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Biomedical subjects

John J Foxe

Publications and source records attributed to John J Foxe.

At least 37 records · Page 2Linked to original sources
PubMed2005

Spatiotemporal dynamics of human object recognition processing: an integrated high-density electrical mapping and functional imaging study of "closure" processes.

Humans are capable of recognizing objects, often despite highly adverse viewing conditions (e.g., occlusion). The term "perceptual closure" has been used to refer to the neural processes responsible for "filling-in" missing information in the visual image under such conditions. Closure phenomena have been linked to a group of object recognition areas, the so-called lateral-occipital complex (LOC). Here, we investigated the spatiotemporal dynamics of perceptual closure processes by coregistering data from high-density electrical recordings (ERPs) and functional magnetic resonance imaging (fMRI) while subjects participated in a perceptual closure task. Subjects were presented with highly fragmented images and control scrambled images. Fragmented images were calibrated to be 'just' recognizable as objects (that is, perceptual closure was necessary), whereas the scrambled images were unrecognizable. Comparison of responses to these two stimulus classes revealed the neural processes underlying perceptual closure. fMRI revealed an object recognition system that mediates these closure processes, the core of which consists of the LOC regions. ERP recordings resulted in the well-characterized N(CL) component (for negativity associated with closure), a robust relative negativity over bilateral occipito-temporal scalp that occurs in the 230-400 ms timeframe. Our investigations further revealed an extended network of dorsal and frontal regions, also involved in perceptual closure processes. Inverse source analysis showed that the major generators of N(CL) localized to the identical regions within LOC revealed by the fMRI recordings and detailed the temporal dynamics across these LOC regions including interactions between LOC and these other nodes of the object recognition circuit.

Adult↗
PubMed2005

The brain uses single-trial multisensory memories to discriminate without awareness.

Multisensory experiences enhance perceptions and facilitate memory retrieval processes, even when only unisensory information is available for accessing such memories. Using fMRI, we identified human brain regions involved in discriminating visual stimuli according to past multisensory vs. unisensory experiences. Subjects performed a completely orthogonal task, discriminating repeated from initial image presentations intermixed within a continuous recognition task. Half of initial presentations were multisensory, and all repetitions were exclusively visual. Despite only single-trial exposures to initial image presentations, accuracy in indicating image repetitions was significantly improved by past auditory-visual multisensory experiences over images only encountered visually. Similarly, regions within the lateral-occipital complex-areas typically associated with visual object recognition processes-were more active to visual stimuli with multisensory than unisensory pasts. Additional differential responses were observed in the anterior cingulate and frontal cortices. Multisensory experiences are registered by the brain even when of no immediate behavioral relevance and can be used to categorize memories. These data reveal the functional efficacy of multisensory processing.

Acoustic Stimulation↗
PubMed2005

Biasing the brain's attentional set: II. effects of selective intersensory attentional deployments on subsequent sensory processing.

This study used high-density mapping of human event-related potentials to examine the brain activity associated with selective information processing when subjects were cued on a trial-by-trial basis to perform a discrimination in either the visual or auditory modality. On each trial, word-cues (S1) instructed subjects to attend to features within one sensory-modality of an impending compound auditory-visual stimulus (S2) that arrived approximately 1-second following the cue. Subjects made a discrimination within the cued modality of the S2 stimulus. The spatio-temporal patterns of activity in response to the compound S2 stimulus were examined as a function of the sensory modality being attended. The earliest effects of intersensory attention on visual processing were seen subsequent to the initial activation of visual cortex, beginning at 80 ms and continuing into the P1 and N1 components of the visual ERP. The scalp-topography of this earliest modulation was consistent with modulation of activity in ventral visual stream areas. Thus, the locus of effects on visual S2 processing differed from the anticipatory parieto-occipital biasing activity that preceded S2 presentation. This pattern of effects strongly suggests that the anticipatory activity (following the cue) associated with sustaining the focus of attention during intersensory attention, at least in the context of this paradigm, does not operate as a simple gain mechanism in early visual sensory areas. Rather, attentional biasing can operate through a higher-order process whereby parieto-occipital cortices influence the subsequent flow of visual processing in the ventral stream.

Acoustic Stimulation↗
PubMed2005

Biasing the brain's attentional set: I. cue driven deployments of intersensory selective attention.

Brain activity associated with directing attention to one of two possible sensory modalities was examined using high-density mapping of human event-related potentials. The deployment of selective attention was based on visually presented symbolic cue-words instructing subjects on a trial-by-trial basis, which sensory modality to attend. We measured the spatio-temporal pattern of activation in the approximately 1 second period between the cue-instruction and a subsequent compound auditory-visual imperative stimulus. This allowed us to assess the flow of processing across brain regions involved in deploying and sustaining inter-sensory selective attention, prior to the actual selective processing of the compound audio-visual target stimulus. Activity over frontal and parietal areas showed sensory specific increases in activation during the early part of the anticipatory period (~230 ms), probably representing the activation of fronto-parietal attentional deployment systems for top-down control of attention. In the later period preceding the arrival of the "to-be-attended" stimulus, sustained differential activity was seen over fronto-central regions and parieto-occipital regions, suggesting the maintenance of sensory-specific biased attentional states that would allow for subsequent selective processing. Although there was clear sensory biasing in this late sustained period, it was also clear that both sensory systems were being prepared during the cue-target period. These late sensory-specific biasing effects were also accompanied by sustained activations over frontal cortices that also showed both common and sensory specific activation patterns, suggesting that maintenance of the biased state includes top-down inputs from generators in frontal cortices, some of which are sensory-specific regions. These data support extensive interactions between sensory, parietal and frontal regions during processing of cue information, deployment of attention, and maintenance of the focus of attention in anticipation of impending attentionally relevant input.

Acoustic Stimulation↗
PubMed2005

How single-trial electrical neuroimaging contributes to multisensory research.

This study details a method to statistically determine, on a millisecond scale and for individual subjects, those brain areas whose activity differs between experimental conditions, using single-trial scalp-recorded EEG data. To do this, we non-invasively estimated local field potentials (LFPs) using the ELECTRA distributed inverse solution and applied non-parametric statistical tests at each brain voxel and for each time point. This yields a spatio-temporal activation pattern of differential brain responses. The method is illustrated here in the analysis of auditory-somatosensory (AS) multisensory interactions in four subjects. Differential multisensory responses were temporally and spatially consistent across individuals, with onset at approximately 50 ms and superposition within areas of the posterior superior temporal cortex that have traditionally been considered auditory in their function. The close agreement of these results with previous investigations of AS multisensory interactions suggests that the present approach constitutes a reliable method for studying multisensory processing with the temporal and spatial resolution required to elucidate several existing questions in this field. In particular, the present analyses permit a more direct comparison between human and animal studies of multisensory interactions and can be extended to examine correlation between electrophysiological phenomena and behavior.

Acoustic Stimulation↗
PubMed2005

Sensory contributions to impaired prosodic processing in schizophrenia.

BACKGROUND: Deficits in affect recognition are prominent features of schizophrenia. Within the auditory domain, patients show difficulty in interpreting vocal emotional cues based on intonation (prosody). The relationship of these symptoms to deficits in basic sensory processing has not been previously evaluated. METHODS: Forty-three patients and 34 healthy comparison subjects were tested on two affective prosody measures: voice emotion identification and voice emotion discrimination. Basic auditory sensory processing was measured using a tone-matching paradigm and the Distorted Tunes Test (DTT). A subset of subjects was also tested on facial affect identification and discrimination tasks. RESULTS: Patients showed significantly impaired performance on all emotion processing tasks. Within the patient group, a principal components analysis demonstrated significant intercorrelations between basic pitch perception and affective prosodic performance. In contrast, facial affect recognition deficits represented a distinct second component. Prosodic affect measures correlated significantly with severity of negative symptoms and impaired global outcome. CONCLUSIONS: These results demonstrate significant relationships between basic auditory processing deficits and impaired receptive prosody in schizophrenia. The separate loading of auditory and visual affective recognition measures suggests that within-modality factors may be more significant than cross-modality factors in the etiology of affect recognition deficits in schizophrenia.

Adult↗
PubMed2005

The case for feedforward multisensory convergence during early cortical processing.

The prevailing hierarchical model of sensory processing in the brain holds that different modalities of sensory information emanating from a single object are analyzed extensively during passage through their respective unisensory processing streams before they are combined in higher-order 'multisensory' regions of the cortex. Because of this view, multisensory interactions that have been found at early, putatively 'unisensory' cortical processing stages during hemodynamic imaging studies have been assumed to reflect feedback modulations that occur subsequent to multisensory processing in the higher-order multisensory areas. In this paper, we consider findings that challenge an exclusively feedback interpretation of early multisensory integration effects. First, high-density electrical mapping studies in humans have shown that multisensory convergence and integration effects can occur so early in the time course of sensory processing that purely feedback mediation becomes extremely unlikely. Second, direct neural recordings in monkeys show that, in some cases, convergent inputs at early cortical stages have physiological profiles characteristic of feedforward rather than feedback inputs. Third, damage to higher-order integrative regions in humans often spares the ability to integrate across sensory modalities. Finally, recent anatomic tracer studies have reported direct anatomical connections between primary visual and auditory cortex. These findings make it clear that multisensory convergence at early stages of cortical processing results from feedforward as well as feedback and lateral connections, thus using the full range of anatomical connections available in brain circuitry.

Animals↗
PubMed2005

Can whole brain nerve conduction velocity be derived from surface-recorded visual evoked potentials? A re-examination of Reed, Vernon, and Johnson (2004).

Reed, Vernon, and Johnson [Reed, T. E., Vernon, P. A., & Johnson, A. M. (2004). Sex difference in brain nerve conduction velocity in normal humans. Neuropsychologia, 42, 1709-1714] reported that "nerve conduction velocity" (NCV) of visual transmission from retina to the primary visual area (V1) is significantly faster in males than females. The authors estimated the NCV by dividing head length (nasion-to-inion distance) by the latency of the well-known P100 component of the visual evoked potential (VEP). Here, we critically examine these metrics and we contend that knowledge of the underlying physiology of neural transmission across the initial stages of the visual processing hierarchy dictates that a number of their assumptions cannot be reasonably upheld. Alternative, and we believe, more parsimonious interpretations of the data are also proposed.

Evoked Potentials, Visual↗
PubMed2005

Filling-in in schizophrenia: a high-density electrical mapping and source-analysis investigation of illusory contour processing.

Evidence is accumulating that patients with schizophrenia exhibit relatively severe deficits in early visual sensory processing within the dorsal stream, while processing within the ventral stream appears to be relatively more intact. Here, illusory contour (IC) processing was investigated in a cohort of schizophrenia patients and age-matched healthy controls using high-density visual evoked potentials (VEPs), spatiotemporal topographic analyses and the Local Auto-Regressive Average distributed linear inverse source estimation. IC processing was assessed because it is now known to be an excellent metric of early processing within regions of the ventral visual stream. Results in the present study show that IC processing (106-194 ms) is spared in patients with schizophrenia, providing strong evidence that early ventral stream processing is essentially normal. This is so despite equally strong evidence that early dorsal stream processing is severely impaired in this population, as indexed by a robust decrement in amplitude of the P1 component in patients and a large topographic difference between groups for this component (54-104 ms). Source analysis confirmed that the flow of activity into the dorsal stream was substantially decreased in patients. As such, these results suggest that some aspects of early ventral processing are not entirely reliant on intact inputs from the dorsal stream. Lastly, we show that later phases of visual processing (240-400 ms) also rely on the activity of different brain networks in controls and patients, with the latter recruiting strong frontal activity perhaps as compensation for impaired ventral stream processing during this period. We interpret the present findings in the context of a two-stage processing model. Under this model, it is suggested that the second stage of ventral stream processing is dependent on the fidelity of inputs from the dorsal visual stream and that impairment of this critical modulatory input may underlie the failure of 'higher-level' ventral stream processes in this population.

Adult↗
PubMed2005

Individual differences discriminate event-related potentials but not performance during response inhibition.

Event-related brain potentials (ERPs) were recorded from 20 normal participants while they completed a Go/NoGo response inhibition task. Previous ERP studies have implicated the N2 and P3 waveforms as the main indices of processing in this task, and functional brain imaging has shown parietal, prefrontal and anterior cingulate cortices to be involved in response inhibition. 32-channel ERP analysis revealed amplitude differences in the N2/P3 components when stimuli that required a button-press (Go stimuli) were compared with stimuli for which the response had to be withheld (No-Go stimuli), and in N2 and P3 latencies when successful withholds to No-Go stimuli were compared with unsuccessful attempts to inhibit. Further differences in the N2/P3 complex emerged when participants were grouped in terms of a measure of absentmindedness (the Cognitive Failures Questionnaire, CFQ); larger and earlier components were found for high CFQ respondents. We conclude that the latencies of the N2 and P3 may be the critical indicators of active inhibitory processes for this task, suggesting that a pattern of sequential activation rather than altered activity level in key structures may mediate success on the task. In addition, highly absentminded participants exhibited larger components for errors than did less absentminded participants when performing at the same level, which implies that the absentminded may require greater activity in the neural substrates of response inhibition in order to accomplish this task at a comparable level of performance to less absentminded participants.

Adolescent↗
PubMed2005

Neural mechanisms involved in error processing: a comparison of errors made with and without awareness.

The ability to detect an error in one's own performance and then to improve ongoing performance based on this error processing is critical for effective behaviour. In our event-related fMRI experiment, we show that explicit awareness of a response inhibition commission error and subsequent post-error behaviour were associated with bilateral prefrontal and parietal brain activation. Activity in the anterior cingulate region, typically associated with error detection, was equivalent for both errors subjects were aware of and those they were not aware of making. While anterior cingulate activation has repeatedly been associated with error-related processing, these results suggest that, in isolation, it is not sufficient for conscious awareness of errors or post-error adaptation of response strategies. Instead, it appears, irrespective of awareness, to detect information about stimuli/responses that requires interpretation in other brain regions for strategic implementation of post-error adjustments of behaviour.

Adult↗
PubMed2005

Neurophysiological markers of alert responding during goal-directed behavior: a high-density electrical mapping study.

The ability to dynamically modulate the intensity of sustained attention (i.e., alertness) is an essential component of the human executive control system, allowing us to function purposefully in accordance with our goals. In this study we examine high-density ERP markers of alert responding during the fixed sequence sustained attention to response task (SART(fixed)). This paradigm has proven to be a sensitive clinical metric in patient populations with deficits in their ability to sustain attention (e.g., attention deficit hyperactivity disorder). In this task subjects withhold a button press to an infrequent no-go target ('3') embedded within a predictable sequence of numbers ('1' to '9'). Our data reveal a complex pattern of effects across the trial sequence of the SART, with clear contributions from frontal and parietal cortices to sustained attentional performance. Over occipito-parietal regions, early visual attention processes were increased during trial 2 (i.e., trial in which the digit '2' was presented) and trial 3, giving rise to the so-called selection negativity (SN). Two prominent late components were manifest during trial 2: LP1 (550-800 ms) and LP2 (850-1150 ms) over occipito-parietal and central sites. We interpret the LP1 component on trial 2 as reflecting retrieval of the task goal and the subsequent LP2 as reflecting competition between the currently relevant go response and the subsequent no-go response. On trial 3, an enhanced "no-go N2" (250-450 ms) was seen fronto-centrally in the absence of the "no-go P3" that typically follows. Fronto-polar activity was also seen across all trials and may be indicative of subgoal processes to integrate the association between stimulus and goal. Prior to a lapse of attention (i.e., failure to inhibit a response to "3") the LP1 was significantly attenuated on the preceding trial 2 indicating a failure of anticipatory goal-directed processing. The results are discussed in terms of models of sustained attention involving frontal and parietal cortices.

Adolescent↗
PubMed2005

Visual spatial attention tracking using high-density SSVEP data for independent brain-computer communication.

The steady-state visual evoked potential (SSVEP) has been employed successfully in brain-computer interface (BCI) research, but its use in a design entirely independent of eye movement has until recently not been reported. This paper presents strong evidence suggesting that the SSVEP can be used as an electrophysiological correlate of visual spatial attention that may be harnessed on its own or in conjunction with other correlates to achieve control in an independent BCI. In this study, 64-channel electroencephalography data were recorded from subjects who covertly attended to one of two bilateral flicker stimuli with superimposed letter sequences. Offline classification of left/right spatial attention was attempted by extracting SSVEPs at optimal channels selected for each subject on the basis of the scalp distribution of SSVEP magnitudes. This yielded an average accuracy of approximately 71% across ten subjects (highest 86%) comparable across two separate cases in which flicker frequencies were set within and outside the alpha range respectively. Further, combining SSVEP features with attention-dependent parieto-occipital alpha band modulations resulted in an average accuracy of 79% (highest 87%).

Adult↗
PubMed2004

Kanizsa subjective figures capture visual spatial attention: evidence from electrophysiological and behavioral data.

Figural binding and attention are two important processes that help to perceive the outside world. Binding is necessary to link together the different features of single objects which are represented in a distributed fashion in the brain. Attention serves to focus onto a small subset of incoming information. It is still not clear how exactly these two mechanisms operate and interact. We performed two experiments employing illusory Kanizsa figures (KFs) to investigate the temporal order of figural binding and spatial attention. In a visual search task, subjects had to detect the presence of a KF among distractor stimuli. We found only a slight increase of reaction times when increasing the number of distractors, indicating that KFs popped out and drew the perceiver's attention. In a further event-related potential (ERP) study, we used displays of the search task as non-informative cue for a subsequent target choice-reaction task. Enhanced contralateral negative amplitudes (starting at about 230 ms) over ventral occipital areas were found for cue displays which included a KF. For target stimuli, faster reaction times and enhanced ipsilateral N1 amplitudes over occipito-parietal areas were observed for validly (target presentation inside a KF) as compared to invalidly cued targets (target presentation outside a KF). Furthermore, enhanced contralateral N1 amplitudes were found for invalidly cued targets. It might be that interactions between perceptual closure processing of the ventral pathway and spatial target processing of the dorsal pathway contributed to the present result. We conclude that KFs automatically capture spatial attention when used as visual cues.

Adult↗
PubMed2004

Grabbing your ear: rapid auditory-somatosensory multisensory interactions in low-level sensory cortices are not constrained by stimulus alignment.

Multisensory interactions are observed in species from single-cell organisms to humans. Important early work was primarily carried out in the cat superior colliculus and a set of critical parameters for their occurrence were defined. Primary among these were temporal synchrony and spatial alignment of bisensory inputs. Here, we assessed whether spatial alignment was also a critical parameter for the temporally earliest multisensory interactions that are observed in lower-level sensory cortices of the human. While multisensory interactions in humans have been shown behaviorally for spatially disparate stimuli (e.g. the ventriloquist effect), it is not clear if such effects are due to early sensory level integration or later perceptual level processing. In the present study, we used psychophysical and electrophysiological indices to show that auditory-somatosensory interactions in humans occur via the same early sensory mechanism both when stimuli are in and out of spatial register. Subjects more rapidly detected multisensory than unisensory events. At just 50 ms post-stimulus, neural responses to the multisensory 'whole' were greater than the summed responses from the constituent unisensory 'parts'. For all spatial configurations, this effect followed from a modulation of the strength of brain responses, rather than the activation of regions specifically responsive to multisensory pairs. Using the local auto-regressive average source estimation, we localized the initial auditory-somatosensory interactions to auditory association areas contralateral to the side of somatosensory stimulation. Thus, multisensory interactions can occur across wide peripersonal spatial separations remarkably early in sensory processing and in cortical regions traditionally considered unisensory.

Adult↗
PubMed2004

The neural circuitry of pre-attentive auditory change-detection: an fMRI study of pitch and duration mismatch negativity generators.

Electrophysiological studies have revealed a pre-attentive change-detection system in the auditory modality. This system emits a signal termed the mismatch negativity (MMN) when any detectable change in a regular pattern of auditory stimulation occurs. The precise intracranial sources underlying MMN generation, and in particular whether these vary as a function of the acoustic feature that changes, is a matter of some debate. Using functional magnetic resonance imaging, we show that anatomically distinct networks of auditory cortices are activated as a function of the deviating acoustic feature--in this case, tone frequency and tone duration--strongly supporting the hypothesis that MMN generators in auditory cortex are feature dependent. We also detail regions of the frontal and parietal cortices activated by change-detection processes. These regions also show feature dependence and we hypothesize that they reflect recruitment of attention-switching mechanisms.

Adult↗