Decision factors affecting line orientation judgments in the method of single stimuli

Just noticeable differences in orientation are smaller at principal standard orientations than at oblique standard orientations when they are measured with the method of single stimuli. We determined whether this oblique effect is due to an anisotropy in decision factors. A first series of experiments showed that the subjects compare the stimulus with an internal criterion, and that this decision rule is used at all standard orientations. A second series of experiments determined the influence on the oblique effect of nonsensorial variables related to criterion setting. The results strongly suggest that the effect is not due to a criterial noise anisotropy and that criterion-setting processes are similar at principal and oblique standard orientations. The latter conclusion was also supported by an analysis of the sequential stimulus and response dependencies in this task. Hence, it appears that the oblique effect in line orientation discrimination, when it is measured with the method of single stimuli, is due not to decision factors but to a sensorially based anisotropy.     

Why does natural scene categorization require little attention? Exploring attentional requirements for natural and synthetic stimuli

It was recently demonstrated that detecting target objects (e.g., animals) in natural scenes can be done in a dual-task paradigm, in the near absence of spatial attention. Under the same conditions, subjects were unable to perform apparently simpler tasks involving synthetic stimuli (e.g., discriminating randomly rotated Ts and Ls, or a bisected colour disc and its mirror image). Classical theories predict that attention is more critical for the recognition of complex stimuli that cannot be easily separated on a single feature dimension. Therefore, these puzzling results have raised a number of questions. If it is not the complexity of a stimulus, what then determines the recognition task's attentional requirements? How does this differ between natural and artificial stimuli? What can these observations tell us about the mechanism of natural scene processing as well as its relation to attention? Here we show that removing colour information, or doubling the amount of information to be analysed, failed to make the natural scene categorization tasks significantly more “attention demanding”. Conversely, increasing discriminability or predictability did not diminish the need for attention in the case of synthetic stimuli. However, when the familiar letters, such as Ts or Ls, were presented upright, full attention was no longer required for discrimination. This suggests that familiarity and meaningfulness might be among the factors that determine attentional requirements for both natural and synthetic stimuli.     

Effects of category learning on the stimulus selectivity of macaque inferior temporal neurons

Primates can learn to categorize complex shapes, but as yet it is unclear how this categorization learning affects the representation of shape in visual cortex. Previous studies that have examined the effect of categorization learning on shape representation in the macaque inferior temporal (IT) cortex have produced diverse and conflicting results that are difficult to interpret owing to inadequacies in design. The present study overcomes these issues by recording IT responses before and after categorization learning. We used parameterized shapes that varied along two shape dimensions. Monkeys were extensively trained to categorize the shapes along one of the two dimensions. Unlike previous studies, our paradigm counterbalanced the relevant categorization dimension across animals. We found that categorization learning increased selectivity specifically for the category-relevant stimulus dimension (i.e., an expanded representation of the trained dimension), and that the ratio of within-category response similarities to between-category response similarities increased for the relevant dimension (i.e., category tuning). These small effects were only evident when the learned category-related effects were disentangled from the prelearned stimulus selectivity. These results suggest that shape-categorization learning can induce minor category-related changes in the shape tuning of IT neurons in adults, suggesting that learned, category-related changes in neuronal response mainly occur downstream from IT.     

Is perception discrete or continuous?

How does conscious perception evolve following stimulus presentation? The idea that perception relies on discrete processing epochs has been often considered, but never widely accepted. The alternative, a continuous translation of the external world into explicit perception, although more intuitive and subjectively appealing, cannot satisfactorily account for a large body of psychophysical data. Cortical and thalamocortical oscillations in different frequency bands could provide a neuronal basis for such discrete processes, but are rarely analyzed in this context. This article reconciles the unduly abandoned topic of discrete perception with current views and advances in neuroscience.     

Attentional sampling of multiple wagon wheels

Attending to a periodic motion stimulus can induce illusory reversals of the direction of motion. This continuous wagon wheel illusion (c-WWI) has been taken to reflect discrete sampling of motion information by visual attention. An alternative view is that it is caused by adaptation. Here, we attempt to discriminate between these two interpretations by asking participants to attend to multiple periodic motion stimuli: The discrete attentional sampling account, but not the adaptation account, predicts a decrease of c-WWI temporal-frequency tuning with set size (with a single periodic motion stimulus the c-WWI is tuned to a temporal frequency of 10 Hz). We presented one to four rotating gratings that occasionally reversed direction while participants counted reversals. We considered reversal overestimations as manifestations of the c-WWI and determined the temporal-frequency tuning of the illusion for each set size. Optimal temporal frequency decreased with increasing set size. This outcome favors the discrete attentional sampling interpretation of the c-WWI, with a sampling rate for each individual stimulus dependent on the number of stimuli attended.     

An oscillatory mechanism for prioritizing salient unattended stimuli

To survive in a complex world, it is important that unattended, but salient, input can still draw one's attention. In this article, we suggest that posterior alpha oscillations (8-13Hz) provide a mechanism for prioritizing and ordering unattended visual input according to 'relevance'. Gamma oscillations (30-100Hz) that are phase-locked to the alpha oscillations keep competing unattended representations apart in time, thus creating a sequence of perceptual cycles. As inhibition gradually lowers within an alpha cycle, the ordered sequence of competing input is activated, producing a temporal phase code for saliency. The proposed mechanism is based on recent experiments indicating that the phase of alpha activity modulates perception and that alpha oscillations are produced by periodic pulses of inhibition.     

The effect of category learning on the representation of shape: dimensions can be biased but not differentiated

Recent studies have suggested a profound influence of category learning on visual perception, resulting in independent processing of previously integral dimensions. The authors reinvestigate this issue for shape dimensions. They first extend previous findings that some shape dimensions (aspect ratio and curvature) are processed in a separable way, whereas others (radial frequency components) are not. They then show that a category-learning phase improved the discrimination of a relevant with respect to an irrelevant dimension, but only for separable dimensions. No similar effect was found on the relative sensitivity for integral shape dimensions. Thus, category learning is capable of biasing separable shape dimensions but does not alter the status of dimensions in the visual system as either separable or integral.     

Is it a bird? Is it a plane? Ultra-rapid visual categorisation of natural and artifactual objects

Visual processing is known to be very fast in ultra-rapid categorisation tasks where the subject has to decide whether a briefly flashed image belongs to a target category or not. Human subjects can respond in under 400 ms, and event-related-potential studies have shown that the underlying processing can be done in less than 150 ms. Monkeys trained to perform the same task have proved even faster. However, most of these experiments have only been done with biologically relevant target categories such as animals or food. Here we performed the same study on human subjects, alternating between a task in which the target category was 'animal', and a task in which the target category was 'means of transport'. These natural images of clearly artificial objects contained targets as varied as cars, trucks, trains, boats, aircraft, and hot-air balloons. However, the subjects performed almost identically in both tasks, with reaction times not significantly longer in the 'means of transport' task. These reaction times were much shorter than in any previous study on natural-image processing. We conclude that, at least for these two superordinate categories, the speed of ultra-rapid visual categorisation of natural scenes does not depend on the target category, and that this processing could rely primarily on feed-forward, automatic mechanisms.     

Timing divided attention

Visual attention can be divided over multiple objects or locations. However, there is no single theoretical framework within which the effects of dividing attention can be interpreted. In order to develop such a model, here we manipulated the stage of visual processing at which attention was divided, while simultaneously probing the costs of dividing attention on two dimensions. We show that dividing attention incurs dissociable time and precision costs, which depend on whether attention is divided during monitoring or during access. Dividing attention during monitoring resulted in progressively delayed access to attended locations as additional locations were monitored, as well as a one-off precision cost. When dividing attention during access, time costs were systematically lower at one of the accessed locations than at the other, indicating that divided attention during access, in fact, involves rapid sequential allocation of undivided attention. We propose a model in which divided attention is understood as the simultaneous parallel preparation and subsequent sequential execution of multiple shifts of undivided attention. This interpretation has the potential to bring together diverse findings from both the divided-attention and saccade preparation literature and provides a framework within which to integrate the broad spectrum of divided-attention methodologies.