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- Jigo, M., Gong, M. & Liu, T. (2018) Neural determinants of task performance during feature-based attention in human cortex. eNeuro, 5(1), e0375-17.2018. pdf Studies of feature-based attention have associated activity in a dorsal frontoparietal network with putative attentional priority signals. Yet, how this neural activity mediates attentional selection and whether it guides behavior are fundamental questions that require investigation. We reasoned that endogenous fluctuations in the quality of attentional priority should influence task performance. Human subjects detected a speed increment while viewing clockwise or counter-clockwise motion (baseline task) or while attending to either direction amid distracters (attention task). In an fMRI experiment, direction-specific neural pattern similarity between the baseline task and the attention task revealed a higher level of similarity for correct than incorrect trials in frontoparietal regions. Using TMS, we disrupted posterior parietal cortex and found a selective deficit in the attention task, but not in the baseline task, demonstrating the necessity of this cortical area during feature-based attention. These results reveal that frontoparietal areas maintain attentional priority that facilitates successful behavioral selection.
- Liu, T., Cable, D. & Gardner, J. L. (2018) Inverted encoding models of human population response conflate noise and neural tuning width. Journal of Neuroscience, 38, 398-408. pdf Channel encoding models offer the ability to bridge different scales of neuronal measurement by interpreting population responses, typically measured with BOLD imaging in humans, as linear sums of groups of neurons (channels) tuned for visual stimulus properties. Inverting these models to form predicted channel responses from population measurements in humans seemingly offers the potential to infer neuronal tuning properties. Here, we test the ability to make inferences about neural tuning width from inverted encoding models. We examined contrast invariance of orientation selectivity in human V1 (both sexes) and found that inverting the encoding model resulted in channel response functions that became broader with lower contrast, thus, apparently, violating contrast invariance. Simulations showed that this broadening could be explained by contrast-invariant single-unit tuning with the measured decrease in response amplitude at lower contrast. The decrease in response lowers the signal-to-noise ratio of population responses that results in poorer population representation of orientation. Simulations further showed that increasing signal-to-noise makes channel response functions less sensitive to underlying neural tuning width, and in the limit of zero noise will reconstruct the channel function assumed by the model regardless of the bandwidth of single-units. We conclude that our data are consistent with contrast invariant orientation tuning in human V1. More generally, our results demonstrate that population selectivity measures obtained by encoding models can deviate substantially from the behavior of single-units because they conflate neural tuning width and noise and are therefore better used to estimate the uncertainty of decoded stimulus properties.
- Pleskac, T. J., Yu, S., Hopwood, C. & Liu, T. (In press) Mechanisms of deliberation during preferential choice: Perspectives from computational modeling and individual differences. Decision. Computational models of decision making typically assume as people deliberate between options they mentally simulate outcomes from each one and integrate valuations of these outcomes to form a preference. In two studies, we investigated this deliberation process using a task where participants make a series of decisions between a certain and an uncertain option, which were shown as dynamic visual samples that represented possible payoffs. We developed and validated a method of reverse correlational analysis for the task that measures how this time-varying signal was used to make a choice. The first study used this method to examine how information processing during deliberation differed from a perceptual analog of the task. We found participants were less sensitive to each sample of information during preferential choice. In a second study, we investigated how these different measures of deliberation were related to impulsivity and drug and alcohol use. We found that while properties of the deliberation process were not related to impulsivity,some aspects of the process may be related to substance use. In particular, alcohol abuse was related to diminished sensitivity to the payoff information and drug use was related to how the initial starting point of evidence accumulation. We synthesized our results with a rank-dependent sequential sampling model which suggests that participants allocated more attentional weight to larger potential payoffs during preferential choice.
- Hao, R, Becker, M. W., Ye, C., Liu, Q., & Liu, T. (2018) The bandwidth of VWM consolidation varies with the stimulus feature： Evidence from event-related potentials. Journal of Experimental Psychology: Human Perception & Performance, 44, 767-777. link Our previous work suggests that two colors can be consolidated into visual short-term memory (VSTM) in parallel without a loss of memory precision, while consolidation of two orientations is performed in a strictly serial manner. Those experiments compared VSTM performance for simultaneously and sequentially presented stimuli. However, there is still controversy about whether the bandwidth for consolidation is determined by the type of information. To further investigate this issue, here we measured electroencephalography while participants attempted to consolidate one, two or four simultaneously presented colors (Experiment 1) or orientations (Experiment 2) under limited presentation times. We used the contralateral delay activity (CDA) as an electrophysiological marker of the number of items that were consolidated. For colored stimuli, the CDA amplitude increased between set-size one and two but did not further increase for set size four. By contrast, for orientation, the CDA amplitude remained at the set size one amplitude as set size increased to two or four items. Furthermore, in a long exposure duration (300 ms) condition that did not limit the consolidation process, the CDA amplitude pattern indicated that VSTM capacity is limited to about three colored items and about two orientation items in our paradigm. Thus, the CDA effects observed in the short presentation time was not limited by VSTM storage, but rather by consolidation. These results are consistent with our previous behavioral research and suggest that the bandwidth of VSTM consolidation is determined by the stimulus feature.
- Liu, T. & Jigo, M. (2017) Limits in feature-based attention to multiple colors. Attention, Perception, & Psychophysics, 79, 2327–2337. pdf Attention to a feature enhances the sensory representation of that feature. Although much has been learned about the properties of attentional modulation when attending to a single feature, the effectiveness of attending to multiple features is not well understood. We investigated this question in a series of experiments using a color detection task while varying the number of attended colors in a cueing paradigm. Observers were shown either a single cue, two cues, or no cue (baseline) before detecting a coherent color target. We measured detection threshold by varying the coherence level of the target. Compared to the baseline condition, we found consistent facilitation of detection performance in the one-cue and two-cue conditions, but performance in the two-cue condition was lower than that in the one-cue condition. In the final experiment, we presented a 50% valid cue to emulate the situation in which observers were only able to attend a single color in the two-cue condition, and found equivalent detection thresholds with the standard two-cue condition. These results indicate a limit in attending to two colors and further imply that observers could effectively attend a single color at a time. Such a limit is likely due to an inability to maintain multiple active attentional templates for colors.
- Qian, C. S., Brascamp, J. W., & Liu, T. (2017) On the functional order of binocular rivalry and blind spot filling-in. Vision Research, 136, 15-20. pdf Binocular rivalry is an important phenomenon for understanding the mechanisms of visual awareness. Here we assessed the functional locus of binocular rivalry relative to blind spot filling-in, which is thought to transpire in V1, thus providing a reference point for assessing the locus of rivalry. We conducted two experiments to explore the functional order of binocular rivalry and blind spot filling-in. Experiment 1 examined if the information filled-in at the blind spot can engage in rivalry with a physical stimulus at the corresponding location in the fellow eye. Participants’ perceptual reports showed no difference between this condition and a condition where filling-in was precluded by presenting the same stimuli away from the blind spot, suggesting that the rivalry process is not influenced by any filling-in that might occur. In Experiment 2, we presented the fellow eye’s stimulus directly in rivalry with the ‘inducer’ stimulus that surrounds the blind spot, and compared it with two control conditions away from the blind spot: one involving a ring physically identical to the inducer, and one involving a disc that resembled the filled-in percept. Perceptual reports in the blind spot condition resembled those in the ‘ring’ condition, more than those in the latter, ‘disc’ condition, indicating that a perceptually suppressed inducer does not engender filling-in. Thus, our behavioral data suggest binocular rivalry functionally precedes blind spot filling-in. We conjecture that the neural substrate of binocular rivalry suppression includes processing stages at or before V1.
- Ye, C., Hu, Z., Li, H., Ristaniemi, T., Liu, Q, & Liu, T. (2017) A two-phase model of resource allocation in visual working memory. Journal of Experimental Psychology: Learning, Memory & Cognition, 43, 1557-1566. link Two broad theories of visual working memory (VWM) storage have emerged from current research, a discrete slot-based theory and a continuous resource theory. However, neither the discrete slot-based theory or continuous resource theory clearly stipulates how the mental commodity for VWM (discrete slot or continuous resource) is allocated. Allocation may be based on the number of items via stimulus-driven factors, or it may be based on task demands via voluntary control. Previous studies have obtained conflicting results regarding the automaticity versus controllability of such allocation. In the current study, we propose a two-phase allocation model, in which the mental commodity could be allocated only by stimulus-driven factors in the early consolidation phase. However, when there is sufficient time to complete the early phase, allocation can enter the late consolidation phase, where it can be flexibly and voluntarily controlled according to task demands. In an orientation recall task, we instructed participants to store either fewer items at high-precision or more items at low-precision. In 3 experiments, we systematically manipulated memory set size and exposure duration. We did not find an effect of task demands when the set size was high and exposure duration was short. However, when we either decreased the set size or increased the exposure duration, we found a trade-off between the number and precision of VWM representations. These results can be explained by a two-phase model, which can also account for previous conflicting findings in the literature.
- Senturk, G., Greenberg, A. S., & Liu, T. (2016). Saccade latency indexes exogenous and endogenous object-based attention. Attention, Perception, & Psychophysics, 78, 1998-2013. pdf Classic studies of object-based attention have utilized keypress responses as the main dependent measure. However, people typically make saccades to fixate important objects. Recent work has shown that attention may act differently when it is deployed covertly versus in advance of a saccade. We further investigated the link between saccades and attention by examining whether object-based effects can be observed for saccades. We adapted the classical double-rectangle cueing paradigm of Egly, Driver, and Rafal (1994), and measured both the first saccade latency and the keypress reaction time (RT) to a target that appeared at the end of one of the two rectangles. Our results showed that saccade latencies exhibited higher sensitivity than did RTs for detecting effects of attention. We also assessed the generality of the attention effects by testing three types of cues: hybrid (predictive and peripheral), exogenous (nonpredictive and peripheral), and endogenous (predictive and central). We found that both RTs and saccade latencies exhibited effects of both space-based and object-based attentional selection. However, saccade latencies showed a more robust attentional modulation than RTs. For the exogenous cues, we observed a spatial inhibition of return along with an object-based effect, implying that object-based attention is independent of space-based attention. Overall, our results revealed an oculomotor correlate of object-based attention, suggesting that, in addition to spatial priority, object-level priority also affects saccade planning.
- Liu, T. (2016). Neural representation of object-specific attentional priority.NeuroImage, 129, 15-24. pdf Humans can flexibly select locations, features, or objects in a visual scene for prioritized processing. Although it is relatively straightforward to manipulate location- and feature-based attention, it is difficult to isolate object-based selection. Because objects are always composed of features, studies of object-based selection can often be interpreted as the selection of a combination of locations and features. Here we examined the neural representation of attentional priority in a paradigm that isolated object-based selection. Participants viewed two superimposed gratings that continuously changed their color, orientation, and spatial frequency, such that the gratings traversed the same exact feature values within a trial. Participants were cued at the beginning of each trial to attend to one or the other grating to detect a brief luminance increment, while their brain activity was measured with fMRI. Using multi-voxel pattern analysis, we were able to decode the attended grating in a set of frontoparietal areas, including anterior intraparietal sulcus (IPS), frontal eye field (FEF), and inferior frontal junction (IFJ). Thus, a perceptually varying object can be represented by patterned neural activity in these frontoparietal areas. We suggest that these areas can encode attentional priority for abstract, high-level objects independent of their locations and features.
- Wang, Y., Miller, J. R., & Liu, T. (2015) Suppression effects in feature-based attention. Journal of Vision, 15(5):15, 1-16. pdf Attending to a feature enhances visual processing of that feature, but it is less clear what occurs to unattended features. Single-unit recording studies in middle temporal (MT) have shown that neuronal modulation is a monotonic function of the difference between the attended and neuron’s preferred direction. Such a relationship should predict a monotonic suppressive effect in psychophysical performance. However, past research on suppressive effects of feature-based attention has remained inconclusive. We investigated the suppressive effect for motion direction, orientation, and color in three experiments. We asked participants to detect a weak signal among noise and provided a partially valid feature cue to manipulate attention. We measured performance as a function of the offset between the cued and signal feature. We also included neutral trials where no feature cues were presented to provide a baseline measure of performance. Across three experiments, we consistently observed enhancement effects when the target feature and cued feature coincided and suppression effects when the target feature deviated from the cued feature. The exact profile of suppression was different across feature dimensions: Whereas the profile for direction exhibited a ‘‘rebound’’ effect, the profiles for orientation and color were monotonic. These results demonstrate that unattended features are suppressed during feature-based attention, but the exact suppression profile depends on the specific feature. Overall, the results are largely consistent with neurophysiological data and support the feature-similarity gain model of attention.
- Miller, J. R., Becker, M. W., & Liu, T. (2014) The bandwidth of consolidation into visual short-term memory (VSTM) depends on the visual feature. Visual Cognition, 22, 920-947. pdf We investigated the nature of the bandwidth limit in the consolidation of visual information into visual short-term memory. In the first two experiments, we examinedwhether previous results showing differential consolidation bandwidth for colour and orientation resulted from methodological differences by testing the consolidation of colour information with methods used in prior orientation experiments. We briefly presented two colour patches with masks, either sequentially or simultaneously, followed by a location cue indicating the target. Participants identified the target colour via buttonpress (Experiment 1) or by clicking a location on a colour wheel (Experiment 2). Although these methods have previously demonstrated that two orientations are consolidated in a strictly serial fashion, here we found equivalent performance in the sequential and simultaneous conditions, suggesting that two colours can be consolidated in parallel. To investigate whether this difference resulted from different consolidation mechanisms or a common mechanism with different features consuming different amounts of bandwidth, Experiment 3 presented a colour patch and an oriented grating either sequentially or simultaneously. We found a lower performance in the simultaneous than the sequential condition, with orientation showing a larger impairment than colour. These results suggest that consolidation of both features share common mechanisms. However, it seems that colour requires less information to be encoded than orientation. As a result, two colours can be consolidated in parallel without exceeding the bandwidth limit, whereas two orientations or an orientation and a colour exceed the bandwidth and appear to be consolidated serially.
- Zeigenfuse, M. D., Pleskac, T. J., & Liu, T. (2014) Rapid decisions from experience.Cognition, 131, 181-194. pdf In many everyday decisions, people quickly integrate noisy samples of information to form a preference among alternatives that offer uncertain rewards. Here, we investigated this decision process using the Flash Gambling Task (FGT), in which participants made a series of choices between a certain payoff and an uncertain alternative that produced a normal distribution of payoffs. For each choice, participants experienced the distribution of payoffs via rapid samples updated every 50 ms. We show that people can make these rapid decisions from experience and that the decision process is consistent with a sequential sampling process. Results also reveal a dissociation between these preferential decisions and equivalent perceptual decisions where participants had to determine which alternatives contained more dots on average. To account for this dissociation, we developed a sequential sampling rank-dependent utility model, which showed that participants in the FGT attended more to larger potential payoffs than participants in the perceptual task despite being given equivalent information. We discuss the implications of these findings in terms of computational models of preferential choice and a more complete understanding of experience-based decision making.
- Ye, C., Zhang, L., Liu, T., Li, H., & Liu, Q. (2014) Visual working memory capacity for color is independent of representation resolution. PLoS One, 9(3), e91681. pdf link Background: The relationship between visual working memory (VWM) capacity and resolution of representation have been extensively investigated. Several recent ERP studies using orientation (or arrow) stimuli suggest that there is an inverse relationship between VWM capacity and representation resolution. However, different results have been obtained in studies using color stimuli. This could be due to important differences in the experimental paradigms used in previous studies.
Methodology/Principal Findings: We examined whether the same relationship between capacity and resolution holds for color information. Participants performed a color change detection task while their electroencephalography was recorded. We manipulated representation resolution by asking participants to detect either a salient change (low-resolution) or a subtle change (high-resolution) in color. We used an ERP component known as contralateral delay activity (CDA) to index the amount of information maintained in VWM. The result demonstrated the same pattern for both low- and high-resolution conditions, with no difference between conditions.
Conclusions/Significance: This result suggests that VWM always represents a fixed number of approximately 3–4 colors regardless of the resolution of representation.
- Liu, T. & Hou, Y. (2013) A hierarchy of attentional priority signals in human frontoparietal cortex. Journal of Neuroscience, 33, 16606-16616. pdf Humans can voluntarily attend to a variety of visual attributes to serve behavioral goals. Voluntary attention is believed to be controlled by a network of dorsal frontoparietal areas. However, it is unknown how neural signals representing behavioral relevance (attentional priority) for different attributes are organized in this network. Computational studies have suggested that a hierarchical organization reflecting the similarity structure of the task demands provides an efficient and flexible neural representation. Here we examined the structure of attentional priority using functional magnetic resonance imaging. Participants were cued to attend to location, color, or motion direction within the same stimulus. We found a hierarchical structure emerging in frontoparietal areas, such that multivoxel patterns for attending to spatial locations were most distinct from those for attending to features, and the latter were further clustered into different dimensions (color vs motion). These results provide novel evidence for the organization of the attentional control signals at the level of distributed neural activity. The hierarchical organization provides a computationally efficient scheme to support flexible top-down control.
- Becker, M. W., Miller, J. R., & Liu, T. (2013) A severe capacity limit in the consolidation of orientation information into visual short-term memory (VSTM).Attention, Perception & Psychophysics, 75, 415-425. pdf Previous research has suggested that two color patches can be consolidated into visual short-term memory(VSTM) via an unlimited parallel process. Here we examinedwhether the same unlimited-capacity parallel process occurs for two oriented grating patches. Participants viewed two gratings that were presented briefly and masked. In blocks of trials, the gratings were presented either simultaneously or sequentially. In Experiments 1 and 2, the presentation of the stimuli was followed by a location cue that indicated the grating on which to base one’s response. In Experiment 1, participants responded whether the target grating was oriented clockwise or counterclockwise with respect to vertical. In Experiment 2, participants indicated whether the target grating was oriented along one of the cardinal directions (vertical or horizontal) or was obliquely oriented. Finally, in Experiment 3, the location cue was replaced with a third grating that appeared at fixation, and participants indicated whether either of the two test gratings matched this probe. Despite the fact that these responses required fairly coarse coding of the orientation information, across all methods of responding we found superior performance for sequential over simultaneous presentations. These findings suggest that the consolidation of oriented gratings into VSTM is severely limited in capacity and differs from the consolidation of color information.
- Liu, T. & Becker, M. W. (2013) Serial consolidation of orientation information into visual short-term memory. Psychological Science, 24, 1044-1050. pdf Previous research suggests that there is a limit to the rate at which items can be consolidated in visual short-term memory (VSTM). This limit could be due to either a serial or a limited-capacity parallel process. Historically, it has proven difficult to distinguish between these two types of processes. In the present experiment, we took a novel approach that allowed us to do so. Participants viewed two oriented gratings either sequentially or simultaneously and reported one of the gratings’ orientation via method of adjustment. Performance was worse for the simultaneous than for the sequential condition. We fit the data with a mixture model that assumes performance is limited by a noisy memory representation plus random guessing. Critically, the serial and limited-capacity parallel processes made distinct predictions regarding the model’s guessing and memory-precision parameters. We found strong support for a serial process, which implies that one can consolidate only a single orientation into VSTM at a time.
- Liu, T., Becker, M. W., & Jigo, M. (2013) Limited featured-based attention to multiple features. Vision Research, 85, 36-44. pdf Attending to a feature (e.g., color or motion direction) can enhance the early visual processing of that feature. However, it is not known whether one can simultaneously enhance multiple features. We examined people’s ability to attend to multiple features in a feature cueing paradigm. Each trial contained two intervals consisting of a random dot motion stimulus. One interval (noise) had 0% coherence (no net motion), while the other interval (signal) moved in a particular direction with varying levels of coherence. Participants reported which interval contained the signal in one of three cueing conditions. In the one-cue condition, a line segment preceded the stimuli indicating the direction of the signal with 100% validity. In the two-cue condition, two lines preceded the stimuli, indicating the signal would move in one of the two cued directions. In the no-cue condition, no line segment appeared before the dot stimuli. In several experiments, we consistently observed a lower detection threshold in the one-cue condition than the no-cue condition, showing that participants can enhance processing of a single feature. However, detection threshold was consistently higher for the two-cue than one-cue condition, indicating that participants could not simultaneously enhance two motion directions as effectively as one direction. This finding revealed a severe capacity limit in our ability to enhance early visual processing for multiple features.
- Solgi, M., Liu, T., & Weng, J. (2013) A computational developmental model for specificity and transfer in perceptual learning. Journal of Vision, 13(1):7,1-23. pdf link How and under what circumstances the training effects of perceptual learning (PL) transfer to novel situations is critical to our understanding of generalization and abstraction in learning. Although PL is generally believed to be highly specific to the trained stimulus, a series of psychophysical studies have recently shown that training effects can transfer to untrained conditions under certain experimental protocols. In this article, we present a brain-inspired, neuromorphic computational model of the Where-What visuomotor pathways which successfully explains both the specificity and transfer of perceptual learning. The major architectural novelty is that each feature neuron has both sensory and motor inputs. The network of neurons is autonomously developed from experience, using a refined Hebbian-learning rule and lateral competition, which altogether result in neuronal recruitment. Our hypothesis is that certain paradigms of experiments trigger two-way (descending and ascending) off-task processes about the untrained condition which lead to recruitment of more neurons in lower feature representation areas as well as higher concept representation areas for the untrained condition, hence the transfer. We put forward a novel proposition that gated self-organization of the connections during the off-task processes accounts for the observed transfer effects. Simulation results showed transfer of learning across retinal locations in a Vernier discrimination task in a double-training procedure, comparable to previous psychophysical data (Xiao et al.,2008). To the best of our knowledge, this model is the first neurally-plausible model to explain both transfer and specificity in a PL setting.
- Hou, Y. & Liu, T. (2012) Neural correlates of object-based attentional selection in human cortex. Neuropsychologia, 50, 2916-2925. pdf Humans can attend to different objects independent of their spatial locations. While selecting an object has been shown to modulate object processing in high-level visual areas in occipitotemporal cortex, where/how behavioral importance (i.e., priority) for objects is represented is unknown. Here we examined the patterns of distributed neural activity during an object-based selection task. We measured brain activity with functional magnetic resonance imaging (fMRI), while participants viewed two superimposed, dynamic objects (left- and right-pointing triangles) and were cued to attend to one of the triangle objects. Enhanced fMRI response was observed for the attention conditions compared to a neutral condition, but no significant difference was found in overall response amplitude between two attention conditions. By using multi-voxel pattern classification (MVPC), however, we were able to distinguish the neural patterns associated with attention to different objects in early visual cortex (V1 to hMTþ) and lateral occipital complex (LOC). Furthermore, distinct multi-voxel patterns were also observed in frontal and parietal areas. Our results demonstrate that object-based attention has a widespread modulation effect along the visual hierarchy and suggest that object-specific priority information is represented by patterned neural activity in the dorsal frontoparietal network
- Mance, I., Becker, M. W., & Liu, T (2012) Parallel consolidation of simple features into visual short-term memory. Journal of Experimental Psychology: Human Perception & Performance, 38, 429- 438. pdf Although considerable research has exa mined the storage limits of visual short-term memory (VSTM), little is known ab out the initial formation (i.e., the consolidation) of VSTM representations. A few previous studies have estimated the capacity of consolidation to be one item at a time. Here we used a sequential-simultaneous manipulation to re-examine the limits of consolidating items into VSTM. Participants viewed briefly presen ted and masked color patches (targets), which were shown either sequentially or simultaneously. A probe color followed the targets and participants decided whether it matched one of the targets or was a novel color. In four experiments, we consistently found equal performance for sequential and simultaneous presentations for two targets. Worse performance in the simultaneous than the sequential condition was observed for larger set sizes (three and four). Contrary to previous results suggesting a severe capacity limit of one item, our results indicate that consolidation into VSTM can occur in parallel and without capacity limits for at least two items.
- Liu, T. & Hou, Y. (2011) Global feature-based attention to orientation. Journal of Vision, 11(10):8, 1-8. pdf link Selective attention to motion direction can modulate the strength of direction selective sensory responses regardless of their spatial locations. Although such spatially global modulation is thought to be a general property of feature-based attention, few studies have examined visual features other than motion. Here we used an adaptation protocol combined with attentional instructions to assess whether attention to orientation, a prominent feature in early visual processing, also exhibit such spatially global modulation. We adapted observers to an orientation by cueing them to attend to the orientation in a compound grating that was presented at a peripheral location. We then assessed the size of the tilt aftereffect at three locations that were never stimulated by the adapter. Attending to orientation produced a tilt aftereffect in these locations, indicating that attention modulated orientation selective mechanisms in remote locations from the adapter. Furthermore, there was no difference in the magnitude of the tilt aftereffect for test stimuli that were located at different distances and hemifields to the adapter. These results suggest that attention to orientation spreads uniformly across the visual field. Thus spatially global modulation seems to be a general property of feature-based attention and it provides a flexible mechanism to modulate feature salience across the visual field.
- Liu, T. & Pleskac, T. J. (2011) Neural correlates of evidence accumulation in a perceptual decision task. Journal of Neurophysiology, 106, 2383-2398. pdf Sequential sampling models provide a useful framework for understanding human decision making. A key component of these models is an evidence accumulation process in which information is accrued over time to a threshold, at which point a choice is made. Previous neurophysiological studies on perceptual decision making have suggested accumulation occurs only in sensorimotor areas involved in making the action for the choice. Here we investigated the neural correlates of evidence accumulation in the human brain using functional magnetic resonance imaging (fMRI) while manipulating the quality of sensory evidence, the response modality, and the foreknowledge of the response modality. We trained subjects to perform a random dot motion direction discrimination task by either moving their eyes or pressing buttons to make their responses. In addition, they were cued about the response modality either in advance of the stimulus or after a delay. We isolated fMRI responses for perceptual decisions in both independently-defined sensorimotor areas and task-defined non-sensorimotor areas. We found neural signatures of evidence accumulation-a higher fMRI response on low coherence trials than high coherence trials-primarily in saccade-related sensorimotor areas (FEF, IPS) and non-sensorimotor areas in anterior insula and inferior frontal sulcus. Critically, such neural signatures did not depend on response modality or foreknowledge. These results help establish human brain areas involved in evidence accumulation and suggest that the neural mechanism for evidence accumulation is not specific to effectors. Instead, the neural system might accumulate evidence for particular stimulus features relevant to a perceptual task.
- Liu, T., Hospadaruk, L., Zhu, D., & Gardner, J. L. (2011) Feature-specific attentional priority signals in human cortex. Journal of Neuroscience, 31, 4484-4495. pdf Human can flexibly attend to a variety of stimulus dimensions, including spatial location and various features such as color and direction of motion. While the locus of spatial attention has been hypothesized to be represented by priority maps encoded in several dorsal frontal and parietal areas, it is unknown how the brain represents attended features. Here we examined the distribution and organization of neural signals related to deployment of feature-based attention. Subjects viewed a compound stimulus containing two superimposed motion directions (or colors), and were instructed to perform an attention-demanding task on one of the directions (or colors). We found elevated and sustained fMRI response for the attention task compared to a neutral condition, without reliable differences in overall response amplitude between attending to different features. However, using multi-voxel pattern analysis, we were able to decode the attended feature in both early visual areas (V1 to hMT+) and frontal and parietal areas (e.g., IPS1-4 and FEF) that are commonly associated with spatial attention. Furthermore, analysis of the classifier weight maps showed that attending to motion and color evoked different patterns of activity, suggesting different neuronal subpopulations in these regions are recruited for attending to different feature dimensions. Thus, our finding suggests that rather than a purely spatial representation of priority, frontal and parietal cortical areas also contain multiplexed signals related to the priority of different non-spatial features.
- Liu, T., Mance, I. (2011) Constant spread of feature-based attention across the visual field. Vision Research, 51, 26-33. pdf Attending to a feature in one location can produce feature-specific modulation in a different location. This global feature-based attention effect has been demonstrated using two stimulus locations. Although the spread of feature-based attention is presumed to be constant across spatial locations, it has not been tested empirically. We examined the spread of feature-based attention by measuring attentional modulation of the motion aftereffect (MAE) at remote locations. Observers attended to one of two directions in a compound motion stimulus (adapter) and performed a speed-increment task. MAE was measured via a speed-nulling procedure for a test stimulus at different distances from the adapter. In Experiment 1, the adapter was at fixation, while the test stimulus was located at different eccentricities. We also measured the magnitude of baseline MAE for each location in two control conditions that did not require feature-based selection necessitated by a compound stimulus. In Experiment 2, the adapter and test stimuli were all located in the periphery at the same eccentricity. Our results showed that attention induced MAE spread completely across the visual field, indicating a genuine global effect. These results add to our understanding of the deployment of feature-based attention and provide empirical constraints on theories of visual attention.
- Liu, T., Abrams, J., & Carrasco, M. (2009) Voluntary attention enhances contrast appearance. Psychological Science, 20, 354-362. pdf Voluntary(endogenous, sustained) covert spatial attention selects relevant sensory information for prioritized processing. The behavioral and neural consequences of such selection have been extensively documented, but its phenomenologu has received little empirical investigation. We asked whether voluntary attention affects the subjective appearance of contrast- a fundamental dimension of visual perception. We used a demanding rapid serial visual presentation(RSVP) task to direct endogenous attention to a given location and measured perceived contrast at the attended and unattended locations. Attention increased perceived contrast of suprathreshold stimuli and also improved performance on a concurrent orientation discrimination task at the cued location. We ruled out response bias as an alternative account of the pattern of results. Thus, this study establishes that voluntary attention enhances perceived contrast. This phenomenological consequence links behavioral and neurophysiological studies on the effects of attention.
- Ling, S., Liu, T., & Carrasco, M. (2009) How spatial and feature-based attention affect the gain and tuning of population responses. Vision Research, 49, 1194-1204. pdf How does attention optimize our visual system for the task at hand? Two mechanisms have been proposed for how attention improves signal processing: gain and tuning. To distinguish between these two mechanisms we use the equivalent-noise paradigm, which measures performance as a function of external noise. In the present study we explored how spatial and feature-based attention affect performance by assessing their threshold-vs-noise (TvN) curves with regard to the signature behavioral effects of gain and tuning. Furthermore, we link our psychophysical results to neurophysiology by implementing a simple, biologically-plausible model to show that attention affects the gain and tuning of population responses differentially, depending on the type of attention being deployed: Whereas spatial attention operates by boosting the gain of the population response, feature-based attention operates by both boosting the gain and sharpening the tuning of the population response.
- Liu, T. (2007) Learning sequences of views of objects: the effect of spatiotemporal coherence on object recognition. Perception, 36, 1320-1333. link How humans recognize objects remains a contentious issue in current research on high-level vision. Here, I test the proposal by Wallis and Bulthoff (1999 Trends in Cognitive Sciences 3 22-31) suggesting that object representations can be learned through temporal association of multiple views of the same object. Participants first studied image sequences of novel, three-dimensional objects in a study block. On each trial, the images were from either an orderly sequence of depth-rotated views of the same object (SS), a scrambled sequence of those views (SR), or a sequence of different objects (RR). Recognition memory was assessed in a following test block. A within-object advantage was consistently observed–greater accuracy in the SR than the RR condition in all four experiments, greater accuracy in the SS than the RR condition in two experiments. Furthermore, spatiotemporal coherence did not produce better recognition than temporal coherence alone (similar or less accuracy in the SS compared to the SR condition). These results suggest that the visual system can use temporal regularity to build invariant object representations, via the temporal-association mechanism.
- Liu, T., Larsson, J., & Carrasco, M. (2007) Feature-based attention modulates orientation-selective responses in human visual cortex. Neuron, 55, 313-323. pdf How does feature-based attention modulate neural responses? We used adaptation to quantify the effect of feature-based attention on orientation-selective responses in human visual cortex. Observers were adapted to two superimposed oblique gratings while attending to one grating only. We measured the magnitude of attention-induced orientation-selective adaptation both psychophysically, by the behavioral tilt aftereffect, and physiologically, using fMRI response adaptation. We found evidence for orientation-selective attentional modulation of neuronal responses—a lower fMRI response for the attended than the unattended orientation—in multiple visual areas, and a significant correlation between the magnitude of the tilt aftereffect and that of fMRI response adaptation in V1, the earliest site of orientation coding. These results show that feature-based attention can selectively increase the response of neuronal subpopulations that prefer the attended feature, even when the attended and unattended features are coded in the same visual areas and share the same retinotopic location.
Accompanying commentary in Neuron: Attending to features inside and outside the spotlight of attention.
Article in Scientific American: How we know where our lost keys are.
- Liu, T., Stevens, S. T., & Carrasco, M. (2007) Comparing the time course and efficacy of spatial and feature-based attention. Vision Research, 47, 108-113. pdf We investigated the time course of feature-based attention and compared it to the time course of spatial attention in an experiment with identical stimuli and task. Observers detected a speed increment in a compound motion stimulus preceded by cues that indicated either the target location or direction. The cue-target stimulus-onset-asynchrony (SOA) was varied to assess the time course of the attentional effect. We found that spatial attention was deployed earlier than feature-based attention and that both types of attention improved performance to a similar extent at a longer SOA. Results indicate that attention is a flexible mechanism allowing us to efficiently select task-relevant information based on either spatial or feature dimensions, but that spatial attention exert its effects faster.
- Liu, T., Heeger, D. J., & Carrasco, M. Neural correlates of visual vertical meridian asymmetry. (2006) Journal of Vision, 6, 1294-1306. pdf link Human visual performance is better below than above fixation along the vertical meridian-a phenomenon we refer to as vertical meridian asymmetry (VMA). Here, we used fMRI to investigate the neural correlates of the VMA. We presented stimuli of two possible sizes and spatial frequencies on the horizontal and vertical meridians and analyzed the fMRI data in subregions of early visual cortex (V1/V2) that corresponded retinotopically to the stimulus locations. Asymmetries in both the spatial extent and amplitude of the fMRI measurements correlated with the behavioral VMA. These results demonstrate that the VMA has a neural basis at the earliest stages of cortical visual processing and imply that visual performance is limited by the pooled sensory responses of large populations of neurons in the visual cortex.
- Liu, T., Fuller, S., & Carrasco, M. (2006) Attention alters the appearance of motion coherence. Psychonomic Bulletin & Review, 13, 1091-1096. pdf Selective attention enhances visual information processing, as measured by behavioral performance and neural activity. However, little is known about its effects on subjective experience. Here, we investigated the effect of transient (exogenous) attention on the appearance of visual motion, using a psychophysical procedure that directly measures appearance and controls for response bias. Observers viewed pairs of moving dot patterns and reported the motion direction of the more coherent pattern. Directing attention (via a peripheral precue) to a stimulus location increased its perceived coherence level and improved performance on a direction discrimination task. In a control experiment, we ruled out response bias by lengthening the time interval between the cue and the stimuli, so that the effect of transient attention could no longer be exerted. Our results are consistent with those of neurophysiological studies showing that attention modulates motion processing and provide evidence of a subjective perceptual correlate of attention, with a concomitant effect on performance.
- Rauschenberger, R., Liu, T., Slotnick, S. D., & Yantis, S. (2006) Temporally unfolding neural representation of pictorial occlusion. Psychological Science, 17, 358-364. pdf The human visual system possesses a remarkable ability to reconstruct the shape of an object that is partly occluded by an interimposed surface. Behavioral results suggest that, under some circumstances, this perceptual process(termed amodal completion) progresses from an initial representation of local image features to a completed representation of a shape that may include features that are not explicitly present in the retinal image. Recent functional magnetic imaging(fMRI) studies have shown that the completed surface is represented in early visual cortical areas. We used fMRI adaptation, combined with brief, masked exposures, to track the amodal completion process as it unfolds in early visual cortical regions. We report evidence for an evolution of the neural representation from the image-based feature representation to the completed representation. Our method offers the possibility of measuring changes in cortical activity using fMRI over a time scale of a few hundred milliseconds.
- Liu, T., Pestilli, F., & Carrasco, M. (2005) Transient attention enhances perceptual performance and fMRI response in human visual cortex. Neuron, 45, 469-477. pdf When a visual stimulus suddenly appears, it captures attention, producing a transient improvement of performance on basic visual tasks. We investigate the effect of transient attention on stimulus representations in early visual areas using rapid event-related fMRI. Participants discriminated the orientation of one of two gratings preceded or followed by a non-predictive peripheral cue. Compared to control conditions, precueing the target location improved performance and produced a larger fMRI response in corresponding retinotopic areas. This enhancement progressively increased from striate to extrastriate areas. Control conditions indicated that the enhanced fMRI respose was not due to sensory summation of cue and target signals. Thus, an uninformative precue increases both perceptual performance and the concomitant stimulus-evoked activity in early visual areas. These results provide evidence regarding the retinotopically specific neural correlate for the effects of transient attention on early vision.
- Liu, T., Slotnick, S. D., & Yantis, S. (2004) Human MT+ mediates perceptual filling-in during apparent motion. NeuroImage, 21, 1772– 1780. pdf During apparent motion, spatially distinct items presented in alternation cause the perception of a visual stimulus smoothly traversing the intervening space where no physical stimulus exists. We used fMRI to determine whether the perceptual ‘filling-in’ that underlies this phenomenon has an early or late cortical locus. Subjects viewed a display comprised of concentric rings that elicited apparent motion (two concentric rings presented in alternation), flicker (the same rings presented simultaneously), or real motion. We independently localized the cortical regions corresponding to the path of apparent motion in early visual areas (V1, V2, VP, V3, V4v, V3A), as well as the human motion processing complex (MT+). Cortical activity in the path of apparent motion in early visual areas was similar in amplitude during both apparent motion and flicker. In contrast, cortical activity in MT+ was higher in amplitude during apparent motion than during flicker, but was lower in amplitude than during real motion. In addition, we observed overlap in the cortical loci of MT+ and the lateral occipital complex (LOC), a region involved in shape and object processing. This overlap suggests that these regions could directly interact and thereby support perceived object continuity during apparent motion.
- Sunness, J. S., Liu, T., & Yantis, S. (2004) Retinotopic mapping of visual cortex using fMRI in a patient with central scotomas from atrophic macular degeneration.Ophthalmology, 111, 1595–1598. pdf Purpose: To describe retinotopic mapping of the visual cortex when a central scotoma is present.
Design: Single observational case report.
Methods: Scanning laser ophthalmoscope perimetry was used to define the site and stability of fixation and the area of dense scotoma. Functional magnetic resonance imaging of the visual cortex was performed while the patient viewed an expanding annular stimulus.
Results: Retinotopic mapping of the visual cortex for a patient with a horseshoe scotoma from geographicatrophy involving the macular region showed a loss of stimulation to the cortical areas representing the site of the atrophic lesion.
Conclusions: Cortical retinotopic mapping can be performed successfully in patients with central scotomas from macular disease. This study can serve as a basis for the future investigation of cortical plasticity in visual cortex.
- Liu, T., Slotnick, S. D., Serences, J. S., Yantis, S. (2003) Cortical mechanisms of feature-based attentional control. Cerebral Cortex, 13, 1334-1343. pdf A network of fronto-parietal cortical areas is known to be involved in the control of visual attention, but the representational scope and specific function of these areas remains unclear. Recent neuro-imaging evidence has revealed the existence of both transient (attention-shift) and sustained (attention-maintenance) mechanisms of space-based and object-based attentional control. Here we investigate the neural mechanisms of feature-based attentional control in human cortex using rapid event-related functional magnetic resonance imaging (fMRI). Subjects viewed an aperture containing moving dots in which dot color and direction of motion changed once per second. At any given moment, observers attended to either motion or color. Two of six motion directions and two of six colors embedded in the stimulus stream cued subjects either to shift attention from the currently attended to the unattended feature or to maintain attention on the currently attended feature. Attentional modulation of the blood oxygenation level dependent (BOLD) fMRI signal was observed in early visual areas that are selective for motion and color. More importantly, both transient and sustained BOLD activity patterns were observed in different fronto-parietal cortical areas during shifts of attention. We suggest these differing temporal profiles reflect complementary roles in the control of attention to perceptual features
- Liu, T., & Cooper, L. A. (2003) Explicit and implicit memory for rotating objects.Journal of Experimental Psychology: Learning, Memory & Cognition, 29, 554-562. pdf Although both the object and the observer often move in natural environments, the effect of motion on visual object recognition has not been well documented. The authors examined the effect of a reversal in the direction of rotation on both explicit and implicit memory for novel, 3-dimensional objects. Participants viewed a series of continuously rotating objects and later made either an old–new recognition judgment or a symmetric–asymmetric decision. For both tasks, memory for rotating objects was impaired when the direction of rotation was reversed at test. These results demonstrate that dynamic information can play a role in visual object recognition and suggest that object representations can encode spatio-temporal information.
- Liu, T., & Cooper, L. A. (2001) The influence of task requirements on priming in object decision and matching. Memory & Cognition, 29, 874-882. pdf We argue that task requirements can be the determinant in generating different results in studies on visual object recognition. We investigated priming for novel visual objects in three implicit memory tasks. A study–test design was employed in which participants first viewed line drawings of unfamiliar objects and later made different decisions about structural aspects of the objects. Priming for both symmetric and asymmetric possible objects was observed in a task requiring a judgment of structural possibility. However, when the task was changed to one requiring a judgment of structural symmetry, only symmetric possible objects showed priming. Finally, in a matching task in which participants made a same–different judgment, only symmetric possible objects exhibited priming. These results suggest that an understanding of object representation will be most fruitful if it is based on careful analyses of both the task demands and their interaction(s) with encoding and retrieval processes.
- Serences, J. T., Liu, T., & Yantis, S. (2005) Parietal mechanisms of attentional control: locations, features, and objects. In Neurobiology of Attention, (Eds.) L. Itti, G. Rees, & J. Tsotsos. San Diego, CA:Elsevier.