying these pictures. Consistent with earlier findings, pictures of faces activated bilateral regions in the lateral fusiform gyri, whereas pictures of buildings activated bilateral regions in the parahippocampal gyri. Analyzing the activation elicited by visually presenting the written names of the pictures, however, we did not find an effect of word meaning on the fMRI signal change in these areas: fMRI signal changes for the names of faces and the names of buildings did not differ in any of the areas selectively activated by the corresponding pictures. In general, both word conditions and non-preferred picture conditions elicited similar signal amplitudes. While presentation of words did not lead to strong activation in object-specific areas, activation for words of both categories was found in the left occipito-temporal cortex, close to the location which has tentatively been called 'visual word form area' (L. Cohen, S. Dehaene, L. Naccache, S. Lehéricy, G. Dehaene-Lambertz, et al., The visual word form are: spatial and temporal characterization of an initial stage of reading in normal subjects and posterior split-brain patients, Brain 123 (2000) 291-307), revealing that words were processed effectively during the experiment. Taken together, these results show that names of pictures do not automatically activate the corresponding object-selective areas.

The neural correlate of very-long-term picture priming

Repetition priming denotes a behavioural change caused by prior exposure to a stimulus. The effect is known to last for weeks. This study addresses the underlying neural mechanisms for very-long-term picture priming by using event-related functional magnetic resonance imaging complemented by a behavioural paradigm. Previous functional imaging studies with shorter retention intervals have shown that priming is associated with changes in the activity of both the occipital and posterior temporal cortex. In this study we compared retention intervals of 1 day and 6 weeks after initial exposure to a picture stimulus. Priming-related decreases in cortical activity in posterior extrastriate and dorsal left inferior frontal areas were found only for the shorter retention interval. In contrast, fMRI activation in the inferior posterior temporal and anterior left inferior frontal cortex was reduced following priming for both retention intervals. In the behavioural paradigm, the priming effect was stable over time. We conclude that the left inferior frontal and inferior posterior temporal cortex play a key role in the very-long-term priming effect.

Behavioral and Neurofunctional Changes Over Time in Healthy and Aphasic Subjects

Background and Purpose— Follow-up neuroimaging studies of aphasia never addressed a comparison between aphasic and healthy subjects. Investigation of changes over time in healthy subjects during language tasks seems a prerequisite before interpretation of longitudinal changes in aphasic patients.

Methods— Six healthy subjects and 8 aphasic patients were PET scanned twice (PET1 and PET2) at a 1-year interval during a word generation task. Using SPM99, language-rest main effect was compared at PET1 and PET2 in each group, whereas group effect was assessed at each session. Correlations were analyzed in each group between performance indexes and changes in regional cerebral flood flow (rCBF) between the 2 sessions.

Results— Language performances were improved in both groups. rCBF decreased from PET1 to PET2 in the healthy group and increased in the aphasic group in perisylvian regions bilaterally. Correlations between performance and rCBF changes across sessions were similar in the 2 groups; positive correlations involved superior temporal cortexes bilaterally, and negative correlations concerned superior frontal and medial temporal regions.

Conclusions— Increased perisylvian activation over time probably reflects improved performance at the expenses of cognitive effort in aphasic patients. Decreased activation in different neural systems suggests a familiarization effect with reduced emotional load.