Temporal summation of visual motion

doi:10.1016/0042-6989(94)90241-0

Vision Research

Volume 34, Issue 19, October 1994, Pages 2547-2559

https://doi.org/10.1016/0042-6989(94)90241-0 Get rights and content

Abstract

The sensitivity of the visual system to changes in velocity over time was investigated using the approach that Rashbass [(1970) Journal of Physiology, 210, 165–186] applied to luminance. Pairs of motion impulses (jumps) were presented, and thresholds for discriminating these pairs of impulses from a stationary display were determined. The results were consistent with a model that posits linear filtering of the input velocity, squaring, and integration over some duration. According to the model, the degree of interaction between the impulses reveals the autocorrelation of the impulse response of the motion system. The data were well fit by a three-element cascade of leaky integrators. In the temporal frequency domain, the visual motion system is a lowpass filter. This means that the visual system is quite insensitive to acceleration.

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Under appropriate stimulus conditions, judgments about the degree of temporal synchrony in sequences containing rapid alternations of colour and motion direction imply a large apparent delay of motion perception relative to colour perception. Whether this colour–motion asynchrony results from the relative processing delay of different visual attributes, or from inappropriate matching of time markers assigned to first-order change of colour and position has been the subject of recent debate. Colour–motion asynchrony is significantly weakened when the angle of direction change is reduced from 180° (direction reversal) to a smaller change in direction. Although this finding has been interpreted to favour the processing delay hypothesis, here we show that it is consistent with the time marker account. First, the reported dependence on the motion direction angle was particularly strong for random-dot stimuli, but our results indicate that this may reflect the introduction of an artefact, motion streaks, that allows subjects to make a colour–orientation synchrony judgement rather than a colour–motion synchrony judgment for direction change angles other than 180°. Second, when we used streak-free plaid stimuli, a certain amount of angle dependence remained regardless of whether we asked the observers to judge the apparent binding or synchrony of colour and motion direction changes. The degree of direction change also affected reaction times, but the effect of apparent asynchrony for a direct comparison of sequences of 90° and 180° motion direction changes was very small, if at all present. These findings with plaid stimuli are consistent with the time marker account; in that we allow that the direction change angle can affect the time course of the recruitment of neural responses to the new direction of motion, which will have a consequential effect on the temporal location of salient features in the sequence of motion changes.
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Rashbass [Rashbass, C. (1970). The visibility of transient changes of luminance. Journal of Physiology, 210, 165–186] presented pairs of flashes having various contrasts separated by a delay, and found that the thresholds for detecting the pairs fell on an ellipse. He fit the data using a model that computed the filtered energy of the pulses. Although this Rashbass model is phase-insensitive, many other experimental results show that humans can perform phase-sensitive detection consistent with a template-matching mechanism. We show that an observer who uses a form of template-matching produces thresholds that fall on an ellipse, just like the Rashbass model. The results from two-pulse experiments are consistent with the idea that humans cross-correlate the stimulus (signal or noise) with a filtered version of the expected signal rather than the signal itself. In symbols, we propose that observers compute $\int r (t) [s (t) * h (t)] d t$ where r(t) is the received stimulus on a given trial [s(t) + n(t) or n(t)], s(t) is the signal, h(t) is the visual filter, and ∗ is convolution.
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This is, at least in part, we believe, because of temporal limits on the allocation of attention needed to link transitions and turning points. There is also considerable evidence showing that changes in speed (second-order changes) are poorly detected by the visual system [24–27]. Given that neural sensors specialized for temporal change exist only for transitions, but not for turning points, it is reasonable that the visual system can use only transitions at high temporal rates.
Background: When simultaneous visual events appear to occur at different times, the discrepancy has generally been ascribed to time differences in neural transmission or cortical processing that lead to asynchronous awareness of the events.
Results: We found, however, that an apparent delay of changes in motion direction relative to synchronous color changes occurs only for rapid alternations, and this delay is not accompanied by a difference in reaction time. We also found that perceptual asynchrony depends on the temporal structure of the stimuli (transitions [first-order temporal change] versus turning points [second-order temporal change]) rather than the attribute type (color versus motion).
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