Psychophysical tasks and performance of human subjects.

A. Illustration of the 2AFC and 3AFC tasks. B. Motion speeds of visual stimuli. The speeds of two stimulus components were plotted versus the log mean speed of each bi-speed stimulus. C. Discriminability of four human subjects performing a standard 2AFC task. D. In the 3AFC task, the percentage of trials that human subjects reported “no two-speeds”. E. Discriminability of the same subjects performing the 3AFC task. B1-E1. X4 speed separation. B2-E2. X2 speed separation. Each color represents data from one subject. The solid line shows the subject-averaged result. Error bars and error bands represent ±STE.

Monkey psychophysics.

A. Behavioral task and visual stimuli. B. Discriminability of a monkey subject performing a 2AFC task. B1. X4 speed separation. B2. X2 speed separation. Error bars and error bands represent ±STE.

Speed tuning curves of four example neurons to bi-speed stimuli and constituent single-speed components.

A. Illustration of the visual stimuli and the response tuning curves of an example neuron. Green and blue dots in the diagram indicate two overlapping achromatic random-dot patterns moving in the same direction at different speeds. Colors are used for illustration purposes only. The abscissas in green and blue show the speeds of the slower and faster components, respectively. The abscissa in black shows the log mean speed of the two speed components. A-D. Four example neurons are sorted by their preferred speeds (PS) from slow to fast. Error bars represent ±STE. For some data points, error bars were comparable to the symbol size. A1-D1. X4 speed separation. A2-D2. X2 speed separation.

Population-averaged speed tuning curves to bi-speed stimuli and constituent single-speed components.

Speed tuning curves averaged across A. 100 neurons in our dataset. B. 10 neurons that had PS lower than 2.5°/s. C. 61 neurons that had PS between 2.5 and 25°/s. D. 29 neurons that had PS greater than 25°/s. Error bars represent ±STE. For some data points, error bars were comparable to the symbol size. A1-D1. X4 speed separation. A2-D2. X2 speed separation.

Response weight for faster component based on linear regression (N = 100)

Relationship between the responses to the bi-speed stimuli and the constituent stimulus components.

A-E. Each panel shows the responses from 100 neurons. Each dot represents the response from one neuron. The ordinate shows the difference between the responses to a bi-speed stimulus and the slower component (R - Rs). The abscissa shows the difference between the responses to the faster and slower components (Rf - Rs). Type II regression line is shown in red. F. Response weights for the faster stimulus component obtained from the slope of the linear regression based on the responses of 100 neurons. A1-F1. X4 speed separation. A2-F2. X2 speed separation.

Timecourse of MT responses averaged across neurons to bi-speed stimuli.

Peristimulus time histograms (PSTHs) were averaged across 100 neurons. The bin width of PSTH was 10 ms. A1-E1. X4 speed separation. A2-E2. X2 speed separation. In A-C, the left dash line indicates the latency of the response to a bi-speed stimulus, and the right dash line and the arrow indicate when the response to a bi-speed stimulus started to diverge from the response to the faster component.

MT responses to bi-speed stimuli moving in different directions.

A. Population-averaged direction tuning curves of 21 neurons in response to stimuli moving at two speeds and in two directions separated by 90° (red). The component direction Dir. 1 (blue) moved at 10°/s and the component direction Dir. 2 (green) moved at 2.5°/s. Dir. 1 was on the clockwise side of Dir. 2. The abscissas in blue and green show the directions of stimulus components Dir. 1 and Dir. 2, respectively. The blue and green axes are shifted by 90° relative to each other. The abscissa in black shows the corresponding VA direction of the two direction components. B. Response weights for the stimulus components obtained using a linear weighted summation fit. Each dot represents the response from one neuron.

Comparison of response weights between attention-away and fixation paradigms.

The red and blue curves indicate the response weights for the faster speed component in an attention-away paradigm and a fixation paradigm, respectively, obtained from the same population of 32 neurons. The black curves are the replot of the data in Figure 5F, obtained from 100 neurons in a fixation paradigm. A. X4 speed separation. B. X2 speed separation.

Population-averaged speed tuning curves to bi-speed stimuli and constituent single-speed components recorded in an attention-away and a fixation paradigm.

Speed tuning curves from one monkey (RG) averaged across A1-D1. 5 neurons that had PS ≤ 2.5°/s, A2-D2. 6 neurons that had PS between 2.5 and 25°/s, A3-D3. 21 neurons that had PS > 25°/s. Error bars represent ±STE. A1-A3 and B1-B3. X4 speed separation; C1-C3 and D1-D3. X2 speed separation. A1-A3 and C1-C3. Attention directed away from the RF; B1-B3 and D1-D3. Fixation paradigm.

Population neural responses elicited by the bi-speed and single-speed stimuli and the performance of a linear classifier.

A population of 100 neurons was constructed by pooling across recordings in different experimental sessions. Each neuron’s response was averaged across experimental trials and normalized by the maximum response of the spline-fitted speed tuning curve to single speeds. Each dot represents the response from one neuron plotted as the neuron’s PS in the natural logarithm scale. The curves represent the spline-fitted population neural responses. Red: response to the bi-speed stimulus; Black: the response to the corresponding single, log-mean speed. A1-F1. X4 speed separation. The speeds of the bi-speed stimuli are 1.25 and 5°/s (A1), 2.5 and 10°/s (B1), 5 and 20°/s (C1), 10 and 40°/s (D1), 20 and 80°/s (E1). A2-F2. X2 speed separation. The speeds of the bi-speed stimuli are 1.25 and 2.5°/s (A2), 2.5 and 5°/s (B2), 5 and 10°/s (C2), 10 and 20°/s (D2), 20 and 40°/s (E2). Two red dots on the X-axis indicate two component speeds; the black dot indicates the log-mean speed. F1, F2. Performance of a linear classifier to discriminate the population neural responses to the bi-speed stimulus and the corresponding single log-mean speed. Error bars represent STE.

Model fit of MT responses to bi-speed stimuli.

A. Speed tuning curve to single-speed stimulus averaged across 100 recorded MT neurons in our data set. B. Population-averaged responses to slower (open circle) and faster (solid circle) speed components. The convention for the speed components is the same as in Figure 1B1, B2. C, D. The response weights for the faster component calculated based on the data (black) and the models of Equation 9 (green in C) and Equation 10 (red in D). B1-D1. X4 speed separation. B2-D2. X2 speed separation.