Article Figures & Data
Figures
- Figure 1.
Stimulus and task design. Each black box represents the computer display visible to the subject in the different periods of each task. A, Macaque monkeys were trained to perform a speed change task while fixating a central red target. On each trial, 200 ms after acquiring fixation, choice targets (green triangle and magenta cross) appeared for 450 ms, followed by a dot stimulus moving in the preferred direction of the recorded neuron. Speeds during this reference period were 8 or 12°/s. After a random delay of 550–5000 ms the speed abruptly changed to a test speed (4–18°/s). Trials with each adaptation speed, and with speed increments and decrements, were randomly interleaved. Behavioral choices could be made at any time by making a saccadic eye movement to one of the two choice targets, which were presented above and below the fixation target and were visible from the start of each trial. Only correct discrimination choices were rewarded. B, Human subjects performed a similar speed change discrimination task while fixating a central target; however, choices were indicated at the end of each trial by keypress. All trials within a block had the same reference duration (13–3240 ms) and test duration (400 ms).
- Figure 2.
Single neuron speed tuning during passive fixation and speed change task. A, Transient (70–170 ms) and sustained (200–500 ms) speed tuning measured during passive fixation. Data points show mean ± SD. Dashed line is mean spontaneous rate evoked by stationary dots in receptive field. B, Speed tuning measured in period 50–150 ms following speed changes from a reference of 8°/s (white circles) and 12°/s (black circles). C, D, The corresponding spike density functions are shown for stepped speed changes from reference 8°/s (C) and 12°/s (D), aligned on the time of the speed change. Four different test-reference speed ratios are shown for each reference speed. The shaded gray panel indicates the period 50–150 ms after the speed change which is used to calculate speed tuning following the speed change (B). Arrows indicate responses to test speeds of 9.6°/s.
- Figure 3.
Quantification of changes in speed tuning. A, B, Speed tuning of 2 neurons following stepped change in speed from a reference of 8°/s and 12°/s. Solid lines show laterally shifted second order polynomial fitted simultaneously to both datasets. Text in the lower corner of each panel indicates lateral shifts (δx) and vertical gain changes (g) between the two datasets, based on fits of Equations 2 and 3. C, Distribution of lateral shifts and vertical gain changes associated with separate fits to the two speed-tuning curves. Data are shown separately for each animal and cortical area. Spearman correlation between lateral shift and vertical gain change was significantly different from 0 for individual animals and areas (rN,MT = 0.83; rN,MST = 0.76; rP,MT = 0.80; p < 0.01).
- Figure 4.
Changes in neural tuning over time. A, B, Comparison of mean normalized spiking rate (A) and Fano factor (B) in sliding 100 ms time windows, shown relative to the onset of reference motion. Shaded regions show mean ± SEM (n = 121). Responses were combined across two animals, both reference speeds and all test speeds. Spiking rates were normalized by subtracting the spontaneous firing rate in the period 200 ms before reference motion onset and scaling to give a normalized peak response of 1 in the first 400 ms of reference motion. C, D, Comparison of speed tuning slope and lateral shift calculated separately from Short and Long trials, corresponding to trials with reference durations shorter or longer than 1 s. Only neurons (n = 87) with at least 4 trials with each reference test condition in each time window were included. Black data points in (D) are those with speed tuning slope >20 spikes/s/octave.
- Figure 5.
Adaptation induces tuning changes. A, B, The size of lateral tuning shifts is compared with the RRI (A) and TSI (B). The RRI quantifies the difference between the neuronal response to the 8 and 12°/s reference speeds. TSI is a measure of short term adaptation that quantifies the size of the change in the response evoked by the neuron's preferred speed over the first 500 ms of motion. Neurons (n = 87) were only included if their speed tuning curve was measured during fixation and was well fit by a skewed log-Gaussian. Diagonal lines, with corresponding r and p values in each panel show the results of linear regression. Only lateral tuning curve shifts showed a significant correlation with TSI. This correlation was significant for neurons from monkey N (r = 0.44, p < 0.01) but not monkey P (r = 0.30, p = 0.09).
- Figure 6.
Neural decoding. A, B, Comparison of neurometric performance calculated using “Relative” and “Absolute” (A) or “Wrong” (B) decoding schemes. Neurometric performance is defined as the proportion of trials in which a neuron's firing rate can be used to correctly discriminate the sign of the speed change. Neurometric performance of 1 represents perfect decoding (every trial is correctly predicted), 0.5 represent chance decoding. With relative decoding, each test response is compared with the mean response evoked by the reference speed presented on that trial. With absolute decoding, each test response is compared with the mean response evoked by both reference speeds. With wrong decoding, each test response is compared with the mean response evoked by the reference speed that was not presented on that trial. For both animals, the relative decoding scheme gives significantly better neurometric performance than either absolute or wrong decoding (p < 0.01, t test). No significant differences in performance were found between animals or between areas MT and MST for monkey N (p > 0.05, t test).
- Figure 7.
Human psychophysical data. A, B, Psychometric tuning functions from subject NP for reference durations of 120 ms (A) and 1080 ms (B). Fits are logistic curves constrained to have the same slope for the data with each reference speed. C–E, Summary metrics for 3 subjects tested with reference durations of 13–3240 ms. With increasing reference duration, the lateral separation in the point of subjective equality (PSE) increased (C), the Weber fraction calculated from the 75% threshold in the fitted logistic curves decreased (D) and the percentage of correct responses for trials with intermediate test speeds in the range 8–12°/s increased (E). Dotted line in (C) shows the expected separation of psychometric curves assuming a purely relative response (0.585 octaves). Open markers in C indicate the median lateral shift in neuronal tuning curves for monkey N (Δ) and monkey P (○). The mean performance of the 3 human subjects is shown overlaid on the performance of the two monkeys (F). Reference durations for the monkey experiments were combined using a sliding 200 ms window.
