Simple reaction time as a measure of the temporal response properties of transient and sustained channels

Interaction between central and peripheral vision: Influence of distance and spatial frequencies

Visual scene perception is based on reciprocal interactions between central and peripheral information. Such interactions are commonly investigated through the semantic congruence effect, which usually reveals a congruence effect of central vision on peripheral vision as strong as the reverse. The aim of the present study was to further investigate the mechanisms underlying central-peripheral visual interactions using a central-peripheral congruence paradigm through three behavioral experiments. We presented simultaneously a central and a peripheral stimulus, that could be either semantically congruent or incongruent. To assess the congruence effect of central vision on peripheral vision, participants had to categorize the peripheral target stimulus while ignoring the central distractor stimulus. To assess the congruence effect of the peripheral vision on central vision, they had to categorize the central target stimulus while ignoring the peripheral distractor stimulus. Experiment 1 revealed that the physical distance between central and peripheral stimuli influences central-peripheral visual interactions: Congruence effect of central vision is stronger when the distance between the target and the distractor is the shortest. Experiments 2 and 3 revealed that the spatial frequency content of distractors also influence central-peripheral interactions: Congruence effect of central vision is observed only when the distractor contained high spatial frequencies while congruence effect of peripheral vision is observed only when the distractor contained low spatial frequencies. These results raise the question of how these influences are exerted (bottom-up vs. top-down) and are discussed based on the retinocortical properties of the visual system and the predictive brain hypothesis.

Backward masking reveals coarse-to-fine dynamics in human V1

Natural images exhibit luminance variations aligned across a broad spectrum of spatial frequencies (SFs). It has been proposed that, at early stages of processing, the coarse signals carried by the low SF (LSF) of the visual input are sent rapidly from primary visual cortex (V1) to ventral, dorsal and frontal regions to form a coarse representation of the input, which is later sent back to V1 to guide the processing of fine-grained high SFs (i.e., HSF). We used functional resonance imaging (fMRI) to investigate the role of human V1 in the coarse-to-fine integration of visual input. We disrupted the processing of the coarse and fine content of full-spectrum human face stimuli via backward masking of selective SF ranges (LSFs: <1.75cpd and HSFs: >1.75cpd) at specific times (50, 83, 100 or 150ms). In line with coarse-to-fine proposals, we found that (1) the selective masking of stimulus LSF disrupted V1 activity in the earliest time window, and progressively decreased in influence, while (2) an opposite trend was observed for the masking of stimulus' HSF. This pattern of activity was found in V1, as well as in ventral (i.e. the Fusiform Face area, FFA), dorsal and orbitofrontal regions. We additionally presented subjects with contrast negated stimuli. While contrast negation significantly reduced response amplitudes in the FFA, as well as coupling between FFA and V1, coarse-to-fine dynamics were not affected by this manipulation. The fact that V1 response dynamics to strictly identical stimulus sets differed depending on the masked scale adds to growing evidence that V1 role goes beyond the early and quasi-passive transmission of visual information to the rest of the brain. It instead indicates that V1 may yield a 'spatially registered common forum' or 'blackboard' that integrates top-down inferences with incoming visual signals through its recurrent interaction with high-level regions located in the inferotemporal, dorsal and frontal regions.

Visual field differences in temporal synchrony processing for audio-visual stimuli

Audio-visual integration relies on temporal synchrony between visual and auditory inputs. However, differences in traveling and transmitting speeds between visual and auditory stimuli exist; therefore, audio-visual synchrony perception exhibits flexible functions. The processing speed of visual stimuli affects the perception of audio-visual synchrony. The present study examined the effects of visual fields, in which visual stimuli are presented, for the processing of audio-visual temporal synchrony. The point of subjective simultaneity, the temporal binding window, and the rapid recalibration effect were measured using temporal order judgment, simultaneity judgment, and stream/bounce perception, because different mechanisms of temporal processing have been suggested among these three paradigms. The results indicate that auditory stimuli should be presented earlier for visual stimuli in the central visual field than in the peripheral visual field condition in order to perceive subjective simultaneity in the temporal order judgment task conducted in this study. Meanwhile, the subjective simultaneity bandwidth was broader in the central visual field than in the peripheral visual field during the simultaneity judgment task. In the stream/bounce perception task, neither the point of subjective simultaneity nor the temporal binding window differed between the two types of visual fields. Moreover, rapid recalibration occurred in both visual fields during the simultaneity judgment tasks. However, during the temporal order judgment task and stream/bounce perception, rapid recalibration occurred only in the central visual field. These results suggest that differences in visual processing speed based on the visual field modulate the temporal processing of audio-visual stimuli. Furthermore, these three tasks, temporal order judgment, simultaneity judgment, and stream/bounce perception, each have distinct functional characteristics for audio-visual synchrony perception. Future studies are necessary to confirm the effects of compensation regarding differences in the temporal resolution of the visual field in later cortical visual pathways on visual field differences in audio-visual temporal synchrony.

Differential neurodynamics and connectivity in the dorsal and ventral visual pathways during perception of emotional crowds and individuals: a MEG study

Reading the prevailing emotion of groups of people ("crowd emotion") is critical to understanding their overall intention and disposition. It alerts us to potential dangers, such as angry mobs or panicked crowds, giving us time to escape. A critical aspect of processing crowd emotion is that it must occur rapidly, because delays often are costly. Although knowing the timing of neural events is crucial for understanding how the brain guides behaviors using coherent signals from a glimpse of multiple faces, this information is currently lacking in the literature on face ensemble coding. Therefore, we used magnetoencephalography to examine the neurodynamics in the dorsal and ventral visual streams and the periamygdaloid cortex to compare perception of groups of faces versus individual faces. Forty-six participants compared two groups of four faces or two individual faces with varying emotional expressions and chose which group or individual they would avoid. We found that the dorsal stream was activated as early as 68 msec after the onset of stimuli containing groups of faces. In contrast, the ventral stream was activated later and predominantly for individual face stimuli. The latencies of the dorsal stream activation peaks correlated with participants' response times for facial crowds. We also found enhanced connectivity earlier between the periamygdaloid cortex and the dorsal stream regions for crowd emotion perception. Our findings suggest that ensemble coding of facial crowds proceeds rapidly and in parallel by engaging the dorsal stream to mediate adaptive social behaviors, via a distinct route from single face perception.

Rapid Temporal Recalibration to Audiovisual Asynchrony Occurs Across the Difference in Neural Processing Speed Based on Spatial Frequency

Audiovisual integration relies on temporal synchrony between visual and auditory stimuli. The brain rapidly adapts to audiovisual asynchronous events by shifting the timing of subjective synchrony in the direction of the leading modality of the most recent event, a process called rapid temporal recalibration. This phenomenon is the flexible function of audiovisual synchrony perception. Previous studies found that neural processing speed based on spatial frequency (SF) affects the timing of subjective synchrony. This study examined the effects of SF on the rapid temporal recalibration process by discriminating whether the presentation of the visual and auditory stimuli was simultaneous. I compared the magnitudes of the recalibration effect between low and high SF visual stimuli using two techniques. First, I randomly presented each SF accompanied by a tone during one session, then in a second experiment, only a single SF was paired with the tone throughout the one session. The results indicated that rapid recalibration occurred regardless of difference in presented SF between preceding and test trials. The recalibration magnitude did not significantly differ between the SF conditions. These findings confirm that intersensory temporal process is important to produce rapid recalibration and suggest that rapid recalibration can be induced by the simultaneity judgment criterion changes attributed to the low-level temporal information of audiovisual events.

Interocular Differences in Spatial Frequency Influence the Pulfrich Effect

The Pulfrich effect is a stereo-motion phenomenon. When the two eyes are presented with visual targets moving in fronto-parallel motion at different luminances or contrasts, the perception is of a target moving-in-depth. It is thought that this percept of motion-in-depth occurs because lower luminance or contrast delays the speed of visual processing. Spatial properties of an image such as spatial frequency and size have also been shown to influence the speed of visual processing. In this study, we use a paradigm to measure interocular delay based on the Pulfrich effect where a structure-from-motion defined cylinder, composed of Gabor elements displayed at different interocular phases, rotates in depth. This allows us to measure any relative interocular processing delay while independently manipulating the spatial frequency and size of the micro elements (i.e., Gabor patches). We show that interocular spatial frequency differences, but not interocular size differences of image features, produce interocular processing delays.

Stimulus-Locked Responses on Human Upper Limb Muscles and Corrective Reaches Are Preferentially Evoked by Low Spatial Frequencies

In situations requiring immediate action, humans can generate visually-guided responses at remarkably short latencies. Here, to better understand the visual attributes that best evoke such rapid responses, we recorded upper limb muscle activity while participants performed visually-guided reaches towards Gabor patches composed of differing spatial frequencies (SFs). We studied reaches initiated from a stable posture (experiment 1, a static condition), or during on-line reach corrections to an abruptly displaced target (experiment 2, a dynamic condition). In both experiments, we detail the latency and prevalence of stimulus-locked responses (SLRs), which are brief bursts of EMG activity that are time-locked to target presentation rather than movement onset. SLRs represent the first wave of EMG recruitment influenced by target presentation, and enable quantification of rapid visuomotor transformations. In both experiments, reach targets composed of low SFs elicited the shortest latency and most prevalent SLRs, with SLR latency increasing and SLR prevalence decreasing for reach targets composed of progressively higher SFs. SLRs could be evoked in either the static or dynamic condition, and when present in experiment 2, were associated with shorter latency and larger magnitude corrections. The results in experiment 2 are consistent with a linkage between the forces produced by SLRs and the earliest portion of on-line reach corrections. Overall, our results demonstrate that stimuli composed of low SFs preferentially evoke the most rapid visuomotor responses that, in the context of rapidly correcting an on-going reaching movement, are associated with earlier and larger on-line reach corrections.

Illusory agency attribution to others performing actions similar to one's own

When people observe others performing actions similar to their own while dancing or playing musical instruments, they sometimes feel as if their actions were subsumed into others’ actions or others’ actions led their own actions. Many studies have been conducted to investigate agency attribution. However, these studies have mainly examined agency attribution in cases where people do not know the true agent. Few studies have focused on how people attribute agency to others despite knowing that they themselves are actual agents. This study investigates agency attribution to others performing actions similar to one’s own when one knows who the actual agent is. We evaluated agency attribution when participants manipulated a mouse to control a cursor while observing another person performing similar actions. Our findings demonstrated that participants could attribute agency to others despite knowing that they themselves were actual agents. We refer to this illusory sense as “illusory agency attribution to others.” We suggest that illusory agency attribution to others is determined by multiple factors including a bottom-up process with a subjective feeling of agency in addition to a top-down process with an interpretative judgement of agency.

Time course of spatial frequency integration in face perception: An ERP study

Face perception is based on the processing and integration of multiple spatial frequency (SF) ranges. However, the temporal dynamics of SF integration to form an early face representation in the human brain is still a matter of debate. To address this issue, we recorded event-related potentials (ERPs) during the presentation of spatial frequency-manipulated facial images. Twenty-six participants performed a gender discrimination task on non-filtered, low-, high-, and band-pass filtered face images, corresponding, respectively, to the full range, spatial frequencies up to 8 cycles/image, above 32 cycles/image, and from 8 to 16 cycles/image. Behaviorally, the task related-performance was more accurate and faster for non-filtered (NF) and mid-range SF (MSF) than for low SF (LSF) and high SF (HSF) stimuli. At both behavioral and electrophysiological levels, response to MSF contained in faces did not differ from the responses to full spectrum non-filtered (NF) facial images. In ERPs, LSF facial images evoked the largest P1 amplitude while HSF facial images evoked the largest N170 amplitude compared with the other three conditions. Since LSFs and HSFs would transmit global and local information respectively, our observations lend further support to the "coarse-to-fine" processing theory of faces. Furthermore, they offer original evidence of the effectiveness and adequacy of the mid-range spatial frequency in face perception. Possible theoretical interpretations of our findings are discussed.

Spatial frequency sensitivity in macaque midbrain

Visual brain areas exhibit tuning characteristics well suited for image statistics present in our natural environment. However, visual sensation is an active process, and if there are any brain areas that ought to be particularly in tune with natural scene statistics, it would be sensory-motor areas critical for guiding behavior. Here we found that the rhesus macaque superior colliculus, a structure instrumental for rapid visual exploration with saccades, detects low spatial frequencies, which are the most prevalent in natural scenes, much more rapidly than high spatial frequencies. Importantly, this accelerated detection happens independently of whether a neuron is more or less sensitive to low spatial frequencies to begin with. At the population level, the superior colliculus additionally over-represents low spatial frequencies in neural response sensitivity, even at near-foveal eccentricities. Thus, the superior colliculus possesses both temporal and response gain mechanisms for efficient gaze realignment in low-spatial-frequency-dominated natural environments.

Influence of learning strategy on response time during complex value-based learning and choice

Measurements of response time (RT) have long been used to infer neural processes underlying various cognitive functions such as working memory, attention, and decision making. However, it is currently unknown if RT is also informative about various stages of value-based choice, particularly how reward values are constructed. To investigate these questions, we analyzed the pattern of RT during a set of multi-dimensional learning and decision-making tasks that can prompt subjects to adopt different learning strategies. In our experiments, subjects could use reward feedback to directly learn reward values associated with possible choice options (object-based learning). Alternatively, they could learn reward values of options' features (e.g. color, shape) and combine these values to estimate reward values for individual options (feature-based learning). We found that RT was slower when the difference between subjects' estimates of reward probabilities for the two alternative objects on a given trial was smaller. Moreover, RT was overall faster when the preceding trial was rewarded or when the previously selected object was present. These effects, however, were mediated by an interaction between these factors such that subjects were faster when the previously selected object was present rather than absent but only after unrewarded trials. Finally, RT reflected the learning strategy (i.e. object-based or feature-based approach) adopted by the subject on a trial-by-trial basis, indicating an overall faster construction of reward value and/or value comparison during object-based learning. Altogether, these results demonstrate that the pattern of RT can be informative about how reward values are learned and constructed during complex value-based learning and decision making.

Simple reaction times to cyclopean stimuli reveal that the binocular system is tuned to react faster to near than to far objects

Binocular depth perception is an important mechanism to segregate the visual scene for mapping relevant objects in our environment. Convergent evidence from psychophysical and neurophysiological studies have revealed asymmetries between the processing of near (crossed) and far (uncrossed) binocular disparities. The aim of the present study was to test if near or far objects are processed faster and with higher contrast sensitivity in the visual system. We therefore measured the relationship between binocular disparity and simple reaction time (RT) as well as contrast gain based on the contrast-RT function in young healthy adults. RTs were measured to suddenly appearing cyclopean target stimuli, which were checkerboard patterns encoded by depth in dynamic random dot stereograms (DRDS). The DRDS technique allowed us to selectively study the stereoscopic processing system by eliminating all monocular cues. The results showed that disparity and contrast had significant effects on RTs. RTs as a function of disparity followed a U-shaped tuning curve indicating an optimum at around 15 arc min, where RTs were minimal. Surprisingly, the disparity tuning of RT was much less pronounced for far disparities. At the optimal disparity, we measured advantages of about 80 ms and 30 ms for near disparities at low (10%) and high (90%) contrasts, respectively. High contrast always reduced RTs as well as the disparity dependent differences. Furthermore, RT-based contrast gains were higher for near disparities in the range of disparities where RTs were the shortest. These results show that the sensitivity of the human visual system is biased for near versus far disparities and near stimuli can result in faster motor responses, probably because they bear higher biological relevance.

Sequential hemifield gating of α- and β-behavioral performance oscillations after microsaccades

Microsaccades are tiny saccades that occur during gaze fixation. Even though visual processing has been shown to be strongly modulated close to the time of microsaccades, both at central and peripheral eccentricities, it is not clear how these eye movements might influence longer term fluctuations in brain activity and behavior. Here we found that visual processing is significantly affected and, in a rhythmic manner, even several hundreds of milliseconds after a microsaccade. Human visual detection efficiency, as measured by reaction time, exhibited coherent rhythmic oscillations in the α- and β-frequency bands for up to ~650-700 ms after a microsaccade. Surprisingly, the oscillations were sequentially pulsed across visual hemifields relative to microsaccade direction, first occurring in the same hemifield as the movement vector for ~400 ms and then the opposite. Such pulsing also affected perceptual detection performance. Our results suggest that visual processing is subject to long-lasting oscillations that are phase locked to microsaccade generation, and that these oscillations are dependent on microsaccade direction.NEW & NOTEWORTHY We investigated long-term microsaccadic influences on visual processing and found rhythmic oscillations in behavioral performance at α- and β-frequencies (~8-20 Hz). These oscillations were pulsed at a much lower frequency across visual hemifields, first occurring in the same hemifield as the microsaccade direction vector for ~400 ms before switching to the opposite hemifield for a similar interval. Our results suggest that saccades temporally organize visual processing and that such organization can sequentially switch hemifields.

Global shape integration and illusory form perception in the absence of awareness

Previous research on perceptual organization operations still provides contradictory evidence on whether the integration of sparse local elements into coherently unified shapes and the construction of the illusory form are accomplished without the need of awareness. In the present study, three experiments were conducted in which participants were presented with masked (Experiment 1, SOA=27ms; Experiment 2; SOA=53ms) and unmasked (Experiment 3) primes consisting of geometric shapes (a square or a diamond) that could be congruent or incongruent with subsequent probe stimuli (square vs. diamond). Furthermore, the primes were divided into: a grouping condition (where local elements may group together into global shapes), an illusory condition (where the arrangement of local elements produced illusory shapes) and a hybrid condition (where both operations were presented simultaneously). While no priming effects were found for the shortest SOA (27ms), both grouping and illusory primes produced significant priming effects in the longer SOA (53ms). On the other hand, results in Experiment 3 (unmasked) showed strong priming effects for the grouping of the inducers in both the grouping and the hybrid conditions, and also a significant but weaker priming effect for the illusory condition. Overall, our results support the possibility of the integration of local visual features into a global shape in the absence of awareness and, likewise, they suggest an early -subliminal- construction of the illusory shape, implying that feedback projections from higher to lower visual areas are not crucial in the construction of the illusory form.

Larger Stimuli Require Longer Processing Time for Perception

The time it takes for a stimulus to reach awareness is often assessed by measuring reaction times (RTs) or by a temporal order judgement (TOJ) task in which perceived timing is compared against a reference stimulus. Dissociations of RT and TOJ have been reported earlier in which increases in stimulus intensity such as luminance intensity results in a decrease of RT, whereas perceived perceptual latency in a TOJ task is affected to a lesser degree. Here, we report that a simple manipulation of stimulus size has stronger effects on perceptual latency measured by TOJ than on motor latency measured by RT tasks. When participants were asked to respond to the appearance of a simple stimulus such as a luminance blob, the perceptual latency measured against a standard reference stimulus was up to 40 ms longer for a larger stimulus. In other words, the smaller stimulus was perceived to occur earlier than the larger one. RT on the other hand was hardly affected by size. The TOJ results were further replicated in a simultaneity judgement task, suggesting that the effects of size are not due to TOJ-specific response biases but more likely reflect an effect on perceived timing.

A neural locus for spatial-frequency specific saccadic suppression in visual-motor neurons of the primate superior colliculus

Saccades cause rapid retinal-image shifts that go perceptually unnoticed several times per second. The mechanisms for saccadic suppression have been controversial, in part because of sparse understanding of neural substrates. In this study we uncovered an unexpectedly specific neural locus for spatial frequency-specific saccadic suppression in the superior colliculus (SC). We first developed a sensitive behavioral measure of suppression in two macaque monkeys, demonstrating selectivity to low spatial frequencies similar to that observed in earlier behavioral studies. We then investigated visual responses in either purely visual SC neurons or anatomically deeper visual motor neurons, which are also involved in saccade generation commands. Surprisingly, visual motor neurons showed the strongest visual suppression, and the suppression was dependent on spatial frequency, as in behavior. Most importantly, suppression selectivity for spatial frequency in visual motor neurons was highly predictive of behavioral suppression effects in each individual animal, with our recorded population explaining up to ~74% of behavioral variance even on completely different experimental sessions. Visual SC neurons had mild suppression, which was unselective for spatial frequency and thus only explained up to ~48% of behavioral variance. In terms of spatial frequency-specific saccadic suppression, our results run contrary to predictions that may be associated with a hypothesized SC saccadic suppression mechanism, in which a motor command in the visual motor and motor neurons is first relayed to the more superficial purely visual neurons, to suppress them and to then potentially be fed back to cortex. Instead, an extraretinal modulatory signal mediating spatial-frequency-specific suppression may already be established in visual motor neurons.NEW & NOTEWORTHY Saccades, which repeatedly realign the line of sight, introduce spurious signals in retinal images that normally go unnoticed. In part, this happens because of perisaccadic suppression of visual sensitivity, which is known to depend on spatial frequency. We discovered that a specific subtype of superior colliculus (SC) neurons demonstrates spatial-frequency-dependent suppression. Curiously, it is the neurons that help mediate the saccadic command itself that exhibit such suppression, and not the purely visual ones.

The Effect of Spatial-Frequency Filtering on the Visual Processing of Global Structure

In three experiments we measured reaction times (RTs) and error rates in identifying the global structure of spatially filtered stimuli whose spatial-frequency content was selected by means of three types of 2-D isotropic filters (Butterworth of order 2, Butterworth of order 10, and a filters with total or partial Gaussian spectral profile). In each experiment, low-pass (LP), band-pass (BP), and high-pass (HP) filtered stimuli, with nine centre or cut-off spatial frequencies, were used. Irrespective of the type of filter, the experimental results showed that: (a) RTs to stimuli with low spatial frequencies were shorter than those to stimuli with medium or high spatial frequencies, (b) RTs to LP filtered stimuli were nearly constant, but they increased in a non-monotonic way with the filter centre spatial frequency in BP filtered stimuli and with the filter cut-off frequency in HP filtered stimuli, and (c) the identification of the global pattern occurred with all visible stimuli used, including BP and HP images without low spatial frequencies. To remove the possible influence of the energy, a fourth experiment was conducted with Gaussian filtered stimuli of equal contrast power ( crms = 0.065). Similar results to those described above were found for stimuli with spatial-frequency content higher than 2 cycles deg−1. A model of isotropic first-order visual channels collecting the stimulus spectral energy in all orientations explains the RT data. A subsequent second-order nonlinear amplitude demodulation process, applied to the output of the most energetic first-order channel, could explain the perception of global structure of each spatially filtered stimulus, including images lacking low spatial frequencies.

No significant effect of transcranial direct current stimulation (tDCS) found on simple motor reaction time comparing 15 different simulation protocols

BACKGROUND

Research exploring the behavioral impact of transcranial direct current stimulation (tDCS) over M1 has produced homogenous results. The most common explanations to address this homogeneity concerns the differential impact of varied tDCS parameters (such as stimulation intensity or electrode montage). To explore this, we systematically examined the effects of 15 different tDCS protocols on a well-elucidated neurobehavioral system: simple visual motor reaction time (smRT).

METHODS

For the initial phase of this study, 150 healthy participants were randomly assigned to one of 5 experimental groups (2mA anodal, 2mA cathodal, 1mA anodal, 1mA cathodal, or sham) across 3 different conditions (orbitofrontal, bilateral, or extracephalic reference electrode location). The active electrode was always placed over M1 and tDCS lasted for 20min. Starting ~5min prior to stimulation and running continuously for ~30min, participants were repeatedly presented with a visual cue centered on a computer monitor and asked to press a response button as quickly as possible at stimulus onset (stimuli number: 100 pre-, 400 during-, and 100-post stimulation - interstimulus interval: 1-3s). Ex-gaussian distribution curves, miss, and error rates were determined for each normalized batch of 100 RTs and compared using a two-way ANOVA. As the largest group differences were seen with 2mA anodal (compared to sham) stimulation using an orbitofrontal montage, an additional 60 healthy participants were recruited to further test for significance in this condition.

RESULTS

No significant impact of tDCS was seen on any parameter of smRT distribution, error rate, or miss rate, regardless of polarity, stimulation intensity, electrode montage, or stimulation-to-task relationship.

CONCLUSION

Our results suggest that tDCS over M1 might not have a predictable or reliable effect on short duration smRT. Our results raise interesting questions regarding the mechanisms by which tDCS might modulate more complex motor behaviors. Additional research utilizing multiple tDCS protocols as undertaken here will help address and clarify these concerns.

Are Spatial Frequencies Integrated from Coarse to Fine?

The existence of a temporal anisotropy in the integration of spatial frequencies, such that spatial frequencies are integrated more effectively if they are available from low to high through time, has been examined in a series of experiments. In the first experiment, the first three harmonics of a square wave were presented in a low-to-high or a high-to-low sequence in a temporal two-interval forced-choice experiment. Subjects were asked to indicate which sequence appeared to resemble a square wave more. A high-to-low sequence of spatial frequencies was judged to more resemble the target than the low-to-high sequence. These results support a temporal anisotropy in the integration of spatial frequencies of exactly the opposite form to that suggested from previous results. Further experiments established that this was not due to task differences or to subjects basing their decision on the final spatial frequency shown. An interpretation is offered in which an isotropic mechanism for spatial-frequency integration is combined with a recency bias.

Predictions of U-Shaped Backward Pattern Masking from Considerations of the Spatio-Temporal Frequency Response

The threshold detectability of a briefly presented target stimulus consisting of a vertical sinusoidal grating was affected not only by the spatial frequency content of an equally briefly presented, two-octave-wide masking noise, but also by the time interval separating the onsets of the target and its mask. Over a range of stimulus onset asynchronies, in which the mask onset either preceded, coincided with, or followed the target onset, a mask with a low spatial frequency content had its greatest masking effect on a high spatial frequency target grating when the mask followed the target by 120–180 ms. When the mask had a high spatial frequency content and the target was of low spatial frequency, or when the target was entered on the mask frequency band, optimal masking effects occurred when the onsets of the mask and target coincided. The results are discussed in relation to previous masking studies, particuarly those in which U-shaped backward pattern masking functions are obtained.

Effects of Endogenous Spatial Attention on the Detection and Discrimination of Spatial Frequencies

Two experiments were designed to explore the relationship between visual attention and spatial-frequency processing using a cuing paradigm. In both experiments, the targets were a sharp-edged line segment with high spatial frequencies present and a blurred line segment with only low spatial frequencies present. In each trial an endogenous cue appeared at fixation indicating the probable location, left or right, in which a stimulus would appear. In experiment 1, a typical cuing effect was found with simple reaction times (RTs) for detecting the stimuli being faster when they appeared at a cued (ie attended) compared to an uncued (ie unattended) location. In experiment 2, choice RTs were measured, with participants indicating whether the sharp-edged line segment or the blurred line segment was presented in each trial. In this case, when it was necessary to process the spatial-frequency content of the stimuli, RTs were significantly faster at the attended location only for the sharp-edged line segment. For the blurred line segment without high spatial frequencies, RTs did not differ for attended and unattended locations. The results indicate that endogenous spatial attention interacts differently with high-spatial-frequency and low-spatial-frequency selective mechanisms depending on whether the task is to detect a stimulus or identify it on the basis of its spatial-frequency content.

Vision, Perception, and Attention through the Lens of Microsaccades: Mechanisms and Implications

Microsaccades are small saccades. Neurophysiologically, microsaccades are generated using similar brainstem mechanisms as larger saccades. This suggests that peri-saccadic changes in vision that accompany large saccades might also be expected to accompany microsaccades. In this review, we highlight recent evidence demonstrating this. Microsaccades are not only associated with suppressed visual sensitivity and perception, as in the phenomenon of saccadic suppression, but they are also associated with distorted spatial representations, as in the phenomenon of saccadic compression, and pre-movement response gain enhancement, as in the phenomenon of pre-saccadic attention. Surprisingly, the impacts of peri-microsaccadic changes in vision are far reaching, both in time relative to movement onset as well as spatial extent relative to movement size. Periods of ~100 ms before and ~100 ms after microsaccades exhibit significant changes in neuronal activity and behavior, and this happens at eccentricities much larger than the eccentricities targeted by the microsaccades themselves. Because microsaccades occur during experiments enforcing fixation, these effects create a need to consider the impacts of microsaccades when interpreting a variety of experiments on vision, perception, and cognition using awake, behaving subjects. The clearest example of this idea to date has been on the links between microsaccades and covert visual attention. Recent results have demonstrated that peri-microsaccadic changes in vision play a significant role in both neuronal and behavioral signatures of covert visual attention, so much so that in at least some attentional cueing paradigms, there is very tight synchrony between microsaccades and the emergence of attentional effects. Just like large saccades, microsaccades are genuine motor outputs, and their impacts can be substantial even during perceptual and cognitive experiments not concerned with overt motor generation per se.

The resilience of object predictions: early recognition across viewpoints and exemplars

Recognition of everyday objects can be facilitated by top-down predictions. We have proposed that these predictions are derived from rudimentary image information, or gist, extracted rapidly from the low spatial frequencies (LSFs) (Bar Journal of Cognitive Neuroscience 15: 600–609, 2003). Because of the coarse nature of LSF representations, we hypothesized here that such predictions can accommodate changes in viewpoint as well as facilitate the recognition of visually similar objects. In a repetition-priming task, we indeed observed significant facilitation of target recognition that was primed by LSF objects across moderate viewpoint changes, as well as across visually similar exemplars. These results suggest that the LSF representations are specific enough to activate accurate predictions, yet flexible enough to overcome small changes in visual appearance. Such gist representations facilitate object recognition by accommodating changes in visual appearance due to viewing conditions, and help generalize from familiar to novel exemplars.

Valuable Orientations Capture Attention

Visual attention has long been known to be drawn to stimuli that are physically salient or congruent with task-specific goals. Several recent studies have shown that attention is also captured by stimuli that are neither salient nor task-relevant, but that are rendered in a color that has previously been associated with reward. We investigated whether another feature dimension-orientation-can be associated with reward via learning and thereby elicit value-driven attentional capture. In a training phase, participants received a monetary reward for identifying the color of Gabor patches exhibiting one of two target orientations. A subsequent test phase in which no reward was delivered required participants to search for Gabor patches exhibiting one of two spatial frequencies (orientation was now irrelevant to the task). Previously rewarded orientations robustly captured attention. We conclude that reward learning can imbue features other than color-in this case, specific orientations-with persistent value.

The neural bases of spatial frequency processing during scene perception

Theories on visual perception agree that scenes are processed in terms of spatial frequencies. Low spatial frequencies (LSF) carry coarse information whereas high spatial frequencies (HSF) carry fine details of the scene. However, how and where spatial frequencies are processed within the brain remain unresolved questions. The present review addresses these issues and aims to identify the cerebral regions differentially involved in low and high spatial frequency processing, and to clarify their attributes during scene perception. Results from a number of behavioral and neuroimaging studies suggest that spatial frequency processing is lateralized in both hemispheres, with the right and left hemispheres predominantly involved in the categorization of LSF and HSF scenes, respectively. There is also evidence that spatial frequency processing is retinotopically mapped in the visual cortex. HSF scenes (as opposed to LSF) activate occipital areas in relation to foveal representations, while categorization of LSF scenes (as opposed to HSF) activates occipital areas in relation to more peripheral representations. Concomitantly, a number of studies have demonstrated that LSF information may reach high-order areas rapidly, allowing an initial coarse parsing of the visual scene, which could then be sent back through feedback into the occipito-temporal cortex to guide finer HSF-based analysis. Finally, the review addresses spatial frequency processing within scene-selective regions areas of the occipito-temporal cortex.

On the 'visual' in 'audio-visual integration': a hypothesis concerning visual pathways

Crossmodal interaction conferring enhancement in sensory processing is nowadays widely accepted. Such benefit is often exemplified by neural response amplification reported in physiological studies conducted with animals, which parallel behavioural demonstrations of sound-driven improvement in visual tasks in humans. Yet, a good deal of controversy still surrounds the nature and interpretation of these human psychophysical studies. Here, we consider the interpretation of crossmodal enhancement findings under the light of the functional as well as anatomical specialization of magno- and parvocellular visual pathways, whose paramount relevance has been well established in visual research but often overlooked in crossmodal research. We contend that a more explicit consideration of this important visual division may resolve some current controversies and help optimize the design of future crossmodal research.

Persistent hemispheric differences in the perceptual selection of spatial frequencies

Previous research has shown that the right hemisphere processes low spatial frequencies more efficiently than the left hemisphere, which preferentially processes high spatial frequencies. These studies have typically measured RTs to single, briefly flashed gratings and/or have directed observers to attend to a particular spatial frequency immediately before making a judgment about a subsequently presented stimulus. Thus, it is unclear whether the hemispheres differ in perceptual selection from multiple spatial frequencies that are simultaneously present in the environment, without bias from selective attention. Moreover, the time course of hemispheric asymmetry in spatial frequency processing is unknown. We addressed both of these questions with binocular rivalry, a measure of perceptual selection from competing alternatives over time. Participants viewed a pair of rivalrous orthogonal gratings with different spatial frequencies, presented either to the left or right of central fixation, and continuously reported which grating they perceived. At the beginning of a trial, the low spatial frequency grating was perceptually selected more often when presented in the left hemifield (right hemisphere) than in the right hemifield (left hemisphere), whereas the high spatial frequency grating showed the opposite pattern of results. This hemispheric asymmetry in perceptual selection persisted for the entire 30-sec stimulus presentation, continuing long after stimulus onset. These results indicate stable differences in the resolution of ambiguity across spatial locations and demonstrate the importance of considering sustained differences in perceptual selection across space when characterizing conscious representations of complex scenes.

Contributions of magno- and parvocellular channels to conscious and non-conscious vision

The dorsal and ventral cortical pathways, driven predominantly by magnocellular (M) and parvocellular (P) inputs, respectively, assume leading roles in models of visual information processing. Although in prior proposals, the dorsal and ventral pathways support non-conscious and conscious vision, respectively, recent modelling and empirical developments indicate that each pathway plays important roles in both non-conscious and conscious vision. In these models, the ventral P-pathway consists of one subpathway processing an object's contour features, e.g. curvature, the other processing its surface attributes, e.g. colour. Masked priming studies have shown that feed-forward activity in the ventral P-pathway on its own supports non-conscious processing of contour and surface features. The dorsal M-pathway activity contributes directly to conscious vision of motion and indirectly to object vision by projecting to prefrontal cortex, which in turn injects top-down neural activity into the ventral P-pathway and there 'ignites' feed-forward-re-entrant loops deemed necessary for conscious vision. Moreover, an object's shape or contour remains invisible without the prior conscious registration of its surface properties, which for that reason are taken to comprise fundamental visual qualia. Besides suggesting avenues for future research, these developments bear on several recent and past philosophical issues.

Global precedence, spatial frequency channels, and the statistics of natural images

A great deal of evidence suggests that early in processing, retinal images are filtered by parallel, spatial frequency selective channels. We attempt to incorporate this view of early vision with the principle of global precedence, which holds that Gestalt-like processes sensitive to global image configurations tend to dominate local feature processing in human pattern perception. Global precedence is inferred from the pattern of reaction times observed when visual patterns contain multiple cues at different levels of spatial scale. Specifically, it is frequently observed that global processing times are largely unaffected by conflicting local cues, but local processing times are substantially lengthened by conflicting global cues. The asymmetry of these effects suggests the dominant role of global configurations. Since global spatial information is effectively represented by low spatial frequencies, global precedence potentially implies a low frequency dominance. The thesis is that low spatial frequencies tend to be available before information carried by higher frequency bands, producing a coarse-to-fine temporal order in visual spatial perception. It is suggested that a variety of factors contribute to the "prior entry" of low frequency information, including the high contrast gain of the magnocellular pathway, the amplitude spectra typical of natural images, and inhibitory interactions between the parallel frequency-tuned channels. Evidence suggesting a close relationship between global precedence and spatial frequency channels is provided by observations that the essential features of the global precedence effect are obtained using patterns consisting of low and high frequency sinusoids. The hypothesis that these asymmetric interference effects are due to interactions between parallel spatial channels is supported by an analysis of reaction times (RTs), which shows that RTs to redundant low and high frequency cues produce less facilitation than predictions that assume the channels are independent. In view of previous work showing that global precedence depends upon the low frequency content of the stimuli, we suggest that low spatial frequencies represent the sine qua non for the dominance of configurational cues in human pattern perception, and that this configurational dominance reflects the microgenesis of visual pattern perception. This general view of the temporal dynamics of visual pattern recognition is discussed, is considered from an evolutionary perspective, and is related to certain statistical regularities in natural scenes. Potential adaptive advantages of an interactive parallel architecture that confers an initial processing advantage to low resolution information are explored.

Temporally advanced dynamic change of receptive field of lateral geniculate neurons during brief visual stimulation: Effects of brainstem peribrachial stimulation

Processing of visual information in the brain seems to proceed from initial fast but coarse to subsequent detailed processing. Such coarse-to-fine changes appear also in the response of single neurons in the visual pathway. In the dorsal lateral geniculate nucleus (dLGN), there is a dynamic change in the receptive field (RF) properties of neurons during visual stimulation. During a stimulus flash centered on the RF, the width of the RF-center, presumably related to spatial resolution, changes rapidly from large to small in an initial transient response component. In a subsequent sustained component, the RF-center width is rather stable apart from an initial slight widening. Several brainstem nuclei modulate the geniculocortical transmission in a state-dependent manner. Thus, modulatory input from cholinergic neurons in the peribrachial brainstem region (PBR) enhances the geniculocortical transmission during arousal. We studied whether such input also influences the dynamic RF-changes during visual stimulation. We compared dynamic changes of RF-center width of dLGN neurons during brief stimulus presentation in a control condition, with changes during combined presentation of the visual stimulus and electrical PBR-stimulation. The major finding was that PBR-stimulation gave an advancement of the dynamic change of the RF-center width such that the different response components occurred earlier. Consistent with previous studies, we also found that PBR-stimulation increased the gain of firing rate during the sustained response component. However, this increase of gain was particularly strong in the transition from the transient to the sustained component at the time when the center width was minimal. The results suggest that increased modulatory PBR-input not only increase the gain of the geniculocortical transmission, but also contributes to faster dynamics of transmission. We discuss implications for possible effects on visual spatial resolution.

Sound-driven enhancement of vision: disentangling detection-level from decision-level contributions

Cross-modal enhancement can be mediated both by higher-order effects due to attention and decision making and by detection-level stimulus-driven interactions. However, the contribution of each of these sources to behavioral improvements has not been conclusively determined and quantified separately. Here, we apply psychophysical analysis based on Piéron functions in order to separate stimulus-dependent changes from those accounted by decision-level contributions. Participants performed a simple visual speeded detection task on Gabor patches of different spatial frequencies and contrast values, presented with and without accompanying sounds. On one hand, we identified an additive cross-modal improvement in mean reaction times across all types of visual stimuli that would be well explained by interactions not strictly based on stimulus-driven modulations (e.g., due to reduction of temporal uncertainty and motor times). On the other hand, we singled out an audio-visual benefit that strongly depended on stimulus features such as frequency and contrast. This particular enhancement was selective to low-visual spatial frequency stimuli, optimized for magnocellular sensitivity. We therefore conclude that interactions at detection stages and at decisional processes in response selection that contribute to audio-visual enhancement can be separated online and express on partly different aspects of visual processing.

"Slight" of hand: the processing of visually degraded gestures with speech

Co-speech hand gestures influence language comprehension. The present experiment explored what part of the visual processing system is optimized for processing these gestures. Participants viewed short video clips of speech and gestures (e.g., a person saying “chop” or “twist” while making a chopping gesture) and had to determine whether the two modalities were congruent or incongruent. Gesture videos were designed to stimulate the parvocellular or magnocellular visual pathways by filtering out low or high spatial frequencies (HSF versus LSF) at two levels of degradation severity (moderate and severe). Participants were less accurate and slower at processing gesture and speech at severe versus moderate levels of degradation. In addition, they were slower for LSF versus HSF stimuli, and this difference was most pronounced in the severely degraded condition. However, exploratory item analyses showed that the HSF advantage was modulated by the range of motion and amount of motion energy in each video. The results suggest that hand gestures exploit a wide range of spatial frequencies, and depending on what frequencies carry the most motion energy, parvocellular or magnocellular visual pathways are maximized to quickly and optimally extract meaning.

Enabling global processing in simultanagnosia by psychophysical biasing of visual pathways

A fundamental aspect of visual cognition is our disposition to see the 'forest before the trees'. However, damage to the posterior parietal cortex, a critical brain region along the dorsal visual pathway, can produce a neurological disorder called simultanagnosia, characterized by a debilitating inability to perceive the 'forest' but not the 'trees' (i.e. impaired global processing despite intact local processing). This impairment in perceiving the global shape persists even though the ventral visual pathway, the primary recognition pathway, is intact in these patients. Here, we enabled global processing in patients with simultanagnosia using a psychophysical technique, which allowed us to bias stimuli such that they are processed predominantly by the intact ventral visual pathway. Our findings reveal that the impairment in global processing that characterizes simultanagnosia stems from a disruption in the processing of low-spatial frequencies through the dorsal pathway. These findings advance our understanding of the relationship between visuospatial attention and perception and reveal the neural mechanism mediating the disposition to see the 'forest before the trees'.

Microgenesis of surface completion in visual objects: evidence for filling-out

Using metacontrast masking we examined the temporal dynamics of surface completion in object vision. By varying the stimulus onset asynchrony between the target object and the flanking mask(s), we obtained estimates of the time required for the entire surface contrast to fill out within the area delimited by the contours/edges of the target. The estimated speed of the filling-out process was 36.0 deg/s. Using existing estimates of cortical magnification, the computed filling-out speed in terms of cortical distance is .385 m/s, a value that approximates the estimated cortical filling-in speed and the speed of horizontal propagation in monkey V1. We discuss our results in relation to (1) prior findings of filling-in and filling-out phenomena, using surface completion in cortical space as the unifying principle, and (2) extant computational models of object vision.

Magnocellular and Parvocellular Pathway Influences on Location-Based Inhibition-Of-Return

The roles of the parvocellular (P) and magnocellular (M) retino-geniculo-cortical pathways during shifts of visual attention were investigated by creating M/dorsal-biased (eg low spatial frequency target, no objects present) and P/ventral-biased (ie high spatial frequency target, the perception of 3-D objects) stimulus conditions and measuring location-based inhibition-of-return (IOR). P/ventral-biased conditions produced the greatest IOR. M/dorsal-biased conditions produced the least IOR, in one instance eliminating it altogether. The results indicate a close relationship between IOR magnitude and relative P/ventral and M/dorsal activity with location-based IOR related more to P/ventral than to M/dorsal activity.

Similar Effects of Visual Perception and Imagery on Simple Reaction Time

A longstanding issue is whether perception and mental imagery share similar cognitive and neural mechanisms. To cast further light on this problem, we compared the effects of real and mentally generated visual stimuli on simple reaction time (RT). In five experiments, we tested the effects of difference in luminance, contrast, spatial frequency, motion, and orientation. With the intriguing exception of spatial frequency, in all other tasks perception and imagery showed qualitatively similar effects. An increase in luminance, contrast, and visual motion yielded a decrease in RT for both visually presented and imagined stimuli. In contrast, gratings of low spatial frequency were responded to more quickly than those of higher spatial frequency only for visually presented stimuli. Thus, the present study shows that basic dependent variables exert similar effects on visual RT either when retinally presented or when imagined. Of course, this evidence does not necessarily imply analogous mechanisms for perception and imagery, and a note of caution in such respect is suggested by the large difference in RT between the two operations. However, the present results undoubtedly provide support for some overlap between the structural representation of perception and imagery.

Coarse-to-fine changes of receptive fields in lateral geniculate nucleus have a transient and a sustained component that depend on distinct mechanisms

Visual processing in the brain seems to provide fast but coarse information before information about fine details. Such dynamics occur also in single neurons at several levels of the visual system. In the dorsal lateral geniculate nucleus (LGN), neurons have a receptive field (RF) with antagonistic center-surround organization, and temporal changes in center-surround organization are generally assumed to be due to a time-lag of the surround activity relative to center activity. Spatial resolution may be measured as the inverse of center size, and in LGN neurons RF-center width changes during static stimulation with durations in the range of normal fixation periods (250-500 ms) between saccadic eye-movements. The RF-center is initially large, but rapidly shrinks during the first ~100 ms to a rather sustained size. We studied such dynamics in anesthetized cats during presentation (250 ms) of static spots centered on the RF with main focus on the transition from the first transient and highly dynamic component to the second more sustained component. The results suggest that the two components depend on different neuronal mechanisms that operate in parallel and with partial temporal overlap rather than on a continuously changing center-surround balance. Results from mathematical modeling further supported this conclusion. We found that existing models for the spatiotemporal RF of LGN neurons failed to account for our experimental results. The modeling demonstrated that a new model, in which the response is given by a sum of an early transient component and a partially overlapping sustained component, adequately accounts for our experimental data.

Estimating the growth of internal evidence guiding perceptual decisions

Perceptual decision-making is thought to involve a gradual accrual of noisy evidence. Temporal integration of the evidence reduces the relative contribution of dynamic internal noise to the decision variable, thereby boosting its signal-to-noise ratio. We aimed to estimate the internal evidence guiding perceptual decisions over time, using a novel combination of external noise and the response signal methods. Observers performed orientation discrimination of patterns presented in external noise. We varied the contrast of the patterns and the delay at which observers were forced to signal their decision. Each test stimulus (patterns and noise sample) was presented twice. Across two experiments we varied the availability of the visual stimulus for processing. Observer model analyses of discrimination accuracy and response consistency to two passes of the same stimulus, suggested that there was very little growth in the internal evidence. The improvement in accuracy over time characterised by the speed-accuracy trade-off function predominantly reflected a decreasing proportion of non-visual decisions, or pure guesses. There was no advantage to having the visual patterns visible for longer than 80 ms, indicating that only the visual information in a short window after display onset was used to drive the decisions. The remarkable constancy of the internal evidence over time suggests that temporal integration of the sensory information was very limited. Alternatively, more extended integration of the evidence from memory could have taken place, provided that the dominant source of internal noise limiting performance occurs between-trials, which cannot be reduced by prolonged evidence integration.

Testing a simplified method for measuring velocity integration in saccades using a manipulation of target contrast

A growing number of studies in vision research employ analyses of how perturbations in visual stimuli influence behavior on single trials. Recently, we have developed a method along such lines to assess the time course over which object velocity information is extracted on a trial-by-trial basis in order to produce an accurate intercepting saccade to a moving target. Here, we present a simplified version of this methodology, and use it to investigate how changes in stimulus contrast affect the temporal velocity integration window used when generating saccades to moving targets. Observers generated saccades to one of two moving targets which were presented at high (80%) or low (7.5%) contrast. In 50% of trials, target velocity stepped up or down after a variable interval after the saccadic go signal. The extent to which the saccade endpoint can be accounted for as a weighted combination of the pre- or post-step velocities allows for identification of the temporal velocity integration window. Our results show that the temporal integration window takes longer to peak in the low when compared to high contrast condition. By enabling the assessment of how information such as changes in velocity can be used in the programming of a saccadic eye movement on single trials, this study describes and tests a novel methodology with which to look at the internal processing mechanisms that transform sensory visual inputs into oculomotor outputs.

The neural substrates and timing of top-down processes during coarse-to-fine categorization of visual scenes: a combined fMRI and ERP study

Spatial frequencies in an image influence visual analysis across a distributed, hierarchically organized brain network. Low spatial frequency (LSF) information may rapidly reach high-order areas to allow an initial coarse parsing of the visual scene, which could then be "retroinjected" through feedback into lower level visual areas to guide finer analysis on the basis of high spatial frequency (HSF). To test this "coarse-to-fine" processing scheme and to identify its neural substrates in the human brain, we presented sequences of two spatial-frequency-filtered scenes in rapid succession (LSF followed by HSF or vice versa) during fMRI and ERPs in the same participants. We show that for low-to-high sequences (but not for high-to-low sequences), LSF produces a first increase of activity in prefrontal and temporo-parietal areas, followed by enhanced responses to HSF in primary visual cortex. This pattern is consistent with retroactive influences on low-level areas that process HSF after initial activation of higher order areas by LSF.

Inverted face processing in body dysmorphic disorder

Individuals with body dysmorphic disorder (BDD) are preoccupied with perceived defects in appearance. Preliminary evidence suggests abnormalities in global and local visual information processing. The objective of this study was to compare global and local processing in BDD subjects and healthy controls by testing the face inversion effect, in which inverted (upside-down) faces are recognized more slowly and less accurately relative to upright faces. Eighteen medication-free subjects with BDD and 17 matched, healthy controls performed a recognition task with sets of upright and inverted faces on a computer screen that were either presented for short duration (500 ms) or long duration (5000 ms). Response time and accuracy rates were analyzed using linear and logistic mixed effects models, respectively. Results indicated that the inversion effect for response time was smaller in BDD subjects than controls during the long duration stimuli, but was not significantly different during the short duration stimuli. Inversion effect on accuracy rates did not differ significantly between groups during either of the two durations. Lesser inversion effect in BDD subjects may be due to greater detail-oriented and piecemeal processing for long duration stimuli. Similar results between groups for short duration stimuli suggest that they may be normally engaging configural and holistic processing for brief presentations. Abnormal visual information processing in BDD may contribute to distorted perception of appearance; this may not be limited to their own faces, but to others' faces as well.