Reference speeds (°/s) are color coded according to the key. Each data point represents data from a single subject for a single reference speed. B: comparison of PSE in the uni-uni and uni-bi conditions for a range of reference speeds.

The intersection of the dotted black lines indicates the PSE for veridical speed perception. The clear rightward shift of the point of subjective equivalence (PSE) in the uni-bi conditions represents a strong overestimation of the perceived speed of the bidirectional reference. In the uni-bi condition, the subjects compared a unidirectional test speed ( x-axis) to a bidirectional reference moving at 10°/s. In the uni-uni condition, the subjects compared 2 patches of unidirectional motion, one with a fixed (reference) speed of 10°/s and the other with the variable speed shown on x-axis. A: psychometric curves for 5 human subjects. The only behavioral requirement for the monkey was to fixate the red spot for the duration of the trial. B: in the physiological experiments 1 pattern was presented in the receptive field (indicated by dashed line). Subjects were instructed to indicate the pattern that moved fastest by a key press (human) or an eye movement to 1 of 2 targets (monkey). In the monkey experiments the bi-uni condition was only shown for a small subset of trials and rewarded randomly. In the monkey experiments there was a third condition in which 2 bidirectional motion patterns were shown (bi-bi). In the second condition 2 unidirectional motion patterns were shown (uni-uni). In the first condition a bidirectional motion pattern (2 dot patterns transparently moving in opposite directions) and a unidirectional motion pattern were shown (bi-uni). In the human behavioral experiments there were 2 conditions. Subjects fixated a red dot, and 2 patterns appeared for 500 ms. A: sequence of events in the paradigm used for the behavioral experiments. Labeled line macaque monkey middle temporal area motion perception speed coding.Įxperimental paradigms. These data provide new insight into the encoding of transparent motion patterns and provide support for the hypothesis that MT activity can be decoded for speed perception with a labeled line model. Third, we showed that these changes in neural responses could explain changes in speed perception with a simple labeled line decoder. As a consequence, bidirectional motion of some speeds typically evoked larger responses than unidirectional motion. In addition, we found that tuning curves for bidirectional motion were shifted to lower preferred speeds. Consistent with previous reports, the response to bidirectional motion was often reduced compared with unidirectional motion at the preferred speed. Second, we determined speed tuning curves of MT neurons to bidirectional motion and compared these to speed tuning curves for unidirectional motion. We first confirmed these behavioral findings in human subjects and extended them to a monkey subject. Moreover, the sensitivity for speed discrimination is reduced for such bidirectional patterns. Here we study this relationship by using a strong perceptual illusion in speed perception: when two transparently superimposed dot patterns move in opposite directions, their apparent speed is much larger than the perceived speed of a single pattern moving at that physical speed. However, there is conflicting evidence for a link between speed selectivity of MT neurons and speed perception. There is strong evidence for a link between the activity of these neurons and direction perception. Visual motion on the macaque retina is processed by direction- and speed-selective neurons in extrastriate middle temporal cortex (MT).

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