Dynamics of unconscious contextual effects in orientation processing

Abstract

Contextual effects abound in the real world; how we perceive an object depends on what surrounds it. A classic example of this is the tilt illusion (TI) whereby the presence of a surround shifts the perceived orientation of a target. Surprisingly, the magnitude and direction of this shift depend on the orientation difference between the target and surround: when their orientations are similar, the perceived difference is amplified and the target appears repelled in orientation from the surround (i.e., the TI). However, when their orientations are close to perpendicular, the difference is decreased and the target appears attracted in orientation toward the surround (i.e., the indirect TI). These misperceptions of orientation have revealed much about the underlying detectors involved in visual processing and how they interact with each other. What remains at stake are the levels of processing involved. To examine this, we designed a reverse-correlation technique whereby observers are blind to the orientation of the surround. We find that the TI and indirect TI occur reliably and over a similar time course, supporting the role of a single mechanism underlying orientation biases that operates in the early stages of visual processing before the conscious extraction of the surround orientation.

Fig. 1.

Stimulus presentation and data analysis. (A) A rapid series of annular surround gratings was presented and, every 2 s, a target appeared in the center of the annulus. The observer reported the orientation of the target with a key press (Lower). (B) Observer R.M.’s CW response histogram, (C) CCW response histogram, and (D) orientation bias, as a function of the surround orientation over 1,170 ms. Time 0 represents the time of target onset. Surrounds before the target are assigned negative time values, and those after the target are assigned positive time values. Intensity is proportional to key presses. (E) An χ² analysis was performed on a frame-by-frame basis over the 1,170 ms. (F) Frames that reached significance from E were averaged to create an ROS and plotted as a function of the absolute orientation of the surround (black circles). Percentage is the number of targets the observer responded to over all trials. Error bars are ±1 SEM. Red squares and curve are the result of averaging over the same frames as the ROS in the control experiment and show that the observer could not access the orientation of the surround at any time within the experiment. (G) Orientation bias over the entire 1,170-ms sequence from the control experiment contains no discernable structure.

Fig. 2.

Orientation bias from regions of significance for four additional observers. Only one observer (S.S.) does not get a clear indirect TI at a surround orientation of 67.5°. Error bars are ±1 SEM. Red squares are the control experiment.

Fig. 3.

Linear filter analysis. (A) The CW (or CCW) response histogram approximates the temporal linear filter. (B) Each 100-frame stimulus sequence can be represented as a 100 * 12 matrix in which the orientation present on a given frame is set to 1 and all other orientations are set to 0. (C) Point-wise multiplying the filter and the stimulus and summing across all 12 * 100 bins yields a single index denoting the time averaged response of the linear filter. (D) The value of the index is stored in the CW (dark blue) or CCW (pale blue) distribution according to the observer’s response to the stimulus. The procedure is repeated for all 100-frame stimuli sequences, and a criterion (arrow) is calculated to optimize discrimination between the two distributions.

Fig. 4.

Time course of orientation biases averaged across observers. (A) Distribution of key presses averaged across all observers and normalized by the maximum number of key presses for ±82 ms around the target. Note the clear emergence of the TI (positive peak) as well as the indirect TI (negative peak). Lower: Distribution of key presses for the TI and indirect TI summed across all observers plotted together over the 100 frames (B) and over 40 frames flanking the target (C). Shaded colored areas encompass the 95% confidence intervals centered on the means.

Fig. 5.

(A) Orientation biases in the ROS measured in the presence (red squares) and absence (blue circles) of a 1° gap, averaged across four observers. Each ROS was symmetric and centered on the corresponding peak for the observer and condition. To combine data, the number of frames comprising the ROS was always the same (n = 5) and was determined by using the smallest ROS obtained by any observer in either condition. Error bars are ±1 SEM averaged within observers. Insets show orientation biases in the two conditions for the entire 100 frames. (B) Orientation biases with different surround spatial frequencies for observer I.M. The ROS for each condition contained seven frames (determined as described earlier), symmetrically centered on the peak for the given condition. Error bars are ±1 SEM.