Noise Reduction and Suppression of Competing Stimuli

Figure 2 shows that for about 200 msec after an exogenous cue and for 600 msec or longer following an endogenous cue, RT to detect the onset of a target is shorter when a target appears at the valid cue location compared to the invalid cue location. This RT effect indicates that attention has been drawn to the location of the cue, thereby facilitating the detection of the target at the valid location compared to the invalid location.

It is generally agreed that the presence of attention due to valid cueing increases the signal-to-noise ratio or sensitivity of information processing at the cued location. Of course, this could happen either if the signal from the attended location were enhanced (a facilitatory process) or if noise that might interfere with that signal were reduced or suppressed (an inhibitory process). The available evidence supports a combination of these two effects.

First, valid cueing can increase sensitivity even when a single target appears in an otherwise blank visual field. Since there are no other stimuli around to produce any noise or distraction, it would appear that attention is enhancing the signal from the attended location. However, the effect of cueing a target in an otherwise blank field is often small and sometimes disappears. The impact of cueing is usually greater when the target is accompanied by distracting stimuli at other spatial locations. From such evidence it has been argued that the primary contribution of spatial attention, especially in real-world visual environments, is to reduce noise or cross talk from unattended stimuli that if unsuppressed would interfere with processing the attended stimulus.

Direct evidence for inhibition of cross talk-based interference from unattended stimuli comes from single-cell recording in extrastriate cortical areas of the ventral object-processing pathway of monkeys. When two stimuli are present in the receptive field of a cell in one of these areas (e.g., V4 or TEO), the cell's response is less than if only one of them was present. This reduction in activity shows that the two stimuli interfere with each another's ability to activate the cell. However, if the monkey attends to one of the stimuli for the purpose of performing an experimental task, the cell's response returns to approximately the same level as if the attended stimulus were presented alone. This restoration of responsiveness shows that the unattended stimulus is being filtered out of the cell's receptive field and hence no longer interferes with processing the other stimulus. Analogous evidence that humans have a similar attention-driven mechanism of noise reduction through suppression of competing stimuli comes from functional magnetic resonance imaging (fMRI) experiments reported by Kastner and colleagues.

Breaking Bulimia

Breaking Bulimia

We have all been there: turning to the refrigerator if feeling lonely or bored or indulging in seconds or thirds if strained. But if you suffer from bulimia, the from time to time urge to overeat is more like an obsession.

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