Perceptual learning on orientation and direction discrimination

Two experiments were conducted to determine the extent to which perceptual learning transfers between orientation and direction discrimination. Naive observers were trained to discriminate orientation differences between two single-line stimuli, and direction differences between two single-moving-dot stimuli. In the first experiment, observers practiced the orientation and direction tasks along orthogonal axes in the fronto-parallel plane. In the second experiment, a different group of observers practiced both tasks along a single axis. Perceptual learning was observed on both tasks in both experiments. Under the same-axis condition, the observers' orientation sensitivity was found to be significantly elevated after the direction training, indicating a transfer of learning from direction to orientation. There was no evidence of transfer in any other cases tested. In addition, the rate of learning on the orientation task was much higher than the rate on the direction task. The implications of these findings on the neural mechanisms subserving orientation and direction discrimination are discussed.     

General and specific perceptual learning in radial speed discrimination

The specificity of learning in speed discrimination was examined in three psychophysical experiments. In Experiment 1, half of the observers trained with inward motion direction on a speed discrimination task, whereas the other half trained on the same task but with outward motion direction. The results indicated that significant training-based improvement transferred from a trained radial direction to an untrained radial direction. Experiment 2 confirmed this transfer by showing that complete transfer was obtained even when stimuli moving in an untrained radial direction were used in the transfer task. In Experiment 3, observers were trained at a viewing distance of 114 cm. The results showed that learning transferred partly to the viewing distances of 57 cm and 228 cm. In summary, the present transfer results indicate that reliable generalizations can be obtained in perceptual learning of radial speed discrimination.     

Motion perceptual learning: when only task-relevant information is learned

The classic view that perceptual learning is information selective and goal directed has been challenged by recent findings showing that subthreshold and task-irrelevant information can induce perceptual learning. This study demonstrates a limit on task-irrelevant learning as exposure to suprathreshold task-irrelevant signals failed to induce perceptual learning. In each trial, two random-dot motion stimuli were presented in a two-alternative forced-choice task. Observers either decided which of the two contained a coherent motion signal (detection task), or whether the coherent motion direction was clockwise or counterclockwise relative to a reference direction (discrimination task). Whereas the exact direction of the coherent motion signal was irrelevant to the detection task, detection of the coherent motion signal was necessary for the discrimination task. We found that the detection trainees improved only their detection but not discrimination sensitivity, whereas the discrimination trainees improved both. Therefore, the importance of task relevance was demonstrated in both detection and discrimination learning. Furthermore, both detection and discrimination training along a single pedestal direction transferred to a broad range of pedestal directions. The profile of the discrimination transfer (as a function of pedestal direction) narrowed for the discrimination trainees.     

Specificity of perceptual learning increases with increased training

Perceptual learning often shows substantial and long-lasting changes in the ability to classify relevant perceptual stimuli due to practice. Specificity to trained stimuli and tasks is a key characteristic of visual perceptual learning, but little is known about whether specificity depends upon the extent of initial training. Using an orientation discrimination task, we demonstrate that specificity follows after extensive training, while the earliest stages of perceptual learning exhibit substantial transfer to a new location and an opposite orientation. Brief training shows the best performance at the point of transfer. These results for orientation-location transfer have both theoretical and practical implications for understanding perceptual expertise.     

Task-specific perceptual learning on speed and direction discrimination

Twenty-two nai;ve undergraduates participated in a psychophysical experiment designed to elucidate the neural events that allow us to see subtle motion differences. Half of the subjects practiced extensively on a direction-discrimination task while the other half practiced extensively on a speed-discrimination task. The stimulus conditions in the two groups were identical. The results indicated that the learning curves for direction discrimination were significantly steeper than those for speed discrimination. Additionally, the significant practice-based improvements on each motion task did not transfer to the other motion task. The different learning rates and the lack of transfer suggest that the neural events mediating speed discrimination are at least partially independent from those mediating direction discrimination, and vice versa, even under identical stimulus conditions.     

Effects of daily training amount on visual motion perceptual learning

Perceptual learning has been widely used to study the plasticity of the visual system in adults. Owing to the belief that practice makes perfect, perceptual learning protocols usually require subjects to practice a task thousands of times over days, even weeks. However, we know very little about the relationship between training amount and behavioral improvement. Here, four groups of subjects underwent motion direction discrimination training over 8 days with 40, 120, 360, or 1080 trials per day. Surprisingly, different daily training amounts induced similar improvement across the four groups, and the similarity lasted for at least 2 weeks. Moreover, the group with 40 training trials per day showed more learning transfer from the trained direction to the untrained directions than the group with 1080 training trials per day immediately after training and 2 weeks later. These findings suggest that perceptual learning of motion direction discrimination is not always dependent on the daily training amount and less training leads to more transfer.     

Learning of interpolation in 2 and 3 dimensions.

We investigated learning of spatio-temporal interpolation in depth and its relation to spatio-temporal interpolation in two dimensions by means of a vernier discrimination task. Performance improved with training but improvement did not or only partially transfer between opposite directions of motion in depth. Improvement was also at least partly specific for the eye and for the direction of two-dimensional motion used during monocular training. This specificity might explain the apparent specificity of interpolation in three dimensions. Training with stimuli moving in two dimensions increased performance for a stimulus moving in depth. The results indicate that improvement in spatio-temporal interpolation occurs at least partly on a stage preceding stereoscopic vision, and are a rare example for transfer of improvement between different perceptual tasks.     

Perceptual learning in object recognition: object specificity and size invariance

A series of four experiments measured the transfer of perceptual learning in object recognition. Subjects viewed backward-masked, gray-scale images of common objects and practiced an object naming task for multiple days. In Experiment 1, recognition thresholds decreased on average by over 20% over 5 days of training but increased reliably following the transfer to a new set of objects. This suggests that the learning was specific to the practiced objects. Experiment 2 ruled out familiarity with strategies specific to the experimental context, such as stimulus discrimination, as the source of the improvement. Experiments 3 and 4 found that learning transferred across changes in image size. Learning could not be accounted for solely by an improvement in general perceptual abilities, nor by learning of the specific experimental context. Our results indicate that a large amount of learning took place in object-specific mechanisms that are insensitive to image size.     

Perceptual learning, aging, and improved visual performance in early stages of visual processing

In the present study, we examined whether perceptual learning methods can be used to improve performance of older individuals. Subjects performed a texture discrimination task in the peripheral visual field and a letter discrimination task in central vision. The SOA threshold was derived by presenting a mask following the stimuli. Older subjects (age greater than 65 years) were either trained for 2 days using near threshold stimuli (experimental group) or were trained with the task with supra-threshold stimuli (older control group). The experimental group showed significant improvement in the task as a result of training whereas the older control group showed no significant improvement. The improved performance post-training equaled that of a younger control group and was maintained for at least 3 months. The results of two additional experiments indicate that the improved performance was not due to changes in divided attention, that the effect of perceptual learning was location specific, and that the pattern of learning was similar to that of younger subjects. These results indicate that perceptual learning with near threshold training can be used to improve visual performance among older individuals, that the improvements are not the result of practice with the visual task, and that the improvements do not transfer to non-trained locations.     

Versatile perceptual learning of textures after variable exposures

Perceptual learning of 10-AFC texture identification is stimulus specific: after practice, identification accuracy drops substantially when textures are rotated 180°, reversed in contrast polarity, or when a novel set of textures is presented. Here we asked if perceptual learning occurs without any repetition of items during training, and whether exposure to greater stimulus variation during training influences transfer of learning. We trained three groups of subjects in a 10-AFC texture identification task on 2 days. The Standard group viewed a fixed set of 10 textures throughout training. The Variable group viewed 840 novel sets of textures. The Switch group viewed different fixed sets of 10 textures on Days 1 and 2. In all groups, transfer of learning was tested by using fixed sets of textures on Days 3 and 4 and having half of the subjects from each group switch to a novel set on Day 4. During training, the most learning was obtained by the Standard group, and gradual but significant learning was obtained by the other two groups. On Day 4, performance of the Standard group was adversely affected by a switch to novel textures, whereas performance of the Variable and Switch groups remained intact. Hence, slight but significant learning occurred without repetition of items during training, and stimulus specificity was influenced significantly by the type of training. Increasing stimulus variability by reducing the number of times stimuli are repeated during practice may cause subjects to adopt strategies that increase generalization of learning to new stimuli. Alternatively, presenting new stimuli on each trial may prevent subjects from adopting strategies that result in stimulus specific learning.     

Spatial frequency discrimination learning in normal and developmentally impaired human vision

Perceptual learning effects demonstrate that the adult visual system retains neural plasticity. If perceptual learning holds any value as a treatment tool for amblyopia, trained improvements in performance must generalise. Here we investigate whether spatial frequency discrimination learning generalises within task to other spatial frequencies, and across task to contrast sensitivity. Before and after training, we measured contrast sensitivity and spatial frequency discrimination (at a range of reference frequencies 1, 2, 4, 8, 16 c/deg). During training, normal and amblyopic observers were divided into three groups. Each group trained on a spatial frequency discrimination task at one reference frequency (2, 4, or 8 c/deg). Normal and amblyopic observers who trained at lower frequencies showed a greater rate of within task learning (at their reference frequency) compared to those trained at higher frequencies. Compared to normals, amblyopic observers showed greater within task learning, at the trained reference frequency. Normal and amblyopic observers showed asymmetrical transfer of learning from high to low spatial frequencies. Both normal and amblyopic subjects showed transfer to contrast sensitivity. The direction of transfer for contrast sensitivity measurements was from the trained spatial frequency to higher frequencies, with the bandwidth and magnitude of transfer greater in the amblyopic observers compared to normals. The findings provide further support for the therapeutic efficacy of this approach and establish general principles that may help develop more effective protocols for the treatment of developmental visual deficits.     

Spatial shifts of audio-visual interactions by perceptual learning are specific to the trained orientation and eye

A large proportion of the human cortex is devoted to visual processing. Contrary to the traditional belief that multimodal integration takes place in multimodal processing areas separate from visual cortex, several studies have found that sounds may directly alter processing in visual brain areas. Furthermore, recent findings show that perceptual learning can change the perceptual mechanisms that relate auditory and visual senses. However, there is still a debate about the systems involved in cross-modal learning. Here, we investigated the specificity of audio-visual perceptual learning. Audio-visual cuing effects were tested on a Gabor orientation task and an object discrimination task in the presence of lateralised sound cues before and after eight-days of cross-modal task-irrelevant perceptual learning. During training, the sound cues were paired with visual stimuli that were misaligned at a proximal (trained) visual field location relative to the sound. Training was performed with one eye patched and with only one Gabor orientation. Consistent with previous findings we found that cross-modal perceptual training shifted the audio-visual cueing effect towards the trained retinotopic location. However, this shift in audio-visual tuning was only observed for the trained stimulus (Gabors), at the trained orientation, and in the trained eye. This specificity suggests that multimodal interactions resulting from cross-modal (audio-visual) task-irrelevant perceptual learning involves so-called unisensory visual processing areas in humans. Our findings provide further support for recent anatomical and physiological findings that suggest relatively early interactions in cross-modal processing.     

Preexposure disrupts learning of location-contingent perceptual biases for ambiguous stimuli

The perception of a bistable stimulus as one or the other interpretation can be biased by prior presentations of that stimulus. Such learning effects have been found to be long lasting even after small amounts of training. The effectiveness of training may be influenced by preexposure to the ambiguous stimulus. Here we investigate the role of preexposure for learning a position-dependent perceptual bias. We used rotating Necker Cubes as the bistable stimuli, which were presented at two locations: above or below fixation. On training trials, additional depth cues disambiguated the rotation direction contingent on the location. On test trials, the rotating cube was presented without disambiguation cues. Without preexposure to the ambiguous stimulus, subjects learned to perceive the cube to be rotating in the trained direction for both locations. However, subjects that were preexposed to the ambiguous stimulus did not learn the trained percept-location contingency, even though the preexposure was very short compared to the subsequent training. Preexposure to the disambiguated stimulus did not interfere with learning. This indicates a fundamental difference between ambiguous test and disambiguated training trials for learning a perceptual bias. In short, small variations in paradigm can have huge effects for the learning of perceptual biases for ambiguous stimuli.     

Comparison of perceptual learning of real and virtual line orientations: An event-related potential study

When investigating perceptual learning (PL), most researchers use real figures as stimuli, but PL can occur when subjects are trained with virtual stimuli or even without any visual stimuli at all. Here, we first demonstrated that virtual lines have the same perceptual attributes as real lines by confirming that there is also an oblique effect in virtual lines (formed by a pair of circles) in an orientation discrimination task. Then, our ERP study showed that orientation discrimination learning and its transfer across real and virtual lines were associated with more negative parietal–occipital P1–N1 (reduced P1 and enhanced N1), which indicated the involvement of high-level stages of visual information processing or the involvement of top-down influences. At the same time, the specific ERP changes in the frontal ERP components were differently associated with real versus virtual line orientation learning. That is, real line learning was characterized by an early and short-lasting frontal N1 (120–140 ms) reduction, in contrast to a much later, widespread, and long-lasting P150–300 decrease in virtual line learning. These results contribute to the understanding of the neural basis of perceptual learning and the distinction between real and virtual stimulus learning.     

Perceptual learning in contrast discrimination: the effect of contrast uncertainty

Performance in perceptual tasks improves with repetition (perceptual learning), eventually reaching a saturation level. Typically, when perceptual learning effects are studied, stimulus parameters are kept constant throughout the training and during the pre- and post-training tests. Here we investigate whether learning by repetition transfers to testing conditions in which the practiced stimuli are randomly interleaved during the post-training session. We studied practice effects with a contrast discrimination task, employing a number of training methods: (i) practice with a single, fixed pedestal (base-contrast), (ii) practice with several pedestals, and (iii) practice with several pedestals that included a spatial context. Pre- and post-training tests were carried out with the base contrast randomized across trials, under conditions of contrast uncertainty. The results showed that learning had taken place with the fixed pedestal method (i) and with the context method (iii), but only the latter survived the uncertainty test. In addition, we were able to identify a very fast learning phase in contrast discrimination that improved performance under uncertainty. We contend that learned tasks that do not pass the uncertainty test involve modification of decision strategies that require exact knowledge of the stimulus.     

Perceptual learning of motion discrimination by mental imagery

Perceptual learning can occur when stimuli are only imagined, i.e., without proper stimulus presentation. For example, perceptual learning improved bisection discrimination when only the two outer lines of the bisection stimulus were presented and the central line had to be imagined. Performance improved also with other static stimuli. In non-learning imagery experiments, imagining static stimuli is different from imagining motion stimuli. We hypothesized that those differences also affect imagery perceptual learning. Here, we show that imagery training also improves motion direction discrimination. Learning occurs when no stimulus at all is presented during training, whereas no learning occurs when only noise is presented. The interference between noise and mental imagery possibly hinders learning. For static bisection stimuli, the pattern is just the opposite. Learning occurs when presented with the two outer lines of the bisection stimulus, i.e., with only a part of the visual stimulus, while no learning occurs when no stimulus at all is presented.     

Perceptual learning of Gabor orientation identification in visual periphery: complete inter-ocular transfer of learning mechanisms

We combined the external noise paradigm, the Perceptual Template Model approach, and transfer tests to investigate the mechanisms and eye-specificity of perceptual learning of Gabor orientation in visual periphery. Coupled with a fixation task, discriminating a 5 from an S in a rapid small character string at fixation, contrast thresholds were estimated for each of eight external noise levels at two performance criteria using 3/1 and 2/1 staircases. Perceptual learning in one eye was measured over 10 practice sessions, followed by five sessions of practice in the new eye to assess transfer. We found that monocular learning improved performance (reduced contrast thresholds) with virtually equal magnitude across a wide range of external noise levels with no significant change in central task performance. Based on measurements of learning effects at two performance criterion levels, we identified a mixture of stimulus enhancement and external noise exclusion as the mechanism of perceptual learning underlying the observed improvements. Perceptual learning in the trained eye generalized completely to the untrained eye. We related the transfer patterns to known physiology and psychophysics on orientation direction coding.     

Co-learning analysis of two perceptual learning tasks with identical input stimuli supports the reweighting hypothesis

Perceptual learning, even when it exhibits significant specificity to basic stimulus features such as retinal location or spatial frequency, may cause discrimination performance to improve either through enhancement of early sensory representations or through selective re-weighting of connections from the sensory representations to specific responses, or both. For most experiments in the literature, the two forms of plasticity make similar predictions (Dosher and Lu, 2009, Petrov et al., 2005). The strongest test of the two hypotheses must use training and transfer tasks that rely on the same sensory representation with different task-dependent decision structures. If training changes sensory representations, transfer (or interference) must occur since the (changed) sensory representations are common. If instead training re-weights a separate set of task connections to decision, then performance in the two tasks may still be independent. Here, we performed a co-learning analysis of two perceptual learning tasks based on identical input stimuli, following a very interesting study of Fahle and Morgan (1996) who used nearly identical input stimuli (a three dot pattern) in training bisection and vernier tasks. Two important modifications were made: (1) identical input stimuli were used in the two tasks, and (2) subjects practiced both tasks in multiple alternating blocks (800 trials/block). Two groups of subjects with counter-balanced order of training participated in the experiments. We found significant and independent learning of the two tasks. The pattern of results is consistent with the reweighting hypothesis of perceptual learning.     

Low-level sensory plasticity during task-irrelevant perceptual learning: Evidence from conventional and double training procedures

Studies of perceptual learning have focused on aspects of learning that are related to early stages of sensory processing. However, conclusions that perceptual learning results in low-level sensory plasticity are controversial, since such learning may also be attributed to plasticity in later stages of sensory processing or in readout from sensory to decision stages, or to changes in high-level central processing. To address this controversy, we developed a novel random dot motion (RDM) stimulus to target motion cells selective to contrast polarity by ensuring the motion direction information arises only from signal dot onsets and not their offsets, and used these stimuli in the paradigm of task-irrelevant perceptual learning (TIPL). In TIPL, learning is achieved in response to a stimulus by subliminally pairing that stimulus with the targets of an unrelated training task. In this manner, we are able to probe learning for an aspect of motion processing thought to be a function of directional V1 simple cells with a learning procedure that dissociates the learned stimulus from the decision processes relevant to the training task. Our results show direction-selective learning for the designated contrast polarity that does not transfer to the opposite contrast polarity. This polarity specificity was replicated in a double training procedure in which subjects were additionally exposed to the opposite polarity. Taken together, these results suggest that TIPL for motion stimuli may occur at the stage of directional V1 simple cells. Finally, a theoretical explanation is provided to understand the data.     

Perceptual learning in visual hyperacuity: A reweighting model

Improvements of visual hyperacuity are a key focus in research of perceptual learning. Of particular interest has been the specificity of visual hyperacuity learning to the particular features of the trained stimuli as well as disruption of learning that occurs in some cases when different stimulus features are trained together. The implications of these phenomena on the underlying learning mechanisms are still open to debate; however, there is a marked absence of computational models that explore these phenomena in a unified way. Here we implement a computational learning model based on reweighting and extend it to enable direct comparison, by means of simulations, with a variety of existing psychophysical data. We find that this very simple model can account for a diversity of findings, such as disruption of learning of one task by practice on a similar task, as well as transfer of learning across both tasks and stimulus configurations under certain conditions. These simulations help explain existing results in the literature as well as provide important insights and predictions regarding the reliability of different hyperacuity tasks and stimuli. Our simulations also shed light on the model’s limitations, for example in accounting for temporal aspects of training procedures or dependency of learning with contextual stimuli, which will need to be addressed by future research.