Huk, Alexander C. and David J. Heeger. Task-related modulation of visual cortex. J Neurophysiol 83: [3525][3526][3527][3528][3529][3530][3531][3532][3533][3534][3535][3536] 2000. We performed a series of experiments to quantify the effects of task performance on cortical activity in early visual areas. Functional magnetic resonance imaging (fMRI) was used to measure cortical activity in several cortical visual areas including primary visual cortex (V1) and the MT complex (MTϩ) as subjects performed a variety of threshold-level visual psychophysical tasks. Performing speed, direction, and contrast discrimination tasks produced strong modulations of cortical activity. For example, one experiment tested for selective modulations of MTϩ activity as subjects alternated between performing contrast and speed discrimination tasks. MTϩ responses modulated in phase with the periods of time during which subjects performed the speed discrimination task; that is, MTϩ activity was higher during speed discrimination than during contrast discrimination. Task-related modulations were consistent across repeated measurements in each subject; however, significant individual differences were observed between subjects. Together, the results suggest 1) that specific changes in the cognitive/behavioral state of a subject can exert selective and reliable modulations of cortical activity in early visual cortex, even in V1; 2) that there are significant individual differences in these modulations; and 3) that visual areas and pathways that are highly sensitive to small changes in a given stimulus feature (such as contrast or speed) are selectively modulated during discrimination judgments on that feature. Increasing the gain of the relevant neuronal signals in this way may improve their signal-to-noise to help optimize task performance. Recent neuroimaging experiments have demonstrated that cortical activity in early visual areas depends not only on which stimulus is being presented, but also on which task the subject is performing (sometimes referred to as effects of featural attention). Instructing subjects to perform a discrimination on a specific feature of a stimulus can selectively increase the activity in the secondary (extrastriate) visual areas that are believed to process information relevant to the task. For example, the human MT/MST complex (MTϩ, also known as V5) responds more strongly to moving than stationary stimuli and is also more active when subjects are instructed to make judgments about stimulus speed than when they are instructed to make judgments about other features such as color or shape (Corbetta et al. 1990 (Corbetta et al. , 1991 ) (see DISCUSSION for further references). The specificity of these effects implies that they do not simply reflect changes in the general arousal level of the observer. Rather, task specificity suggests a close relationship between specific tasks and specific brain areas or pathways. We emphasize, however, that the cortical mechanisms underlying task-specific, featural attention may be different from those underlying spatial attention. Even primary visual cortex (V1) can be affected by task demands. V1 activity can be modulated by instructing subjects to alternately "attend to" and "passively view" a stimulus (Watanabe et al. 1998a) . While these and other results confirm that early visual areas are affected by the behavioral state of the observer, the varieties of task instructions, stimuli, and experimental designs fall short of providing a clear and systematic understanding of how and when task performance can modulate cortical activity in early visual areas. To clarify and quantify the relationships between task performance and activity in visual cortex, we performed a series of functional magnetic resonance imaging (fMRI) experiments with the following goals: 1) to replicate task-related modulations of V1 with task instructions that exert a clear control over the subject's behavioral state (i.e., using a 2-interval forcedchoice threshold-level discrimination task, as opposed to instructing subjects simply to attend); 2) to test whether task performance reduces the variability of responses by controlling the subject's attentional state; 3) to evaluate the consistency of task-related modulations within and between subjects; 4) to replicate the motion specificity of task-related modulations in MTϩ; and 5) to test for specificity of task-related modulations in V1 and other early visual areas. M E T H O D S Stimuli were either random dot fields or radial gratings that moved with an average speed of either 8 or 0°/s. To minimize eye movements, subjects fixated a small, high contrast fixation mark (1°square, centered in the display) that was displayed continuously throughout each scan. In addition, the gratings and dots moved radially inward/ outward (toward or away) from the fixation mark, avoiding a single, powerful optokinetic stimulus. DOTS. The dot stimulus was a field of 75 white dots on a black background placed randomly inside a circular aperture that subtended 14°of visual angle. Each dot was shaped as a two-dimensional (2D) gaussian (with standard deviation ϭ 0.15°of visual angle). The dots moved radially inward or outward from the fixation mark. GRATINGS. Radial sinusoidal gratings (concentric rings) subtending the central 14°of visual angle moved inward or outward from the fixation mark. Spatial frequency was randomly varied (0.4 -0.6 cycles/deg). Temporal frequency was selected, according to the randomized spatial frequency, to produce the desired speed. Subjects performed one of four tasks (speed discrimination, direction discrimination, contrast discrimination, or passive fixation) during each block of each fMRI scan. All four tasks were performed in
For example, area MT+ is more active during a speed discrimination task whereas area V1 shows increased activation during a contrast discrimination task REF .
Task-Related Modulation of Visual Cortex
Journal of Neurophysiology
Psychology Medicine