Keeping It Short: A Comparison of Methods for Brief Picture Presentation

Further evidence and theoretical framework for a subliminal sensory buffer store (SSBS)

We recently provided evidence that strongly masked stimuli are not erased or overwritten but are briefly stored in a subliminal sensory buffer store (SSBS), where information can accumulate through repetition and become consciously accessible. SSBS supports a direct prediction made by the global workspace theory of consciousness (GWT) and has implications on discussions about conscious overflow and the problem of the criterion. Here we show that the presentation sequence and the time from the target presentation to evaluation does not significantly impact perception. We suggest that selected information from this subliminal sensory buffer store is transferred into a type of supraliminal short-term memory that keeps stable representations for longer durations with full conscious access. We argue that the level of conscious access of memory storage has a greater impact on subsequent reportability than initial phenomenology and needs to be included more prominently in discussions on perception and consciousness.

Luminance and timing control during visual presentation of natural scenes

In the study of visual cognition, accurate control of stimulus presentation is of primary importance yet is complicated by hardware malfunctioning, software variability, and visual materials used. Here, we describe VISTO 2.0, a low-cost and open-source device which is capable to measure the timing and temporal luminance profile of visual stimuli. This device represents a major improvement over VISTO (De Cesarei, Marzocchi, & Loftus, 2021), as it is only sensitive to a light spectrum in the visible range, is easier to assemble, and has a modular design that can be extended to other sensory modalities.

The Hypothesis of Subliminal Cue Reactivity in Addiction Revisited: An fMRI Study

INTRODUCTION

Exposure to conditioned cues is a common trigger of relapse in addiction. It has been suggested that such cues can activate motivationally relevant neurocircuitry in individuals with substance use disorders even without being consciously perceived. We aimed to see if this could be replicated in a sample with severe amphetamine use disorder and a control group of healthy subjects.

METHODS

We used fMRI to test the hypothesis that individuals with amphetamine use disorder, but not healthy controls, exhibit a specific neural reactivity to subliminally presented pictures related to amphetamine use. Twenty-four amphetamine users and 25 healthy controls were recruited and left data of sufficient quality to be included in the final analysis. All subjects were exposed to drug-related and neutral pictures of short duration (13.3 ms), followed by a backward visual mask image. The contrast of interest was drug versus neutral subliminal pictures.

RESULTS

There were no statistically significant differences in BOLD signal between the drug and neutral cues, neither in the limbic regions of primary interest nor in exploratory whole-brain analyses. The same results were found both in amphetamine users and controls.

DISCUSSION/CONCLUSION

We found no evidence of neural reactivity to subliminally presented drug cues in this sample of subjects with severe amphetamine dependence. These results are discussed in relation to the earlier literature, and the evidence for subliminal drug cue reactivity in substance use disorders is questioned.

VISTO: An open-source device to measure exposure time in psychological experiments

The study of higher cognitive processes often relies on the manipulation of bottom-up stimulus characteristics such as exposure time. While several software exist that can schedule the onset and offset time of a visual stimulus, the actual exposure time depends on several factors that are not easy to control, resulting in undesired variability within and across studies. Here we present VISTO, a simple device built on the Arduino platform that allows one to measure the exact onset and offset of a visual stimulus, and to test its synchronization with a trigger signal. The device is used to measure the profile of luminance waveforms in arbitrary analog/digital (AD) units, and the implications of these luminance profiles are discussed based on a model of information accumulation from visual exposure. Moreover, VISTO can be calibrated to match the brightness of each experimental monitor. VISTO allows for control of stimulus timing presentation, both in classical laboratory settings and in more complex settings as technology allows to use new display devices or acquisition equipment. In sum, VISTO allows one to:•

measure the profile of luminance curves.

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determine the exposure time of a visual stimulus.

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measure the synchronization between a trigger signal and a visual stimulus.

Unconscious processing of subliminal stimuli in panic disorder: A systematic review and meta-analysis

Attentional biases to threat exist in panic disorder (PD), probably related to altered subliminal processing. We systematically reviewed studies investigating subliminal processing in PD. Studies were retrieved from MEDLINE and Scopus®. We meta-analytically compared PD (n = 167) and healthy controls (HC, n = 165) for processing of masked panic-related and neutral words. We also compared subliminal and supraliminal presentations of panic-related words relative to neutral words within PD subjects and HC. We found a significantly enhanced Stroop interference to masked panic-related words in PD vs HC (Hedges' g = 0.60, p = 0.03; Q = 14.83, I² = 66.3 %, p = 0.01). While both PD subjects and HC tended to be slower to respond to supraliminal threat words than to neutral words, PD subjects only showed a marginally significant slower response to subliminal panic-related words vs neutral words. Findings remain inconclusive regarding comparison to other mental disorders, neural correlates, and the effect of psychotherapy. Even if possibly flawed by methodological weaknesses, our findings support the existence of a sensitivity to subliminal threat cues in PD, which could be targeted to improve treatment.

Strongly masked content retained in memory made accessible through repetition

A growing body of evidence indicates that information can be stored even in the absence of conscious awareness. Despite these findings, unconscious memory is still poorly understood with limited evidence for unconscious iconic memory storage. Here we show that strongly masked visual data can be stored and accumulate to elicit clear perception. We used a repetition method across a wide range of conditions (Experiment 1) and a more focused follow-up experiment with enhanced masking conditions (Experiment 2). Information was stored despite being masked, demonstrating that masking did not erase or overwrite memory traces but limited perception. We examined the temporal properties and found that stored information followed a gradual but rapid decay. Extraction of meaningful information was severely impaired after 300 ms, and most data was lost after 700 ms. Our findings are congruent with theories of consciousness that are based on an integration of subliminal information and support theoretical predictions based on the global workspace theory of consciousness, especially the existence of an implicit iconic memory buffer store.

LCD Monitors as an Alternative for Precision Demanding Visual Psychophysical Experiments

Precise timing and presentation of stimuli is critical in vision research, still, the limiting factor in successful recognition is often the monitor itself that is used to present the stimuli. The most widespread method is the use of monitors controlled by personal computers. Traditionally, most experiments used cathode-ray tubes but they are more and more difficult to access, and instead, liquid-crystal displays are getting more and more popular. The two types have fundamentally different working principles and limitations in displaying the stimulus.In our experiments, the temporal precision of the stimulus presentation was in focus. We investigated whether liquid-crystal displays, which are not considered to be fit to display fast successive stimuli, can represent an alternative choice for cathode-ray tubes. We used the double flash and the flicker illusion to compare the technical capabilities of the two monitor types. These illusions not only do require a precise timing but also a very short exposure to the stimuli. At the same time, the interstimulus interval is also of extreme importance. In addition, these illusions require peripheral stimulation of the retina, which is more sensitive to the temporal aspects of the visual stimulus. On the basis of previous studies and our own psychophysical results, we suggest that liquid-crystal displays might be a good alternative for precise, frame-to-frame stimulus presentation even if parts of the stimuli are projected on the peripheral retina.

Delaying in vivo exposure to a tarantula with very brief exposure to phobic stimuli

BACKGROUND AND OBJECTIVES

Research has documented the very brief exposure (VBE) effect: the reduction of phobic fear by continuous presentation of masked phobic pictures. In prior studies, phobic participants approached a live tarantula immediately after the masked stimuli were presented. This study tested the hypothesis that VBE would reduce phobic avoidance of the tarantula 24 h later.

METHOD

86 spider-phobic participants were identified with a fear questionnaire and Behavioral Avoidance Test (BAT) with a live tarantula indicative of a DSM-IV diagnosis of Specific Phobia. One week later, they were randomly assigned in double-blind fashion to presentation of a continuous series of 25 trials of masked images of either spiders or flowers (33-ms each), i.e., to VBE or control exposure. The participants gave subjective distress ratings just before and after these exposures. Then they engaged in the BAT again either immediately thereafter or 24 h later to measure changes in avoidance of the tarantula.

RESULTS

Masked images of spiders reduced avoidance of the tarantula both immediately after exposure and 24 h later without causing subjective distress. The effect sizes at these two time points did not significantly differ from each other.

LIMITATIONS

We did not manipulate awareness of the spider images by presenting them unmasked to a third group. Conclusions about the effect of awareness of the stimuli cannot be drawn.

CONCLUSIONS

VBE induces a process of fear reduction before phobic individuals engage in in vivo exposure, which is more distressing. Thus, VBE may help phobic-resistant individuals start treatment more gradually.

Submillisecond unmasked subliminal visual stimuli evoke electrical brain responses

Subliminal perception is strongly associated to the processing of meaningful or emotional information and has mostly been studied using visual masking. In this study, we used high density 256-channel EEG coupled with an liquid crystal display (LCD) tachistoscope to characterize the spatio-temporal dynamics of the brain response to visual checkerboard stimuli (Experiment 1) or blank stimuli (Experiment 2) presented without a mask for 1 ms (visible), 500 µs (partially visible), and 250 µs (subliminal) by applying time-wise, assumption-free nonparametric randomization statistics on the strength and on the topography of high-density scalp-recorded electric field. Stimulus visibility was assessed in a third separate behavioral experiment. Results revealed that unmasked checkerboards presented subliminally for 250 µs evoked weak but detectable visual evoked potential (VEP) responses. When the checkerboards were replaced by blank stimuli, there was no evidence for the presence of an evoked response anymore. Furthermore, the checkerboard VEPs were modulated topographically between 243 and 296 ms post-stimulus onset as a function of stimulus duration, indicative of the engagement of distinct configuration of active brain networks. A distributed electrical source analysis localized this modulation within the right superior parietal lobule near the precuneus. These results show the presence of a brain response to submillisecond unmasked subliminal visual stimuli independently of their emotional saliency or meaningfulness and opens an avenue for new investigations of subliminal stimulation without using visual masking.

Direct measurement of the system latency of gaze-contingent displays

Gaze-contingent displays combine a display device with an eyetracking system to rapidly update an image on the basis of the measured eye position. All such systems have a delay, the system latency, between a change in gaze location and the related change in the display. The system latency is the result of the delays contributed by the eyetracker, the display computer, and the display, and it is affected by the properties of each component, which may include variability. We present a direct, simple, and low-cost method to measure the system latency. The technique uses a device to briefly blind the eyetracker system (e.g., for video-based eyetrackers, a device with infrared light-emitting diodes (LED)), creating an eyetracker event that triggers a change to the display monitor. The time between these two events, as captured by a relatively low-cost consumer camera with high-speed video capability (1,000 Hz), is an accurate measurement of the system latency. With multiple measurements, the distribution of system latencies can be characterized. The same approach can be used to synchronize the eye position time series and a video recording of the visual stimuli that would be displayed in a particular gaze-contingent experiment. We present system latency assessments for several popular types of displays and discuss what values are acceptable for different applications, as well as how system latencies might be improved.

Changes in the neural correlates of implicit emotional face processing during antidepressant treatment in major depressive disorder

An emerging hypothesis regarding the mechanisms underlying antidepressant pharmacotherapy suggests that these agents benefit depressed patients by reversing negative emotional processing biases (Harmer, 2008). Neuropsychological indices and functional neuroimaging measures of the amygdala response show that antidepressant drugs shift implicit and explicit processing biases away from the negative valence and toward the positive valence. However, few studies have explored such biases in regions extensively connected with the amygdala, such as the pregenual anterior cingulate cortex (pgACC) area, where pre-treatment activity consistently has predicted clinical outcome during antidepressant treatment. We used functional magnetic resonance imaging (fMRI) to investigate changes in haemodynamic response patterns to positive vs. negative stimuli in patients with major depressive disorder (MDD) under antidepressant treatment. Participants with MDD (n = 10) underwent fMRI before and after 8 wk sertraline treatment; healthy controls (n = 10) were imaged across an equivalent interval. A backward masking task was used to elicit non-conscious neural responses to sad, happy and neutral face expressions. Haemodynamic responses to emotional face stimuli were compared between conditions and groups in the pgACC. The response to masked-sad vs. masked-happy faces (SN-HN) in pgACC in the depressed subjects was higher in the pre-treatment condition than in the post-treatment condition and this difference was significantly greater than the corresponding change across time in the controls. The treatment-associated difference was attributable to an attenuated response to sad faces and an enhanced response to happy faces. Pre-treatment pgACC responses to SN-HN correlated positively with clinical improvement during treatment. The pgACC participates with the amygdala in processing the salience of emotional stimuli. Treatment-associated functional changes in this limbic network may influence the non-conscious processing of such stimuli by reversing the negative processing bias extant in MDD.

Superior recognition performance for happy masked and unmasked faces in both younger and older adults

In the aging literature it has been shown that even though emotion recognition performance decreases with age, the decrease is less for happiness than other facial expressions. Studies in younger adults have also revealed that happy faces are more strongly attended to and better recognized than other emotional facial expressions. Thus, there might be a more age independent happy face advantage in facial expression recognition. By using a backward masking paradigm and varying stimulus onset asynchronies (17–267 ms) the temporal development of a happy face advantage, on a continuum from low to high levels of visibility, was examined in younger and older adults. Results showed that across age groups, recognition performance for happy faces was better than for neutral and fearful faces at durations longer than 50 ms. Importantly, the results showed a happy face advantage already during early processing of emotional faces in both younger and older adults. This advantage is discussed in terms of processing of salient perceptual features and elaborative processing of the happy face. We also investigate the combined effect of age and neuroticism on emotional face processing. The rationale was previous findings of age-related differences in physiological arousal to emotional pictures and a relation between arousal and neuroticism. Across all durations, there was an interaction between age and neuroticism, showing that being high in neuroticism might be disadvantageous for younger, but not older adults’ emotion recognition performance during arousal enhancing tasks. These results indicate that there is a relation between aging, neuroticism, and performance, potentially related to physiological arousal.

LCDs are better: psychophysical and photometric estimates of the temporal characteristics of CRT and LCD monitors

Many cognitive and perceptual phenomena, such as iconic memory and temporal integration, require brief displays. A critical requirement is that the image not remain visible after its offset. It is commonly believed that liquid crystal displays (LCD) are unsuitable because of their poor temporal response characteristics relative to cathode-ray-tube (CRT) screens. Remarkably, no psychophysical estimates of visible persistence are available to verify this belief. A series of experiments in which white stimuli on a black background produced discernible persistence on CRT but not on LCD screens, during both dark- and light-adapted viewing, falsified this belief. Similar estimates using black stimuli on a white background produced no visible persistence on either screen. That said, photometric measurements are available that seem to confirm the poor temporal characteristics of LCD screens, but they were obtained before recent advances in LCD technology. Using current LCD screens, we obtained photometric estimates of rise time far shorter (1-6 ms) than earlier estimates (20-150 ms), and approaching those of CRTs (<1 ms). We conclude that LCDs are preferable to CRTs when visible persistence is a concern, except when black-on-white displays are used.

Temporal properties of liquid crystal displays: implications for vision science experiments

Liquid crystal displays (LCD) are currently replacing the previously dominant cathode ray tubes (CRT) in most vision science applications. While the properties of the CRT technology are widely known among vision scientists, the photometric and temporal properties of LCDs are unfamiliar to many practitioners. We provide the essential theory, present measurements to assess the temporal properties of different LCD panel types, and identify the main determinants of the photometric output. Our measurements demonstrate that the specifications of the manufacturers are insufficient for proper display selection and control for most purposes. Furthermore, we show how several novel display technologies developed to improve fast transitions or the appearance of moving objects may be accompanied by side–effects in some areas of vision research. Finally, we unveil a number of surprising technical deficiencies. The use of LCDs may cause problems in several areas in vision science. Aside from the well–known issue of motion blur, the main problems are the lack of reliable and precise onsets and offsets of displayed stimuli, several undesirable and uncontrolled components of the photometric output, and input lags which make LCDs problematic for real–time applications. As a result, LCDs require extensive individual measurements prior to applications in vision science.

An LCD Monitor with Sufficiently Precise Timing for Research in Vision

Until now, liquid crystal display (LCD) monitors have not been used widely for research in vision. Despite their main advantages of continuous illumination and low electromagnetic emission, these monitors had problems with timing and reliability. Here we report that there is at least one new inexpensive 120 Hz model, whose timing and stability is on a par with a benchmark cathode-ray tube monitor, or even better. The onset time was stable across repetitions, 95% confidence interval (the error) of which was <0.01 ms. Brightness was also delivered reliably across repeated presentations (<0.04% error) and across blocks with different durations (<3% error). The LCD monitor seems suitable for many applications in vision research, including the studies that require combined accuracy of timing and intensity of visual stimulation.

The volatility of the amygdala response to masked fearful eyes

Recently, it has been suggested that backwardly masked, and thus subliminally presented, fearful eyes are processed by the amygdala. Here, we investigated in four functional magnetic resonance imaging experiments whether the amygdala responds to subliminally presented fearful eyes per se or whether an interaction of masked eyes with the masks or with parts of the masks used for backward masking might be responsible for the amygdala activation. In these experiments, we varied the mask as well as the position of the target eyes. The results show that the amygdala does not respond to masked fearful eyes per se but to an interaction between masked fearful eyes and the eyes of neutral faces used for masking. This finding questions the hypothesis that the amygdala processes context-free parts of the human face without awareness.

Usability of liquid crystal displays for research in the temporal characteristics of perception and attention

Recently, the use of liquid crystal displays (LCDs) in computer monitors has increased in popularity. Can LCDs produce results similar to those obtained in cathode-ray tube (CRT) displays in studies of temporal attention and perception tasks? Performance in two tasks (metacontrast masking and attentional blink) was examined using an LCD, a CRT oscilloscope, and a raster scan CRT display. Experiment 1 focused on metacontrast masking where a typical metacontrast function emerged irrespective of monitor type. Experiments 2 and 3 examined whether differences in monitors influence the attentional blink. Again, all displays elicited similar performance profiles for both the attentional blink and the trade-off between identification accuracy of the two targets. Although our results may not generalize to all LCD applications and all experimental paradigms, they indicate that LCDs can reproduce results similar to those found in metacontrast masking and attentional blink studies that were originally identified with CRT displays.

Relationship between amygdala responses to masked faces and mood state and treatment in major depressive disorder

CONTEXT

Major depressive disorder (MDD) is associated with behavioral and neurophysiological evidence of mood-congruent processing biases toward explicitly presented, emotionally valenced stimuli. However, few studies have investigated such biases toward implicitly presented stimuli.

OBJECTIVE

To investigate differential amygdala responses to sad, happy, and neutral faces presented below the level of explicit conscious awareness using a backward masking task in unmedicated participants with MDD and healthy controls (HCs).

DESIGN

Initial cross-sectional design followed by a longitudinal treatment trial using functional magnetic resonance imaging.

SETTING

Psychiatric outpatient clinic at the National Institute of Mental Health.

PARTICIPANTS

We studied 22 unmedicated, currently depressed people with MDD (dMDD), 16 unmedicated individuals with MDD in full remission (rMDD), and 25 HCs.

INTERVENTION

Ten dMDD participants underwent 8 weeks of antidepressant treatment with the selective serotonin reuptake inhibitor sertraline hydrochloride.

MAIN OUTCOME MEASURES

Amygdala region-of-interest and whole-brain analyses evaluated the hemodynamic response during exposure to masked sad vs masked happy faces, to masked sad vs neutral faces, and to masked happy vs neutral faces.

RESULTS

The dMDD participants showed greater amygdala responses than HCs to masked sad faces, whereas HCs showed greater amygdala responses to masked happy faces. The bias toward sad faces also was evident in rMDD participants relative to HCs and did not differ between dMDD and rMDD participants. This processing bias reversed toward the normative pattern in dMDD participants after sertraline treatment.

CONCLUSIONS

Emotional-processing biases occur in amygdala responses to sad faces presented below the level of conscious awareness in dMDD or rMDD individuals and to happy faces in HCs. By influencing the salience of social stimuli, mood-congruent processing biases in the amygdala may contribute to dysfunction in conscious perceptions and social interactions in MDD. Our data suggest, however, that the negative bias resolves and a positive bias develops in patients with MDD during selective serotonin reuptake inhibitor treatment.

A solution for measuring accurate reaction time to visual stimuli realized with a programmable microcontroller

This article presents a new solution for measuring accurate reaction time (SMART) to visual stimuli. The SMART is a USB device realized with a Cypress Programmable System-on-Chip (PSoC) mixed-signal array programmable microcontroller. A brief overview of the hardware and firmware of the PSoC is provided, together with the results of three experiments. In Experiment 1, we investigated the timing accuracy of the SMART in measuring reaction time (RT) under different conditions of operating systems (OSs; Windows XP or Vista) and monitor displays (a CRT or an LCD). The results indicated that the timing error in measuring RT by the SMART was less than 2 msec, on average, under all combinations of OS and display and that the SMART was tolerant to jitter and noise. In Experiment 2, we tested the SMART with 8 participants. The results indicated that there was no significant difference among RTs obtained with the SMART under the different conditions of OS and display. In Experiment 3, we used Microsoft (MS) PowerPoint to present visual stimuli on the display. We found no significant difference in RTs obtained using MS DirectX technology versus using the PowerPoint file with the SMART. We are certain that the SMART is a simple and practical solution for measuring RTs accurately. Although there are some restrictions in using the SMART with RT paradigms, the SMART is capable of providing both researchers and health professionals working in clinical settings with new ways of using RT paradigms in their work.

Development of a light-emitting diode tachistoscope

This paper describes a new method for rapid visual stimulus delivery, the light-emitting diode (LED) tachistoscope. An array of white LEDs provided a luminous intensity greater than 1,000,000 mcd. This array was placed behind a liquid crystal display (LCD) to function as a backlight; switching it on and off determined the visibility of the display. Commands to illuminate for periods from 1 ms to continuously on were relayed from a computer to the LED array. Changes in luminous intensity at the surface of a LED and the LCD were recorded via oscilloscope. The required duration of light pulses consistently matched the durations displayed, with only microsecond discrepancies due to turn-on and turn-off delays. Images were illuminated on the LCD screen for as little as 1 ms, with the amplitude of the luminance consistent across trials. The LED tachistoscope can be used with any computer to display images extremely briefly, potentially at the submillisecond level, providing superior performance to traditional and computer monitor tachistoscopes.

Multiple serial picture presentation with millisecond resolution using a three-way LC-shutter-tachistoscope

Throughout recent years there has been an increasing interest in studying unconscious visual processes. Such conditions of unawareness are typically achieved by either a sufficient reduction of the stimulus presentation time or visual masking. However, there are growing concerns about the reliability of the presentation devices used. As all these devices show great variability in presentation parameters, the processing of visual stimuli becomes dependent on the display-device, e.g. minimal changes in the physical stimulus properties may have an enormous impact on stimulus processing by the sensory system and on the actual experience of the stimulus. Here we present a custom-built three-way LC-shutter-tachistoscope which allows experimental setups with both, precise and reliable stimulus delivery, and millisecond resolution. This tachistoscope consists of three LCD-projectors equipped with zoom lenses to enable stimulus presentation via a built-in mirror-system onto a back projection screen from an adjacent room. Two high-speed liquid crystal shutters are mounted serially in front of each projector to control the stimulus duration. To verify the intended properties empirically, different sequences of presentation times were performed while changes in optical power were measured using a photoreceiver. The obtained results demonstrate that interfering variabilities in stimulus parameters and stimulus rendering are markedly reduced. Together with the possibility to collect external signals and to send trigger-signals to other devices, this tachistoscope represents a highly flexible and easy to set up research tool not only for the study of unconscious processing in the brain but for vision research in general.

Methods for dichoptic stimulus presentation in functional magnetic resonance imaging - a review

Dichoptic stimuli (different stimuli displayed to each eye) are increasingly being used in functional brain imaging experiments using visual stimulation. These studies include investigation into binocular rivalry, interocular information transfer, three-dimensional depth perception as well as impairments of the visual system like amblyopia and stereodeficiency. In this paper, we review various approaches of displaying dichoptic stimulus used in functional magnetic resonance imaging experiments. These include traditional approaches of using filters (red-green, red-blue, polarizing) with optical assemblies as well as newer approaches of using bi-screen goggles.

Attentional shift by gaze is triggered without awareness

Reflexive attentional shift in response to another individual's gaze direction has been reported, but it remains unknown whether this process can occur subliminally. We investigated this issue using facial stimuli consisting of drawings (Experiment 1) and photographs (Experiment 2). The gaze direction was expressed by the eye gaze direction (Experiment 1), and the eye gaze and head direction (Experiment 2). The gaze cue was presented either supraliminally or subliminally in the center of the visual field, before target presentation in the periphery. The task for participants was to localize the target as soon as possible. The reaction time needed to localize the target was consistently shorter for valid than invalid gaze cues for both types of gaze cues in both subliminal and supraliminal conditions. These findings indicate that attentional shift can be triggered even without awareness in response to another individual's eye gaze or head direction.

Temporal limitations in object processing across the human ventral visual pathway

Behavioral studies have shown that object recognition becomes severely impaired at fast presentation rates, indicating a limitation in temporal processing capacity. Here, we studied whether this behavioral limit in object recognition reflects limitations in the temporal processing capacity of early visual areas tuned to basic features or high-level areas tuned to complex objects. We used functional MRI (fMRI) to measure the temporal processing capacity of multiple areas along the ventral visual pathway progressing from the primary visual cortex (V1) to high-level object-selective regions, specifically the fusiform face area (FFA) and parahippocampal place area (PPA). Subjects viewed successive images of faces or houses at presentation rates varying from 2.3 to 37.5 items/s while performing an object discrimination task. Measures of the temporal frequency response profile of each visual area revealed a systematic decline in peak tuning across the visual hierarchy. Areas V1-V3 showed peak activity at rapid presentation rates of 18-25 items/s, area V4v peaked at intermediate rates (9 items/s), and the FFA and PPA peaked at the slowest temporal rates (4-5 items/s). Our results reveal a progressive loss in the temporal processing capacity of the human visual system as information is transferred from early visual areas to higher areas. These data suggest that temporal limitations in object recognition likely result from the limited processing capacity of high-level object-selective areas rather than that of earlier stages of visual processing.

Subliminal emotion perception in brain imaging: findings, issues, and recommendations

Many theories of emotion propose that emotional input is processed preferentially due to its relevance for the organism. Further, because consciousness has limited capacity, these considerations imply that emotional input ought to be processed even if participants are perceptually unaware of the input (subliminal perception). Although brain imaging has studied effects of unattended, suppressed (in binocular rivalry), and visually masked emotional pictures, conclusions regarding subliminal perception have been mixed. The reason is that subliminal perception demands a concept of an awareness threshold or limen, but there is no agreement on how to define and measure this threshold. Although different threshold concepts can be identified in psychophysics (signal detection theory), none maps directly onto perceptual awareness. Whereas it may be tempting to equate unawareness with the complete absence of objective discrimination ability (d'=0), this approach is incompatible with lessons from blindsight and denies the subjective nature of consciousness. This review argues that perceptual awareness is better viewed as a continuum of sensory states than a binary state. When levels of awareness are characterized carefully in terms of objective discrimination and subjective experience, findings can be informative regarding the relative independence of effects from awareness and the potentially moderating role of awareness in processing emotional input. Thus, because the issue of a threshold concept may never be resolved completely, the emphasis is to not prove subliminal perception but to compare effects at various levels of awareness.

Visual masking in magnetic resonance imaging

When a brief target picture is followed by another picture (mask), people often report that they are not consciously aware of the target. Thus, visual masking can be used to manipulate perceptual awareness of target pictures. To avoid interference with magnetic resonance imaging, pictures have been presented on liquid crystal device (LCD) and thin film transistor (TFT) projectors that were placed outside of the scanner room. However, we found that display devices with LCD/TFT technology exhibit poor accuracy in presenting pictures at brief durations [Wiens, S., Fransson, P., Dietrich, T., Lohmann, P., Ingvar, M., Ohman, A., 2004. Keeping it short: A comparison of methods for brief picture presentation. Psychological Science, 15, 282--285]. In this paper, we present a reliable and valid masking procedure involving two LCD/TFT projectors in combination with mechanical shutters. Because LCD/TFT projectors present pictures in steady state at longer durations (e.g., after 70 ms), picture presentation is more ecologically valid than for common cathode ray tube (CRT) monitors that present pictures in multiples of refresh cycles. Also, because picture presentation with mechanical shutters is instantaneous and reliable in terms of onset, rise time, and duration, shutters can be used to control picture durations precisely in steps of milliseconds. In this paper, we also discuss risks for confounding effects from unreliable picture presentations in masking. Our findings and arguments recommend the use of mechanical shutters in front of LCD/TFT projectors in imaging studies of visual masking.

Imaging implicit perception: promise and pitfalls

The study of implicit perception - perception in the absence of awareness - has a long history. Decades of behavioural work have identified crucial theoretical and methodological issues that must be considered when evaluating claims of implicit perception. Neuroimaging methods provide an important new avenue for illuminating our understanding of perception both with and without awareness, but most imaging experiments have not met the rigorous conditions that the behavioural work has shown are necessary for inferring implicit perception. Here, we review the literature of both behavioural and neuroimaging studies, and note the pitfalls of studying implicit perception as well as the promise that neuroimaging studies have for providing insights about implicit perception when combined with appropriately rigorous behavioural measures of awareness.