Academic journal article Cognitive, Affective and Behavioral Neuroscience

The Role of Selective Attention in Visual Awareness of Stimulus Features: Electrophysiological Studies

Academic journal article Cognitive, Affective and Behavioral Neuroscience

The Role of Selective Attention in Visual Awareness of Stimulus Features: Electrophysiological Studies

Article excerpt

Attention and awareness are closely related, but the nature of their relationship is unclear. The present study explores the timing and temporal evolution of their interaction with event-related potentials. The participants attended to specific conjunctions of spatial frequency and orientation in masked (unaware) and unmasked (aware) visual stimuli. A correlate of awareness appeared 100-200 msec from stimulus onset similarly to both attended and unattended features. Selection negativity (SN), a correlate of attentional selection, emerged in response to both masked and unmasked stimuli after 200 msec. This double dissociation between correlates of awareness and SN suggests that the electrophysiological processes associated with feature-based attentional selection and visual awareness of features can be dissociated from each other at early stages of processing. In a passive task, requiring no attention to the stimuli, early electrophysiological responses (before 200 msec) related to awareness were attenuated, suggesting that focal attention modulates visual awareness earlier than does selective feature-based attention.

There is a controversy concerning the relationship between visual attention (the selection of visual input for detailed processing) and visual awareness (the subjective experience of seeing the stimulus) (Lamme, 2003, 2004; Rees & Lavie, 2001; Rees, Russell, Frith, & Driver, 1999). According to the dominant view, attention is the gateway to visual awareness. Recent demonstrations of various forms of functional blindness for unattended stimuli in persons with normal vision have been interpreted as supporting this view. For example, the phenomenon of inattentional blindness is reflected as an inability to consciously report seeing an unexpected, unattended stimulus (Mack & Rock, 1998). Similarly, change blindness is manifested as a considerable difficulty in detecting changes in unattended stimuli, when two versions of the same scene are separated by a brief interruption (Simons & Levin, 1997). Attentional blink is demonstrated in rapid serial presentation as a failure to detect the second target stimulus when attention is engaged with the first target stimulus presented about half a second earlier (Shapiro, Araell, & Raymond, 1997). All these phenomena seem to imply that we must pay attention to an object in order to be aware of it-that is, that the contents of awareness are identical to the contents of attention. Consequently, it has been stated that every theory of consciousness should incorporate the assumption that attention is a prerequisite of consciousness (Dehaene & Naccache, 2001). However, an alternative and equally plausible interpretation is that we visually experience more than what is in the focus of attention but, because coding the stimulus into memory requires focused attention, only the attended stimuli can be consciously reported (Sperling, 1960; Wolfe, 1999). Thus, although it is clear that awareness and attention are closely linked, it is less clear how and at which processing stage they are linked.

Studies in which online measures of brain activity have been used may clarify the relationship between visual awareness and attention. Neuroimaging studies have shown that the ventral visual pathway from Vl to the temporal cortex, known to be important in visual object recognition, is associated with changes in visual awareness (Bar et al., 2001; Beck, Rees, Frith, & Lavie, 2001; Moutoussis & Zeki, 2002; Pins & fiytche, 2003; Vanni, Revonsuo, Saarinen, & Hari, 1996). For example, in binocular rivalry, the alternating subjective perception of stimuli presented to different eyes is accompanied by alternating activations of neurons specialized for processing the corresponding types of stimuli, while the physical stimuli remain constant (Logothetis, 1998; Tong, Nakayama, Vaughan, & Kanwisher, 1998). In addition, it has been argued that the activation of the ventral pathway alone may not be sufficient for subjective visual perception. …

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