Figure 1.
Linking propositions for motion discrimination and visual search. (A) Diagram of visual displays. For the motion discrimination task, a field of randomly moving dots appears. Monkeys signal the perceived direction of motion by shifting gaze to one of two peripheral stimuli (rightward arrow). For a visual search task multiple fields of randomly moving dots appear. Monkeys signal the location of the stimulus moving in the direction opposite all of the others by shifting gaze to it (rightward arrow). (B) Discharge rate in visual processing area MT as a function of time from stimulus presentation. Diagram of encoding of preferred (thick) and non-preferred (thin) motion directions. As far as we know, the encoding is equivalent across tasks. (C) Discharge rate in parietal area LIP. During the motion discrimination task (left), neurons in LIP on average exhibit a transient suppression followed by progressively increasing activity that reaches a particular level (dashed horizontal line labeled ART to indicate the level of activity at RT). The rate of this accumulation varies from rapid (thicker) to slower (thinner) according to the clarity of the motion stimulus. This is often interpreted as accumulating the evidence provided by area MT neurons. However, as indicated by the thin vertical line spanning left panels B and C, the accumulation begins well after area MT neurons have encoded motion direction. During visual search tasks (right), neurons in LIP show an initially indiscriminate response followed by elevated discharge rate if the oddball stimulus is in the receptive field (thick) and reduced discharge rate otherwise (thin). This is interpreted as representing the salience of the objects in the array. (D) Discharge rate of presaccadic movement neurons in ocular motor structures FEF and SC. According to a model of the motion discrimination task [100], when discharge rates of LIP neurons reach ART, a subsequent process is triggered that produces the saccade after a stochastic period of accumulating discharge rate occupying ~150 ms. The model identifies this process with the activity of presaccadic movement neurons in FEF and SC, which project to the brainstem saccade generator and initiate saccades 10 ms after reaching ART. According to a model of the visual search task [40], the dynamics of the presaccadic movement neurons correspond to the accumulation of salience evidence. The colored arrows highlight questions about relationships that are elaborated in the text. Cyan, how can neurons represent the evidence in one task and accumulate evidence in another task? Green, how does the reaching of ART by neurons in LIP reliably initiate the subsequent response preparation process? Yellow, how can neurons be the response stage after evidence accumulation in one task and do evidence accumulation (followed by another response stage?) in another task?