Perceptual learning of motion direction discrimination with suppressed and unsuppressed MT in humans: an fMRI study

Abstract

The middle temporal area of the extrastriate visual cortex (area MT) is integral to motion perception and is thought to play a key role in the perceptual learning of motion tasks. We have previously found, however, that perceptual learning of a motion discrimination task is possible even when the training stimulus contains locally balanced, motion opponent signals that putatively suppress the response of MT. Assuming at least partial suppression of MT, possible explanations for this learning are that 1) training made MT more responsive by reducing motion opponency, 2) MT remained suppressed and alternative visual areas such as V1 enabled learning and/or 3) suppression of MT increased with training, possibly to reduce noise. Here we used fMRI to test these possibilities. We first confirmed that the motion opponent stimulus did indeed suppress the BOLD response within hMT+ compared to an almost identical stimulus without locally balanced motion signals. We then trained participants on motion opponent or non-opponent stimuli. Training with the motion opponent stimulus reduced the BOLD response within hMT+ and greater reductions in BOLD response were correlated with greater amounts of learning. The opposite relationship between BOLD and behaviour was found at V1 for the group trained on the motion-opponent stimulus and at both V1 and hMT+ for the group trained on the non-opponent motion stimulus. As the average response of many cells within MT to motion opponent stimuli is the same as their response to non-directional flickering noise, the reduced activation of hMT+ after training may reflect noise reduction.

Figure 1. A schematic representation of a single trial demonstrating a counter-clockwise change in motion axis about a bisecting motion axis of 45°.

The bisecting axis is depicted with solid grey lines (not present in the real stimuli). The lower panel illustrates the two possible relative phases of dot motion. Counter-phase dots provided locally balanced motion directions and therefore putatively suppressed MT. In-phase dots did not have locally balanced motion signals and therefore MT was presumed to be activated normally. Arrows are shown for illustrative purposes only.

Figure 2. Localization of hMT+.

The hMT+ localization stimulus is shown in panel A with yellow arrows representing the centripetal oscillations that occurred during the dynamic phase. Example localization results for one participant are shown in panel B. hMT+ localization data were acquired pre (top row) and post (middle row) training. The hMT+ ROI used for analysis was derived from the intersection of the pre and post training ROIs (bottom row). The FDR corrected (q<0.001) statistical maps are rendered on inflated representations of the participant’s left and right cerebral hemispheres.

Figure 3. Improvement in behavioural thresholds as a result of learning for participants trained on in-phase dots (A) and counter-phase dots (B).

Each training session consisted of 400 trials. If 75% correct or better was achieved during a training session, the angular deviation was decreased by 1° for the subsequent training session. For two participants trained on counter-phase dots, the angular deviation was increased for two sessions mid-training in an attempt to facilitate additional learning. Each line represents an individual participant.

Figure 4. Average pre and post training psychometric functions for participants trained on in-phase dots (n = 7) (A) and counter- phase dots (B) (n = 11).

A subset of 5 participants trained on counter- phase dots also completed psychometric functions for in- phase dots pre and post training. These data are shown in panel C. For each set of functions pre training data are collapsed across the two motion axis orientations as they did not differ. Error bars show within subjects standard error of the mean , in this figure and all subsequent figures that represent within subject effects.

Figure 5. The response of hMT+ and V1 to counter-phase vs. in-phase dot stimuli before training (n = 13).

Data for individual participants are shown in panel A for hMT+ and panel B for V1. Data points lying above the unity line indicate a greater response to in-phase dots, consistent with motion opponency. Group averages are shown in panel C. * indicates a statistically significant difference (p<0.01).

Figure 6. Behavioural accuracy during scanning (A and C) and the average angular threshold that gave rise to 75% correct accuracy outside of the magnet (B and D) for the group of participants trained on counter-phase dot stimuli (n = 9) (A and B) and those trained on in-phase dot stimuli (n = 7) (C and D).

Data are shown for measurements made pre and post training. Post training stimuli were presented either at the same angular size shown pre training (“same angles” in panels A and C which correspond to pre-training measurements in panels B and D) or at the 75% correct angular threshold as measured outside of the magnet post training (“smaller angles” in A and C which correspond to the post training measurements in panels B and D). The dashed grey lines in A and C indicate 75% correct behavioural accuracy. Asterisks indicate a significant difference from pre training data unless otherwise indicated by a bracket (paired samples t-tests, p<0.05).

Figure 7. The response of hMT+ (%BOLD change) pre and post training with counter-phase dots (filled bars) or in-phase dots (open/textured bars).

* indicates a significant reduction in the activation of hMT+ for the counter-phase dot stimulus presented along the trained motion-axis. Post training data are collapsed across angle size (“same” and “smaller” angles) as there were no differences in the BOLD response for this variable.

Figure 8. The response of V1 (%BOLD change) pre and post training with counter-phase dots (filled bars) or in-phase dots (open/textured bars).

There were no reliable training-related changes in activation. Post training data are collapsed across angle size (“same” and “smaller” angles) as there were no differences in the BOLD response for this variable.

Figure 9. The relationship between the change in BOLD response and the amount of learning.

The percent reduction in BOLD response pre to post training for the trained motion axis is shown on the Y axis and the percent improvement in 75% correct threshold a result of learning is shown on the X axis. Each data point represents an individual participant. Data are shown for hMT+ (top row) and V1 (bottom row) for counter-phase dots (left column) and in-phase dots (right column). Post training data are collapsed across angle size (“same and “smaller” angles).

Figure 10. The effect of counter-phase dot training on the response of hMT+ and V1 to counter-phase and in-phase dot stimuli.

Filled bars indicate counter-phase dots and open bars in-phase dots. Training increased the difference between in-phase and counter-phase dot responses at hMT+ for the trained motion axis.

Figure 11. Behavioural accuracy during scanning for participants trained on counter-phase dots and scanned post training with both counter-phase and in-phase dot stimuli (n = 7).

Pre training data are collapsed across motion axis orientation. The dashed line indicates 75% correct accuracy measured outside of the magnet. Behavioural accuracy during scanning did not differ between the two dot phase conditions (p>0.05) or between the trained or untrained motion axis (p>0.05), however post training behavioural accuracy during scanning was lower than pre training performance, F_1,7 = 6.4, p = 0.04.