View from the top: hierarchies and reverse hierarchies in the visual system

We propose that explicit vision advances in reverse hierarchical direction, as shown for perceptual learn-ing. Processing along the feedforward hierarchy of areas, leading to increasingly complex represen-tations, is automatic and implicit, while conscious perception begins at the hierarchy’s top, gradually re-turning downward as needed. Thus, our initial con-scious percept—vision at a glance—matches a highlevel, generalized, categorical scene interpretation, identifying “forest before trees.” For later vision with scrutiny, reverse hierarchy routines focus attention to specific, active, low-level units, incorporating into conscious perception detailed information available there. Reverse Hierarchy Theory dissociates between early explicit perception and implicit low-level vision

Stimulus uncertainty and perceptual learning: Similar principles govern auditory and visual learning

AbstractWe examined the impact of variability in speech stimuli on improvement of general performance and on accessibility to low-level information as a function of practice. Listeners had to discriminate between two similar words in noise in two configurations that differed only in their low-level binaural information, which was either null or maximal. The difference in performance quantifies the use of binaural low-level information. These configurations were presented in three training protocols: in separate blocks; in a consistently interleaved manner; and in a randomly mixed manner. The first protocol enabled optimal use of the low-level binaural cues already at the first training session. The second, consistently interleaved protocol required more than one training session to reach the same performance. The final, mixed protocol did not enable optimal use of the low-level cues even after multi-session training. Interestingly, training with the first two protocols transferred to the mixed one. These results are in line with recent findings in the visual modality. In both modalities, the effects of variability on learning can be explained by the introduction of obstructions to a search mechanism going down along the sensory processing hierarchy, as suggested by the Reverse Hierarchy Theory

Low-Level Information and High-Level Perception: The Case of Speech in Noise

Auditory information is processed in a fine-to-crude hierarchical scheme, from low-level acoustic information to high-level abstract representations, such as phonological labels. We now ask whether fine acoustic information, which is not retained at high levels, can still be used to extract speech from noise. Previous theories suggested either full availability of low-level information or availability that is limited by task difficulty. We propose a third alternative, based on the Reverse Hierarchy Theory (RHT), originally derived to describe the relations between the processing hierarchy and visual perception. RHT asserts that only the higher levels of the hierarchy are immediately available for perception. Direct access to low-level information requires specific conditions, and can be achieved only at the cost of concurrent comprehension. We tested the predictions of these three views in a series of experiments in which we measured the benefits from utilizing low-level binaural information for speech perception, and compared it to that predicted from a model of the early auditory system. Only auditory RHT could account for the full pattern of the results, suggesting that similar defaults and tradeoffs underlie the relations between hierarchical processing and perception in the visual and auditory modalities

The Spread of Attention and Learning in Feature Search: Effects of Target Distribution and Task Difficulty

We examined the roles of two determinants of spatial attention in governing the spread of perceptual learning, namely, stimulus location distribution and task difficulty. Subjects were trained on detection of a target element with an odd orientation imbedded in an array of light bars with otherwise uniform orientation. To assess the effects of target distribution on attention and learning, target positions were distributed so that attention was allocated not only to the target positions themselves, but also to intermediate positions where the target was not presented. Target detection performance substantially improved and improvement spread to match the induced window of spatial attention rather than only the actual target locations. To assess the effect of task difficulty on the spread of attention and learning, the target-distractor orientation difference and the time interval available for processing were manipulated. In addition, we compared performance of subjects with more versus with less detection difficulty. A consistent pattern emerged: When the task becomes more difficult, the window of attention shrinks, and learning becomes more localized. We conclude that task-specific spatial attention is both necessary and sufficient to induce learning. The spread of spatial attention, and thus of learning, is determined by the integrated effects of target distribution and task difficulty. We propose a theoretical framework whereby these factors combine to determine the cortical level of the focus of attention, which in turn enables learning modifications

Musical experience, auditory perception and reading-related skills in children.

BACKGROUND: The relationships between auditory processing and reading-related skills remain poorly understood despite intensive research. Here we focus on the potential role of musical experience as a confounding factor. Specifically we ask whether the pattern of correlations between auditory and reading related skills differ between children with different amounts of musical experience. METHODOLOGY/PRINCIPAL FINDINGS: Third grade children with various degrees of musical experience were tested on a battery of auditory processing and reading related tasks. Very poor auditory thresholds and poor memory skills were abundant only among children with no musical education. In this population, indices of auditory processing (frequency and interval discrimination thresholds) were significantly correlated with and accounted for up to 13% of the variance in reading related skills. Among children with more than one year of musical training, auditory processing indices were better, yet reading related skills were not correlated with them. A potential interpretation for the reduction in the correlations might be that auditory and reading-related skills improve at different rates as a function of musical training. CONCLUSIONS/SIGNIFICANCE: Participants' previous musical training, which is typically ignored in studies assessing the relations between auditory and reading related skills, should be considered. Very poor auditory and memory skills are rare among children with even a short period of musical training, suggesting musical training could have an impact on both. The lack of correlation in the musically trained population suggests that a short period of musical training does not enhance reading related skills of individuals with within-normal auditory processing skills. Further studies are required to determine whether the associations between musical training, auditory processing and memory are indeed causal or whether children with poor auditory and memory skills are less likely to study music and if so, why this is the case

Learning Pop-out Detection: Specificities to Stimulus Characteristics

Training induces dramatic improvement in the performance of pop-out detection. In this study, we examined the specificities of this improvement to stimulus characteristics. We found that learning is specific within basic visual dimensions: orientation, size and position. Accordingly, following training with one set of orientations, rotating target and distracters by 30 deg or more substantially hampers performance. Furthermore, rotation of either target or distracters alone greatly increases threshold. Learning is not transferred to reduced-size stimuli. Position specificity near fixation may be finer than 0.7 deg. On the other hand, learning transfers to the untrained eye, to expanded images, to mirror image transformations and to homologous positions across the midline (near fixation). Thus, learning must occur at a processing level which is early enough to maintain fine separability along basic stimulus dimensions, yet sufficiently high to manifest the described generalizations. We suggest that the site of early perceptual learning is one of the cortical areas which receive input from primary visual cortex, VI, and where top-down attentional control i

Sequential Spatial Frequency Discrimination is Consistently Impaired Among Adult Dyslexics

The degree and nature of dyslexics' difficulties in performing basic visual tasks have been debated for more than thirty years. We recently found that dyslexics' difficulties in detecting temporally modulated gratings are specific to conditions that require accurate comparisons between sequentially presented stimuli [Brain 124 (2001) 1381]. We now examine dyslexics' spatial frequency discrimination (rather than detection), under simultaneous (spatial forced choice) and sequential (temporal forced choice) presentations. Sequential presentation (at SOAs of 0.5, 0.75 and 2.25 s) yielded better discrimination thresholds among the majority of controls (around 0.5 c/# reference), but not among dyslexics. Consequently, there was a (large and significant) group e#ect only for the sequential conditions. Within the same dyslexic group, performance on a sequential auditory task, two-tone frequency discrimination, was impaired in a smaller proportion of the participants. Taken together, our findings indicate that visual paradigms requiring sequential comparisons are difficult for the majority of dyslexic individuals, perhaps because deficits either in visual perception or in visual memory could both lead to difficulties on these paradigms