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Hippocampal neurogenesis in adult Old World primates
No full textThe production of new hippocampal neurons in adulthood has been well documented in rodents. Recent studies have extended these findings to other mammalian species, such as tree shrews and marmoset monkeys. However, hippocampal neurogenesis has not been demonstrated in adult Old World primates. To investigate this possibility, we injected 11 adult Old World monkeys of different ages (5–23 years) with the thymidine analog bromodeoxyuridine and examined the fate of the labeled cells at different survival times by using neuronal and glial markers. In the young-adult and middle-aged monkeys, we found a substantial number of cells that incorporated bromodeoxyuridine and exhibited morphological and biochemical characteristics of immature and mature neurons. New cells located in the dentate gyrus expressed a marker of immature granule neurons, Turned On After Division 64 kDa protein, as well as markers of mature granule neurons including neuron specific enolase, neuronal nuclei, and the calcium-binding protein calbindin. Fewer new cells expressed the astroglial marker glial fibrillary acidic protein. Evidence of neurogenesis was observed in the oldest monkeys (23 years) as well, but it appeared to be less robust. These results indicate that the adult brains of Old World monkeys produce new hippocampal neurons. Adult macaque monkeys may provide a useful primate model for studying the functional significance of adult neurogenesis
Attention to visual motion suppresses neuronal and behavioral sensitivity in nearby feature space
No full textBACKGROUND: Feature-based attention prioritizes the processing of the attended feature while strongly suppressing the processing of nearby ones. This creates a non-linearity or “attentional suppressive surround” predicted by the Selective Tuning model of visual attention. However, previously reported effects of feature-based attention on neuronal responses are linear, e.g., feature-similarity gain. Here, we investigated this apparent contradiction by neurophysiological and psychophysical approaches. RESULTS: Responses of motion direction-selective neurons in area MT/MST of monkeys were recorded during a motion task. When attention was allocated to a stimulus moving in the neurons’ preferred direction, response tuning curves showed its minimum for directions 60–90° away from the preferred direction, an attentional suppressive surround. This effect was modeled via the interaction of two Gaussian fields representing excitatory narrowly tuned and inhibitory widely tuned inputs into a neuron, with feature-based attention predominantly increasing the gain of inhibitory inputs. We further showed using a motion repulsion paradigm in humans that feature-based attention produces a similar non-linearity on motion discrimination performance. CONCLUSIONS: Our results link the gain modulation of neuronal inputs and tuning curves examined through the feature-similarity gain lens to the attentional impact on neural population responses predicted by the Selective Tuning model, providing a unified framework for the documented effects of feature-based attention on neuronal responses and behavior. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12915-022-01428-7
Feature-based attention induces non-linearities in neuronal tuning and behavior during visual motion perception
No full textThe Impact Of Color on Response Inhibition
Full text linkResponse inhibition is an important cognitive function that affects decision-making and action selection. Impairments in it occur in neurodegenerative diseases therefore, ways to support response inhibition are important for quality of life. One possibility is the use of color, as color has been shown to modulate inhibitory processes. The overall objective of this work was to determine the prefrontal networks underlying response inhibition that can be modulated through an automatic attentional process such as color. A series of three studies were performed whereby young adults performed a stop-signal task (SST) or a Go/No-go task (GNGT) with colored stimuli. In our first study, the SST, a reactive response inhibition task, was performed to determine whether the effect of color on response inhibition was due to color opponency, attentional color hierarchy, or visual associations. We found that while red stop signals produced faster response inhibition compared to green, blue and yellow stop signals did not differ from each other. This pattern of results was not consistent with color opponency or the attentional color hierarchy of red > green > yellow > blue. Therefore, red facilitating and green impairing response inhibition suggested that response inhibition was modulated by visual color associations where red means stop and green means go. In our second study, we tested if the color modulations between red and green extended beyond countermanding to more general inhibitory control by using a proactive response inhibition task, the GNGT. Indeed, participants were more successful on red in comparison to green No-go trials. Based on these results, a modified accumulator model and putative neural circuitry of color modulation response inhibition was proposed. In our third study, event-related potentials (ERPs) were recorded while participants performed a GNGT to test the putative underlying neural network. While the P300 was not modulated by color, we observed reduced N200 amplitudes and earlier N200 latencies over the prefrontal areas proposed in study 2 in response to red No-go stimuli over green, yellow, and blue. The increased accuracy was argued to be an advantage conferred by learned and evolutionary associations to the colour red. The decreased N200 amplitudes suggested reduced conflict on No-go trials with red No-go stimuli compared to other colours. These findings bring us a step closer to mapping out the differential colour modulated neural circuitry involved in response inhibition and such research will help pave the way for efficient decision-making and staving off cognitive decline
Color Opponency Modulates Feature Integration Through Bayesian Priors
Full text linkStimulus color, even irrelevant to the task, has been shown to modulate high-level cognitive functions, producing different behavioral outcomes. However, the effect of color on lower-level perceptual processes remains unclear. To address this gap, we investigated whether color affects feature integration, an early-stage visual process, using the flash-jump illusion. Our results demonstrated that color modulates the integration of color and motion features, as red and blue flashes resulted in more veridical estimates of flash location, compared to green and yellow. We provide a novel interpretation of our current results in respect to a Bayesian perceptual framework, where the color of the flash is inherently assigned different Bayesian weights, resulting in different levels of perceptual shifts. Furthermore, in reviewing pertinent literature and empirical evidence, we have proposed a novel theory outlining three putative mechanisms, predictions and underlying neural circuitry using Bayesian frameworks to explain such color-dependent modulations in visual processing
SacTrace: an Interpretable Architecture for Assessing Contributions of Saccade Trajectories to Fixation Prediction
Full text linkHuman eye movements reflect an interplay between image-driven selection and motor planning, yet most salience and scanpath models focus only on fixation-based features. This study presents the SacTrace architecture, an interpretable fixation-prediction framework for the systematic investigation of feature use throughout the modelling process. The primary point of interest for the study is the evaluation of saccade trajectories and their utility for the prediction of human eye movement patterns. The architecture uses structured feature categories to explicitly condition visual representations on each category via Feature-wise Linear Modulation (FiLM) modules and explores their spatial relevance through cross attention. SacTrace also integrates temporal context with a Convolutional Long-Short Term Memory (ConvLSTM) network and produces probabilistic fixation estimates with a Mixture Density Network (MDN) readout. Using a dataset that provided raw eye-tracking data, we perform ablative, FiLM-parameter, and attention-map diagnostics to evaluate how trajectory features groups contribute to localization accuracy and saccade plausibility. Across analyses, trajectory inputs acted as an image-conditioned balancing signal: ablating trajectories had minimal impact on localization but substantially degraded amplitude/angle plausibility, whereas spatial feature ablation sharply reduced localization performance. Temporal-FiLM and attention diagnostics revealed that trajectory conditioning interacts differentially with hierarchical visual representations. The potential underlying neural signals captured by the trajectories are discussed along with dataset limitations, and avenues for improving trajectory encoding. The results suggest that incorporating learned, trajectory-derived priors alongside spatial image features yields more behaviorally consistent fixation predictions and provides a diagnostic pathway for linking model components to plausible oculomotor mechanisms
Modulation of Brain Activity by the Integration of Color into Dorsal Stream Object Files
Full text linkTwo superimposed surfaces of dots are perceived as separate objects when rotating in two different directions. When one surface is cued, there is a larger suppression of the attentional ERP components of the unattended surface than the attended surface when two objects are perceived versus when one object is perceived. We hypothesized that the strength of object-based attention was dependent on the differentiation of the two object representations. We tested this hypothesis by determining if two oppositely rotating superimposed surfaces of differing colors would produce a greater cueing effect than if the two surfaces were the same color. This additional color feature would allow for object files with stronger neural representation, leading to a greater suppression of the uncued surface in the task. It was found that there was a greater cueing effect in the bicolored condition compared to the unicolored condition both behaviorally and in event related potentials
Visual and Oculomotor Integration: Representations and Temporal Mechanisms
Full text linkThe visual system recruits the oculomotor system to enhance processing at a particular location of interest with the use of saccadic eye movements. This involves the transfer of visual information from the visual system to the oculomotor system so that the correct location or object may be fixated at the expense of all othersa process called target selection. However, the relative extent of visual processing between the visual and oculomotor systems to facilitate this process is disputed. Here, this question is examined by specifically investigating the extent of oculomotor processing prior to a saccade. First, the nature of object representations in the ventral stream of the visual system is examined to gain insight into how complex visual representations are encoded. Next, target selection was examined in a visual context requiring extremely complex visual computations in order to select the correct stimulus. Last, the temporal factors that affect oculomotor target selection were examined.
This research demonstrated that objects of considerable complexity elicit similar perceptual behaviours as do simple visual features. This elucidates that there are very robust modes of encoding object representations, which generalize to objects of varying complexity and familiarity. Furthermore, when these same complex visual representations were utilized on a target selection task (visual search), there was evidence of oculomotor competition between them. Given the complexity of these stimuli and the limitations of oculomotor visual processing, it was reasoned that the visual system performed these computations, as observed in the previous experiment, and the results of this computation were output to the oculomotor system. Finally, an analysis of the target selection time course suggested that the oculomotor competition observed previously is likely due to cortical top-down input, further elucidating the role of the visual system in mediating oculomotor target selection
The Effects of Target-Distractor Competition on the Oculomotor System
Full text linkWe use our eyes to investigate the world around us and decide what we want to act on. In a crowded scene, we direct our gaze to a single object of interest to gather more information about it. We do this effortlessly, but the underlying neural circuitry is complex. In this thesis, I look at temporal, spatial, and object identity factors that feed into the oculomotor system to drive eye movements in a target selection task. We varied the distance and similarity of complex objects to examine the effects of target-distractor competition on saccade trajectories and pupil size. We found that the effects of distance and similarity on saccade trajectories depended on the development of target-distractor competition. However, these factors did not modulate pupil size. Overall, these findings show that information processed in higher order visual areas is projected to the oculomotor system for saccade planning but not pupillary control
The Effect of Colour on Response Execution and Inhibition in the Stop Signal Paradigm
Full text linkRecent investigations have shown that smooth pursuit target selection is biased according to a hierarchy of red, green, yellow, and blue (e.g. red is always selected over green). This implies that colours higher on the hierarchy have greater attentional salience. Using the stop signal task, we conducted experiments in which go signal colour was manipulated (exps. 1 and 2) and in which stop signal colour was manipulated (exp. 3) to determine whether the hierarchy also applied to response execution and inhibition. When colour was either irrelevant (exp. 1) or relevant (exp. 2) to response execution there was no effect on reaction times or individual RT variance. When colour was relevant to response inhibition (exp. 3) estimated stop signal reaction times were significantly faster for red (~225ms) relative to green (~250ms) stop signals. This suggests that response inhibition, but not execution, networks are sensitive to differences in colour salience
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