Predicting perceptual learning from higher-order cortical processing

Training method and difficulty modulate electrophysiological correlates of visual perceptual learning

The present study focused on the influence of training methods and task difficulty on event-related potentials (ERPs) at early and later visual perceptual learning (VPL) on a coherent motion identification task. Sixty participants were randomly divided into four groups for training with an adaptive stimulus (staircase group) and three constant stimuli (moderate, easy and difficult intensity groups). Visual performance improved in the staircase and moderate training groups but not in the easy or difficult training groups. ERP results revealed a decreased P1 amplitude in all groups. Additionally, staircase training increased the frontal P2 amplitude; accordingly, moderate constant stimulus training reduced the frontal P2 amplitude and increased the frontal N2 amplitude. Importantly, the change in frontal P2 amplitude was correlated with improved performance, indicating the involvement of cortices responsible for higher-order cognitive processes in VPL. Additionally, the difference in frontal P2 amplitude changes suggests the modulation of training methods (adaptive and consistent) on the role of attention in VPL. Furthermore, although behavior changes were not observed, the brains in the easy and difficult groups still presented different ERP changes. In summary, the results provide electrophysiological evidence for the modulation of training methods and task difficulty in VPL-related neuroplasticity.

Using EEG microstates to examine whole-brain neuronal networks during offline rest consolidation after visual perceptual learning

Visual perceptual learning (VPL) leads to improvements in visual skills after practice or training in visual perceptual tasks. Evidence suggests that newly formed skills are preferentially consolidated by the brain during offline task-free periods. Additionally, VPL can lead to changes in brain areas associated with higher cognitive functions. Thus, training may result in changes in whole-brain networks during the offline consolidation period. To test this inference, electroencephalography (EEG) microstates were used to explore the dynamic characteristics of the whole-brain network during consolidation periods after training. Forty-five healthy young adults were randomly divided into three groups for training with moderate, easy and difficult intensity. The participants were trained on a coherent motion discrimination task, and the coherence threshold and resting EEG were measured before and after training. The results showed that visual performance improved only in the moderate training group and not in the easy or difficult training groups. Microstate analyses revealed significant decreases in the duration and occurrence rate of microstate C (often associated with the default mode network) during offline consolidation following moderate training. Moreover, the duration of microstate D (often associated with the dorsal attention network) significantly increased. However, moderate training did not change the duration or occurrence rate of microstate B (often associated with the visual network). This study revealed the activity of whole-brain networks in the consolidation period after VPL.

Brief memory reactivation may not improve visual perception

Visual perceptual learning often requires a substantial number of trials to observe significant learning effects. Previously Amar-Halpert et al. (2017) have shown that brief reactivation (5 trials/day) is sufficient to improve the performance of the texture discrimination task (TDT), yielding comparable improvements to those achieved through full practice (252 trials/day). The finding is important since it would refine our understanding of learning mechanisms and applications. In the current study, we attempted to replicate these experiments using a larger number of observers and an improved experimental design. Using between-group comparison, we did find significant improvements in the reactivation group and the full-practice group as Amar-Halpert et al. (2017) showed. However, these improvements were comparable to those of the no-reactivation group with no exposure to the TDT task over the same period. Importantly, our within-group comparison showed that both the reactivation and no-reactivation groups exhibited additional significant improvements after further practicing the TDT task for an additional three days, demonstrating that the full-practice effect was significantly superior to the effects of brief memory reactivation or simple test-retest. Besides, when refining the constant stimuli method with fewer stimulus levels and more trials per level, we still observed comparable improvements brought by the reactivation and no-reactivation groups. Therefore, our results suggested that brief memory reactivation may not significantly contribute to the improvement of perceptual learning, and traditional perceptual training could still be a necessary and effective approach for substantial improvements.

Fast perceptual learning induces location-specific facilitation and suppression at early stages of visual cortical processing

Tens of minutes of training can significantly improve visual discriminability of human adults, and this fast perceptual learning (PL) effect is usually specific to the trained location, with little transfer to untrained locations. Although location specificity is generally considered as a hallmark of visual PL, it remains unclear whether it involves both facilitation of trained locations and suppression of untrained locations. Here we developed a novel experimental design to investigate the cognitive neural mechanism of location specificity of fast PL. Specifically, we manipulated attentional settings and recorded event-related potentials (ERPs) in both the training and tests. To get reliable location-specific PL effects on early ERPs, we adopted a new approach involving analysis of contralateral-minus-ipsilateral P1 (P1c-i). ERP results showed that tens of minutes of training not only increased the late P1c-i (~100-120 ms) evoked by targets at the trained location, but also decreased the early P1c-i (~75-95 ms) evoked by distractors at the untrained location, both of which were location specific. Moreover, comparison between the pretest and posttest revealed that the suppression effect of early P1c-i preserved even when the untrained location became target location, whereas the facilitation effect of late P1c-i appeared only when the trained location remained actively attended. These findings provide the first evidence that fast PL induces both location-specific facilitation and location-specific suppression at early stages of visual cortical processing. We speculate that while the facilitation effect indicates more efficient allocation of voluntary attention to the trained location induced by fast PL, the suppression effect may reflect learning-associated involuntary suppression of visual processing at the untrained location. Several confounding factors with regard to the early ERP effects of PL are discussed, and some important issues worth further investigation are proposed.

Deep networks may capture biological behavior for shallow, but not deep, empirical characterizations

We assess whether deep convolutional networks (DCN) can account for a most fundamental property of human vision: detection/discrimination of elementary image elements (bars) at different contrast levels. The human visual process can be characterized to varying degrees of "depth," ranging from percentage of correct detection to detailed tuning and operating characteristics of the underlying perceptual mechanism. We challenge deep networks with the same stimuli/tasks used with human observers and apply equivalent characterization of the stimulus-response coupling. In general, we find that popular DCN architectures do not account for signature properties of the human process. For shallow depth of characterization, some variants of network-architecture/training-protocol produce human-like trends; however, more articulate empirical descriptors expose glaring discrepancies. Networks can be coaxed into learning those richer descriptors by shadowing a human surrogate in the form of a tailored circuit perturbed by unstructured input, thus ruling out the possibility that human-model misalignment in standard protocols may be attributable to insufficient representational power. These results urge caution in assessing whether neural networks do or do not capture human behavior: ultimately, our ability to assess "success" in this area can only be as good as afforded by the depth of behavioral characterization against which the network is evaluated. We propose a novel set of metrics/protocols that impose stringent constraints on the evaluation of DCN behavior as an adequate approximation to biological processes.

Non-sensory Influences on Auditory Learning and Plasticity

Distinguishing between regular and irregular heartbeats, conversing with speakers of different accents, and tuning a guitar-all rely on some form of auditory learning. What drives these experience-dependent changes? A growing body of evidence suggests an important role for non-sensory influences, including reward, task engagement, and social or linguistic context. This review is a collection of contributions that highlight how these non-sensory factors shape auditory plasticity and learning at the molecular, physiological, and behavioral level. We begin by presenting evidence that reward signals from the dopaminergic midbrain act on cortico-subcortical networks to shape sound-evoked responses of auditory cortical neurons, facilitate auditory category learning, and modulate the long-term storage of new words and their meanings. We then discuss the role of task engagement in auditory perceptual learning and suggest that plasticity in top-down cortical networks mediates learning-related improvements in auditory cortical and perceptual sensitivity. Finally, we present data that illustrates how social experience impacts sound-evoked activity in the auditory midbrain and forebrain and how the linguistic environment rapidly shapes speech perception. These findings, which are derived from both human and animal models, suggest that non-sensory influences are important regulators of auditory learning and plasticity and are often implemented by shared neural substrates. Application of these principles could improve clinical training strategies and inform the development of treatments that enhance auditory learning in individuals with communication disorders.

The effect of initial performance on motion perception improvements is modulated by training method

Repeated practice of a perceptual task, termed "perceptual learning," can improve visual performance. Previously, the training thresholds were determined in two ways. One is that the stimulus corresponding to a certain level in individually set psychometric functions was selected as the training threshold. The other is that the certain stimulus was selected as the training threshold without consideration of individual differences. However, little is known about how the two training methods modulate perceptual learning. This study aimed to evaluate the effect of initial performance on patterns of motion perceptual learning under two methods-individually set or group averaged-for setting the training threshold. Thirty-six observers were randomly divided into individual and group thresholds. Psychometric functions, with the percentage correct as a function of coherence, were measured using the coherent motion direction identification task. For the individual threshold, each observer was trained at individualized coherence level, targeting 60% correct for each observer's psychometric function. For the group threshold, each observer was trained at one coherence level, targeting 60% correct in the group-averaged psychometric function. The threshold was reduced after training with the method of constant stimulus in both groups, indicating improvements following perceptual learning. Furthermore, observers with a poorer initial performance exhibited greater learning gains independent of the training method. Importantly, the correlation between the initial performance and learning gains was larger in the individual threshold than in the group threshold, suggesting the influence of the initial performance on the learning amount depends on the training method.

Sleep is more than rest for plasticity in the human cortex

Sleep promotes adaptation of behavior and underlying neural plasticity in comparison to active wakefulness. However, the contribution of its two main characteristics, sleep-specific brain activity and reduced stimulus interference, remains unclear. We tested healthy humans on a texture discrimination task, a proxy for neural plasticity in primary visual cortex, in the morning and retested them in the afternoon after a period of daytime sleep, passive waking with maximally reduced interference, or active waking. Sleep restored performance in direct comparison to both passive and active waking, in which deterioration of performance across repeated within-day testing has been linked to synaptic saturation in the primary visual cortex. No difference between passive and active waking was observed. Control experiments indicated that deterioration across wakefulness was retinotopically specific to the trained visual field and not due to unspecific performance differences. The restorative effect of sleep correlated with time spent in NREM sleep and with electroencephalographic slow wave energy, which is thought to reflect renormalization of synaptic strength. The results indicate that sleep is more than a state of reduced stimulus interference, but that sleep-specific brain activity restores performance by actively refining cortical plasticity.

Failure of resting-state frontal-occipital connectivity in linking visual perception with reading fluency in Chinese children with developmental dyslexia

It is widely accepted that impairment in visual perception impedes children's reading development, and further studies have demonstrated significant enhancement in reading fluency after visual perceptual training. However, the mechanism of the neural linkage between visual perception and reading is unclear. The purpose of this study was to examine the intrinsic functional relationship between visual perception (indexed by the texture discrimination task,TDT) and reading ability (character reading and reading fluency) in Chinese children with developmental dyslexia (DD) and those with typical development (TD). The resting-state functional connectivity (RSFC) between the primary visual cortex (V1, BA17) and the entire brain was analyzed. In addition, how RSFC maps are associated with TDT performance and reading ability in the DD and TD groups was examined. The results demonstrated that the strength of the RSFC between V1 and the left middle frontal gyrus (LMFG, BA9/BA46) was significantly correlated with both the threshold (SOA) of the TDT and reading fluency in TD children but not in DD children. Moreover, LMFG-V1 resting-state connectivity played a mediating role in the association of visual texture discrimination and reading fluency, but not in character reading, in TD children. In contrast, this mediation was absent in DD children, albeit their strengths of RSFC between V1 and the left middle frontal gyrus (LMFG) were comparable to those for the TD group. These findings indicate that typically developing children use the linkage of the RSFC between the V1 and LMFG for visual perception skills, which in turn promote fluent reading; in contrast, children with dyslexia, who had higher TDT thresholds than TD children, could not take advantage of their frontal-occipital connectivity to improve reading fluency abilities. These findings suggest that visual perception plays an important role in reading skills and that children with developmental dyslexia lack the ability to use their frontal-occipital connectivity to link visual perception with reading fluency.

Visual motion perception improvements following direct current stimulation over V5 are dependent on initial performance

Transcranial direct current stimulation (tDCS) can improve visual perception. However, the effect of tDCS on visual perception is largely variable, possibly due to individual differences in initial performance. The goal of the present study was to evaluate the dependency of visual motion perception improvements on initial performance. Twenty-eight observers were randomly divided into two groups. Anodal tDCS and sham stimulation were separately applied to V5 (1.5 mA, 20 min), while observers performed a coherent motion direction identification task. The results showed that compared to sham stimulation, anodal tDCS induced a significant improvement in motion perception that lasted at least 20 min. In addition, the degree of improvement was dependent on initial performance, with a greater improvement magnitude observed for those with poorer initial performance. These results may have implications for understanding the nature of the stimulation rule and for the use of a customised stimulation protocol to enhance tDCS efficiency in practical applications.

Perceptual learning induces changes in early and late visual evoked potentials

Studies of visual cortical responses following visual perceptual learning (VPL) have produced diverse results, revealing neural changes in early and/or higher-level visual cortex as well as changes in regions responsible for higher cognitive processes such as attentional control. In this study, we investigated substrates of VPL in the human brain by recording visual evoked potentials with high-density electroencephalography (hdEEG) before (Session 1) and after (Session 2) training on a texture discrimination task (TDT), with two full nights of sleep between sessions. We studied the following event-related potential (ERP) components: C1 (early sensory processing), P1 and N1 (later sensory processing, modulated by top-down spatial attention), and P3 (cognitive processing). Our results showed a significant decrease in C1 amplitude at Session 2 relative to Session 1 that was positively correlated with the magnitude of improvement in behavioral performance. Although we observed no significant changes in P1 amplitude with VPL, both N1 amplitude and latency were significantly decreased in Session 2. Moreover, the difference in N1 latency between Session 1 and Session 2 was negatively correlated with behavioral improvement. We also found a significant increase in P3 amplitude following training. Our results suggest that VPL of the TDT task may be due to plasticity in early visual cortical areas as well as changes in top-down attentional control and cognitive processing.

Effects of Stimulus Size and Contrast on the Initial Primary Visual Cortical Response in Humans

Decades of intracranial electrophysiological investigation into the primary visual cortex (V1) have produced many fundamental insights into the computations carried out in low-level visual circuits of the brain. Some of the most important work has been simply concerned with the precise measurement of neural response variations as a function of elementary stimulus attributes such as contrast and size. Surprisingly, such simple but fundamental characterization of V1 responses has not been carried out in human electrophysiology. Here we report such a detailed characterization for the initial "C1" component of the scalp-recorded visual evoked potential (VEP). The C1 is known to be dominantly generated by initial afferent activation in V1, but is difficult to record reliably due to interindividual anatomical variability. We used pattern-pulse multifocal VEP mapping to identify a stimulus position that activates the left lower calcarine bank in each individual, and afterwards measured robust negative C1s over posterior midline scalp to gratings presented sequentially at that location. We found clear and systematic increases in C1 peak amplitude and decreases in peak latency with increasing size as well as with increasing contrast. With a sample of 15 subjects and ~180 trials per condition, reliable C1 amplitudes of -0.46 µV were evoked at as low a contrast as 3.13% and as large as -4.82 µV at 100% contrast, using stimuli of 3.33° diameter. A practical implication is that by placing sufficiently-sized stimuli to target favorable calcarine cortical loci, robust V1 responses can be measured at contrasts close to perceptual thresholds, which could greatly facilitate principled studies of early visual perception and attention.