Alais (1999) Neural strength of visual attention gauged by motion adaptation

1015 From everyday experience, we intuitively know that visual attention enables us to select from a cluttered visual scene only those objects or events of immediate relevance. In the case of vision, it is well established that attention modulates neural activity in several areas within visual cortex1–3, including V1, the earliest cortical stage4–7. This attentional modulation seems to involve a boost in the gain of responses of cells to their preferred stimuli, not a sharpening of their stimulus selectivity8–11. Neuroimaging experiments in humans point to comparable attentional effects: brain activity evoked by visual stimuli is enhanced when attention is focused on those stimuli12. Although it is straightforward to gauge attention’s influence on cognitive and perceptual performance, it is less clear how to specify the gain in neural response presumably underlying attention’s influence. We have developed a psychophysical strategy that quantifies attentional gain in the case of visual motion perception, thereby, permitting meaningful comparisons between perception and physiology. Specifically, our strategy used the same metric—motion coherence level—to reveal a striking equivalence of attention’s effects measured psychophysically and physiologically. To probe the visual system’s response to motion, we used the motion aftereffect (MAE). The MAE is the illusory motion of a stationary test pattern viewed following prolonged adaptation to a moving stimulus: the test pattern temporarily appears to move in the direction opposite that of the adapting motion. It is already well known that attention can modulate the duration13 and the perceived strength14 of the MAE. In our study, we used bivectorial motion stimuli composed of two superimposed fields of drifting random dots (Fig. 1). A viewer of these stimuli clearly sees two fields of dots transparently moving across one another. The MAE direction generated by these bivectorial motion fields has a single direction corresponding to the vector sum of the MAEs generated by the separate component motions of the inducing stimulus15,16. Thus, varying the strength of one of the components systematically changes the direction of the MAE. If attention to a given direction of motion indeed increases gain in direction-selective neurons8, then attention alone should be able to induce changes in the perceived direction of the MAE. Specifically, attending to one of the component motions in a bivectorial motion field should shift the direction of the MAE closer to a direction opposite that of the attended motion. Our first experiment tested this prediction. The ‘adapting’ dots (Fig. 1a) drifted continuously at 100% coherence during adaptation, whereas the ‘attentional’ dots alternated between 0% coherence and brief periods of weak, coherent motion inserted randomly throughout adaptation (Fig. 1b; see Methods). Detecting these motion inserts constituted the attentional task during ‘active attention’ trials. Passive trials involved observing the same stimuli without performing the detection task. Following adaptation to these two motion vectors, observers indicated the direction of the resulting MAE. As the ‘adapting’ motion (arbitrarily assigned a direction of 0°) was far stronger than the weak ‘attentional’ motion, MAE direction should generally have been close to opposite the direction of the adapting motion (180°). Depending on which of the 8 ‘attentional’ motion directions was combined with the adapting motion, small systematic deviations in MAE direction should follow a sinusoidal pattern oscillating around 180° (Fig. 1c). The amplitude of the sinusoid would depend on the weights of the vectors, with stronger attentional-motion components producing larger amplitudes. If attention boosted the neuronal response to an attended motion, then active conditions would yield a larger-amplitude sinusoid than passive conditions. In other words, the deviations in MAE direction opposite the adapting motion would be enhanced by active attention17.