Unattended Objects Research Articles

Neural basis of auditory-induced shifts in visual time-order perception

Attended objects are perceived to occur before unattended objects even when the two objects are presented simultaneously. This finding has led to the widespread view that attention modulates the speed of neural transmission in the various perceptual pathways. We recorded event-related potentials during a time-order judgment task to determine whether a reflexive shift of attention to a sudden sound modulates the speed of sensory processing in the human visual system. Attentional cueing influenced the perceived order of lateralized visual events but not the timing of event-related potentials in visual cortex. Attentional cueing did, however, enhance the amplitude of neural activity in visual cortex, which shows that attention-induced shifts in visual time-order perception can arise from modulations of signal strength rather than processing speed in the early visual-cortical pathways.

Implicit Attentional Selection of Bound Visual Features

Traditionally, research on visual attention has been focused on the processes involved in conscious, explicit selection of task-relevant sensory input. Recently, however, it has been shown that attending to a specific feature of an object automatically increases neural sensitivity to this feature throughout the visual field. Here we show that directing attention to a specific color of an object results in attentional modulation of the processing of task-irrelevant and not consciously perceived motion signals that are spatiotemporally associated with this color throughout the visual field. Such implicit cross-feature spreading of attention takes place according to the veridical physical associations between the color and motion signals, even under special circumstances when they are perceptually misbound. These results imply that the units of implicit attentional selection are spatiotemporally colocalized feature clusters that are automatically bound throughout the visual field.

Dynamic Interaction of Object- and Space-Based Attention in Retinotopic Visual Areas

We investigated the interaction between object- and space-based attention by measuring activity in early visual cortex. After central cueing, when subjects directed attention to a spatially defined part of an object, activity in early visual areas was enhanced at corresponding retinotopic representations but also at representations of other locations covered by the object. Different from the assumption of automatic attentional "spreading" within an object, however, activity was greater for representations of cued than of uncued locations on the same object. These findings support an interaction of object-based spatial selection with object-independent spatial mechanisms in directing attention. When the target stimulus did not appear at the expected location, we found higher activation in areas representing other locations on the same object than equidistant locations on other objects. Objects, hence, also guide spatial search, and this may account for the behaviorally observed delay in processing parts of an unattended object.

Transient neural activity in human parietal cortex during spatial attention shifts.

Observers viewing a complex visual scene selectively attend to relevant locations or objects and ignore irrelevant ones. Selective attention to an object enhances its neural representation in extrastriate cortex, compared with those of unattended objects, via top-down attentional control signals. The posterior parietal cortex is centrally involved in this control of spatial attention. We examined brain activity during attention shifts using rapid, event-related fMRI of human observers as they covertly shifted attention between two peripheral spatial locations. Activation in extrastriate cortex increased after a shift of attention to the contralateral visual field and remained high during sustained contralateral attention. The time course of activity was substantially different in posterior parietal cortex, where transient increases in activation accompanied shifts of attention in either direction. This result suggests that activation of the parietal cortex is associated with a discrete signal to shift spatial attention, and is not the source of a signal to continuously maintain the current attentive state.

Complementary solutions to the binding problem in vision: Implications for shape perception and object recognition

Behavioural, neural, and computational considerations suggest that the visual system may use (at least) two approaches to binding an object's features and/or parts into a coherent representation of shape: Dynamically bound (e.g., by synchrony of firing) representations of part attributes and spatial relations form a structural description of an object's shape, while units representing shape attributes at specific locations (i.e., a static binding of attributes to locations) form an analogue (image-like) representation of that shape. I will present a computational model of object recognition based on this proposal and empirical tests of the model. The model accounts for a large body of findings in human object recognition, and makes several novel and counter intuitive predictions. In brief, it predicts that visual priming for attended objects will be invariant with translation, scale, and left-right reflection, whereas priming for unattended objects will be invariant with translation and scale, but sensitive to left-right reflection. Five experiments demonstrated the predicted relationships between visual attention and patterns of visual priming as a function of variations in viewpoint. The implications of these findings for theories of visual binding and shape perception will be discussed.

Contrast dependence of perceptual grouping in brain-damaged patients with visual extinction.

Extinction is manifested in conditions of bilateral simultaneous stimulation, as a failure to detect the stimulus contra-lateral to the side of a cerebral lesion, while the same stimulus is correctly detected there when presented in isolation. The phenomenon is usually interpreted in terms of impaired mobilization of attention from an attended to an unattended object. We have recently shown, using pairs of Gabor patches as stimuli, that pair detection is maximally improved in conditions where the two stimuli presented simultaneously to the two halves of the visual field are co-oriented and co-axial and their location is not too eccentric. Here we add new information by showing that contrast isotropy of the stimulus pair is important in producing this orientation-similarity gain. The further advantage of co-oriented co-linear stimuli over co-oriented parallel (vertical) stimuli was shown exclusively with iso-contrast stimulus pairs, and was significantly enhanced when the contrast level of the stimulus pair was low. Stimulus properties producing reduced extinction seem to correlate with the selectivity pattern and contrast dependence of (a) spatial lateral facilitation observed in psychophysical studies with normal observers, and (b) long-range interactions observed in the primary visual cortex. Thus, two remote visual stimuli seem to be processed as a single object when the corresponding neuronal activities are linked via long-range lateral interactions. The present demonstration of contrast dependency in such processing, strengthens our previous conjecture that even in the presence of significant, extinction producing, parietal damage, the primary visual cortex preserves the capacity to encode, using long-range lateral interactions, an image description in which visual objects are already segregated from background.

Gorillas in our midst: sustained inattentional blindness for dynamic events.

With each eye fixation, we experience a richly detailed visual world. Yet recent work on visual integration and change direction reveals that we are surprisingly unaware of the details of our environment from one view to the next: we often do not detect large changes to objects and scenes ('change blindness'). Furthermore, without attention, we may not even perceive objects ('inattentional blindness'). Taken together, these findings suggest that we perceive and remember only those objects and details that receive focused attention. In this paper, we briefly review and discuss evidence for these cognitive forms of 'blindness'. We then present a new study that builds on classic studies of divided visual attention to examine inattentional blindness for complex objects and events in dynamic scenes. Our results suggest that the likelihood of noticing an unexpected object depends on the similarity of that object to other objects in the display and on how difficult the priming monitoring task is. Interestingly, spatial proximity of the critical unattended object to attended locations does not appear to affect detection, suggesting that observers attend to objects and events, not spatial positions. We discuss the implications of these results for visual representations and awareness of our visual environment.

Perceiving visually presented objets: recognition, awareness, and modularity

Object perception may involve seeing, recognition, preparation of actions, and emotional responses — functions that human brain imaging and neuropsychology suggest are localized separately. Perhaps because of this specialization, object perception is remarkably rapid and efficient. Representations of componential structure and interpolation from view-dependent images both play a part in object recognition. Unattended objects may be implicitly registered, but recent experiments suggest that attention is required to bind features, to represent three-dimensional structure, and to mediate awareness.

Inattention and the perception of visual features

Subjects selectively attended to one of two interleaved, novel figures while ignoring the other figure. In subsequent tests administered to determine the extent to which the ignored figure was perceived, recognition of shape and the location of contour gaps was at the chance level. Moreover, recognition of the presence of contour gaps was significantly below the chance level. These results indicate that preattentive visual processing of unattended objects is too crude to encode global shape and local features such as contour gaps. It is suggested that preattentive processing creates visual representations of unattended objects that contain very limited information about features.

Visual offsets facilitate saccadic latency: does predisengagement of visuospatial attention mediate this gap effect?

Saccadic reaction time (RT) is reduced when a fixation stimulus is extinguished 200 ms before a target appears. An attentional predisengagement theory (APT) may explain this gap effect: When covert attention is engaged (e.g., on fixation), the saccadic system is inhibited and RT is delayed; when the attended stimulus is extinguished, attention is disengaged, the inhibition is removed, and RT is facilitated. In 3 experiments covert attention was endogenously or exogenously cued to an object on the vertical meridian. Onset of a saccadic target on the horizontal meridian could be preceded by the offset of an attended or unattended object. Contrary to APT, RTs were identical after attended and unattended offsets. Results suggest that the gap effect has 2 components, and covert visual attention plays no role. One component is motor system preparation; the other is a fixation offset effect specific to the oculomotor system.

The use of single-frequency Decca Navigator signals for remote position monitoring

The paper describes an extension to the Tracer technique for monitoring and tracking the positions of remote and unattended objects, allowing the signals transmitted by Decca Navigator stations to be used. The problems of retransmitting the multi-frequency Decca Navigator signals have been overcome by receiving a single frequency and decoding the time-multiplexed ‘multipulse’ signals in time-division form.A number of applications of the system are described and results presented. The accuracy, fix repeatability and skywave sensitivity of the system are seen to compare favourably with those of conventional receivers.