In the extreme, this model predicts that a stimulus that is directionally ambiguous or composed of dynamic noise will yield a percept of directional motion when the imaginal component is directionally this website strong (Figure 6B). Support

for this mechanistic interpretation comes in part from an experiment by Backus and colleagues (Haijiang et al., 2006). These investigators used classical conditioning to train associations between two directions of motion and two values of a covert second cue (e.g., stimulus position). Following learning, human subjects were presented with directionally ambiguous (bistable) motion stimuli along with one or the other cue value. Subjects exhibited marked biases in the direction of perceived motion, which were dictated by the associated cue, even though subjects professed no awareness of the cue or its meaning. The discovery of recall-related activity in area MT (Schlack and Albright, 2007) suggests that these effects of association-based recall on perception are mediated through integration of bottom-up (ambiguous stimulus) and top-down (reliable implicit imagery) signals at the level of individual cortical neurons. One important prediction of this mechanistic hypothesis is that the influence of top-down

associative recall on perception should, under normal circumstances, be inversely proportional to the “strength” of the bottom-up sensory signal (Figure 6). To test this prediction, A. Schlack et al. (2008, Soc. Neurosci., abstract) designed an experiment in which the influence of associative CX-5461 price recall on reports of perceived direction of motion could be systematically quantified over a range of input strengths. The visual stimuli used for this experiment consisted of dynamic dot displays, in which the fraction of dots moving in the same direction (i.e., “coherently”) could be varied from 0% to 100%, while the remaining (noncoherent) dots moved

randomly. By varying the motion coherence strength, the relative influence of bottom-up and top-down signals could be evaluated over a range of input conditions. These stimuli lend the additional advantage that there is an extensive literature in which they have been used to quantify perceptual Digestive enzyme and neuronal sensitivity to visual motion (e.g., Britten et al., 1992, Croner and Albright, 1997, Croner and Albright, 1999 and Newsome et al., 1989). The experiment conducted by Schlack et al. (2008, Soc. Neurosci., abstract) consisted of three phases. In the first (“pretrain”) phase, human subjects performed an up-down direction discrimination task using stimuli of varying motion signal strength. The observed psychometric functions confirmed previous reports: the point of subjective equality (equal frequency of responses in the two opposite directions) occurred where the motion signal was at or near 0%.