Physiological Evidence

Neurophysiological and electrophysiological studies have overwhelmingly supported the early locus of selection and sensory gain effects of spatial attention. An early locus for spatial attention has been demonstrated with event-related potential (ERP) studies in which the electroencephalogram is time locked to the appearance of a visual stimulus. The experimental paradigm used most often in these studies is illustrated in Fig. 3a. At the beginning of each trial block, observers are instructed to attend to either the left or the right visual field while maintaining fixation at the center of the screen. Stimuli are then presented rapidly and sequentially to the left and right visual fields, and observers are required to respond when they detect an infrequently occurring target stimulus in the attended field. By maintaining the same sequence of stimuli from trial block to trial block and varying whether the left or right field is attended, it is possible to compare

Figure 3 ERP and neurophysiological results from a spatial attention. (a) Observers attend to the left or right side of space, and targets appear at either the attended or ignored location. (b) ERP results show larger P1 and N1 components when targets appear at the attended location than at the ignored location. (c) Neurophysiological results from a representative neuron in area V4. When a target falls within the neuron's receptive field, the neuron fires more vigorously when the target's location is attended than when it is ignored.

Figure 3 ERP and neurophysiological results from a spatial attention. (a) Observers attend to the left or right side of space, and targets appear at either the attended or ignored location. (b) ERP results show larger P1 and N1 components when targets appear at the attended location than at the ignored location. (c) Neurophysiological results from a representative neuron in area V4. When a target falls within the neuron's receptive field, the neuron fires more vigorously when the target's location is attended than when it is ignored.

the response to the same physical stimulus when it is presented at an attended versus an ignored location.

As shown in Fig. 3b, the ERP waveform recorded over occipital scalp sites in this paradigm consists of a series of positive and negative peaks or components. The earliest component, which is called the "C1 wave'' and is observed only under certain conditions, is typically unaffected by attention. Although it is usually difficult to determine the neuroanatomical site at which an ERP component is generated, it is known that the C1 wave is generated in primary visual cortex (area V1), and the finding that the C1 wave is unaffected by spatial attention indicates that the initial volley of V1 activity is not influenced by attention (although V1 activity appears to be modulated by attention at later time points, presumably due to feedback).

The C1 wave is followed by the P1 and N1 waves, both of which are typically larger in amplitude for attended location stimuli than for unattended location stimuli. The P1 effect typically begins before 100 msec poststimulus and combined PET/ERP studies have indicated that it is probably generated in the ventral extrastriate cortex. Moreover, this effect is present for target stimuli, nontarget stimuli, and completely task-irrelevant probe stimuli. Together, these factors indicate that the P1 modulation reflects an effect of attention on sensory processing, supporting early selection models of visual-spatial attention. The N1 effect appears to reflect a modulation of visual discrimination processes, although the precise nature of this effect and its neural origins are not clear.

This paradigm has also been modified for use with single-neuron recordings in monkeys, and a similar pattern of results was obtained (Fig. 3c). Attention had no effect on responses in area V1, but in area V4 (an intermediate visual processing region) attended location stimuli evoked higher rates of neural firing than did ignored location stimuli. Moreover, the effect of attention began at 60 msec poststimulus, which was the same time as the onset of stimulus-related activity in these neurons. However, spatial attention produces no changes in the tuning curves of the neurons. Thus, under certain conditions, attention acts as a preset filter that controls the amplitude of the sensory response in intermediate-level areas of visual cortex.

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